Male vs. Female Brain Differences & How They Arise From Genes & Hormones | Dr. Nirao Shah

25:57So many genes you can take from the human genome and put in the mouse and can get, you know, mouse mutations rescued. But you can't do that with SRI. So it's been mutating so fast because it's sort of important for speciation and protecting the sort of species advantages that led to the development of that species. So you can't take SRI instead of moving between species. Not only that, but as you said, we're alluding to that, many species in which invertebrates in which SRI is not even relevant for sexual differentiation and determination. What happens is, as you put out population densities, can regulate that.

26:32Temperature can regulate that. Sex differentiation. I think it's true in alligators and crocodiles maybe. And suddenly adult fish can transfer from female to male as well. Wow, I didn't realize it was that common. But it makes sense if for these ectotherms that regulate their temperature based on the environment, every species main goal is to make more of itself and protect it's young. And protect the advantages that has the species in the ecological environment and find itself in. Right. So you sort of close, you don't exchange gene pools between species. For example, right. Yeah, there's a whole other discussion years ago, I think you and I were attended a talk where you can, you know, somebody working on Drosophila, you know, species of flies, that you know, Drosophila prefer to mate with Drosophila as opposed to other species.

27:22And this gets a little bit kind of gross slash edgy when you start thinking about, yeah, like why is it that species maintain reproduction basically within species, one hopes, as well as sex behavior within species. And as you point out, every species is vying for itself. In fact, we had a plant biologist on here recently, the plants are making things to kill off their predators, you know, limit their their for -cundity. Let's talk about how hormones downstream of S .R .Y. or the absence of those hormones shaped the brain. Because I think people listening to this certainly know people of both sexes, right.

28:00And I don't think it's that politically edgy to say that most people probably believe that men and women, boys and girls, even, respond very differently to the same stimuli. And the stereotype here is, you know, she started playing with dolls from the beginning. So he picked up a stick and pretended it was a weapon from the moment that he picked up a stick prior to puberty, prior to the testes secreting testosterone. So what is known about hormone -based differentiation of the brain in terms of maleness and femaleness? And let's just for the moment suspend all politics, all stereotypes, just as like, what does the biology say?

28:50So there are a couple of classic experiments in the field, done in the 1950s that really speak to this, the sort of organizational differentiation effect of hormones. And then we've done some additional work in the mouse that also relates to this. And then there are human conditions that can inform this discussion as well. So the first experiment I would like to talk about is by Charles Phoenix in 1959, I think. And he did this experiment in guinea pigs, and guinea pigs become female, mass -monizer, feminized, in utero, prenatally, just like humans do. And if he gave testosterone to the pregnant female, then females that were born to that mother had received testosterone, had seen testosterone that brings it, seen testosterone in development in utero.

29:34And when they were born and became adults, they had probability of mating like a male, like sexual, having sexual behaviors like a male. So a thrusting behavior. And they had very little receptivity, sort of female type of behaviors, which in rodents is typically lordosis. The archer of the back. People have cats know about this, right? For example, cats in heat will lardose. So that, and even if he gave the females adult females who had seen testosterone early on, if he gave these adult females boosts with estrogen and progesterone to sort of increase female sexual behavior in these females, then very little displays of female sexuality.

30:17This amounted like males. Okay, so the exposure of females to testosterone in utero sets up a program whereby their sexual behavior appears more male -like. Thrusting behavior and lack of lordosis. So there's a, there's a presence of something in the absence of something. What about aggression? Were they more aggressive? That paper didn't look at aggression. We've done that in the mouse. And you basically see the same thing. Now in mouse sexual differentiation, as we talked about earlier, happens right at birth or just around birth. So we could take day one pups and if you give them testosterone, these females became territorial -like males as adults.

30:59Interesting. So territorialism is a male -specific trait. Mice. Mice, male -mice, territorial female mice, at least in laboratory don't fight as much except when their mothers are nursing a letter. Maternal aggression is very real. Is it testosterone -mediated? We don't know. Interesting. Okay, so exposure to testosterone in utero sets up male -like behaviors in female offspring. This one I'm hearing. I'm aware of at least one condition in humans where this might occur, which is when there's either a tumor or stress -induced stimulation of the, or over stimulation of the adrenal glands. And of course the adrenals make adrenaline and cortisol, but also they have a layer of cells that produce interesting dion, which is an, an angrogen.

31:49What is the outward appearance of female babies born to women who had an overactive adrenal during pregnancy? Yes, I think you're referring to congenital adrenal hyperplasia, which is a mutation in an enzyme that typically makes cortisol. And this happens in the baby itself. So the baby is a mutant for this enzyme. Or so the babies adrenal are the ones that are disrupted in this case. And because they can't make cortisol, these sort of precursors to cortisol get shunted into making as you pointed out androgens. Because as excess precursors it just gets shunted off into a different pathway. So these babies, these females, are born with sort of mass -renized external tanotalia.

32:33Based not on the presence of testes or testosterone. Or S or Y. But presence of testosterone of androgens, right? Because the adrenals are now pumping out androgens rather than cortisol. Are the androgens that come from the adrenals the same in terms of they bind the androgens receptor just like testosterone? So they look like testosterone. Actually some years ago androstein dion was the topic of a lot of news stories because of Mark McGuire, the baseball player, was accused of taking androstein dion. I mean it's not hard to see the differences in his physical size from one season to the next.

33:08Whether or not he did that or not, I don't know if it was ever confirmed. I think it was. I see. We can ask him. I don't want to put anything on him that wasn't true, but that's what the news claimed. And you could buy androstein dion in the GNC. But the adrenals make testosterone -like substances in this person that doesn't have the capacity to make enough claims. Cortisol. So what does the female offspring look like? She has a sort of massacred external dionitalia. And that can be surgically corrected because now doctors are aware of this condition. So they can surgically set off correct that and you can give the baby when she's born cortisol because that's absolutely essential for survival.

33:51So she's XX genetically female. She has no SRIG might. She made too much testosterone in utero. So the clitoris resembles a penis more or less. Yes. And the reason I say more or less is not to be facetious. It's that there's a continuum there. It depends on exactly when the adrenals kicked in from the adrenals. So it could be an enlarged clitoris or it could be a small penis or it could be a normal size penis. It just depends on how much androgynous. She's paralyzed, as we say. Vuralized. Does she have facial hair? As a baby no. Okay, later. No, I mean, the surgically was corrected for right? As a point it out.

34:31You can give cortisol to the female. But she's fertile as a female because she still makes ovaries because she doesn't have the SRIG. That's correct. Wow. All right. What about stress -induced androgen release in the pregnant mother? Does that arrive to the fetus? So let's assume there's a female fetus. Everything's progressing normally. She has normal adrenal function. But mom, who also has normal adrenals, no CH mutation, goes through a period of extreme stress. It's making a lot of cortisol, but also a lot of interesting dion. Or maybe she has a challenge stress that requires she produce more androgens, which happens.

35:12Does the baby see those androgens and does it partially masculinize or viralize, as you said, the fetus? There's no reason why the baby won't see that testosterone or the androgens. Because it's lipid, it should cross over into cells. Whether or not it affects a behavior, I don't know actually the human data on that. Or whether or not it viralizes, I don't know the data on that. But we know stress during pregnancy is not good. It's not good. It's associated with high incidence of schizophrenia and things like that. But we don't know that it's because of stress -induced release of androgens. Right.

35:42Yeah, okay. It's out of the mouth, right. I think it's just an important thing to distinguish because if people will hear a goodness, I had a stressful second trimester or something of that sort. To step back for a moment before going into more of these kind of naturally occurring experiments. I don't know if that's the proper way to think about it, but they are. They're naturally occurring outcomes. How much variation is there in terms of masculine to feminine phenotypes at birth? Has anyone ever looked at that? I mean, we sort of present like you. It's a baby girl. It's a girl. It's a boy, right?

36:17You know, the gender reveal thing or whatever. You know, on the ultrasound, it's a boy. Okay, there's an opinus. It's a girl. And there's other markers too. You know, that people have gotten quite good at recognizing male versus female fetus on the basis of a number of different things. But most notably, the absence of opinus is generally the driving the conclusion that's a female until chromosomal typing is done. That's right. But what is the range in terms of phenotypes? Has anyone ever actually explored that? I think John Hopkins had a program to do that back in the, you know, about 50 years ago.

36:56And I think back then, at least it was just the size of the penis. That said, this is the boy or not. From the extraordinary to tell you, I don't know what the current criteria are. I'm not a practicing MD. You are an MD though. I am an MD. I don't practice that. Yes. So don't put the current criteria. But with carrier type, man, you can easily tell. You look whether or not it's XX or XY. That's right. Okay. Well, thank you for saying that because the reason I asked that question is that some years ago, there were these reports of people who had grown up being treated as a male, having received testosterone injections or something like that.

37:32And then later discovered that they have XX chromosomes. Other people reported having XX chromosomes never been treated with anything, but they thought they were, they were, you know, appeared male because they had one of these conditions that increased testosterone. And my understanding at the time was that the level of OK -ness, I don't even know what the word is, the level of OK -ness of the person with how they were raised, oriented very strongly with whether or not they were XX or XY. Not which hormones they had seen during development. In other words, if somebody had XX chromosomes, no S -R -Y gene, but was exposed to a lot of Androgens, maybe from their Adrenals or elsewhere, a drug that the mom was treated with during pregnancy, perhaps, that they would hit puberty and they didn't feel, quote, unquote, right.

38:23And in fact, genetically, they were female. And then the reverse cases were also true. And oftentimes these people would seek corrective hormone therapy or surgeries. So what I'm talking about here is actually the opposite of what we hear so much controversy about today, where people want to switch. These are people who were forced by their parents and their doctors to be raised a certain way that did not match their chromosomes. And it generally did not feel good to them. That's why. What does that tell us about the role of genes in establishing male -ness or female -ness of the brain? So we can go back to the condition we talked about earlier, you know, where the puberty used proud of penis, because you had a deficiency in alpha -rock days, so you're not picking DHT.

39:05So these kids were racist girls, because there's no, the external genitalia looked like they're feminized. But as soon as, you know, the had puberty and they start getting paralyzed, they get this proud of penis as you put it, many of them switch over to being boys and becoming men. Happily, I guess so. I mean, they switch, right? It's not forced on them. Okay, so they voluntarily go in the direction of their XY chromosomes. Right. Because in theory, they could, well, it's tricky because they're now making testosterone. So they're sort of in a... Well, they've always been making testosterone, it's just they had not been making DHT.

39:41Sorry to interrupt, but... No, no, no. Please, you're being accurate. So testosterone can still act on the brain, remember, doing development. So what you're basically saying is that the growth of the penis is largely determined by DHT. Right. Early on, yes. Pupu -Bertle. And then after puberty, it's controlled by testosterone. testosterone's sufficient to drive penis out of it. Got it. Okay. Goodness, what does this tell us? Does this tell us again that XX versus XY is really the driver of one's own sex preference? And I don't mean sexual preference for partner. I mean sex preference of their own sexual identity.

40:23At least that's what these natural variations tell us, right? As you put it, natural experiments tell us. The same is true for complete androgen -insensitivity syndrome in which humans have this mutation in the androgen receptor. So they can't see testosterone. And as we discussed, they are completely feminized externally. But they have testes because they have XY. Right. So they have testes. But they're feminized and they think of themselves as females. They raise as females. They look like females. It's just that puberty, you know, they don't start menstruating. So they go to the clinic that diagnosed as XY with an S .R .Y.

40:59But not responsive to testosterone. So they're inability to respond to testosterone, sort of mask -in -wise. Feminize them. This is a tricky topic because we haven't injected kind of how people are socialized. We haven't talked about, you know, pink versus blue clothing, which is socialization. It's a choice, obviously. But a strong choice that's very, you know, statistically, you just see that almost across the board, unless people deliberately go against that. It all seems so clear and straightforward based on the presence or absence of this S .R .Y. gene. Until I start looking at the genetics, which I did in anticipation of this episode, and I discovered that one in 12 people, which is a very high number, is heterozygous for congenital adrenal hyperplasia, meaning they have one mutant copy, one healthy copy, their fertile, which is probably why it's so prevalent.

41:57And yet those people make less cortisol and more androgen in response to a stressor. So then you say, well, okay, maybe as a fetus, they were making a bit more androgen. So is that going to drive a kind of hyper -mailness, or is it going to in an XY baby, and it's maybe going to drive a little bit more mailness, a little bit less -femaleness, and an XX baby? I mean, it starts getting really tricky. It is very tricky. What is known is that boys who have congenital adrenal hyperplasia seem to be completely like boys. They're fertile. The fertile, and the behavior seems to be unchanged as well. So it's not as if they're hyper.

42:41They're not hyper -mastoralized. They're not hyper -masculinized. At least that's what the data suggests, yes. But of course, we don't know what the measures are. We don't know what the measures are, and we don't know what social cultural exposures they had as well in the environment. Having grown up in a very conventional home with respect to these things, I mean, it's like looking back and comparing to what I see now. It's so vastly different, and I was born in 1975, so it kind of blows my mind. How different things are, even in the last 20, 30 years, in terms of how boys and girls are socialized.

43:10I mean, things were... I remember the first television show coming out in the... I forget when it came out exactly, but all in the family where the mother is going to work. This was like a revolutionary thing at the time, right? But it wasn't terribly long ago. Okay, so let's talk about hormones shaping brain structure and function. What are some of the anatomical and or functional differences in brains? Let's say with the most typical scenario. XY chromosomes, makes testosterone, makes DHT, all the receptors are functional, versus XX, no SRY gene, all the stuff, testosterone and estrogen are functional, receptors are functional.

43:59The typical pattern. How are the brains of those babies and later adults different? What do we know about that? There are a lot of cells in the brain that express the receptors for testosterone, endogen receptor, and estrogen and progesterone. So people have looked over the last 40, 50 years to see how these cells are responding to these hormones. And it seems that at least one major thing that emerges is that early on, at least in the mouse right, you can still see that the brain is bi -potential at the first day of life. It looks somewhat neutral. And then if you have testosterone, then in some brain regions, more neurons will survive.

44:42And in those regions, in the female, those neurons will die. So then as adults, you end up with a male brain that has more neurons in one region compared to a female. And conversely, in the female brain, there are structures that survive, and the males you lose cells. So in those structures in the adult, females will have more neurons than males, or cells than males. So you have cell death that can be sex -specific, you know, female -specific or male -specific. Actually, I should say that, it's not specific. It's more statistical, that there are more cell death in one than the other. So you end up with different numbers of neurons in the adult animal.

45:19And you're not getting those neurons back. You're not getting those neurons back. So it's the same as true for connectivity. So it's fair to say that as a consequence of genes and hormones in you to grow, males have certain neurons in circuits that females don't have, and females have certain neurons in circuits that males don't have. And it doesn't matter how much testosterone or estrogen, you put into the adult of those people. They're not getting those circuits back. Right. And you draw the other saying, where once you've been exposed to testosterone or estrogen -progesterone, you get cell loss in one or the other sex.

46:01And once you get that cell loss, you're not going to cover that as an adult. Is there any evidence in humans or in mouse that the loss of the cells or the maintenance of these cells we can look at it through either lens is along a continuum, or is it pretty strict divide? Like, if we were to plot the number of cells in one of these brain areas, would it be a binary distribution where you get a big pile of neurons? On one side of the graph and many fewer in the female with a big trough between, or are we talking about a more single -hand? In some regions, it looks pretty binary, and these are regions that control innate behaviors, like mating or aggression, for example.

46:45But others, there's going to be overlap. And the animals we work in in the mouse, they sort of specifically bred to be genetically identical to each other, so we can sort of really parse out what the differences look like. And if you will, there are more extreme examples of these animals. And there, in some regions, we can really see that, you know, there's always about two to three -fold more cells in one sex compared to the other. And that's pretty much true for all animals, for that region. But other regions that might be more overlap. I'd like to take a quick break and acknowledge our sponsor, AG1.

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50:06I love the mall. If you'd like to try Element, you can go to drinkelement .com slash Huberman, spelled drinklmnt .com slash Huberman, to claim a free Element sample pack with a purchase of any Element drink mix. Again, that's drinkelement .com slash Huberman to claim a free sample pack. To remove some of the sociological, political and other sorts of biases that understandably kind of get into people's minds when you start talking about this. If you just look back in history, were there examples of intersex people just born of, you know, without any knowledge of chromosomes, without any knowledge of hormones, people intuitively understood hormones, but based on damage to the testes or things like that, right?

50:55What would happen? But I think you get the idea, were there examples that were cultures where it was kind of understood that this was along the continuum, because everything you're describing makes it sound pretty darn binary. And, you know, again, this isn't a political discussion, it's a biological discussion. S -R -Y, yes or no. Yes, it seems to be pretty much what it's about. Yeah, so, but there are cultures, I mean, we mentioned about these contying with us marriages where people, you know, would have kids where they would look feminized early on, because they have a deficiency into five -authority, no DHT production.

51:32And then at 12, they would become, you know, masculinized, they would sprout a penis. But never in the other direction, you know, males converting to females, right? Yeah. Physically, no, right? So at least in these cultures, it's a known thing that there will be a subset of kids who are born with this fuel intersex condition. Right? And there are descriptions of, you know, what people used to call it, it's no longer politically correct to say, call them hermaphrites, but there are examples of, you know, intersex individuals across history. Hermaphrodite is not a politically correct, that's what I'm told.

52:03Intersex is the medically self -accepted term, got it. And people have also known that testosterone or hormones, sex hormones, play a huge role in regulating behavior, right? So unics and castrates, gastroaties. I've been used in palaces and courts, sort of to guard herms, for example. That was the motivation. Yep. Wouldn't you favor a more aggressive, testicularly intact male, if you think goal is protection? I think the idea was that if you had, you know, a cast with guarding a hand of females, then they can't sort of, you know, have sexual behavior with them. They can't have sex with them. Oh, then we're going to do what the cuttlefish do.

52:40Cuttlefish males will pretend they're females, befriend females, and then they'll mate with them. And also in opera singing, right? You would have gastroaties who would have a high pitched voice. And they were castrated early in life to maintain the high pitched voice. Yes. Anyway, I'm just going to refrain from any, I mean, the poor kids that presumably they didn't get a choice. Presumably, yeah. Yeah, ex. Okay. So here's where I'm stuck. I can hear all this biology. And it's very clear that the genes and hormones are affecting peripheral, what we call phenotypes. Presums are absensitum, as presums are absensitum, descended testes, presums are absensitum, menstruation.

53:29But in the brain, it just seems that there are different circuits that kind of pile up more neurons or maintain more neurons in males versus females. In females, what are the circuits that get favored? Are they circuits for lactation, for child rearing? What is the behavior, for example? Obitation. So that's a control ovulation, for example. It would be very dimorphic. But not in terms of behavior, right? Like it seems like it's the presence or absence of rough and tumble play. Presence or absence of thrusting behavior. Maybe this is for historical reasons or maybe it's for biological reasons.

54:06But I guess what I'm getting at here is what are the things that babies that are exx, that are females, how are their brains specialized? I mean, or is it just the absence of copulatory thrusting and aggressive behavior? It seems to me that there would be circuits that were female specific. That's right. So there are circuits that are specific female sexual behavior. So you can take an adult male, for example, and you can move testosterone, you can castrate them, and it can give them female hormones, estrogen and progesterone, and ask, this is in mice now. You can ask, will he now be sexually receptively large dose, like a female house with?

54:44Arched back, sexually receptively posture. And most, in most cases, he won't. No, he won't, because the circuit's missing. Right, the neurons just aren't there. At least they're not responsive to the hormones. We don't know if the circuits there, but it's not responding to hormones, or we don't know if the circuit's not there. We now know that there are connections in the female brain that are simply missing in the male brain. And these connections are from neurons that regulate sexual behavior. So we know that some circuits are missing in the male brain. For sexual, female sexual behavior. So, lordosis behavior in females seems to be a very XX chromosome driven outcome.

55:22But it's not as black and white like that. There are circuits that seem to be conserved in both sexes for the behavior of the opposite sex. And I'll give you two examples of that. If you take an adult female mouse, and this is an experiment done in the 70s by David Edwards and Catherine Bergey, it's a really beautiful experiment. And it came around because he was doing a control experiment. He was simply giving testosterone to adult females, adult female mice. And the idea was to sort of see if he got the same results as Charles Phoenix did with kidney packs. So he gave testosterone to young females at birth, as well as to adult females, and the adult females were controls.

56:05The idea was, will these females mount like males if they've seen testosterone early on? The surprising result that he got was that adult females given testosterone, mounted like males. So they have the circuit for male sexual behavior, but it's not activated because there's no testosterone. Similarly, if you take, and this is worked by Catherine Delac at Harvard, if you take mice and you sort of remove pheromone sensing from them, you know, pheromones are these chemical cues that animals use to sort of recognize sex and social status of other individuals with their species. If you sort of disable pheromone sensing in mice, females will now show male -taps sexual behavior.

56:45And so as if that pheromonal input is inhibiting male sexual behavior. But if you take away the pheromone sensing capacity, then the females will start mounting like males. So you have at least two sort of controlled mechanisms, if you will, to inhibit adult male sexual behavior in adult female mice. What is the essence of testosterone or very low levels of testosterone? And the other is the sort of pheromonal input, this is chemo sensory or factory input. That is inhibiting male sexual behavior. You take either one of those, I mean, you give testosterone or you take away the inhibition from those pheromones, you get male sexual behavior.

57:19So it's seen that parts of the circuit for male sexual behavior to display the behavior are there in the adult female brain. So in some cases, the circuit seems to be missing, like the female sexual behavior circuit. Because you can give an adult male estrogen and progesterone to mimic estros or heat, and he doesn't learn those. We can take an adult female and give it testosterone, and she will have, you know, she'll show sexual behavior like a male. And because it's probably in the back of people's minds and because I'm very familiar with this literature, we should just point out that all data point to the fact that you don't see market differences in androgens or estrogen if you were to look between women who define themselves as heterosexual versus homosexual.

58:11So heterosexual women versus lesbians or heterosexual men versus homosexual men. And if anything, the data point to homosexual men having higher levels of testosterone, it's been difficult to tease apart from some lifestyle and behavioral things. But when teased apart, and it's been done, you're not going to find anything that screams hormone levels define sexual orientation. You just don't find that. You don't see that now. You see a lot of data that points to changes in utero that may be hormone driven, but nothing as adults. And in fact, if we can take, you know, what we call wild type male mice, if you will, right?

58:52Meaning that it's sort of completely typical or normal male mice. And you can measure their testosterone levels. And you get a huge range of circulating testosterone and otherwise normal mice of, you know, are five to ten full difference in testosterone. Or humans for that matter or humans for that matter. And they still, you know, these mice will still behave like males. I won't out this person, but I'm not talking about sexual orientation. The CEO of one of the most successful media companies in the world came up to me at a gathering two years ago. And he said, listen, I have this ever problem.

59:31So usually when a guy says that to me, it's going to be something about testosterone or sexual dysfunction or something. And he said, his testosterone is down in the 300s, kind of lower end of reference range. He said, but I feel great. He's like, he's saying, my libido is great. My work drive is great. I feel great. And I said, well, your free testosterone is probably normal and high. And he goes, no, that's also low. But I feel great. Should I take testosterone? And I said, listen, I'm not an endocrinologist, but my advice will be no. And I point this out, I think he's probably in his late 50s, early 60s.

1:00:06And what he was revealing was unique among the questions I typically get around testosterone. But I think it points to the fact that who knows, maybe he has a higher than normal receptor density that can make use of those levels of testosterone. I mean, there's so many ways in which hormone levels can play out in one direction or another or something in between. I think it's worth it people knowing that. That's right. I have so many questions, but this feels like thorny territory. And I've learned when doing this podcast, whenever something feels like thorny territory that to go right into it.

1:00:41These days, we hear a lot endlessly, it seems, about the debate as to whether or not sex differentiation and gender are biologically determined or are more important. We're certainly not going to resolve that question here, certainly not for everybody, sure you have your stance and I have mine. But how is it that we bring together our understanding of sex differentiation versus this gender word? It seems to me that in a lot of talks you've given, you use the word gender. I know because I've listened to those talks and we've revealed it. Now, we've been friends for a long time. That's right. And you'll sometimes say sex and you'll sometimes say gender.

1:01:24And I understand that sex is a confusing word because the moment they hear it, they think of the verb sex. How do we think about sex versus gender when it comes to understanding brain and, yeah, just brain. Let's just stay with that not even body because clearly the data and mice and humans point to the fact that the administration of hormones can change the body. It can shift things in one direction or the other. Given at the right time. Given at the right time. And we can talk about that. But what about the brain piece? How mutable is this? And what are your thoughts on the controversy? And how should we be thinking about this?

1:02:04Forgive me for stumbling, but it is not that I'm trying to avoid upsetting anyone. It's like we don't have a good language to differentiate these things. And I think part of the issue, part of the problem for not having a good language and good understanding is we don't have an animal model for it. Gender is such a human -specific construct. You know, as these sort of constellation of behaviors and expectations generated from within and by our society and culture. But what gender is and gender sort of includes not only sort of identification of yourself as a male or a female or something in between having sort of attraction for one sex or the other or not having any attraction for any body.

1:02:42Or sort of having this sort of comportment of behaviors like dressing in a particular way, sort of speaking in a particular way or having meeting societal expectations. All of those sort of comprised gender. And it's hard to do that in the mouse. We don't know enough about mice. We don't even know about mice enough to say they have a gender. We know that they have sexes, females and males based on SRI, testosterone, estrogen and testosterone. Okay. Well, it's hard to have an animal model for something like this, which is so complex and so it seems human -specific. Well, you said one thing that at least my understanding checks off one box, which is that sexual orientation and how people self -identify in terms of maleness or femaleness is separable.

1:03:30We know that because there are people who are homosexual. And we know that because there are people who switch gender by way of hormones, obviously not from birth, but later in life. And in many cases, they don't change sexual orientations. Sometimes they do, but my read of the data is that usually they don't. In other words, if somebody preferred females before, they might administer hormones, change their body. But they'll continue to like females or vice versa. That's my understanding of the data. And I went into the data looking prior to this conversation. And there are a lot of data now. The problem is it's difficult to find unbiased data.

1:04:11I'll be very honest. I feel like the data are biased on both sides. People seem to be arguing for something going in. Okay. So sexual orientation and how people self -identify, we know is separable. That's not a controversial thing. We just know because that's what happens. But when it comes to when people are administered hormones, how that changes the brain inhuman, what do we know? You said it depends on whether or not they're administered hormones early versus later in life. Well, I think the early data, and we talked about congenital adrenal hyperplasia. We talked about antigen and sensitivity syndrome.

1:04:46Those data really say that hormones at a point in development, maybe neutro, have a profound effect. On masculinization, of feminization, external, as well as of the brain. Right. These kids that make, that don't make DHT that are raised as girls, but later sprouted penis are, at least as you described it, for all the world, raised as girls and happy being raised as girls, identify as girls until testosterone kicks in. And then it, but it's interesting, right, because their body changes, so it's unclear to what extent the bodily changes are driving the psychological changes. But presumably if the brain is organized male, because they're X, Y, and they have the S, R, Y, Gene, and they have testosterone.

1:05:29And they have testosterone. There's a substrate for it. Like it's waiting for that testosterone. There's something for it to act on. And similarly, if you're, if you're insensitive to testosterone, if you have antigen and sensitivity syndrome, then you've not seen testosterone sort of biologically. It's present in the circulation, but your brain, for example, can't respond to it. So you're feminized externally, and you're also behaving as a female, all the way through adult life. So that's the early action of testosterone, right. So I think what you're referring to is people deciding to sort of take hormones, at a later point in life after birth.

1:06:08Much later after birth. To switch genders. Right. And maybe the starting place to really understand this is when people take hormones, but don't want to switch genders. So these days it's very common for more common now, for men typically, but women also. But let's just say men taking testosterone or augmenting testosterone. Or for women to augment estrogen. This is now because of the increasing attention on menopause and perimenopause and the women's health initiative and trials that looked at this. It's very clear that there are some advantages to estrogen therapy and women who identify as women.

1:06:48I'm just making this like trying to simplify this as much as possible. Our colleague, Robert Sapolsky, who knows a lot about testosterone, has written books about it, said when somebody increases the testosterone, if it's their testosterone pharmacologically, it just makes them more the way they are. If they're an aggressive jerk, it makes them more aggressive jerk. If they're altruistic, it makes them more altruistic. But it's really about hierarchy. It's really about a willingness to lean into effort, to suppress a migdala activation, and to lean into effort within the domains where they feel a lot of agency.

1:07:22It's kind of what he describes as the main effect of testosterone. It's a little unclear what the main effect of estrogen is when given to a woman in adulthood. Besides the ones that have been described, like preservation of cognitive function, skin texture, vaginal lubrication, a bunch of things that are kind of youthful restoration type phenotypes. I don't think there are a lot of data about the psychological changes, but they seem to be in the direction of feeling better, because there are a lot of women now who are seeking estrogen replacement therapy. With menopause, there's a sharp increase in the incidence of Alzheimer's disease in women.

1:08:03As you pointed out, taking estrogen after menopause, if it's medically fine, once you've consulted your doctor, then that will at least prevent the decline in cognition, because you now have estrogen on board. That's the thinking behind some hormone replacement therapies, for cognition at least. Coming back to the testosterone, I think that you mentioned from Robert Spoltsky, we did a similar experiment in the mouse, where we just mutated the antigen receptor only in the brain. This is going to get complicated, I think. No, it's cool experiment. So, penis can respond to testosterone, muscle can respond to testosterone, connective tissue can respond to testosterone, brain can't respond.

1:08:46Is it going to get into that from birth in these meals? It's going to get interesting, because we're going to talk about aromatization now. So, these males are still masculinized. They just mate and fight less than normal males would. Okay, so their brains are a little, again, there's a dearth of language here, but these mice that don't have testosterone acting on their brain are a little less stereotypically male. That's right. They're less like, but they don't like to fight as much. They don't, they're mock territory, but not so much. Interesting. So, someone's in the comments already saying beta male.

1:09:26That's the kind of YouTube speak. YouTube, by the way, because it's male dominated in terms of its audience, is if you look at the comments on YouTube, not just for this podcast but other podcasts, it's a rich data set for how males compete when anonymous and when physical strength is not involved. Very interesting. A lot of hierarchies in comment sections that are removed from the stereotypical kind of notions of how hierarchies were played out, because aggression is, it's all words. Right. And memes. Well, you mentioned a Romanization, so we should tell people what a Romanization is. This always throws people for a loop.

1:10:14When you tell men that they're very male like because of estrogen, freaks them out. We'll go ahead, freaks them out. So, this all started with classic work by Frank Nathdalen in the 70s, when he was sort of working on human embryonic tissue, brain tissue. And he realized that the embryonic human brain contained an enzyme that converted androgen into estrogen. And enzymes called aromatase. And this is, in fact, the primary way that the ovaries make estrogen, they first make the testosterone, then gets aromatized by the enzyme aromatase and gets made into estrogen. So, it turns out that Nathdalen's sort of discovery was exactly right, even in the mouse brain, in the mouse male brain, we and others have shown.

1:11:00But there is aromatase, the enzyme expressed in very specific circuits in the brain. Can I just W. You mentioned this early experiment by this gentleman, was drawn on human brain tissue? Yes. And rats and you know those. It's a very important point. I think she will appreciate hearing this, but a long while ago, I mentioned this thing about aromatization of testosterone to estrogen is really what mask and lines is the male brain. And a very prominent author in the testosterone space, a female author, wrote to me and said, it's just mice. But she's very scholarly and I think she'll appreciate hearing that the original data come from human.

1:11:37Great. Thank you. So, it's not just mice yet another way that we're conserved to be fair though. I think the idea with what she might have been referring to is that aromatization in the human brain may not be playing as dominant a role in mask -renizing the brain as it does in rodents and other animals. So, that, you know, we can't really speak to that because you can do those experiments in humans. But if you have a male mouse lacking aromatase, so he can't make estrogen, then, you know, his behaviors won't be masternized. He appears more female, not appears, behaves more like it doesn't behave like a male.

1:12:15Because he's not converting testosterone into estrogen. And this happens very early at birth in mice. So, testosterone gets made by the testes, gets in the brain, gets converted into estrogen. And then, you know, as we talked about earlier, there are some cells that die or survive depending on the sex. And this conversion of testosterone into estrogen enables specific sets of cells in the male brain to survive. This is probably a good place for us to inform people that the steroid hormones to testosterone and estrogen are very interesting because they can have immediate effects and they can also change gene expression.

1:12:50This is a good opportunity for you to teach us some cell biology. So, is it by virtue of the fact that they are lipid soluble, they can go all the way into the nucleus of a cell? I mean, you know, this is very different than like dopamine, right? Dopamine can impact cells, you know, don't do this, folks, but, you know, if you were to take methamphetamine or something, your brain would go very dopamine orgic very fast. But it's not going to change gene expression in the short term, maybe in the long term, but not in the short term. But testosterone administration or estrogen administration is literally changing the genes that are expressed in the cells they interact with.

1:13:25How does that work? What's going on? What are they actually controlling? So, the receptors for these hormones, testosterone, estrogen, progesterone, they sit in the saddle plasmoth cells, not in the nucleus. And as you pointed out, these are, you know, steroid hormones are lipids, they can cross -subboundary cell membranes. And once they bind it to the receptor, the receptor bound to the hormone is translocated into the nucleus, where it finds stretches of DNA that it recognizes, it sort of sits on them, binds them, and then changes a regular gene expression of what we call target genes. And that's how, you know, you get gene expression changes by these hormones.

1:14:02So, this is why whenever I hear like the Sapolsky argument, which I totally agree with, that, you know, you give someone testosterone and they become a lot more like themselves, they don't, if they're a nice person, they become that much nicer, if they're aggressive, they become that much more aggressive. But those are short term studies. Right, so we don't really know how the administration of hormones, just testosterone or estrogen, to a self -declared male or female, or x, y, x, x doesn't matter. The point is that we don't know how the long -term administration of these hormones literally change the genes, and therefore the thought patterns and behaviors and feelings of these people.

1:14:39So, you're basically changing the molecular fingerprints of specific sets of cells in the brain with hormone action. A big debate these days is whether or not people, if they seek to change their gender identity, whether or not they're in a position to make that decision, because they're a minor, right, Myers, are not legally allowed to make all sorts of decisions like vote, drive a car, all sorts of things. Yeah, but work in this country anyway, work a job, I think you have to be, used to be 14, I don't know what it is now. But it's an interesting biological question when you just say, okay, at forgetting all of that and just asking, okay, what is the condition of a, like a 10 year old brain versus a 14 year old brain that's entering puberty versus a 16 year old brain that's still transitioning through puberty maybe in late phases of puberty versus 25, which is when we know brain development is more or less coming to a, to a close, although brain development continues forever.

1:15:40I mean, how is anyone going to eventually come to an agreement one way or the other on this? Is there a real biology that we can look at in mice or in humans and say, like, okay, here's the dynamic tension. The dynamic tension out there is, there are people saying there are kids that are too young to know what they are, let alone choose what they want to be. And then on the other side, you've got people battling saying, no, it's essential to get in early because then the trajectory is more malleable and then you don't want somebody to end up in a place where change isn't possible. And then you have people saying, well, wait, they changed gender and then now they want to reverse later because, and they're angry that they were allowed to make the decision.

1:16:25So it's a mess. It's a, it's a, it's a genuine mess in terms of defining what the key parameters are. Do you think it will ever be resolved? Let me step back and say, we don't even know much about this in the mouse yet. Right. So we don't know what happens to the mouse brain and puberty. Really? There are experiments being done, but not certainly not the same detail as in the adult mouse brain. So how circuits are made plastic or how they're malleable at puberty is still sort of being worked out in the mouse. So that's the first answer. The second one, the reason I think it's contentious is it's both deeply personal.

1:17:01What the kids are feeling, but also there's these huge sort of societal political forces that come into play. So I think the tension there has to be resolved, I think politically and sort of socially rather than, you know, just resorting to science. I think the science will give you data, but you will still have to make a decision as to whether or not, you know, that'll be allowed. So I think that's the reason it is so contentious. The data is not there in terms of at least in the mouse or their animal models or it's coming out, it's coming out slowly. And socially and politically it's very volatile.

1:17:35Because it's not clear how you sort of, you know, have kids, white parental rights, societal expectations intersect and give a result that is satisfactory to everybody. So that's where we are. I'm not saying I'm pro one or against the other. I'm just saying that's why it's so contentious in my mind. Today is a biological discussion because that's what we can say things about for sure. We can talk about biology for sure that the other pieces are, they're even prone to trip wires related to language and that for biologists is no fun. And the whole reason to become a biologist as opposed to a psychologist is because while I have tremendous respect for the field, biologists have nomenclature committees.

1:18:22We agreed. This is, because you could make this argument about anything. Again, by way of example, I mean, you can say, oh, the S .R .Y. gene is the S .R .Y. gene. But what if it's just two amino acids different and it's still functional? Is it still the S .R .Y. gene? Well, there are nomenclature committees where people decide yes or no. You have a community agreement in order to go forward. And you don't have that in terms of the discussion around gender. But you have it around the discussion of sex. And circuits. And circuits. So let's talk about circuits for sex. Start there. Let's start with a recent discovery your laboratory made, which is about sexual behavior.

1:19:01In males. And the frequency of sexual behavior. I think most everyone who has gone through sex education in one form or another understands that males have a refractory period after ejaculation in which they don't made again. And in some cases can't made again. What did you discover about the neural circuits responsible for mating and the refractory period? Yeah, so this is in the mouse and we were working in male mice and we sort of hit upon these neurons. We identified these neurons using genetics that expressed the specific set of genes in the hypothalamus that if we activate them, male mice no longer have a refractory period.

1:19:45And the strain of mouse we were working on has a post ejaculation refractory period of about four to five days. Typically, typically. So he won't mate for up to four days with a female after ejaculation. So if he is presented a female and they may he ejaculates, you remove that female, you give them a new female, he won't mate with her for four or five days. He's content or he's not able or whatever. So we sort of switched these cells on with optogenetics. You know, we sort of electrically activate these cells with light and they lose their refractory period. They start mating within a second.

1:20:19As soon as the light comes on, the cells start firing, they start mating again and they get ejaculated again. So you reduce the refractory period from four to five days to one second. That's right. How long can they keep this up? No pun intended. As long as the light is on, they'll keep mating. And you're not talking about light presented to the eyes. You're talking about, like basically a light driven way to stimulate the neurons. That's right. What are these neurons? What are they called? They're in the hypothalamus. They're in the preoptic area, which is one of the most sexually differentiated areas in the brain across vertebrates.

1:20:50And they express the gene tachycanin receptor one, tacharone. I thought tachycanin is associated with aggression, social behaviors, depending on the circuit. It's a fly that's been shown, David Anderson's shown, for example, the tachycanin gene regulates aggression. In this circuit in the male mouse, it regulates sexual behavior. How many neurons? Maybe about 1 ,250, not on each side, but 2 ,200, 2500 cells total. I'd like to take a quick break and acknowledge one of our sponsors, Function. Last year, I became a function member after searching for the most comprehensive approach to lab testing.

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1:22:36In contrast, I've been super impressed by function simplicity and, at the level of cost, it is very affordable. As a consequence, I decided to join their scientific advisory board and I'm thrilled that they're sponsoring the podcast. If you'd like to try Function, you can go to functionhealth .com slash huberman. Function currently has a wait list of over 250 ,000 people, but they're offering early access to huberman podcast listeners. Again, that's functionhealth .com slash huberman to get early access to function. If we were to scale the size of the preoptic area from the mouse to the human, back of the envelope calculation, how many neurons is this in humans?

1:23:13Probably the same range because the human hypothalamus hasn't expanded that much. It's a human cortex that's expanded. We should remind people of this or let them know. The hypothalamus in your brain is what the size of a couple of marbles, sitting above the roof of your mouth, controlling all of this stuff. In the mouse, these cells account for about 3 ,000 cells. I can't for, I don't know, 1 -10 ,000th of the mouse brain. So take the same number of the human brain, which is 80 billion neurons. It's really a tiny, tiny subset of cells. So a few thousand, maybe 100 ,000 of the human, 10 ,000 of the human.

1:23:54So if stimulation of this of these cells reduces the refractory period to essentially zero, one second. It's not zero seconds, but, and that's with the same female or you can replace females. He'll just keep mating. Without delight, without the activation, you wouldn't have ejaculated it again for four or five days. So this tells us that these neurons control the entire circuit down to ejaculation. So because the words refractory period encompass a bunch of things, right? The difficulty in achieving erection as easily as one did prior to the first mating.

1:24:32Presumably this bypasses all the dopamine aspect of it. What about prolactin controlling the refractory period? You don't think the data on that is super strong. I think Susanna Lima has done some work and she doesn't find any sort of relation with prolactin and refractive period. Although in humans, there's a practice of people taking, I forget what the, I'm not pretending to forget, what the drug is. It's cable -goline, which is a dopaminergic agonist which is used to treat hyper -prolactinemia, to reduce prolactin. And it seems to be very pro -libido in males and females. People, and I do not recommend this, people take it recreationally.

1:25:18There's actually a slippery slope of this where people will take it in an effort to have more sex, but they can't achieve orgasm. And so it drives them crazy and they're institutionalized. I'm just kidding, they're not institutionalized, but it drives them crazy. And they decide it's not a good choice. So yeah, I think that's a great point. Let me circle back to the same circuit. And also sort of take you on attention. I think people with Parkinson's taking aldopa. Also augmenting dopamine levels because they are giving the precursor to dopamine, right? And there are reports in literature saying that there is an increase in hyper -cycel -type behavior.

1:25:56You see this in the case that I heard years ago on the radio was of a woman who was taking aldopa to treat her Parkinson's and she became a gambling addict. That's right. So part of the spectrum of sort of taking aldopa and Parkinson's as you become sort of, you get these compulsive behaviors coming out, or hyper -sexual behaviors coming out. And coming back to our circuit, the TACR1, the TACC -CAN and the CEPTOS circuit, we also show, we also found that activating these cells leads to dopamine release in the nucleus compounds. Oh, interesting. Which it will make sense. But these neurons themselves are not responsive to dopamine, are they?

1:26:37No, they're not expressive receptors for dopamine. They project to the ventral -tech mental area, which is dopaminorgic, which has a lot of dopamine neurons. And they activate these cells, which then release dopamine in the nucleus compounds. So these cells are like switches. Yes. And they're also, we think encoding the rewarding aspects of sexual behavior. Tell me more about that. So, you know, people who describe sexual behavior as pleasurable. It is pleasurable. And about 70 years ago, James Old and Peter Miller in classic studies showed that there were areas in the brain that if you put an electrode in that region, and you gave a rat an option to press lever to deliver electric current into that brain region.

1:27:23Many areas were identified by Old and Miller, where the rat would keep pressing the lever to get ahead of the current, if you will. And he identified such a sort of rewarding center or reinforcing center in the hypothalamus of the rat. And he said, this must be the pleasure center for sex. He had a piece in the scientific American on this. And the identity of these cells wasn't known as, you know, as we talked about the hypothalamus of super complex, it regulates not only mating and aggression and maternal behaviors that regulates body temperature, thirst, feeding, regulates many different behaviors.

1:27:59So which cells are sort of encoding rewarding properties of sexual behavior. So these tack R1 cells. If you give my see opportunity to activate these cells with opera genetics, so instead of pressing a lever, they just spoke up the nose, the nose in a hole. And if they poke their nose in a hole that has the correct hole, they get light stimulation into these neurons. So these mice once they learn, once they figure out that this port or hole delivers light and therefore electrical activation of these cells, the tack R1 cells will keep doing that repeatedly. So it says like it, they love it. And in fact, they could be sexually naive, they could be virgins, and they still love it.

1:28:44So this rewarding property of these neurons doesn't depend on past sexual experience. These neurons are naturally encoding some form of reward or reinforcing behavior. Does it require sexual behavior itself? No, that's what I said. So Virgin males will do it too. Oh, you mean while they're still virgins? I thought you meant having never had sexual experience before. This is important because as you and I know, Dio Lins work from NYU showed that these neurons in the ventramedial hypothalamus when stimulated mice will attack another mouse. They'll even attack a glove. We can put a link to these videos.

1:29:21They're very dramatic to see this. The stimulation of these neurons goes on and they just will attack the glove, attack the other mouse, stop the stimulation, they stop. It's like a rage switch. But if there's no glove or mouse to attack, they don't attack anything at all. They just cruise around their cage. These neurons are different. These neurons seem to make what you're calling virgin males. They'll work to stimulate these cells. But are they... I can't get around this. Are they masturbating? What are they doing? The brain's getting activated. It's with the center for matings getting activated.

1:29:57But what are they doing with that activation? They're not doing anything else. They're just going into the port again and again and again. Okay, so in a lot of ways, it's like these ventramedial hypothalamus neurons. They need something to mate with. It's not like they start mating with the whole in the wall. It's not like they start mating with inanimate objects. They like the feeling of these neurons being stimulated. But the neurons themselves don't trigger mating. Let me step back. I think we are confusing two things. We are conflating two things. One is, do the mice like activation of the neurons?

1:30:31And the answer is yes. They love it because they keep doing it. Even if they've never made it before. Okay, so the analogous experiment for the diulin stuff would be will animals work for stimulation of the VMH? And we know the answer is yes. Male animals will work to fight. They like to fight. But if you activate these neurons, just like if you activate the VMH, you get aggression towards the glove. If you activate these neurons and you give them an object, they will try and mount with it. As long as it looks like a mouse. So if you give it a toy mouse, the male will try and mount the toy mouse.

1:31:02But if you give them say a, I don't know, a marble? No. A beaker? No. A block of, a wooden block? No. But if you take a test tube, they won't mount it. But if you give, if you take a toy mouse tail and glue it to the test tube, so it now has a, you know, has some mouse like elements, they will turn mount it. Very low threshold for activating the behavior. Yeah, I think what it says, just like the aggression of experiments says, that there are these innate circuits, these hardwired circuits, that if you activate them and you have the right stimulus, the animals will attempt to do the behavior that these circuits are wired for.

1:31:41Or even the wrong stimulus, but one that resembles it just barely. But see, I mean, tail on a test tube. Come on. By stimulus, I mean, I'm actually looking at some pretty low standards for who they'll mate with. No, no, what they'll mate with. But that's pretty low. By right stimulus, I mean, by activating these cells with the opportunetics. So if you activate these cells and you give them an inanimate object, if it roughly resembles something that they're familiar with, that looks like a mouse in this case, they'll try to mount it. But if you give a mouse with no stimulation of these neurons, a test tube with a tail, nothing happens.

1:32:15They'll sniff it. They'll, they'll sort of, you know, maybe play with it and then they'll walk away. That's a significant result to reduce the refractory period from four days to, or five days to, to one second. What is the theory as to why there's a refractory period at all? Is this female driven? Is it based on the female sexual behavior preferences or non -preferences? Or is it something related to controlling population numbers like you would end up with, I don't know, too many pregnancies from one male? What's the idea there? Actually, every species has a different refractive period. And in the mouse, you know, because of genetic inbreeding, there are lots of strains of mice, you know, people have been raising breeding mice for pets and whatnot.

1:32:57And different strains of mice will also have different refractive periods. So there's definitely sort of a genetic basis for refractive period that may be species specific and also strain specific in the mouse. As to why you've genetically sort of selected for a specific refractive period of species, I think it's generally unknown. It could depend on the kinds of mating strategies different species use. Well, in humans who may not just reproduce, but also for pleasure, you know, what is known about the relationship between age and the refractory period duration? Some years ago, let's reading this book, as I did again this weekend, about hormones and behavior.

1:33:50And it's really interesting when you look at the distribution of testosterone levels in males from age, say, 20 up to 90. There's a big range at any given age. And it's not clear that absolute testosterone numbers are that informative anyway, but they point in a certain direction. But you also look at sort of copulatory frequency, sex frequency as a function of age. And it's also highly variable. I mean, they're these famous slash infamous cases of like Frank Lloyd Wright, who was purportedly having sex up to, you know, four, five, six, seven times a day, and did that well into his 80s. You know, to the point where his wife at one point was really concerned, like, is this okay for his health?

1:34:32And he was also an incredibly productive person. Another domains of life, also by the way, an incredible procrastinator. Apparently did all his sketches on the like cab ride over to the deadline. Like he would, he sort of functioned in this like kind of thoughtful slash impulsive manner, so people say, but he certainly never contested these rumors. And then some people probably just have lower libido, right? But as a function of age, it is the idea that it's all testosterone driven. If testosterone levels drop, then frequency of mating, assuming someone has a partner that they mate with, drops off, like what's known about this?

1:35:13As you pointed out, testosterone levels vary all over the place, right? And it's not just, you could have normal levels of testosterone, quote, unquote normal levels. And there's already a huge range of normal titers circulating levels of testosterone. But also, you could have different receptor densities in different regions. So it's hard to just take one parameter testosterone levels and say that that correlates with libido or with the desired mate in humans. Why such a behavior changes or the fractal period changes? I don't, I don't think it's generally known. It could be biological, it could be social, it could be many things.

1:35:52I'd neglected to ask the obvious question, which is, do these neurons also exist in the female brain? Yes they do. And what are they controlling in the female brain? We don't know yet. But each our way, a postdoctoral fellow in my lab, when he was a graduate student, activated a larger subset of these cells in the pre -optic hepatitis, in the females. And they're all expressed estrogen receptor. Estrogen receptor alpha, ESR1. And these females also made it like males. So this sort of hawks back to something we talked about earlier that the circuit for males actually behavior is present in the female brain.

1:36:33And he sort of identified a node in the female brain that lets them mate like males if he activates this optogenetically. Whether the attack R1 cells that we identified do the same we don't know yet. Without getting too down in the nitty gritty of circuit biology, but also getting down into the nitty gritty of circuit biology, I have to know. So where do these cells connect to? You mentioned that they're in communication with the dopamine system to activate this kind of sense of reward, pleasure, and reinforcement to drive more of the behavior. Where else are these cells projecting? I mean, it's a long way from a couple, you know, from 1200 neurons to the penis.

1:37:15What's in between? Yeah, so one big area they project to a really dense projection from these cells is to the periacryptal dwe. And they're involved in pain regulation. And many other sort of innate behavioral displays. It's a fight or flight, freezing behavior, and also sort of floridosis behavior. And for folks that aren't familiar with neuroanatomy, the periacryptal dwe, Dr. Gray sits in the back of the brain and back ish of the brain. And I always imagine it kind of like a pizza. It's got these like segments. It has these segments. Like you activate one brain area that's involved in suppressing the pain response.

1:37:49You activate another area that's involved in female lardosis. You activate another area. It's involved in kind of fleeing. You activate another area that's in approach. So either it hasn't been parsed finely enough or it's, in fact, it's kind of like a, it's almost like a mirror of the hypothalamus further back in the brain. That's right. So they project you the P .A .G. And then from there, and the P .A .G. goes to the brainstem, you're part, it's already, already in the back of the brain as you pointed out, and then goes further down. So multiple connections to the spinal cord. And then it integrates with the bubble cover noses or whatever controls are.

1:38:24The thoracic muscles involved in thrusting. It's a program that's innate program. I mean, most animals have to learn the socialization of mating, dating, consent, all other things, but they don't have to learn the motor programs. The motor programs are activated during puberty, is that right? Yeah. Some years ago, I recall a paper showing that mounting behavior could be both aggressive or reproductive. What's the story there? Because females do it too. So you're saying, by aggressive, you mean like a form of dominance display. Yeah, like G .G .2. So that's what people have suddenly said that it could be a dominance display because males are sometimes mount males.

1:39:12Although once males start fighting and once they've had sexual experience, they tend less often to mount other males. They just go straight for the kill, if you will. And in other species, many nonhuman primates, sort of animals will just mount each other, sort of a play behavior, or also for giving pleasure. So that's a known thing. Some female male, male, male amounts, they will do it as play behavior and nonhuman primates. So there are many, presumably, many reasons to engage in that sort of behavior. So it's not always sexual. Yeah, I did. Not necessarily, right? So what other collections of neurons live in this part of our brain that when activated, give critters us or otherwise, these kind of supernatural, let's just say extreme.

1:39:59Functions. Third neurons feeding neurons, right? So you can activate specific sets of cells that express a GRP, for example, or other sets of cells. Animals start drinking water or start eating. Thank you. I mean, within the context of mating and sexual behavior, are there, are there, for instance, like neurons that when you stimulate them, my start building this? Yes, we don't have those sets of neurons, yeah, but there are so many sets of neurons in the same vicinity that regulate parenting behaviors. Right? So they start taking care of pups, for example. So you can take virgin mice who don't normally take care of pups and can activate these circuits and can prevent these mice from hurting the pups.

1:40:38So normally mice will hurt, yes, will hurt other mice pups. Yes, not their own, right? That sucks. Yeah, doesn't say much for mice. Right. Well, a lot of animals species do that. Yeah. Right? They sort of like zip it and fantasize on a behavior where there are other species like wolves that take a show fostering behavior that they care of pups not around. So it depends on the species you're talking about. Yeah, years ago I worked with ferrets and they're probably happy to raise other ferrets. They don't even seem to notice if it's theirs or some other ferrets, pups, kits, interesting. Do you think when people get dogs, bulldogs in particular, I'm just joking, dogs, Niro has one of the world's cutest French bulldogs that some of the caretaking of dogs activates some of the same circuitries in the brain that are responsible for rearing our own species.

1:41:34I don't know, to be honest. I'm disappointed here you say that. I must say I'm disappointed when I got Costello as a puppy. I'll never forget that for the first, I don't know, three weeks that I had him, I had very little appetite. I, my work drive certainly still there, but I just felt like it 99 % of my cognition was on his well -being. But that's certainly true. I could have sworn it was a surge in oxytocin or prolactin or something. You mean, you're doing oxytocin or prolactin? No, I didn't, I would have had I had the means to do it, but there aren't very good tests to do that that are sold over the counter.

1:42:11I should have. No, sir. But if I get another puppy, I'll do it. Although now I think I'll go about it a little bit more differently. But it was my first dog and I was just, it was like all about him. Nothing else really mattered except the basics of maintain. But that's the part of how parents describe having a newborn. So that's certainly true. Cooper's our first dog as well. And, you know, if you need something, it basically, you know, takes precedence over everything else. Like feed, if you need food or if you need to go out for a walk, then, you know, I have to, I do drop things and I just take care of them.

1:42:43Yeah, that part is not only true. So if you inhibit selfishness or inhibits your doing other things, yeah, makes it more altruistic. This was really just my ploy to bring up oxytocin. Okay. We hear that oxytocin is the chemical responsible for bonding between romantic partners, bonding between mother and infant, maybe even bonding between friends, etc. What's the real deal on oxytocin? Because I think like so many things in neuroscience that were first discussed in roughly the 90s, early 2000s, we're getting a lot more data now. So what's the real deal on oxytocin? I'm not trying to burst any oxytocin bubbles, but what's the deal with oxytocin?

1:43:27So the paradigm that people have mostly used to study the role of oxytocin and pair bonding in animal models has been the prairie vol. So these are like mouse -sized rodents with very short tails. And unlike mice or rats for that matter, voles after having sex with one another, they will pair bond for life. They've found these long -term enduring relationships completely monogamous. Well, they actually just like humans, they will have extra pair meetings as well. So they will cheat if you will? At a cold -play concert. Exactly. Right. Or for the most part, they're monogamous. Right? So if you give them a potential mate of opposite sex, they will reject it aggressively.

1:44:07Right? So they have this monogamous behaviors and classic work from many labs had shown that oxytocin was maybe a really huge driver of the sort of monogamous bonding. So the last ten, it took us about ten, fifteen years to develop the technology to make knockout voles. And we've done that. And this is the work of really heroic post -toxin my laboratory. And knocking out the oxytocin receptor and prairie voles, we saw that these voles continued to form pair bonds. They were just as monogamous as their wild -type siblings were. So in fairness to oxytocin and to experimental biology generally, when you see an experiment like that, you go darn, everything we thought about oxytocin is wrong, or you say, pair bonding is so important that there's redundancy in the system, that other things can compensate.

1:45:03Which one do you think it is? So the most likely other candidate is going to be vasopressin, because the same folks who had certified oxytocin as being sort of really important for pair bonding had also suggested vasopressin might play sort of a similar role. And vasopressin like oxytocin is a neuropeptide hormone. So it's about nine amino acids. So it's a short peptide and it binds a different receptor, vasopressin receptor 1A, that regulates pair bonding behavior. So that's an ex -experiment for us, is that vasopressin receptor and vasopressin that's required for pair bonding behavior. So we shouldn't give up on oxytocin just yet.

1:45:38Let me also step back. Let me push back against this idea of, I'm going to get some heat for this, for saying that if it's so important you want to sort of have redundancy split into the system. We just talked a while ago about SRI. You just have one copy, you just have one SRI. In fact, it's only on one chromosome. So you only have one copy. If you don't have it, you're not going to become a male. So there's no redundancy for perhaps the most important decision. The embryo is going to make male or female. And there's no redundancy built in there. So I think it depends on what process we are talking about.

1:46:10If there are going to be redundancies or not, for something extremely critical, if you don't have redundancy, then I think it could be that other processes also don't have as many redundancies as we thought. Did you think you were going to get some heat because somebody would say, well, that implies the SRI gene is not important and therefore nails aren't important? No, I think as you and I both were taught doing developmental biology classes that we took as gas students, redundancies and sort of multiple pathways regulating a process is a thing. And it's definitely true for many things as we've learned during development and developmental biology, but also there may be processes where you don't have redundancies that are equally important for life.

1:46:47But if you don't have the gene, you're done because evolution is agnostic, right? If you're not successful, it doesn't just move on. So you won't reproduce. Evolution doesn't care. If you're not fit, you aren't fit. Yeah. The bad ideas died literally. Not the bad experiments died, right? Okay, so speaking of hormones and behavior and language and where language can be a little bit complicated, let's talk about libido. Most people know what that word means. It's a drive to have sex for reproduction or pleasure or both. And you discovered these neurons that effectively eliminate the refractory period.

1:47:29I don't know how an animal could mate any faster than once a second. After one second, I guess, yeah, okay. So I mean, there needs to be some time in between one seconds about as short a refractory period as possible. But we don't really know what's going on in the mind of the mouse. But when a discovery like this is made and because of the conservation between the mouse hypothalamus and the human hypothalamus, I think many people probably thinking, oh, you know, is this a drugable target? Is this the sort of thing that could be used to enhance libido or reduce the refractory period in males? And that opens up a larger discussion, I think, about biology, drugable targets and sex behavior in humans.

1:48:18So there is an FDA -approved drug that targets the melanocortin pathway, I believe. That's used to enhance libido in females. That's right. Although I hear, I would say if I had, but I've never tried it, but I hear that men take it also and it has a similar effect, although not as pronounced as in women. Tell us about melanocortin and why a drug that stimulates melanocortin would increase libido. And then we'll talk about whether or not the tacky kind of neurons that you discovered represent a good drugable target for increasing male libido. So actually removing melanocortin signaling in the mouse brain in male or female mice does impact sexual behavior in both sexes.

1:49:07It does, yes, it does. Okay. So it seems to be playing a role in sexual behavior in both sexes. The effect of the drug seems to be pretty small. It helps a subset of women, not all women, I think, and then a significant side of facts as well. What's it called, the lacy, the lacy? I think the drug is called the lacy. And my understanding is melanocortin comes from the medial pituitary and is involved in pigmentation of the skin as well. So it tends to darken people's skin. It can cause hyperpigmentation in some women digging as it's injectable, I think. So it definitely seems to help a subset of women.

1:49:50So I think that's one of the few libido enhancing drugs out there. And it's very different than viagra, which works in men as you know, right? Which, because viagra acts on a more peripheral vascular thing. It doesn't act on libido. It acts on the ability to have interaction. Right. It's pro erectile. And I think women will take some of these vasodilators as well. Right. For enhanced sexual function. That's right. But libido is pretty separable from erectile function. Right. As you pointed out, libido is more the desire to engage in sexual behavior. Because erectile function is the ability to enact on that desire.

1:50:30So those are pretty separable. And I don't think there are very many good libido enhancing drugs for men or for women. We talked about the strong vileacy. That's helpful. And it seems to have a positive effect. But there's only a big dirt out there of drugs that were enhanced libido. So is there any one habit libido for that matter? Right. I think when people think about drugs that inhibit libido, it's naturally occurring experiments like opioid use. Does that excessive alcohol intake? Right. Anything that diminishes dopaminergic function. We'll do that. So after you made this discovery, did people approach you about developing a drug to enhance libido and men and or women?

1:51:19Yeah. Called Mutual Friend of Hours. Mike Eisenberg gets Stanford. Oh, yeah. He was a guest on this podcast. Our head of male sexual health in your realm. Exactly. He approached me and he says, can we do something about this target? And I said, there's no agonist. There's no drug that would activate the attack on the receptor that we know about. That's clinically proven to be safe. There is an antagonist for it. That's clinically approved. That's used for other purposes. But that would diminish libido. It would diminish libido. It did my approach you because he has a lot of patients that have diminished libido who want to enhance libido.

1:51:50That's right. That's exactly right. Why do you think there's such a dearth of drugs to enhance libido? I think for a long time pharmaceutical companies have stayed away from drugs that act on the CNS because back in the 90s, there are a lot of studies developing drugs to enhance different functions of the brain. And there are always some off -dragate effects. So companies have typically stayed away from those. Not SSRIs. I mean, SSRIs were a boom industry until recently when everybody turned on them. That's right. And I say this every time SSRIs come up, yes, they can have pronounced side effects.

1:52:27No, I don't think they are always the solution. They believe in libido. Encuring decreasing libido. For certain populations of people who have clinically diagnosed OCD, SSRIs have been very helpful. So we don't want to completely... I'm just saying that's why there's a general drug. Many, many drugs being developed for different conditions that affect different functions. So drug companies don't want to make drugs that act on the brain? I think now there's a change with Glipp R1 and with the Glipp Agnes coming out. There's a huge interest suddenly. Which drug? Vigoby and a Zempik. Oh, what did...

1:53:01For people who lose body fat. That's right. But those act on the brain as well, right? So there's now a sudden surge in interest again, developing agonists, a few of all, are antagonists to budgetate different pathways on the brain. Because this is a huge success story. So now people are energized again, I think. Well, and if nothing else, those drugs prove that one of the main reasons, perhaps the main reason why so many people are overweight or obese is that they eat more than they burn. You know what? People debated that until very recently. Now hardly anyone debates that. People will say, oh, well, you need to think about blood sugar regulation and you know...

1:53:34But when it comes down to it, you need to adjust roughly less than you burn. There's some noise there, but it's clear that that set of experiments, the heel monster that doesn't eat very much, which makes a peptide, which then is turned into a drug, makes people not eat as much, boom, you have a trillion dollar industry. So here you have a discovery where you discover an animal that when these neurons are stimulated can... has kind of an insatiable libido. So it seems that the appropriate dose of a drug that targets the tacky kind of one neurons might... might think of reasonable drugable target.

1:54:15Oh, thanks, though, yeah. Well, someone listening to this will take interest. What does it take to go from a desire to make a drug like that to a drug that can go into humans? I mean, first of all, you go preclinical testing obviously. First, we actually make sure that the circuit exists, that those same neurons in the human brain express the same receptors. Well, that's easier to do nowadays, right? There's some brain banks. You take some brain sections from some deceased people who said it's okay with them and you do the mRNA and CQs. Okay, all right. So the neurons are there. And then you do what dose response curves in mice.

1:54:52That's right. And then you do, right, then you go into preclinical trials and ask, is there other agonists who can develop that are safe? And then of the desired effects with minimal off -target effects. I promise you that just by virtue of this discussion, somebody someplace, and I'm not recommending this, is going to develop or acquire a tacky kind in peptide and inject that peptide. The reason I say that is that these GLP agonists that many people are now using, were used for many years in the fitness industry by people who would read a couple papers based on animal models and be willing to acquire or develop the peptide and inject the peptide.

1:55:29Not something I recommend, but you can be absolutely sure that someone will try this. The reason I say that is that there's a peptide in the hypothalamus called KISS -Peptin, I think which regulates puberty. And there is a subculture of people that take KISS -Peptin as a peptide as a libido enhancer. I can't avoid asking because we're on the topic, but do we know what switches on puberty? KISS -Peptin is certainly important, right? So mutations in the receptor focus peptides, he'll block puberty in humans. And in mice as well. So there are people that never undergo puberty? That's right. Really.

1:56:04And it's a mutation in KISS -Peptin receptor. Do they grow in size despite not being like sexually able to... I think if you don't undergo puberty, then you are not going to make the hormones... The sex hormones that you normally make. So you don't get the boost in testosterone or estrogen or progesterone. So this is where gene therapy is going to be a huge boon to medicine. I'm curious about the regulation of brain function, changes in brain circuitry, as female hormones change. During, say, the menstrual cycle. What is known about that? How different is the brain at one stage of the cycle versus another?

1:56:44Okay. Stepping back in the rodents where a lot of this work has been done, we know that the estro cycle is not rats or mice don't menstruate. But they still have the auditory cycle. The ovulate ones have been four to five days. And their hormones, estrogen, progesterone, do change correspondingly, just like they would in non -human primates or in women. So you have the same hormonal cycle, roughly, and you have the periodic ovulation. So it's just compressed into five days. Okay. And rats has been known for a while for about 20, 30 years now, that there are very specific sets of neurons that are responsive to estrogen, that change the number of dendritic spines.

1:57:23So these are sort of processes on neurons that receive information from other neurons, as we know, neurons act in circuits. So neurons are listening to neurons upstream of them, and then transmitting information to other neurons downstream of them. So some of these connections, the presynaptic connections that are receiving information from other neurons, those spines seem to increase the wax and wane across the estro cycle. And we showed in a different finding more recently, that neurons that transmit information, when they are transmitting information downstream to other neurons, those pathways also change pretty dramatically.

1:58:01We saw about a three -fold increase or decrease every five days in the adult female brain of the circuit. Wow. That's huge. That's huge. And this seemed to be functionally relevant, because when the circuit was fully on or was fully mature, when she was ovulating, if we inhibited this pathway, she stopped meeting. And coming back to sort of going back to an earlier part of the discussion, the circuit seems to be very demographic. This pathway essentially doesn't exist in the male brain. Which makes sense. Which makes sense. They don't, they don't ovulate. Are there hormonal fluctuations in males across the day or the week?

1:58:39I mean, we assume that testosterone is highest in the morning. That's right. My read of the literature is that there's a subset of men for which testosterone is actually higher in the afternoon, but in most men it's going to be highest in the morning. But we don't think of hormones as fluctuating in men very much. Core does all yes, but testosterone not so much. Is there any evidence of hormonal fluctuations in males that are meaningful? Or is it just pretty much of, you know, a flat rate? And they found that we've done in mice that doesn't seem to be the case. So you can just give testosterone to, you know, male males.

1:59:10If you've castrated them, you can basically inject it at any given time of day and allow the same effect. Right. But in females, if you give estrogen and progesterone, it has to be at a very specific time point for you to see the effects of that hormone. So during the menstrual cycle, it sounds like there's profound changes in neural circuitry in the female brains. So it's a very dynamic... Circuits are growing, circuits are disappearing, circuits are growing, circuitry. And people have seen in women also, women on the pill, for example, or not on the pill across the menstrual cycle, you do see changes in MRI imaging in women as well.

1:59:45So what's known about that in terms of blocking ovulation with world contraception? No, so I think what I'm just saying is that the brain seems all to be all to dynamic, as visualized by imaging in women. So it's not just a rodent sort of phenomenon. It seems to be this dynamic processes going on in humans as well, across the menstrual cycle. What about during pregnancy? We don't know. We don't know. There are a couple of reports that say that are circuits that are changing in the adult... In the mouse brain when she's pregnant? In my surprise... HIPAA Campus grows. I don't know that. Maybe you do.

2:00:20I ever call... There was a guy who did a sabbatical in our colleague, Leach & Lowe's lab. I forget now. He was from Larry Katz's lab. Well, factory guy. Adi Mizrahi. Adi Mizrahi. That's right. He showed the auditory cortex. The circuit and auditory cortex changes, I think. Mother, so is there more attuned to pop vocalizations? That's right. They're auditory cortex changes so they could hear their pops better. And that's... That's the mother, so yeah. That wasn't during pregnancy. I was in... The study might have started in pregnancy. I'm pretty sure the experiments, the assays were done when she was nursing.

2:00:55I definitely need more science on how the brain changes during pregnancy. How the mother's brain changes during pregnancy. What about menopause? You know, these days there is appropriately, I think, increasing attention on perimenopause and menopause as very important stages of human development. That have not been entirely ignored, but that we're largely ignored for a long time. Now there's a lot of attention about it. What's known in terms of brain circuitry changing during menopause? Because my understanding is one of the most market changes. Formanally is a reduction in estrogen. So, again, these studies have just been done in mice.

2:01:34Just starting to be done in a self -vehicle, molecular way in the mouse. And I think the jury's still out. But it's clear that cognitive changes happen with menopause. So the estrogen going down is definitely affecting cognitive performance. And this is sort of, you know, reported by women too, is the mood changing, the appetite changing, and also the steep increase in Alzheimer's incidence in women. In mice, I think there's going to be a lot of focus on the hippocampus, which is involved in learning and memory and the frontal cortex, where in the non -agid mouse, female mouse, people have seen these dendritic spines waxing and waning across the ester cycle.

2:02:12So what happens there, and what happens to those circuits, and the downstream behavior is something that's still being investigated? Yeah, I think we often hear about estrogen, and we think only in terms of ovarian function and ovulation, and that talks right in with menopause. But when we hear about the effect of estrogen in preserving brain function, my understanding is it's also true for men, and that one of the ways that it helps preserve brain function is that it helps keep the blood vessels and capillaries very pliable. It's very good for the cardiovascular system. Do we know if any of the reductions in estrogen that occur during menopause are acting directly on neurons, or is this all downstream of reduced blood flow, for instance?

2:02:57Yeah, I don't know the answer to that, to be honest. I suspect there's going to be both. There's only going to be direct effects on neurons, because neurons express to the septifestrogen. Many neurons are all expressed receptors for estrogen, so estrogen going down is certainly going to affect their function. Every MD that I've had on this podcast, who has a specialization in endocrine stuff, will say the goal is to keep your estrogen as high as possible without running into side effects. That's good for your brain. So it's said that when people quash estrogen, or when you get males that have, for instance, very high DHT levels and T levels and their estrogen is very low, it's not a good picture cognitively, certainly not in terms of cognitive longevity.

2:03:41So estrogen is pretty interesting, I think, from the standpoint of its effects on the body, but also as a neuroprotective agent, right, in men and women. I have all sorts of questions about why that might be. I solicited for some questions from the internet, always a dangerous thing to do, but a lot of fun. And so I'll ask you some of the more frequent questions. Feel free to pass on any of these if you don't feel like you have an answer or one answer. One was whether or not men's hormones cycled throughout the day and talked about an early morning pecan testosterone, which by the way is very correlated with the amount of REM sleep that people get.

2:04:22It seems like that, if you don't get enough REM sleep, that might blunt some of that testosterone increase. Okay, there's a speculative question. If male and female brains are wired so differently, does that mean they experience reality in fundamentally different ways? Like maybe we're not at all having the same experience of life. Let me answer that from our studies in the mouse. A fundamental feature of social interactions is the ability to recognize potential mates from potential competitors. Recognize sex of other individuals. Female male, you know, we do that subconsciously. You walk into a bar, you subconsciously processing.

2:05:06Female male, female male. We all do that automatically. Might also seem to do that. And we identified a region of the brain, a set of neurons of the brain, that if you record from these cells, you and I, if you're just looking at the activity of these cells, we can say he's thinking that's a female or a male. So that's sex recognition going on in the male mouse brain. If you record from the same cells in the female brain, those cells seem to be quite acid. So it seems that male mice and female mice are using different circuits for recognizing females and males within their species. So they're wired differently and they're recognizing females and males using different pathways.

2:05:53So that's in one sense having a very different intake of reality. If that makes sense. That makes sense. I'm remembering an early discussion that you and I had, meaning many years ago, where for whatever reason you said exactly what you said here, minus the difference between males and females, where you said, you know, as you walk down the street, there's a process happening beneath your conscious awareness, where you're going male female, male female, male female. You're you're batching people into these two compartments based on maleness or femaleness. And it in the mind, it's just happening.

2:06:39And you said it's because you need to know whether or not someone's a potential mate or a potential foe or a potential collaborator. Based on what you just told us that females aren't necessarily making the same calculation the same way. I have to speculate a bit. One, they have to know male versus female, right? Because males could be a threat. Females could be a threat too, but males are more often a threat to females than other females. Females can be a collaborator, a friend, or a threat. Maybe a physical threat, but could be a sociological threat. I've observed this.

2:07:19And so it makes sense that one of the most fundamental calculations we make as we move through life is batching people into these different compartments. How plastic do you think that process is? Like, this sounds like a pretty hardwired thing that is difficult to get people's minds around. I mean, now it would never air. But in the old Saturday night live, they had this character Pat, right? Which was supposed to be neither male nor female, or you weren't supposed to be clear on what Pat was. And that was the whole basis of the the skit. That was the whole basis of the character that was a repeated character on Saturday night live.

2:07:55I don't think they're going to reintroduce Pat. But that character was an interesting experiment at the time, because it introduced this kind of circuit confusion where people didn't quite know where to place Pat, the whole basis of the script for it was exactly that. So how do you think about these things? I mean, most circuits in the brain are push pull, they're binary. Mate or fight, right? Eat or don't eat. There isn't a whole lot of middle ground. I think it is more nuanced. It is female male. But as you pointed out in humans, you're going to say, okay, potential male, potential foe, collaborator, friend, unknown person.

2:08:35So there are other recognitive pathways feeding into your initial binary classification of female or male. So it's not a simple go -nogal decision in humans. In the mouse world, it's simpler, at least in the assays that we design. So in the instance I was telling you about, if you take the male mouse, the sex recognition happens in the first 10, 15, 5 to 10 seconds. Just like in humans. Just like in humans. If you can make the distinction, your brain makes it automatically. So first 5 to 10 seconds, right? And that signal of female or male persists for about 90 seconds. And it's much larger feasing of female than a male.

2:09:17So if we artificially optogenetically activate these cells in the male brain, only for 90 seconds. And then give them a male. For the next 15 to 20 minutes, he thinks it's a female and he'll try and mate with them. So that recognitive process has induced a state in the male that says it's a female. Although the sensory input that's coming in, the pheromones that are coming in, the size, the way the animals walking around, all screams male. He thinks it's a female. He tries to mate with them. So he's different even though the outside world isn't right. And if we inactivate these cells, if we silence these cells or if we kill the cells, and again, we're talking of maybe 2000 cells.

2:09:58If we kill the cells, he cannot recognize females from males. Typically prefers a smell of a female that preference is gone. And because he can't say that's a male or a female, he needs a mate with females, nor attacks other males. He will interact with them, he'll hang out with them, he'll be pretty chill. He simply won't mate or fight with them. So that says that there are some hardwired things in the mouth, but at least, right? Where you can't sort of convert those with experiments into yes, go, no, go signals. But I imagine if you said a more complicated assays, where if the other male is a sibling, then you won't attack the male, but you won't mate with them either, as long as you don't touch them or on it.

2:10:47Right? So right now, we just try to understand the basic decisions these cells are making, the basic sort of information they're processing. And that seems to be, you know, go, no, go mate, don't mate, fight. It seems you want context to matter, but not when survival and reproduction are critical. I like watching nature shows for a variety of reasons, but there's an incredible one where these hyenas are attacking a lion, and they're trying to rip off its testicles. It's a pretty convenient way to limit lion numbers, as long as they're going to kill this lion and eat it. But even if they don't succeed in that, they try and castrate the animal.

2:11:27And another male lion shows up, and it's really interesting because typically those lions would fight. But in this case, the second male lion is willing to risk his fertility and his life in order to protect the other. So there's this higher order calling. It's like it suddenly has a mission that overrides his desire to be the dominant lion, and it's just about preserving lions more generally. It's pretty incredible that, you know, as unsophisticated as a lion brain may be, it's able to just completely switch over. And I raise this because what you're describing and what this nature show reveals is that it's almost like hormones activate circuits, activate repertoire of behaviors that were sort of a repertoire machine as opposed to just having like switches in the brain, which is how we were talking about them earlier.

2:12:23It's tempting to think about them as switches, but the context really matters. Context matters, and you know, people of Tinburg, for example, is proposed that there's a hierarchy of behaviors, right? So you have mating, aggression, protection of young, or defense from predation. So all of those are sort of have nested regulatory structures when it imagines, as you pointed out with this lion, that if you have a different context, then a different set of behaviors is sort of, you know, activated. And the same things true for, you know, even aggression, right? If you take these VMA cells that we've talked about before, if you activate them, the animals will attack other males, or females for that matter, or a glove for that matter.

2:13:03But if you change the context that the animals in, your experimental animals in, and you activate these cells, he may not attack. Because in this case, the context is overriding activation of these cells and telling him, no, it might be too dangerous to not attack. So if you put him in another resident's cage, you know, different animals cage, so it's no longer his turf. And you activate the cells, he's much less likely to attack now. And then there are these experiments, right, that females will kill the offspring of other females. Females will kill the offspring of other females. Right. Unless certain conditions are met, like they've already had a litter of their own, they've happily raised that litter.

2:13:43Or they've been hanging out with the other female anaerobic. It's worth mentioning because, you know, I'm not trying to equally distribute violence here, but so often we think about, you know, males and violence. But maternal aggression is one of the most robust things one will ever observe. But female female aggression does exist, and it usually exists in the context of who gets to have and raise successful offspring. That's when you see real nastiness emerge. Yeah, which is, you know, in the context of sexual behavior, we're yet to get this guest on here. But there's someone out there that studies female sexual behavior in an interesting way in terms somewhat evolutionary terms.

2:14:22But saying that, you know, one of the more pronounced effects that you see is depending on whether or not someone has had and raised children, how they behave towards other women. Or the more salient experiment. And I need to verify this is actually true that when, apparently, there's a study where they sort of scale the level of attractiveness of women coming in for their to get a haircut from another heterosexual woman. And the more attractive a woman is who comes in to get her haircut, the more hair the, the hair stylist, the female hair stylist cuts off. Almost as if there's a competition and they're trying to actually damage the competition.

2:15:09And then other examples where the whole notion of women shaming other women for being promiscuous, the notion being, well, if men can get sex without having to invest much, then that will change the standard of what men expect. And we'll make it less likely that they'll be able to find a, you know, a safe, happy mate situation to raise kids. I mean, these are the ideas that spin in the background and you can go, okay, well, that's a just so story. I probably could explain those data five different other ways. But then you hear the animal data and you go, wow, a lot of this is really about extension and preservation of our species.

2:15:49All right, more questions. This is interesting, given our earlier discussion of periac, Waductal Gray and its involvement in sexual behavior and in pain management. Is there a difference in the way that males and females experience and attempt to relieve pain? Do we know anything about their interaction between hormones and pain management as it relates to males and females? There are a lot of reports saying that males and females have different pain thresholds. But I think it's been really challenging to dissect out where those differences arise from. I mean, that's how I have to say it. So I don't know much about this.

2:16:26Because people will say because of the pain of childbirth that women have a higher pain threshold and that's been revealed in some studies, at least to my knowledge. But that could also be because they're in a different hormonal state than having a baby, you know, so a lot of natural endorphins released. There were a lot of questions about environmental toxins in food, in water. You know, some of this gets to the atrazine data from Tyrone Hayes from Berkeley years ago said that atrazine present in the water and that frogs were being exposed to was causing an inversion of sexual behavior in these frogs and disrupting sexual differentiation that was taken and run within a variety of directions, some accurate, some far from accurate.

2:17:14But I think nowadays people are very concerned about endocrine disruptors, especially during pregnancy and in early childhood. And a lot of people are speculating as to whether or not this is one reason that there's a fair amount of discussion about confusion about gender identity and sexual differentiation. What are your thoughts on this? Is it conceivable that things in food in the environment, which act as endocrine disruptors, are smearing some of the... of previously clear outcomes for human fetuses? I think you have to ingest large amounts of these hormones at the right time to... or these modifies these modulators to have an effect.

2:17:58So I don't know what the kinds of exposures there are. You know, with plastic bottles and whatnot, I mean maybe, but it's... you'd have to have a large exposure. That's not to say it doesn't happen. There might be species in which it's really sensitive, so it could happen. But here's one thing I know for sure, our former friend and colleague Ben Barris, who was born Barbara Barris, was an identical twin. Has an identical twin sister that is perfectly happy being a woman. Ben was definitely not happy being a woman from an early age. Switched to being Ben. And for a long time, and I know this because he told me directly, but it's been documented.

2:18:39He claimed that his mother was treated with an anti -miscarriage drug that had Androgenic protestosterone properties. And he thought that perhaps that had an impact on his gender preference, which is interesting, right? Because he's speaking to hormonal influence on gender preference that at least his idea... Gender identity. Right, and he can't know, but he was an MD and a PhD, and he was thoughtful about the biology of sex differentiation, obviously. So it's conceivable, right? He passed away in 2017, so I can't get his thoughts on this now. But, you know, he was pretty vocal about the fact that he thought that there were things that medications and other things that could certainly impact gender identity.

2:19:27What he was referring to was a pretty powerful hormonal modulator that he was exposed to, right? So that is very different dosage than, you know, you might presumably get from these days, from environmental, you know, plastics with modulators that could impact hormone signaling. That's a pharmacological dose, he was presumably exposed to. Right. And I think that's a big question nowadays to what extent these endocrine disruptors are impacting the fetus. I mean, it has been shown that microplastics are present in the first fecal matter that a baby, you know, excretes. Whether or not those microplastics are effective endocrine disruptors in the sense that they are causing endogen disruption or estrogen disruption isn't clear.

2:20:14Lots to consider. I mean, there are so many conflicting data, you know, it's easy to paint a picture where it's all about endocrine disruptors pushing things one way or the other. But our colleague Mike Eisenberg has done studies showing that indeed testosterone levels and sperm counts are dropping, but according to data from his lab, penis sizes are going up. So, you know, the data don't always fall squarely into a, like a news article type framework, you know, and typically news articles on the stuff pick one or the other side to push for. What do you want to know most going forward about how sex differences in the brain come about?

2:20:53Like what are you most excited about lately? There are many questions, right. One is we still don't have the identity of all the different social behaviors that animals engage in, that mice engage in the innate behaviors, right. So, what are these circuits? How do they interact with each other? So, if you're mating, how do you assess threats and stop mating, for example. So, that's one level of questions. What are the circuits and how do they interact with each other? And at the same time, how are they interacting with higher order circuits that, you know, let you navigate, let you make decisions.

2:21:30What is interaction between cortical cells and hypothalamic cells? So, that's a big question, I think. The other is this thing, this plasticity, this adult dynamic circuit feature that we, you know, this have run into in the female brain. How widespread is it in the brain? Do males also have such dynamic plasticity in the adult animal? We don't know. And if so, what are the conditions in which the male brain requires? And, you know, females undergo different, as we've talked about, undergo many different life stages that are pretty unique to females, right. Lactations, one of them, menopause, and other pregnancies, and other, or relations, and other.

2:22:09So, how are these circuits different across these stages compared to, say, the female who's not gone through any of those yet? Those are very interesting questions, especially given the divergence of life choices that you see out there now. Not everyone is getting married, having kids, and doing that. I mean, many people still are, but my understanding is birth rates are going way down. So, certainly some people are opting out or for whatever reason are having kids. Neral, thank you so much for coming here today for sharing with us all your incredible knowledge and experiments. For me, it was especially gratifying because I think these topics are not just timely, but it's fundamental to who we are.

2:22:55I mean, as you pointed out, perhaps one of the most important distinctions that we make in life is determining who we are and who others are, and the male -female distinction is a critical one that arises, at least as early as conception in terms of the chromosomes are involved. And then the hormones are acting on that, of course. So, I want to thank you for the work you're doing. You do really hard experiments. You do beautiful experiments. They're super clean. And you get really incredible outcomes, which you shared with us today. And it's also wonderful that you took the time to be a public educator, come here and share with us on this set of not trivial topics when it comes to navigating the landscape of sex and gender and hormones and all this stuff.

2:23:46So, you're brave, and we appreciate your bravery, and then the way you approach these questions. Thanks, Andrew. It was a pleasure being here. Thanks for having me on the show. Yeah, well, we'll have you back again. And thanks for also being a bulldog owner. I love that you got Cooper. Next time I've been Cooper. That you got Cooper. And next time, bring him. He's an amazing French bulldog, and you know, just makes me appreciate you that much more. Thank you, Andrew. Thank you for joining me for today's discussion with Dr. Nierau Shaw. To learn more about his work, please see the links in the show note captions.

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