Sex Differences, Drugs & the Brain: Psychedelics, Psilocybin, Alcohol & Nicotine | Melissa Herman | #170

About the guest: Melissa Herman, PhD is a neuroscientist and associate professor at the University of North Carolina. Her lab studies the effects of drugs on the brain and behavior in rodents.

Episode summary: Nick and Dr. Herman discuss: sex differences in the brain between males and females; behavioral variability among individuals; the effects of psychedelics like psilocybin on the amygdala and hypothalamus; effects of alcohol and vaporized nicotine; and more.

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* Episode transcript below.

Full AI-generated transcript below. Beware of typos & mistranslations!

Melissa Herman 3:42

Thanks for having me.

Nick Jikomes 3:43

Can you tell everyone a little bit about who you are and what your lab studies? Yes.

Melissa Herman 3:48

So I'm an associate professor in the Department of Pharmacology and the bulls Center for Alcohol Studies at the University of North Carolina, Chapel Hill. I run a really fun research group that asks a lot of different questions, but mainly centering on how different substances change activity in defined brain regions, and how that activity change translate to changes in behavior in physiologic contexts and as dysregulated in different pathological contexts.

Nick Jikomes 4:18

And so what are you working in rodents?

Melissa Herman 4:22

Yes, we're preclinical labs, so only rodents, rats and mice.

Nick Jikomes 4:26

And what give people just a general bird's eye view of the lab. What substances Do you work on and what, what are some of the major tools you use?

Melissa Herman 4:39

So historically, my my interest in science really started with stress and then bridged pretty quickly into alcohol. And so I think the longest substance we've worked with has been alcohol, many, many years and really fun discoveries related to alcohol effects in the brain, we've since branched in two. To nicotine vaping, the vaporization of the nicotine compound. This is all again, only in rats and mice. And then, most recently, we've branched out into the effects of psychedelic compounds in defined brain regions, and how those compounds change brain activity and behavioral responding over time course.

Nick Jikomes 5:22

And you've done, you've done some recent, recent work with psychedelics. I want to start there. So psilocybin. Many listeners will be familiar with it. We can give them just a brief overview if you want. But this one gets a lot of attention. It's one of the more popular ones, both as an object of study and in terms of how frequently it's used. It's the thing that's in Magic Mushrooms when, when human studies are done, right? The thing that gets the most attention are the FMRI pictures of the brain letting up in different ways, and people looking with frankly, very coarse grained tools at how the brain is behaving differently when under the influence of silos and but in your lab, you can do more invasive studies. You can look deeper in the brain, literally. So what have you done recently with regard to silos, and can you give people a picture of like, sort of where in the brain you were looking and why you chose to look in some of the places that you looked?

Melissa Herman 6:20

Yes, so I have to freely admit that it was some of the exciting work coming out of human studies that prompted our interest in silos and but it also, for me, brought out why we need more work, not in humans, for silos, and that's because there are some very human, specific complications to the research into psychedelics that rodents don't have as much. So that was one of the central questions, is just, how do these drugs change activity in different brain regions? And the first region that we started with, our very first publication in this realm, was the central amygdala. And that was for a number of reasons, but probably most relevantly, because the central amygdala is dysregulated in a lot of the disorders that psychedelics are supposed to be potentially useful in treating. And so we wanted to know, do these drugs differentially change central amygdala activity, and how does that change change behavior? And how do those things relate? And does that provide any insight into human results? So we went through that study, and we found that there are indeed changes in central and make delivery activity, both acutely in the presence of the drug and then over long term. So I should probably take a step back when I say brain activity. There's a whole lot of ways to measure it, and I'm sort of a classically trained physiologist, so the most straightforward way is recording from live neurons and looking at their firing. That's the most direct physiologic readout of activity. That's useful. And we have studies in that line, and that's very meaningful from a mechanistic cellular level, but we want when we want to connect it to behavior, and some of our more recent studies have used a technique called fiber photometry. So fiber photometry, which is what I think maybe you were alluding to, is the fiber implant into a brain. The central amygdala is somewhat deep. It is a subcortical structure, and we can talk about why we were interested in subcortical but so we put this fiber optic electrode in, and we use it to measure changes in fluorescence from genetically encoded calcium sensors. And when a brain reaches an act breach, when a brain region is activated, there's an influx of calcium, and that causes these genetically encoded calcium sensors, or G camps, to fluoresce. And we can detect that fluorescent signal as a proxy for neuronal activity. Compare that change in activity with changes in behavior that are happening in the same time frame.

Nick Jikomes 8:47

So you have ways to so every time a neuron fires, calcium is coming into the cell. And you have ways of putting special things inside of cells that light up, literally emit light every time that a neuron fires. And you stick a, literally a fiber optic, into the brain of a mouse or a rat, and it can detect that light when it's emitted. So you have a you have a readout, a proxy for how active neurons are, then you're detecting that using light by literally sticking something inside the brain.

Melissa Herman 9:22

Yes, that's correct, and that is one benefit to using rodents. Rodents are incredibly resilient. We can do this surgery. It's a viral mediated delivery of the G camps. So the animals receive the surgery for the viral infection of the G camps and the fiber optic implant, and then they wait for two to four weeks to recover, and then they walk around as if nothing happened. That is something I think is very unique to rodents humans. If we had a fiber optic stuck out of our head for a long time, we might change how we behave. Our rodents are very reliable, and that they'll walk around and behave fairly normally despite. Having a fiber optic.

Nick Jikomes 10:01

So using that technique, you were able to get information about what the central amygdala was doing in response to various manipulations involving silos. And can you give us a little bit more of a picture of where? Where is the central amygdala and what is it sort of known for? What do we associate it with in the neuroscience world?

Melissa Herman 10:24

So central amygdala is in the ventral, ventral temporal region of the brain. So it's in this, in the immediate amygdala complex that includes the basal lateral amygdala, medial amygdala. But the central amygdala is really unique in that it sort of is known to be through its connections with other brain regions, given the role of assigning emotional relevance, emotional salience to a number of internal and external stimuli. So everything that you're constantly perceiving within yourself or within your environment is being continuously tracked by the central amygdala, and sort of a sign of Valence. So, good, bad. I like it. I don't like it. I should change my behavior accordingly. That's all the job of the central amygdala, and that's something where you can maybe speculate why psychedelics would potentially perturb this area, and it's also more pathologically been associated dysregulation of the central amygdala has been associated with psychiatric with psychiatric conditions, including anxiety, depression, PTSD, things of that nature for which psychedelics are currently in testing form. I

Nick Jikomes 11:31

see so would the idea be that you know, when you look at some of the clinical work in humans with psilocybin, with MDMA, with related compounds, people are often reporting improvements with things like PTSD or anxiety disorders, etc, and each of those involves someone assigning, usually, a negative value to some aspect of their experience, like a memory. And the way that people like to think these things are working is they're helping the brain sort of unassigned that negative value to something that isn't adaptive.

Melissa Herman 12:04

Yes, it's somewhat like that, and somewhat more complicated things like PTSD, you have the assignment of a negative value to something that might be neutral to somebody else, and so you have hyper reactivity of your amygdala in that tagged context or stimulus, but it's But, and this is where the CEA does generally get ascribed aversive processing. So it's meant to detect a negative situation. That's part of the story, and the CEA definitely does that, but it also does signal for some positive valence. So you can have a positive, rewarding, motivated signal that goes through the amygdala as well. And in the case of anxiety or depression, you can have a dampening of that, and that's actually part of the emotional flatness that is sometimes reported in some people with treatment resistant depression.

Nick Jikomes 12:55

I see, I see, okay, so, so you've got some, you've got good reasons to think that this, this might be a region worth looking at in the brain. Central amygdala is fairly well studied historically. We know quite a bit about it compared to many other brain regions. You've got a way in rodents that you can measure what's going on in in the brain, in that part of the brain, while animals are awake and doing stuff, and then you can give them silos and under certain conditions. So what exactly were the conditions and and what did you find?

Melissa Herman 13:25

So we were in the interest of looking at something that could be relevant to anxiety or depression. I should say one of the limits of rodents are that we are not trying to recapitulate human anxiety or depression. It's a complex disorder. It's complex in between subjects. But we did want to find a facet of an experience we could administer to the animal, one that we could repeat reliably, that would potentially be relevant to the conditions of anxiety and depression. And for that, we wanted to look at an aversive stimulus processing. So there's a lot of aversive stimuli, there's foot shock, there's different pain models, but we wanted to avoid those because we wanted something that would be low, low injury to the animal, and especially something that we could repeat over time. So for that, we selected just an air puff. So these animals get an 85 psi air puff into their face, and it's an aversive experience. They don't like it. And importantly, for our work, it reliably induces a response in the central amygdala. And by response, I mean it induces that burst of fluorescence associated with increased activity of that nucleus. So basically,

Nick Jikomes 14:36

something unpleasant happens to the animal, and you can detect a burst of activity in the central amygdala,

Melissa Herman 14:42

yes. And so then the question was, if we administer silos in or a vehicle control, does that change reactivity in the central amygdala to this stimulus? And how does that change? Reactivity change over time? And importantly, we also really wanted to look in males and females, because. Pre clinically, a lot of research has focused solely on males, and that's sort of unfortunate for the human population, because we are both males and females, and there are important sex differences in how disorders like anxiety and depression manifest present or change over time, and that's key to improving treatment. So we did male and female rats. We did this in vehicle and silos and injected animals, and we found some really interesting sex differences. So when we put the animal in their behavior chamber and air puff them, there were no difference in the central amygdala reactivity at baseline in these animals. So a male and female rat CEA response to an air puff about the same. But when we gave the animal silos and the female central amygdala in the presence of the drug, so this was like 1020, minutes after the drug was injected, during full drug experience, these at the female central amygdala reaction was much higher than at baseline, where the males was unchanged. So that's just acutely so interestingly, even though this is a very psychoactive drug that does basally increase activity in the central amygdala, it didn't change the response to the stimulus in males, and it magnified the response to the stimulus on top of that basal increase in reactivity in females. So

Nick Jikomes 16:17

so the central amygdala is reacting in the same general type of way, there's a burst of activity, but, but the magnitude of the response is higher, but only in female, not in male rodents,

Melissa Herman 16:29

yep, and only in females that were injected with silos and not vehicle.

Nick Jikomes 16:33

And I mean, is that surprising? Or do you sort of expect, or do we know that these types of sex differences are actually fairly common in the rodent brain.

Melissa Herman 16:43

Well, nobody had really looked at Central amygdala reactivity in psychedelics. So until we saw these data, this was not something that was really well known. It's not entirely surprising. There are instances of increased reactivity of the central amygdala in females in other contexts. So in that realm, this is not something that's out of left field and it but, but it is interesting, because for our purposes, when I mentioned the drug effect, we really wanted to make sure and look at acutely when these animals are on this very active, psychoactive drug. And that's why I really wanted to highlight that even though they're having this very significant drug experience, this is a full psychedelic dose, the males are unchanged, and they're responding, and the females are magnified. So I guess I phrased it where the females were aberrantly increased, but it's also worth the opposite consideration, where it's interesting that the males, despite this very active drug, aren't changing their central amygdala reactivity at all.

Nick Jikomes 17:44

And can you give us a picture first? So you give a full dose to these animals qualitatively, like, like, Can Can you just tell by looking at them that something's different? What is their sort of gross behavior like when you give them silos?

Melissa Herman 18:00

Well? So this is really interesting. And I get this question a lot we did. We gave two makes per kg, which has been shown by their labs to be a full agonist, fully psychedelic dose. For that case, they're actually not doing too much different. They do walk somewhat less, but not they're not lies. Yeah, so

Nick Jikomes 18:23

naive observer. If they were looking at the rodents, they wouldn't necessarily notice anything.

Melissa Herman 18:28

No, not necessarily. If you're familiar with rats, though they do do this thing, which we did not report in the paper, because it's a little bit harder to quantify. But there were some of them that would have a lower crouch, which is an interesting, maybe more vigilant posture, but that was not something that we quantified, and we didn't notice any differences between males and females, and it didn't impair their ability to respond to the stimulus or walk around freely. They didn't appear otherwise significantly impaired.

Nick Jikomes 19:01

Okay, so the acute effects were that the females had this larger response in the central amygdala the males did not show that. What about any lasting effects that outlasted the drug?

Melissa Herman 19:12

Yes, that was the subsequent follow up. Studies were to just repeat the same procedure, put them back in the box, administer an air puff, and we did this at two days and six days and up to and 28 days after administration. So this is long past time where there's drug in the system. It is, it is well and truly metabolized out, but the changes that that acute drug experience might have created are potentially still there. And so interestingly, while the females had that heightened reaction on the drug. Soon as drug was gone, they were back to normal. So there, and that's not to say that they didn't respond, but to say that their central amygdala responsiveness was back to baseline and stayed consistent at two, six and 28, days after the males, in contrast, that didn't have anything while they were on the drug as early. Like these two days, we started to see a reduction in the central amygdala response, and that continued six days and lasted up to 28 days. 28 days was the last time point we checked. It's possible that that response could have been like that. Decrease could have gone further or could have lasted longer. We just ended the study there, but

Nick Jikomes 20:18

still a pretty large amount of time, 28 days in the span of a rodent, that would probably translate to weeks of time in a human, I would imagine,

Melissa Herman 20:26

yeah, yeah. It's a our lab rats live about like two years. So 28 Yeah. And

Nick Jikomes 20:35

so what? What like? How do you interpret those results? What might they mean in terms of how we might think about the human side of this, and clinical studies in terms of sex differences, in terms of, you know, measuring things at baseline compared to after the drug. What is the relevance here for humans?

Melissa Herman 20:53

So I think there's a couple relevant points. I want to be careful, because rats are not humans, and there's some advantages to rats and disadvantages to humans, and vice versa. So an advantage to rats is that they did not know that they were getting these compounds. So they had they and they cannot control their central amygdala activity. So these neurons are behaving as they would behave the rats aren't changing them consciously in any way, and we can't interpret we aren't making them change in any way. And so I think that for the acute effect, I think this is one of the things that may be important to keep in mind. So in human clinical trials, that increased central amygdala reactivity in females could mean that during the active drug sessions in human patients, females might be more at risk for an aberrant over response during the administration, and that's meant it should be monitored for I

Nick Jikomes 21:49

see, yeah. So this could indicate that some people, whether it's males versus females or just individual some individuals compared to some other individuals, they could be more prone to a bad trip or an anxiety response, or something could be different.

Melissa Herman 22:05

Well, again, I don't necessarily want to directly link the CEA with bad because we don't know that, but that hyper reactivity, it just, it just could be, mean, an exaggerated response

Nick Jikomes 22:16

of some kind, of some kind. There's another thing here. So like in the human studies that have been done, and I'm really not a I haven't looked at all of them, maybe someone has done this, but I have not noticed any human studies where they're doing fMRI or other measures of cortical activity. I have not noticed people bin the data, male and female and look for differences. Could that be? Could that be something Where? Where? I mean, you're the work that you've done, could indicate that there could be a difference there. That's worth knowing about.

Melissa Herman 22:49

Yeah, and I My hope is that as the human studies progress and get larger, I think one of the issues in some of the early studies, 1020, participants, you're likely not powered to detect differences, and that's understandable, so I think that that's some some reason why we maybe haven't gotten to that point yet in human studies. I do think that they're important, and they're things that I would hope my human researcher colleagues are considering and following up on. But admittedly, it's something it's easier for me to work in the animal numbers and controlled settings of male and female rats. And so this is where we can actually, hopefully help our human researcher colleagues.

Nick Jikomes 23:28

Yeah, have you? Have you looked at other brain regions that in response to silos that might be relevant to, say, a stress response or something like that? Yes.

Melissa Herman 23:38

So that would be the more recent paper before I shift to the other paper, though, there were two other key findings from the central amygdala that will be relevant in our other studies. So the increased female responsiveness on the drug is something to be mindful of for drug treatment, but I would say therapeutically, the dampened reactivity in the males that occurred over time is more consistent with the long, lasting symptom reductions that are seen in human patients. So these rodents are not patients. They're not even perturbed. These are standard rat models. So I'm not saying that these are depressed rats or anxious rats. They're just rats, but that dampened central amygdala reactivity is something that potentially could be a neurobiological mechanism by which the human symptom reduction are occurring,

Nick Jikomes 24:22

yeah, in humans, you see this sustained response that outlasts the drug treatment by quite a long time. And so there must be some kind of sustained change in some parts of the brain. And this would be a potential observation of something like that,

Melissa Herman 24:36

right? That could be one. So that's important to keep in mind that sex difference and time difference in treatment effect. The other thing that was really interesting that began in our central amygdala study didn't study and has carried through and is now a main focus of the lab, is that when we were looking at how these animals were responding to the air pump stimulus, the very the graduate student working on this a really talented. Devin effinger, he noticed that some animals would run away from the stimulus and some would sort of sit still. And when he went back and looked at who ran and who sat still, he and this is informed by a lot of sex differences work by Becca shansky and others, with darting and freezing, he found that the females were, by and large, strong active responders, so they would Dart or have a high velocity escape movement in response to the stimulus. And then males are about equally split in this case. So when he looked and said, Okay, if you're a active or a passive responder at baseline, are you more or less sensitive to the effects of the drug? And when his data that way, he found that the treatment effect, he saw that reduction in reactivity was predominantly by those animals that were active responders at baseline.

Nick Jikomes 25:53

So you did this work looking at the central amygdala, you saw differences between males and females in terms of their response, how that part of the brain was responding to a stressor. I know that you've looked at other brain regions in the hypothalamus, for example, that are hooked up to some of the important stress centers in the brain. What did you find there?

Melissa Herman 26:14

Yeah, so our next study focused on the periventricular nucleus of the hypothalamus, the PBN. It's a big area for stress responding, the originating side of the endocrine stress response. So that was another place where we thought potentially, psychedelics could be changing your reactivity, and that could be a key contributing factor to some of the therapeutic effects that are seen. So using the same approach we did in the central amygdala, we planted that fiber optic, infected the G camps and then allowed the animals to recover, and then administered that air puff again. And when we started these studies, we did find, interestingly that there was a baseline sex difference in PDN reactivity, so the male PDN at baseline, so no no administrations, no manipulations, nothing, but just at rest, our males had a heightened reactivity to the airpuff stimulus and the PBN as compared to the females. So that's something it's we did not predict or expect, but was important as we kept our studies going forward and now when we administered the drug and then put them back into the chambers to give them the air puff. Again, instead of the females have the heightened responding on drug. Now, in the PBN perspective, the males were heightened reactivity. So now on this drug, when there was an air puff to the face, the pVn had an exaggerated reaction to that air puff. And only in males, and only in the silos in group, not in the vehicle treated group,

Nick Jikomes 27:41

and so that's when they're on silos, and that's just strictly

Melissa Herman 27:44

in the presence of the drug. When we did the follow up days, this time we went two days and seven days there, everything was back to normal, and by back to normal, there was still a response that the PBN could generate to this air puff. So it was still sensitive to the air puff. It didn't lose that sensitivity, but it was no different from baseline. So

Nick Jikomes 28:05

two different, two different brain regions, you see different types of effects. One of them, the the deviation from baseline, is seen in females. In another region, you see it males. So different things are happening in different places, depending on males versus females, and depending on the brain region you're looking

Melissa Herman 28:21

at right. And then for this one, because we had a key that that behavioral respondent was really important, we now had, in collaboration with a colleague, Clyde Hodge, at the bull center, we could actually monitor their behavior in a more granular detail, as it happens organically, in the same time, temporal time frame as the brain reactivity. And we found that just as the PBN was increased in reactivity, the males showed a dampening of that that active threat responding. So they had a deep velocity of their flight and a decreased distance traveled. And so it's of a brain being more engaged while the behavior is actually depressed.

Nick Jikomes 29:01

I see, I see, so, so this particular brain region, the PBN, is showing more activity, basically, but this threat, this behavioral stress response is is

Melissa Herman 29:12

weaker, yes, yes.

Nick Jikomes 29:16

And were there any, and remind me, what were the changes you noticed that were more long term or after the drug was gone. So

Melissa Herman 29:22

in the PBN it was they it went back to normal and that way and that, and it stayed that way. So males had that increased reaction in the drug, but then went back to normal, and females kept that stable, responding throughout. So the PBN reactivity was not changed by drug, and was not changed at any of the follow up time points after drug.

Nick Jikomes 29:43

So when were you able to look at so in the the amygdala studies, you mentioned that you could further, you could you could subdivide individuals based on the kind of behavior they had in response to the stressor the active versus the more passive responders. That was something similar.

So, you know, you might do an experiment in five rodents, 10 rodents, 15, even 15 or 20 is not that many. And, you know, I wonder how many null results have been found in the lab over the years, where you're actually just averaging together, you know, something that goes in two different directions, in two different subpopulations, and you just don't see a phenomenon that is, in fact, there?

Melissa Herman 35:21

Yeah, no, I think that this is something that goes back to the original point of in some instances, you have these big, robust assays that that have consistent outcomes, and those are beautiful, and we love those for many reasons. But when you're getting into more nuanced, more complex disorders like anxiety, when you're getting into complex substances like psychedelics, you can't ignore the details. And I think you miss things when you look for big, big, consistent outcomes, because in a system that has that level of complexity, there's a lot going on.

Nick Jikomes 35:56

Do we understand? So you know, outside of the context of just the individual experiments that you did that we were just talking about in terms of thinking about this inter individual variability, that certain animals respond behaviorally in different ways from other animals in response to the same stimulus. How much do we know about the origins of that variability in rodents in general? So for example, is it random and stochastic? To some extent, is some of it. Does some of it map on to things like birth order or social rank? Do we have any sense for how that variability might arise?

Melissa Herman 36:31

So no, but not completely. This is where I think it's important to to reflect on what we can control. So the rodents in our studies are a standard strain. These are Sprague Dolly. So they're they're bred to be as similar as they can be. They're from the same vendor. They get shipped to us the same. They're in the same housing conditions. They're all treated the same. So absent something like maternal care, rank order we've controlled for everything else. So these animals have had roughly the same life experience as each other and have a very similar genetic makeup, but they have these inherent baseline differences. So I started off by saying that I really love working in preclinical models, because we're we can control so many variables, but it's interesting to still reflect that even in that highly controlled experiment, there's instances where the brains react differently and behavior is different.

Nick Jikomes 37:28

Yeah. I mean, one of the most interesting things I've learned recently on the podcast is, you know, I talked to a man called Christian lucer. He studies addiction in rodents, basically, and there's this very peculiar result that you see that is really interesting, that I think they're following up on, and it kind of relates to what we're talking about. So basically, in the addiction literature, what you find is that in rodents, again, these are inbred animals that are more or less genetically homogenous. They are not nearly as genetically variable as human beings. They are not human beings. They're very different from us. What you find is the classic result in the cocaine addiction literature, is that about one in five animals, about 20% will become addicted to cocaine. If you give animals repeated instances of cocaine, about one in five will engage in compulsive drug seeking behavior. And what's interesting is you see that in rats, you see that mice, you see that in humans, even though the rats, in the mice are inbred strains that have these very controlled lives, humans are not that at all. And so the speculation there is that it could have some there could be something conserved there in terms of social rank, birth order effects that that get established early on.

Melissa Herman 38:38

Yeah. And it could be, I mean, I I think maternal care, social rank, diet enrichment, but I also think there's a lot that happens when any brain is wired, and any brain is wired and developed and changed by their experience, and even in an instance where I described where our Animals have essentially the same history. Some of those brains could have been wired differently, and that likely manifests in different outcomes. And so this maybe comes from somebody who likes working in the amygdala, which is really complicated and likes to be very confounding. That complexity doesn't bother me scientifically. It actually interests me. I think that there's opportunity there for asking more detailed questions. So that goes back to where with substance use, with vulnerability to anxiety, with behavioral manifestations associated with depression. You know, if there's individuality that drives any of those outcomes, we can still see it, even in our preclinical models, and that absolutely has relevance. And that's where I think getting away from collapsing by group or saying something's not different when it might be different you just didn't ask the right question, is a really, really key concept in neuroscience right now,

Nick Jikomes 39:56

and just out of curiosity. So when you do these experiments, when you like the central. Amygdala, stuff, the paraventricular nucleus of the hypothalamus, stuff, what are the the rough sample sizes that that you're using, where you where you can see these effects? So

Melissa Herman 40:10

it's a good question. For both of our studies, we use an n of 10 per group, and for a lot of our behavioral analysis, that's about 10 to 12 is about on average

Nick Jikomes 40:20

10, meaning 10 to 12. Males, 10 to 12 females, 10 to 12

Melissa Herman 40:24

per group, per subject. So yes, 10 male vehicle, 10 male silos, and 10 female, 10 females. I listen, yes, I see it's a lot of work, but I think that that's why you need that. And that's that, again, goes back to where I think some of the limitations of the human work is you need numbers of a certain level to be able to ask questions like we can ask right, right, right?

Nick Jikomes 40:50

So you've worked on a number of drugs. You mentioned that the one that you've worked on the longest is alcohol. That's actually something I've not talked about much on the podcast. And so I'm wondering if you tell me a little bit about what the basic setup is in your lab for studying things like alcohol consumption and compulsive alcohol taking in rodents and what that looks like, sort of in comparison to humans.

Melissa Herman 41:13

So rodents are actually an excellent model. They will drink, as you mentioned. So I'm familiar with Christian Luger's work. They drugs like cocaine and heroin are readily adaptive for a lot of rodents. Alcohol is less so they do have a very protective mechanism where if something tastes different, they'll avoid it. It's called neophobia. So it is not that they go binge immediately, voluntarily. You do have to get them to become used to it, understand that it's not going to kill them immediately, and then they'll drink. So we have studies where they will voluntarily drink in their home cage. We have studies where we experiment or administer alcohol, and we can do that through intragastric means, through an oral gavage. We also have vapor inhalation, so they'll sit in chambers and just breathe in alcohol vapor and get to very high BLS in a very relatively stress free environment. But in all those cases, there's pluses and minuses. So with the home cage drinking, they have the option of choice and volition, which are very essential and most relevant human behavior.

Nick Jikomes 42:17

And what literally, are they drinking? Is it like a 10% ethanol solution? Is it?

Melissa Herman 42:23

So we use a 20% ethanol solution, and this is 20% lab grade ethanol, so no flavors, no liquor types. It's just the 20% ethanol that tastes like ethanol, and they'll still drink it. So that, I think is interesting, because certainly it's not super palatable, and there is, I think, some relevance to be made for some of the drinking that we likely see in the lab setting is because they're bored, yeah, with something new. I don't think that that doesn't relate to the human condition entirely, but, but importantly, yeah, they will consume alcohol. And Interestingly though, there are strain differences. So we often will say rats and mice together, but it's been well known in the field for a long time. Mice will drink quite a bit, and it is strain specific. So we work with a strain called C, 57 black, 6j, it's a lab strain that's been used for a very long time they consume very, very high quantities of alcohol voluntarily. And there's a sex difference there. The females will consume considerably more than males. Females

Nick Jikomes 43:30

consume more than males. I would guess that it's the opposite of human beings, but I don't actually know for sure. Well, so human

Melissa Herman 43:38

this is, this is, again, this is where humans are a little more problematic than rodents. Rodents don't have any cultural bias. They don't have any like, cultural expectations for alcohol or what it might be to them. This is just pure male and female, and the females, yeah, they drink, they drink more. They do also, in a lot of instances, drink more water. So there could be drives for fluid intake that come into play for females that aren't as relevant in males.

Nick Jikomes 44:04

Interesting. Yeah. And can you give us a sense for so in human beings, like, what? How do we define? How do psychologists or psychiatrists define alcohol use disorder? What does that look like in terms of how much people drink before it's clinically considered to be a problem? And how do you then start to model that in the rodents? Well,

Melissa Herman 44:23

so this is a really good question, and this is something where a lot of progress has been made in the field appreciating the complexity of alcohol use disorder, because historically, people would use very stigmatized language, like alcoholic, drunk. This really was a problem clinically, because it kind of suggested that these people had problems controlling themselves. They had issues of, you know, discipline and self control and moral failings. That's all that. That's all a cultural baggage that humans have, and we sort of progress from there, based in large part by pre clinical work. Rodents have none of this. They have no motivation for any of this, other than what they can choose to do on their own, and they will drink. And you can see in rodents, as well as other models, like non human primates, they will regularly consume and they'll achieve some of the same things that are now in the diagnostic criteria for human alcohol use disorder. So that would be escalated intake that would be somatic withdrawal signs in the absence of the drug that would be increased, seeking and taking escalation, the development of tolerance. It's a very multifaceted disorder, and that's why I think currently it's not a yes or no question. It's of these range of criteria, how many? How many resonate with you, and past a certain limit, you are now qualified for an alcohol use disorder diagnosis. So it's not one thing. It's sort of a spectrum, a constellation of

Nick Jikomes 45:51

effects. But so it's a spectrum of effects that I think the two key ingredients here it sounds like, are one dependence or tolerance. So you develop a dependency such that in the absence of the substance, you feel bad there's measurable things happening. And the other would be like compulsive seeking, like giving up doing certain things that would be considered ordinary in order to acquire the substance. Yeah,

Melissa Herman 46:15

I think the term compulsivity is really controversial in the field right now, because that is harder to model in animals, but they will say more like habitual, so repeated, consistent intake. And this is something we can absolutely see in our models, is the voluntary models you give these animals access. So when I mentioned home cage drinking, they're not forced to drink. There's a bottle of alcohol and there's a bottle they can choose to leave the alcohol entirely alone, and some animals do. So, yeah, differences you mentioned, it's absolutely the case that there are some that will will sample it and then decide it's not for them and choose to consume almost nothing or very low levels forever. There are some that will sample and increase their intake over time. There are some that will start high and stay high. I mean, how it looks, even in a reduced rodent model, is as variable as it is for humans.

Nick Jikomes 47:10

Interesting, yeah, I was gonna ask that next actually the individual variability piece. So there's quite a bit of variability individually, also strain by strain, species by species, so all of the above well, and this is

Melissa Herman 47:23

where you get at some really interesting things, because going back to some of these psychiatric conditions, an area of intense study is the overlap and comorbidity and relationship between things like anxiety and depression and substance use, and that shapes your landscape. Because if you're an individual that has a pre existing anxiety like disorder, you may consume alcohol for partial relief of that anxiety. And absolutely, that's something that's been reported correctly with social anxiety. Alcohol is really well known for being what's called a social lubricant, and this would help somebody that is trying to relieve their negative feelings. Yeah,

Nick Jikomes 48:03

and I guess I'm just going off the cuff here, but I think you said, I think you said, mice tend to drink more voluntarily than rats do. And you know, if you think about this, like, like an ecologist, right? These, there's these are both prey species, but, but rats will prey on mice, and mice are smaller and more vulnerable, so you might expect them to have higher baseline anxiety, and maybe that is potentially a tie in here.

Melissa Herman 48:29

Yeah, we don't. So this is actually where it's in our lab. We look in rats and mice, and we perform a lot of anxiety like or depressive like tests in rats and mice, and I haven't seen what you're describing this idea of these mice being more anxious and that that might drive their intake. I'm not sure that we can reliably make that conclusion. And both rats and mice, if you stress them out, they will. You can, you can induce a stress like relapse phenomenon in both rats and mice. So I think it's, I'm not sure it's as ecologically simple as lower order higher order actually do this is where going back to the complexity I forgot to mention so that C, 57 black 6j strain that we used was sequence, and there's a genetic polymorphism in those animals, in the GABA, a receptor that drives their intake. Other strains of mice drink considerably less and do not have this polymorphism. What

Nick Jikomes 49:23

is the effect of the polymorphism? In terms of is it's GABA a receptor. We're talking about inhibitory neurotransmission. Are they? Is there? Is there less baseline GABA activity in the strain or more, which what's the actual effect there? No.

Melissa Herman 49:37

So there, it's a sub unit of the GABA, a receptor. So it's not as easy as less or more. It's receptor distribution, receptor function, which changes the brain area

Nick Jikomes 49:46

I see, but ethanol directly acts on GABA receptors. Is that true? Yeah,

Melissa Herman 49:51

the GABA a receptor, is one of the primary, probably the Hallmark, primary target of alcohol, and alcohol will generally exacerbate the. Effects of GABA a receptor, which the GABA a receptor is the primary inhibitory amino acid transmitter. So if you think about that inhibition, a lot of the effects we see with alcohol, sedation, slowing of locomotor responses, eventual loss of consciousness. A lot of those are very much under the purview of the GABA a receptor.

Nick Jikomes 50:19

One last little question here. So some mice, some rats, will voluntarily ingest some amount of alcohol, depending on the individual. Do they get drunk? Do they stumble around?

Melissa Herman 50:30

Well? So yes, yes, and no, you can't. You absolutely can see motor impairments. There's something called a loss of writing reflex. So mice being preying, animals are not like puppy dogs. They don't like to be turned on their back and have their bellies exposed, because that's a very vulnerable, unsafe position for them. So it's an adaptive response to flip yourself back over as quickly as you can. And when animals have a certain level of intoxication, they lose that ability to write themselves, or they will write themselves much slower.

Nick Jikomes 51:00

And so what are some of the things that you guys have found in terms of the neural basis of the ethanol intake and the development of high levels of ethanol consumption? And rodents, are there particular brain regions that are involved in this

Melissa Herman 51:14

well, so this is a perfect circle to where we started with the psychedelic work, a region that is also a central anatomical target of alcohol effects of the central amygdala, and that, again, goes back to the good and bad of the central amygdala. Canonically, this gel amygdala has been sort of ascribed to this negative affect that emerges following chronic alcohol exposure. But you can even see that with initial alcohol exposure, the central amygdala is engaged so that job of assigning Valence is happening from the first drink you take, and changes in the activity of the central amygdala will change your drinking behavior. And that is something where I think, just to tie everything together, one of the components of the central amygdala that's very sensitive to alcohol and changes with acute and chronic alcohol exposure is the corticotropin releasing factor receptor, one system, which is the primary receptor system for the primary stress peptide corticotropin releasing factor, or CRF. But this is also a case where there was a lot of excitement about the really, really beautiful, definitive pre clinical data looking at the CR one receptor. They put this into clinical trials, and they use syrup one antagonists in human alcohol use disorder patients, and they found release relief of some of the anxiety related behavior, but there was not substantial reductions in voluntary drinking. So this is something that had a strong preclinical backing. It looked beautiful. You could give a serf one antagonist, and you can stop volunteer Well, significantly reduce voluntary drinking in rodents and happen in humans.

Nick Jikomes 52:50

Yeah, and that's that's pretty common, right? Like, we see a lot of really promising preclinical stuff, and it just never ends up translating to humans most of the time. Well, you know, just like, like, how you know, when we stepping back from any sort of particular set of experiments you guys or anyone else has done, when we think about using species like rodents to try and model things like brain disorders, anxiety related behavior, depression like behavior and so forth. How well can we do? You think? You know, how well can we actually model stuff in humans with the rodents, when we think about the the very low frequency with which the preclinical work actually ends up translating to human outcomes?

Melissa Herman 53:39

So when you phrase it that way, it seems like preclinical work has no value. As a preclinical researcher, I strongly disagree. I think it's actually a challenge for us, because I think a lot of the failures have been issues with data replication and issues with an oversimplification of outcomes. So especially when you're talking about complex things like human depression, human anxiety. A lot of studies that say, Okay, we fixed it. We reduced anxiety, or anxiety like behavior in rodents. All I can ever think is in that context, in this one test, in males, in adults, in animals with thi