Scientists still aren’t sure why women are diagnosed with depression at nearly twice the rate of men: probably some mix of hormones, life experiences, and biology, is the going theory.
Then, this month, researchers at Michigan State University announced they’ve identified a mechanism in the brain that makes female mice get depressed when they’re stressed out, but not male mice.
They also say they’ve figured out how to turn that specific brain circuit on and off like a light switch, either turning off the depression in females or turning it on in males.
While no one’s claiming this is some kind of silver bullet, and plenty of findings in mice don’t hold true for humans, it is part of a growing, relatively new field of research into sex differences. And what those scientists are finding is that sex plays a much bigger role in disease, and possibly future treatments, than we ever knew.
For decades, scientists only studied male mice. Now that’s changing.
“So this is a new mom who just had babies last week,” says Ph.D. student Claire Manning, holding up a plastic compartment where five baby mice are burrowed in the sawdust next to their mom. “So we don’t disturb her at all.”
She and her boss, Professor A.J. Robison, are surrounded by mice in the chilly basement of the MSU Biomedical and Physical Sciences building. Robison runs a neuroscience lab up a few floors from here (appropriately called the Robison Lab; not to be confused with his wife’s lab, the Mazei-Robison Lab, just across the hall. “We go home at night and I whisper sweet science experiments in her ear,” is the line Robison likes to give visitors.)
Studying female mice at all is still a relatively new thing. For generations, researchers stuck to mostly male mice, thinking that female mice would be too complicated and throw off results because of their estrous cycles (similar to the human menstrual cycle.)
“But guess what? Female humans have estrous cycles,” Robison says. “Should we just ignore that as part of our disease treatment? No! That's a good model for male and female humans, which is who were trying to actually figure this stuff out for. So it’s a short-sighted and silly argument, and we’ve kind of, as a field, started to squash it.”
Since the National Institutes of Health started requiring its researchers to include female animals and cells in their work (unless they can provide “strong justifications” not to do so) in 2016, that has started to change.
And as scientists are studying female mice more, they’re seeing these really surprising sex differences. It’s not just in who is susceptible to this disease or that addiction, but even how sex can determine how diseases show up in the body and the brain, whether it’s Down syndrome or heart disease or cancer.
“Now that all groups are using both sexes, we are seeing a growing number of examples of very different biology between males and females,” Dr. Eric Nestler, a leading expert in the field of animal research and an editor at the Biological Psychiatry journal, which published the Robison team’s findings. “...The new inclusion of female rodents in most studies is revealing a lot of new things about the female brain for the first time.”
What’s more, scientists are also finding that males are more complicated than previously thought, Nestler says, “...because they exhibit regular fluctuations in testosterone (the male sex hormone) just as females show fluctuations in estrogens and progesterones (female sex hormones)."
How two grad students made this discovery possible
Professor A.J. Robison knew he wanted to “poach” Claire Manning to come work in his lab. When she was applying to do her doctoral work at MSU, she was looking at several labs, and wanted to focus on her own area of interest: sex differences.
Robison says he was straightforward: he really wanted to hire her, but that’s not the kind of work he does. His lab focuses on what’s happening in the brain, on a molecular and cellular level, to create diseases like addiction and depression. We don’t do sex difference research, he told her.
Manning was undeterred.
“When I came in, I posited to A.J., ‘You know, maybe these male and female mice are doing the exact same thing (in terms of depression and addiction),’” she says. “‘Their brains were the exact same way with these type of symptoms. But maybe they don't. And we don't know! That's a whole open field!’”
Robison laughs about it now.
“But then somehow, I was able to convince her that my lab was the right spot for her,” he says “And then somehow, she was able to convince me that I should be doing sex differences research. And then that's what happened. Because I knew she was smart and I listened to what she said.”
What Robison’s lab has been looking at for years is this one particular circuit in your brain, that we know has something to do with regulating depression, especially after stress.
In technical terms, it’s the pathways that ventral hippocampus neurons take to the nucleus accumbens. But if you’re not a neuroscientist, picture a highway, running from the emotional memory part of your brain to the part that handles rewards and motivation.
One day at a lab meeting, another doctoral student, Liz Williams, presented some weird results she’d been getting about this brain circuit. Some of the mice were showing a lot of electrical activity in that area, while others weren’t. The data was all over the place.
“And of course my thought is, ‘Liz, what’d you do wrong? What mistake did you make?” Robison says.
Williams remembers it differently.
“I don’t know that I remember him saying I was wrong, necessarily,” she says. But she went back to her results and confirmed, yes, they were right. And then she realized, if they separated the results by sex (male and female) they suddenly looked pretty clear: the female mice were showing a lot of activity in this circuit of the brain. And the male mice weren’t.
“And then she separated the two sexes, and reanalyzed the data as males-only and females-only, and compared them and showed me those data,” Robison says. “And of course, both sets of data were beautiful and tight. And it was that there was a sex difference. And she is the discoverer of that sex difference. Liz Williams. She's the one who corrected me in that.”
The brain circuit that can turn depression on and off
Knowing this brain circuit plays a role in depression, they thought, could this have something to do with why women are diagnosed with depression twice as often as men?
So they ran an experiment. They put both male and female mice through some stress tests, and then watched their reactions. Would the stress make them act depressed?
In female mice, the answer was, yes. Stress set off all this activity in this one circuit of their brains. And while mice can’t tell you, “I’m depressed,” they can demonstrate a loss of interest in something they used to love. In this case, sugar water.
“Or in other words, they didn't really care about this thing that was probably, you know, tasty and induced joy to some degree,” Williams says. “As much as a mouse can experience joy.” (The team also looked for other behaviors that can signify anxiety or depression, like changes in social interaction or self-grooming, but didn't find any.)
It was a completely different story for the male mice. Stress did not set off a lot of activity in that brain circuit. They still loved the sugar water.
It’s like their brains were literally resilient to the stress.
But why? Claire Manning thought, "okay, maybe it’s hormones?" But manipulating estrogen levels didn’t have any impact. So she took one last shot. “And so you know, maybe we’re wrong, but let’s see if testosterone might be playing a role.”
That turned out to be key. When they removed testosterone from the male mice, they were no longer resilient to stress. This critical brain circuit was suddenly very active, and they lost interest in sugar water.
For the first time, they were...kind of depressed.
“And if we gave testosterone to females, they were resilient to this stress paradigm,” Manning says. “They still liked sugar just the same as their control counterparts after stress.”
But in order to be certain it was, in fact, this particular brain circuit creating the depressive behaviors, the team used chemogenetic tools to reach into the brains of these mice,and turn this circuit on and off like a light switch. Same results: when they turned the circuit off in females, they were resilient to the stress.
This doesn’t mean women are just less resilient in general
“That would be a very silly and spurious interpretation,” Robison says.
Rather, he says, this illustrates one of the neurological responses female mice have to stress, that male mice don’t. So while the female mice may react to stress by losing interest or pleasure, the male brain has its own responses to stress.
“Males are having a different anxiety response, and basically express their stress responses differently…there are many behavioral responses in males that don’t exist in females,” including a post-traumatic stress response and other vulnerabilities. Often, those responses are expressed later, Robison says.
“While females are more susceptible to the type of stress used in this study, they are not generally more susceptible to stress,” says Dr. Nestler. “And in fact, show the same or even greater resilience than males in response to other forms of stress.”
The best way to look at it, he says, is that sex differences make males and females respond differently to stress, and that those reactions may be because it was evolutionarily beneficial to them to react in a certain way.
“I’m not an expert on the human research literature, but to my knowledge, experience with humans is similar to what’s been observed in animals,” Nestler says. “Namely, men and women respond differently but one sex is not inherently more susceptible or resilient than the other. The key is the behavioral or physiological endpoint being examined, where one sex might develop a given symptom more than the other sex, but with that other sex developing other symptoms preferentially.”
Now obviously, the Robison team isn’t saying they’ve got a depression cure that’s ready for market. You definitely can’t just give women a lot of testosterone and say, you’re welcome. “I mean, testosterone is not the answer,” says Manning, who successfully defended her dissertation this summer and says she's headed to a post-doc job at Stanford. “We know that, you know, anabolic steroids are a huge problem in their own right.”
So Robison’s lab is trying to understand what, exactly, testosterone is doing in the brain to create that resilience. And once they can understand those mechanisms, hopefully, they might eventually lead to better treatments.
For now, their work is further evidence that looking at sex differences is like looking at a new planet that still needs to be mapped and explored.
And doing that, Robison says, will be crucial.
“We know for a fact, especially in something like depression, that there are huge differences in male and female biology that drive these disease effects. And if we ignore that, we ignore lots of opportunities to find out new and important things about these diseases.”