Ever heard of a vomeronasal system? If you answered no, you’re not alone. The vomeronasal system is apparently present in many animal species and is involved in relaying external signals to the main olfactory system. It is described as an auxiliary olfactory sense organ and derives its name fittingly from its location, which is near the vomer and nasal bones. I had no idea it existed before attending Dr. Catherine Dulac’s eye opening, or should I say nose opening lecture (admit it, you laughed). Dr. Catherine Dulac is a Higgins Professor of Molecular and Cellular Biology at Harvard University and a Howard Hughes Medical Institute Investigator. To say she is highly decorated is an understatement, and I was fortunate enough to hear her talk at the Presidential Lecture Series during the SfN 2018 meeting.
Catherine Dulac, PhD
I’ve never thought much about the neurobiology of social behaviors, such as sex, aggression, and parenting. As it were, I study substance use disorders. However, Dr. Dulac’s presentation was fabulously engaging. My eyes were glued to the screen that projected part of her life’s work, as I watched her navigate effortlessly through taboo subjects that would later put her at odds with her fellow colleagues; I marveled at how she boldly challenged accepted views on male and female behavioral circuits and forced the endocrinology field to think about the possibility that the systems that make us “male” or “female” are a lot more complicated and intertwined than once believed. I don’t want to sound too much like a groupie, but I do want to emphasize that her “science story”, as I like to call them, was marvelously captivating and refreshing.
One of the main points that lingered in my mind was the idea of the fluidity between sexes. The classical idea of gender development dictates that between 8-24 weeks of in utero development, there is a spike of testosterone that is present in males, but not females. There is another, smaller spike shortly after birth (sometime between birth and the first year) that is again, present in males, but not females. And after that, hormonal life is fairly uneventful until the onset of puberty. These early hormonal spikes wire the brain as “female” or “male”, and that’s the end of it, or so we thought. But Dr. Dulac posited a different take on the story. She offered that males may possess female-typical behavior circuits, revealing the possibility that there are “silent” nodes in the brain that possess the capability to make males more “female-like”, or rather, engage in female-specific behaviors. Further, there may be processes that exist to modulate these circuits that are triggered by environmental stimuli as well as internal cues. The idea may have sounded farfetched at the time, but was it really? We know there exists many, many examples of gender fluidity in the animal kingdom. Males and females of certain species can undergo a behavior switch, usually triggered by sexual maturation and parenting, that rewires their sex-specific social behaviors. As it were, remember that vomeronasal system? I didn’t just mention it to get your attention, although that was a clever little trick wasn’t it? The vomeronasal system is involved in gating sex-specific behaviors of both males and females. First, impairments in the vomeronasal systems resulted in males mounting both males and females, suggesting that the vomeronasal system is involved in discrimination of sexes (Stowers et al, 2002, Science). Second, females with impaired vomeronasal systems also displayed mounting behaviors with both males and females, which indicates that the vomeronasal system is involved in repression of male-like behaviors in females (Kimchi et al, 2007, Nature). Further, virgin male mice are known to be infanticidal towards foreign pups, but when male mice have pups with a female, over time they are no longer infanticidal and take on a parental role. Interestingly, when the vomeronasal system was genetically hindered, virgin mice took on a parental role towards pups that didn’t even belong to them (Wu et al, 2014, Nature). I think taken together, this drives a strong case for thinking about the neural networks that separate “male” from “female”, or more precisely, the networks that inherently program us to perform sex-specific roles/tasks, as more malleable than previously thought.
I’ll stop here. I can’t stress enough that this is all really outside of my field, and I don’t want to butcher explaining the amazing science that Dr. Dulac has produced throughout her career. I also can’t stress enough how inspired I was by her. She presented ideas that, at the time, went against the accepted views on social behaviors and the hard-wiring of the male and female brains. Her fellow colleagues later had no choice but to respect her science as it became clear that she was a solid and steady force of great ideas and sound science practices combined. I do encourage everyone to explore other aspects of her work, including the molecular and spatial single-cell profiling technique she uses to fully characterize cell types. Thanks for reading, my fellow mad scientists.