It is well known that social interactions are essential for animals to survive and reproduce, but did you know those behaviors are rooted in specific neurons and proteins in the brain? Harvard neurobiologist Catherine Dulac, Ph.D., discussed the neural circuits responsible for instinctive social behaviors in mice, during the last NIEHS Distinguished Lecture of the year Dec. 12.
Dulac began her work years ago by studying the vomeronasal system, which is a sensory circuit that detects chemical signals called pheromones in most mammals. At the time, she thought that the vomeronasal system drove mating behavior and that the olfactory system handled the sense of smell. Her thinking changed after her team discovered a protein in the vomeronasal system called TRPC2.
One gene, different impacts
"If you remove the TRPC2 gene from a male mouse, the animal is still able to mate, but it is unable to determine if the other mouse is a male or female," Dulac said. "Surprisingly, males with mutations in the olfactory system were unable to breed."
The data suggested that mating behavior requires both the vomeronasal system to detect pheromones that distinguish gender and the olfactory system to detect pheromones responsible for mating.
When TRPC2 was removed from female mice, the story was different. The females behaved like males by mounting other mice. Dulac searched the old scientific literature and found examples of females of a particular species occasionally behaving like males, and vice versa. It was a watershed moment, because it suggested that the template for behavior for the other sex existed in both sexes.
Serena Dudek, Ph.D., deputy chief of the NIEHS Neurobiology Laboratory hosted the seminar and said having Dulac come to the institute to talk about her work was timely. "Many of us in the Neurobiology Laboratory are studying social behaviors, so it is a great pleasure to have her here," Dudek said.
Driven to nurture
Parenting is a huge investment in time and resources, so Dulac wondered why animals do it. She explained that the neural circuits that drive the behavior have a strong genetic component. Animals are driven to nurture infants.
However, the behavior of males toward the young varies among species. According to Dulac, in about half of all mammalian species, typical male behavior includes killing the babies of other adults. Another scientist characterized the infanticide in primates as a reproductive strategy, because killing the young prompts the females to re-enter the estrus cycle, making them sexually receptive.
Another deep dive into the literature told Dulac that becoming a parent changes the behavior of males. Male mice become temporarily parental after they mate with a female.
A graduate student in Dulac’s lab identified the neurons for parental behavior in both males and females as being the medial preoptic area (MPOA) in the hypothalamus. MPOA neurons from parents were very active, whereas the same neurons from infanticidal males were not. Dulac and her team are currently expanding their research to include more work on the neural circuitry of parents.
Jerry Yakel, Ph.D., chief of the NIEHS Neurobiology Laboratory said, "Dulac’s pioneering work on the mechanism behind sex differences in paternal behavior was extremely fascinating and inspiring to many of us."