New evidence points to “hardwired” parental behavior in our brains
Most female mammals give birth, and then care for their offspring, while males often breed with multiple partners and play little role in parenting once mating is over. Yet researchers have had a hard time pinpointing where, exactly, in the brain these differences between the sexes are located and how they translate into behavior. For years, the extent of “hardwired parental behavior” has been hotly disputed.
But now new research from the Weizmann Institute of Science offers new insight into this issue, showing that the same network of brain cells operates differently in male and female mice. In particular, the study found that neurons containing a protein known as tyrosine hydroxylase, or TH, are more numerous in mothers than in virgin females or any males, in a small part of the hypothalmus. When the researchers increased and decreased the quantity of TH in those neurons the parenting style of the mice changed accordingly. That is, more TH meant more traditionally “motherly” and less TH meant more aggressive behavior.
The study, published in Nature in October, was led by Dr. Tali Kimchi and her graduate student Niv Scott, in collaboration with Dr. Ofer Yizhar and Dr. Matthias Prigge, a postdoctoral fellow in his lab, all of the Weizmann Institute’s Department of Neurobiology.
Female mice, even those that have never had pups, act in ways that can be defined as maternal. They will carry a pup left in the corner of the cage back to the nest, for example, and spend time grooming and huddling over their newborns. This tendency becomes amplified once those mice turn into mothers. Males, in contrast, exhibit aggressive, territorial behavior and often ignoring strange pups or else violently attack them. However, males will become parental for a short period after mating with a female, starting around the birth of their pups.
The team employed optogenetic technology, in which neurons are activated by light, to precisely manipulate the activity of TH-containing neurons—literally, at the flip of a light switch. By elevating the TH levels or increasing the activity of these TH neurons, they were able to promote maternal actions. Further tests revealed that these manipulations enhanced blood levels of oxytocin—a hormone associated with lactation and female reproductive behavior in general, among other things. Decreasing the number of TH-containing neurons in females lowered their levels of oxytocin and severely impaired their maternal instincts.
When the scientists used optogenetics to activate TH-containing neurons in male mice, there was no effect on oxytocin levels or pup-caring. However, there was a significant drop in aggressive behaviors toward unfamiliar pups and adult males, both of which they would normally have attacked. Decreasing the number of TH-containing neurons, on the other hand, led to a profound increase in the males’ aggression toward both.
“By controlling the amount and activities of these unique neurons, we were able to manipulate the maternal behavior of the females and the aggression of the males, says Dr. Kimchi. “Our results hint that maternal behavior arises from neuronal networks that are largely hard-wired. These are different from those of males, and they are at least partly regulated by the hormone oxytocin.”
These findings may, in the future, provide insight into the ways that male and female brains function when it comes to such conventional gender-related activities as tending to infants, and other innate reproductive and social behaviors. Many psychiatric disorders have unique gender differences. These include postpartum depression and autism spectrum disorders. Dr. Kimchi hopes that this discovery may ultimately advance our understanding of the biological factors that contribute to these disorders, which have a huge social impact.
Dr. Tali Kimchi’s research is supported by the Adelis Foundation; the Joan and Jonathan Birnbach Family Laboratory Fund; the Abisch Frenkel Foundation for the Promotion of Life Sciences; the Peter and Patricia Gruber Awards; Mike and Valeria Rosenbloom through the Mike Rosenbloom Foundation; the Hymen T. Milgrom Trust donation fund; and the Ethel and Harry Reckson Foundation. Dr. Kimchi is the incumbent of the Jenna and Julia Birnbach Family Career Development Chair.
Dr. Ofer Yizhar’s research is supported by the Grodetsky Center for Higher Brain Functions; the Henry Chanoch Krenter Institute for Biomedical Imaging and Genomics; the Clore Center for Biological Physics; the Adelis Foundation; the Carolito Stiftung; the Iby and Aladar Fleischman Foundation; the Candice Appleton Family Trust; the Minna-James-Heineman Stiftung; the Irving B. Harris Fund for New Directions in Brain Research; the Joseph D. Shane Fund for Neurosciences; the Corinne S. Koshland Equipment Endowment Fund; the European Research Council; the Lord Sieff of Brimpton Memorial Fund; the Irving Bieber, M.D. and Toby Bieber, M.D. Memorial Research Fund; and Jean-Charles Schwartz and Marc-Antoine Schwartz. Dr. Yizhar is the incumbent of the Gertrude and Philip Nollman Career Development Chair.