Feminization of social play behavior depends on microglia

Abstract

Many sex differences in brain and behavior are established developmentally by the opposing processes of feminization and masculinization, which manifest following differential steroid hormone exposure in early life. The cellular mechanisms underlying masculinization are well-documented, a result of the fact that it is steroid-mediated and can be easily induced in newborn female rodents via exogenous steroid treatment. However, the study of feminization of particular brain regions has largely been relegated to being "not masculinization" given the absence of an identified initiating trigger. As a result, the mechanisms of this key developmental process remain elusive. Here we describe a novel role for microglia, the brains innate immune cell, in the feminization of the medial amygdala and a complex social behavior, juvenile play. In the developing amygdala, microglia promote proliferation of astrocytes equally in both sexes, with no apparent effect on rates of cell division, but support cell survival selectively in females through the trophic actions of Tumor Necrosis Factor (TNF). We demonstrate that disrupting TNF signaling, either by depleting microglia or inhibiting the associated signaling pathways, prevents the feminization of astrocyte density and increases juvenile play levels to that seen in males. This data, combined with our previous finding that male-like patterns of astrocyte density are sculpted by developmental microglial phagocytosis, reveals that sexual differentiation of the medial amygdala involves opposing tensions between active masculinization and active feminization, both of which require microglia but are achieved via distinct processes. Significance StatementThe cellular mechanisms by which sex differences in the brain arise provide insight into the cellular basis of behavior. Most mechanistic studies have focused on the process whereby regions of the male brain are differentiated from the female in response to elevated gonadal steroid in development due to the tractability of inducing masculinization by blocking steroid action in males or providing exogenous steroids to newborn females. As such, feminization is usually defined as "not masculinized". Here, we demonstrate the active feminization of astrocyte density in a brain region modulating complex social behavior, rough-and-tumble play in juveniles. These findings indicate that lower levels of playfulness in females is an actively regulated process as opposed to simply being a lack of masculinization.