A sexually dimorphic neuronal cluster in the mouse medial amygdala responds to male sexual status

Increasing scientific interest has been directed toward understanding sexual dimorphism in the brain. Although several brain structures exhibit masculine or feminine characteristics, strictly binary anatomical feature, comparable to those observed in genitalia, has been rarely identified. In this study, we identified a dense, sexually dimorphic cluster of neurons in the posterodorsal medial amygdala (MeApd), which we named DIMPLE (Dimorphic IEGs Medial Posterodorsal amygdala Labeled Ensemble) that exhibited a remarkable binary pattern of c-fos promoter activation. Using the TRAP2 (Targeted Recombination in Active Populations) transgenic mouse model, we found that it was consistently labeled in all females, regardless of age or sexual experience. In contrast, DIMPLE labeling was absent in adult virgin males but present both prior to weaning and following mating. Surgical removal of gonads (ovariectomy or orchiectomy) did not alter the labeling pattern of DIMPLE in either sex. Interestingly, a single intraperitoneal injection of prolactin, a hormone that increases in males after mating, induced DIMPLE labeling in virgin males. However, treatment with cabergoline, a potent inhibitor of prolactin secretion, did not prevent DIMPLE labeling in females or in postmating males. Given the established role of the MeApd in social and reproductive behaviors, we hypothesize that DIMPLE may support neural mechanisms underlying female-typical behavior and potentially contribute to postmating behavioral shifts in males.