Natural fetal testosterone variation is associated with preoptic development and partner preference in male mice

Individual variation in sexually differentiated brain structures may contribute to variation in sex-related behavior, but the developmental origins of such individual differences remain unclear. We first examined several sexually dimorphic nuclei in the preoptic area and bed nucleus of the stria terminalis to identify brain structures associated with individual variation in partner preference in adult mice. In males, female-directed partner preference was consistently associated with the characteristics of the sexually dimorphic nucleus of the preoptic area (SDN-POA), including its volume, number of calbindin-D28K (Calb)-positive neurons, and neuronal activity. Therefore, we examined whether natural variation in fetal testosterone was associated with development of this nucleus. At embryonic day 18.5, higher fetal testosterone was associated with increased expression of genes related to GABAergic neuronal differentiation and phenotype in hypothalamic tissue and with altered spatial organization of developing GABAergic neurons around the presumptive SDN-POA. A longitudinal follow-up showed that males exposed to higher fetal testosterone concentrations during the late fetal period subsequently developed a larger SDN-POA and stronger female-directed partner preference. They also showed more frequent tail rattling toward unfamiliar males and initiated sexual behavior more readily toward receptive females. The difference in partner preference disappeared after orchiectomy and re-emerged following testosterone replacement despite comparable circulating testosterone concentrations between the fetal testosterone groups. These findings suggest that natural variation in fetal testosterone during brain sexual differentiation contributes to individual differences in SDN-POA development and the expression of adult male social behaviors.