A male‐dominant cell group expressing calbindin‐ <scp>D28K</scp> and androgen receptor in the mouse preoptic area requires postnatal testicular androgens and histone deacetylation

The mouse brain is masculinized by postnatal testicular androgens, which are active after conversion to estrogens and modulate gene expression epigenetically, at least in part. The preoptic area contains a sexually dimorphic nucleus (SDN) comprising calbindin D-28K (Calb) neurons with a male-biased sex difference in cell number (Calb-SDN), although the mechanisms responsible for the sex difference are not fully understood. We have previously demonstrated that Calb neurons expressing the androgen receptor (AR) are a male-dominant cell group of the Calb-SDN in pubertal mice, while Calb neurons without AR exist in both sexes with equal cell numbers. In this study, we investigated the mechanisms by which more Calb/AR neurons emerge in the male Calb-SDN than in the female one. Immunohistochemistry for Calb and AR was performed using the brain sections from pubertal male mice subjected to sham surgery or neonatal orchidectomy, from pubertal female mice treated with vehicle, testosterone, or estradiol during the postnatal period, and from pubertal male mice whose brains were treated with trichostatin A, a histone deacetylase inhibitor, during the postnatal period. Immunostained brain sections were analyzed stereologically to determine the numbers of Calb-immunopositive and AR-immunopositive cells (Calb⁺/AR⁺ cells) and Calb-immunopositive and AR-immunonegative cells (Calb⁺/AR^- cells) in the Calb-SDN. The number of Calb⁺/AR⁺ cells in the Calb-SDN during the pubertal period was significantly decreased in neonatally orchidectomized males compared with sham males and increased in testosterone- or estradiol-treated females compared with vehicle-treated females; however, the number of Calb⁺/AR^- cells remained unchanged. Trichostatin A treatment significantly reduced the number of Calb⁺/AR⁺ cells, but not the number of Calb⁺/AR^- cells, in the Calb-SDN of males. These findings suggest that estrogens synthesized from postnatal testicular androgens act selectively on the AR-expressing subpopulation of Calb neurons, contributing to the sex difference in the number of Calb neurons in the mouse Calb-SDN. Epigenetic regulation of gene expression, possibly mediated by histone deacetylation, may be involved in the emergence of the AR-expressing subpopulation of Calb neurons.