Serotonin promotes feminization of the sexually dimorphic nucleus of the preoptic area, but not the calbindin cell group

Testosterone and its metabolites masculinize the brain during a critical perinatal window, including the relative volume of sexually dimorphic brain areas such as the sexually dimorphic nucleus of the preoptic area (SDN), which is larger in males than females. Serotonin (5HT) may mediate this hormone action, since 5HT given during the second week of life decreases (i.e., feminizes) SDN volume in males and testosterone-treated females. Although previous work indicates that the 5HT2A/2C receptor is sufficient to induce feminization, it is unclear whether other serotonin receptors are required and which subpopulation(s) of SDN cells are specifically organized by 5HT. Therefore, we injected male and female Sprague-Dawley rat pups with saline, a nonselective 5HTR agonist, a 5HT2A/2C agonist, or a 5HT2A/2C antagonist over several timecourses in early life, and measured the Nissl-SDN as well as a calbindin+ subdivision of the SDN, the CALB-SDN. When examined on postnatal day 18 or early adulthood, the size of the Nissl-SDN was feminized in males treated with any of the serotonergic drugs, eliminating the typical sex difference. In contrast, the sex difference in CALB-SDN size was maintained regardless of serotoninergic drug treatment. This pattern suggests that although gonadal hormones shape the whole SDN, individual cellular phenotypes respond to different intermediary signals to become sexually dimorphic. Specifically, 5HT mediates sexual differentiation of non-calbindin population(s) within the SDN. The results also caution against using measurement of the CALB-SDN in isolation, as the absence of an effect on the CALB-SDN does not preclude an effect on the overall nucleus. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1241-1253, 2016.