Estrogen configures sexual dimorphism in the preoptic area of C57BL/6J and ddN strains of mice.

Immunohistochemistry using a calbindin D28k antibody revealed a marked sex difference in neuronal distribution in the central portion of the medial preoptic area in C57BL/6J and ddN strains of mice when the animals were sacrificed on D65 (D1 = the day of birth). Male mice had a distinct ellipsoidal cell aggregate, whereas females lacked such a structure. This sex difference was not observed in Nissl-stained sections. Co-localization of calbindin D28k and the neuron-specific nuclear protein NeuN confrmed that the cells in the aggregate were neurons. The aggregates were larger in males than in females in both strains. When observed on D65, males orchidectomized on D1 had smaller aggregates. However, daily injections of 2 microg estradiol benzoate through D1-D5 as well as a single injection of 100 microg testosterone propionate on D1 enlarged the aggregates in females, but a single injection of 100 microg dihydrotestosterone on D1 had no effect on the female phenotype. Similar endocrine manipulations had no effects in adult animals of both sexes. Thus, the calbindin-immunoreactive cell aggregates in the preoptic area of C57BL/6J and ddN mice are homologous to the sexually dimorphic nucleus of the rat preoptic area in terms of the morphology and sex steroid-dependent organization.