Possible involvement of preoptic estrogen receptor beta positive cells in luteinizing hormone surge in the rat

The anteroventral periventricular nucleus (AVPV) of the preoptic area has been implicated in the induction of spontaneous ovulation. In the AVPV, we found a striking sex difference in the distribution of estrogen receptor (ER) positive cells. In females, a significantly larger number of ER mRNA-positive cells were visualized than in males using in situ hybridization in the most medial part of the AVPV next to the ependymal lining of the third ventricle. In males, the labeled cells were dispersed into more lateral region. Immunohistochemistry revealed a similar sexual dimorphism in the ER protein. The dimorphism persisted from Day 7 to Day 60. Orchidectomy of male neonates or estrogen treatment of female pups had reversed the brain phenotype when examined on Day 14. No gross sex difference was detected in the pattern of ER expression in the medial preoptic nucleus and the bed nucleus of the stria terminals. Estrogen receptor immunoreactive cells co-localization in 83% of ER mRNA positive cells in the AVPV of adult females. Infusion of an ER antisense oligonucleotide into the third ventricle resulted in a significantly longer period of successive vaginal estrus and 50% reduction in the number of ER-immunoreactive cells in the AVPV. These findings suggest an important role of ER AVPV in the female-typical estrogen-dependent induction of the luteinizing hormone surge.