Absence of estrogen receptor immunoreactivity in somatostatin (SRIF) neurons of the periventricular nucleus but sexually dimorphic colocalization of estrogen receptor and SRIF immunoreactivities in neurons of the bed nucleus of the stria terminalis.

The mechanisms by which somatostatin (SRIF) neurons in the periventricular nucleus and bed nucleus of the stria terminalis (BNST) are differentially regulated by gonadal steroids are unknown. Studies have shown an overlap in the distribution of estrogen receptor and SRIF immunoreactivities in these areas, giving rise to the possibility that SRIF neurons are regulated directly by gonadal steroids. In this study we have used double labeling immunocytochemistry to address the question of whether SRIF neurons in the periventricular region and BNST possess estrogen receptors in male and female rats. Short-term (2- to 4-day) gonadectomized rats with or without colchicine pretreatment for 16-18 h were processed for immunocytochemical staining using monoclonal estrogen receptor (H222) and polyclonal SRIF antisera. Single staining for SRIF and the estrogen receptor in rostral hypothalamic areas showed that the only sites of significant overlap between the two immunoreactivities were in the dorsal half of the periventricular region and posterior BNST. No sex differences were detected in the numbers of SRIF- or estrogen receptor-immunoreactive cells in the dorsal periventricular region. Sequential double-staining techniques demonstrated that periventricular SRIF neurons do not possess estrogen receptor immunoreactivity in either the male or female rat. However, double-labeled cells were readily detected in the posterior division of the BNST in male rats treated with colchicine. No colocalization was detected in the BNST of the female. These results show that the periventricular SRIF neurons controlling GH secretion do not possess estrogen receptor immunoreactivity in either sex and suggest that effects of testosterone and estradiol on SRIF messenger RNA expression in these cells through the estrogen receptor are indirect. In contrast, we provide evidence of a substantial sex difference in the localization of estrogen receptors within SRIF neurons of the BNST.