Do gonadotropin-releasing hormone, tyrosine hydroxylase-, and beta-endorphin-immunoreactive neurons contain estrogen receptors? A double-label immunocytochemical study in the Suffolk ewe.

We used double label immunocytochemistry to examine the brains of ovariectomized ewes and determine whether GnRH, tyrosine hydroxylase-(TH), and beta-endorphin-immunoreactive (IR) neurons contain IR-estrogen receptors (ER). Because of their possible importance as a target for the feedback actions of estradiol, we also examined the presence of nuclear ER in LH-IR cells of the pars tuberalis of the pituitary. Although preoptic GnRH neurons were frequently in close proximity to ER-IR cells, only one out of approximately 1000 GnRH cells examined was found to coexpress ER. In contrast, in the arcuate nucleus and vicinity, 3-5% of TH cells and 15-20% of beta-endorphin cells contained ER. Virtually all LH-IR cells, seen predominantly in the ventral portion of the pars tuberalis, coexpressed ER. These results suggest that in sheep as in rodents, the influence of estradiol on the reproductive neuroendocrine system is not directly mediated by GnRH neurons, but instead is conveyed to GnRH cells via presynaptic afferents. Subsets of TH- and beta-endorphin-IR cells which coexpress ER are two candidates for relaying gonadal steroid signals to GnRH cells. At the level of the pituitary, the feedback actions of estradiol may be expressed directly upon the gonadotroph.