A SUBSET OF ESTROGEN RECEPTOR-CONTAINING NEURONS PROJECT TO THE MEDIAN-EMINENCE IN THE EWE

The neural pathways responsible for conveying the steroid feedback sig nals that ultimately affect reproductive neuroendocrine function remai n largely undefined, One possibility involves a direct projection from estrogen receptor (ER)-containing neurons to the median eminence (ME) , a site of neuroendocrine peptide release. To examine this possibilit y, 8 ewes received stereotaxic injections of the retrograde neuronal t ract-trading compound cholera toxin-beta subunit (CT beta) into the ME . Neurons sending projections to the ME and containing ER were identif ied using a dual-label immunoperoxidase method. Double-labeled cells w ere found in distinct regions: (1) the ER-rich arcuate nucleus (ARC) t hat contained the greatest number of double-labeled cells, and (2) the organum vasculosum of the lamina terminalis (OVLT) which contained a very consistent, but low, number of double-labeled cells, While a fair ly large number of retrogradely-labeled ARC neurons containing ER were identified, the majority of ER-containing ARC neurons were unlabeled and thus send projections elsewhere, Other regions containing high con centrations of ER-positive cells such as the medial preoptic area (MPO A), anterior hypothalamic area, and ventrolateral portion of the ventr omedial hypothalamic nucleus, were devoid of double-labeled cells. Sim ilarly, regions rich in neuroendocrine neurons such as the periventric ular hypothalamus and paraventricular and supraoptic hypothalamic nucl ei contained no double-labeled cells, These results suggest that modul ation of neuroendocrine secretory activity may occur directly at the l evel of the ME by ER-containing neurons located within restricted regi ons of the hypothalamus and forebrain, However, the relatively low pro portion of ER-containing neurons projecting to the ME suggests that th e influence of estradiol upon neuroendocrine function also may include target sites other than the ME.