EFFECTS OF OVARIECTOMY ON GNRH NEURONAL MORPHOLOGY IN RHESUS-MONKEY (MACACA-MULATTA)

Gonadotropin releasing hormone (GnRH) neurons are typically simple, fu siform cells; however, over the course of prepubertal development incr easing numbers take on a 'spiny' appearance. Following gonadectomy the re is a decrease in the frequency of these spiny GnRH neurons, These o bservations which were made in the rat suggest that GnRH neurons are d irectly affected by the gonadal steroid milieu, though they do not the mselves contain receptors for these steroidal hormones. In that there are important species differences in the hypothalamic-pituitary-gonada l axis between rats and primates, the present study was undertaken to determine whether a reduction in ovarian hormones would produce simila r changes in the morphology of GnRH neurons in the monkey, A further a im was to determine whether such changes were localized to a specific brain region. Immunocytochemically defined GnRH neurons were compared in adult rhesus macaques which had been ovariectomized for 6 weeks to 2 years (n=7) and intact, cycling animals (n=8). Within the intact gro up, there were significantly more spiny GnRH neurons in the medial bas al hypothalamus (MBH) than in the preoptic area (POA) (about 50% of th e total in the MBH compared to 33% in the POA). Following ovariectomy the frequency of spiny cells in the MBH dropped to less than 30%, but was not significantly reduced in the POA, These results suggest that c hanges in systemic gonadal steroid levels result in changes in the mor phology of GnRH neurons preferentially in the MBH, a region that is co nsidered critical in the generation of GnRH pulsatile release in the m onkey.