Effects of ovariectomy on GnRH neuronal morphology in rhesus monkey (Macaca mulatta)

Gonadotropin releasing hormone (GnRH) neurons are typically simple, fusiform cells; however, over the course of prepubertal development increasing numbers take on a 'spiny' appearance. Following gonadectomy there is a decrease in the frequency of these spiny GnRH neurons. These observations which were made in the rat suggest that GnRH neurons are directly affected by the gonadal steroid milieu, though they do not themselves contain receptors for these steroidal hormones. In that there are important species differences in the hypothalamic-pituitary-gonadal axis between rats and primates, the present study was undertaken to determine whether a reduction in ovarian hormones would produce similar changes in the morphology of GnRH neurons in the monkey. A further aim 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 group, there were significantly more spiny GnRH neurons in the medial basal hypothalamus (MBH) than in the preoptic area (POA) (about 50% of the 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 changes in systemic gonadal steroid levels result in changes in the morphology of GnRH neurons preferentially in the MBH, a region that is considered critical in the generation of GnRH pulsatile release in the monkey.