Neuropeptide Y System of the Female Monkey Hypothalamus: Retrograde Tracing and Immunostaining

Neuropeptide Y(NPY) stimulates the release of hypothalamic gonadotropin-releasing hormone (GnRH) as well as pituitary gonadotropins in the presence of ovarian steroids, but inhibits release in their absence. In pπmates, however, the effects of NPY depend largely upon the site and method of administration. In ovariectomized monkeys, NPY infusion into the stalk-median eminence reportedly causes a dose-response increase in GnRH secretion in the absence of gonadal steroids. To help elucidate these findings, we investigated the NPY system and its neuroendocrine (NEU) component in the primate brain by retrograde tracing and immunostaining. One adult female and 1 juvenile female cynomolgus monkey were given microinjections of retrograde tracer into the median eminence (ME). Two weeks later, they were perfused with fixative, and series of 40-µm frontal vibratome sections were collected at 500-µm intervals through 4 mm of the forebrain. Injection sites were not visible in the juvenile female monkey ME, so this animal served as a neurosurgical and injection control. Sections were immunostained using a polyclonal NPY antiserum and the peroxidase antiperoxidase (PAP) technique. NPY immunostaining in another adult female cynomolgus monkey and in a late fetal female and a neonatally castrated adult male rhesus monkey gave essentially similar results. NPY-immunoreactive (NPY-IR) neurons were widely distributed throughout the caudate nucleus, but appeared concentrated within specific hypothalamic areas. Their number, as well as the number of NEU neurons, was nearly equal in bilaterally paired areas and on both sides of the hypothalamus overall. Ratios of retrogradely labeled NPY-IR neurons to the number of NPY-IR somata were expressed as percentages of NEU NPY-IR neurons for each side and in each area. These averaged 65% in the supraoptic nucleus (SON), 41% in the paraventricular nucleus (PVN), 32% in the medial preoptic area (MPOA), which has only one quarter of their number of NPY-IR cells, and 11% in the medial basal hypothalamus (MBH). NPY-IR fiber densities were highest in the area olfactoria, medial septal and ventromedial nuclei. They were high in the tuberculum olfactorium, lateral septum, nucleus accumbens, MPOA, PVN, dorsomedial nucleus and regions of the MBH including the arcuate nucleus, tuber cinereum and ventral hypothalamic tract (VHT). NPY fiber densities were moderate in the vertical portion of the diagonal band of Broca, the ventral part of the caudate nucleus, the anterior commissural nucleus and the lateral preoptic area, as well as the anterior and lateral hypothalamic areas, the anterior ventral periventricular area, the suprachiasmatic nucleus and the dorsolateral SON. Lastly, they were low in the nucleus and horizontal portion of the diagonal band of Broca, in the dorsal part of the caudate nucleus and in SON. In a series of sections double-immunostained for both NPY and GnRH, we failed to find evidence of contacts or interactions between NPY- and GnRH-IR cell bodies and processes in MBH, MPOA or PVN by either light or electron microscopy. Our results indicate that NPY-IR neurons in SON, PVN, MBH and to a lesser extent, in MPOA project to the ME and may perform direct hypophysiotropic functions. However, almost two thirds of all NPY-IR neurons do not project to the ME and may participate in other, neuromodulatory roles. Together, these NPY-IR neurons contribute to an extensive fiber network in the monkey hypothalamus. Although the NPY-IR system is partially coextensive with the GnRH-IR system, the locale and mechanism of their interactions could not be ascertained from our study. Whether NPY affects GnRH release directly or acts through other neuronal systems in primates remains to be determined.