Galanin's functional significance in the regulation of the neuroendocrine reproductive axis of the monkey

Galanin stimulates the neuroendocrine reproductive axis in the rat, but whe ther galanin acts similarly in primate species is unknown. To test the hypo thesis that galanin acts within the hypothalamo-hypophyseal axis to stimula te luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) secre tion in the primate, galanin was administered either systemically or direct ly into the arcuate nucleus-median eminence of ovariectomized macaques (pig tailed or rhesus, respectively) that were maintained on estradiol. The mean plasma levels of LH were significantly elevated in pigtailed macaques afte r peripheral injection of galanin (2 mg) as compared with vehicle treatment . In rhesus monkeys, galanin (80 mu M) administered by push-pull perfusion into the arcuate nucleus-median eminence did not significantly alter either GnRH or LH release. To determine whether in the monkey, as in the rat, sub populations of medial forebrain GnRH neurons coexpress galanin mRNA, we use d single- and double-label in situ hybridization and computerized imaging t echniques. GnRH mRNA-containing cells were identified in both the medial an d lateral forebrain of the female pigtailed macaque. No galanin mRNA expres sion was detectable in GnRH neurons located in either the medial preoptic a rea or mediobasal hypothalamus; however, within the substantia innominata a subset of GnRH mRNA-expressing neurons did coexpress galanin mRNA. Taken t ogether, these results suggest that galanin induces LH release in primates, but galanin may not act directly on hypothalamic GnRH neurons. Presently, we have confirmed in another primate species the existence of GnRH gene exp ression in the lateral forebrain and discovered that a small subset of thes e neurons coexpress galanin. These particular cells may have a unique and a s of yet undefined physiological function that is distinct from those GnRH neurons serving a hypophysiotropic function. copyright (C) 2000 S. Karger A G, Basel.