INDUCTION OF GALANIN GENE-EXPRESSION IN GONADOTROPIN-RELEASING-HORMONE NEURONS WITH PUBERTY IN THE RAT

The onset of puberty reflects the developmental activation of GnRH neu rons whose secretory activity awakens the reproductive axis; however, the cellular mechanisms involved in this activational process remain p oorly understood. GnRH neurons coexpress the neuropeptide galanin, and we have previously shown that galanin's level;of coexpression is link ed to the activity state of GnRH neurons. We theorized that altered ex pression of galanin by GnRH neurons may be an important mechanism rela ted to activation of GnRH neurons at puberty. We examined two hypothes es related to this idea. First, we tested the hypothesis that expressi on of galanin messenger RNA (mRNA) in GnRH neurons is induced across t he transition from prepubertal to adult life in the rat. To accomplish this, we used double label in situ hybridization and image analysis t o compare cellular levels of galanin mRNA in GnRH neurons between grou ps of prepubertal and adult male and female rats. Levels of galanin mR NA within GnRH neurons increased significantly across puberty in both sexes. In females, galanin mRNA signal in GnRH neurons increased appro ximately 8-fold, whereas in males, cellular galanin mRNA signal levels increased about 8-fold. The number of identifiable GnRH neurons was n ot significantly different among the experimental groups. Next, we exa mined the hypothesis that pubertal induction of galanin mRNA in GnRH n eurons reflects the activational effects of gonadal hormones associate d with the onset of puberty. To test this, we killed groups of prepube rtal male and female rats together with adult male and female animals that had been either castrated or sham castrated at a prepubertal age. In animals that had been prepubertally castrated, no developmental in crease in galanin mRNA in GnRH neurons was observed, whereas in sham c astrated animals, levels of galanin mRNA in GnRH neurons were again sh own to be higher in adult compared to prepubertal animals of both sexe s, as had been demonstrated in the first experiment. We conclude that galanin message expression in GnRH neurons is induced during the trans ition from the juvenile to the adult state through a gonad-dependent p rocess. This developmental increase in galanin gene expression is one mechanism by which the capacity for the synthesis and secretion of gal anin by GnRH neurons may be enhanced, which, in turn, could facilitate the functional activity of GnRH neurons and amplify their trophic eff ect on the pituitary.