Induction of galanin gene expression in gonadotropin-releasing hormone neurons with puberty in the rat

Abstract

The onset of puberty reflects the developmental activation of GnRH neurons whose secretory activity awakens the reproductive axis; however, the cellular mechanisms involved in this activational process remain poorly understood. GnRH neurons coexpress the neuropeptide galanin, and we have previously shown that galanin's level of coexpression is linked to the activity state of GnRH neurons. We theorized that altered expression of galanin by GnRH neurons may be an important mechanism related to activation of GnRH neurons at puberty. We examined two hypotheses related to this idea. First, we tested the hypothesis that expression of galanin messenger RNA (mRNA) in GnRH neurons is induced across the transition from prepubertal to adult life in the rat. To accomplish this, we used double label in situ hybridization and image analysis to compare cellular levels of galanin mRNA in GnRH neurons between groups of prepubertal and adult male and female rats. Levels of galanin mRNA within GnRH neurons increased significantly across puberty in both sexes. In females, galanin mRNA signal in GnRH neurons increased approximately 8-fold, whereas in males, cellular galanin mRNA signal levels increased about 2-fold. The number of identifiable GnRH neurons was not significantly different among the experimental groups. Next, we examined the hypothesis that pubertal induction of galanin mRNA in GnRH neurons reflects the activational effects of gonadal hormones associated with the onset of puberty. To test this, we killed groups of prepubertal 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 increase in galanin mRNA in GnRH neurons was observed, whereas in sham-castrated animals, levels of galanin mRNA in GnRH neurons were again shown to be higher in adult compared to prepubertal animals of both sexes, as had been demonstrated in the first experiment. We conclude that galanin message expression in GnRH neurons is induced during the transition from the juvenile to the adult state through a gonad-dependent process. This developmental increase in galanin gene expression is one mechanism by which the capacity for the synthesis and secretion of galanin by GnRH neurons may be enhanced, which, in turn, could facilitate the functional activity of GnRH neurons and amplify their trophic effect on the pituitary.