SEXUAL-DIFFERENTIATION OF GALANIN GENE-EXPRESSION IN GONADOTROPIN-RELEASING-HORMONE NEURONS

In adult rats, only females are capable of generating an LH surge in r esponse to high levels of ovarian steroids. This is because the abilit y to generate LH surges in adulthood in response to ovarian steroids i s suppressed by high levels of testosterone during neonatal life. We h ave previously shown that galanin gene expression in GnRH neurons is i nduced by ovarian steroids in association with an LH surge, suggesting that galanin plays a role in the generation of the LH surge. If this is the case, then the induction of galanin gene expression in GnRH neu rons in response to ovarian steroids might, like the LH surge itself, be sexually differentiated during the neonatal period. To test this hy pothesis, we manipulated gonadal steroid levels in four groups of rat pups on the first day of life and looked at the ability of ovarian ste roids to induce an LH surge and stimulate galanin gene expression in G nRH neurons of adult animals. The four groups consisted of female pups that were injected with either 10 mu g of testesterone propionate (n = 6; testosterone-treated females) or vehicle (n = 6; control females) and male pups that were either castrated (n = 6; castrated males) or sham-castrated (n = 6; sham-castrated males). All animals were then go nadectomized at 59 or 60 days of age, except for the males that had be en castrated neonatally. Seven weeks later, the animals were challenge d with estradiol (E(2)) benzoate and progesterone (P) in a paradigm th at reliably produces an LH surge. (The rats were injected sc with 1530 mu g of E(2) on day 0 at 1030 h and 5 mg of P on day 2 at 1200 h.) Th e animals were then killed between 1800 h and 1840 h on day 2 at the t ime ofthe expected LH surge, and their brains were processed for doubl e-label in situ hybridization to estimate cellular levels of galanin m essenger RNA (mRNA) in GnRH neurons. We reconfirmed the classical obse rvation that the ability to generate an LH surge in response to E(2)/P is suppressed by neonatal exposure to testosterone. Galanin mRNA leve ls in GnRH neurons showed a similar pattern: after E(2)/P priming, gal anin mRNA levels in GnRH neurons were significantly higher in normal f emales and in males castrated neonatally than in normal males and in f emales treated with testosterone neonatally. These results demonstrate that the E(2)/P induction of galanin gene expression in GnRH neurons at the time of the LH surge is sexually differentiated during the neon atal period and suggest that the increase in galanin gene expression i n GnRH neurons could be one of the mechanisms underlying the sex diffe rence in gonadotropin release seen in rats.