Regulation of Pro-Gonadotropin-Releasing Hormone Gene Expression by Sex Steroids in The Brain of Male and Female Rats

The fine modulation of gonadotropin gene expression and secretion is well recognized to be regulated by sex steroids through their direct action both at the anterior pituitary level and on the pulsatile pattern of GnRH secretion at the hypothalamic level. Since the influence of sex steroids on hypothalamic GnRH mRNA levels remains to be elucidated, quantitative in situ hybridization was used to study the effect of sex steroids on cellular levels of pro-GnRH mRNA in adult rats of both sexes. The effects of 14-day gonadectomy as well as administration of 17β-estradiol (E2, 0.25 µg) or dihydrotestosterone (DHT, 100 μg) twice a day during 14 days to gonadectomized animals were evaluated. In addition, the effect of progesterone (P, 2 mg, twice daily) alone or in the presence of E2 was also studied in ovariectomized animals. Hybridization was performed using a 35Slabeled cDNA probe encoding rat pro-GnRH and the corresponding mRNA levels were assessed by counting the number of silver grains overlying labeled neurons. In male rats, castration induced a highly significant 65% increase (compared to intact rats) in the mean number of grains per neuron. Administration of E2 or DHT to castrated animals completely prevented the post castration rise in pro-GnRH mRNA levels. In female animals, the effect of ovariectomy was less striking than in the male, a 25% increase (P < 0.001) being observed. Treatment with E2 or DHT also completely prevented the increase in pro-GnRH mRNA levels induced by ovariectomy. Moreover, treatment with P in ovariectomized animals markedly potentiated the inhibitory effect of E2 on pro-GnRH mRNA levels. The present data clearly demonstrate the inhibitory effect of androgens and estrogens on pro-GnRH mRNA levels in gonadectomized male and female rats as well as the marked inhibitory effect of P in the presence of E2 on pro-GnRH mRNA levels in ovariectomized animals. The present study thus suggests that variationsin gonadotropin gene expression and secretioninduced by gonadectomy and sex steroid administration might be related, at least to some extent, to parallel changes in the amount of neuronal GnRH mRNA in a restricted population of GnRH-containing neurons.