Although hypothalamic GnRH release is known to be modulated by neural and hormonal factors, the relationship between altered GnRH secretion and GnRH synthesis remains unclear. In an attempt to address this question, we examined GnRH gene expression in the rat hypothalamus using in situ hybridization histochemistry. An25S-labeled antisense RNA probe was used to identify neurons expressing GnRH mRNA in an area that included the diagonal band of Broca, the organum vasculosum of the lamina terminalis, and the preoptic area. The number of GnRH mRNA-expressing cells was determined at various times during the rat estrous cycle. During proestrus, the number of GnRH mRNA-expressing cells decreased somewhat at 1400-1600 h, increased significantly at 1800 h (the time of the LH surge), then gradually returned to basal levels at 2200 h. Expression did not change substantially at other times during the estrous cycle. To understand this close temporal relationship between the LH surge and increased GnRH mRNA levels, we examined GnRH gene expression in proestrous animals in which the LH surge was blocked with pentobarbital. Pentobarbital treatment blocked the increase in the number of GnRH mRNA- expressing cells normally observed at 1800 h in saline-treated controls, suggesting that the increase in GnRH gene expression is closely coupled to secretion of GnRH from the hypothalamus. Finally, we addressed the question of whether ovarian steroids have direct effects on GnRH gene expression by examining GnRH mRNA levels in ovariectomized steroid-treated rats at a time before (1100 h) and a time after (1800 h) hypothalamic GnRH hypersecretion. At 1100 h, no significant changes were observed, but at 1800 h, estrogen-treated rats showed a significant increase in both the number of GnRH mRNA-expressing cells and serum LH levels. This suggests that estrogen influences GnRH gene expression indirectly, perhaps by altering hypothalamic GnRH release. Our results in each of these models suggest that GnRH mRNA levels increase in response to GnRH hypersecretion at the time of the LH surge.
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