REGULATION OF HIPPOCAMPAL GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTOR MESSENGER-RNA AND GNRH-STIMULATED INOSITOL PHOSPHATE PRODUCTION BYGONADAL-STEROID HORMONES

The purpose of the present study was to determine if gonadal steroids can alter the amounts of GnRH receptor mRNA in the pyramidal and granu le neurons of the hippocampus of female and male rats and if GnRH caus es a change in the production of inositol phosphates in hippocampal sl ices in vitro. The results show that in the ovariectomized rat the amo unt of GnRH receptor mRNA is increased to 137% in area CA(1) and to 14 7% in area CA(3) and in the dentate gyrus when compared to the ovariec tomized, estradiol-progesterone treated animal. Similarly, in the orch idectomized male rat the amount of GnRH receptor mRNA is increased to 155% in area CA(1), to 146% in area CA(3) and to 145% in the dentate g yrus when compared to the intact male rat. There was no significant di fference in the relative amounts of GnRH receptor mRNA when gonadectom ized male and female rats were compared. Addition of GnRH (100 pM-1 mu M) to hippocampal slices in vitro caused a dose-dependent increase in the production of [H-3]inositol phosphate which was abolished by co-a dministration of a GnRH antagonist. The increase in inositol phosphate production was significantly higher at low doses of GnRH (100 pM-1 nM ) in estradiol-progesterone treated female and in intact male rats whe n compared to gonadectomized rats. The results suggest that the amount of GnRH receptor mRNA in the hippocampus is at least in part regulate d by gonadal steroids and that the steroid hormones can sensitize the GnRH target neurons to respond more robustly to a GnRH stimulus.