Central regulation of luteinizing hormone-releasing hormone release by nitric oxide in female rsts.

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Nitric oxide (NO), a novel diffusible intercellular or intracellular messenger, is known as one of the factors regulating LHRH release in the hypothalamus. Recent study demonstrated that estrogen modulates excitatory amino acid-induced in vitro LHRH release from the arcuate nucleus-median eminence (ARC-ME) fragment in rats. The present study, therefore, was designed to test whether estrogen modulates glutamate-induced in vitro LHRH release by altering NO action and where the action site of NO is. The role of cyclic GMP in mediating NO action was also examined.ARC-ME or ME fragments taken from ovariectomized (OVX) or OVX and estradiol-primed (OVX+E2) rats were incubated in the medium containing glutamate with or without NG-monomethyl-L-arginine (NMMA), a NO synthase inhibitor, or hemoglobin (Hb), a NO scavenger, or in the medium containing sodium nitroprussid (SNP), a NO donor, with or without Hb for 3 min. At the end of the incubation, tissue fragments were homogenized for measurement … More of cGMP contents. In vitro LHRH release from ME fragments taken from both OVX and OVX+E2 rats was significantly increased by incubation with a combination of glutamate and NMMA compared to glutamate treatment alone without changing cGMP contents. NO also seems to have inhibitory effect on glutamate-induced LHRH release in ARC-ME fragments taken from OVX rats as well as in ME fragments. It was further revealed that glutamate increased cGMP level in the ARC-ME fragment taken from OVX rats and the glutamate-induced cGMP increase was blocked by NMMA or Hb. On the other hand, glutamate-induced in vitro LHRH release from ARC-ME fragments taken from OVX+E2 rats was completely suppressed by NMMA or Hb not through the cGMP production. These results clearly demonstrated that estrogen modulates glutamate-induced in vitro LHRH release from the ARC-ME fragment by altering NO tone which mediates the effects of glutamate. On the other hand, NO plays an inhibitory role in mediating glutamate-induced vitro LHRH release from the ME fragment. These results also suggest that NO mediates the action of glutamate on LHRH release in the ARC-ME or ME region in an estrogen-dependent manner, and has a dual role in mediation glutamate-induced LHRH release : one is stimulatory and the other is inhibitory. In addition, NO itself seems to be directly involved in inducing LHRH release from both ARC-ME and ME fragments in the presence of estrogen but not in the absence of estrogen not through cGMP production, since SNP-induced vitro LHRH release from both ARC-ME and ME fragments taken from OVX+E2 rats was block by Hb without changing cGMP contents. In conclusion, the present study suggests that NO mediates the effects of gllutamate on LHRH release at the ARC-ME opr ME region and estrogen alters the effect of NO on LHRH release. NO may, therefore, play an important role in the feedback mechanism of estrogen in the ARC by swithing the inhibitory effect of estrogen to the stimulatory one. Less