Neuropeptide-Y Stimulation of Luteinizing Hormone- Releasing Hormone Secretion from the Median Eminencein Vitroby Estrogen-Dependent and Extracellular Ca2+- Independent Mechanisms*

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Abstract
The roles of estrogen and extracellular calcium (Ca2+) in neuropeptide-Y (NPY)-stimulated LHRH release from median eminence (ME) fragments in vitro were examined. Ovariectomized (OVX) rats received one or several sc implants of Silastic tubes containing estradiol benzoate (235 .mu.g/ml sesame oil) or vehicle. Plasma estrogen concentrations were similar to levels during the estrous cycle. These estrogen treatments were equally effective in reducing the elevated plasma levels of LH in vehicle-treated OVX rats. Animals were killed 3 days after implantation, and ME fragments were incubated in medium for 30 min (control), followed by a second 30-min period (test) in medium containing NPY or potassium chloride (K+). Estrogen treatment increased the basal release of LHRH and the ME concentration of LHRH in a dose-related fashion. NPY (0.1-10 .mu.M) increased LHRH secretion in a dose-related manner from ME fragments obtained from estrogen-treated OVX rats, but had no effect on MEs from hormonally untreated OVX rats. Treatment with higher doses of estrogen enhanced the LHRH secretory response of ME fragments to NPY (1-10 UmM). K+-stimulated LHRH release from ME fragments from estrogen-treated rats was completely eliminated in Ca2+-free medium containing EGTA. In contrast, LHRH release elicited by NPY (10 .mu.M) was unchanged Ca2+-free medium in both the absence and presence of cobalt chloride (Co2+). Decreasing the Ca2+ concentration from 2.5 to 0.25 mM reduced K+-stimulated LHRH release 7-fold, while NPY-stimulated LHRH secretion was not affected. These results indicate that NPY stimulation of LHRH release from the ME in vitro is related to prior circulating levels of estrogen, but does not require extracellular Ca2+ in the incubation medium. NPY may enhance LHRH release in an estrogen-dependent manner during the estrus cycle and before the LH surge on proestrous.

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