Abstract
Neural substances mediating the negative feedback effects of estrogen on LHRH release in the stalk-median eminence (S-ME) were studied using the in vivo push-pull perfusion method in female rhesus monkeys. Twelve ovariectomized animals were implanted with Silastic capsules containing 17β-estradiol two weeks prior to the experiments and on the day of the experiment estradiol benzoate (EB, 50 μg/kg, sc) or oil was injected. Perfusate samples from the S-ME were collected in 10-min fractions from 4 h before to 18-20 h after EB or oil. LHRH and neuropeptide Y (NPY) levels in the same perfusates were measured by RIA, and glutamate and γ-amino-butyric acid (GABA), also in the same perfusates, were assessed by HPLC. The results indicate that EB suppressed LHRH release (p<0.005) starting within 2 h after EB, and continued until the end of the experiment. Pulse analysis suggested that estrogen suppressed the pulse amplitude, but not pulse frequency, of LHRH release. In contrast, EB did not alter any parameters (mean release, pulse amplitude or frequency) of pulsatile NPY release throughout the experiment. HPLC analysis further suggested that neither glutamate nor GABA levels in the S-ME were changed with estrogen-induced LHRH suppression. Oil treatment did not alter LHRH, NPY, GABA or glutamate levels. In conclusion, estrogen induces suppression of pulsatile LHRH release within 2 h, but negative feedback effect of estrogen does not appear to be mediated by NPY, GABAergic, or glutamatergic neurons, at least, at the level of S-ME. [Jpn J Physiol 55 Suppl:S211 (2005)]
