Abstract
Although estrogen can induce LH hypersecretion in ovariectomized (ovx) rats, the magnitude and duration of the LH surge are invariably far less than those of the preovulatory LH surge in cycling rats. Recently, the crucial roles of hypothalamic nitric oxide (NO) and neuropeptide Y (NPY) in the induction of LHRH and the LH surge have been recognized. In this study, we have tested the hypothesis that low grade stimulatory feedback effects of 17 beta-estradiol (E2) on LH secretion in ovx rats is due to a deficit in NO signaling. As the NO substrate, L-arginine (L-Arg), can enhance NO-dependent physiological responses, the effects of L-Arg on the LH surge in E2-primed ovx rats was studied. Ovx rats bearing permanent cannulas in the lateral cerebroventricle received 15-mm long SILASTIC capsules filled with E2 (300 micrograms/ml oil, s.c.) at 1000 h. Two days later, saline (SAL) or L-Arg (0.1 or 1 nmol in 5 microliters SAL) or D-Arg (1.0 nmol) was injected intracerebroventricularly every hour from 1100-1400. Blood samples were collected at 1100 h and at hourly intervals from 1400-1800 h via intraatrial cannulae implanted the day before. Whereas an expected moderate LH surge occurred in SAL-injected control rats, L-Arg, but not D-Arg,...Continue Reading
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