EVIDENCE IN SUPPORT OF NITRIC-OXIDE (NO) INVOLVEMENT IN THE CYCLIC RELEASE OF PROLACTIN AND LH SURGES

Studies were undertaken to determine whether nitric oxide (NO) is invo lved in induction of the prolactin surge on proestrus and in that indu ced by ovarian steroids in ovariectomized (ovx) rats, by using inhibit ors of NO synthase, the enzyme that generates NO. Two week-ovariectomi zed rats were treated either with estradiol benzoate (EB, 30 mu g/rat, s.c.) alone, or with EB and 2 days later with progesterone (P, 2 mg/r at, s.c.) to evoke a prolactin surge in the afternoon. Injections of t he NO synthase inhibitor NW-nitro-L-arginine methyl ester (40 mg/kg, s .c.) at 10.00, 12.00, and 14.00 h completely suppressed the steroid-in duced prolactin surge in the afternoon. Similarly, another NO synthase inhibitor, N-W-nitro-L-arginine (100 mg/kg, s.c.), injected at 1000, 1200, and 1400 h on proestrus suppressed prolactin and luteinizing hor mone (LH) surges, but failed to alter the daily increase of corticoste rone. These studies confirm our earlier report and show that NO is inv olved in the complex processes that initiate the afternoon prolactin a nd LH surges of proestrus.