8-OH-DPAT REGULATES THE AMPLITUDE AND THE PHASE OF LH SURGE IN OVARIECTOMIZED STEROID-PRIMED RATS

Precise interactions between ovarian steroids and neurotransmitters are required for the secretion of phasic LH surge. Previous data suggested the existence of an interactive stimulatory effect of progesterone (P) and serotonin (5-HT) on LH release. In the present work the effects of 8-OH-DPAT, a selective 5-HT1A agonist, on phasic LH secretion were tested in ovariectomized rats implanted for 6 days with a pellet of 17 beta estradiol (OVX-E(2)) and in OVX-E(2) treated with progesterone (OVX-E(2)-P). Intraperitoneal injection of 8-OH-DPAT at 11.00 h in the morning of the expected LH surge had no effect on circadian plasma levels of LH in OVX-E(2) rats, whereas it induced a phase advance and an increase in LH surge in OVX-E(2)-P rats. Administration of the antiprogestin RU 38486 in OVX-E(2)-P rat, totally abolished the combined effects of P and 8-OH-DPAT on phasic LH release. SDZ 216-525, a specific 5-HT1A antagonist administered 60 min before 8-OH-DPAT, inhibited the stimulatory effect of the 5-HT1A agonists on the amplitude of LH surge. The present data suggest that progesterone is required for the regulation of phasic LH release by 5-HT1A agonists and that under this hormonal condition the activation of 5-HT1A receptors induces a phase advance and an increase in LH surge.