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

Precise interactions between ovarian steroids and neurotransmitters ar e required for the secretion of phasic LH surge. Previous data suggest ed the existence of an interactive stimulatory effect of progesterone (P) and serotonin (5-HT) on LH release. In the present work the effect s of 8-OH-DPAT, a selective 5-HT1A agonist, on phasic LH secretion wer e 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 th e morning of the expected LH surge had no effect on circadian plasma l evels of LH in OVX-E(2) rats, whereas it induced a phase advance and a n increase in LH surge in OVX-E(2)-P rats. Administration of the antip rogestin RU 38486 in OVX-E(2)-P rat, totally abolished the combined ef fects 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 surg e. The present data suggest that progesterone is required for the regu lation of phasic LH release by 5-HT1A agonists and that under this hor monal condition the activation of 5-HT1A receptors induces a phase adv ance and an increase in LH surge.