EVIDENCE FOR 5-HT2 RECEPTOR INVOLVEMENT IN THE STIMULATION OF PREOVULATORY LH AND PROLACTIN-RELEASE AND OVULATION IN NORMAL CYCLING RATS

The participation of serotonin (5-HT) in the control of LH secretion and ovulation has been reported in numerous pharmacological experiments, although the results have been contradictory. In this study to determine the physiological involvement of 5-HT2 receptors in the control of preovulatory LH and prolactin (PRL) release and ovulation, ketanserin (KET), a selective 5-HT2 receptor antagonist, was administered to normal cycling rats. Oral administration of KET at 10 mg/kg BW between 0900h and 1300h on the day of proestrus completely blocked ovulation, while the administration after 1500h did not. The ovulation was confirmed 2 days after treatment with KET. There was no consistent effect on the number of ova ovulated. Co-administration of KET with 5-methoxy-N, N-dimethyltryptamine (5-MeDMT), a 5-HT agonist, at a dose of 2 mg/kg BW ip reversed KET-induced inhibition of ovulation. The highest rate (89%) of reversal was obtained by co-administration at 1500h. KET administered at 1100h completely blocked the preovulatory surge of LH and PRL observed in control animals at 1800h. The present study suggests that 5-HT stimulates preovulatory LH and PRL surge and ovulation via 5-HT2 receptor under physiological conditions and that the effect of 5-HT depends on the critical period and the 24-hour periodicity.