REGULATION OF LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) SECRETION BY MELATONIN IN THE EWE .1. SIMULTANEOUS DELAYED INCREASE IN LHRH AND LUTEINIZING-HORMONE PULSATILE SECRETION

This study was performed to test the hypothesis that a short-day-like melatonin treatment would induce simultaneous stimulations of LHRH and LH secretion. Simultaneous serial samples of jugular and portal blood to assay LH and LHRH, respectively, were carried out at three differe nt periods according to the expected changes in LH secretion. Twenty-s ix ovariectomized ewes bearing a subcutaneous implant of estradiol and exposed to a long-day photoperiod (16L:8D) received s.c. implants of melatonin on Day 0. LHRH and LH pulsatile secretion was studied in the first group during inhibition of LH secretion by long days (on Day -1 , n = 6). A second group (n = 9) was sampled on Day 39. Moreover, seve n animals from this group were sampled on Day 46 to focus on the begin ning of LH increase. The third group (n = 6) was sampled during maxima l LH secretion (Day 74, n = 6). A fourth group (n = 5) composed of non cannulated animals was sampled for LH secretion only as a control for the effects of surgical procedures. On each occasion, blood was collec ted every 10 min for 6 h. Between Day -1 and Day 39, LHRH pulse freque ncy was low and did not differ. In the same animals, this value became 2.5-fold higher between Day 39 and Day 46. Finally, on Day 74, LHRH p ulse frequency was about 6-fold higher than on Day -1. The modificatio ns of LH pulse frequency paralleled those of LHRH with no change betwe en Day -1 and Day 39, a 3-fold increase on Day 46 compared to Day 39, and a 4.5-fold increase on Day 74 compared to Day -1. This experiment demonstrated that the increases in LHRH and LH pulsatile secretion fol lowing a short-day-like melatonin treatment occurred simultaneously, w ith a 40-60-day delay.