Regulation of luteinizing hormone-releasing hormone (LHRH) secretion by melatonin in the ewe. I. 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 different periods according to the expected changes in LH secretion. Twenty-six 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, seven animals from this group were sampled on Day 46 to focus on the beginning of LH increase. The third group (n = 6) was sampled during maximal LH secretion (Day 74, n = 6). A fourth group (n = 5) composed of noncannulated animals was sampled for LH secretion only as a control for the effects of surgical procedures. On each occasion, blood was collected every 10 min for 6 h. Between Day -1 and Day 39, LHRH pulse frequency 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 pulse frequency was about 6-fold higher than on Day -1.(ABSTRACT TRUNCATED AT 250 WORDS)