Regulation of luteinizing hormone-releasing hormone (LHRH) secretion by melatonin in the ewe. II. Changes in N-methyl-D,L-aspartic acid-induced LHRH release during the stimulation of luteinizing hormone secretion by melatonin.

Injection of N-methyl-D,L-aspartic acid (NMDA), a glutamatergic agonist, cause an acute release of LH. The amount of LH released is modulated by photoperiod in sheep; it is larger in animals in which reproductive activity has been inhibited by long days than in those photostimulated by short days. The purpose of the present experiment was to determine whether this effect of photoperiod on LH response to NMDA is mediated by melatonin and whether it reflects a change in the amount of LHRH released in response to NMDA. Twenty-one ovariectomized ewes, bearing an s.c. implant of estradiol and kept in long days (16L:8D), received s.c. implants of melatonin on Day 0. The effect of a single injection of NMDA (5 mg/kg i.v.) on LHRH and simultaneous LH secretion was studied at three different periods of the stimulation of LH secretion by the melatonin treatment: first, during inhibition of LH secretion by long days (on Day -1, n = 6); secondly, during the onset of the LH increase (on Day 39, n = 9); and thirdly, during maximum LH secretion (on Day 74, n = 6). The stimulation of LHRH secretion by administration of NMDA was characterized by a large and long increase in LHRH concentration on Day -1 and Day 39. There was no significant difference between these two periods in terms of duration and amplitude of the LHRH release. On Day 74, the stimulation was much shorter than on Day -1 and Day 39 and was followed by a phase of decreased pulsatility of LHRH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)