REGULATION OF LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) SECRETION BY MELATONIN IN THE EWE .2. 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 agonis t, causes an acute release of LH. The amount of LH released is modulat ed by photoperiod in sheep; it is larger in animals in which reproduct ive activity has been inhibited by long days than in those photostimul ated by short days. The purpose of the present experiment was to deter mine whether this effect of photoperiod on LH response to NMDA is medi ated by melatonin and whether it reflects a change in the amount of LH RH released in response to NMDA. Twenty-one ovariectomized ewes, beari ng an s.c. implant of estradiol and kept in long days (16L:8D), receiv ed s.c. implants of melatonin on Day 0. The effect of a single injecti on of NMDA (5 mg/kg i.v.) on LHRH and simultaneous LH secretion was st udied 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 inc rease (on Day 39, n = 9); and thirdly, during maximum LH secretion (on Day 74, n = 6). The stimulation of LHRH secretion by administration o f NMDA was characterized by a large and long increase in LHRH concentr ation on Day -1 and Day 39. There was no significant difference betwee n these two periods in terms of duration and amplitude of the LHRH rel ease. On Day 74, the stimulation was much shorter than on Day -1 and D ay 39 and was followed by a phase of decreased pulsatility of LHRH sec retion. The differences between time points were similar for the induc ed LH release. The results show that a short-day-like melatonin treatm ent caused a decrease in LH release in response to NMDA as short days do. Furthermore, this modification in LH response to NMDA reflects a s imilar change in LHRH release.