SHORT-DAY EFFECTS OF MELATONIN ON LUTEINIZING-HORMONE SECRETION IN THE EWE - EVIDENCE FOR CENTRAL SITES OF ACTION IN THE MEDIOBASAL HYPOTHALAMUS

Experiments were designed to localize the central sites of action of m elatonin involved in the control of seasonal reproduction. Ewes were e xposed to long days and received microimplants of melatonin in the pre optic area (n = 9), anterior hypothalamus (n = 4), dorsolateral hypoth alamus (n = 4), or mediobasal hypothalamus (n = 12). The effects of im plants were determined by comparison with control ewes (untreated or s ham-operated, n = 10) and with ewes treated with an s.c. implant of me latonin (n = 8) or ewes subjected to short days (n = 8). All ewes were ovariectomized and treated s.c. with a silastic capsule of estradiol. Melatonin released in the preoptic area as well as in the anterior an d lateral hypothalamus did not cause any difference as compared with t he controls (no stimulation of LH secretion and no inhibition of prola ctin secretion). In contrast, melatonin implanted in the mediobasal hy pothalamus caused an increase in LH secretion in 7 of the 12 ewes on D ay 53.0 +/- 4.2 after implantation (mean +/- SEM). Their response was not different compared with that of ewes treated s.c. with melatonin o r exposed to short days either in terms of timing (Day 56.3 +/- 6.2 an d 59.5 +/- 3. 1, respectively, for controls) or of amplitude of the LH response. Similarly, melatonin caused only a reduction of prolactin s ecretion in the mediobasal, s.c., and short-day groups. It is conclude d that the mediobasal hypothalamus or the surrounding tissues could be the sites of action of melatonin involved in the control of seasonal reproduction.