EVIDENCE THAT MELATONIN ACTS IN THE PREMAMMILLARY HYPOTHALAMIC AREA TO CONTROL REPRODUCTION IN THE EWE - PRESENCE OF BINDING-SITES AND STIMULATION OF LUTEINIZING-HORMONE SECRETION BY IN-SITU MICROIMPLANT DELIVERY

Melatonin transduces the effect of day length on LH secretion by actin g on the hypothalamus. However, the precise hypothalamic site is unkno wn. Two studies were undertaken to clarify where melatonin acts in the hypothalamus. Using autoradiographic methods, the hypothalami of 5 ew es were screened to determine whether specific regional densities in m elatonin binding existed. A higher density of binding was observed in the premammillary area of the hypothalamus (PMH) (3- to 5-fold higher than the rest of the hypothalamus). This binding area is delimited ros trally by the infundibular recess, caudally by the mammillary bodies, dorsally by the fornix, and ventrally by the base of the brain; and it encompasses the premammillary and tuberomammillary nuclei. To test th e functional importance of the identified area, 3 groups of animals re ceived bilateral melatonin microimplants: 1) in the PMH (n = 11); 2) i n the anterior/mediobasal hypothalamus (AH/MBH; n = 8); and 3) sham-op erated animals received empty microimplants in the PMH (SHAM; n = 6). All ewes were ovariectomized and treated sc with a 20-mm SILASTIC bran d capsule of estradiol and exposed to long days (16-h light, 8-h dark) . At the end of the 80-day experiment, no animal of the SHAM group and only 2 of the 8 ewes of the AH/MBH group displayed a stimulation of L H secretion. In contrast, melatonin implanted in the PMH stimulated LH secretion in 10 of the 11 ewes on day 44.5 +/- 5.3 (mean +/- SEM). AN OVA revealed that the changes in LH secretion were not different betwe en the SHAM and the AH/MBH groups but the PMH group differed from the other 2 groups (P < 0.0001). This study suggests that the PMH is an im portant target for melatonin to regulate reproductive activity.