Abstract:
The effects of melatonin and isoproterenol injections on estrous cyclicity, the ovarian, adrenal and pineal gland weights, and on locomotor activity were examined in Syrian hamsters maintained in constant light (LL). Isoproterenol effects on the same parameters also were studied by injections 15 minutes before lights-out in hamsters maintained on a 14 hour photoperiod (14L:lOD). Melatonin (25 ug), isoproterenol (0.2mg), or oil-control (0.1 ml) injections were administered daily, between 1930-2100 EST, except in two groups which also received daily, morning injections of melatonin or oil at 0700 EST. Single-melatonin injections, and single-or double-oil injections had no significant effects on estrous cyclicity, organ weights or locomotor free-running activity. Double injections of melatonin, however, induced estrous acyclicity in 66.7% of those hamsters and severely depressed uterine weights, as well. Double-melatonin injections did not affect the locomotor activity since the free-running rhythm was near 24 hours. Isoproterenol injections in LL-exposed hamsters, ~n contrast, induced persistent estrus in two groups of animals. Conversely, isoproterenol injections in animals maintained in l4L:lOD induced acyclicity and marked uterine atrophy. The present findings suggest that LL affects the hamster in a manner similar to pinealectomy since single-melatonin injections
neither induced estrous acyclicity nor entrained locomotor activity. Injections of isoproterenol in LL-exposed animals are postulated to affect the luteinizing-hormone cyclic center in the anterior
hypothalamus, because of the induction of persistent estrus. In contrast, isoproterenol treatment in l4L:lOD-exposed animals appears to suppress gonadal function by inducing melatonin synthesis. The daily double-melatonin injections are theorized to be antigonadal due to the "melatonin rhythm" established by the protocol employed in the hamsters to LL. The results of this study further support the hypothesis that melatonin is the antigonadal hormone of the pineal gland.
