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The object of the following study was to compare pineal melatonin rhythms of hamsters housed in outdoor versus laboratory conditions during five consecutive seasons. For each season, 72 adult male Syrian hamsters were caged under controlled laboratory conditions and 72 were caged in a three-sided shelter outdoors. The light:dark cycle for the animals kept in the laboratory approximated the corresponding day:night lengths of each season. After hamsters were exposed to their respective environments for 3 weeks, pineal glands were collected from 8 animals from each group at 08.00, 12.00, 17.00, 20.00, 22.00, 24.00, 02.00, 04.00 and 06.00 h. Radioimmunoassay was used to determine pineal melatonin content. All groups of animals displayed a circadian rhythm of pineal melatonin with peak nighttime levels of melatonin being 8- to 12-fold greater than daytime levels. Compared to animals kept in the laboratory, hamsters exposed to natural seasonal conditions appear to produce significantly more melatonin during the winter and significantly less melatonin during the summer and fall. A seasonal rhythm of melatonin synthesis was observed in animals kept in the laboratory and outdoors.

Pineal levels of melatonin exhibit a circadian rhythm in the rat. To determine if this rhythm varies during the estrous cycle, adult rats were sacrificed every 2 hr from 2000 hr (prior to lights off) until 0800 hr (2 hr after lights on) throughout each day of the 4-day estrous cycle. Pineal glands were assayed for melatonin by radioimmunoassay. A significant circadian rhythm in the pineal content of melatonin was evident each day of the cycle with peak levels exhibited during the dark phase. There were significant differences in titers of melatonin present at 0200, 0400, and 0600 hr between various stages of the cycle. In general, highest nighttime levels occurred during the evenings of metestrus and diestrus, with levels at a minimum on the evening of estrus. These data indicate that the circadian pineal melatonin rhythm is a function of estrous cycle stage in the rat, and, thus, important differences exist in the pineal biosynthetic dynamics of this species relative to those of seasonal breeders such as the hamster and sheep. Additionally, the results suggest that hydroxyindole-0methyltransferase, and not serotonin N-acetyltransferase, is the regulatory enzyme responsible for these estrous stage differences in pineal melatonin content. 416

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In rats, anosmia induced by olfactory bulbectomy sensitizes the neuroendocrine-reproductive axis to the inhibitory effects of the pineal gland. Thus, combined blinding (which stimulates pineal antigonadotrophic activity) and anosmia cause a marked delay in the sexual maturation of female rats. The present study tested whether anosmia would also sensitize the neuroendocrine axis to the reproductive inhibitory effects of melatonin. In the present study, young female rats were rendered anosmic and maintained under 1ight:dark cycles of 14: 10; they were given melatonin injections daily (25 pg) a t either 0900 hr (AM-melatonin) or at 1800 hr (PM-melatonin)-i.e., either 3 or 12 hr after lights on, respectively. The melatonin injections were begun when the rats were 23 days of age and were continued for 35 days. By comparison with AM-melatonin, PM-melatonin injections were considerably more effective in inhibiting bodily growth as well as the weights of the anterior pituitary, ovaries, and uterus of anosmic rats. Similarly, the changes in pituitary and plasma LH and PRL levels were more obvious in the PM-treated anosmic rats than in those given melatonin in the morning. Thus, anosmia does increase the sensitivity of the neuroendocrine-reproductive axis to melatonin, provided the indoleamine is administered late in the light phase of the 1ight:dark cycle. Anosmia by itself had no influence on the daytime levels of radioimmunoassayable melatonin within the pineal gland, whereas in blinded control rats, melatonin levels were markedly augmented.Compared to some other species, the reproductive systems of rats are insensitive to the inhibitory influence of the pineal gland (Reiter, '68). However, the sensitivity can be substantially increased by any of several perturbations, including rendering the animals anosmic by surgical removal of the olfactory bulbs (Reiter and Ellison, '70), early neonatal injection of gonadal steroids (Reiter et al., '68), or restricting the food intake of the rats (Sorrentino et al., '71; Osman et al., '72). It is believed that these factors act by lowering the threshold of inhibition of some site within the central nervous system rather than by any direct action on the antigonadotrophic potential of the pineal gland.The sexual organs of intact rats are also com-