Male rats, 28 days of age, housed on a 14L:10D cycle (lights on 0600–2000 h) were fed either a standard laboratory chow ad libitum or 50% of the food intake of comparably aged normal rats. On the first day of underfeeding, groups of fed and underfed animals began receiving single daily afternoon (1700–1800 h) s.c. injections of either 50 µg melatonin (Mel) or diluent for 5 weeks. By the end of this period, underfeeding resulted in a dramatic reduction in absolute pituitary, testicular and body weights; however, relative to body weight, testicular and pituitary weights were actually increased in underfed animals compared with fed controls. Both the absolute and relative accessory organ weights were significantly reduced in food deprived animals. The testes of underfed rats treated with Mel were 76% and 56% smaller than those of the fed or underfed controls, respectively. Similarly, the accessory sex organ weights of underfed Mel treated rats were 94% and 72% smaller than those of the fed or underfed controls, respectively. Both Mel treated fed rats and saline treated underfed animals experienced a significant reduction in pituitary prolactin (Prl) content; however, the concentration of Prl in underfed animals was elevated above that of the fed controls. The pituitary content of Prl in underfed-Mel treated rats was the lowest of any group. The pituitary Prl concentration in these animals was significantly less than that of the underfed control animals alone. There were no statistically significant differences in serum Prl levels among any of the groups. It is concluded that chronic nutritional deprivation is yet another physiological state in which the neuroendocrine-reproductive axis is more sensitive to the antigonadotropic effects of exogenously administered Mel.
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Blask et al. (1980) studied this question.