Nutritional status, time of day and pinealectomy: factors influencing the sensitivity of the neuroendocrine-reproductive axis of the rat to melatonin.

Afternoon but not morning injections of melatonin (Mel) resulted in significant 68, 50 and 20% reductions in seminal vesicle, ventral prostate and testicular weights, respectively, as compared with underfed controls. As in the animals injected in the morning, reproductive organ weights in underfed-pinealectomized rats were unaffected by afternoon injections of Mel. Both pituitary and serum prolactin levels were significantly depressed in underfed rats receiving Mel in the afternoon; however, Mel was ineffective when given either in the morning or to pinealectomized rats. These data indicate that there is a diurnal rhythm of sensitivity to Mel in undernourished rats which is abolished by pinealectomy.