Daily melatonin injections have only minor effects on gonadotropins of intact or castrated male rats kept under constant or periodic light.

Intact and castrated adult male Wistar rats were kept under constant or periodic (lights on 0600 and off 1800 h) light for 1 wk. During the study they received melatonin or saline injections daily either at 0900 or 1600 h. After each experiment, serum samples and the adenohypophyses were collected between 1000 and 1100 h and the gonadotropin concentrations were measured radioimmunologically. We had previously found changes in light sensitivity of the hypothalamo-pituitary axis of castrated rats and hypothesized that this axis could also involve changes in sensitivity to exogenous melatonin. However, the results of the present study do not support the hypothesis.