Antigonadal Effects of Timed Melatonin Infusion in Pinealectomized Male Djungarian Hamsters (Phodopus sungorus sungorus): Duration is the Critical Parameter*

To determine which parameter of the day/night pattern of pineal melatonin secretion is the critical component signaling daylength information in the Djungarian hamster, we have developed a method for giving timed sc melatonin infusions in pinealectomized juvenile males. When given for 12 h daily, as little as 10 ng melatonin (14 pg/min) consistently induced testicular regression within 12 days. However, 10 ng melatonin infused for 4 or 6 h daily did not inhibit gonadal development. The effects of these infusions on the reproductive system did not depend on the time of day at which melatonin was administered. In complementary experiments, the minimal daily infusion duration and the critical daylength for induction of testicular regression were determined. The critical length of infusion (7-8 h) was in close agreement with the estimated duration of melatonin secretion during the critical scotophase. These findings support the hypothesis that melatonin mediates the pineal-antigonadal effects of short day exposure in the Djungarian hamster. Furthermore, the data strongly suggest that duration is the feature of nighttime melatonin release that is most important for photoperiodic time measurement in this species.