Progonadal role of the pineal in the Djungarian hamster (Phodopus sungorus sungorus): mediation by melatonin.

A progonadal role of the pineal in both adult and juvenile male Djungarian hamsters has been demonstrated which confirms and extends previous reports. In short day-raised hamsters, stimulation of testicular development was achieved by transfer to long days. This stimulatory effect of long days was blocked by pinealectomy. Gonadal stimulation could be elicited in pinealectomized juveniles by timed daily infusions of melatonin. When given for 4 or 6 h daily at a dose rate of 0.83 or 83 ng/h, melatonin infusions stimulated testicular development and body growth. Measurement of plasma FSH and PRL indicated that this pubertal response resulted at least in part from a stimulatory effect of melatonin on the secretion of these hormones. Longer duration infusions (8 or 12 h) did not stimulate testicular development or body growth, and plasma FSH and PRL levels remained low. The efficacy of the short duration infusions did not appear to depend upon the time of day of administration. These findings are in close agreement with those of a previous study which investigated the antigonadal effect of long duration melatonin infusions. The present results demonstrate that melatonin administration can mimic the progonadal effects of the pineal in this species and add further support to the hypothesis that the pineal influences the reproductive system through changes in the duration of daily melatonin production.