EFFECTS OF PHOTOPERIOD, PINEALECTOMY, AND MELATONIN IMPLANTS ON TESTICULAR DEVELOPMENT IN JUVENILE SIBERIAN HAMSTERS (PHODOPUS-SUNGORUS)

When transferred from a long to short photoperiod, Siberian hamsters ( Phodopus sungorus) undergo a number of physiological and morphological changes including suppression of gonadal activity, a change in pelage color, a decrease in body weight, and, in response to a simultaneous decrease in ambient temperature, physiological changes associated with the induction of daily torpor. All these functions can be affected by photoperiod and melatonin treatment. To investigate the interactive e ffects of photoperiod, pinealectomy, and melatonin on gonadal developm ent, two experiments were performed using juvenile Siberian hamsters. In experiment 1, animals born in a long photoperiod (16L:8D) either re mained in a long photoperiod or were transferred to a short photoperio d (8L:16D) from 15 days of age, when surgeries (pinealectomy and/or me latonin implantation) were performed. Testicular development was inhib ited in all animals bearing melatonin implants irrespective of the pre sence or absence of the pineal gland. Pinealectomy blocked the inhibit ory effect of short photoperiod on maturation of the reproductive syst em. Therefore, the pineal gland must be involved in the short photoper iod-induced inhibition of testicular maturation of juvenile Siberian h amsters. In experiment 2, a similar experimental design was employed e xcept that the hamsters mere born and raised to 15 days of age in 8L:1 6D. Exogenous melatonin, pinealectomy, or both retarded gonadal develo pment in hamsters born in 8L:16D and transferred on Day 15 of age to 1 6L:8D. All hamsters maintained in a short photoperiod had small testes irrespective of the presence or absence of the pineal gland or of mel atonin implants. Hamsters transferred to a long photoperiod after pine alectomy and/or melatonin implantation had small testes compared to th ose of the control group. These results suggest that melatonin implant s inhibit prepubertal testicular development irrespective of ambient p hotoperiod.