A test of the coincidence and duration models of melatonin action in Siberian hamsters. II. The effects of 4-and 8-hr melatonin infusions on testicular development of pinealectomized juvenile Siberian hamsters (Phodopus sungorus)

In a previous paper we demonstrated that properly timed 1-hr infusions of 5 0 ng melatonin effectively suppressed testicular development in juvenile Si berian hamsters, Only melatonin infused between 20:00 and 21:00 hr was effe ctive in animals exposed to 16L (lights off 20:00 hr), In this paper we fur ther investigate the importance of the coincidence and duration hypotheses of daily exposure of melatonin. Prepubertal Siberian hamsters received eith er 4- or 8-hr melatonin infusions at various times either on long photoperi od (LD 16:8 = 16L) or on short photoperiod (LD 10:14 = 10L). Daily 8-hr mel atonin infusions suppressed testicular development in both photoperiods. Da ily 4-hr, 50 ng/hr, melatonin infusions at 17:00-21:00 hr inhibited testicu lar growth in 16L and daily 4-hr melatonin infusions (either 50 ng/h or 50 ng/day) inhibited testicular growth at 17:00-21:00 hr in 10L. We also teste d the efficacy of an interrupted melatonin infusion of long duration (8 hr) , Pinealectomized prepubertal male Siberian hamsters, burn on 16L, were inf used with two signals of 4 hr separated by an interval of 2 hr. Melatonin-i nfused groups had significantly inhibited testicular growth compared to veh icle-infused animals. Testicular development was maximally inhibited only i n those groups in which the period of melatonin sensitivity identified in t he previous paper (20:00-21:00 hr) overlapped or immediately followed a per iod of melatonin infusion. Considering the restrictions of the experimental design employed in these studies, the results are best explained by the hy pothesis that the photoperiodic gonadal response in juvenile Siberian hamst ers is regulated by the coincidence in time of exogenously administered mel atonin with an intrinsic rhythm of sensitivity to melatonin, which occurred at 20:00-21:00 hr, The duration of the melatonin signal alone can not expl ain the results.