We examined the effects of daily melatonin (Mel) infusions in pinealectomized prepubertal male Siberian hamsters in three different conditions. In one study we investigated the body weight maturation response to one hour daily infusions of 10 ng, 25 ng, or 50 ng of Mel in pinealectomized hamsters. Animals received, at day 15 of life, programmed subcutaneous infusions of Mel or vehicle at one of five time points (1900-2000, 2000-2100, 2100-2200, 2400-0100, and 0300-0400 hours) for 30 days. In long-day-born (16L) animals, Mel infusion right after lights off (2000-2100 hours) significantly inhibited body growth; this dose was ineffective at other times. Doses of 10 ng and 25 ng Mel were ineffective at all time points. In a second study, hamsters received either 4-or 8-h infusions of Mel (either 50 ng/h or 50 ng/day) at various times throughout the day and night of a 16L or 10L photoperiod. Daily 4-h, 50 ng/h, Mel infusions at 1700-2100 hours inhibited body weight growth in 16L and daily 4-h Mel infusions (either 50 ng/h or 50 ng/day) inhibited body weight growth at 1700-2100 hours in 10L. In all cases, daily 8-h infusions suppressed body weight development. In a third study, long-day-born pinealectomized hamsters were infused with two signals of four hours separated by an interval of two hours. Mel infused groups had significantly inhibited body growth compared to vehicle infused animals. Body weight development was maximally inhibited only in those groups in which the period of Mel sensitivity was identified in the first study (2000-2100 hours) overlapped or immediately followed a period of Mel infusion These results show that the photoperiodic body weight response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered Mel with an intrinsic rhythm of sensitivity to Mel, and the duration of the Mel signal alone cannot explain the results.

We examined the effects of daily melatonin (Mel) infusions in pinealectomized prepubertal male Siberian hamsters in three different conditions. In one study we investigated the body weight maturation response to one hour daily infusions of 10 ng, 25 ng, or 50 ng of Mel in pinealectomized hamsters. Animals received, at day 15 of life, programmed subcutaneous infusions of Mel or vehicle at one of five time points (1900-2000, 2000-2100, 2100-2200, 2400-0100, and 0300-0400 hours) for 30 days. In long-day-born (16L) animals, Mel infusion right after lights off (2000-2100 hours) significantly inhibited body growth; this dose was ineffective at other times. Doses of 10 ng and 25 ng Mel were ineffective at all time points. In a second study, hamsters received either 4-or 8-h infusions of Mel (either 50 ng/h or 50 ng/day) at various times throughout the day and night of a 16L or 10L photoperiod. Daily 4-h, 50 ng/h, Mel infusions at 1700-2100 hours inhibited body weight growth in 16L and daily 4-h Mel infusions (either 50 ng/h or 50 ng/day) inhibited body weight growth at 1700-2100 hours in 10L. In all cases, daily 8-h infusions suppressed body weight development. In a third study, long-day-born pinealectomized hamsters were infused with two signals of four hours separated by an interval of two hours. Mel infused groups had significantly inhibited body growth compared to vehicle infused animals. Body weight development was maximally inhibited only in those groups in which the period of Mel sensitivity was identified in the first study (2000-2100 hours) overlapped or immediately followed a period of Mel infusion These results show that the photoperiodic body weight response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered Mel with an intrinsic rhythm of sensitivity to Mel, and the duration of the Mel signal alone cannot explain the results.