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The impact of photoperiodic history was studied in male hamsters kept from birth to 6 wk of age (Week 0) in long-day (LD; 16 h light per day) or short-day (SD; 10 h light per day) photoperiods. At Week 0, hamsters were transferred to constant light and were s.c. infused daily with melatonin (MEL; 4 ng/h) until Week 12. Groups of SD hamsters received 10-h MEL infusions or MEL infusions of durations that decreased linearly by 1-2 min/day from 10 h at Week 0 to 7.5 h at Week 12. LD hamsters were infused for 5 h/day or in patterns that decreased from 10 to 7.5 h or increased from 5 to 7.5 h during Weeks 0-12. MEL signals of decreasing duration produced opposite effects on the gonads of LD and SD hamsters: decreasing MEL durations induced complete gonadal regression in LD hamsters (testis weight = 42 +/- 3 mg) but stimulated testis growth in SD hamsters (testis weight = 276 +/- 57 mg; p &lt; 0.05). MEL infusions that were of shorter but increasing duration induced gonadal regression in LD hamsters. The present experiments indicate that MEL signals of a given sequence and duration can produce either gonadal growth or regression depending on the hamster's photoperiodic history. The pattern of change in MEL secretion, as well as absolute duration of the MEL signal, is important for implementing seasonal cycles of reproduction.

Pattern of Change in Melatonin Duration Determines Testicular Responses in Siberian Hamsters, Phodopus sungorus 1