PATTERN OF CHANGE IN MELATONIN DURATION DETERMINES TESTICULAR RESPONSES IN SIBERIAN HAMSTERS, PHODOPUS-SUNGORUS

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, hamst ers were transferred to constant light and were s.c. infused daily wit h melatonin (MEL; 4 ng/h) until Week 12. Groups of SD hamsters receive d 10-h MEL infusions or MEL infusions of durations that decreased line arly by 1-2 min/day from 10 h at Week 0 to 7.5 h at Week 12. LD hamste rs 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 de creasing duration produced opposite effects on the gonads of LD and SD hamsters: decreasing MEL durations induced complete gonadal regressio n in LD hamsters (testis weight = 42 +/- 3 mg) but stimulated testis g rowth in SD hamsters (testis weight = 276 +/- 57 mg; p < 0.05). MEL in fusions that were of shorter but increasing duration induced gonadal r egression in LD hamsters. The present experiments indicate that MEL si gnals of a given sequence and duration can produce either gonadal grow th or regression depending on the hamster's photoperiodic history. The pattern of change in MEL secretion, as well as absolute duration of t he MEL signal, is important for implementing seasonal cycles of reprod uction.