Comparison of pineal melatonin rhythms in young adult and old Djungarian hamsters (Phodopus sungorus) under long and short photoperiods.

The pattern of the pineal melatonin rhythm was studied in 3-6- and 17-22-month-old female Djungarian hamsters under light-dark (LD) schedules, LD 16:8 and LD 8:16. Under both photoperiods, the amplitude of the melatonin rhythm was 2.5 times higher in young adult hamsters than in old ones. Two weeks after the change from LD 16:8 to LD 8:16, the period of elevated nocturnal melatonin levels lengthened by about 3 h in both age groups. It is suggested that old Djungarian hamsters, as well as young adults, might be able to recognize the length of a photoperiod and its change.