Effects of photoperiod on hamster pineal melatonin concentrations

1. 1. Pineal melatonin concentrations were determined by radioimmunoassy at various intervals throughout a 24 hr period in adult male Syrian hamsters (Mesocricetus auratus) maintained in light: dark cycles of either 14 hr light and 10 hr darkness (LD 14:10), 6hr light and 18 hr darkness (LD 6:18), or of 2 hr light and 22 hr darkness (LD 2:22). 2. 2. A marked diurnal rhythmicity in pineal melatonin concentrations was present in hamsters maintained in each photoperiod with daytime levels of 70-230 pg/pineal gland rising to concentrations of 700-1300 pg/pineal gland during the night. 3. 3. In a LD cycle of 14:10, of 6:18 and of 2:22, pineal melatonin concentrations reached peak values 8 hr, 13 hr and 16-20 hr after the onset of darkness, respectively.