The rate of melatonin synthesis within pineal glands of female hamsters maintained in an 8-h light, 16-h dark photoperiod has been calculated. When exposed to these photoperiodic conditions, female hamsters exhibited a distinct circadian rhythm in pineal melatonin content, with basal concentrations of 40-90 pg/gland found throughout the day and during the first 10 h of darkness and a peak concentration of 847 pg/ gland found near the end of the dark phase 1 h before lights on. The rates of melatonin synthesis required to produce this concentration profile ranged from 1.6 pg/min during the daylight hours to a maximum of 70.6 pg/min 14 h after the onset of darkness. The total quantity of melatonin synthesized throughout the 24-h cycle was calculated to be 18.6 ng. Exposure of hamsters to an unexpected source of white light at the time of the nocturnal peak in pineal melatonin content resulted in a rapid decay in pineal melatonin concentrations. The rate constant obtained (−0.0788 min−1) corresponded to a t1/2 of approximately 9 min. Exposure of rats to a similar experimental paradigm resulted in a 6 min t1/2 for pineal melatonin content and a 4 min t1/2 for pineal N-acetylserotonin transferase activity. The quantity of exogenously administered melatonin required to induce vaginal acyclicity in female hamsters when injected sc 0.5 h before lights out in a 14-h light, 10-h dark photoperiod was determined. Dosages of 1.6-12.8 μg melatonin/day induced acyclicity in 50% of the female hamsters within 40-52 days. Dosages less than 1.6 μg/day did not interrupt vaginal cyclicity. Hamsters exposed to an 8-h light, 16-h dark photoperiod required 104 days of exposure before acyclicity was found in 50% of the population. At the end of 105 days of treatment, mean uterine weights averaged between 133 ± 24 and 215 ± 33 mg/100 g BW (mean ± sem) in hamsters treated with dosages greater than 1.6 μg melatonin/day and between 311 ± 35 and 369 ± 6 mg/100 g BW in hamsters treated with dosages less than 1.6 μg/day. Hamsters receiving diluent had mean uterine weights of 363 ± 15 mg/100 g BW. Those exposed to an 8-h light, 16-h dark photoperiod had mean uterine weights of 252 ± 22 mg/100 g BW.
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