Ontogeny of the Pineal Melatonin Rhythm in Golden Hamsters1

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Abstract
Pineal melatonin concentrations have been quantified in groups of six to eight male Syrian hamsters on Days 7, 9, 11, 13, 15, 16, 17, 19, and 21 postpartum (Day 1 = day of birth). Samples were obtained at 2 h intervals throughout each 14L:10D cycle. On Days 7, 9, and 11, pineal melatonin concentrations were usually low with significantly elevated peak values as apt to occur during periods of light as during periods of darkness. On Days 13 and 15, melatonin concentrations increased just prior to the onset of darkness and remained elevated throughout the dark phase. By Day 17, elevated concentrations were restricted to the latter half of the dark phase, as occurs in adults. From these data we conclude that an adultlike circadian rhythm in pineal melatonin content is found on and after Day 17 of postnatal development.