Pineal melatonin concentrations exhibited a marked diurnal rhythmicity in golden hamsters maintained in a light-dark cycle of 14 h of light and 10 h of darkness. When tissue was collected at 3-h intervals throughout a 24-h period, daytime levels of 95–232 pgpineal gland rose to concentrations of 760–1335 pg/pineal gland (P < 0.001 vs. all other values) at 0400 h, 8 h after the onset of darkness. When tissue was collected sequentially during the dark phase, pineal melatonin concentrations remained significantly elevated from 0200–0500 h (P < 0.01 and P < 0.001 vs. daytime values, respectively). Superior cervical ganglionectomy abolished the rhythm of pineal melatonin concentrations, and the concentrations were maintained at 60–105 pg/pineal gland throughout a 24-hr period.
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