Free-running activity rhythms in the rat: entrainment by melatonin

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Science  04 Mar 1983:
Vol. 219, Issue 4588, pp. 1089-1091
DOI: 10.1126/science.6823571


The pineal gland hormone melatonin may play a role in synchronization of rat circadian rhythms. Free-running activity rhythms of the rat were entrained by a daily melatonin injection, with entrainment occurring when the onset of activity coincided with the time of daily injections. When injections were stopped, activity rhythms became free-running again. Thus in pharmacological experiments, the time of day of melatonin administration is crucial.

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