The pineal gland and its hormone, melatonin, have been implicated in the regulation of rat circadian rhythmicity. The present study was designed to evaluate whether the pineal has a similar role in the hamster, and to clarify whether the marked rhythm responses to constant light (LL) previously seen in serotonin-depleted hamsters might be attributable to a functional pinealectomy. The results demonstrated that young, but not old, hamsters showed loss of the circadian wheel-running rhythm (mostly via splitting) in LL, and that young hamsters had longer circadian periods in LL than old animals. Neither effect was related to the presence of a pineal gland. In LD 14:10 old animals ran much less than young animals, regardless of pineal status, and the suppression of running by LL was greater in old animals. The activity phase duration was only modestly related to age and not to pineal presence. The data support a previous report of an age effect on incidence of rhythm splitting and circadian period length, but do not support the view that the pineal gland helps modulate circadian rhythmicity in the hamster.
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