Effect of a phase advance of the light/dark cycle on pineal function and circadian running activity in individual rats.

Circadian rhythms of urinary 6-sulphatoxy-melatonin excretion and spontaneous wheel-running activity were monitored in individual male Wistar albino and hooded rats entrained to a 12 h L:12 h D photoperiod, before and after an 8 h phase advance of the light cycle. The pigmented hooded strain commenced melatonin metabolite excretion 2 h after darkness, whereas the albino rats did not excrete detectable amounts until 4 h of darkness had elapsed (p < 0.05). There was no correlation between the time of onset of spontaneous wheel-running activity and the onset of excretion of 6-sulphatoxymelatonin under entrained conditions. When the photoperiod was advanced by 8 h, the albino rats took a median of 4 days (range 2-10 days) to establish a normal phase angle for the onset of 6-sulphatoxymelatonin excretion in contrast to the pigmented rats, which took in excess of 10 days to entrain. Albino rats reentrained running activity significantly earlier than pigmented rats (8 days: range 6-9 days), compared to 10 days (range 8 to > 12 days). There was no consistent relationship between the pineal and running rhythms with respect to the time taken to entrain following a lighting phase shift. All rats entrained the 6-sulphatoxymelatonin rhythm by advancing the onset of excretion in contrast to the running rhythm where three rats reentrained by advancing, three by delaying, and in four, the direction of entrainment could not be accurately determined. In two of the animals (both albino), there was unequivocal evidence that the 6-sulphatoxymelatonin rhythm and running reentrained in different directions.(ABSTRACT TRUNCATED AT 250 WORDS)