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 spon taneous wheel-running activity were monitored in individual male Wista r albino and hooded rats entrained to a 12 h L:12 h D photoperiod, bef ore and after an 8 h phase advance of the light cycle. The pigmented h ooded strain commenced melatonin metabolite excretion 2 h after darkne ss, whereas the albino rats did not excrete detectable amounts until 4 h of darkness had elapsed (p < 0.05). There was no correlation betwee n the time of onset of spontaneous wheel-running activity and the onse t of excretion of 6-sulphatoxymelatonin under entrained conditions. Wh en 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 reentrai ned running activity significantly earlier than pigmented rats (8 days : range 6-9 days), compared to 10 days (range 8 to > 12 days). There w as no consistent relationship between the pineal and running rhythms w ith respect to the time taken to entrain following a lighting phase sh ift. All rats entrained the 6-sulphatoxymelatonin rhythm by advancing the onset of excretion in contrast to the running rhythm where three r ats reentrained by advancing, three by delaying, and in four, the dire ction of entrainment could not be accurately determined. In two of the animals (both albino), there was unequivocal evidence that the 6-sulp hatoxymelatonin rhythm and running reentrained in different directions . The results obtained during entrainment suggest that the pineal and spontaneous running rhythms can be dissociated and may even be control led by different rhythm centres.