PERSISTENCE OF CIRCADIAN OSCILLATION WHILE LOCOMOTOR-ACTIVITY AND PLASMA MELATONIN LEVELS BECAME APERIODIC UNDER PROLONGED CONTINUOUS LIGHTIN THE RAT

In order to examine the mechanism for a loss of circadian rhythms in s everal functions under prolonged continuous light (LL), rats were blin ded following LL over 5 months, and the mode of reappearance of circad ian rhythms were analyzed in locomotor activity and plasma melatonin l evels. Locomotor activity and plasma melatonin levels in individual ra ts became aperiodic after the exposure to LL. On the day of blinding, plasma melatonin levels showed circadian rhythms having a peak coincid ed with the activity time of locomotor rhythm which was restored after blinding. The time of melatonin peak was not related to the time of b linding (onset of darkness) nor to the initial time of blood sampling. Circadian rhythm in plasma melatonin levels reappeared faster than th ose in locomotor activity. The findings suggest that aperiodism develo ped in these functions under prolonged LL is not due to disruption of the circadian oscillation but to uncoupling of overt functions from th e circadian pacemaker.