Prolonged food deprivation abolishes the circadian adrenocortical rhythm, but not the endogenous rhythm in rats.

An abolishment of 24 h periodicity of the adrenocortical activity was observed in rats subjected to a prolonged food deprivation of 7--8 days. Total locomotor activity progressively decreased and its circadian rhythm faded out in a similar fashion as the adrenocortical rhythm. The circadian rhythms of both locomotor and adrenocortical activities were recovered after the rats were blinded and allowed free access to food. At the end of the first week, a peak elevation of blood corticosterone levels took place at the time corresponding to the light-dark transition before blinding. Similar patterns of locomotor activity and food intake were also noticed. The recovered rhythm of these activities free-run throughout the observation period of 5 weeks. This fact indicates that the endogenous time-keeping system (clock) remained intact and synchronized with light-dark alternation during the period of prolonged food deprivation.