Comparison of synchronization of circadian corticosteroid rhythms by photoperiod and food

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Abstract
Under conditions of feeding at will and normal light-dark alternation, rats consume the major portion of their daily food intake during the dark period. The circadian peak of plasma corticosteroid concentrations and of body temperature levels occur just prior to or subsequent to the time of light-dark transition. Both light-dark transition and time of food presentation have been implicated as zeitgebers in determining the phase of these 2 circadian rhythms. The time of food presentation appears to be a more potent synchronizer of the phase of plasma corticosteroid levels than is the light-dark cycle. Rats maintained in constant dim light with free access to food exhibit aperiodic feeding behavior; plasma corticosteroid concentrations and body temperature levels are also aperiodic. Imposition of a restricted period of food access under such constant light conditions is associated with the appearance of a circadian periodicity of both plasma corticosteroid concentrations and body temperature levels, with peaks, respectively, just before and after the time of food presentation. An additional example of food entrainment of previously aperiodic functions, similar to the food entrainment in animals rendered aperiodic by lesions of the suprachiasmatic nucleus is represented.