Parallel shift in circadian rhythms of adrenocortical activity and food intake in blinded and intact rats exposed to continuous illumination.

Twenty-four hour patterns of plasma corticosterone levels and food intake were determined at 3-4 h intervals weekly for 7 consecutive weeks in blinded and intact female adult rats exposed to continuous illumination. Plasma samples were obtained from the tail vein. Resting levels and circadian periodicity of plasma corticosterone levels determined by this method were essentially similar to those obtained by decapitation. Circadian periodicity of adrenocortical activity and food intake shifted in both blinded and intact rats. In each group, the patterns of shift in these two activities were essentially similar. Between the two groups, however, the patterns of shift were markedly different. In blinded rats under either constant light or regular light-dark alternation a fairly constant phase shift was observed after the 3rd week and phase reversal of rhythms was found at the end of the 7th week. An additional determination of 48 h pattern of plasma corticosterone levels at the end of the 7th week demonstrated a clear periodicity in the majority of the blinded rats when the patterns were determined individually. On the other hand, in the intact rats exposed to constant light the parallel shift in the two activities was observed for only two weeks. Although a significant difference between the peak and trough values was observed up to the 6th week in food intake and the 4th week for corticosterone levels, a plot of the plasma corticosterone levels determined at 4 h intervals for 48 h did not show any periodicity in individual rats at the 4th and 7th week. In the case of partial food deprivation (feeding time, 1900-2100 h), peak elevation of plasma corticosterone levels occurred at 1900 h for two consecutive weeks in the intact rats exposed to continuous illumination. These results indicate that rhythmicities of adrenocortical activity and food intake persist longer in the blinded rats than in the intact rats kept under constant light. The fact that the phase shift of adrenocortical activity was prevented by the artificial fixation of the feeding period indicates that the shift of adrenocortical activity is a secondary phenomenon to the shift in the periodicity of food intake.