Limited Effect of Three Types of Daily Stress on Rat Free-Running Locomotor Rhythms

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Abstract
The effects of three types of stress/arousal on rat free-running circadian locomotor rhythms in constant darkness were investigated over a 93-day treatment period. Rats were subjected to 30-min daily immobilisation stress or 30-min novelty or to brief handling (n = 10/group). Seven of the 30 rats exhibited some changes in circadian parameters. Three rats (two immobilised, one handling) showed entrainment, three rats (one from each group) showed a change in tau, and one rat (novelty) showed a phase advance. Thus, in total, 30% immobilisation, 20% percent novelty, and 20% handled rats showed circadian changes. These group changes paralleled changes in faecal boli and body weight, which were taken as indirect indices of the level of stress. Five of the seven changes took place when the end of the active phase (alpha), i.e., subjective dawn, coincided with the time of treatment and the other two when the onset of sigma, i.e., subjective dusk, coincided. Rat circadian locomotor rhythms appear much less susceptible to stress/arousal than those reported for hamsters.