Circadian effects of escapable and inescapable shock on the food intake and wheelrunning of rats.

The feeding and running activities of rats were used to characterize the stressful effects of footshock. A 12-hour light/dark cycle was continuously in effect. During the preshock stage, all rats lived in individual cages where they could drink water, press a lever once for each 45 mg food pellet or run in an attached running wheel. During the shock stage, rats were moved to cages where they could drink and eat, but not run. Five s of light followed by 5 s of a light + tone compound preceded five progressively more intense shock levels. Rats in group AE were trained to pull a ceiling chain to avoid/escape from footshocks that were scheduled 24 hours per day. In group YS, the frequency, intensity and pattern of shocks for a rat were yoked to a rat in group AE. Rats in group CN were not shocked. During recovery, rats were allowed to resume running. During the dark, but not the light of the first day of shock, the intake of groups AE and YS was less than the intake of group CN. By the third day of shock, the food intake of group AE recovered and was no longer different from the intake of group CN. Intake of group YS remained less than the intakes of groups AE and CN during the remainder of the shock stage. When shock was terminated, the intake of the YS group recovered, but neither running nor food intake differed among groups. These results suggest that although inescapable shock disrupts food intake more than escapable shock, this disruption is specific to the duration of the shock stage and depends on rat's circadian pattern of intake.