Repeated rat exposure inhibits the circadian activity patterns of C57BL/6J mice in the home cage.

Exposing male C57BL/6J mice repeatedly, in an unpredictable and uncontrollable fashion to rats, alters their cognitive performance and the neuroendocrine stress response, weeks to months after the rat stress. Continuous observation of the behavioural activity of male C57BL/6J mice in their home cage before (baseline) and after rat exposure could reveal if repeated rat exposure leads to changes in circadian activity patterns, which is a key feature of chronic stress and stress-related disorders in humans. Rat stress (1) decreased exploratory and foraging activity as characterized by increased time spent in the shelter and less time in the open area; (2) reduced sucrose consumption and inhibited the development of sucrose preference, suggesting changes in the reward system and (3) the exploration pattern in a novel environment included more behavioural perseverations, but no change in general locomotor activity. Comparison to baseline activity pattern, i.e., before any intervention, revealed that already the control procedure to rat exposure (spending the same amount of time in another cage) disrupted the organization of behavioural activity patterns, albeit to a different and lesser degree than observed in rat stressed mice. While only the longitudinal design of the study allowed detecting these dynamic patterns of circadian activities, the distinct behavioural changes in foraging and explorative activities support our notion that repeated rat exposure might serve as mouse model of chronic stress.