Interactions between psychosocial stress and the circadian endogenous clock.

Animals are exposed to regular daily challenges in their environment as well as to unpredictable stressful events, so-called stressors. Both measures require adequate physiological responses that are controlled by the circadian system and the stress system. The two systems are highly linked with each other. The circadian system prepares the animal for the regular daily challenges by upregulating the stress system before the animal's activity phase every day. In addition, unpredicted environmental challenges upregulate the stress system independently of the circadian system. This can happen at any time, even in the middle of the animal's sleeping phase. Although the stress system feeds back on the brain, the circadian master clock in the suprachiasmatic nuclei (SCN) of the hypothalamus is usually not disturbed by stressful events, but instead continues to activate the animal in a regular daily manner. However, the stability of the SCN clock appears to fade away after chronic stress, in particular after chronic psychosocial stress. Chronic psychosocial stress is the major risk factor for major depression and certain anxiety disorders in humans and therefore deserves attention. Here, I review the interactions between the circadian and stress systems after chronic psychosocial stress in an animal model, the social defeat paradigm.