The effects of social defeat and other stressors on the expression of circadian rhythms.

Most biological functions display a 24 h rhythm that, in mammals, is under the control of an endogenous circadian oscillator located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The circadian system provides an optimal temporal organization for physiological processes and behavior in relation to a cyclic environment imposed upon organisms by the regular alternation of day and night. In line with its function as a clock that serves to maintain a stable phase-relationship between endogenous rhythms and the light-dark cycle, the circadian oscillator appears to be well protected against unpredictable stressful stimuli. Available data do not provide convincing evidence that stress is capable of perturbing the central circadian oscillator in the SCN. However, the shape and amplitude of a rhythm is not determined exclusively by the SCN and certain stressors can strongly affect the output of the clock and the expression of the rhythms. In particular, social stress in rodents has been found to cause severe disruptions of the body temperature, heart rate and locomotor activity rhythms, especially in animals that are subject to uncontrollable stress associated with defeat and subordination. Such rhythm disturbances may be due to effects of stress on sub-oscillators that are known to exist in many tissues, which are normally under the control of the SCN, or due to other effects of stress that mask the output of the circadian system. These disturbances of peripheral rhythms represent an imbalance between normally precisely orchestrated physiological and behavioral processes that may have severe consequence for the health and well being of the organism.