Abstract
In mammals, molecular circadian clocks are present in most cells of the body, and this circadian network plays an important role in synchronizing physiological processes and behaviors to the appropriate time of day. The hypothalamic–pituitary–adrenal endocrine axis regulates the response to acute and chronic stress, acting through its final effectors – glucocorticoids – released from the adrenal cortex. Glucocorticoid secretion, characterized by its circadian rhythm, has an important role in synchronizing peripheral clocks and rhythms downstream of the master circadian pacemaker in the suprachiasmatic nucleus. Finally, glucocorticoids are powerfully anti-inflammatory, and recent work has implicated the circadian clock in various aspects and cells of the immune system, suggesting a tight interplay of stress and circadian systems in the regulation of immunity. This mini-review summarizes our current understanding of the role of the circadian clock network in both the HPA axis and the immune system, and discusses their interactions.
Highlights
Life on Earth has evolved in the context of a rhythmic environment, characterized largely by the regular succession of night and day
The HPA axis is a key in the regulation of stress responses, with glucocorticoids mediating intermediate and chronic adaptation to stressful stimuli, complementing the rapid response of catecholamines, both secreted from the adrenal gland
Given the influence that circadian rhythmicity of glucocorticoids may have on peripheral clock function, it is perhaps not surprising that the HPA axis, which is acutely activated in stressful situations, is unlikely to be the main driver of the circadian rhythm of these hormones
Summary
Life on Earth has evolved in the context of a rhythmic environment, characterized largely by the regular succession of night and day. This has led to the evolution of intrinsic circadian (from Latin circa diem – about the day) clock systems, in order to optimally time physiological and behavioral processes. Less well studied is the interaction of the HPA axis and immune system with regard to the circadian clock This mini-review will summarize current knowledge regarding the role of the circadian clock in each of these systems, and the interactions that can occur in the context of disrupted circadian rhythmicity
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