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Anthony H Tsang, Mariana Astiz, Maureen Friedrichs and Henrik Oster

Endogenous circadian clocks regulate 24-h rhythms of behavior and physiology to align with external time. The endocrine system serves as a major clock output to regulate various biological processes. Recent findings suggest that some of the rhythmic hormones can also provide feedback to the circadian system at various levels, thus contributing to maintaining the robustness of endogenous rhythmicity. This delicate balance of clock–hormone interaction is vulnerable to modern lifestyle factors such as shiftwork or high-calorie diets, altering physiological set points. In this review, we summarize the current knowledge on the communication between the circadian timing and endocrine systems, with a focus on adrenal glucocorticoids and metabolic peptide hormones. We explore the potential role of hormones as systemic feedback signals to adjust clock function and their relevance for the maintenance of physiological and metabolic circadian homeostasis.

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C E Koch, M S Bartlang, J T Kiehn, L Lucke, N Naujokat, C Helfrich-Förster, S O Reber and H Oster

In modern societies, the risk of developing a whole array of affective and somatic disorders is associated with the prevalence of frequent psychosocial stress. Therefore, a better understanding of adaptive stress responses and their underlying molecular mechanisms is of high clinical interest. In response to an acute stressor, each organism can either show passive freezing or active fight-or-flight behaviour, with activation of sympathetic nervous system and the hypothalamus–pituitary–adrenal (HPA) axis providing the necessary energy for the latter by releasing catecholamines and glucocorticoids (GC). Recent data suggest that stress responses are also regulated by the endogenous circadian clock. In consequence, the timing of stress may critically affect adaptive responses to and/or pathological effects of repetitive stressor exposure. In this article, we characterize the impact of predictable social defeat stress during daytime versus nighttime on bodyweight development and HPA axis activity in mice. While 19 days of social daytime stress led to a transient reduction in bodyweight without altering HPA axis activity at the predicted time of stressor exposure, more detrimental effects were seen in anticipation of nighttime stress. Repeated nighttime stressor exposure led to alterations in food metabolization and reduced HPA axis activity with lower circulating adrenocorticotropic hormone (ACTH) and GC concentrations at the time of predicted stressor exposure. Our data reveal a circadian gating of stress adaptation to predictable social defeat stress at the level of the HPA axis with impact on metabolic homeostasis underpinning the importance of timing for the body’s adaptability to repetitive stress.