Circadian Clocks in the Regulation of Neurotransmitter Systems.

To anticipate and adapt to daily recurring events defined by the earth's rotation such as light-dark and temperature cycles, most species have developed internal, so-called circadian clocks. These clocks are involved in the regulation of behaviors such as the sleep-wake cycle and the secretion of hormones and neurotransmitters. Disruptions of the circadian system affect cognitive functions and are associated with various diseases that are characterized by altered neurotransmitter signaling. In this review, we summarize the current knowledge about the interplay of the circadian clock and the regulation of psychiatric health and disease.

An adipokine feedback regulating diurnal food intake rhythms in mice.

Endogenous circadian clocks have evolved to anticipate 24 hr rhythms in environmental demands. Recent studies suggest that circadian rhythm disruption is a major risk factor for the development of metabolic disorders in humans. Conversely, alterations in energy state can disrupt circadian rhythms of behavior and physiology, creating a vicious circle of metabolic dysfunction. How peripheral energy state affects diurnal food intake, however, is still poorly understood. We here show that the adipokine adiponectin (ADIPOQ) regulates diurnal feeding rhythms through clocks in energy regulatory centers of the mediobasal hypothalamus (MBH). Adipoq-deficient mice show increased rest phase food intake associated with disrupted transcript rhythms of clock and appetite-regulating genes in the MBH. ADIPOQ regulates MBH clocks via AdipoR1-mediated upregulation of the core clock gene Bmal1. BMAL1, in turn, controls expression of orexigenic neuropeptide expression in the MBH. Together, these data reveal a systemic metabolic circuit to regulate central circadian clocks and energy intake.

Mutual influence of sleep and circadian clocks on physiology and cognition.

The 24-h sleep-wake cycle is one of the most prominent outputs of the circadian clock system. At the same time, changes in sleep-wake behavior feedback on behavioral and physiological circadian rhythms, thus altering the coordination of the body's clock network. Sleep and circadian rhythm disruption have similar physiological endpoints including metabolic, cognitive, and immunologic impairments. This raises the question to which extent these phenomena are causally linked. In this review, we summarize different physiologic outcomes of sleep deprivation and mistimed sleep and discuss the experimental evidence for a mediating role of the circadian clock machinery in this context.

Tissue-Specific Dissociation of Diurnal Transcriptome Rhythms During Sleep Restriction in Mice.

Study objectives: Shortened or mistimed sleep affects metabolic homeostasis, which may in part be mediated by dysregulation of endogenous circadian clocks. In this study, we assessed the contribution of sleep disruption to metabolic dysregulation by analysing diurnal transcriptome regulation in metabolic tissues of mice subjected to a sleep restriction (SR) paradigm. Methods: Male mice were subjected to 2 × 5 days of SR with enforced waking during the first 6 hours of the light phase. SR and control mice were sacrificed at different time points of the day and RNA preparations from the mediobasal hypothalamus (MBH), liver, and epididymal white adipose tissue (eWAT) were subjected to whole-genome microarray hybridization. Transcriptional rhythms were associated with changes in behavioral and physiological parameters such as sleep, body temperature, and food intake. Rhythm detection was performed with CircWave and transcription profiles were compared by 2-way analysis of variance and t-tests with Benjamini-Hochberg corrections. Results: Clock gene rhythms were blunted in all tissues, while transcriptome regulation was associated with either clock gene expression, sleep patterns, or food intake in a tissue-specific manner. Clock gene expression was associated with apoptosis pathways in the MBH and with tumor necrosis factor alpha signalling in liver. Food intake-associated genes included cilium movement genes in the MBH and lipid metabolism-associated transcripts in liver. Conclusions: In mice, repeated SR profoundly alters behavioral and molecular diurnal rhythms, disrupting essential signalling pathways in MBH, liver, and eWAT, which may underlie the metabolic and cognitive disturbances observed in sleep-restricted humans such as shift workers.

Circadian Rhythms in Adipose Tissue Physiology.

The different types of adipose tissues fulfill a wide range of biological functions-from energy storage to hormone secretion and thermogenesis-many of which show pronounced variations over the course of the day. Such 24-h rhythms in physiology and behavior are coordinated by endogenous circadian clocks found in all tissues and cells, including adipocytes. At the molecular level, these clocks are based on interlocked transcriptional-translational feedback loops comprised of a set of clock genes/proteins. Tissue-specific clock-controlled transcriptional programs translate time-of-day information into physiologically relevant signals. In adipose tissues, clock gene control has been documented for adipocyte proliferation and differentiation, lipid metabolism as well as endocrine function and other adipose oscillations are under control of systemic signals tied to endocrine, neuronal, or behavioral rhythms. Circadian rhythm disruption, for example, by night shift work or through genetic alterations, is associated with changes in adipocyte metabolism and hormone secretion. At the same time, adipose metabolic state feeds back to central and peripheral clocks, adjusting behavioral and physiological rhythms. In this overview article, we summarize our current knowledge about the crosstalk between circadian clocks and energy metabolism with a focus on adipose physiology. © 2017 American Physiological Society. Compr Physiol 7:383-427, 2017.