Abstract
In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Individual SCN cells exhibit intrinsic oscillations, and their circadian period and robustness are different cell by cell in the absence of cellular coupling, indicating that cellular coupling is important for coherent circadian rhythms in the SCN. Several neuropeptides such as arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are expressed in the SCN, where these neuropeptides function as synchronizers and are important for entrainment to environmental light and for determining the circadian period. These neuropeptides are also related to developmental changes of the circadian system of the SCN. Transcription factors are required for the formation of neuropeptide-related neuronal networks. Although VIP is critical for synchrony of circadian rhythms in the neonatal SCN, it is not required for synchrony in the embryonic SCN. During postnatal development, the clock genes cryptochrome (Cry)1 and Cry2 are involved in the maturation of cellular networks, and AVP is involved in SCN networks. This mini-review focuses on the functional roles of neuropeptides in the SCN based on recent findings in the literature.
Highlights
Creatures on earth anticipate cyclic changes in the surrounding environment, such as day–night, seasonal, and annual changes, to adapt their own physiological functions so that they can minimize risks to survival
It took longer to restore the circadian rhythms of VPAC2 KO suprachiasmatic nucleus (SCN) as compared to vasoactive intestinal polypeptide (VIP) KO SCN. These results indicate that VIP and other molecules, such as arginine vasopressin (AVP) and gastrin-releasing peptide (GRP), are crucial for the synchronization of circadian rhythms in the SCN
Avp expression was attenuated in Cry1/Cry2deficient SCN in both neonatal and adult SCN (Ono et al, 2016; Figure 2B). These results indicate that VIP and AVP are responsible for CRY-independent and -dependent cellular coupling, respectively, and that these neuropeptides are differentially involved in the maturation of circadian networks in the SCN
Summary
Creatures on earth anticipate cyclic changes in the surrounding environment, such as day–night, seasonal, and annual changes, to adapt their own physiological functions so that they can minimize risks to survival. Rhythmic changes adapting to environmental changes caused by the rotation of the earth are driven by intrinsic oscillatory mechanisms called the circadian clock. The circadian system comprises a hierarchical structure involving the master clock in the brain and a number of peripheral clocks situated throughout the body. Almost all cells in our body possess a circadian clock in which the molecular machinery, namely, transcription– translation feedback loops involving several clock genes and their protein products, generates circadian rhythmicity (Reppert and Weaver, 2002). Circadian rhythms of cells and tissues are coordinated by the master circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus (Weaver, 1998). The circadian clock in the SCN regulates rhythms within a variety of physiological output functions such as sleep/wakefulness, body temperature, and endocrine functions
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