International Journal of Molecular Zoology 2024, Vol.14, No.3, 166-181 http://animalscipublisher.com/index.php/ijmz 171 Figure 3 Diagram of connections between brain regions and the pineal gland in the circadian rhythm system (Adopted from Sanchez et al., 2021) Image caption: Simplified diagram of connections between the master circadian clock and sleep-wake circuitry. Diagram is color-coded as follows: Red=SCN, Green=generally wakepromoting, Purple=generally sleep promoting, orange=both, blue=neutral/other role, dotted lines=sparse projections. Abbreviations: ipRGCs, intrinsically photosensitive retinal ganglion cells; RHT, retinohypothalamic tract; SCN, suprachiasmatic nucleus; ARC, arcuate nucleus of the hypothalamus; VLPO, ventrolateral preoptic nucleus; dSPZ, dorsal subparaventricular zone; vSPZ, ventral subparaventricular zone; DMH, dorsomedial hypothalamus; TMN, tuberomammillary nucleus; LH, lateral hypothalamus; PVN, paraventricular nucleus; IGL, intergeniculate leaflet; PAG, periaqueductal gray; LC, locus coeruleus; SCG; spinal cervical ganglion (Adopted from Sanchez et al., 2021) The sleep-wake cycle is influenced by rhythmic hormones such as melatonin and cortisol, which are regulated by the circadian clock. Melatonin, often referred to as the "sleep hormone," is secreted in response to darkness and promotes sleep, while cortisol levels peak in the early morning to promote wakefulness (Koop and Oster, 2021). The synchronization of sleep-wake cycles with the external light-dark cycle ensures that animals are active during optimal times for feeding and other activities, thereby enhancing survival and reproductive success (Singh and Kumar, 2018; Koop and Oster, 2021). 4.2 Feeding and foraging behaviors influenced by circadian rhythms Feeding and foraging behaviors in animals are also under circadian control. The circadian clock regulates the release of hormones such as leptin, ghrelin, insulin, and orexin, which influence hunger and satiety (Koop and Oster, 2021). These hormonal rhythms ensure that feeding occurs at times when food is most likely to be available and when the body is best prepared to metabolize nutrients (Panda, 2016). In Drosophila, for example, the circadian clock in the brain's pars intercerebralis (PI) region regulates feeding rhythms. Neurons expressing the neuropeptide SIFamide (SIFa) are crucial for maintaining these rhythms, while neurons expressing Drosophila insulin-like peptides (DILPs) influence overall food consumption but not the rhythmicity of feeding (Dreyer et al., 2019). This distinction highlights the complex interplay between circadian and homeostatic mechanisms in regulating feeding behavior. Moreover, time-restricted feeding, which aligns feeding times with the natural circadian rhythm, has been shown to sustain robust diurnal rhythms and alleviate metabolic diseases in experimental animal models (Panda, 2016). This suggests that maintaining proper circadian timing of feeding can have significant health benefits.
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