Regulation of Photic Signaling Peptides in the Mammalian SCN Circadian Clock: In Vivo Measurements of VIP and GRP

Jessica M. Francl, MS

PhD Candidate
Kent State University
Kent, Ohio

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Alternating daylight with nighttime darkness influences the circadian rhythm signals generated by the suprachiasmatic nucleus (SCN), a region in the hypothalamus that controls other biological clocks in the body.1 Among the molecular mechanisms responsible for circadian rhythm regulation is the interaction of 2 neuropeptides: vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP).2 These neuropeptides signal the SCN, which in turn activates pacemaker neurons to facilitate normal biological rhythms in other tissues and organ systems. Using a hamster model, the levels of VIP and GRP were measured in the SCN over 24 hours. The study found that the highest levels of both VIP and GRP were released in the middle of the day—expected, since these light-sensitive peptides function best at the time of day when there is the most light. The study also found, however, that the peptides were also released in total darkness, suggesting a role for VIP and GRP that is unrelated to environmental light. Further analyses implicated serotonergic signaling in the modulation of VIP and GRP levels during periods of darkness. Interestingly, serotonin regulates sleep and mood. It is established that poor sleep and associated excessive sleepiness can promote and exacerbate mood disorders. Thus, understanding the mechanisms associated with both sleep and mood may facilitate management approaches to optimize care in patients with comorbid sleep/wake and mood disorders.

References

Okamura H. Suprachiasmatic nucleus clock time in the mammalian circadian system. Cold Spring Harb Symp Quant Biol. 2007;72:551-556.
Maywood ES, Reddy AB, Wong GK, et al. Synchronization and maintenance of timekeeping in suprachiasmatic circadian clock cells by neuropeptidergic signaling. Curr Biol. 2006;16:599-605.

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