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S.M. Yom, D.L. Nickla; There Is a Circadian Rhythm in Nitric Oxide Synthesis in Isolated Chick Choroid . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1985.
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Purpose: There are circadian rhythms in axial elongation and choroidal thickness in chick eyes. At least part of the underlying mechanism(s) controlling the rhythm in elongation is the circadian rhythm in scleral proteoglycan synthesis (Nickla et al., 1999). Evidence suggests that nitric oxide (NO) might be involved in the compensatory changes in choroidal thickness in response to defocus (Nickla et al., 2004). We hypothesized that if NO plays such a role, then it might also play a role in the diurnal changes in choroidal thickness, and if so, itself express a diurnal rhythm in synthesis. Methods: Pieces of choroid (7 mm) from the back of the eye of 1–2 week old untreated (n=5) or positive lens–wearing (n=3) chicks were dissected and put into growth chambers in a temperature–controlled flow–through perifusion system for 24 (n=2) or 48 (n=6) hours. Medium was collected at 2 hour intervals over the culture period. Nitric oxide was assayed in the aliquots using the in–NO amperometric system (Innovative Instruments, Inc.) by an indirect method (nitrates/nitrites). Results: There is a rhythm in NO synthesis in isolated choroidal tissue that has a period of approximately 24 hours and persists for at least two cycles. The peak occurs during the early night, between approximately 8 pm and 11 pm. The trough to peak excursion is approximately 3 fold. The mean NO concentration is 14 nmoles. In this small sample size, there are no apparent differences in choroidal NO concentration, rhythm amplitude or phase between untreated and positive lens–wearing eyes. Conclusions: There is a diurnal rhythm in NO synthesis in choroidal tissue that persists in culture, in the absence of temporal cues, indicating the presence of a circadian oscillator. The peak in synthesis is approximately coincident with the peak in choroidal thickness in normal eyes, suggesting that NO might mediate the diurnal changes in thickness. The source of the NO could be either intrinsic choroidal neurons (nNOS) or vascular endothelial cells (eNOS). Possible sites of action include the vasculature, which might effect changes in fluid dynamics, and/or smooth muscle, both of which could cause changes in choroidal thickness.
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