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A.F. Wiechmann, J.A. Rada; Diurnal regulation of ocular growth by melatonin. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4294.
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Purpose: The rate of ocular elongation exhibits a diurnal rhythm with peak growth occurring during the light period. The purpose of this study was to examine the role of the circadian signaling molecule, melatonin, on the rate of ocular growth. Methods: Chicks maintained on a 12:12 light–dark cycle were form vision deprived in one eye with a translucent occluder for five days. During the five–day treatment period, chicks were injected systemically during either the mid–light or early dark period with approximately 50 mg/kg of melatonin or 2% ethanol vehicle control. Axial elongation of the occluded and contralateral control eyes were examined by high frequency A–scan ultrasound. Expression of melatonin receptors in ocular tissues were examined by Western blot analysis and immunocytochemistry. Results: In both the occluded and contralateral control eyes, melatonin treatment during the day resulted in a significantly greater axial length as compared with eyes from animals receiving melatonin treatment at night (p < 0.02). The occluded eyes showed a more pronounced response to melatonin than did the contralateral control eyes, with average day–night differences in vitreous chamber depth of 2.3 fold, as compared with the day–night differences of contralateral control eyes. Western blot and immunocytochemical analysis of chick ocular tissues revealed expression of three melatonin receptor subtypes in the sclera and retina. Conclusions: These observations suggest that melatonin exerts a differential effect on ocular elongation depending on the circadian time of administration. We speculate that the action of melatonin, combined with expression of melatonin receptors in ocular and/or extraocular tissues regulates the diurnal rhythm of ocular growth.
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