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Laura Ann Goldberg, Frances J Rucker; Antagonistic effects of atropine and timolol on the color and luminance emmetropization mechanisms. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2150.
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© ARVO (1962-2015); The Authors (2016-present)
The role of the autonomic nervous system in the color and luminance emmetropization mechanisms is unknown. This study analyzed the response to the non-selective, parasympathetic antagonist, atropine, and the sympatholytic, beta-adrenergic antagonist, timolol, in chicks subjected to illumination conditions that selectively stimulate the color and luminance emmetropization mechanisms.
Chicks were binocularly exposed eight hours each day, for four days, to one of three illumination conditions: 2Hz sinusoidal luminance flicker (LUM), 2Hz sinusoidal color flicker (B/Y), or steady light. Mean illuminance was 680 lux. Eyes received daily injections of either 20μl atropine (18nmol) (N=8), 2 drops of 0.5% timolol (N=8), 20μl phosphate-buffered saline (N=8), or no injection (N=8). Measurements of the axial dimensions of ocular components and refraction were performed using A-scan ultrasonography, photorefraction and a Hardinger Refractometer. In each illumination condition, the saline effect was subtracted from the drug effect [Drug (ΔX-ΔN) - Saline (ΔX-ΔN)].
LUM flicker demonstrated opposite effects on eye growth and refraction with atropine and timolol treatment. Atropine caused a reduction in eye growth (-0.08 ± 0.02 mm, p=0.01) and a reduction in vitreous chamber depth (-0.10 ± 0.02 mm, p=0.004), evoking a hyperopic shift in refraction (3.40 ± 1.77 D), despite an antagonistic increase in lens thickness (0.14 ± 0.05 mm, p=0.004). In contrast, timolol elicited a myopic shift in refraction (-4.07 ± 0.92 D, p=0.001), due to an increase in eye length (0.045 ± 0.030 mm). Color flicker induced choroidal compensation for eye growth, preventing refractive shifts with atropine and timolol. With atropine, hyperopia was not observed, because a reduction in eye length (-0.05 ± 0.02 mm, p=0.01) was compensated for by choroidal thinning (-0.05 ± 0.02 mm, p=0.03). With timolol, myopia did not occur because a reduction in eye length (-0.05 ± 0.018 mm, p=0.02) was also compensated for by choroidal thinning (-0.052 ± 0.015 mm, p=0.01).
The opposing growth and refractive effects of atropine and timolol with luminance flicker, and the compensatory choroidal compensation with color flicker, suggest a precise balancing mechanism between the parasympathetic and sympathetic nervous system, and the visual environment, in achieving emmetropization.
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