Opposing effects of atropine and timolol on the color and luminance emmetropization mechanisms in chicks

- The autonomic nervous system appears to influence color and luminance emmetropization.
- Blocking parasympathetic input with atropine reduces eye growth.
- Blocking sympathetic input with timolol increases eye growth under luminance flicker.
- The effects of atropine are heightened under luminance flicker conditions.

This study analyzed the luminance and color emmetropization response in chicks treated with the nonselective parasympathetic antagonist atropine and the sympathetic β-receptor blocker timolol.Chicks were binocularly exposed (8 h/day) for 4 days to one of three illumination conditions: 2 Hz sinusoidal luminance flicker, 2 Hz sinusoidal blue/yellow color flicker, or steady light (mean 680 lux). Atropine experiments involved monocular daily injections of either 20 μl of atropine (18 nmol) or 20 μl of phosphate-buffered saline. Timolol experiments involved monocular daily applications of 2 drops of 0.5% timolol or 2 drops of distilled H2O. Changes in the experimental eye were compared with those in the fellow eye after correction for the effects of saline/water treatments.Atropine caused a reduction in axial length with both luminance flicker (−0.078 ± 0.021 mm) and color flicker (−0.054 ± 0.017 mm), and a reduction in vitreous chamber depth with luminance flicker (−0.095 ± 0.023 mm), evoking a hyperopic shift in refraction (3.40 ± 1.77 D). Timolol produced an increase in axial length with luminance flicker (0.045 ± 0.030 mm) and a myopic shift in refraction (−4.07 ± 0.92 D), while color flicker caused a significant decrease in axial length (−0.046 ± 0.017 mm) that was associated with choroidal thinning (−0.046 ± 0.015 mm).The opposing effects on growth and refraction seen with atropine and timolol suggest a balancing mechanism between the parasympathetic and β-receptor mediated sympathetic system through stimulation of the retina with luminance and color contrast.

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