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J.M. Schartau, B. Sjögreen, R.H. H. Kröger; Light–Dark Adaptive Optical Changes in Fish Lenses . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1212.
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Chromatic aberration causes severe defocus in species that are capable of color vision and have short depth of focus because of large pupils. In vertebrates, the problem is solved by multifocal lenses that focus several spectral bands on the retina. Multiple focal lengths are achieved by spherical aberration of complex shape. In many fishes the color–differentiating cones are at dusk moved out of the focal plane and replaced by highly light–sensitive rods (retinomotor movements). In the dark adapted state vision is monochromatic such that a monofocal lens is the best optical solution. We hypothesized therefore that the lens may be able to adjust its optical properties in response to light–dark adaptive signals. We studied the effects of natural dark adaptation and dopamine depletion. Dopamine plays a major role in light–dark adaptive processes and has low levels at night.
The optical properties of fish lenses (Aequidens pulcher, Cichlidae) were determined with three different optical methods: photorefractometry, schlieren photography and laser scanning. The experimental groups consisted of lenses taken from (i) dark–adapted animals at night and (ii) during daytime from animals in which the eyes had been depleted of dopamine by bilateral injections of 6–hydroxy–dopamine two weeks before measurements were performed. In the control group, data from untreated and sham–injected light–adapted animals were pooled because no differences could be detected.
We detected significant differences in the optical properties between controls and dark–adapted as well as dopamine–depleted animals. Changes occurred between about 60 and 100 % lens radius. In the dark adapted state, there was less spherical aberration than in light adapted animals. Lenses from dopamine–depleted eyes had strong positive spherical aberration in the periphery.
The results indicate that the optical properties of fish lenses are regulated and that changes can occur within hours. Dopamine seems to be directly or indirectly involved since depletion of dopamine induces a similar but stronger effect than dark adaptation which naturally reduces dopamine level in the eye
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