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J Neurosci. 1994 Mar;14(3 Pt 1):1091-105.

Light adaptation and photopigment bleaching in cone photoreceptors in situ in the retina of the turtle.

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Department of Psychology, University of Minnesota, Minneapolis 55455.


Light adaptation and photopigment bleaching in cone photoreceptors were studied in the intact, superfused retina of the turtle (Pseudemys scripta elegans). A new method for measuring changes in the photopigment of cones is described. Action spectrum measurements indicate that the signals arise from the red-sensitive cones. Measurements of steady-state bleaching are well described by the monomolecular bleaching equation with a half-bleaching constant of about 5.5 log photons sec-1 microns-2. Quantitative data on light adaptation were obtained by intracellular recording from 15 red-sensitive cones over nearly 8 decades of background illumination obtained from a helium-neon laser (632.8 nm). The steady-state membrane potential, Rs, and the rate of photoisomerization of the photopigment, Pi, rose in parallel with background illumination and then stabilized over the upper 4 decades of illumination. These results are described by the relation Rs = k Pi0.27, and suggest that about 5 x 10(6) photoisomerizations sec-1 lead to the closure of half the cone's light-sensitive channels in the steady state. A full range of decremental and incremental flashes was used to investigate stimulus-response relations. Cones tended to generate responses of approximately constant amplitude to flashes of constant contrast over a substantial range of contrast (< or = 3 x) and background illumination (approximately 3-4 decades). This suggests that a substantial component of contrast constancy in vertebrate vision may originate in cones. Over nearly 7 decades, the small-signal step sensitivity was found to conform closely to Weber's law (sensitivity is inversely proportional to background illumination). Thus, Weber's law extends into the ultra-high-intensity realm, some 3 decades higher than previously known for vertebrate cones. Over the upper 3-4 decades of illumination, Weber's law behavior can be explained by the depletion of photopigment (reduced probability of the photon catch). There remains a substantial low-intensity domain for which light adaptation and Weber's law behavior are presumably mediated by other mechanisms within the cone. These might be the calcium- and/or cGMP-dependent mechanisms recently suggested by others.

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