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Journal of Neuroscience, Vol 14, 1091-1105, Copyright © 1994 by Society for Neuroscience
Light adaptation and photopigment bleaching in cone photoreceptors in situ in the retina of the turtle
DA Burkhardt
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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