Journal of Neuroscience, Vol 10, 1967-1973, Copyright © 1990 by Society for Neuroscience
Activation kinetics of retinal cones and rods: response to intense flashes of light
S Hestrin and JI Korenbrot
Department of Physiology, School of Medicine, University of California, San Francisco 94143.
Cone photoreceptors are less sensitive to light and the duration of their
photoresponse is shorter than that of rods. In salamander rods and cones,
we identified 3 components in membrane currents activated by bright flashes
of light: an early receptor current (ERC) resulting from charge
displacement within visual pigments, a saturation photocurrent generated by
the closure of the cGMP-sensitive channels, and a putative Na-Ca exchanger
current. The time courses of both the ERC and the onset of the saturation
photocurrent were similar in rods and cones. The putative Na-Ca exchanger
current, on the other hand, is 4- to 8-fold faster in cones. The onset of
the saturation photocurrent consisted of a delay followed by a fast
relaxation with an exponential time course. In both photoreceptor types the
delay and the time course of the fast relaxation are dependent on light
intensity and reach a limiting value when about 1% of the photopigment is
bleached. The limiting value of the delay, about 8 msec, and of the
relaxation time constant, about 2 msec, are nearly identical in rods and
cones. The near identity of these parameters implies that at least 2
kinetic steps in the activation response of rods and cones are
quantitatively similar. These findings suggest that the functional
differences between rods and cones may arise from disparities in the
processes that restore the components of the phototransduction cascade to
their dark level and not from differences in the activation processes.
Previous Article | Next Article
