Activation kinetics of retinal cones and rods: response to intense flashes of light

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.