CAMP-Dependent Regulation of the Phototransduction Cascade in Cones

Abstract

Retinal photoreceptors – cones and rods – are able to undergo light adaptation over a wide range of illumination levels due to a complex of regulatory mechanisms. Among these, the best studied are calcium feedback circuits, which can explain about 50% of the actual regulation of light sensitivity. There are other regulatory mechanisms also able to adjust photoreceptor light responses depending on the illumination level, in particular regulation of the phototransduction cascade through the diurnal rhythm. During the dark phase of the diurnal cycle, cAMP levels in photoreceptors increase and rod sensitivity increases, which can be regarded as an adaptive action. In the case of cones, which operate at high illumination levels and make virtually no contribution to vision in twilight conditions, the increase in sensitivity in the dark phase may not have adaptive value. We report here our studies of how changes in [cAMP]in affect the operation of the phototransduction cascade in carp cones. Increases in [cAMP]in were obtained by incubating cells with the adenylate cyclase activator forskolin. Forskolin was found to slow both phases – the rise phase and the descending phase – of the light response in cones. As a result, cones, in contrast to rods, did not respond to forskolin with increases in light sensitivity but showed an almost two-fold reduction in the dark current. Thus, the reaction of the cone phototransduction cascade to increases in [cAMP]in was significantly different from the reaction in rods. This effect of [cAMP]in in cones may also have adaptive value – not in the form of increased sensitivity but as a reduction in the metabolic load on cells not functioning in the dark phase.