cAMP-Mediated Modulation of Functions of Green- and Blue-Sensitive Cones in Zebrafish

Although cyclic adenosine monophosphate (cAMP) is not a major secondary messenger in the visual transduction cascade in vertebrates, it may modulate photoreceptor functions. The effects of cAMP have been extensively studied in rods; however, its role in cones remains less understood. The aim of this study was to investigate the effects of increased levels of cAMP on the photoresponses of isolated blue- and green-sensitive cones in adult zebrafish (Danio rerio). To examine the effects of elevated cAMP on individual cone spectral types, photoreceptor currents were recorded using a suction pipette method. The adenylate cyclase activator forskolin was used to increase intracellular cAMP levels. Sensitivity and photoresponse parameters were compared before and after forskolin application. An increase in cAMP levels has similar effects on photoresponses of blue- and green-sensitive cones. Forskolin application to both types of cones resulted in a slight increase in sensitivity, with significant slowing of the phototransduction cascade shutdown processes and a marked increase in the integration time of photoresponses. These findings suggest that intracellular cAMP levels, which fluctuate in the retina during the diurnal cycle, can modulate cone function. The observed effects of cAMP are consistent with its action on one of its main putative targets, opsin kinases.