The visual transduction pathway in vertebrate photoreceptors transforms a light stimulus entering the photoreceptor outer segments into an electrical response at the synapses of rod and cone photoreceptor cells. This process is mediated by complex biochemical pathways that precisely regulate cGMP levels, thereby controlling the extent, duration, and adaptation of the photoreceptor to the light stimulus. This review first summarizes the major mechanisms of regulating cytoplasmic cGMP levels (synthesis, degradation, buffering, and efflux) as well as the primary targets of action of cGMP (cyclic nucleotide-gated ion channels, cGMP-dependent protein kinase, and cGMPregulated phosphodiesterases). This information is applied to our current understanding of how these processes operate in the signal-transducing outer segment of rod and cone photoreceptors to carry out visual excitation, recovery, and adaptation in response to light stimulation.
