ABSTRACT
Phosphodiesterase (PDE6) inactivation is temporally coordinated with activation of the photoreceptor cyclase so that cyclic nucleotide levels recover quickly to restore the dark-adapted state of the photoreceptor. Rod and cone photoreceptors of the vertebrate retina utilize cyclic nucleotides as the primary intracellular messenger for converting photic stimuli into an electrical response. The first step in vertebrate vision is the photoisomerization of 11-cis retinal to the all-trans isomer that is covalently bound to the G-protein-coupled receptor, rhodopsin. The initial events of visual perception occur in the retina where two distinct classes of photoreceptor cells are responsible for converting photon absorption into an electrical response that is propagated to second-order retinal neurons and eventually to the optic nerve. Reinhibition of PDE6 catalysis is accompanied by stimulation of photoreceptor guanylate cyclase activity to restore guanosine 3',5' -monophosphate levels to their dark-adapted state. Low levels of PDE6 catalytic subunit transcript have been reported in nonphotoreceptive tissues.
