A cGMP-gated cation channel and phototransduction in depolarizing photoreceptors of the lizard parietal eye

Photoreceptors of the lizard parietal eye, unlike rods and cones but like most invertebrate photoreceptors, respond to light under dark-adapted conditions with a depolarization. Using excised-patch recordings: we have nonetheless found a cGMP-gated, non-selective cation channel present at high density at the presumptive light-sensitive part (the outer segment) of these cells. This channel resembles the rod cGMP-gated channel in its activation characteristics, and by showing a relative non-selectivity among alkali monovalent cations, a high permeability to Ca²⁺, a high sensitivity to L-cis-diltiazem, as well as a negative modulation by Ca²⁺-calmodulin. This channel appears to mediate phototransduction by opening in the light to produce the depolarizing response.