Cone-Like Functional Properties of cGMP-Gated Channels in Rod-Like Retinal Photoreceptors of Nocturnal Geckoes

Retinas of nocturnal geckoes only contain scotopic photoreceptors with rod-shaped outer segments and sensitivities to light similar to the one of retinal rods from other species of lower vertebrates. However, these cells are not rods, but ” originally derived from cones in cone-only retinas of ancestral diurnal geckoes as a consequence of evolutionary pressure that forced adaptation to nocturnal behavior. Several interesting adaptations of these rod-like cones have been studied to date; molecular biology and functional studies confirmed that several proteins of the phototransductive cascade display structural and functional properties that indicate their origin from cones rather than from rods. In this paper, we investigate, with whole cell voltage clamp in the detached outer segment preparation, the voltage rectification properties of photoreceptor cGMP-gated channels in three species, Gekko gecko, Tarentola mauritanica, and Hemidactylus frenatus. We show that the currentvoltage properties in the physiological voltage range are reminiscent of the ones of cGMP-gated channels from cones rather than from rods of other cold-blooded vertebrates. The origin and the relevance of the mechanisms investigated are discussed. In conclusion, in this paper, we provide a novel, functional evidence of the presence of mechanisms in rod-shaped photoreceptors from nocturnal geckos that are reminiscent of their origin from ancestral cones. The molecular analysis of the retinal cGMP gated channel recently performed in Gekko gecko identified cone-like structural features, which agree with our observations.