Cone-Like Functional Properties of cGMPGated 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.