Nocturnal geckoes’ retinas contain only scotopic photoreceptors with rod-shaped outer segments and light sensitivity close to that of retinal rods of other lower vertebrate organisms. These cells, however, are not rods, but are originally derived from cones of ancestral diurnal geckoes in cone-only retinas as a result of evolutionary pressure that forced adaptation to nocturnal behaviour. To date, several fascinating modifications of these rod-like cones have been studied; molecular biology and functional studies have reported that several phototransductive cascade proteins exhibit structural and functional properties suggesting their origin from cones rather than rods. In this paper, we examine the voltage rectification properties of photoreceptor cGMP-gated channels in three species, Gekko gecko, Tarentola mauritanica, and Hemidactylus frenatus, with whole cell voltage clamp in the detached outer segment preparation. In the physiological voltage spectrum, we show that the current-voltage properties are reminiscent of those of cGMP-gated channels from cones rather than from rods of other cold-blooded vertebrates. The sources and the importance of the examined processes are discussed. In conclusion, we provide novel, functional evidence in this paper of the existence of mechanisms in rod-shaped nocturnal gecko photoreceptors that remember their origin from ancestral cones. Recently performed in Gekko gecko, the molecular analysis of the retinal cGMP gated channel identified cone-like structural features, which agree with our observations.
Dipartimento di Scienze Biomediche Metabolismo e Neuroscienze, Università di Modena e Reggio Emilia, via Campi 287, 41125 Modena, Italy.
Università degli Studi della Repubblica di San Marino, via Consiglio dei Sessanta 99, 47891 Dogana, Repubblica di San Marino.
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