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

J. T. Finn, E. C. Solessio and K.-W. Yau
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J. T. Finn: Johns Hopkins University
E. C. Solessio: University of Utah Health Science Center
K.-W. Yau: Johns Hopkins Univeristy School of Medicine

Nature, 1997, vol. 385, issue 6619, 815-819

Abstract: Abstract Rods and cones of the two vertebrate lateral eyes hyperpolarize when illuminated, a response generated by a cyclic GMP cascade leading to cGMP hydrolysis and consequently the closure of cGMP-gated, non-selective cation channels that are open in darkness1–4. Lizards and other lower vertebrates also have a parietal (third) eye5, which contains ciliary photoreceptors that under dark-adapted conditions depolarize to light instead6. Depolarizing light responses are characteristic of most invertebrate rhabdomeric photoreceptors, and are thought to involve a phosphoinositide signalling pathway (see, for example, refs 7–9). Surprisingly, we have found in excised membrane patches a cGMP-gated channel that is selectively present at high density on the outer segment (the presumptive light-sensitive part) of the parietal eye photoreceptor. Like the light-activated channel of the cell, it is non-selective among cations. Inositol trisphosphate (InsP3) had no effect on the same membrane patches. These findings suggest that the photoreceptors of the parietal eye, like rods and cones, use a cGMP cascade and not an InsP3-mediated pathway for phototransduction, but in this case light increases cGMP. A unifying principle of evolutionary significance emerges: that phototransductions in various ciliary photoreceptors, whether hyperpolarizing or depolarizing, uniformly use a cGMP cascade and a cGMP-gated channel to generate the light response, although there are rich variations in the details.

Date: 1997
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DOI: 10.1038/385815a0

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