A cGMP-gated cation channel in depolarizing photoreceptors of the lizard parietal eye.
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 darkness. Lizards and other lower vertebrates also have a parietal (third) eye, which contains ciliary photoreceptors that under dark-adapted conditions depolarize to light instead. 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.
Finn
J T
JT
Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21205, USA.
Solessio
E C
EC
Yau
K W
KW
D000818:Animals; D002413:Cations, Divalent / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D006152:Cyclic GMP / Q000502:physiology*; D054815:Cyclic Nucleotide-Gated Cation Channels; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D007473:Ion Channels / Q000502:physiology*; D008027:Light; D008116:Lizards / Q000033:anatomy & histology / Q000502:physiology*; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D010786:Photoreceptor Cells / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*
D002413:Cations, Divalent; D054815:Cyclic Nucleotide-Gated Cation Channels; D007473:Ion Channels; D006152:Cyclic GMP