Modification of Cyclic Nucleotide–Gated Ion Channels by Ultraviolet Light

Abstract

We irradiated cyclic nucleotide–gated ion channels in situ with ultraviolet light to probe the role of aromatic residues in ion channel function. UV light reduced the current through excised membrane patches from Xenopus oocytes expressing the α subunit of bovine retinal cyclic nucleotide–gated channels irreversibly, a result consistent with permanent covalent modification of channel amino acids by UV light. The magnitude of the current reduction depended only on the total photon dose delivered to the patches, and not on the intensity of the exciting light, indicating that the functionally important photochemical modification(s) occurred from an excited state reached by a one-photon absorption process. The wavelength dependence of the channels' UV light sensitivity (the action spectrum) was quantitatively consistent with the absorption spectrum of tryptophan, with a small component at long wavelengths, possibly due to cystine absorption. This spectral analysis suggests that UV light reduced the currents at most wavelengths studied by modifying one or more “target” tryptophans in the channels. Comparison of the channels' action spectrum to the absorption spectrum of tryptophan in various solvents suggests that the UV light targets are in a water-like chemical environment. Experiments on mutant channels indicated that the UV light sensitivity of wild-type channels was not conferred exclusively by any one of the 10 tryptophan residues in a subunit. The similarity in the dose dependences of channel current reduction and tryptophan photolysis in solution suggests that photochemical modification of a small number of tryptophan targets in the channels is sufficient to decrease the currents.

Reversible modulation of GABAA receptor‐mediated currents by light is dependent on the redox state of the receptor

“…Additional evidence from studies involving other ion channels, strengthens our hypothesis that light and redox modulation of ion channels are structurally linked. For example, it has been shown that cyclic nucleotide gated channel currents are depressed following both light and exposure to reducing agents (Broillet & Firestein, 1996; Middendorf et al ., 2000). We have shown in this study that glycine receptor‐mediated responses, which have been demonstrated to be somewhat depressed following exposure to a reducing agent (Pan et al ., 1995), may also be decreased following a brief pulse of light (Fig.…”

Temperature-dependent Activation of Neurons by Continuous Near-infrared Laser

“…Photons in this wavelength range carry the right amount of energy to break chemical bonds, especially those of aromatic amino acids such as tryptophan, tyrosine and phenylalanine, or to catalyze crosslinking of DNA molecules [6]. Irradiation by UV-laser has been shown to quickly destroy tryptophan residues of cyclic nucleotide-gated channels in cell-free membrane patches, leading to irreversible reduction of ligand-induced channel currents [7, 8]. UV-laser irradiation can also reduce membrane excitability of myelinated nerve fibers, likely due to both a loss of Na permeability and Na channel inactivation [9].…”

G protein activation by uncaging of GTP-γ-S in the leech giant glial cell

“…Since we studied effects that were relatively slow in this large cell, we used moderate UV illumination to avoid the side effects of strong UV light. For example, ion channels can be sensitive to UV, and strong illumination may therefore result in a change in neuronal activity (Hof and Fox, 1983;Middendorf et al, 2000). We could observe changes in membrane potential and firing frequency of several neurons in the leech central nervous system when we used strong UV pulses.…”