NOISE-ANALYSIS OF THE GLUTAMATE-ACTIVATED CURRENT IN PHOTORECEPTORS

The glutamate-activated current in photoreceptors has been attributed both to a sodium/glutamate transporter and to a glutamate-activated ch loride channel. We have further studied the glutamate-activated curren t in single, isolated photoreceptors from the tiger salamander using n oise analysis on whole-cell patch-clamp recordings. In cones, the curr ent is generated by chloride channels with a single-channel conductanc e of 0.7 pS and an open lifetime of 2.4 ms. The number of channels per cell is in the range of 10,000-20,000. Activation of the channels req uires the presence of both glutamate and sodium. The single-channel co nductance and the open lifetime of the channel are independent of the external concentration of glutamate and sodium. External glutamate and sodium affect only the opening rate of the channels. D,L-Threo-3-hydr oxyaspartate (THA), a glutamate-transport blocker, is shown to be a pa rtial agonist for the channel. The single-channel conductance is the s ame regardless of whether glutamate or THA is the ligand, but the open lifetime of the channel is only 0.8 ms with THA as ligand. The glutam ate-activated current in rods has a similar single-channel conductance (0.74 pS) and open lifetime (3 ms). We propose a kinetic model, consi stent with these results, to explain how a transporter can simultaneou sly act both as a sodium/glutamate-gated chloride channel and a glutam ate/sodium cotransporter.