Glutamate-gated chloride channel with glutamate-transporter-like properties in cone photoreceptors of the tiger salamander

Abstract

1. Using the patch-clamp technique, we investigated whether the glutamate-elicited current in mechanically isolated cone photoreceptors from the salamander retina is generated by a Cl- channel or a glutamate transporter. 2. The current reversed near the equilibrium potential for Cl-, was decreased by three Cl- channel blockers, 5-nitro-2-(3-phenyl-propylamino) benzoic acid, 4,4'-diisothiocyanostilbene-2,2'-disulfonate, and diphenylamine 2,2'-dicarboxylic acid, and was eliminated when gluconate was substituted for both internal and external Cl-, features consistent with the current being mediated by a Cl- channel. 3. The single-channel conductance of the Cl- channel was estimated by noise analysis of the glutamate-elicited current fluctuations to be 0.7 pS with an open time of 2 ms. 4. The magnitude of the current was dependent on both internal and external Na+ and K+, features consistent with the current being related to the activation of a glutamate transporter. Yet changes in their concentrations did not affect the reversal potential of the current. 5. Taken together with earlier reports on this current showing that it has a glutamate-transporter-like pharmacology, our results suggest that the glutamate-elicited current is carried by a Cl- channel but gated by a glutamate receptor whose pharmacology and ionic requirement resemble those previously described for glutamate transporters.