A GLUTAMATE-ELICITED CHLORIDE CURRENT WITH TRANSPORTER-LIKE PROPERTIES IN ROD PHOTORECEPTORS OF THE TIGER SALAMANDER

Glutamate, when puffed near the synaptic terminals, elicits a current in rod photoreceptors. The current is strongly dependent upon both the intracellular and extracellular chloride concentration: its reversal potential follows the predicted Nernst potential for a chloride permea ble channel. The glutamate-elicited current also requires the presence of extracellular sodium. This glutamate-elicited current is pharmacol ogically like a glutamate transporter: it is elicited, in order of eff icacy, by L-glutamate, L-aspartate, L-cysteate, D-aspartate, and D-glu tamate, all shown to activate glutamate transport in other systems. Fu rthermore, it is reduced by the glutamate transport antagonists dihydr okainate (DHKA) and D,L-threo-3-hydroxyaspartate (THA). THA, when appl ied alone, elicits a current similar to that elicited by glutamate. Th e current cannot be activated by the glutamate receptor agonists kaina te, quisqualate, NMDA and APB, nor can it be blocked by the glutamate receptor antagonists CNQX and APV. Thus, the current does not appear t o be mediated by a conventional glutamate receptor. Taken together, th e ionic dependence and pharmacology of this current suggest that it is generated by glutamate transporter coupled to a chloride channel.