Ionotropic glutamate receptors in isolated horizontal cells of the rabbit retina

With the use of the whole-cell voltage-clamp technique, we have recorded the currents induced by ionotropic glutamate receptor agonists on isolated axonless horizontal cells (HC) of rabbit retina. Bath application of the non-N-methyl-D-aspartate receptor agonists: kainate (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and L-glutamate (GLU) produced an increase in the conductance for non-selective cations. All the isolated horizontal cells responded to GLU, AMPA and KA. Responses elicited by GLU and AMPA but not KA exhibited a concentration-dependent desensitization. Application of N-methyl-D-aspartate (NMDA) evoked no responses. The rank order affinities of the agonists as estimated from EC50 values were AMPA > GLU > KA. Whereas KA had the lowest affinity of the agonists tested, it produced the largest currents. Hill coefficients of the concentration-response data were near 1 for AMPA, and 2 for KA and GLU. Coapplication of AMPA with cyclothiazide (CTZ) blocks AMPA receptor desensitization, and enhanced its effects on conductance. However, CTZ did not change the KA -induced conductances. In all cells tested, 6,7-dinitroquinoxaline (DNQX) completely and reversibly blocked the effects of KA and AMPA. The KA- and AMPA-induced currents were also completely blocked by 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), a selective AMPA receptor antagonist. These results indicate that the responses to glutamate agonists in HC were mediated almost exclusively by AMPA receptors. Our study indicates that AMPA receptors play a fundamental role in mediating the synaptic input into rabbit horizontal cells.