AMPA-preferring receptors mediate excitatory synaptic inputs to retinal ganglion cells

Pharmacological studies were performed to determine whether α-amino-3- hydroxy-5-methyl-4-isoazoleprionic acid (AMPA)- and/or kainate (KA)- preferring receptors mediate excitatory synaptic inputs to tiger salamander retinal ganglion cells. Excitatory postsynaptic currents (EPSCs), evoked either by light or by stimulating bipolar cells with puffs of K⁺, were measured using whole cell recording techniques in the tiger salamander retinal slice. The AMPA/KA component of the EPSCs was isolated by including antagonists of glycine-, γ-aminobutyric acid (GABA)- and NMDA-receptors in the bath. The AMPA-preferring receptor antagonists, 1-(4-amino-phenyl)-4- methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI-52466) and l-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4- dihydro-5H-2,3-benzodiazepine (GYKI-53665), reduced light-evoked EPSCs and K⁺ puff-evoked EPSCs amplitudes in a concentration-dependent manner. The IC₅₀, values for GYK1-52466 were 3.6 and 4.2 μM for the light- and puff- evoked responses, respectively. The more potent GYKI-53665 had IC₅₀ values of 0.7 μM for both the light- and puff evoked responses. KA activates both KA- and AMPA-preferring receptors. KA-evoked currents were completely blocked by 1040 μM GYKI-53665, indicating that little or no excitatory synaptic current was mediated by KA-preferring receptors. Concanavalin A, a compound that preferentially potentiates responses mediated by KA-preferring receptors, did not enhance either EPSCs or glutamate-evoked responses. By contrast, cyclothiazide, which selectively enhances AMPA-preferring receptor mediated responses, was found to enhance both EPSCs and glutamate-evoked currents. Our results indicate that the non-NMDA component of ganglion cell EPSCs is mediated by AMPA-preferring receptors and not significantly by KA- preferring receptors.