Light-Evoked Excitatory Synaptic Currents of X-Type Retinal Ganglion Cells

Abstract

The excitatory amino acid receptor (EAAR) types involved in the generation of light-evoked excitatory postsynaptic currents (EPSCs) were examined in X-type retinal ganglion cells. Using isolated and sliced preparations of cat and ferret retina, the light-evoked EPSCs of X cells were isolated by adding picrotoxin and strychnine to the bath to remove synaptic inhibition. N-methyl-d-aspartate (NMDA) receptors contribute significantly to the light-evoked EPSCs of on- andoff-X cells at many different holding potentials. An NMDA receptor contribution to the EPSCs was observable when retinal synaptic inhibition was either normally present or pharmacologically blocked. NMDA receptors formed 80% of the peak light-evoked EPSC at a holding potential of −40 mV; however, even at −80 mV, 20% of the light-evoked EPSC was NMDA-mediated. An α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor–mediated component to the light-evoked EPSCs predominated at a holding potential of −80 mV. The light-evoked EPSC was blocked by the AMPA receptor-selective antagonist GYKI52466 (50–100 μM). The AMPA receptor–mediated EPSC component had a linear current-voltage relation. AMPA receptors form the main non-NMDA EAAR current on bothon- and off- X ganglion cell dendrites. When synaptic transmission was blocked by the addition of Cd²⁺to the Ringer, application of kainate directly to ganglion cells evoked excitatory currents that were strongly blocked by GYKI52466. Experiments using selective EAAR modulators showed the AMPA receptor–selective modulator cyclothiazide potentiated glutamate-evoked currents on X cells, while the kainate receptor–selective modulator concanavalin A (ConA) had no effect on kainate-evoked currents. Whereas the present study confirms the general notion that AMPA EAAR-mediated currents are transient and NMDA receptor–mediated currents are sustained, current-voltage relations of the light-evoked EPSC at different time points showed the contributions of these two receptor types significantly overlap. Both NMDA and AMPA EAARs can transmit transient and sustained visual signals in X ganglion cells, suggesting that much signal shaping occurs presynaptically in bipolar cells.