Functional characteristics of non‐NMDA‐type ionotropic glutamate receptor channels in AII amacrine cells in rat retina

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Abstract
The properties of non-NMDA glutamate receptor channels in AII amacrine cells were studied by patch-clamp recording from rat retinal slices. Application of AMPA or kainate to intact cells evoked currents with no apparent desensitization (EC50 of 118 μM for AMPA and 169 μM for kainate). Application of AMPA to patches evoked desensitizing responses with an EC50 of 217 and 88 μM for the peak and steady-state responses, respectively. Kainate-evoked responses of patches displayed no desensitization (EC50= 162 μM). Cyclothiazide strongly potentiated AMPA-evoked responses and the AMPA-receptor antagonist GYKI 53655 inhibited both AMPA- and kainate-evoked responses (IC50= 0.5–1.7 μM). Pre-equilibration with GYKI 53655 completely blocked the response to kainate and pretreatment with concanavalin A did not unmask a response mediated by kainate receptors. AMPA- and kainate-evoked currents reversed close to 0 mV. AMPA-evoked peak and steady-state response components in patches displayed linear and outwardly rectifying I–V relations with an RI (ratio of the slope conductances at +40 mV and -60 mV) of 0.96 ± 0.11 and 5.6 ± 1.3, respectively. AMPA-evoked currents displayed a voltage-dependent relaxation after steps to positive or negative membrane potentials, indicating that the outward rectification of the steady-state response is caused by a voltage-dependent kinetic parameter of channel gating. Under bi-ionic conditions ([Ca2+]out= 30 mm, [Cs+]in= 171 mm), the reversal potentials of AMPA- and kainate-evoked currents indicated channels with significant Ca2+ permeability (PCa/PCs= 1.9–2.1). Stationary noise analysis indicated that kainate activated channels with an apparent chord conductance of ∼9 pS. Non-stationary noise analysis indicated that AMPA and glutamate activated channels with apparent chord conductances of ∼9, ∼15, ∼23 and ∼38 pS. Discrete single-channel gating corresponding to chord conductances of ∼23 pS could be directly observed in some responses. Thus, our results indicate expression of high-affinity, voltage-sensitive AMPA receptors with significant Ca2+ permeability and relatively large single-channel chord conductances in AII amacrine cells.