| OFF cone bipolar cell | Hartveit (H) | For the types CB1, CB2, CB3, and CB4, kainate (or AMPA) evoked a short-latency inward current, characteristic of a direct response, in every cell tested. In addition, for a number of cells there was evidence that kainate evoked an indirect response mediated by GABAergic input onto the axon terminals of these cells. This was observed as a gradual drift of the Erev from ~0 mV toward ECl during longer-lasting application of kainate. Indeed, with perfect voltage clamp, the observed I-V curve would simply be the linear sum of the I-V curves for the two response components (direct and indirect). This drift of Erev was blocked when kainate was coapplied with the GABAA receptor antagonist picrotoxin and the GABAC receptor antagonist 3-APMPA, and it was not observed in axotomized bipolar cells. In both of these conditions the evoked current reversed close to 0 mV, as expected for nonselective cation channels integral to ionotropic glutamate receptors. Furthermore, no cells classified as CB1-CB4 ever responded to kainate with an outward current at -70 mV, the response expected for an APB type of glutamate receptor. Taken together, these results suggest that the cone bipolar cells morphologically classified as CB1-CB4 correspond physiologically to OFF cone bipolar cells, expressing conventional ionotropic non-NMDA glutamate receptors.
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