Functional organization of cone bipolar cells in the rat retina

The responses of cone bipolar cells in slices of rat retina to ionotropic glutamate receptor agonists were recorded with the whole cell voltage-clamp technique in the presence of 5 mM Co2+ and nominally 0 mM Ca2+ extracellularly. Application of the non-N-methyl-D-aspartate (non-NMDA) receptor agonists kainate and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate had a series of effects on cone bipolar cells (types 1-9), and the different cell types could be classified as ON- or OFF-type cells according to which type(s) of responses they displayed. First, direct responses were observed in cell types 1-4 as short-latency inward currents at -70 mV with reversal potentials (E(rev)s) close to 0 mV, characteristic of nonselective cation channels. Second, some cells, among types 5-9, did not display short-latency inward currents to kainate at -70 mV. Other type 5-8 cells displayed short-latency kainate responses, but the currents could not be reversed (E(rev) of +40 mV or greater). I suggest that these responses are conveyed to the cone bipolar cells through gap junctions, most likely with AII amacrine cells. The lack of reversal is likely due to a substantial voltage drop across the gap junctions resulting in an inadequate voltage control of AII amacrine cells when the recording pipette is on the cone bipolar cell. Kainate responses recorded directly from AII amacrine cells had E(rev) approximately 0 mV. Third, long-latency indirect responses selective for chloride ions (E(rev) approximately chloride equilibrium potential) were observed in many cone bipolar cells during longer-lasting application of kainate. The long-latency response component was suppressed by coapplication of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist picrotoxin and the GABA(C) receptor antagonist 3-aminopropyl(methyl)phosphinic acid. This long-latency component was absent in axotomized bipolar cells, suggesting that it was due to external Ca2+-independent release of GABA onto the axon terminals of the cone bipolar cells. All kainate-evoked response components were blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Application of NMDA evoked no response in cone bipolar cells. These results suggest that cone bipolar cells types 1-4 are OFF cone bipolar cells, whereas cone bipolar cells types 5-9 are ON cone bipolar cells.