1. With the use of the whole cell voltage-clamp technique, I have recorded the current responses to ionotropic glutamate receptor agonists of rod bipolar cells in vertical slices of rat retina. Rod bipolar cells constitute a single population of cells and were visualized by infrared differential interference contrast video microscopy. They were targeted by the position of their cell bodies in the inner nuclear layer and, after recording, were visualized in their entirety by labeling with the fluorescent dye Lucifer yellow, which was included in the recording pipette. To study current-voltage relationships of evoked currents, voltage-gated potassium currents were blocked by including Cs+ and tetraethylammonium+ in the recording pipette. 2. Pressure application of the non-N-methyl-D-aspartate (non-NMDA) receptor agonists kainate and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) from puffer pipettes evoked a long-latency conductance increase selective for chloride ions. When the intracellular chloride concentration was increased, the reversal potential changed, corresponding to the change in equilibrium potential for chloride. The response was evoked in the presence of 5 mM Co2+ and nominally O mM Ca2+ in the extracellular solution, presumably blocking all external Ca2(+)-dependent release of neurotransmitter. 3. The long latency of kainate-evoked currents in bipolar cells contrasted with the short-latency currents evoked by gamma-aminobutyric acid (GABA) and glycine in rod bipolar cells and by kainate in amacrine cells. 4. Application of NMDA evoked no response in rod bipolar cells. 5. Coapplication of AMPA with cyclothiazide, a blocker of agonist-evoked desensitization of AMPA receptors, enhanced the conductance increase compared with application of AMPA alone. Coapplication of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the response to kainate and AMPA, indicating that the response was mediated by conventional ionotropic glutamate receptors. 6. The conductance increase evoked by non-NMDA receptor agonists could not be blocked by a combination of 100 microM picrotoxin and 10 microM strychnine. Application of the GABAC receptor antagonist 3-aminopropyl (methyl)phosphinic acid (3-APMPA) strongly reduced the response, and coapplication of 500 microM 3-APMPA and 100 microM picrotoxin completely blocked the response. These results suggested that the conductance increase evoked by non-NMDA receptor agonists was mediated by release of GABA and activation of GABAC receptors, and most likely also GABAA receptors, on rod bipolar cells. 7. Kainate responses like those described above could not be evoked in bipolar cells in which the axon had been cut somewhere along its passage to the inner plexiform layer during the slicing procedure. This suggests that the response was dependent on the integrity of the axon terminal in the inner plexiform layer, known to receive GABAergic synaptic input from amacrine cells. 8. The results indicate that ionotropic glutamate receptors are not involved in mediating synaptic input from photoreceptors to rod bipolar cells and that an unconventional mechanism of GABA release from amacrine cells might operate in the inner plexiform layer.
Functional NMDA receptors are expressed by both AII and A17 amacrine cells in the rod pathway of the mammalian retina