Membrane currents evoked by ionotropic glutamate receptor agonists in rod bipolar cells in the rat retinal slice preparation. Hartveit, E. University of Oslo, Department of Neurophysiology, Oslo, Norway.
APStracts 3:0029N, 1996.
SUMMARY AND CONCLUSIONS
1. Using the whole-cell patch clamp technique, I have recorded the current responses of rod bipolar cells in vertical slices of rat retina to ionotropic glutamate receptor agonists. Rod bipolar cells constitute a single population of cells and were visualized by infra-red differential interference contrast video microscopy. They were targeted by the position of their cell bodies in the inner nuclear layer and, following recording, were visualized in their entirety by labeling with the fluorescent dye Lucifer yellow which was included in the recording pipette. In order 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 (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 Co 2+ and nominally 0 mM Ca 2+ in the extracellular solution, presumably blocking all external Ca 2+ -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. Co-application of AMPA with cyclothiazide, a blocker of agonist- evoked desensitization of AMPA receptors, enhanced the conductance increase compared to application of AMPA alone. Co-application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) 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 [mu] M picrotoxin and 10 [mu] M strychnine. Application of the GABA C receptor antagonist 3- aminopropyl(methyl)phosphinic acid (3-APMPA) strongly reduced the response and co-application of 500 [mu] M 3-APMPA and 100 [mu] M 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 GABA C receptors, and most likely also GABA A 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.

Received 13 October 1995; accepted in final form 8 January 1995.
APS Manuscript Number J690-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 February 96