ANALYSIS OF EXCITATORY AND INHIBITORY SPONTANEOUS SYNAPTIC ACTIVITY IN MOUSE RETINAL GANGLION-CELLS

Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs an d sEPSCs) were identified and characterized with whole cell and perfor ated patch voltage-clamp recordings in adult mouse retinal ganglion ce lls. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and gamma-aminobuty ric acid-A (GABA(A)) receptors. One-half (7/14) of the cells also rece ived glycinergic spontaneous synaptic inputs. Both GABA(A) and glycine receptor-mediated sIPSCs had rise times (10-90%) of <1 ms. The decay times of the GABA(A) receptor-mediated sIPSCs were comparable with tho se of the glycine receptor-mediated sIPSCs. The average decay time con stant for monoexponentially fitted sIPSCs was 63.2 +/- 74.1 ms (mean /- SD, n = 3278). Glutamate receptor-mediated sEPSCs had an average ri se time of 0.50 +/- 0.20 ms (n = 109) and an average monoexponential d ecay time constant of 5.9 +/- 8.6 ms (n = 2705). Slightly more than tw o-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexpo nential. The amplitudes of the spontaneous synaptic events were not co rrelated with rise times, suggesting that the electrotonic filtering p roperties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the deca y time constants of the glutamate and GABA(A)/glycine receptor-mediate d spontaneous synaptic events. The amplitudes of sEPSCs were similar t o those recorded in tetrodotoxin (TTX), consistent with the events mea sured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control s aline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size gre ater than one. The rates of sIPSCs and sEPSCs were determined under eq uivalent conditions by recording with perforated patch electrodes at p otentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSC s frequency ratio of 0.96 +/- 0.77 (n = 28) and another group had an a verage ratio of 6.63 +/- 0.82 (n = 7). These findings suggest that a s ubset of cells is driven much more strongly by excitatory synaptic inp uts. We propose that this subset of cells could be OFF ganglion cells, consistent with the higher frequency of spontaneous action potentials found in OFF ganglion cells in other studies.