Multiple types of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

Spontaneous and light-evoked excitatory postsynaptic currents (sEPSCs and leEPSCs) in retinal ganglion cells of the larval tiger salamander were recorded under voltage clamp conditions from living retinal slices. sEPSCs were isolated from the spontaneous inhibitory postsynaptic currents (sIPSCs) by application of 100 M picrotoxin+1 microM strychnine. In addition to the previously reported sEPSCs [K. Matsui, N. Hosoi, M. Tachibana, Excitatory synaptic transmission in the inner retina: pair recordings of bipolar cells and neurons of the ganglion cell layer, J. Neurosci. 18 (1998) 4500-4510; W.R. Taylor, E. Chen, D.R. Copenhagen, Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells, J. Physiol. 486 (1995) 207-221] [which are equivalent to our fast AMPA receptor-mediated sEPSCs (fAMPAsEPSCs)], we found another type of AMPA receptor-mediated sEPSC with slower rise and decay time courses and larger peak amplitudes (sAMPAsEPSCs), and the NMDA receptor-mediated sEPSCs (NMDAsEPSCs) in ON-OFF ganglion cells. The frequency of all three types of sEPSCs is greatly reduced by cobalt (with zero calcium) and increased by hyperosmotic solution, suggesting that these events are mediated by calcium-dependent exocytosis of glutamatergic synaptic vesicles. The amplitude histograms of sEPSCs do not show multiple peaks, suggesting that larger events are not discrete multiples of elementary events, or quanta, of similar neurotransmitter contents, as in the neuromuscular junction [P. Fatt, B. Katz, Spontaneous subthreshold activity at motor nerve endings, J. Physiol. 117 (1952) 109-128]. The average I-V relations of the fAMPAsEPSCs and sAMPAsEPSCs were outward rectified with reversal potentials at -12.2 mV and -10.8 mV, and that of the NMDAsEPSCs was N-shaped with a reversal potential at -5.8 mV. The average conductance increase associated with a single fAMPAsEPSC, a single sAMPAsEPSC, and a single NMDAsEPSC were 163. 26+/-51.02 pS, 233.33+/-163.64 pS, and 37.5+/-50.0 pS at -110 mV; 241.67+/-22.92 pS, 444.90+/-469.94 pS, and 25.93+/-70.37 pS at -60 mV; and 440.48+/-183.33 pS, 1,192.68+/-651.22 pS, and 517.71+/-238. 24 pS at +30 mV, respectively. The average frequency of the three sEPSCs at +30 mV were 15 Hz, 3.7 Hz and 3.6 Hz, respectively. The rise time (time to peak) of fAMPAsEPSCs was 1.5+/-1.05 ms and the decay time could be fitted with a single exponential with an average time constant of 3.4+/-4.1 ms. The rise and decay time course of the sAMPAsEPSCs and NMDAsEPSCs were much slower and sawtooth-shaped, and each 'sawtooth' had time course and amplitude similar to those of individual fAMPAsEPSCs. We propose that each fAMPAsEPSC is mediated by single or synchronized multiples of glutamatergic synaptic vesicles from bipolar cells, and each sAMPAsEPSC or NMDAsEPSC is mediated by larger clusters of synaptic vesicles triggered by spontaneous calcium spikes in bipolar cell axon terminals [J. Burrone, L. Lagnado, Electrical resonance and calcium influx in the synaptic terminal of depolarizing bipolar cells from the goldfish retina, J. Physiol. 505 (1997) 571-584; D. Zenisek, G. Matthews, Calcium action potentials in retinal bipolar neurons, Vis. Neurosci. 15 (1998) 69-75].