Characterization of spontaneous excitatory synaptic currents in salamander XXXX cells.

@entity7343 @entity2227 @entity162 @entity1025 @entity2228 @entity2479 @entity41980 @entity143
1. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage-clamp conditions. Consistent with activation of non- @entity143 -type @entity2227 receptors, the sEPSCs reversed at potentials above 0 mV, were blocked by 1 microM CNQX and prolonged by 2 mM @entity41980 . 2. The peak conductance of the averaged sEPSCs (n = 70-400) was 130 +/- 60 pS (mean +/- S.D.; 17 cells, ranging from 70 to 290 pS). Amplitude distributions were skewed towards larger amplitudes. 3. The decay of individual and mean sEPSCs was exponential with a mean time constant (tau d) of 3.75 +/- 0.84 ms (n = 13), which was voltage independent. The 10-90% rise time of the sEPSCs was 1.30 +/- 0.44 ms (n = 13). There was no correlation between sEPSC rise time and tau d suggesting that dendritic filtering alone did not shape the time course of sEPSCs. 4. Light-evoked EPSCs in these @entity2479 cells are mediated by concomitant activation of @entity143 and non- @entity143 receptors; however, no @entity143 component was discerned in the sEPSCs, even when recording at -96 mV in @entity1025 -free solutions. The decay time course was not altered by 20 microM AP7, an @entity143 antagonist, nor was an @entity143 component unmasked by adding @entity2228 or @entity7343 . These results suggest that @entity143 and non- @entity143 receptors are not coactivated by a single vesicle of transmitter during spontaneous release, and thus are probably not colocalized in the postsynaptic membrane at the sites of spontaneous release. 5. The sEPSCs were an order of magnitude faster than the non- @entity143 receptor-mediated EPSCs evoked by light stimuli, and it is proposed that the EPSC time course is determined largely by the extended time course of release of synaptic vesicles from @entity162 . The quantal content of a light-evoked non- @entity143 receptor-mediated EPSC in an on-off cell is about 200 quanta.
