AMPLITUDE AND TIME-COURSE OF EVOKED AND SPONTANEOUS SYNAPTIC CURRENTSIN RAT SUBMANDIBULAR GANGLION-CELLS

1. Excitatory postsynaptic currents (EPSCs) were recorded in rat subma ndibular ganglion cells in vitro using the two-electrode voltage clamp technique. 2. The peak amplitude of EPSCs evoked by nerve impulses in single presynaptic fibres varied between 1.2 and 9.8 nA in different cells (mean = 4.6 +/- 2.6 nA; n = 23; -80 mV membrane potential; 22-25 degrees C). 3. Experiments were performed to re-investigate a previou s hypothesis that different mechanisms underlie the generation of evok ed versus spontaneous quantal EPSCs in submandibular cells. This hypot hesis was based on the observation of different time courses of evoked and spontaneous EPSCs. 4. In agreement with previous studies, the tim e course of the decay phase of evoked EPSCs was described by the sum o f two exponentials, with time constants tau(1) and tau(2) of 6.9 +/- 0 .7 and 34.4 +/- 7.7 ms, respectively (n = 23; -80 mV membrane potentia l). 5. The double-exponential decay of evoked EPSCs persisted when tra nsmitter release was reduced by bath addition of 100 mu M cadmium chlo ride to the level of failures, one or several quanta. 6. Spontaneous E PSCs exhibited mean amplitudes of 81 +/- 24 pA (n = 5 cells; -80 mV me mbrane potential), and displayed an extremely wide range of peak ampli tudes in the same cell (mean coefficient of variation (c.v.) = 0.37 +/ - 0.09; n = 5 cells). In contrast to a previous report (see below), th e decay phase of spontaneous EPSCs was found to exhibit a double-expon ential time course with time constants similar to those of the evoked EPSC recorded in the same cell. These results indicate that evoked and spontaneously released quanta of transmitter most probably act on the same population of postsynaptic receptors in submandibular ganglion c ells. There is a large variability in the peak amplitudes of quantal E PSCs recorded in the same cell. This large variability is not due to e lectrotonic effects, since these cells lack dendrites.