THE QUANTAL SIZE AT RETINOGENICULATE SYNAPSES DETERMINED FROM SPONTANEOUS AND EVOKED EPSCS IN GUINEA-PIG THALAMIC SLICES

1. To determine the quantal size at retinogeniculate synapses, spontaneous and evoked excitatory postsynaptic currents (EPSCs) were recorded in twelve neurones of the dorsal lateral geniculate nucleus in guinea-pig thalamic slices using the whole-cell patch-clamp technique. We limited our study to the fast non-N-methyl-D-aspartate (NMDA) component of the EPSCs by adding the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid to the perfusion medium. 2. Spontaneous EPSCs occurred at a frequency between 0.5 and 6.6 Hz (mean 2.5 Hz). The modal value of the peak conductance change of spontaneous excitatory events varied between cells from 102 to 179 pS. 3. EPSCs were evoked by electrical stimulation in the optic tract. The peak conductance change of EPSCs evoked by stimulation of a putative single input fibre ranged from 0.6 to 3.4 nS (mean 1.7 nS). 4. To resolve the quantal components of evoked EPSCs the external Ca2+ concentration was reduced and the external Mg2+ concentration increased for four cells. In this condition failures occurred and the amplitude histograms were multimodal with approximately eguidistant peaks. 5. These multimodal histograms could be fitted by a sum of Gaussian functions with mean values corresponding to integer multiples of the modal value of the spontaneous EPSCs for the same cell. Thus, the quantal size of evoked EPSCs was the same as the modal value of spontaneous EPSCs. The mean of the apparent quantal conductance change was 138 pS. The estimated number of quanta released by stimulating a putative single input fibre in the control condition ranged from 4 to 27 (mean 13). 6. Paired-pulse facilitation was demonstrated with low external Ca2+ concentration. The quantal size remained unchanged from the first to the second response, consistent with a presynaptic mechanism of facilitation.