Unitary EPSCs of corticogeniculate fibers in the rat dorsal lateral geniculate nucleus in vitro.

To investigate unitary corticogeniculate excitatory postsynaptic currents (EPSCs), whole cell patch-clamp recordings were obtained from 20 principal cells in slices of the dorsal lateral geniculate nucleus (dLGN) of DA-HAN rats. EPSCs, evoked by electrical stimulation of corticogeniculate axons, had size distributions with one or more quantal peaks. Gaussian curves fitted to such distributions gave a mean quantal size (q) of -5.0 +/- 0.7 (SD) pA for the EPSCs. Paired-pulse ratio (EPSC2/EPSC1) was 3.3 +/- 0.9 for stimuli separated by 40 ms. The mean quantal size was similar for facilitated EPSCs (-5.2 +/- 0.8 pA), implying an increase in mean quantal content (m). Most corticogeniculate axons were capable of releasing only one or two quanta onto individual principal cells. Mean resting release probability (p) was low, 0.09 +/- 0.04. Binomial models, with the same n but increased p, could account for both the basal and facilitated EPSC size distributions in 6/8 cells. It is suggested that the low resting efficacy of corticogeniculate synapses serves to stabilize this excitatory feedback system. The pronounced facilitation in conjunction with large convergence from many corticogeniculate cells would provide a transient, potent excitation of dLGN cells, compliant with the idea of a visually driven neuronal amplifier.