Quantal properties of spontaneous EPSCs in neurones of the guinea-pig dorsal lateral geniculate nucleus.

1. Spontaneous non-NMDA glutamate receptor-mediated EPSCs were recorded with the whole-cell patch-clamp technique from twenty-six neurones in the dorsal lateral geniculate nucleus in thalamic slices from guinea-pig. 2. Amplitude distributions of the EPSCs were skewed towards larger values. The skewness could be accounted for by multiquantal properties. The multiquantal properties were most clearly demonstrated in four cells that had prominent peaks in the amplitude distribution, and peak separation approximately corresponding to the modal value. The amplitude distribution for all cells could be adequately fitted by a quantal model consisting of a sum of Gaussians with means equal to integer multiples of a quantal unit. The variance of each Gaussian was equal to the sum of the noise variance of the recordings and an additional non-negative variance which increased linearly with the number of the Gaussian in the series. The estimated mean quantal size was 152 +/- 37 pS. The estimated mean quantal coefficient of variation was 15%. Addition of tetrodotoxin did not significantly change any of the quantal parameters (n = 5). 3. The waveform of the EPSCs was similar for small and large events, and similar to that of events evoked by stimulation of retinal input fibres. There was a positive correlation between peak amplitude and rise time. This is the opposite of that expected if differences in electrotonic distances were to explain differences in amplitude. 4. The spontaneous EPSCs occurred randomly at an average frequency of 3.1 Hz. The events with amplitudes approximately equal to multiples of the quantal size were, in most cells, more numerous than could be accounted for by coincidence of randomly occurring events of quantal size. 5. The results indicate that spontaneous EPSCs can reflect more than a single quantum, and suggest that quantal events may be coupled due to action potential-independent near-synchronous multiquantal release of transmitter.