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Fig. 7. Quantal analysis of GABA-mediated glial ePSCs suggests a small number of release sites. (A) Averaged response (120 stimuli, 150 microseconds) in the presence of NBQX (10 µM) and D-APV (25 µM). Peak amplitude (4.8 pA) was reached 5.4 milliseconds after the stimulus. (B) ePSC amplitudes were plotted against time (left), and the cumulative fraction was calculated (right). (C) Eight successively recorded exemplary traces document the presence of failures and ePSCs in GluR cells. (D) The ePSC amplitude histogram resembles a binomial distribution, characteristic of a small number of release sites. The individual ePSC amplitudes were determined as the difference between the averaged currents measured between 4.9 and 5.9 milliseconds after stimulation, and the average of 1 milliseconds corresponding baseline current. Inset depicts the amplitude histogram of current noise, taken from baseline traces before stimulation (–100 milliseconds to 0 milliseconds in C). Evaluation of 600 milliseconds baseline noise (7373 points) revealed a Gaussian distribution around 0 pA with a half width of 1.03 pA. (E) shows the superposition of ePSC amplitude histograms of 4 GluR cells (interstimulus interval, 10 seconds). [Cl^–]_i was always 27 mM.

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