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Department of Pharmacology, University College London.

1. Single-channel currents evoked by excitatory amino acids have been examined in outside-out patches from large cerebellar neurones (including Purkinje cells) in tissue culture. L-Glutamate (3-10 microM), L-aspartate (3-10 microM), NMDA (N-methyl-D-aspartate, 10-50 microM), ibotenate (50 microM), quisqualate (3-50 microM), and kainate (3-50 microM) all produced single-channel currents with multiple amplitudes. 2. Single-channel currents recorded over a range of patch potentials had a mean interpolated reversal potential of -3.8 +/- 0.5 mV. The directly resolvable multiple conductance levels could be classified into five main groups, with mean values (averaged for all agonists) of: 47.9 +/- 0.7, 38.5 +/- 0.8, 27.8 +/- 1.4, 18.2 +/- 0.5 and 8.3 +/- 0.6 pS. 3. From the relative areas under current amplitude histograms it was estimated that the percentage of openings with conductances greater than 30 pS was about 83% with NMDA, 79% with glutamate and 78% with aspartate. In some patches, the majority of greater than 30 pS events evoked by these agonists were to the maximum conductance of 48 pS, whereas in other patches there were more 38 pS openings than 48 pS openings. Only 27% of quisqualate openings, and about 10% of kainate openings, were greater than 30 pS. 4. Of the small amplitude (less than 20 pS) events, 93% of quisqualate openings were to the 8 pS level whereas approximately 87% of less than 20 pS currents produced by NMDA, glutamate and aspartate were to the 18 pS level (the remainder being 8 pS). Direct transitions could occur between certain levels (including events above and below 30 pS) suggesting that these are sublevels of multiple-conductance channels. The most frequently occurring transitions were between the 48 and 38 pS levels, and the 38 and 18 pS levels. 5. Channel openings occurred in bursts, within which individual openings were separated either by brief closures (gaps), or by direct transitions between the multiple conductance levels. The briefest of these gaps (less than 200-400 microseconds) could represent a mixture of transitions to lower conductance levels as well as partially resolved complete shuttings. The mean duration of the longer gaps within bursts, thought to represent complete but partially resolved shuttings was 1.05 +/- 0.25 ms (pooled for all agonists). 6. Burst-length distributions could be fitted with the sum of three exponentials. The briefest component may have arisen from brief single openings. The two slower components probably reflect the existence of two kinetically distinct open states.(ABSTRACT TRUNCATED AT 400 WORDS)

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