1. Application of non-NMDA (non-N-methyl-D-aspartate) receptor agonists onto outside-out patches of cerebellar granule cells gave two characteristic types of response (in different patches) which we have referred to as 'high conductance' and 'low conductance' responses. At a qualitative level these patches could be readily distinguished by the size of the noise increase accompanying their membrane currents. 2. In high conductance patches both AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate gave discrete single-channel conductances (10-30 pS), while in low conductance patches, AMPA produced small discrete events (6-10 pS), and kainate opened channels with conductances too small to be directly resolved. All patches examined contained NMDA receptor channels with characteristic 50 and 40 pS conductance levels. 3. Despite the marked differences in single-channel conductances, kainate dose-response curves constructed for high and low conductance patches had similar EC50 values of approximately 150 muM. 4. Spectral analysis of low conductance kainate responses gave an estimated channel conductance of approximately 1.5 pS. In these same low conductance patches AMPA produced discrete openings with two conductance levels; their mean conductances (and relative proportions) were 6 (87 %) and 10 pS (13 %). 5. In high conductance patches, glutamate (10-30 muM), AMPA (3-10 muM), and kainate (10-30 muM), each activated non-NMDA channels with three multiple conductance levels. The amplitudes of these conductance levels (approximately 10, 20 and 30 pS) were similar for each of the agonists, and their relative proportions (i.e. areas in the amplitude histograms) were constant for all three agonists. In addition, the relative proportion of levels was constant between patches, and all three levels were invariably present. These observations are all consistent with the idea that the three multiple conductances originate from a single receptor channel, activated by AMPA, kainate and glutamate. 6. Non-NMDA single-channel current-voltage (I-V) plots showed outward rectification in high conductance patches. For all three multiple conductance levels the ratio of outward to inward single-channel slope conductance was 1.8 +/- 0.1 and this rectification remained present in symmetrical Na+ solutions. 7. In high conductance patches, the events produced by a rapid application of 20-50 muM glutamate were compared with those activated during steady-state application. There was no significant difference either in the amplitudes of the various multiple conductance levels opened, or in the relative proportions of these levels, under the two conditions. 8. Kinetic analysis of single channels in high conductance patches indicated that the mean apparent open times were brief (approximately 0.5 ms for all agonists). In low conductance patches, AMPA opened channels with a mean apparent open time of 1.1 ms. 9. Distributions of channel shut times could be described by the sum of three or four exponential components; the mean time constants (and areas) were not significantly different for glutamate, AMPA or kainate. Pooling data for these agonists gave mean values for the time constants (and areas) of tau1 = 147 mus, (22 tau2 = 539 mus (3 %), tau3 = 27.3 ms (35 %) and tau4 = 127 ms (40 %). 10. Burst length distributions of non-NMDA channels in high conductance patches, contained a main component with a time constant similar to that of the apparent openings. This suggests that the majority of activations of non-NMDA channels apparently resulted in single openings or that gaps were brief. 11. We conclude that cerebellar granule cells possess more than one type of non-NMDA receptor channel. Our data suggest that high conductance responses originate from a single receptor-channel complex displaying 10, 20 and 30 pS levels activated by glutamate, AMPA and kainate, and that this channel is the likely candidate for mediating most of the synaptic current in granule cells.
