CURRENTS THROUGH SINGLE GLUTAMATE RECEPTOR CHANNELS IN OUTSIDE-OUT PATCHES FROM RAT CEREBELLAR GRANULE CELLS

1. Single-channel currents evoked in outside-out membrane patches from rat cerebellar granule cells by glutamate, aspartate, N-methyl-D-aspartate (NMDA), kainate and quisqualate were studied. Each agonist produced openings to five discrete amplitude levels. At a membrane potential of -70 mV, these levels correspond to single-channel conductances of about 8, 17, 30, 40 and 50 pS. NMDA, aspartate and glutamate evoked mainly 50 pS openings and also substantial numbers of 40 pS events. Kainate evoked primarily 8 and 17 pS openings. 2. The relative proportion of openings to each conductance level showed no dependence on membrane potential. At membrane potentials negative to -100 mV, current-voltage plots for 30, 40 and 50 pS openings showed substantial inward rectification. 3. With NMDA, aspartate and glutamate, the most common type of direct transition was between the 50 pS open level and the shut level. Transitions between the 30, 40 and 50 pS levels were also relatively common. With few exceptions, 8 and 17 pS openings appeared to arise directly from, and return directly to, the shut level. The differences between granule cells and certain other central neurones, in the types of transitions associated with NMDA receptor channels, provide evidence for the existence of more than one type of NMDA receptor. 4. Four exponential components were identified consistently in the shut-time distributions that were obtained with NMDA, aspartate and glutamate. Mean time constants for the briefest two components were 30 to 65-mu-s and 0.65 to 1.00 ms. The mean duration of these 'gaps within bursts' differed for different agonists, but did not vary with membrane potential. 5. Two exponential components were distinguished in most open-time distributions for the 50 pS level (time constants, 0.9-1.2 and 3.2-3.9 ms at -100 mV), whereas open-time distributions for 30 and 40 pS events were described adequately by single exponentials with time constants below 1.0 ms. The duration of 50 pS openings decreased with hyperpolarization. 6. Mean open times for 8 and 17 pS events produced by NMDA, aspartate and glutamate were 0.3 to 0.7 ms. The longest such openings were observed with quisqualate. 7. Three exponential components were present in distributions of burst length, and of total open time per burst, that were obtained with NMDA, aspartate and glutamate. The slowest two burst-length components had mean time constants of 1.7-2.4 and 10.6-13.0 ms and originated from the kinetic behaviour of the 50 pS state. Distributions of the number of open periods per burst contained two geometric components, one of which had a mean close to unity. 8. Bursts of openings were observed to occur in clusters of long duration and high probability of being open (high p(o)). Clusters were seen with each of the five agonists, but were most common with NMDA. The durations of open times and 'gaps within bursts' that occurred within clusters were similar to the durations of the corresponding events that occurred between clusters.