On the kinetics of large-conductance glutamate-receptor ion channels in rat cerebellar granule neurons

Ion channels activated by glutamate, aspartate, and N-methyl-D-aspartate (NMDA) have been investigated in outside-out patches from cultured cerebellar granule neurons of the rat. Openings of these channels occur in bursts, within which the individual openings are separated by brief shuttings or gaps. The shut-time distributions obtained with each agonist were fitted with four exponential components. The briefest two components were considered as 'gaps within bursts'. Their mean time-constants were: glutamate, 58.0 microseconds and 592 microseconds; aspartate, 31.3 microseconds and 644 microseconds; NMDA, 40.5 microseconds and 903 microseconds. Distributions of burst durations were fitted with three exponential components. The mean time-constants obtained for the longest two components were: glutamate, 1.33 ms and 10.5 ms; aspartate, 2.15 ms and 10.3 ms; NMDA, 2.42 ms and 10.5 ms. Evidence is given that these two components of burst duration reflect the gating kinetics of 50 pS openings and not the fact that each agonist produces openings to more than one conductance level. Not only do openings occur in bursts, but these bursts were observed to occur in clusters, which can be hundreds of milliseconds long. We discuss the relation between the kinetics of single-channel openings observed in patches and the spectral components detected in whole-cell current noise.