Recordings of glutamate-gated ion channels in outside-out patches from Drosophila larval muscle.

Outside-out patches of membrane were excised from muscle fibers 6 and 7 of third-instar wild-type Drosophila larvae. Channels were observed to open in response to short pulses of L-glutamate. At a holding potential of -60 mV, the channels opened to one main conductance level of about 120 pS. The current-voltage plot for the channels was linear and reversed around 0 mV holding potential. The channels were also activated by quisqualate but not by aspartate, N-methyl-D-aspartate (NMDA), kainate, glycine, gamma-aminobutyric acid (GABA) and acetylcholine. At high glutamate concentrations (3 or 10 mM), channel activation reached a peak within 0.3 ms. The channels opened in 'bursts' flickering between open and closed states. The channels opened only for a few milliseconds after switching on the glutamate and channel activity declined after the initial surge to zero with time constants between 5 and 20 ms. During applications of low glutamate concentrations (0.2-0.5 mM) to the same patch the channels opened much less frequently and during most applications no openings were observed. The openings observed were short and 'bursts' of openings were rare. Two exponential components were identified in the open-time distribution obtained with pulsed applications of glutamate (0.5-10 mM) with time constants of about 0.1 and 2.0 ms. The kinetics of the channels seem to be similar to the kinetics of certain glutamate gated channels found on muscle of crayfish and locust.