High-resolution measurements of single-channel currents activated by glutamate in crayfish muscle.

Patch-clamp pipettes filled with 50-5000 microM glutamate were placed on crayfish muscle fibers treated with collagenase, formed G omega seals and elicited single-channel currents with a main amplitude of about -8 pA at -70 mV membrane potential, representing a conductance of about 100 pS (19 degrees C). Evaluation of the channel openings longer than 1 ms yielded three sublevels of this conductance. The channels opened in bursts, the durations of which were distributed in two exponential components with time constants of about 0.1 and 0.3 ms at low glutamate concentrations, which rose to about 0.4 and 1.8 ms, respectively, at high glutamate concentrations. The distributions of closed times could be described by three time constants which also varied with glutamate concentration. Comparison of the burst durations with the decay time constants of natural synaptic currents indicates effective glutamate concentrations in the millimolar range during transmission.