Kinetic properties of glutamate receptor channels in cultured embryonic Drosophila myotubes

Glutamate receptor channels are ubiquitous agonist-gated channels, Pharmacologically they are classified into several subtypes but may have common fundamental channel properties, To build foundation for future molecular biological and genetic studies, we studied kinetics of the glutamate receptor channel in embryonic Drosophila myotubes in culture using the patch clamp technique. There were many brief lasting channel events together with prolonged ones. Brief events were frequently observed in low concentrations whereas the frequency of prolonged events increased with agonist concentrations, Long openings (>5 ms) were often interrupted by brief closures, most of which lasted less than 100 mu s, thus showing a bursting behavior, At all agonist concentrations, the burst duration was fitted with three exponential components (brief, intermediate and long). The mean duration of the long component increased linearly with the glutamate concentration, The mean closed time and number of brief closures per ms within long bursts were independent of agonist concentration. The mean burst durations of the brief (30-250 mu s) and intermediate component (300-1050 mu s) did not change significantly with agonist concentration, The closing episodes within bursts were rare in the brief and intermediate burst components, The ratio of the fractional areas of the brief or intermediate and long burst components increased linearly with agonist concentration in the log-log plot with a slope of one, These findings suggest that the brief and intermediate components are due to singly-liganded openings and the long component is the result of doubly-liganded openings.