A voltage clamp study of the glutamate responsive neuromuscular junction in Drosophila melanogaster

The point and single electrode voltage clamp methods have been used to study the characteristics of junctional currents in Drosophila melanogaster larvae muscle fibers and the modulation of these currents by excitatory amino acids, short and long chain n-alkanols, and pentobarbital. The decay phase of junctional currents in Drosophila was found to be dominated by cooperativity in transmitter binding associated with reverberation, that is, repeated binding of transmitter with receptors as the transmitter molecules diffuse away from the active region. The current decay does not directly reflect the closure of ion channels and is qualitatively similar to the decay of miniature end-plate currents at the mouse neuromuscular junction after poisoning of acetylcholinesterase by paraoxon. In Drosophila an increase in membrane hyperpolarization both slows the time course of current decay and increases the degree of reverberation. The application of excitatory amino acids including glutamate, N-methyl-D-aspartate, quisqualate, and kainate causes a significant decrease in the amplitude of the junctional currents, a prolongation of the decay time course, and a reduction in reverberation of transmitter. The height of junctional currents is also diminished by the n-alkanols ethanol, pentanol, and octanol and by the barbiturate pentobarbital; ethanol also hastened the time course of decay of the currents.