Chloride channels gated by extrajunctional glutamate receptors (H-receptors) on locust leg muscle

Outside-out patches of extrasynaptic membrane were isolated from leg muscles of locusts. l-Glutamate and its agonists were applied to such patches either continuously or in rapidly switched pulses. When the pipette contained a high chloride concentration, 2.5 × 10^-5 M glutamate triggered single-channel currents (gated by H-receptors) with a conductance of 25 pS which were carried by chloride, in addition to cationic channels (gated by D-receptors). For the chloride channels, the distribution of channel open times had components of about 2 and 12 ms. Pulses of higher glutamate concentrations elicited many superimposed channel openings, and the approximately saturating concentration of 10^-3 M glutamate opened 100-200 channels simultaneously. When the pipette contained low chloride, channel conductance was reduced, and the current voltage relation was shifted towards the now negative chloride equilibrium potential. H-Receptor-gated chloride channels were activated by glutamate, ibotenate and aspartate, but not by GABA, quisqualate, kainate, N-methyl-d-aspartate and carbachol. The currents declined in the continued presence of agonist showing a time constant of desensitization > 1 s. Recovery from desensitization after removal of the agonist was tested with double pulses and was found to have a time constant of about 300 ms.