Dose-response curve of glutamate applied by superfusion to crayfish muscle synapses

Single muscle fibers were space clamped to a membrane potential of -75 to -80 mV, and the synaptic currents elicited by L-glutamate (gEPSCs) were recorded. The bathing solutions flowing across the fibers at high speed could be switched rapidly and repeatedly through valves actuated by solenoids. Glutamate solutions were applied for periods of 7 s or 1 s, and the responses to repeated applications were averaged. For glutamate concentrations of 10-50 mumol/l, applied for 7 s, the gEPSCs reached a steady state. In this concentration range the amplitude of the gEPSC rose steeply proportional to the power n=2.5 to n=6 (average of 12 experiments n=4.0) of the glutamate concentration. At higher concentrations, after rising for a few seconds the gEPSC was reduced by desensitization. At 500 mumol/l glutamate complete desensitization was reached with an approximate time constant of less than 1 s. The glutamate concentration that elicited a half maximum gEPSC was K=70 mumol/l. If glutamate was superfused only for 1 s, similar dose-response curves were observed. In these experiments n was between 4 and 6. The results obtained by superfusion agree quantitatively with those published for electrophoretic applications.