Potentiation and desensitization after glutamate induced postsynaptic currents at the crayfish neuromuscular junction.

Glutamate was applied iontophoretically in short (10 to 20 ms) pulses through micropipettes of 10 to 30 M Omega resistance to synaptic spots on crayfish muscle fibres. The resulting glutamate induced postsynaptic currents (gEPSCs) were measured using a voltage clamp of the muscle fiber. Dose-response curves for the action of glutamate were constructed summing g-EPSCs elicited by two separate iontophoretic pipettes at the same synapse. The dose-response curves showed a log log slope of 2, the g-EPSC rising with the second power of the glutamate concentration. Following a small conditioning g-EPSC with delay of up to 1 s, a second g-EPSC was potentiated. For short delays maximum potentiation was 3-fold. For large g-EPSCs potentiation gave way to desensitization: following a large conditioning gEPSC a second one was reduced for delays of up to several seconds. Possible mechanisms of potentiation and desensitization are discussed.