Kinetics of postsynaptic action of glutamate pulses applied iontophoretically through high resistance micropipettes.

Glutamate induced postsynaptic currents (g-EPSCs) were elicited by 2-5 ms glutamate pulses applied iontophoretically through high resistance (80-200 M Omega) micropipettes. Such g-EPSCs decayed with time constants in the range of 5 ms. The dose-response curves for the action of glutamate were S-shaped with a limiting log log slope of 4-6, the half maximum response concentration of glutamate was about 10(-4) Mol/l and the maximum synaptic current in the range of 30 nA. In pairs of small g-EPSCs following each other with delays between 3 and 100 ms, the second one was potentiated. For short delays potentiation was up to 60 fold. The time course of the g-EPSCs and of their potentiation can be described quantitatively with the following assumptions: 1. The simultaneous reaction of 4-6 glutamate molecules with a receptor triggers postsynaptic current flow. 2. The synaptic glutamate concentration is determined by diffusion from a point source. 3. Potentiation is due to summation of the synaptic glutamate concentrations generated by pulse 1 and by pulse 2. The relevance of these results for the interpretation of the natural EPSC is discussed.