Postsynaptic factors controlling the shape of potentials at the squid giant synapse

The roles of rectification and cable properties of the squid giant axon in determining the shape of synaptic potentials generated at the giant synapse were investigated. Excitatory postsynaptic potentials were recorded in response to selective stimulation of the main presynaptic axon at various temperatures. Excitatory postsynaptic potentials elicited at low temperatures (less than 18 degrees C) exhibited a marked after-hyperpolarization or undershoot, while those recorded at higher temperatures did not. The postsynaptic current, recorded under voltage clamp conditions, did not show an undershoot. Furthermore, intracellular injection of tetraethylammonium chloride, to block the voltage-dependent rise in potassium conductance, also eliminated the undershoot of the excitatory postsynaptic potential. These results indicate that the duration of synaptic potentials at the squid giant synapse is reduced by rectification due to a delayed rise in potassium conductance. Computer simulations of these synaptic potentials suggested that the effects of rectification will be more prominent in spherical (isopotential) cells than in cells with more complicated geometries.