Spatial control of membrane potential: a method for improved voltage clamping of the squid giant synapse.

In voltage clamp experiments at the squid giant synapse anisopotentiality of the presynaptic terminal can distort measurements of synaptic transfer. Local compression of the presynaptic cell, at the point where the terminal joins its axon, appears to reduce this problem by electrically isolating the terminal from the axon. Because compression reduced anisopotentiality in the presynaptic terminal and altered the apparent voltage dependency of transmitter release, this procedure may be a simple way to improve voltage clamp control in this and other long cells. Compression of the presynaptic cell also reduced the diffusion of injected molecules out of the terminal region. This increased the effective concentration of given quantities of tetraethylammonium and Fast green injected into the presynaptic terminal. Thus, compression should also facilitate experiments in which molecules are microinjected into this and other cells because the effective volume of the cell is reduced and less of the injected substance will be required.