The mechanism of rectification at the electrotonic motor giant synapse of the crayfish.

The synapse between the giant interneurone and the motor giant axon of the crayfish is a well-known example of the rare class of current-rectifying electrotonic synapses. One early proposal for the basis of this rectification was that rectifying junctions are like diodes. Biological correlates of diodes can exist, such as constant-field channels which rectify by very high-speed rearrangements of charge carriers, but these require high selectivity and large concentration gradients. Electrotonic synapses are believed to be composed of wide-bore (1-2 nm) gap-junction channels which have poor selectivity and bridge similar intracellular compartments. An alternative mechanism for rectification would be by voltage-dependent gates that sense trans-synaptic potential. These two mechanisms can be distinguished because a diode should rectify instantaneously (on a biological time-scale) while a gated channel should show kinetic processes. Although a gating model is more consistent with the known behaviour of channels than a diode model, previous work has failed to find any time course for the rectification. We have now developed a high-quality voltage clamp and by working at reduced temperatures we are able to demonstrate channel kinetics. These results support the hypothesis that this rectifying synapse contains voltage-dependent gates.