Inhibitory postsynaptic current in voltage-clamped crayfish muscle

Abstract

THE current–voltage relationship provides important information about the mechanism of ionic permeation through membranes. In excitatory synapses the relationship between membrane potential and excitatory postsynaptic current (e.p.s.c.) has been extensively studied using the voltage-clamp method^1–6. In the frog endplate it has been shown that the e.p.s.c. was smaller than would be expected from a linear relationship during hyperpolarisation of the membrane^3,4. On the other hand, after iontophoretic application of acetylcholine, the synaptic current increased more than expected at hyperpolarised membrane potentials⁷. In investigating the current–voltage relationship it is important to measure the current on both sides of the equilibrium potential. Inhibitory postsynaptic current (i.p.s.c.) is more suitable for this purpose, because the reversal potential is near the resting potential and so the current–voltage relationship on both sides of the reversal potential can be measured using relatively small potential changes. Little attention has been paid, however, to inhibitory postsynaptic membranes, possibly because the inhibitory synapses are distributed diffusely over the surface of the cell or situated on dendrites remote from the cell body where the electrical changes are measured. In these conditions it is difficult to measure accurately the current–voltage relationship. We have cannulated the opener muscle of the claw in the crayfish (Cambarus clarkii) using a stainless steel wire, the membrane potential being clamped at various levels as described previously⁶. Although the inhibitory synapses are distributed over the whole surface of the muscle fibre, the space clamp condition of the muscle fibre was maintained well using this technique. We have found that the relationship between membrane potential and i.p.s.c. was highly nonlinear on hyperpolarisation of the membrane.