Anion interaction at the inhibitory post-synaptic membrane of the crayfish neuromuscular junction.

1. The membrane potential and the membrane conductance of the crayfish muscle fibre in solutions containing various anions were measured with intracellular micro-electrodes.2. When Cl(-) in the solution was replaced by Br(-), NO(3) (-), I(-) or CNS(-), the addition of gamma-amino butyric acid (GABA) produced a transient hyperpolarization of the membrane.3. The reversal potential of the inhibitory junctional potentials (i.j.p.s) was at a slightly depolarized level relative to the resting potential in the normal Cl(-) solution. When Cl(-) in the bathing solution was replaced by foreign anions, the reversal potential shifted towards the hyperpolarized level. The hyperpolarization was in the order CNS(-) > I(-) > NO(3) (-) > Br(-).4. When a part of Cl(-) in the bathing solution was substituted by Br(-), the inhibitory membrane conductance activated by GABA was increased as the concentration of Br(-) increased. The inhibitory membrane conductance decreased when one quarter or a half of Cl(-) was replaced by CNS(-), NO(3) (-) or I(-), but it increased again in higher concentrations of these anions.5. Ca(2+), Mg(2+) and Ba(2+) showed no appreciable effect on the inhibitory membrane conductance activated by GABA, while they decreased the conductance of the resting muscle membrane.6. It was suggested that, at the activated inhibitory membrane, there is an interaction between anions and the permeability of Cl(-) is decreased by the presence of foreign anions, such as CNS(-), I(-) and NO(3) (-). The present results support the idea that the activated inhibitory membrane is charged positively and anions penetrate the membrane interacting with the charge sites.