Anion permeability of the inhibitory post‐synaptic membrane of the crayfish neuromuscular junction

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Abstract
The membrane conductance of the crayfish muscle fibre was calculated and the increase in the membrane conductance brought about by the application of gamma-aminobutyric acid (GABA) was investigated under various conditions. The permeability of the activated inhibitory postsynaptic membrane was estimated from the conductance change. When the concentration of Cl- was changed by substituting Cl- with methylsulphate, a linear relation was obtained between the membrane conductance change produced by GABA and the concentration of Cl-. Cations have no appreciable effect on the GABA induced conductance change (less than 0.4%). The conductance change produced by GABA on the inhibitory postsynaptic membrane was measured in various solutions in which the Cl- was replaced by various anions. The degree of permeability depended on the anion species in the following order: Br > Cl- > SCN- > I- > NO3- > HCOO- > ClO3- > ClO4- > BrO3- . At high pH the conductance change produced by GABA was decreased, while at low pH it increased. These results show that the activated inhibitory post-synaptlc membrane of crayfish muscle fibres has the characteristics of an anion selective membrane, suggesting the fixed positive charges in the inhibitory membrane.