Abstract

Dose-response curves of the action of γ-aminobutyric acid to increase the conductance of postsynaptic membrane were measured in fibers of the flexor tibialis muscle of the metathoracic leg of Locusta migratoria. The sigmoid shape of the curves is interpreted in terms of the cooperative binding of more than 1 γ-aminobutyric acid molecule to the receptor. The "function of state," which describes the particular case in which receptor activation is induced only after the binding of more than 1 ligand molecule, is compared with some other formulations of cooperative binding, and is shown to give a better fit to the experimental data. This function uniquely predicts that the limiting Hill slope at small concentrations equals n, the number of molecules required to activate a receptor. In the present experiments n appears to be 3.