Aplysia acetylcholine receptors: Blockade by and binding of α-bungarotoxin

αBT is a snake toxin which in vertebrate systems binds with great specificity to ACh receptors. We have studied the effects of αBT on the electrophysiological response to ACh of identifiable cells in the nervous system of the marine mollusc, Aplysia, and the binding of [¹²⁵I]αBT to a ganglionic preparation. Aplysia has 3 pharmacologically distinct ACh responses and each causes a different conductance change. αBT blocks all 3 responses of iontophoretically applied ACh. In all cases the inhibition is reversed on washing. Binding of [¹²⁵I]αBT to the ganglionic preparation is a saturable process. The dissociation constant of binding calculated from rates of association and dissociation of the toxin-receptor complex is 0.8 × 10^-9 M. Binding of [¹²⁵I]αBT is inhibited by unlabeled toxin, ACh agonists, and antagonists as well as by eserine, ouabain, and TEA but not by the transmitters serotonin and dopamine.