'Aplysia' Acetylcholine Receptors: Blockade by and Binding of alpha- Bungarotoxin

Alpha-Bungarotoxin (alphaBT) is a snake toxin which in vertebrate systems binds with great specificity to acetylcholine (ACh) receptors. The authors have studied the effects of alpha BT on the electrophysiological response to ACh of identifiable cells in the nervous system of the marine mollusc Aplysia and the binding of 125I-alpha BT to a ganglionic preparation. Aplysia has three pharmacologically distinct ACh responses and each causes a different conductance change. Alpha BT blocks all three responses of iontophoretically applied ACh. In all cases the inhibition is reversed on washing. Binding of 125I-alpha BT to the ganglionic preparation is a saturable process. The dissociation constant of binding was calculated from rates of association and dissociation of the toxin-receptor complex. Binding of 125I- alpha BT was inhibited by unlabeled toxin, ACh agonists and antagonists as well as by eserine, ouabain, and tetraethylammonium but not by the transmitters serotonin and dopamine.