Kappa-Bungarotoxin: a probe for the neuronal nicotinic receptor in the avian ciliary ganglion

The interaction of snake α-neurotoxins with neuronal membranes has been examined in the chick ciliary ganglion. Some, but not all, α-neurotoxins block nicotinic transmission in this ganglion. α-Bungarotoxin (ABgT), the major α-neurotoxin in the venom of Bungarus multicinctus, does not block transmission at high concentrations (1.2 μM) although it binds ( K d = 1nM ) to a pharmacologically nicotinic site in the ganglion. A toxin (kappa-bungarotoxin, KBgT) has been purified from the venom of Bungarus multicinctus. KBgT has a molecular weight of 6500 daltons and a pI of 9.1 KBgT is a potent inhibitor of nicotinic transmission in the ciliary ganglion, producing a reversible (overal several hours) blockade at 75 nM. Pre-exposure of ganglia to 1.2 μM ABgT does not prevent the effects of KBgT, indicating that the blockade occurs at a site distinct from that recognized by ABgT. Binding of [ 125I]KBgT to ciliary ganglia reveals two binding sites: one which has previously been characterized by [ 125I]ABgT and one which is not identified by [ 125I]ABgT. Both of these [ 125I]KBgT binding sites are blocked following pre-treatment of ganglia with the irreversible nicotinic affinity agent bromoacetylcholine. A two-site model is proposed to account for these observations. One site (the ABgT binding site) is seen by both ABgT and KBgT, and has as yet no physiological function associated with it. The second site is recognized only by the physiologically active KBgT, and may represent binding of the toxin to the physiologically detected nicotinic receptor.