kappa-Bungarotoxin: binding of a neuronal nicotinic receptor antagonist to chick optic lobe and skeletal muscle.

kappa-Bungarotoxin, a snake venom kappa-neurotoxin, is a potent neuronal nicotinic receptor antagonist. kappa-Neurotoxins are structurally related to the long-type alpha-neurotoxins (including alpha-bungarotoxin), which often fail to block neuronal nicotinic transmission, but which are potent antagonists of nicotinic receptors found on vertebrate skeletal muscle. The binding of kappa-bungarotoxin has now been examined in homogenates of chick skeletal muscle and optic lobe. In muscle, kappa-bungarotoxin binds to nicotinic receptors with 200-fold lower affinity than does alpha-bungarotoxin. The weakest known alpha-neurotoxin, L.s. III, is found to be 6.5-fold more potent than kappa-bungarotoxin. These findings support the conclusion that kappa-neurotoxins are selective for neuronal nicotinic receptors. In the optic lobe, 125I-alpha-bungarotoxin and 125I-L.s. III. A second nicotinic site, detected with high affinity by both alpha-neurotoxins, is only weakly bound by kappa-bungarotoxin. No evidence for a unique 125I-kappa-neurotoxin site is observed. Furthermore, kappa-bungarotoxin does not recognize the high affinity L-[3H]nicotine binding site in chick optic lobe which is distinct from the alpha-neurotoxin binding sites. Three subtypes of nicotinic sites can thus be defined in chick optic lobe, although which of these subtypes is involved in nicotinic transmission in the lobe remains to be conclusively determined.