κ-Bungarotoxin blocks an α-bungarotoxin-sensitive nicotinic receptor in the insect central nervous system

Abstract

Snake venom κ-neurotoxins are selective antagonists of nicotinic acetylcholine responses in avian, murine and bovine neurons, and have been used as probes for functionally defined vertebrate neuronal nicotinic receptors. The actions of κ-bungarotoxin, a κ-neurotoxin, have now been examined at a central invertebrate nicotinic receptor. κ-Bungarotoxin is a potent antagonist (IC 50 = 100nM) of nicotinic responses, producing a long-lasting blockade of insect nicotinic acetylcholine receptors. The blockade appears to be competitive, and voltage-clamp experiments on an identified cockroach motorneuron indicate that the actions of κ-bungarotoxin are not dependent on membrane potential. α-Bungarotoxin is also a potent antagonist at the cockroach central nicotinic receptor, and binds ( K d= 4.3nM) to a nicotinic site in cockroach nervous tissue. κ-Bungarotoxin recognizes this invertebrate nicotinic site with high aff ( K i = 27nM). A comparison of the pharmacological properties of insect nicotinic receptors with those of functionally defined receptors identified by κ-neurotoxins in avian autonomic ganglia reveals several similarities. However, a striking exception is α-bungarotoxin, which is the most potent antagonist examined at cockroach nicotinic receptors, but fails to recognize functional autonomic ganglia nicotinic receptors even at very high concentrations. It is concluded that κ-neurotoxins can be used as selective probes for neuronal nicotinic receptors in both vertebrates and invertebrates. Although invertebrates diverged from vertebrates over 600 million years ago, the results indicate that the neuronal nicotinic receptors found in species as diverse as cockroach and chick retain considerable structural similarity, and thus neuronal nicotinic receptors appear to be highly conserved membrane proteins.