Block of neuromuscular transmission by conotoxins GI and GII.

Two similar peptide neurotoxins from the venom of the marine snail Conus geographus were found to block neuromuscular transmission in the frog and in the mouse. Initial experiments were done with an equimolar mixture of Conotoxins GI and GII. The GI-GII mixture was found to block nerve-evoked muscle contractions, but have no effect on contractions of directly stimulated muscle. Conotoxins GI-GII blocked endplate potentials and miniature endplate potentials apparently at a postsynaptic site because the toxins reduced endplate potential amplitude while having no significant effect on the quantal content of endplate potentials. In addition, Conotoxins GI-GII inhibited the binding of (alpha)-Bungarotoxin to acetylcholine receptors at the neuromuscular junction suggesting that Conotoxins GI-GII block neuromuscular transmission at a postsynaptic site by blocking acetylcholine receptors. Further experiments examined the effects of a synthetic form of Conotoxin GI on the postsynaptic membrane at the neuromuscular junction. Conotoxin GI reduced the sensitivity of muscle to ionophoretically applied acetylcholine further suggesting a postsynaptic site of action. This reduction in the sensitivity of the muscle to acetylcholine caused by Conotoxin GI was not due to changes in muscle fibre resting membrane potential or input resistance. Conontoxin GI reduced the amplitude of extracellularly recorded endplate currents and miniature endplate currents, but had no apparent effect on the rate of decay of endplate or miniature endplate currents, suggesting that the toxin has no effect on the lifetime of ion channels opened by acetylcholine. When acetylcholinesterase is blocked, miniature endplate currents decay five to six times more slowly than normal, and this rate of decay reflects the rate of diffusion of acetylcholine from the synaptic cleft. This rate of diffusion is prolonged compared to free diffusion due to repeated binding of acetylcholine to receptors as it diffuses from the cleft. Conotoxin GI significantly reduced the amplitude and increased the rate of decay of extracellularly recorded miniature endplate currents with acetylcholinesterase blocked, suggesting that Conotoxin GI reduces the binding of acetylcholine to receptors at the neuromuscular junction. In summary, Conotoxin GI blocks vertebrate neuromuscular transmission apparently by binding to acetylcholine receptors and thereby reducing the binding of acetylcholine to its receptor.