Nicotinic acetylcholine receptors from muscle contain two functionally active and pharmacologically distinct acetylcholine-binding sites located at the alpha/gamma and alpha/delta subunit interfaces. The alpha-conotoxins are competitive antagonists of nicotinic receptors and can be highly site-selective, displaying greater than 10,000-fold differences in affinities for the two acetylcholine-binding sites on a single nicotinic receptor. The higher affinity site for alpha-conotoxins GI, MI, and SI is the alpha/delta site on mouse muscle-derived BC3H-1 receptors. However, alpha-conotoxins GI and MI exhibit higher affinity for the other site (alpha/gamma site) on nicotinic receptors from Torpedo californica electric organ. alpha-Conotoxin SI does not distinguish between the two acetylcholine-binding sites on Torpedo receptors. In this study, alpha-conotoxins [K10H]SI and [K10N]SI displayed wild-type affinity for the two acetylcholine-binding sites on BC3H-1 receptors but a 10-20-fold decrease in apparent affinity at one of the two acetylcholine-binding sites on Torpedo receptors. alpha-Conotoxin [P9K]SI displayed a selective and dramatic increase in the apparent affinity for the alpha/delta site of BC3H-1 receptors and for the alpha/gamma site of Torpedo receptors. alpha-Conotoxin [R9A]GI displayed a reduction in affinity for both acetylcholine-binding sites on BC3H-1 receptors, although the extent of its selectivity for the alpha/delta site was retained. alpha-Conotoxin [R9A]GI also displayed a loss of affinity for the two acetylcholine-binding sites on Torpedo receptors, but its site-selectivity was apparently abolished. These results indicate that positions 9 and 10 in alpha-conotoxins GI and SI are involved in complex species- and subunit-dependent interactions with nicotinic receptors.

Groebe, DR., et al. (1997) Determinants involved in the affinity of alpha-conotoxins GI and SI for the muscle subtype of nicotinic acetylcholine receptors, Biochemistry. PMID: 9174364

The ability of the marine snail toxin, alpha-conotoxin GI, to produce blockade of singly evoked twitches and to produce tetanic and train-of-four fade has been determined in the isolated rat hemidiaphragm preparation. Results were compared to those obtained with a reversible (vecuronium) and an irreversible (alpha-bungarotoxin) nicotinic acetylcholine antagonist and have been interpreted in terms of relative effects on post- and prejunctional nicotinic acetylcholine receptors at the neuromuscular junction. alpha-Conotoxin GI (0.5-2 microM) produced a concentration-dependent, readily reversible, decrease in the peak amplitude of single twitches and 50 Hz tetani, and an increase in tetanic and train-of-four fade. alpha-Conotoxin GI was consistently 2-3-fold more potent than vecuronium with respect to all of the measured tension parameters. Both alpha-conotoxin GI and vecuronium were approximately 2-fold more potent in producing tetanic fade and in blocking tetanic contractions than in blocking single twitches. In contrast to both alpha-conotoxin GI and vecuronium, alpha-bungarotoxin (0.13 microM) reduced the peak amplitude of both single twitches and 50 Hz tetani to the same extent without the appearance of a large degree of tetanic or train-of-four fade. Based on a comparison of the in vitro time course of neuromuscular block and of the relative effects of vecuronium, alpha-conotoxin GI and alpha-bungarotoxin on twitches, tetani and trains-of-four, we conclude that alpha-conotoxin GI has both pre- and postjunctional activity at the neuromuscular junction. In this respect, alpha-conotoxin GI resembles the clinically used competitive neuromuscular blocking drugs rather than the irreversible snake alpha-neurotoxins.

Blount K., et al. (1992) Alpha-Conotoxin GI produces tetanic fade at the rat neuromuscular junction. Toxicon. PMID: 1355934

Conotoxin GI, a peptide neurotoxin contained in the venom of the marine snail Conus geographus, was applied to the cutaneous pectoris muscle of the frog, and the effects on the postsynaptic response to acetylcholine were examined. Conotoxin GI reversibly blocked nerve-evoked muscle contractions at concentrations greater than or equal to 3 to 4 microM. Micromolar concentrations of conotoxin GI significantly reduced the amplitude of miniature endplate potentials and membrane depolarizations produced by ionophoretic application of acetylcholine, suggesting that the toxin reduced the postsynaptic sensitivity to acetylcholine. The reduction in the sensitivity of the muscle to acetylcholine was not due to changes in muscle fiber resting membrane potential or input resistance. Conotoxin GI reduced the amplitudes but did not affect the rates of decay of focal, extracellularly recorded endplate currents or miniature endplate currents, suggesting that the toxin did not affect the lifetime of ion channels opened by acetylcholine. Miniature endplate currents decay five to six times more slowly than normal when acetylcholinesterase is blocked with neostigmine methyl sulfate due to repeated binding of acetylcholine to receptors as it diffuses from the synaptic cleft. Conotoxin GI reduced the amplitude and increased the rate of decay of miniature endplate currents recorded in the presence of neostigmine methyl sulfate, suggesting that the toxin reduced the binding of acetylcholine to endplate receptors. These results are consistent with the hypothesis that conotoxin GI blocks neuromuscular transmission at the frog endplate by reducing the binding of acetylcholine to receptors.

McManus OB., et al. (1985) Postsynaptic block of frog neuromuscular transmission by conotoxin GI. J Neurosci. PMID: 2981295

The effects of a mixture of two peptides (GI and GII), purified from the venom of the marine gastropod, Conus geographus, were studied on neuromuscular transmission in the isolated mouse phrenic nerve–diaphragm and frog sciatic nerve–sartorius muscles. The GI–GII mixture rapidly blocked nerve-evoked contractions of the mouse diaphragm at bath concentrations greater than or equal to 0.2 microM but had no effect on contractions elicited by direct muscle stimulation. Paralytic concentrations of GI–GII had no significant effect on the compound nerve action potential of the bullfrog sciatic nerve. Similar concentrations of GI–GII produced a rapid reduction of endplate potential (epp) and miniature endplate potential amplitudes, apparently by a postsynaptic effect because the decrease in epp amplitude produced by subparalytic doses was not accompanied by significant alteration in the epp quantal content. The GI–GII mixture also inhibited [125I]alpha-bungarotoxin binding to endplate regions of the mouse diaphragm in a dose-dependent manner and was at least 10 times more potent than d-tubocurarine. We conclude that the blockage of vertebrate neuromuscular transmission by GI–GII is in part due to antagonism of acetylcholine binding to its receptor at motor endplates.

McManus OB., et al. (1981) Peptides isolated from the venom of Conus geographus block neuromuscular transmission. Neurosci Lett. PMID: 6269031