Multiple effects of alpha-toxins on the nicotinic acetylcholine receptor

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Presynaptic inhibitory effects of rocuronium and SZ1677 on [3H]acetylcholine release from the mouse hemidiaphragm preparation

It has been shown that nondepolarizing muscle relaxants may have effects on nicotinic acetylcholine receptors (nAChRs) other than those located on the skeletal muscle: some of them possess inhibitory effects on neuronal nAChRs [Anesth. Analg. 59 (1980) 935; Trends Pharmacol. Sci. 9 (1988) 16; Pharmacol. Ther. 73 (1997) 75]. It was shown that, e.g. (+)-tubocurarine and pancuronium are able to inhibit ACh release from the axon terminals of hemidiaphragm preparations and produce tetanic fade indicating their presynaptic effect. In this study rocuronium, a nondepolarizing steroidal muscle relaxant with shorter onset of action, and SZ1677 [1-(3alpha-hydroxy-17beta-acetyloxy)-2beta-(1.4-dioxa-8-azaspiro-[4,5]-dec-8-yl)-(5alpha-androstane-16beta-yl)-1-(2-propenyl) pyrrolidinium bromide], a short-acting muscle relaxant [Ann. New York Acad. Sci. 757 (1995b) 84] inhibited the release of ACh in response to axonal stimulation, while alpha-bungarotoxin failed to reduce the stimulation evoked release of ACh and did not produce tetanic fade. These results indicate that in addition to their postsynaptic effect, rocuronium and SZ1677 have presynaptic inhibitory effects on neuronal nAChRs at the neuromuscular junction. The finding that alpha-bungarotoxin does not inhibit the release and does not produce tetanic fade indicates that it possesses affinity only for the postsynaptic muscle nAChRs.

Mechanisms underlying the vecuronium-induced tetanic fade in the isolated rat muscle

The cellular mechanisms underlying the effects of vecuronium on the tetanic contraction were studied in vitro with a combination of myographic and electrophysiologic techniques. We used the isolated sciatic nerve extensor digitorum longus muscle preparation of the rat. Indirect twitches were evoked at 0.1 Hz pulses and tetani at 50 Hz pulses. Trains of end-plate potentials were generated at 50 Hz. The electrophysiological variables used in the analysis of the end-plate potentials were: amplitude, tetanic run-down, quantal size and quantal content. The myographic study demonstrated that vecuronium at 0.4 microM caused tetanic fade, but left the twitch unaffected. Regarding electrophysiology, vecuronium (0.4 microM) decreased the amplitude of end-plate potentials and increased their tetanic run-down. These changes were due to significant reductions in both the quantal content of the end-plate potentials and the quantal size. It is concluded that vecuronium has both pre- and postsynaptic effects at the neuromuscular junction, and that it induces fade of the tetanic contraction via a summation of these effects.

Run-down of neuromuscular transmission during repetitive nerve activity by nicotinic antagonists is not due to desensitization of the postsynaptic receptor

1. Whether the function of the postsynaptic acetylcholine receptor is use-dependently affected by repetitive nerve stimulation in the presence of competitive antagonists was studied in the mouse phrenic nerve-hemidiaphragm preparation. 2. For electrophysiological experiments, the preparation was immobilized by synthetic mu-conotoxin, which preferentially blocks muscular Na-channels causing neither depolarization of the membrane potential, inhibition of quantal transmitter release, nor depression of nicotinic receptor function. 3. High concentrations of cobratoxin depressed indirect twitches and endplate potentials (e.p.ps) without inducing waning of contractilities or run-down of trains of e.p.ps evoked at 10-100 Hz. However, waning and run-down were accelerated after washout of the toxin despite diminished postsynaptic receptor blockade. Once the run-down of e.p.ps was produced by washout or low concentrations of cobratoxin, further depression of e.p.p. amplitude with high concentrations of cobratoxin did not attenuate the e.p.p. run-down. 4. The degrees of waning of tetanus and trains of e.p.ps produced by a very high concentration of tubocurarine (20 microM) were also less than that caused at a 100 fold lower concentration, albeit the amplitudes of twitches and the first e.p.p. were depressed more rapidly and markedly. 5. Tubocurarine, like cobratoxin, depressed the amplitude of miniature endplate potentials (m.e.p.ps) more than e.p.ps. 6. In contrast to the steepened run-down of successive e.p.ps in the presence of low concentrations of either nicotinic antagonists, the amplitude of m.e.p.ps observed during repetitive stimulation was uniform and was not different from that before stimulation. 7. The results suggest that the e.p.p. run-down and tetanic fade induced by nicotinic antagonists are due to a slow kinetic blockade of presynaptic receptors and confirm that the e.p.p. run-down is not produced by a use-dependent failure of postsynaptic nicotinic receptors. The roles of the presynaptic nicotinic receptor in positive or negative feedback modulations of transmitter release are discussed.

Effects of chronic nicotinic ligand exposure on functional activity of nicotinic acetylcholine receptors expressed by cells of the PC12 rat pheochromocytoma or the TE671/RD human clonal line

Studies were conducted to ascertain the temporal and dose-dependent effects of nicotinic ligand exposure on functional activity of different nicotinic acetylcholine receptor (nAChR) subtypes, as expressed by cells of the PC12 rat pheochromocytoma (ganglia-type nAChR) or the TE671/RD human (muscle-type nAChR) clonal line. Chronic (3-72-h) agonist (nicotine or carbamylcholine) treatment of cells led to a complete (TE671) or nearly complete (PC12) loss of functional nAChR responses, which is referred to as "functional inactivation." Some inactivation of nAChR function was also observed for the nicotinic ligands d-tubocurarine (d-TC), mecamylamine, and decamethonium. Half-maximal inactivation of nAChR function was observed within 3 min for TE671 cells and within 10 min for PC12 cells treated with inactivating ligands. Functional inactivation occurred with dose dependencies that could not always be reconciled with those obtained for acute agonist activation of nAChR function or for acute inhibition of those responses by d-TC, decamethonium, or mecamylamine. Treatment of TE671 or PC12 cells with the nicotinic antagonist pancuronium or alcuronium alone had no effect on levels of expression of functional nAChRs. However, evidence was obtained that either of these antagonists protected TE671 cell muscle-type nAChRs or PC12 cell ganglia-type nAChRs from functional inactivation on long-term treatment with agonists. Recovery of TE671 cell nAChR function following treatment with carbamylcholine, nicotine, or d-TC occurred with half-times of 1-3 days whether cells were maintained in situ or harvested and replated after removal of ligand. By contrast, 50% recovery of functional nAChRs on PC12 cells occurred within 2-6 h after drug removal. In either case the time course for recovery from nAChR functional inactivation is much slower than recovery from nAChR "functional desensitization," which is a reversible process that occurs on shorter-term (0-5-min) agonist exposure of cells. These results indicate that ganglia-type and muscle-type nAChRs are similar in their sensitivities to functional inactivation by nicotinic ligands but differ in their rates of recovery from and onset of those effects. The ability of drugs such as the agonists d-TC, decamethonium, and mecamylamine to induce functional inactivation may relate to their activities as partial/full agonists, channel blockers, and/or allosteric regulators. Effects of drugs such as pancuronium and alcuronium are likely to reflect simple competitive inhibition of primary ligand binding at functional activation sites.(ABSTRACT TRUNCATED AT 400 WORDS)

The alpha-neurotoxin erabutoxin b causes fade at the rat end-plate

d-Tubocurarine and the alpha-neurotoxins from snake venom are antagonists at the nicotinic acetylcholine receptor. It is well established that d-tubocurarine causes fade in neuromuscular transmission during repetitive nerve stimulation but paradoxically there are many reports which indicate that the alpha-neurotoxins do not cause such fade. We found that high concentrations of erabutoxin b (100-150 nM) from the venom of Laticauda semifasciata did not cause much fade in the rat diaphragm preparation. However, low concentrations of toxin (5 nM) caused severe fade which was similar to the effects of d-tubocurarine. The data suggest that fade may be caused by toxin binding to a high-affinity site on the postsynaptic acetylcholine receptor.

Control of transmitter release from the motor nerve by presynaptic nicotinic and muscarinic autoreceptors

Until recently, release studies have failed to indicate the existence of autoreceptors on motor nerves. Ignaz Wessler now reports on a refinement of the technique - the measurement of newly synthesized [3H]acetylcholine released from the phrenic nerve - which provides clear evidence in support of release-modulating autoreceptors. Presynaptic nicotinic receptors mediate a positive feedback mechanism, can rapidly be desensitized and appear to differ in their pharmacological profile from the postsynaptic receptors. In addition, inhibitory and facilitatory muscarinic receptors appear to be involved in the presynaptic control of transmitter release from the phrenic nerve.