Transmitter-mediated local contracture of the endplate region of the focally innervated mouse diaphragm treated with anticholinesterase

Donepezil inhibits neuromuscular junctional acetylcholinesterase and enhances synaptic transmission and function in isolated skeletal muscle

BACKGROUND AND PURPOSE

Donepezil, a piperidine inhibitor of acetylcholinesterase (AChE) prescribed for treatment of Alzheimer's disease, has adverse neuromuscular effects in humans, including requirement for higher concentrations of non-depolarising neuromuscular blockers during surgery. Here, we examined the effects of donepezil on synaptic transmission at neuromuscular junctions (NMJs) in isolated nerve-muscle preparations from mice.

EXPERIMENTAL APPROACH

We measured effects of therapeutic concentrations of donepezil (10 nM to 1 μM) on AChE enzymic activity, muscle force responses to repetitive stimulation, and spontaneous and evoked endplate potentials (EPPs) recorded intracellularly from flexor digitorum brevis muscles from CD01 or C57BlWld^S mice.

KEY RESULTS

Donepezil inhibited muscle AChE with an approximate IC₅₀ of 30 nM. Tetanic stimulation in sub-micromolar concentrations of donepezil prolonged post-tetanic muscle contractions. Preliminary Fluo4-imaging indicated an association of these contractions with an increase and slow decay of intracellular Ca²⁺ transients at motor endplates. Donepezil prolonged spontaneous miniature EPP (MEPP) decay time constants by about 65% and extended evoked EPP duration almost threefold. The mean frequency of spontaneous MEPPs was unaffected but the incidence of 'giant' MEPPs (gMEPPs), some exceeding 10 mV in amplitude, was increased. Neither mean MEPP amplitude (excluding gMEPPs), mean EPP amplitude, quantal content or synaptic depression during repetitive stimulation were significantly altered by concentrations of donepezil up to 1 μM.

CONCLUSION AND IMPLICATIONS

Adverse neuromuscular signs associated with donepezil therapy, including relative insensitivity to neuromuscular blockers, are probably due to inhibition of AChE at NMJs, prolonging the action of ACh on postsynaptic nicotinic acetylcholine receptors but without substantively impairing evoked ACh release.

"Calcium bombs" as harbingers of synaptic pathology and their mitigation by magnesium at murine neuromuscular junctions

Excitotoxicity is thought to be an important factor in the onset and progression of amyotrophic lateral sclerosis (ALS). Evidence from human and animal studies also indicates that early signs of ALS include degeneration of motor nerve terminals at neuromuscular junctions (NMJs), before degeneration of motor neuron cell bodies. Here we used a model of excitotoxicity at NMJs in isolated mouse muscle, utilizing the organophosphorus (OP) compound omethoate, which inhibits acetylcholinesterase activity. Acute exposure to omethoate (100 μM) induced prolonged motor endplate contractures in response to brief tetanic nerve stimulation at 20-50 Hz. In some muscle fibers, Fluo-4 fluorescence showed association of these contractures with explosive increases in Ca²⁺ ("calcium bombs") localized to motor endplates. Calcium bombs were strongly and selectively mitigated by increasing Mg²⁺ concentration in the bathing medium from 1 to 5 mM. Overnight culture of nerve-muscle preparations from Wld^S mice in omethoate or other OP insecticide components and their metabolites (dimethoate, cyclohexanone, and cyclohexanol) induced degeneration of NMJs. This degeneration was also strongly mitigated by increasing [Mg²⁺] from 1 to 5 mM. Thus, equivalent increases in extracellular [Mg²⁺] mitigated both post-synaptic calcium bombs and degeneration of NMJs. The data support a link between Ca²⁺ and excitotoxicity at NMJs and suggest that elevating extracellular [Mg²⁺] could be an effective intervention in treatment of synaptic pathology induced by excitotoxic triggers.

Impaired neuromuscular function by conjoint actions of organophosphorus insecticide metabolites omethoate and cyclohexanol with implications for treatment of respiratory failure

BACKGROUND

Ingestion of agricultural organophosphorus insecticides is a significant cause of death in rural Asia. Patients often show acute respiratory failure and/or delayed, unexplained signs of neuromuscular paralysis, sometimes diagnosed as "Intermediate Syndrome". We tested the hypothesis that omethoate and cyclohexanol, circulating metabolites of one agricultural formulation, cause muscle weakness and paralysis.

METHODS

Acetylcholinesterase activity of insecticide components and metabolites was measured using purified enzyme from eel electroplaque or muscle homogenates. Mechanomyographic recording of pelvic limb responses to nerve stimulation was made in anaesthetized pigs and isometric force was recorded from isolated nerve-muscle preparations from mice. Omethoate and cyclohexanol were administered intravenously or added to physiological saline bathing isolated muscle. We also assessed the effect of MgSO₄ and cooling on neuromuscular function.

RESULTS

Omethoate caused tetanic fade in pig muscles and long-lasting contractions of the motor innervation zone in mouse muscle. Both effects were mitigated, either by i.v. administration of MgSO₄ in vivo or by adding 5 mM Mg²⁺ to the medium bathing isolated preparations. Combination of omethoate and cyclohexanol initially potentiated muscle contractions but then rapidly blocked them. Cyclohexanol alone caused fade and block of muscle contractions in pigs and in isolated preparations. Similar effects were observed ex vivo with cyclohexanone and xylene. Cyclohexanol-induced neuromuscular block was temperature-sensitive and rapidly reversible.

CONCLUSIONS

The data indicate a crucial role for organophosphorus and solvent metabolites in muscle weakness following ingestion of agricultural OP insecticide formulations. The metabolites omethoate and cyclohexanol acted conjointly to impair neuromuscular function but their effects were mitigated by elevating extracellular Mg²⁺ and decreasing core temperature, respectively. Clinical studies of MgSO₄ therapy and targeted temperature management in insecticide-poisoned patients are required to determine whether they may be effective adjuncts to treatment.

Correct expression and localization of collagen XIII are crucial for the normal formation and function of the neuromuscular system

Transmembrane collagen XIII has been linked to maturation of the musculoskeletal system. Its absence in mice (Col13a1^-/- ) results in impaired neuromuscular junction (NMJ) differentiation and function, while transgenic overexpression (Col13a1^oe ) leads to abnormally high bone mass. Similarly, loss-of-function mutations in COL13A1 in humans produce muscle weakness, decreased motor synapse function and mild dysmorphic skeletal features. Here, analysis of the exogenous overexpression of collagen XIII in various muscles revealed highly increased transcript and protein levels, especially in the diaphragm. Unexpectedly, the main location of exogenous collagen XIII in the muscle was extrasynaptic, in fibroblast-like cells, while some motor synapses were devoid of collagen XIII, possibly due to a dominant negative effect. Concomitantly, phenotypical changes in the NMJs of the Col13a1^oe mice partly resembled those previously observed in Col13a1^-/- mice. Namely, the overall increase in collagen XIII expression in the muscle produced both pre- and postsynaptic abnormalities at the NMJ, especially in the diaphragm. We discovered delayed and compromised acetylcholine receptor (AChR) clustering, axonal neurofilament aggregation, patchy acetylcholine vesicle (AChV) accumulation, disrupted adhesion of the nerve and muscle, Schwann cell invagination and altered evoked synaptic function. Furthermore, the patterns of the nerve trunks and AChR clusters in the diaphragm were broader in the adult muscles, and already prenatally in the Col13a1^oe mice, suggesting collagen XIII involvement in the development of the neuromuscular system. Overall, these results confirm the role of collagen XIII at the neuromuscular synapses and highlight the importance of its correct expression and localization for motor synapse formation and function.

Looking back on the discovery of alpha-bungarotoxin

This review is a personal narration by a retiring pharmacologist from Taiwan who looks back at his discovery of alpha-bungarotoxin from the historical perspective of Taiwan during the last 50 years, with accounts of his experiences and his efforts to overcome hardship. How the alpha-toxin was isolated and characterized as an irreversible specific nicotinic acetylcholine (ACh) receptor antagonist, and how it subsequently became a useful experimental probe are presented here. The dilemma of differentiating the actions of tubocurarine and alpha-bungarotoxin is analyzed. The author also outlines findings based on work done in his laboratory using alpha-bungarotoxin as a tool on particular aspects of synaptic transmission. These include presynaptic receptor for positive feedback of transmitter release, explosive release of ACh, up- and downregulation of ACh receptors after chronic drug treatment, autodesensitization of junctional ACh receptors, differences in action between natural transmitter and exogenous agonists and that between junctional and extrajunctional ACh receptors. Some experimental pitfalls, in which biomedical scientists are frequently trapped, are raised. Finally, some anecdotes are appended from which the reader may further understand scientific life in the 20th century, including its joys and regrets.

Acquired slow-channel syndrome: a form of myasthenia gravis with prolonged open time of the acetylcholine receptor channel

A 32-year-old female presented with a 2-year history of fluctuating generalized weakness including extraocular, bulbar, and limb muscles, suggesting myasthenia gravis, but with poor response to pyridostigmine and unusual electromyographic findings. After rest, power increased on repeated maximal contractions, followed by progressive weakness. There were decremental responses at low-frequency stimulation, but incremental responses at high frequencies, and single stimuli evoked repetitive compound muscle action potentials. Plasmapheresis was ineffective. In a conventional assay, antibodies against acetylcholine receptors (AChRs) were borderline. However, in an assay using cells expressing mainly adult-type human AChRs, the patient's serum was positive. Thymectomy revealed a hyperplastic thymus. An intercostal muscle specimen revealed small miniature end-plate potentials, 0.22+/-0.02 mV instead of 0.56+/-0.05 mV in controls. The number of 125I-alpha-bungarotoxin binding sites was normal. The decay time constant of end-plate potentials was increased from 5.3+/-0.6 msec in controls to 23+/-3.6 msec in the patient. Ultrastructurally, there was no destruction of the end plate. Transfer of the patient's plasma to mice in vivo produced similar physiological changes in their diaphragms. We conclude that the patient has an immune-mediated disorder, in which an antibody specific to the adult form of the AChRs alters the channel properties, reducing total current and slowing the closure. We propose the name "acquired slow-channel syndrome" for this variant of myasthenia gravis.

Physiological and regenerative acetylcholine release from motor nerve: differential inhibitions by vesamicol and omega-agatoxin IVA

Stimulation of mammalian motor neurons can elicit Ca(2+)-dependent regenerative release of acetylcholine and prolonged endplate depolarization when the enzymatic degradation of the neurotransmitter is inhibited. Unlike physiological phasic release of acetylcholine, the regenerative release is sensitive to L-type Ca2+ channel blockers. We studied the effects of vesamicol (an inhibitor of active transport of acetylcholine into synaptic vesicles) and omega-agatoxin IVA (a blocker of the motor nerve P-type Ca2+ channel) on these two types of acetylcholine release to compare the vesicle pools and Ca2+ channels responsible for the release. When coupled with repetitive stimulations, vesamicol decreased mean amplitude of miniature endplate potentials, resulting in a skewed distribution to lower amplitude, reduced quantal content of endplate potentials and decreased immediate available pool of acetylcholine. omega-Agatoxin IVA had no effect on miniature endplate potential but inhibited quantal content of endplate potential. The mean inhibitory concentration was around 5-10 nM. Vesamicol and omega-agatoxin IVA decreased the probability of triggering regenerative release. However, the magnitude and duration of regenerative release, once triggered, were not depressed by either agent. It appears that the majority of Ca2+ necessary for regenerative release is translocated via omega-agatoxin IVA-insensitive Ca2+ channels, which can be activated by prolonged depolarization of nerve terminals induced by accumulated acetylcholine. The results suggest that different Ca2+ channels are activated in the regenerative (L-type) and phasic (P-type) acetylcholine release, which utilize different pools of synaptic vesicles.

Facilitation of nicotinic receptor desensitization at mouse motor endplate by a receptor-operated Ca2+ channel blocker, SK&F 96365

When acetylcholinesterase was inhibited by neostigmine, SK&F 96365 (1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride) at 10 microM caused no effect on the amplitude of single endplate potentials (e.p.p.s) but shortened the decay time in mouse phrenic nerve-diaphragm preparations. However, SK&F 96365 inhibited high-frequency stimulation-evoked long-lasting depolarization of the endplate region and accelerated the run-down of trains of e.p.p.s which were eliminated within 1 s. After a train of stimulation, SK&F 96365 produced a post-tetanic depression of single e.p.p.s. The post-tetanic effect gradually dissipated with full restoration in 10-15 s. During a train of stimulation, SK&F 96365 also depressed miniature endplate potentials (m.e.p.p.s), which were restored after termination of stimuli in parallel with the recovery of e.p.p. The decay times of miniature endplate currents during recovery phases changed slightly. In control preparations not treated with neostigmine, however, SK&F 96365 did not alter the amplitude and decay time of m.e.p.p.s or e.p.p.s but accelerated the decay of succinylcholine-induced endplate depolarizations. The results suggest that SK&F 96365 facilitates nicotinic receptor desensitization in addition to blocking receptor-operated Ca2+ channels.