Effects of chronic treatment with various neuromuscular blocking agents on the number and distribution of acetylcholine receptors in the rat diaphragm

Adaptations, Safety Factors, Limitations and Trade-Offs in Human Exercise Performance

Objectives

This review will describe how human exercise performance at the highest level is exquisitely orchestrated by a set of responses by all body systems related to the evolutionary adaptations that have taken place over a long history. The review will also describe how many adaptations or features are co-opted (exaptations) for use in different ways and have utility other than for selective advantage.

Methods

A review of the literature by relevant search engines and reference lists in key published articles using the terms, performance, limitations, regulation, trade-offs as related to exercise, indicates that there are at least three areas which could be considered key in understanding the evolutionary basis of human exercise performance.

Results

First, there is a basic assumption that exaptations have limitations or capacities which cannot be exceeded which in turn will limit our physical performance. Second, it is thought that some biological systems and tissues have additional capacity which is rarely fully accessed by the organism; referred to as a safety factor. Third, there are biological trade-offs which occur when there is an increase in one trait or characteristic traded for a decrease in another.

Conclusions

Adaptations have resulted in safety factors for body systems and tissues with trade-offs that are most advantageous for human performance for a specific environment.

Extracorporeal shock wave treatment can selectively destroy end plates in neuromuscular junctions

INTRODUCTION

This study assesses the effect of radial extracorporeal shock wave (rESW) exposure on neuromuscular transmission and neuromuscular junction (NMJ) morphology.

METHODS

We applied 2,000 rESWs at 0.18 mJ/mm² and a frequency of 15 Hz to the right calf of male rats, measured the compound muscle action potential (CMAP), and examined NMJ morphology using electron microscopy. Left calf muscles were used as controls.

RESULTS

rESW exposure significantly reduced CMAP amplitude without delayed latency in exposed muscles compared with controls. All rESW-exposed muscles exhibited NMJs with irregular end plates. Mean interjunctional fold interval was significantly increased compared with controls. However, axon terminals and muscle fibers surrounding NMJs with irregular end plates were unchanged.

DISCUSSION

This localized destruction of end plates may be caused by differences in acoustic impedance induced by the density of acetylcholine receptors. These results provide a possible mechanism for the effectiveness of rESW treatment for spasticity and dystonia. Muscle Nerve 57: 466-472, 2018.

Acquired Muscle Weakness in the Surgical Intensive Care Unit

Muscle weakness is common in the surgical intensive care unit (ICU). Low muscle mass at ICU admission is a significant predictor of adverse outcomes. The consequences of ICU-acquired muscle weakness depend on the underlying mechanism. Temporary drug-induced weakness when properly managed may not affect outcome. Severe perioperative acquired weakness that is associated with adverse outcomes (prolonged mechanical ventilation, increases in ICU length of stay, and mortality) occurs with persistent (time frame: days) activation of protein degradation pathways, decreases in the drive to the skeletal muscle, and impaired muscular homeostasis. ICU-acquired muscle weakness can be prevented by early treatment of the underlying disease, goal-directed therapy, restrictive use of immobilizing medications, optimal nutrition, activating ventilatory modes, early rehabilitation, and preventive drug therapy. In this article, the authors review the nosology, epidemiology, diagnosis, and prevention of ICU-acquired weakness in surgical ICU patients.

Treatment of Acquired Autoimmune Myasthenia Gravis: a Topic Review

We propose a new approach to staging the disease based on clinical and immunological response to treatment. We oppose clinical remission to immunological remission and define total clinical remission as the goal of therapy. We describe the use, side effects and indications of established therapies. Acetycholine esterase inhibitors are only a symptomatic treatment as is plasma exchange. Usefulness and limits of thymectomy, corticosteroids and immunosuppressants are described here. Their goal is to reduce the auto-immune process. Long-term hazards from these medications are described and methods to reduce their potential risks are suggested. We suggest the number of patients having life threatening complications while undergoing aggressive immunosuppression can be reduced by a systematic approach to follow-up. In the second part of this review article, adapting management to specific situations is emphasized in refractory disease, respiratory failure, neonatal and juvenile forms of the disease. The special situation of seronegative myasthenia is discussed.

Conditioned medium from amniotic membrane-derived cells prevents lung fibrosis and preserves blood gas exchanges in bleomycin-injured mice-specificity of the effects and insights into possible mechanisms

BACKGROUND AND AIMS

We recently demonstrated that injection of conditioned medium (CM) generated from cells of the mesenchymal region of human amniotic membrane (AMTCs) reduces bleomycin-induced lung fibrosis in mice, suggesting a crucial role of paracrine factor(s) secreted by AMTCs in these beneficial effects. We further investigated this hypothesis, the mechanisms involved, the effects on some lung functional parameters and whether AMTC-secreted effector(s) are specific to these cells and not produced by other cell types, extending the time of analysis up to 28 days after treatment.

METHODS

Bleomycin-challenged mice were either treated with AMTC-CM or CM generated from human skin fibroblasts, human peripheral blood mononuclear cells or Jurkat cells, or were left untreated. Mouse lungs were analyzed for content of pro-inflammatory and pro-fibrotic molecules, presence of lymphocytes and macrophages and for fibrosis level (through histological semi-quantitative evaluation and quantitative measurement of collagen content). Arterial blood gas analysis was also performed.

RESULTS

Up to 28 days after delivery, AMTC-CM-treated mice developed reduced lung fibrosis with respect to mice treated with other CM types. AMTC-CM-treated mice had comparatively better preservation of blood gas parameters and showed lower lung content of interleukin-6, tumor necrosis factor-α, macrophage inflammatory protein-1α, monocyte chemoattractant protein-1 and transforming growth factor-β associated with reduced lung macrophage levels.

CONCLUSIONS

AMTC-CM prevents lung fibrosis in bleomycin-challenged mice, improving survival and preserving lung functional parameters such as blood gas exchanges. The specificity of AMTC-CM action was indicated by the absence of fibrosis reduction when other CM types were used. Finally, we provide some insights into the possible mechanisms underlying AMTC-CM-mediated control of fibrosis.

In vitro neurotoxic effects of Pseudechis spp. venoms: A comparison of avian and murine skeletal muscle preparations

Two common in vitro skeletal muscle preparations used for the study of venom neurotoxicity are the indirectly stimulated chick isolated biventer cervicis nerve-muscle preparation and the rat isolated phrenic nerve-diaphragm preparation. The aim of the current study was to compare the in vitro neurotoxicity of six Pseudechis spp. (Black snakes) venoms in both avian (chicken) and mammalian (rat) skeletal muscle preparations to determine differences in sensitivity. All Pseudechis spp. venoms significantly inhibited indirect twitches, in both preparations, indicating the presence of post synaptic neurotoxins. The inhibitory effects of all venoms were more rapid in the avian preparation, except for Pseudechis colletti venom where no significant difference was seen between the murine and avian muscles. Time taken to produce 50% reduction in stimulated twitches (i.e. t(50)) was markedly shorter in the avian preparation. We have shown that the avian in vitro preparation is more sensitive to the neurotoxic activity of Pseudechis spp. than the murine preparation. This difference is likely to be due to species differences in the interaction between the neurotoxins and the nicotinic receptor binding sites as well as differences in the 'safety factor' between the preparations.

Truncated dystrophins can influence neuromuscular synapse structure

Duchenne muscular dystrophy (DMD) is characterized by muscle degeneration and structural defects in the neuromuscular synapse that are caused by mutations in dystrophin. Whether aberrant neuromuscular synapse structure is an indirect consequence of muscle degeneration or a direct result of loss of dystrophin function is not known. Rational design of truncated dystrophins has enabled the design of expression cassettes highly effective at preventing muscle degeneration in mouse models of DMD using gene therapy. Here we examined the functional capacity of a minidystrophin (minidysGFP) and a microdystrophin (microdystrophin(DeltaR4-R23)) transgene on the maturation and maintenance of neuromuscular junctions (NMJ) in mdx mice. We found that minidysGFP prevents fragmentation and the loss of postsynaptic folds at the NMJ. In contrast, microdystrophin (DeltaR4-R23) was unable to prevent synapse fragmentation in the limb muscles despite preventing muscle degeneration, although fragmentation was observed to temporally correlate with the formation of ringed fibers. Surprisingly, microdystrophin(DeltaR4-R23) increased the length of synaptic folds in the diaphragm muscles of mdx mice independent of muscle degeneration or the formation of ringed fibers. We also demonstrate that the number and depth of synaptic folds influences the density of voltage-gated sodium channels at the neuromuscular synapse in mdx, microdystrophin(DeltaR4-R23)/mdx and mdx:utrophin double knockout mice. Together, these data suggest that maintenance of the neuromuscular synapse is governed through its lateral association with the muscle cytoskeleton, and that dystrophin has a direct role in promoting the maturation of synaptic folds to allow more sodium channels into the junction.

Safety factor for neuromuscular transmission at type-identified diaphragm fibers

The safety factor (SF) for neuromuscular transmission varies across limb muscles of different fiber-type composition. Using intracellular recordings in rat diaphragm fibers, we found that SF varies across muscle fiber types (even within a single muscle), being larger for type IIx or IIb fibers than for type I or IIa fibers. Fiber-type differences in activation history or mechanical load may contribute to differences in SF and are important determinants of neuromuscular plasticity.

Up-regulation of acetylcholine receptors during subchronic infusion of pancuronium is caused by a posttranscriptional mechanism related to disuse

OBJECTIVE

Contrasting with the classic theory that competitive block of the acetylcholine receptor induces up-regulation of the receptor, recent studies show that irreversible block of acetylcholine receptors with alpha-bungarotoxin decreases acetylcholine receptor number within hours. This study investigated the early effects of competitive acetylcholine receptor block with the reversible, competitive muscle relaxant, pancuronium.

DESIGN

Prospective, randomized, placebo-controlled experimental study.

SUBJECTS

Healthy adult Sprague-Dawley rats.

SETTING

Animal laboratory in a university hospital.

INTERVENTIONS

After internal review board approval, Sprague-Dawley rats were anesthetized and received pancuronium at a rate to completely suppress neuromuscular twitch. The control group received saline. Infusion times were 0, 3, 6, or 12 hrs (n = 8 per group). One sciatic nerve was stimulated to induce muscle twitch, and the other nerve remained unstimulated. Total acetylcholine receptor expression, as well as expression of messenger RNA of the five subunits, was assayed.

MEASUREMENTS AND MAIN RESULTS

There were no differences in acetylcholine receptor number between groups at time points 0, 3, and 6 hrs. At 12 hrs, acetylcholine receptor numbers in both the stimulated (35.2 +/- 4.8 fmol acetylcholine receptor/mg protein) and nonstimulated (38.3 +/- 4.8) pancuronium group, as well as the nonstimulated control saline group (37.5 +/- 4.6), were significantly increased compared with stimulated controls (27.6 +/- 4.0). Pancuronium did not potentiate the acetylcholine receptor up-regulation of the nonstimulated control group at 12 hrs. There were no changes in messenger RNA expression between groups.

CONCLUSIONS

Infusion of the reversible competitive inhibitor pancuronium up to 12 hrs does not reduce acetylcholine receptor number and therefore contrasts with the irreversible acetylcholine receptor blocker alpha-bungarotoxin. This study documents that 12 hrs of disuse per se leads to an increased expression of the acetylcholine receptor number by a posttranscriptional mechanism that can be prevented by nerve-evoked muscle contraction.

Transients in acetylcholine receptor site density and degradation during reinnervation of mouse sternomastoid muscle

The degradation rates of acetylcholine receptors (AchRs) were evaluated at the neuromuscular junction during and just after reinnervation of denervated muscles. When mouse sternomastoid muscles are denervated by multiple nerve crush, reinnervation begins 2-4 days later and is complete by day 7-9 after the last crush. In fully innervated muscles, the AChR degradation rate is stable and slow (t1/2 approximately 10 days), whereas after denervation the newly inserted receptors degrade rapidly (t1/2 approximately 1.2 days). The composite profile of degradation, which a mixture of the stable and the rapid receptors would give, is not observed during reinnervation. Instead, the receptors inserted between 2.5 and 7.5 days after the last crush all have an intermediate degradation rate of t1/2 approximately 3.7 days with standard error +/- 0.3 days. The total receptor site density at the endplate was evaluated during denervation and during reinnervation. As predicted theoretically, the site density increased substantially, but temporarily, after denervation. An analogous deleterious substantial decrease in density would be expected during reinnervation, without the intermediate receptor. This decrease is not observed, however, because of a large insertion rate at intermediate times (3000 +/- 700 receptor complexes per micro m2 per day). The endplate density of receptors thus remains relatively constant.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

Prednisolone-induced muscle dysfunction is caused more by atrophy than by altered acetylcholine receptor expression

Large doses of glucocorticoids can alter muscle physiology and susceptibility to neuromuscular blocking drugs by mechanisms not clearly understood. We investigated the effects of moderate and large doses of prednisolone on muscle function and pharmacology, and their relationship to changes in muscle size and acetylcholine receptor (AChR) expression. With institutional approval, 35 Sprague-Dawley rats were randomly allocated to receive daily subcutaneous doses of 10 mg/kg prednisolone (P10 group), 100 mg/kg prednisolone (P100 group), or an equal volume of saline (S group) for 7 days. A fourth group of rats was pair fed (food restricted) with the P100 rats for 7 days (FR group). On Day 8, the nerve-evoked peak twitch tensions, tetanic tensions, and fatigability, and the dose-response curves of d-tubocurarine in the tibialis cranialis muscle were measured in vivo and related to muscle mass or expression of AChRs. Rate of body weight gain was depressed in the P100, FR, and P10 groups compared with the S group. Tibialis muscle mass was smaller in the P100 group than in the P10 or S groups. The evoked peak twitch and tetanic tensions were less in the P100 group than in the P10 or S groups, however, tension per milligram of muscle mass was greater in the P100 group than in the S group. The 50% effective dose of d-tubocurarine (microg/kg) in the tibialis muscle was smaller in the P10 (33.6 +/- 5.4) than in the S (61.9 +/- 5.0) or the P100 (71.3 +/- 9.6) groups. AChR expression was less in the P10 group than in the S group. The evoked tensions correlated with muscle mass (r(2) = 0.32, P < 0.001), however, not with expression of AChR. The 50% effective dose of d-tubocurarine did not correlate with muscle mass or AChR expression. Our results suggest that the neuromuscular dysfunction after prednisolone is dose-dependent, and derives primarily from muscle atrophy and derives less so from changes in AChR expression.

IMPLICATIONS

The mechanisms by which chronic glucocorticoid therapy alters neuromuscular physiology and pharmacology are unclear. We suggest that the observed effects are dose-dependent and derive primarily from muscle atrophy and derive less from changes in acetylcholine receptor expression.

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.

Purinoceptors in neuromuscular transmission

At the neuromuscular junction, P2-purinoceptors mediate the actions of the co-transmitter ATP and P1-purinoceptors, those of its degradation product adenosine. The classification of the subtypes of P1- and P2-purinoceptors and their signal transduction routes is presented. Purinoceptor-mediated effects on the prejunctional release of acetylcholine and the postjunctional desensitization and expression of nicotinic receptors are discussed in depth. An additional section on the reversal action of the P2-purinoceptor antagonist suramin on neuromuscular block underscores the importance of testing purinoceptor-targeted drugs once they will be marketed, to avoid adverse effects in patients.

The contribution of postsynaptic folds to the safety factor for neuromuscular transmission in rat fast- and slow-twitch muscles

1. At the rat neuromuscular junction, the postsynaptic folds and the voltage-gated sodium channels (VGSCs) within them are thought to amplify the effects of postsynaptic currents. In this study, the contribution of this effect to the safety factor for neuromuscular transmission, the ratio of the normal quantal content to the number of quanta required to reach threshold, has been estimated. 2. Normal quantal content was determined in isolated nerve-muscle preparations of rat soleus and extensor digitorum longus (EDL) muscles in which muscle action potentials were blocked by mu-conotoxin. The quantal content estimated from voltage recordings was 61.8 and 79.4 in soleus and EDL, respectively, and from charge measurements derived from current recordings was 46.3 (soleus) and 65.1 (EDL). 3. The threshold for action potential generation in response to nerve stimulation was determined from endplate potentials (EPPs) and endplate currents (EPCs) in preparations partially blocked with d-tubocurarine. The number of quanta required to reach threshold was estimated from voltage recordings to be 19.7 (soleus) and 23.2 (EDL) and from charge measurements derived from current recordings to be 13.3 (soleus) and 13.0 (EDL). 4. When intracellular electrodes were used to inject current into the muscle fibre, the total charge required to reach threshold was approximately twice that of the nerve-evoked threshold EPC. 5. The safety factor for nerve-evoked responses at the junction was 3.5 (soleus) and 5.0 (EDL). In the extrajunctional region the safety factor estimated from injected currents was 1.7 (soleus) and 2.5 (EDL). 6. It is concluded that the effect of the postsynaptic folds and the VGSCs within them is to double the safety factor. At normal frequencies of nerve impulse activity in vivo, this effect is likely to be crucial for ensuring effective neuromuscular transmission.

Properties of fibres, endplates and acetylcholine receptors in the diaphragm, masseter, laryngeal, abdominal and limb muscles in the goat

PURPOSE

Although differences in fibre composition, fibre size or acetylcholine receptor (AChR) density between muscles have often been proposed to explain the unequal sensitivities of muscles to muscle relaxant drugs, it is not clear whether or how these parameters differ among muscles or are related to one another, In this study, several muscles were examined to determine the composition and cross-sectional area (CSA) of types I and II fibres, the surface area of their motor endplates (ESA), and their AChR density.

METHODS

Biopsies from the thyroarytenoideus, cricoarytenoideus dorsalis, masseter, diaphragm, transversus abdominis, rectus abdominis, gastrocnemius and soleus muscles of goats were processed by muscle histochemistry and morphometry and the ESA:CSA ratio was computed. The number and density of AChRs per endplate were estimated by 125I-alpha-bungarotoxin binding studies.

RESULTS

The mean type I fibre composition (range: 0-100%), fibre diameter (28-50 microns) and the ESA:CSA ratio (0.27-1.01) differed among muscles (P = 0.0001), but there were no significant differences (P > 0.05) in the mean endplate size (577-725 microns 2) AChR number (6.6-14.5 x 10(6)) or AChR density (8,900-22,300 microns-2) probably because of marked individual variations. Fibre size increased and the ESA:CSA ratio decreased in the order laryngeal, diaphragm, jaw, limb and abdominal muscles.

CONCLUSION

It is concluded that between muscles fibre size varies more than endplate size or AChR number.

Enhancement by benzodiazepines of the inhibitory effect of adenosine on skeletal neuromuscular transmission

1. Interactions of benzodiazepines with adenosine on the neuromuscular transmission were studied in mouse diaphragm preparations. 2. In tubocurarine (0.6-0.8 microM)-partially paralyzed preparations, diazepam (35 microM) and Ro 5-4864 (3-30 microM), a peripheral type benzodiazepine receptor agonist, potentiated the inhibitory effect of adenosine on indirect twitch responses. 3. The central type receptor agonist, clonazepam did not affect the inhibitory effect of adenosine. 4. The peripheral benzodiazepine receptor antagonist, PK11195 (1-10 microM) attenuated the adenosine inhibition and antagonized the potentiation by Ro 5-4864. 5. Ro 5-4864 failed to enhance further the inhibitory effect of adenosine in the presence of dipyridamole, an adenosine uptake inhibitor that also potentiated adenosine inhibition. 6. Neither Ro 5-4864 nor PK 11195 affected the inhibition produced by a stable adenosine analogue, 2-chloroadenosine, which is not a substrate for the adenosine uptake system. 7. Ro 5-4864 did not affect endplate potentials (e.p.ps) in the absence of adenosine, but reduced the amplitude of e.p.ps in the presence of adenosine without affecting miniature e.p.ps. 8. It is suggested that benzodiazepines potentiate the adenosine-effected presynaptic inhibition of neuromuscular transmission by an inhibition of adenosine uptake through activation of peripheral type benzodiazepine receptors.

Neurotoxins in the study of neural regulation of membrane proteins in skeletal muscle

The discovery and purification of several neurotoxins, including alpha-bungarotoxin and tetrodotoxin has provided very high-affinity ligands which have proved to be central to the elucidation of the neural control of skeletal muscle membrane proteins and to the purification and characterization of the nicotinic acetylcholine receptor (AChR) and the Na+ channel, respectively. This review describes the use of neurotoxins for quantification and localization of receptors and ion channels in normal and denervated skeletal muscles with particular emphasis on the appropriateness of the muscle preparation and ligand used in the studies. It is now clear that the nerve controls the synthesis and spatial distribution of AChRs and Na+ channels by regulating gene expression in extrajunctional and subjunctional nuclei. The down-regulation of extrajunctional AChRs is primarily mediated by neuromuscular activity and the concentration of AChRs and Na+ channels in specific membrane domains at the neuromuscular junction is controlled by a number of neurotrophic substances at the neuromuscular junction. These include agrin, ARIA, and CGRP.

Induction of Na+/K(+)-ATPase activity by long-term stimulation of nicotinic acetylcholine receptors in C2C12 myotubes

1. To investigate the role of long-term stimulation of nicotinic acetylcholine receptors (AChRs) on the regulation of membrane potential, non-contracting C2C12 myotubes were stimulated for 1-4 days with carbachol (10 microM) and membrane potentials were measured by the intracellular microelectrode technique after washing out of the drug. 2. The membrane potential (-45.7 mV) gradually increased by 10.1 mV to -55.8 mV during 4 days treatment, which was caused by enhanced electrogenic Na+/K(+)-pumping. 3. The concentration-dependent enhancement of Na+/K(+)-ATPase activity in long-term carbachol-treated myotubes (4 days, EC50 = 5.3 microM) was prevented by co-treatment with the competitive nicotinic AChR antagonist, pancuronium but not by the muscarinic antagonist, atropine. 4. Enhanced Na+/K(+)-ATPase activity still developed in carbachol-stimulated myotubes during co-treatment (4 days) with the nicotinic AChR-channel blocker, chlorpromazine (1 microM). Membrane depolarization as such, obtained by incubation in high K+ medium (40 mM, 4 days) did not enhance Na+/K(+)-ATPase activity. 5. Non-treated myotubes possessed a high-affinity ouabain binding site (Kd = 119 nM) in association with the low Na+/K(+)-pumping activity. Long-term stimulation of myotubes (4 days) with carbachol or with a combination of carbachol and chlorpromazine was accompanied by the development of an additional low-affinity ouabain binding site (Kd = 13 microM). 6. Binding of monoclonal antibodies directed against either alpha 1- or alpha 2-subunit of Na+/K(+)-ATPase were both increased in myotubes treated with carbachol (4 days). 7. These results support the concept that nicotinic AChRs regulate Na+/K(+)-ATPase activity, independent of the functionality of the receptor-operated ion-channel.

Effect of myasthenic IgG on degradation of junctional acetylcholine receptor

We investigated the effect of the IgG from patients with myasthenia gravis (MG) on the degradation of normal rat junctional acetylcholine receptor (AChR) labeled with 125I-alpha-bungarotoxin (BuTx) and calculated the degradation rate (DR). The DR for the IgG from these patients was significantly higher than that from healthy volunteers and patients with other autoimmune diseases. For MG, DR was significantly correlated with the severity of the disease but not with anti-AChR antibody titer. DR was accelerated by IgG from patients with generalized MG whose antibody titers were in the normal range and by IgG from patients with ocular MG. These results indicate that measurement of the DR of junctional AChR in normal rats is more closely correlated with the severity of the disease than is measurement of anti-AChR antibody and that the former is a sensitive and confirmatory method for evaluating MG.

Stabilization of acetylcholine receptors at the neuromuscular synapse: the role of the nerve

The majority of acetylcholine receptors (AChRs) at innervated neuromuscular junctions (NMJs) are stable, with half-lives averaging about 11 days in rodent muscles. In addition to the stable AChRs, approximately 18% of AChRs at these innervated junctions are rapidly turned over (RTOs), with half lives of less than 24 h. We have postulated that RTOs may be precursors of stable AChRs, and that the motor nerve may influence their stabilization. This hypothesis was tested by: (i) labeling AChRs in mouse sternomastoid (SM) muscles with 125I-alpha-BuTx; (ii) denervating one SM muscle in each mouse, and (iii) following the fate of the labeled AChRs through a 5-day period when RTOs were either stabilized or degraded. The hypothesis predicts that denervation should preclude stabilization of RTOs, resulting in a deficit of stable AChRs in denervated muscles. The results showed a highly significant (P less than 0.002) deficit of stable AChRs in denervated as compared with innervated muscles. Control experiments excluded the possibility that this deficit could be attributed to independent accelerated degradation of either RTOs or pre-existing stable AChRs. The observed deficit was quantitatively consistent with the deficit predicted by a mathematical model based on interruption of stabilization following denervation. We conclude that: (i) the observed deficit after denervation of NMJs is due to failure of stabilization of pre-existing RTOs; (ii) RTOs at normally innervated NMJs are precursors of stable AChRs; (iii) stabilization occurs after the insertion of AChRs at NMJs, and (iv) motor nerves play a key role in stabilization of RTOs. The concept of receptor stabilization has important implications for understanding the biology of the neuromuscular junction and post-synaptic plasticity.

Mechanisms of the synergistic interactions between organic calcium channel antagonists and various neuromuscular blocking agents

The effects of Mn2+, neomycin and four organic Ca2(+)-channel antagonists (OCA): nicardipine, nifedipine, diltiazem and verapamil on the neuromuscular blocking activities of tubocurarine, succinylcholine (SCh), decamethonium and neomycin were studied in isolated mouse phrenic nerve-diaphragm preparations. The effective concentration of SCh for 50% inhibition (IC50) of single indirect twitch responses were reduced markedly by more than 3-fold when the preparations were pretreated with OCA at 10 microM; the latter alone did not appreciably affect the indirect twitch response or the amplitude of miniature endplate potentials. The neuromuscular blocking effect of decamethonium was also enhanced synergistically by OCA to a similar extent. On the other hand, under the comparable condition. the combined uses of OCA plus tubocurarine or neomycin, neomycin plus tubocurarine or SCh, and Mn2+ plus tubocurarine, SCh or neomycin all resulted in insignificant potentiation. These results suggest that OCA have a specific effect to enhance the agonist effect of depolarizing agents on nicotinic acetylcholine receptors. Nicardipine at 2 microM non-competitively inhibited depolarizations of endplates elicited by SCh and decamethonium and abolished them completely at 10 microM nicardipine. The IC50's in inhibiting endplate potentials and miniature endplate potentials by SCh and decamethonium were also reduced 2 to 3.5-fold by nicardipine. It is inferred that OCA are endowed with a unique capability to allosterically affect the postsynaptic nicotinic acetylcholine receptor, promoting its desensitization liability, hence synergistic interaction with depolarizing agents. Presynaptic effects of OCA are probably not involved.

Selective antagonism to succinylcholine-induced depolarization by alpha-bungarotoxin with respect to the mode of action of depolarizing agents

1. The interactions of alpha-bungarotoxin or tubocurarine with the neuromuscular block and endplate depolarization induced by succinylcholine (SCh) in the phrenic nerve-diaphragm preparation of mice were studied in order to elucidate the role of depolarization by SCh in the neuromuscular blockade. 2. The SCh concentrations required to depress the indirect twitch response by 20% and the evoked endplate potential in cut muscle preparations by 80% were 10 microm and 6 microM, respectively, while only 2 microM SCh was needed to induce maximal endplate depolarization from -80 mV to about -60 mV. 3. SCh blocked the neuromuscular transmission synergistically with either alpha-bungarotoxin or tubocurarine. There was an initial partial reversal of the neuromuscular inhibition caused by tubocurarine, but not that by alpha-bungarotoxin. 4. alpha-Bungarotoxin (0.025 microM) antagonized SCh (10 microM)-induced depolarization more effectively than it depressed miniature endplate potentials and the antagonism was insurmountable by increasing SCh concentration. By contrast, tubocurarine preferentially depressed miniature endplate potentials and antagonized SCh-depolarization competitively. 5. The above difference was attributed to the irreversible nature of alpha-bungarotoxin binding to acetylcholine receptors, to the slow diffusion of the toxin molecule into the synaptic cleft and thus to the more rapid binding with perijunctional receptors compared with junctional ones. 6. It is concluded that the sustained depolarization of the endplate by SCh results largely from an action on the perijunctional receptor in mice and, unlike cats, the neuromuscular block by SCh is not due to the depolarization per se but rather to a direct attenuation of endplate potential.

Rapid synthesis of acetylcholine receptors at neuromuscular junctions

The rate of acetylcholine receptor (AChR) degradation in mature, innervated mammalian neuromuscular junctions has recently been shown to be biphasic; up to 20% are rapidly turned over (RTOs; half life less than 1 day) whereas the remainder are lost more slowly ('stable' AChRs; half life 10-12 days). In order to maintain normal junctional receptor density, synthesis and insertion of AChRs should presumably be sufficiently rapid to replace both the RTOs and the stable receptors. We have tested this prediction by blocking pre-existing AChRs in the mouse sternomastoid muscle with alpha-bungarotoxin (alpha-BuTx), and monitoring the subsequent appearance of 'new' junctional AChRs at intervals of 3 h to 20 days by labeling them with 125I-alpha-BuTx. The results show that new receptors were initially inserted rapidly (16% at 24 h and 28% at 48 h). The rate of increase of 'new' 125I-alpha-BuTx binding sites gradually slowed down during the remainder of the time period studied. Control observations excluded possible artifacts of the experimental procedure including incomplete blockade of AChRs, dissociation of toxin-receptor complexes, or experimentally induced alteration of receptor synthesis. The present demonstration of rapid synthesis and incorporation of AChRs at innervated neuromuscular junctions provides support for the concept of a subpopulation of rapidly turned over AChRs. The RTOs may serve as precursors for the larger population of stable receptors and have an important role in the metabolism of the neuromuscular synapse.

Neuromuscular block by verapamil and diltiazem and inhibition of acetylcholine release

The effects of Ca2+-channel antagonists, verapamil, diltiazem and nifedipine, on the neuromuscular transmission were studied in the isolated mouse phrenic nerve-diaphragm preparations. All 3 drugs increased the twitch response evoked by direct single stimulation at 10-100 microM. The neuromuscular transmission at 0.1 Hz was blocked by verapamil and diltiazem, but not by nifedipine, only at very high concentrations (greater than or equal to 100 microM). In the time course of block, no endplate potential (e.p.p.) could be recorded, whenever the junction failed to elicit an action potential, suggesting that the block is due to an axonal conduction failure. Conduction block became apparent in both axon and muscle at low concentrations (greater than 10 microM) of verapamil and diltiazem at 100 Hz. When the safety margin of neuromuscular transmission was reduced by tubocurarine or low Ca2+ plus high Mg2+, verapamil and diltiazem, but not nifedipine, reduced the single twitch response to nerve stimulation at concentrations that did not cause axon conduction block. The inhibition was dependent on the frequency of nerve stimulation, enhanced by low-Ca2+ and antagonized by high-Ca2+. Verapamil (50 microM) inhibited the mean amplitude of the median size miniature e.p.p. by only 8%, whereas it increased the frequency by 4-5-fold and the proportion of both small and giant miniature e.p.p.s. The e.p.p. amplitude was inhibited by verapamil by about 67% in low-Ca2+ media and by about 38% in normal Tyrode. Similar but somewhat lesser effect was obtained with diltiazem. It is concluded that verapamil and diltiazem, but not nifedipine, inhibit the transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of extrajunctional acetylcholine receptors on a vertebrate muscle: evaluated by using a scanning electron microscope autoradiographic procedure

A scanning electron microscope (SEM) autoradiographic technique was calibrated and used to determine the site density of acetylcholine receptors within 250 micron of the neuromuscular junction in innervated as well as 3- and 10-d denervated sternomastoid muscle of the mouse. In all these groups sharp gradients of receptor site density are seen around the endplates in the first 2-7 micron, continuing less sharply to between 25 and 50 micron. Beyond 50 micron (to 250 micron) a spatial density gradient is present 3 d after denervation, but none exist by 10 d. These results suggest that the postdenervation steady-state extrajunctional receptor site density is reached sooner near the junction than away from the junction. The usefulness of SEM autoradiography to study the expression and distribution of membrane molecules at high resolution is demonstrated.

Acetylcholine receptors at mature and aged mouse neuromuscular junctions

The density and distribution of junctional and perijunctional ACh receptors (AChR) were studied in young (8-12 months) and old (24-25 months) C57 mice to determine: (1) if increased amplitude of spontaneous postsynaptic potentials previously reported in old C57 muscle was due to increased junctional AChR; (2) if increased extrajunctional AChR would be found in association with previously reported nerve terminal complexity; and (3) if extrajunctional AChR was present as in disused or denervated muscle. Microdissection of individual muscle fibers combined with I125-alpha-bungarotoxin labeling, gamma counting, measurement of surface area, cholinesterase stains, and autoradiography were used to obtain the results. In both young and old mice there was a sharp gradient in AChR between the end-plate and the perijunctional region. End-plate AChR densities and total AChR per end-plate were the same at old and young end-plates, as were perijunctional values. Thus, neither end-plate nor extrajunctional AChR density changes with age. An increased mepp amplitude reported previously in old CB57 animals must be due to other factors. The perijunctional AChR in old mice show no changes characteristic of disuse or denervation, or those which might give rise to the observed nerve terminal complexity.

Reversal of snake neurotoxin binding to mammalian acetylcholine receptor by specific antiserum

Snake curaremimetic toxins are known to bind to the nicotinic acetylcholine receptor (AcChoR) [Changeux et al. (1970) Proc. Natl Acad. Sci. USA, 67, 1241-1247], thus blocking neuromuscular transmission, and producing respiratory failure in mammals. In the present paper we show that the toxic effects of Naja nigricollis toxin alpha to mammals can be efficiently reversed by toxin-alpha-specific antibodies. In vivo we observed that return to normal breathing in toxin-alpha-intoxicated and ventilated rats was 12 times faster after injection of specific antiserum or monoclonal antibody (M-alpha 1) as compared with control animals. Ex vivo we observed that return to normal contraction of a toxin-alpha-blocked phrenic nerve-hemidiaphragm preparation was 14 times more rapid after treatment with specific antiserum than after washings. In vitro we observed that antibodies accelerated the reversal of binding of [3H]toxin alpha to AcChoR prepared from rat diaphragm. The observation made in vitro furthermore indicates that antibodies are capable of destabilizing the [3H]toxin-AcChoR complex. A similar destabilization phenomenon occurs also in vivo, as inferred from measurements of receptor occupancy by [3H]toxin alpha in diaphragm of anaesthetized rats in the presence or absence of antibodies. The property of antibodies to reverse neurotoxin binding to AcChoR may be considered as a critical test for evaluation of the quality of a neurotoxin-specific antisera.

In vitro autoradiography of [3H]acetylcholine binding in rat hind limb muscles

The morphological distribution of acetylcholine receptors in normal and denervated rat tibialis anterior and soleus muscles was studied by in vitro quantitative autoradiography with [3H]acetylcholine (ACh). In normal muscles [3H]ACh binding was restricted to the neuromuscular junction, while denervated muscles showed binding also in extrajunctional areas. After denervation there was also an increase in the junctional binding.

Mechanism of action of lithium on acetylcholine receptor metabolism in skeletal muscle

Changes in the levels of cations within skeletal muscle are thought to mediate the neural regulation of turnover of extrajunctional acetylcholine receptors (AChRs). We have used lithium as a probe of these cation influences because of its resemblance to calcium and other ions. In the present experiments we studied the mechanism of action of lithium on AChR metabolism in cultured mammalian skeletal muscle. We measured the effects of lithium on AChR turnover (using [125I]alpha-bungarotoxin binding), and evaluated the resemblance of lithium and calcium in producing their effects on AChR metabolism. Our results provide insight into the mechanisms of action of lithium and the cellular processes controlling AChR metabolism in muscle. Lithium reduces the number of AChRs in skeletal muscle in vitro to a degree similar to that which we previously reported in vivo. Lithium appears to enter cells via both sodium and calcium channels. It then produces its effect on levels of AChRs primarily by selectively reducing AChR synthesis and insertion into the surface membrane. Lithium induces this change in AChR metabolism in a manner resembling neural and calcium-mediated effects on AChRs. Phosphoinositide pathways may be involved in the lithium-induced effects. Further analysis of the effects of lithium on AChR turnover should provide new information about the mechanisms underlying the cellular control of receptor metabolism.

Acetylcholine receptors and sodium channels in denervated and botulinum-toxin-treated adult rat muscle

1. The number of acetylcholine (ACh) receptors and Na channels was measured in adult rat hind-limb muscles after denervation or injection of botulinum toxin type A (BoTX), using specific binding of radiolabelled neurotoxins. 2. Denervation by sciatic nerve section increased the number of [125I]iodo-alpha-bungarotoxin ([125I]BTX) binding sites from low, unmeasurable levels to 39 +/- 3 fmol of toxin bound per milligram muscle protein at 21 days. 3. Subcutaneous injection of BoTX produced complete neuromuscular blockade for 11-14 days over which time the number of [125I]BTX binding sites increased with the same time course and to the same extent as following denervation. 4. Neither denervation nor BoTX treatment significantly altered the number of tritiated saxitoxin ([3H]STX) binding sites from normal values of 7.8 fmol/mg muscle weight or 57 +/- 3 fmol/mg homogenate protein. This may, however, correspond to a lower density of [3H]STX sites in the muscle membrane. 5. It was concluded that neuromuscular blockade with BoTX is equivalent to denervation in its effects on synthesis of ACh receptors. Numbers of Na channels are more stable than ACh receptors but may also be modulated by neuromuscular activity.

Denervation supersensitivity as a model for the neural control of muscle

A review about the neural regulation of the membrane distribution of muscle acetylcholine receptors is presented on the basis of author's and literature data. The main evidence in support of the role of nerve impulses and of chemical factors ("trophic" factors, acetylcholine, nerve breakdown products) as neural signals involved in the origin of denervation supersensitivity, is concisely described and evaluated. The contribution of breakdown products of the nerve, their interaction with muscle inactivity are illustrated and a unifying hypothesis on the neural control of extrajunctional and junctional acetylcholine receptors is presented.

Rapid degradation of "new" acetylcholine receptors at neuromuscular junctions

Acetylcholine receptors at innervated neuromuscular junctions are very stable, with half-lives reported to be 6 to 13 days. Their turnover is described as a first-order process, implying a single population of receptors. In this study, two subpopulations of acetylcholine receptors at normally innervated junctions have been identified. One has a rapid turnover rate with a half-life of 18.7 hours, similar to that of extrajunctional receptors, and the other has a slow turnover rate with a half-life of 12.4 days. The rapidly turned over subpopulation represents approximately 20 percent of the total junctional receptors. This finding may account for the discrepancies in previous reports of turnover rates and may explain the rapid reversibility in vivo of agents that "irreversibly" block acetylcholine receptors. This finding also implies that the synthesis rate of junctional acetylcholine receptors may be higher than previous estimates. The rapidly turned-over subpopulation may represent receptors that were newly inserted into the neuromuscular junction and that were not yet stabilized by an influence of the motor nerve.

Properties of junctional acetylcholine receptors that appear rapidly after denervation

It was previously found that the number of junctional acetylcholine receptors of rat diaphragm, as measured with [125I]alphabungarotoxin binding, suddenly increased 2 days after denervation in vivo or in vitro. Organ culture was used here to characterize further this unusual class of junctional receptors. The 'new' acetylcholine receptors were physiologically functional and were functionally located only in the junctional region. The rate of degradation of new receptors was slower than that of extrajunctional receptors and similar (in the first 24 h) to that of typical junctional receptors. In addition, the appearance of new junctional receptors was inhibited by cycloheximide and actinomycin D given at critical periods, implicating a protein synthetic step. Finally, nerve stimulation in the presence of a post-synaptic blocker (pancuronium) advanced the time of appearance of new junctional receptors. This last finding coupled with our previous report of nerve stump length effects on junctional acetylcholine receptors reinforces the suggestion that under certain conditions the level of junctional receptors can be regulated by the motor neuron.

The acetylcholine receptors of human muscles are not stained with rhodamine-labeled erabutoxin-b

The skeletal muscles from man and other vertebrates were incubated in rhodamine-labeled erabutoxin-b(TMR-Eb), and the distribution of the acetylcholine receptors (AChRs) at the neuromuscular junctions was examined under a fluorescence microscope. The AChRs of human muscles were not stained with TMR-Eb, whereas those of fish, frog, chicken, mouse, rat, cat and monkey muscles were stained under the same conditions. It was concluded that human AChRs have low or no binding ability for TMR-Eb.

A comparison of nerve transection and chronic application of beta-bungarotoxin on acetylcholine receptor distribution and other nerve-muscle properties

beta-Bungarotoxin (beta-BuTX), a snake venom neurotoxin which acts presynaptically to inhibit acetylcholine (ACh) release at the neuromuscular junction, was applied to the rat phrenic nerve-diaphragm muscle preparation to determine its effectiveness to mimic denervation. The distribution of junctional and extrajunctional ACh receptors on the muscle were assayed biochemically by [125I]alpha-bungarotoxin ( [125I]alpha-BuTX) binding and electrophysiologically by iontophoretic application of ACh. Spontaneous transmitter release and muscle membrane potential were measured under conditions of denervation, beta-BuTX treatment, and bee venom phospholipase A2 exposure. Within 7 days after treatment with a single dose (5 micrograms/kg) of enzymatically active beta-BuTX, extrajunctional [125I]alpha-BuTX binding increased fivefold, and there was a decrease in miniature end-plate potential (MEPP) frequency and in resting membrane potential (RMP) to values less than those of control muscles but greater than those of denervated.

Immunopathology of acetylcholine receptors in myasthenia gravis

It is now clear that the muscular weakness and fatigability seen in MG result from an antibody-mediated immune response to AChR. The mechanisms by which antibodies impair transmission are moderately well understood and detection of antibodies in patient's sera is a reliable diagnostic test for the disease. The spectrum of antibody specificities produced in MG is also beginning to be understood, largely through the use of antibodies produced in the experimental model EAMG. Treatment for MG continues to rely heavily on the symptomatic relief afforded by acetylcholinesterase inhibitors. However, the recent recognition of the autoimmune nature of MG has led to increased emphasis on immunosuppression and antibody removal with some beneficial effects. Despite all that has been learned, the level of ignorance has just been pushed back one step--from the neuromuscular junction to the immune system. What initiates the immune response to AChR in MG and how to specifically suppress this aberrant response remain completely unknown.

Influence of presynaptic receptors on neuromuscular transmission in rat

The presence and physiological significance of acetylcholine (ACh) receptors on motor nerve terminals was examined at the rat diaphragm neuromuscular junction. Intracellular recording techniques were used to monitor end-plate potentials (EPP), miniature end-plate potentials (MEPP), and resting potentials of the muscle fibers. Muscle action potentials were blocked by the cut-muscle technique. Quantal release was determined by the ratio EPP/MEPP, after correcting for nonlinear summation. Blockade of acetylcholinesterase with eserine and neostigmine was tested to determine the influence of residual ACh on transmitter release. Partial blockade of ACh receptors with curare was examined to further clarify the role of these presynaptic receptors. The experiments demonstrate that residual ACh inhibits transmitter release and that blockade of ACh receptors enhances transmitter release. It is concluded that presynaptic ACh receptors exist and that they serve an important physiological function. It is suggested that the presynaptic ACh receptors normally serve to limit transmitter release in a negative feedback pathway.

A transient increase in junctional acetylcholine receptors after denervation

An improved method for assaying acetylcholine (ACh) receptors at the neuromuscular junction has been used to examine the effects of denervation in the rat diaphragm. An early increase of junctional ACh receptors occurred after two days of denervation followed by a decline at 14 days. Possible mechanisms responsible for this transient increase in junctional ACh receptors are discussed.