The actions of acetylcholine on denervated mammalian and frog's muscle

Removal of endogenous neuromodulators in a small motor network enhances responsiveness to neuromodulation

We studied the changes in sensitivity to a peptide modulator, crustacean cardioactive peptide (CCAP), as a response to loss of endogenous modulation in the stomatogastric ganglion (STG) of the crab Cancer borealis Our data demonstrate that removal of endogenous modulation for 24 h increases the response of the lateral pyloric (LP) neuron of the STG to exogenously applied CCAP. Increased responsiveness is accompanied by increases in CCAP receptor (CCAPr) mRNA levels in LP neurons, requires de novo protein synthesis, and can be prevented by coincubation for the 24-h period with exogenous CCAP. These results suggest that there is a direct feedback from loss of CCAP signaling to the production of CCAPr that increases subsequent response to the ligand. However, we also demonstrate that the modulator-evoked membrane current (I_MI) activated by CCAP is greater in magnitude after combined loss of endogenous modulation and activity compared with removal of just hormonal modulation. These results suggest that both receptor expression and an increase in the target conductance of the CCAP G protein-coupled receptor are involved in the increased response to exogenous hormone exposure following experimental loss of modulation in the STG.NEW & NOTEWORTHY The nervous system shows a tremendous amount of plasticity. More recently there has been an appreciation for compensatory actions that stabilize output in the face of perturbations to normal activity. In this study we demonstrate that neurons of the crustacean stomatogastric ganglion generate apparent compensatory responses to loss of peptide neuromodulation, adding to the repertoire of mechanisms by which the stomatogastric nervous system can regulate and stabilize its own output.

History, Mechanisms and Clinical Value of Fibrillation Analyses in Muscle Denervation and Reinnervation by Single Fiber Electromyography and Dynamic Echomyography

This work reviews history, current clinical relevance and future of fibrillation, a functional marker of skeletal muscle denervated fibers. Fibrillations, i.e., spontaneous contraction, in denervated muscle were first described during the nineteenth century. It is known that alterations in membrane potential are responsible for the phenomenon and that they are related to changes in electrophysiological factors, cellular metabolism, cell turnover and gene expression. They are known to inhibit muscle atrophy to some degree and are used to diagnose neural injury and reinnervation that are occurring in patients. Electromyography (EMG) is useful in determining progress, prognosis and efficacy of therapeutic interventions and their eventual change. For patients with peripheral nerve injury, and thus without the option of volitional contractions, electrical muscle stimulation may be helpful in preserving the contractility and extensibility of denervated muscle tissue and in retarding/counteracting muscle atrophy. It is obvious from the paucity of recent literature that research in this area has declined over the years. This is likely a consequence of the decrease in funding available for research and the fact that the fibrillations do not appear to cause serious health issues. Nonetheless, further exploration of them as diagnostic tools in long-term denervation is merited, in particular if Single Fiber EMG (SFEMG) is combined with Dynamic Echomyography (DyEM), an Ultra Sound muscle approach we recently designed and developed to explore denervated and reinnervating muscles.

Fibrillation potential onset in peripheral nerve injury

INTRODUCTION

Fibrillation potentials are an accepted electrical marker of muscle denervation that occur in axonal nerve injury. Clinically, they are used to determine the type of, and prognosis for nerve injuries. The time of occurrence after nerve injury plays a critical role in clinical decision making. This study explores the evolution of the generally accepted guideline that fibrillation potentials occur 1 to 4 weeks after axonal nerve injury.

METHODS

Pubmed, Ovid, and EMBASE, and current textbooks were reviewed. References were recursively followed back to the initial description of fibrillation potentials.

RESULTS

The majority of our understanding regarding the timing of onset of fibrillation potentials appears to arise from animal experiments in the mid-20th century.

CONCLUSIONS

Despite frequent use in human clinical care, published evidence for the 1 to 4 week guideline comes almost entirely from animal studies. An appreciation of this background and resulting limitations aids clinical application of this guideline.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.

[Spontaneous electromyographic activity. Practical importance]

Spontaneous activities are a major semeiologic sign in electromyography. The present article deals with the different aspects recorded in practice in normal and pathological cases. There are two types of spontaneous activities, those related to motor unit hyperactivity (fasciculations and myokymia) and those related to the hyperactivity of one or more muscle fibers: fibrillations, positive sharp waves, myotonic discharges and complex repetitive discharges. In the first case the lesion is located in the axone and in the second in the membrane of the muscle fibers; All theories related to the cells' abnormalities share a common feature: spontaneous activities result from abnormal firing of the membrane action potential of muscular fibers. This functional abnormality may results from different types of lesions within the cells' membrane and determines the aspects of spontaneous activities. Impaired function of muscular cells' membranes can be produced by denervation or lesion of the membrane structure itself. The latter can be multiple and linked with the membrane proteins (such as laminine or dystrophin as in AIDS diseases) or with ion channel disturbances. Multiple membrane cell alterations may produce the same kind of spontaneous activity; for instance, myotonic discharges have the same morphology in Thomsen and Steinert's disease despite their different mechanisms and fibrillations seen in denervations and myopathies. The practical consequences are discussed and a new classification of these spontaneous activities is proposed.

Effects of cross-suture and of injected acetylcholine on the supersensitivity of denervated striated muscle

The sensitivity of the tongue muscles to acetylcholine was examined in repeated experiments on cats anaesthetized with hexobarbitone. The supersensitivity which developed after section of the hypoglossal nerve was found to decrease when the muscles, after cross-suture, became re-innervated from the vagus or the chorda-lingual nerve. The supersensitivity caused by denervation could also be reduced by injecting acetylcholine repeatedly during six hours in acute experiments.

The concept of transmitter receptors: 100 years on

It is nearly one hundred years since John Langley of Cambridge developed the idea of the 'receptive substance' or 'receptors' as we now call them. This historical review traces the background to his introduction of this concept of the transmitter receptor and of how succeeding generations built on his ideas to generalise the applicability of this concept to synapses in general. It starts with a consideration of the discovery by Bernard (1844) that curare could paralyse rabbits without affecting their hearts because, as Vulpian (1866) suggested, curare acts on some intermediate zone between nerve and muscle. No further progress could be made without establishing the idea of chemical transmission, which Elliott (1904) then achieved, building on observations concerning sympathetic transmission to smooth muscle made previously by his mentor Langley (1901). Then between 1905 and 1907 Langley, in a wonderful act of creative ability, carried out a series of experiments on the somatic neuromuscular junction which established the idea of transmitter receptors. This review gives details of the experiments which persuaded both Langley and a recalcitrant Ehrlich that pharmacological substances could possess the necessary structure for them to combine with appropriate molecules on cells. The subsequent identification by Dale and his colleagues (1936) of acetylcholine as the transmitter acting on the receptors first discovered by Langley at the somatic neuromuscular junction as well as of acetylcholine on receptors in the heart by Loewi (1921) is then detailed. The review concludes with the triumph of the first recordings of the electrical signs of single channel openings by Neher and Sakmann (1976) at the receptors which Langley had first described.

Facilitation of neuromuscular transmission by anticholinesterase drugs

A close correlation has been shown to exist between the in vitro anticholinesterase potencies of ambenonium, neostigmine, methoxyambenonium and edrophonium chloride and their abilities to increase muscle contractions produced by close-arterial injections of acetylcholine. No correlation was found between the anticholinesterase potencies of the drugs and their potentiations of the maximal twitch in response to electrical stimulation, or their antagonisms of tubocurarine. It is concluded that some action, in addition to inhibition of cholinesterase, contributed to their facilitation at the neuromuscular junction, and it is suggested that this action may be at the prejunctional site.

A comparison between the effects of edrophonium and choline in the skeletal muscles of the cat

The effects of edrophonium and choline have been compared with those of the depolarizing substances acetylcholine, decamethonium, and suxamethonium, in both innervated and chronically denervated tibialis anterior muscles of cats under chloralose anaesthesia. Both edrophonium and choline were more potent antagonists to paralysis by tubocurarine than could be accounted for by their ability to stimulate the motor end-plates directly. It appeared likely that direct depolarization of the end-plate played no part in the anti-curare action of edrophonium and only some part in the anti-curare action of choline. A paralysis produced by the neuromuscular blocking agent, benzoquinonium, was more readily antagonized by a tetanus or by acetylcholine, suxamethonium, and decamethonium than a similar paralysis produced by tubocurarine. The tetraethyl ammonium ion was also slightly more effective against a paralysis by benzoquinonium. On the other hand, edrophonium was about 300 times and choline about five times less potent as an antagonist to benzoquinonium than to tubocurarine. Furthermore, the previous administration of benzoquinonium abolished the antagonistic action to tubocurarine of normally effective doses of edrophonium and reduced that of choline. These results were similar to those previously obtained with neostigmine, physostigmine and ethyl pyrophosphate and suggested that there was some similarity in the mechanism of action of all of these substances. Benzoquinonium, therefore, showed promise as a useful pharmacological tool for distinguishing compounds with this particular type of action. These anti-curare compounds did not appear to act by cholinesterase inhibition, not by an increase in the sensitivity of the motor end-plates. In common with other workers, we suggest that there is a pre-synaptic mechanism of action.

A developmental and component analysis of active sleep

A wide variety of hypotheses have been put forth that address the functional significance of active sleep. Despite the well-accepted fact that active sleep expresses itself predominantly in the perinatal period, the vast majority of these functional hypotheses are applicable largely, if not exclusively, to the adult. We build on the developmental approaches of previous researchers and propose that the individual components of active sleep (e.g., myoclonic twitches, rapid eye movements) exhibit unique developmental and phylogenetic histories and may serve independent functions in the developing organism. This dynamic perspective leads to specific experimental approaches aimed at the developmental roles of these components in the neonate, their maintenance roles in the adult, and the means by which these various components coalesce temporally in what is commonly referred to as a behavioral state.

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.

Effects of atropine, pirenzepine, imipramine and phenothiazines on the mammalian neuromuscular junction

1. Trifluoperazine (EC50 = 11.5 nM), chlorpromazine (13.8 microM), imipramine (15 microM), atropine (75.8 microM), and pirenzepine (316.2 microM), all produced neuromuscular facilitation and antagonized the blockade produced by oxotremorine (20 and 30 microM) in the rat isolated diaphragm. 2. These antagonists did not change the responses of curarized diaphragms to direct stimulation, or the twitch tension produced by retrograde injection of acetylcholine. 3. Trifluoperazine (2.5 ng, intra-arterially) reduced the tetanic fade produced by further intra-arterial injection of d-tubocurarine (10 micrograms/kg) in the in situ cat tibial muscle. 4. These results indicate that these antagonists may interact with muscarinic autoreceptors to increase acetylcholine output in the neuromuscular junction.

Synapse formation by autonomic nerves in the previously denervated neuromuscular junctions of the feline intrinsic laryngeal muscles

Nerve terminals of unknown origin at the previously denervated neuromuscular junctions (NMJ) of the feline intrinsic laryngeal muscles were investigated. Until 3 weeks after the transection of the recurrent laryngeal nerve (RLN), no axons were observed in the Büngner's bands and the NMJ, accompanied by a marked decrease of autonomic nerves around the blood vessels. At 5-6 weeks nerve varicosities labeled by 5-hydroxydopamine (5-OHDA) were observed in the Büngner's bands together with an increase of autonomic nerves around the blood vessels. At 9-30 weeks (8 cases) nerve terminals were found at the NMJ in all cases. Even if the ipsilateral vagosympathetic trunk was transected at 17 weeks, nerve terminals were found at all the NMJ 3 weeks after this treatment, indicating that nerve terminals were not from the original RLN. These nerve terminals were considered to be autonomic because nerve terminals labeled by 5-OHDA were observed. Furthermore, in the case of the removal of the superior cervical ganglion (SCG) and the nodose ganglion at 21 weeks, though nerve varicosities were found in the Büngner's bands, nerve terminals were not found 9 days after this treatment, suggesting that the ipsilateral SCG possibly played an important role. In electromyographic findings fibrillation-like activities were recognized in 8/11 cases after 5 weeks. The relationship of this phenomenon to fibrillation and muscle atrophy was discussed.

The effects of fructose 1,6-diphosphate, caffeine and dantrolene sodium on suxamethonium-induced contractures in denervated rat skeletal muscle

Previously unidentified forms of suxamethonium-induced contractures have been investigated in chronically denervated rat extensor digitorum longus (EDL) muscle at 20 degrees C. Contractures were assigned to groups 1-6 on the basis of the peak tension (Tp1) during 0-10 min exposure to the drug (3.0 x 10(-5) M), (7.0 x 10(-6) M), and (3.5 x 10(-6) M) and the subsequent retention, increase, or decrease in tension (Tp2), during the further 10 min. It is proposed that four stages exist in the development of contractile changes at 1-7, 8-35, 36-70 and 70-130 days after denervation (DPD) and that contractility is lost at 147 days after denervation. Initial changes, although present in EDL muscles in group 1 at 2.0 DPD s.d. +/- 1 (n = 7) in response to the drug (3.0 x 10(-5) M), were more apparent in EDL muscles in group 2 which were identified at 5.5 DPD s.d. +/- 1.6 (n = 7) by an excessive contracture response (Tp2) to the drug (3.0 x 10(-5) M), 18.3 mN s.d. +/- 10.6. At 5.0 DPD s.d. +/- 2.7 (n = 5) contracture tension (Tp2) was commensurate with membrane depolarization, 13.1 mN/33.1 mV, but residual tension increased to 23.3 mN during the Krebs wash (80 min) whilst membrane depolarization decreased to 9.2 mV. Also, at 4.3 DPD s.d. +/- 2.3 (n = 5) tension (Tp2) increased significantly (P less than or equal to 0.05) in the presence of caffeine (4.1 x 10(-3) M).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of cold on a partially denervated muscle

The abductor digiti minimi of a patient had undergone partial denervation with some subsequent re-innervation. Cooling of the muscle caused a severe reduction in the EMG and force which accompanied either voluntary effort or tetanic nerve stimulation, but it had little effect on the response to single stimuli. The ineffectual attempts at voluntary contraction of the cooled muscle did, however, block the response to single nerve stimuli. Administration of cholinesterase inhibitors did not improve neuromuscular transmission in the cooled muscle, but usually made it worse. It is concluded that depolarisation block may contribute to the weakness of cooled denervated muscles.

gamma-Hexachlorocyclohexane inhibition of the calcium fluxes at the desensitized mouse neuromuscular junction

The peripheral neurotoxicity of lindane was studied in vitro on mouse phrenic-diaphragm preparation. The experiments were performed on normal and denervated preparations. Twitches evoked by direct or indirect electrical stimulations and contractions evoked by external application of cholinergic agonists were compared in the presence or absence of lindane in the bath medium. Histochemical localizations of calcium were made on fibers after chemical stimulations. The results indicated that, at the normal endplate or at extrajunctionnal sites after denervation, there was a change in the intracellular Ca2+ distribution in response to external application of concentrated cholinergic agonists. This modification caused a transient contracture and could be visualized either by 45Ca autoradiography or by Alizarin Red S intracellular precipitations. Lindane inhibited the ACh-dependent contracture (I50 less than 10(-5) M) and suppressed the histochemical localizations of calcium. These lindane effects were observed on both normal and denervated muscles. It was concluded that lindane inhibited the Ca2+ influx presented by the desensitized ACh-sensitive areas of the muscle.

Caffeine contractures in denervated frog muscle

Caffeine contracture tension, effect of caffeine on the resting membrane potential, and caffeine influx in normal and denervated frog sartorius muscle have been investigated. Peak caffeine contracture tension is increased after denervation at all caffeine concentrations. The percentage increases in tension are highest for lower caffeine concentrations. The caffeine concentration required for half maximum tension is decreased from about 3.6 mM in control muscles to 2.6 mM in denervated muscles. Caffeine at 3.5 mM produces a depolarization of about 6 mV in control muscles and 16mV in denervated muscles. The large contracture tensions observed in denervated muscles are not due to the greater depolarization produced by the drug in denervated muscles since innervated muscles depolarized to the same level by external K+ do not enhance caffeine contracture tension. Both control and denervated muscles are highly permeable to caffeine. The increases in sarcoplasmic reticulum development ( Moscatello et al. 1965) and calcium content ( Picken and Kirby 1976) promoted by denervation may explain the larger tension elicited by caffeine in denervated muscles.

The effect of hydrolytic enzymes on the acetylcholine sensitivity of the skeletal muscle cell membrane

Isometric tension developed by rat soleus and extensor digitorum longus (EDL) muscles in response to acetylcholine (Ach) applied in vitro was recorded. Tension of contractures elicited in response to Ach increased after muscles had been incubated with phospholipase C, pepsin, or soluble fractions prepared from muscle homogenate. Using intracellular microelectrodes, resting membrane potential (RMP) and depolarisation in response to Ach added to the bathing medium were recorded in endplate-free regions of the muscle fibres. No significant change in RMP was observed in muscles incubated with soluble muscle fraction or phospholipase C, but depolarisation in response to Ach or carbachol was significantly increased. The time course for the increase in depolarisation and the contracture response to Ach was similar. When all available receptors were blocked with alpha-bungarotoxin prior to incubation so that no response to Ach could be elicited, with subsequent incubation in muscle soluble fraction or phospholipase C, both contractures and depolarisation in response to Ach returned. These results support the hypothesis that receptors, not previously available to interact with Ach or alpha-bungarotoxin were revealed following incubation.

The role of extracellular calcium in the contractions produced by acetylcholine in chronically denervated muscle

1 Acetylcholine-induced contractions of the isolated chronically denervated soleus muscle of the mouse consist of two phases, but both phases are equivalent to the contracture phase seen in vivo.2 Low [Ca(2+)](0) (0.5-1.5 mM) augmented peak tension, as well as the rate of relaxation, of the first phase, but inhibited the second phase. Ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or La(3+) (2 mM) also inhibited the second phase, but not the first.3 It was concluded that the first phase requires Ca(2+) release from the sarcoplasmic reticulum, and is terminated by inactivation of the contractile process. The second phase is caused by the entry of activator Ca(2+) from the extracellular space.4 Increasing [Ca(2+)](o) to 5 or 10 mM after the addition of acetylcholine caused a contraction, starting after a delay of about 50 seconds. EGTA or La(3+) added during the second phase of the acetylcholine contraction caused relaxation after a much shorter lag time.5 It is concluded that most of the Ca(2+) entering from the extracellular fluid is taken up by the sarcoplasmic reticulum.6 The acetylcholine second phase was augmented in low (25 mM) [Na(+)](0). It is concluded that Na(+) and Ca(2+) compete for the acetylcholine controlled ionic channels.7 Isolated chronically denervated diaphragm muscles were less sensitive to acetylcholine and the contraction usually consisted of a first phase only.8 It is concluded that sequestration of Ca(2+) entering from the extracellular fluid is more complete in the diaphragm.

The effect of catecholamines on the influx of calcium and the development of tension in denervated mouse diaphragm muscle

1 The nature of the catecholamine-induced contracture of chronically denervated mouse diaphragm muscle has been investigated and compared with the contractural response evoked by acetylcholine. 2 The time course of onset of catecholamine-sensitivity in denervated diaphragm muscles was similar to the development of acetylcholine sensitivity. However, catecholamine contractures were absent in tissues denervated for periods longer than 90 days whereas acetylcholine-sensitivity was still evident several months after denervation. 3 The catecholamine-induced contracture of the denervated muscle was inhibited specifically by beta-receptor blocking drugs and was unaffected by alpha-receptor blocking drugs and cholinoceptor antagonists. 4 Catecholamine-induced contractures of denervated muscles, unlike contractures to acetylcholine, were dependent upon the presence of spontaneous fibrillation and the amplitude of spontaneous fibrillation was increased by catecholamines. Fibrillation was absent in the presence of tetrodotoxin (1 muM), 2,4-dinitrophenol (10 muM), potassium cyanide (10 muM), ouabain (100 muM), in lithium chloride Ringer solution and at low temperature. Under these conditions catecholamine-induced contractures, but not those to acetylcholine, were abolished. 5 Labelled calcium was found progressively to enter denervated muscle fibres and this entry of calcium was increased by catecholamines. It is suggested that this calcium entry may represent either an increased calcium permeability of denervated muscle fibres which is increased further by catecholamines or the presence of a calcium current that occurs during the fibrillatory potentials of denervated muscle.

The effect of muscle extracts on the contracture response of skeletal muscle to acetylcholine

Preincubation of normal rat soleus muscles in vitro with homogenates prepared from mixed leg muscles which had been denervated 4 days previously resulted in an increase in the contracture response to acetylcholine. After 30 min incubation a 1.5-fold increase was observed. Homogenates of normally innervated muscles did not increase the response. The active principles of the denervated muscles were found to reside in the "cytosol" fraction. An approximately 2-fold increase was observed upon incubation with the cytosol for 30 min; incubation for longer periods resulted in a subsequent decrease in the response. The effect of the denervated muscle cytosol was concentration-dependent and heat-labile. Normal muscle cytosol also increased the soleus muscle response to acetylcholine but this fraction was less effective than denervated muscle cytosol. The response of control muscles incubated in Krebs-Henseleit solution was found to decrease with time. Commercially obtained phospholipases C and D increased the response of normal soleus muscles approximatley 2-fold. Phospholipase A, lipase, trypsin, collagenase and a bacterial protease had no effect, lysozyme produced a small but consistent increase in the response to acetylcholine.