Mechanisms of the inhibition by neostigmine of tetanic contraction in the mouse diaphragm

Discovery and pharmacological characterization of novel positive allosteric modulators acting on skeletal muscle-type nicotinic acetylcholine receptors

Skeletal muscle-type nicotinic acetylcholine receptors (m-nAChRs) are ligand-gated ion channels that open after activation by ACh and whose signals cause muscle contraction. Defects in neurotransmission are reported in disorders such as myasthenia gravis (MG) and congenital myasthenia syndromes (CMS). Although treatments for these disorders exist, therapies which significantly increase muscle strength have yet to be reported. Positive allosteric modulators (PAMs), which promote ACh signaling through AChRs, are expected to be promising therapeutic agents. In this study, we identified an m-nAChR PAM called AS3513678 by high-throughput screening using human myotube cells and modified it to obtain novel compounds (AS3566987 and AS3580239) that showed even stronger PAM activity. AS3580239 caused a leftward shift in the ACh concentration-response curve and was 14.0-fold potent at 10 μM compared with vehicle. Next, we examined the effect of AS3580239 on electrically-induced isometric contraction of the extensor digitorum longus (EDL) muscle in wild-type (WT) and MG model rats. AS3580239 enhanced EDL muscle contraction in both WT and MG model rats at 30 μM. These data suggest that AS3580239 improved neurotransmission and enhanced muscle strength. Thus, m-nAChR PAMs may be a useful treatment for neuromuscular diseases.

Novel Organoruthenium(II) Complex C1 Selectively Inhibits Butyrylcholinesterase without Side Effects on Neuromuscular Transmission

Enzyme butyrylcholinesterase (BChE) shows increased activity in some brain regions after progression of Alzheimer's disease and is therefore one of the therapeutic targets for symptomatic treatment of this neurodegenerative disorder. The organoruthenium(II) complex [(η⁶-p-cymene)Ru(II)(1-hydroxy-3-methoxypyridine-2(1H)-thionato)pta]PF₆ (C1) was designed based on the results of our previous structure-activity studies. Inhibitory activity toward cholinesterase enzymes shows that this complex selectively, competitively, and reversibly inhibits horse serum BChE (hsBChE) with an IC₅₀ value of 2.88 µM. When tested at supra-pharmacological concentrations (30, 60, 90, and 120 µM), C1 had no significant effect on the maximal amplitude of nerve-evoked and directly elicited single-twitch and tetanic contractions. At the highest tested concentration (120 µM), C1 had no effect on resting membrane potential, but significantly decreased the amplitude of miniature end-plate potentials (MEPP) without reducing their frequency. The same concentration of C1 had no effect on the amplitude of end-plate potentials (EPP), however it shortened the half-decay time of MEPPs and EPPs. The decrease in the amplitude of MEPPs and shortening of the half-decay time of MEPPs and EPPs suggest a possible weak inhibitory effect on muscle-type nicotinic acetylcholine receptors (nAChR). These combined results show that, when applied at supra-pharmacological concentrations up to 120 µM, C1 does not importantly affect the physiology of neuromuscular transmission and skeletal muscle contraction.

β2-Adrenergic receptor agonists ameliorate the adverse effect of long-term pyridostigmine on neuromuscular junction structure

Acetylcholine receptor deficiency is the most common form of the congenital myasthenic syndromes, a heterogeneous collection of genetic disorders of neuromuscular transmission characterized by fatiguable muscle weakness. Most patients with acetylcholine receptor deficiency respond well to acetylcholinesterase inhibitors; however, in some cases the efficacy of acetylcholinesterase inhibitors diminishes over time. Patients with acetylcholine receptor deficiency can also benefit from the addition of a β2-adrenergic receptor agonist to their medication. The working mechanism of β2-adrenergic agonists in myasthenic patients is not fully understood. Here, we report the long-term follow-up for the addition of β2-adrenergic agonists for a cohort of patients with acetylcholine receptor deficiency on anticholinesterase medication that demonstrates a sustained quantitative improvement. Coincidently we used a disease model to mirror the treatment of acetylcholine receptor deficiency, and demonstrate improved muscle fatigue, improved neuromuscular transmission and improved synaptic structure resulting from the addition of the β2-adrenergic agonist salbutamol to the anticholinesterase medication pyridostigmine. Following an initial improvement in muscle fatiguability, a gradual decline in the effect of pyridostigmine was observed in mice treated with pyridostigmine alone (P < 0.001). Combination therapy with pyridostigmine and salbutamol counteracted this decline (P < 0.001). Studies of compound muscle action potential decrement at high nerve stimulation frequencies (P < 0.05) and miniature end-plate potential amplitude analysis (P < 0.01) showed an improvement in mice following combination therapy, compared to pyridostigmine monotherapy. Pyridostigmine alone reduced postsynaptic areas (P < 0.001) and postsynaptic folding (P < 0.01). Combination therapy increased postsynaptic area (P < 0.001) and promoted the formation of postsynaptic junctional folds (P < 0.001), in particular in fast-twitch muscles. In conclusion, we demonstrate for the first time how the improvement seen in patients from adding salbutamol to their medication can be explained in an experimental model of acetylcholine receptor deficiency, the most common form of congenital myasthenic syndrome. Salbutamol enhances neuromuscular junction synaptic structure by counteracting the detrimental effects of long-term acetylcholinesterase inhibitors on the postsynaptic neuromuscular junction. The results have implications for both autoimmune and genetic myasthenias where anticholinesterase medication is a standard treatment.

Structural and functional characterization of an organometallic ruthenium complex as a potential myorelaxant drug

In addition to antibacterial and antitumor effects, synthetic ruthenium complexes have been reported to inhibit several medicinally important enzymes, including acetylcholinesterase (AChE). They may also interact with muscle-type nicotinic acetylcholine receptors (nAChRs) and thus affect the neuromuscular transmission and muscle function. In the present study, the effects of the organometallic ruthenium complex of 5-nitro-1,10-phenanthroline (nitrophen) were evaluated on these systems. The organoruthenium-nitrophen complex [(η⁶-p-cymene)Ru(nitrophen)Cl]Cl; C₂₂H₂₁Cl₂N₃O₂Ru (C1-Cl) was synthesized, structurally characterized and evaluated in vitro for its inhibitory activity against electric eel acetylcholinesterase (eeAChE), human recombinant acetylcholinesterase (hrAChE), horse serum butyrylcholinesterase (hsBChE) and horse liver glutathione-S-transferase. The physiological effects of C1-Cl were then studied on isolated mouse phrenic nerve-hemidiaphragm muscle preparations, by means of single twitch measurements and electrophysiological recordings. The compound C1-Cl acted as a competitive inhibitor of eeAChE, hrAChE and hsBChE with concentrations producing 50 % inhibition (IC₅₀) of enzyme activity ranging from 16 to 26 μM. Moreover, C1-Cl inhibited the nerve-evoked isometric muscle contraction (IC₅₀ = 19.44 μM), without affecting the directly-evoked muscle single twitch up to 40 μM. The blocking effect of C1-Cl was rapid and almost completely reversed by neostigmine, a reversible cholinesterase inhibitor. The endplate potentials were also inhibited by C1-Cl in a concentration-dependent manner (IC₅₀ = 7.6 μM) without any significant change in the resting membrane potential of muscle fibers up to 40 μM. Finally, C1-Cl (5-40 μM) decreased (i) the amplitude of miniature endplate potentials until a complete block by concentrations higher than 25 μM and (ii) their frequency at 10 μM or higher concentrations. The compound C1-Cl reversibly blocked the neuromuscular transmission in vitro by a non-depolarizing mechanism and mainly through an action on postsynaptic nAChRs. The compound C1-Cl may be therefore interesting for further preclinical testing as a new competitive neuromuscular blocking, and thus myorelaxant, drug.

Organoruthenium Prodrugs as a New Class of Cholinesterase and Glutathione-S-Transferase Inhibitors

A small library of 17 organoruthenium compounds with the general formula [Ru^II (fcl)(chel)(L)]ⁿ⁺ (in which fcl=face capping ligand, chel=chelating bidentate ligand, and L=monodentate ligand) were screened for inhibitory activity against cholinesterases and glutathione-S-transferases of human and animal origins. Compounds were selected to include different chelating ligands (i.e., N,N-, N,O-, O,O-, S,O-) and monodentate ligands that can modulate the aquation rate of the metal species. Compounds with a labile ruthenium chloride bond that provided rapid aquation were found to inhibit both sets of enzymes in reversible competitive modes and at pharmaceutically relevant concentrations. When applied at concentrations that completely abolish the activity of human acetylcholinesterase, the lead compound [(η⁶ -p-cymene)Ru(pyrithionato)Cl] (C1 a) showed no undesirable physiological responses on the neuromuscular system. Finally, C1 a was not cytotoxic against non-transformed cells at pharmaceutically relevant concentrations.

Discorhabdin alkaloids from Antarctic Latrunculia spp. sponges as a new class of cholinesterase inhibitors

The brominated pyrroloiminoquinone alkaloids discorhabdins B, L and G and 3-dihydro-7,8- dehydrodiscorhabdin C, isolated from methanol extracts of two specimens of Latrunculia sp. sponges collected near the Antarctic Peninsula, are here demonstrated for the first time to be reversible competitive inhibitors of cholinesterases. They showed K_i for electric eel acetylcholinesterase of 1.6-15.0 μM, for recombinant human acetylcholinesterase of 22.8-98.0 μM, and for horse serum butyrylcholinesterase of 5.0-76.0 μM. These values are promising when compared to the current cholinesterase inhibitors used for treatment of patients with Alzheimer's disease, to counteract the acetylcholine deficiency in the brain. Good correlation was obtained between IC₅₀ data and results by molecular docking calculation on the binding interactions within the acetylcholinesterase active site, which also indicated the moieties in discorhabdin structures involved. To avoid unwanted peripheral side effects that can appear in patients using some acetylcholinesterase inhibitors, electrophysiological experiments were carried out on one of the most active of these compounds, discorhabdin G, which confirmed that it had no detectable undesirable effects on neuromuscular transmission and skeletal muscle function. These findings are promising for development of cholinesterase inhibitors based on the scaffold of discorhabdins, as potential new agents for treatment of patients with Alzheimer's disease.

Synthetic analogs of stryphnusin isolated from the marine sponge Stryphnus fortis inhibit acetylcholinesterase with no effect on muscle function or neuromuscular transmission

The current study describes the AChE inhibitory effect of natural marine neuroactive compounds.

Fenitrothion, an organophosphorous insecticide, impairs locomotory function and alters body temperatures in Sminthopsis macroura (Gould 1845) without reducing metabolic rates during running endurance and thermogenic performance tests

Endemic Australian mammal species are exposed to pesticides used for locust control as they occupy the same habitat as the target insect. The authors examined the impact of an ultra-low volume formulation of the organophosphorous insecticide fenitrothion (O,O-dimethyl-O-[3-methyl-4-nitrophenol]-phosphorothioate) on a suite of physiological measures that affect the ability of animals to survive in free-living conditions: locomotory and thermogenic functions, metabolic performance, body mass, and hematocrit and hemoglobin levels. Plasma and brain cholinesterase activity in relation to time since exposure to pesticide were also determined. An orally applied dose of 90 mg kg(-1) fenitrothion reduced running endurance in the stripe-faced dunnart, Sminthopsis macroura, by 80% the day after exposure concomitantly with a reduction of approximately 50% in plasma and 45% in brain acetylcholinesterase activity. These adverse effects disappeared by 10 d postexposure. Maximal metabolic rates reached during running were unaffected by pesticide, as were body mass and hemoglobin and hematocrit levels. Maximal cold-induced metabolic rate (measured as peak 2 min metabolic rate attained during cold exposure), time taken to reach peak metabolic rate on cold exposure, cumulative total oxygen consumed during shivering thermogenesis, and body temperature before and after cold exposure were unaffected by fenitrothion. Dunnart rectal temperatures showed a reduction of up to 5 °C after exposure to fenitrothion but returned to pre-exposure levels by 10 d postdose. Such physiological compromises in otherwise asymptomatic animals demonstrate the importance of considering performance-based measures in pesticide risk assessments.

Neurophysiological characteristics of MuSK antibody positive myasthenia gravis mice: focal denervation and hypersensitivity to acetylcholinesterase inhibitors

Myasthenia Gravis (MG) patients with antibodies against the muscle specific tyrosine kinase (MuSK+) typically present with focal fatigue and atrophy of the facial and bulbar muscles, along with unbeneficial reactions upon administration of acetylcholinesterase inhibitors (AChEIs). This study addresses the neurophysiological characteristics in facial versus limb muscles, before and after intraperitoneal injection of AChEIs, in mice immunized with MuSK. We performed in-vivo neurophysiological examinations in the masseter and gastrocnemius muscles of mice with MuSK+experimental autoimmune MG (EAMG) and in healthy control mice before and after administration of AChEIs. Abnormal spontaneous activity (fibrillations) was observed in the masseter muscle of MuSK+mice. Furthermore, 94% of MuSK-immunized mice displayed so called extra discharges (EDs) upon administration of a therapeutic AChEI dose, in contrast to 22% of the control mice, indicating neuromuscular hyperactivity. These findings support functional denervation in the masseter muscle and neuromuscular hypersensitivity already at a standard dose of AChEIs in MuSK+EAMG.

Fenitrothion, an organophosphate, affects running endurance but not aerobic capacity in fat-tailed dunnarts (Sminthopsis crassicaudata)

We measured aerobic metabolism during cold exposure and exercise performance (run duration and oxygen consumption while running at 1 m s(-1)) in the fat-tailed dunnart Sminthopsis crassicaudata, a dasyurid marsupial, before and after ingestion of 30 mg kg(-1) of fenitrothion, an organophosphate (OP) pesticide. Running endurance of OP-exposed animals was less than half that of control animals over the first 3 days after dosing and 55% of control animal endurance on day 5 post-dose. Despite these declines, peak metabolic rate at this running speed (9.3 times basal metabolic rate; BMR) was unaffected by OP exposure. Peak metabolic rate (PMR) and cumulative oxygen consumption during a 1-h exposure to conditions equivalent to -20 degrees C did not differ between OP-treated and control dunnarts, with PMR averaging 11 times BMR. We conclude that fenitrothion-induced exercise fatigue is not due to limitations in oxygen or substrate delivery to muscle or in their uptake per se, but more likely relates to decreased ability to sustain high-frequency neuromuscular function. The persistence of locomotor impairment following OP exposure in otherwise asymptomatic animals emphasizes the importance of using performance-based measures when characterising sublethal effects of pesticide exposure in an ecological context.

Reduction of quantal size and inhibition of neuromuscular transmission by bafilomycin A

The energy for uphill transport of neurotransmitters into synaptic vesicles is created by bafilomycin A- and concanamycin A-sensitive vacuolar H(+)-ATPase (V-ATPase). Both blockers (at 0.1-5 microM) depressed twitch tension and induced tetanic fade of mouse diaphragm on stimulation of the phrenic nerve. Axonal impulse conduction and depolarization of motor endplate by exogenous acetylcholine were not inhibited. The IC(50)s for bafilomycin A and concanamycin A were 1.1+/-0.2 and 0.7+/-0.1 microM, respectively. Contractile response evoked by stimulation of diaphragm, muscle resting membrane potential and membrane resistance were not altered. V-ATPase blockers decreased quantal size and shifted the distribution of miniature endplate potentials (mepps) to low amplitude direction. The increase of mepp events in high KCl medium was suppressed slightly. The blockers depressed endplate potentials (epps) with IC(50)s of 0.7+/-0.2 microM (bafilomycin A) and 0.4+/-0.1 microM (concanamycin A). On high frequency stimulation, the coefficient of variance and run-down of epps were increased. The inhibitory effects on mepps and epps were irreversible and augmented by nerve stimulation. The results suggest that inhibition of V-ATPase reduces the acetylcholine content of synaptic vesicles, leading to suppression of neuromuscular transmission.

Fatigability of rat hindlimb muscles after acute irreversible acetylcholinesterase inhibition

The purpose of this study was to investigate the functional impact of acute irreversible inhibition of acetylcholinesterase (AChE) on the fatigability of medial gastrocnemius and plantaris muscles of Sprague-Dawley rats. After treatment with methanesulfonyl fluoride (a lipid-soluble anticholinesterase), which reduced their AChE activity by >90%, these muscles were subjected to an in situ indirect stimulation protocol, including a series of isolated twitch and tetanic contractions preceding a 3-min fatigue regimen (100-ms trains at 75 Hz applied every 1.5 s). During the first minute of the fatigue regimen, the effects of AChE inhibition were already near maximal, including marked reductions in peak tension and the force-time integral (area), as well as a decrement of compound muscle action potential amplitudes within a stimulus train. Neuromuscular transmission failure was the major contributor of the force decreases in the AChE-inhibited muscles. However, despite this neuromuscular transmission failure, muscles of which all AChE molecular forms were nearly completely inhibited were still able to function, although abnormally, during 3 min of intermittent high-frequency nerve stimulation.

Inhibition of quantal release from motor nerve by wortmannin

1. The effects of wortmannin, an inhibitor of phosphatidylinositol (PI) kinases and myosin light chain kinase, on the quantal release of neurotransmitter from mouse phrenic nerve were investigated. 2. Wortmannin (10 - 100 microM) initially enhanced, thereafter progressively depressed spontaneous quantal discharge (miniature endplate potential, mepp). The mean amplitude and the amplitude distribution of mepp were not altered. 3. The compound inhibited and prevented the intensive quantal release evoked by high KC1 solution as well as the mepp burst induced by alpha-latrotoxin, a polypeptide toxin that possesses Ca2+-independent synaptic action to trigger quantal release. The inhibitory actions of wortmannin were partially reversible. 4. Wortmannin depressed the amplitude of endplate potentials (epps) and increased the coefficient of variance of epps. The profile of epps in response to high frequency nerve stimulation exhibited fluctuations between run-down and run-up. The phenomenon is thus different from the consistency of run-up characteristic as the motor nerve Ca2+ channel is blocked by omega-agatoxin IVA. 5. LY294002, another inhibitor of PI 3-kinase, raised mepp frequency without causing late phase suppressions. The compound did not inhibit KC1-, alpha-latrotoxin- or nerve stimulation-evoked quantal release. 6. The results suggest that wortmannin could depress quantal release beyond the step of Ca2+ channel blockade, probably by interfering with the exocytotic cascade.

Evaluation of intrinsic modulation of synaptic transmission by ATP in mouse fast twitch muscle

This study aims to evaluate whether endogenous ATP or adenosine modulates the neurotransmission and contractile function of mouse phrenic nerve-diaphragm. Bath application of ATP (1 mM) and alpha, beta-methylene ATP (m-ATP, 0.1 mM) elevated muscle tones, depressed contractions (approximately 12%), and depolarized muscle membranes (approximately 20 mV). Adenosine (1 mM) or low concentrations of ATP (0.1 mM) had no effect. In a low Ca2+ media, ATP caused prolonged inhibitions of endplate potentials (EPPs), whereas m-ATP augmented EPPs while both agents produced slight effects in normal Tyrode solution. When applied by puff ejection, ATP and m-ATP additionally elicited fast transient suppressions of EPPs in association with inhibitions of high K+-evoked releases of miniature EPPs. Blockades of P2 purinoceptors with suramin antagonized all the effects of ATP and m-ATP except the prolonged inhibitions of EPPs induced by ATP, which were antagonized instead by 8-cyclopentyl-1,3-dipropylxanthine (CPDPX), an A1 adenosine receptor antagonist. Suramin and CPDPX did not change contractions nor alter EPPs evoked by a low- or high-frequency nerve stimulation. The results indicate that exogenously applied ATP and m-ATP, via activations of distinct pre- and postsynaptic purinoceptors, exert inhibitory and facilitatory pharmacological modulations on the mature neuromuscular junction. However, because of intrinsic high efficiency of the synaptic transmission under physiological conditions, endogenously released ATP and its degradation product-adenosine-do not build up to concentrations high enough to alter motor functions.

The in vivo effect of lipopolysaccharide on neuromuscular transmission in the mouse

The in vivo effect of lipopolysaccharide (endotoxin) on nerve-evoked muscle contractions and neuromuscular transmission was studied in the mouse phrenic nerve-diaphragm preparations. In lipopolysaccharide-treated mouse diaphragms it was observed that indirectly induced twitch tension was unchanged while tetanic tension significantly decreased. Neostigmine (50 nM) increased the amplitude of nerve evoked muscle contractions, while it caused partial fade of tetanic contractions (Wedensky inhibition) and accelerated the run-down of end-plate potentials (e.p.ps) evoked by repetitive nerve stimulation, in the diaphragm of saline-control mice, but not of lipopolysaccharide-treated mice. These effects of neostigmine could be abolished by ouabain (5 microM). Measurement of the quantal contents of e.p.ps revealed that ouabain (5 microM) significantly increased it in the diaphragm of saline-control mice to an extent similar to that in diaphragm of lipopolysaccharide-treated mice. Moreover, ouabain-sensitive Na+, K(+)-ATPase activity in the sciatic nerve of lipopolysaccharide-treated mice was markedly decreased. The alterations in neuromuscular transmission of the diaphragm during endotoxemia could be reversed by the administration of polymyxin B (a lipopolysaccharide neutralizer) and NG-nitro-L-arginine (a nitric oxide (NO) synthase inhibitor), suggesting that NO is involved in these lipopolysaccharide-induced alterations of neuromuscular transmission mediated by an impairment of ouabain-sensitive Na+, K(+)-ATPase activity in mouse motor nerves.

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.

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.

beta-Eudesmol as an antidote for intoxication from organophosphorus anticholinesterase agents

beta-Eudesmol, a sesquiterpenol present in Chinese herbs, improved the tetanic contraction impaired by diisopropylfluorophosphate in isolated mouse diaphragm preparations by an inhibition of the regenerative acetylcholine release. The antagonism was enhanced when a small concentration of obidoxime was present. Neither enzyme reactivation nor curare-like action was evident. beta-Eudesmol (300 mg/kg, i.p.) elevated the LD50 of diisopropylfluorophosphate (s.c.) in control mice from 4.2 to 6.4 mg/kg and in mice pretreated with atropine from 7.8 to 10.6 mg/kg. In mice pretreated with atropine and obidoxime, beta-eudesmol showed a greater synergistic effect, increasing the LD50 from 281 to more than 800 mg/kg. beta-Eudesmol also markedly alleviated diisopropylfluorophosphate-induced muscle fasciculation, tremor and convulsion and prolonged the time to death. It is proposed that beta-eudesmol may be added to the standard antidotal regimen (atropine plus obidoxime) for treating organophosphate intoxication.

Obidoxime Antagonizes the Neuromuscular Failure Induced by Neostigmine and Diisopropyl Fluorophosphate via Different Mechanisms

The efficacies and mechanisms of obidoxime in antagonizing the neuromuscular failure induced by neostigmine and diisopropyl fluorophosphate (DFP) were studied in mouse phrenic nerve/diaphragm preparations. Obidoxime antagonized neostigmine-induced tetanic fade (EC(50): 300 &mgr;M) by inhibiting the regenerative and sustained depolarization during repetitive stimulation. The antagonism was associated with a depression and shortening of single endplate potentials (EPPs) and miniature EPPs (MEPPs). In contrast, the neuromuscular failure induced irreversibly after treatment with DFP and followed by washout was restored by obidoxime at concentrations (EC(50): 0.6 &mgr;M) 500-fold lower than that against neostigmine. The regenerative depolarization was abolished with no depression of single EPPs and MEPPs, and the antagonistic action persisted after washout of obidoxime. The EC(50) of obidoxime was proportionately increased in the presence of increasing concentrations of DFP. Nevertheless, the EC(50) against DFP, at a concentration (30 &mgr;M) 15-fold in excess of that which caused tetanic fade, was still 10-fold lower than that which antagonized neostigmine. In both cases, the amplitudes of train EPPs were increased. It is concluded that obidoxime antagonizes neostigmine-induced neuromuscular failure by a curare-like action but antagonizes DFP by an enzyme reactivation. Copyright 1994 S. Karger AG, Basel

Pharmacological relevance of peripheral type benzodiazepine receptors on motor nerve and skeletal muscle

1. Effects of agonists and antagonists of peripheral and central benzodiazepine receptors (pBZR and cBZR) on neuromuscular transmission were studied in mouse isolated phrenic nerve-diaphragm preparations. 2. Ro5-4864, a pBZR agonist, had no effect on the neuromuscular transmission but increased muscle contractility and antagonized the tetanic fade induced by neostigmine. 3. Ro5-4864 inhibited the regenerative tonic endplate depolarization caused by repetitive stimulation in the presence of neostigmine without affecting the amplitude and decay time of miniature and evoked single endplate potentials. 4. All the effects of Ro5-4864 were shared by PK11195, a pBZR antagonist, but not by clonazepam and flumazenil, a cBZR agonist and antagonist, respectively. 5. It is suggested that peripheral type benzodiazepine receptors modulate presynaptic function and muscle contraction.

Improvement by diazepam of neuromuscular transmission blocked by anticholinesterase agents in mouse diaphragms

Effects of diazepam on the neuromuscular transmission blocked by neostigmine were studied in isolated mouse phrenic nerve-diaphragm preparations. Diazepam, in the absence of neostigmine, had no significant effect on the neuromuscular transmission at concentrations lower than 100 microM, except to enhance muscle contractility. Single and train pulses-evoked endplate potentials (e.p.p.s) and miniature e.p.p.s (m.e.p.p.s) were also unaffected. At concentrations of 175 microM or higher, diazepam caused an axonal conduction block. However, neostigmine-induced twitch potentiation, spontaneous fasciculation and tetanic fade were antagonized by diazepam at 3.5-35 microM. Diazepam did not decrease the amplitude of neostigmine-augmented e.p.p.s and m.e.p.p.s but slightly reduced their decay time. The incidence of regenerative depolarization of endplates induced by repetitive stimulation in the presence of neostigmine, was decreased from 92% to 35% junctions and the duration shortened from 650 ms to 230 ms. The amplitude of train e.p.p.s was increased. It is suggested that diazepam reverses the muscle paralysis induced by anticholinesterase agents by inhibiting the regenerative release of acetylcholine.

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

1. Local contraction of the endplate region in response to nerve stimulation was studied in isolated mouse diaphragms. In normal preparations, muscle contractions involved the whole length of the muscle fibre with rise and decay times in the order of tens of ms whether evoked with a single or train of pulses. 2. When acetylcholinesterase was inhibited with neostigmine, tetanic stimulation produced a twitch-like phasic contraction and a delayed tonic contracture. A brief train of pulse (10 ms, 300 Hz) was enough to trigger a full size tonic contracture which reached an amplitude about one tenth that of control tetanus and had a duration of about 4 s. 3. Tetanic stimulation evoked a non-propagating prolonged depolarization at the endplate region lasting for about 1 s following a few muscle action potentials. 4. mu-Conotoxin, a specific inhibitor of muscle Na+ channel, selectively abolished the phasic contraction and the muscle action potentials leaving the tonic contracture and the prolonged depolarization unaffected. 5. Both the tonic contracture and the prolonged depolarization were highly sensitive to blockade by tubocurarine (IC50 0.05-0.1 microM) and vesamicol (1 microM, an inhibitor of packaging acetylcholine into synaptic vesicles), were attenuated by increasing Ca2+ concentration and were prolonged by decreasing Ca2+. 6. The results suggest that prolonged activation of endplate nicotinic receptors by endogenously released transmitter can produce substantial contractions of the endplate region when acetylcholinesterase are inhibited. The source of Ca2+ for the contraction seems to come mainly from intracellular stores.

Effects of velnacrine, tacrine and physostigmine on tetanic twitch responses at the rat neuromuscular junction

The effects of velnacrine (1-hydroxytacrine), tacrine and physostigmine on indirectly elicited twitch at low and high stimulation frequencies were analyzed in the rat phrenic hemidiaphragm preparation. At 0.2 Hz, velnacrine and physostigmine behaved in a similar manner, the latter showing a higher potentiating effect. This potentiation was observed at 3-100 microM velnacrine, whereas a slight depression appeared at higher concentrations. When tetanic responses were studied, the drug concentrations needed to depress tetanic tension and tetanic fade were quite different in the case of velnacrine (depression of tetanic tension from 1 microM and tetanic fade from 170 microM), whereas physostigmine and tacrine were able to affect these parameters at very similar concentrations. The results suggest that some effects of velnacrine could differ from those of tacrine in spite of the chemical similarity.

Antagonism by beta-eudesmol of neostigmine-induced neuromuscular failure in mouse diaphragms

beta-Eudesmol, a sesquiterpenol extracted from a Chinese herb, Atractylodes lancea, at 10-80 microM, did not affect muscle action potentials, miniature and evoked endplate potentials and acetylcholine-induced depolarization in the presence or absence of neostigmine in mouse phrenic nerve-diaphragms. However, the tetanic fade, muscle fasciculation and twitch potentiation induced by neostigmine were effectively antagonized by 20 microM beta-eudesmol. When trains of pulses were applied to the nerve in the presence of neostigmine, beta-eudesmol reduced the incidence of explosive depolarization of the endplate from 95% to 35-67% of junctions, and shortened the duration when it occurred. Moreover, both the maximal and steady-state depolarizations during repetitive stimulation were reduced while the amplitudes of steady-state endplate potentials were increased. The results suggest that beta-eudesmol antagonized neostigmine-induced neuromuscular failure mainly by a presynaptic action to depress the regenerative release of acetylcholine during repetitive stimulation. The mechanism of antagonism is obviously not tubocurarine-like and it is unrelated to desensitization of acetylcholine channels.

Inhibition by neosurugatoxin and omega-conotoxin of acetylcholine release and muscle and neuronal nicotinic receptors in mouse neuromuscular junction

Neosurugatoxin and omega-conotoxin, known to be specific ligands for the neuronal nicotinic receptor and Ca2+ channel, respectively, were previously claimed to exert no depressant action on the mouse neuromuscular junction. It was found that in preparations partially blocked with tubocurarine or with low Ca(2+)-high Mg2+ Tyrode's, both toxins, at 3-10 microM, depressed indirect twitches and either produced wanings (neosurugatoxin) or waxings (omega-conotoxin) of indirectly elicited tetanic contractions whilst in normal Tyrode's the contractile forces were not changed. In normal Tyrode's, neosurugatoxin decreased the amplitudes of spontaneous and evoked endplate potentials and enhanced the run-down of endplate potentials as did tubocurarine though with lesser potency. By contrast, omega-conotoxin (10 microM) decreased the amplitude of the evoked but not of the spontaneous endplate potential in low Ca(2+)-high Mg2+ Tyrode's, and produced facilitation of endplate potentials, instead of run-down, on repetitive stimulations. Higher concentrations of omega-conotoxin appeared to depress quantal release in normal Tyrode's. The effects were all reversible. The prolonged endplate depolarization found in preparations treated with neostigmine or 3,4-diaminopyridine, was partially depressed by both toxins. The results suggest that neosurugatoxin blocks the neuron and muscle nicotinic receptors in the neuromuscular junction with comparable potency. The pharmacology of the nicotinic receptor on motor nerve terminal seems more similar to the muscle nicotinic receptor than to that on autonomic ganglia or brain. On the other hand, omega-conotoxin seems to block a small fraction of Ca2+ channels on the motor nerve and decreases the quantal release of evoked endplate potentials.

On the mechanism of spontaneous recovery of neuromuscular transmission after acetylcholinesterase inhibition in the rat neuromuscular junction

Neuromuscular transmission shows a significant degree of spontaneous recovery after being impeded by acetylcholinesterase inhibition. Part of this recovery can be ascribed to de novo synthesis of acetylcholinesterase but another part is independent of enzyme activity. To unravel the mechanism underlying this synaptic adaptation to acetylcholinesterase inhibition we have compared a number of electrophysiological parameters in diaphragms taken from animals that were sacrificed within 15 min after a 2 x LD50 dose of the acetylcholinesterase inhibitor diisopropylfluorophosphate and from similarly treated animals killed after being kept alive for 3 h under artificial respiration. We found no differences in the quantal content. There was a significantly smaller degree of endplate potential rundown at tetanic stimulation and the miniature endplate potential amplitude was smaller in the 3-h adapted animals. In addition, the desensitization induced by carbachol appeared to be less in this group. It is likely that these synaptic changes, demonstrating the plasticity of the neuromuscular synapse, are involved in the spontaneous recovery of neuromuscular transmission after acetylcholinesterase inhibition.

On the mechanism whereby HI-6 improves neuromuscular function after oxime-resistant acetylcholinesterase inhibition and subsequent impairment of neuromuscular transmission

Experiments were performed to elucidate the mechanism of action by which the oxime HI-6 causes a recovery of neuromuscular function after oxime-resistant inhibition of acetylcholinesterase by the organophosphate S27. In the presence of HI-6 (1-3 mM), the ability of isolated rat diaphragm muscle strips to sustain tetanic contractions after inhibition by S27 was markedly improved, as was the electrophysiological response to indirect tetanic stimulation. At lower concentrations (0-1 mM), HI-6 reduced the amplitude of the miniature endplate potentials and their decay time constant in a dose-dependent manner without having any effect on the resting membrane potential. In addition, HI-6 dose dependently increased the quantal content. It is likely that these post- and presynaptic effects of HI-6 are responsible for the improvement of muscle contractions after inhibition of acetylcholinesterase and they could well be of value in the therapy of organophosphate poisoning.

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

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

Nicotinic actions of oxotremorine on murine skeletal muscle. Evidence against muscarinic modulation of acetylcholine release

The effects of oxotremorine, arecoline and muscarine on neuromuscular transmission of mouse or rat phrenic nerve-diaphragm were investigated. For some studies of endplate potentials (e.p.p.s) the preparation was immobilized by cutting muscle fibers. Oxotremorine (0.3-10 microM) depolarized endplate membranes, reduced miniature e.p.p. amplitudes but increased frequency, induced spontaneous neural discharges and muscle fasciculations, and produced contracture of denervated mouse diaphragm. In mouse and young rat preparations pretreated with Mn2+, Co2+, Ni2+, Cd2+ or low Ca2+ Tyrode to depress evoked acetylcholine release, oxotremorine 0.3-1 microM increased indirect twitches as well as amplitudes and quantal contents of e.p.p.s. These increases were not observed when the synaptic transmission was not depressed, nor in adult rat preparations. The augmentation by oxotremorine of evoked acetylcholine release persisted in preparations pretreated with neostigmine (1 microM) and tetrodotoxin (20 nM), which inhibited acetylcholinesterase and oxotremorine-induced spontaneous neural discharges. These effects of oxotremorine were mimicked by arecoline but not by muscarine and were antagonized by tubocurarine (0.3 microM) but not by atropine (0.1-10 microM). Atropine alone did not affect indirect twitches, synaptic transmission, tetanic responses evoked by direct stimulation of diaphragms, nor the durations of muscle action potential. The direct twitch responses were only slightly increased by oxotremorine at 2-3 microM. Oxotremorine at high concentrations (greater than 2 microM), depressed indirect twitches and e.p.p. amplitude, and accelerated the run-down of trains of e.p.p.s. The IC50 on indirect twitches was reduced by pretreatment with diltiazem or proadifen, which are known to promote receptor desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Organic calcium channel antagonists provoke acetylcholine receptor autodesensitization on train stimulation of motor nerve

The effects of nicardipine and other organic Ca2(+)-channel antagonists on the responses induced by indirect train stimulation (3 s, 50-100 Hz) were studied in mouse phrenic nerve diaphragm preparations. Nicardipine at 1-10 microM, which alone did not affect single or tetanic contractions or the amplitude of evoked endplate potentials and spontaneous miniature endplate potentials, caused tetanic contraction to fade completely in the presence of 0.3 microM neostigmine or 50 microM diisopropylfluorophosphate. In combination with these anticholinesterases, nicardipine caused a severe run-down and shortening of endplate potentials in 1-2 s. This effect on endplate potentials was dependent on stimulus frequency and on extracellular Ca2+. The effect was accelerated by intracellular injection of Ca2+, but retarded by injection of EGTA. The amplitudes of miniature endplate potentials and the evoked endplate depolarization were also depressed during repetitive stimulation. On termination of repetitive stimulation, all postsynaptic responses, including evoked endplate potentials, miniature endplate potentials and single twitches, recovered to pre-train level in 3-10 s. These results suggest that the postsynaptic nicotinic receptors had lost the functional activity during repetitive stimulation. The time-courses of the aforementioned changes initiated by repetitive stimulation were similar to the fast phase of desensitization induced by acetylcholine. The irreversible action of alpha-bungarotoxin on acetylcholine receptor was attenuated in the presence of nicardipine and neostigmine if repetitive stimulation was applied. The same effects were observed with other organic Ca2(+)-channel antagonists (diltiazem, verapamil and nifedipine) as well as agonist (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine- 5-carboxylate, BAY K8644), but not with Mn2+, theophylline or caffeine. It is inferred that organic Ca2(+)-channel antagonists interact directly with acetylcholine receptor ion channel, enhance its autodesensitization liability and thus cause extinction of endplate potentials on repetitive stimulation.

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.

Use of geographutoxin II (mu-conotoxin) for the study of neuromuscular transmission in mouse

1. Endplate potentials (e.p.ps) were investigated in the presence of geographutoxin II (GTXII) in the mouse phrenic nerve diaphragm preparation. This toxin preferentially blocks muscle Na+ channels which allows the study of e.p.ps in the absence of nicotinic receptor antagonists or substances to depress acetylcholine release. 2. GTXII abolished muscle action potentials and antagonized the depolarization of the muscle membrane produced by the crotamine-induced opening of Na+ channels. 3. E.p.ps as large as 19-25 mV were observed after 2-4 micrograms ml-1 GTXII. These concentrations of GTXII did not cause discernible changes of resting membrane potential and frequency and amplitude of miniature e.p.ps. 4. Lower concentrations (1-2 micrograms ml-1) of GTXII caused incomplete blockade of the muscle Na+ channel resulting in exaggerated 'e.p.ps', while higher concentrations of GTXII (8 micrograms ml-1) abolished e.p.ps by a prejunctional effect. 5. Trains of e.p.ps on repetitive stimulation after GTXII neither ran down, as in tubocurarine-treated preparations, nor facilitated, as in low Ca2+ and/or high Mg2+-treated preparations, and were indistinguishable from those of untreated cut muscle preparation. 6. In cut muscle preparations, GTXII did not affect the rise and decay times, amplitude or rundown of e.p.ps. 7. It is concluded that GTXII is a useful agent for studying neuromuscular transmission. This method provides e.p.ps which are neither attenuated nor modified because manipulations that alter transmitter release and postjunctional receptor responses are avoided.

Neostigmine after spontaneous recovery from neuromuscular blockade. Effect on depth of blockade monitored with train-of-four and tetanic stimuli

The effect of neostigmine on neuromuscular function was examined after spontaneous recovery from an atracurium-induced neuromuscular blockade, which reached a train-of-four ratio of either 0.5 or 0.9. Two doses of neostigmine 2.5 mg were given 5 minutes apart. Neuromuscular recovery was assessed with train-of-four and tetanic stimuli. The first dose of neostigmine antagonised the neuromuscular blockade. The second dose diminished tetanic height and increased tetanic fade. The train-of-four measured mechanically was adversely affected to a small degree, but when measured with the electromyograph no significant change occurred. Neostigmine may adversely affect neuromuscular function after spontaneous recovery from a non-depolarising block. This is unlikely with a single modest dose and any effects are probably short-lived.

Antagonism by tubocurarine and verapamil of the regenerative acetylcholine release from mouse motor nerve

The role of presynaptic acetylcholine receptors and Ca2+ channels in the regenerative acetylcholine release was studied in the cut muscle preparation of mouse phrenic nerve hemidiaphragm. The regenerative release shown as a prolonged endplate depolarization was evoked by stimulation of the nerve with a train of pulse at 75-300 Hz when acetylcholinesterase activity was depressed with neostigmine or by lowering temperature. Tubocurarine, cobratoxin, verapamil, diltiazem and nifedipine at low concentrations, which had a negligible effect on the endplate potential, shortened the duration of regenerative depolarization while leaving the amplitude unaffected. In contrast, Mn2+ at concentrations that markedly reduced the amplitude of single endplate potentials caused little suppression of the regenerative depolarization though intensive stimulation was needed to trigger the response. On the other hand, atropine inhibited the regenerative depolarization only at high concentrations which also depressed endplate potentials. These results indicate that the mechanism for evoking the regenerative release involves the activation of acetylcholine receptors and Ca2+ channels which are sensitive to tubocurarine and Ca2+ channel blockers. The Ca2+ channel concerned, however, appears to differ from that involved in the normal quantal release of acetylcholine.

Reversals of the neostigmine-induced tetanic fade and endplate potential run-down with respect to the autoregulation of transmitter release

1. In order to shed more light on the role of presynaptic cholinoceptors in the modulation of transmitter release, the effects of tubocurarine, choline and hexamethonium on neostigmine-induced tetanic fade and run-down of endplate potentials (e.p.ps) in response to indirect stimulation with trains of pulses were studied in the intact and cut isolated phrenic nerve-diaphragm preparation of the mouse, respectively. 2. Tubocurarine, choline and hexamethonium reduced both the tetanic fade and e.p.p. run-down caused by neostigmine, despite the fact that they themselves also induced these two effects. 3. At a given degree of postsynaptic inhibition, choline and hexamethonium caused less e.p.p. run-down and reversed the neostigmine-induced tetanic fade and e.p.p. run-down better than tubocurarine. Moreover, the e.p.p. run-down caused by choline or hexamethonium, but not that induced by tubocurarine, was reciprocally reversed by neostigmine. 4. Tubocurarine, choline and hexamethonium significantly decreased the endplate depolarization induced by repetitive nerve stimulation in the presence of neostigmine. The remaining depolarization continued to grow during repetitive stimulation in the presence of choline or hexamethonium, but not, however, in the presence of tubocurarine; a finding which suggests that choline and hexamethonium but not tubocurarine may be displaced from the receptor by the accumulated acetylcholine. 5. The mutual reversal by neostigmine and cholinoceptor antagonists of e.p.p. run-down may implicate the presence of a positive (physiological) and a negative (pharmacological) feedback regulation for evoked transmitter release via nicotinic cholinoceptors in the mammalian motor nerve, depending on the concentration of acetylcholine within the synaptic cleft.

The nature of the presynaptic effects of (+)-tubocurarine at the mouse neuromuscular junction

1. The effects of (+)-tubocurarine (TC) on tetanic run-down and quantum content of end-plate potentials (EPPs) were investigated in cut-fibre preparations of mouse diaphragm. 2. (+)-Tubocurarine, 0.15 microM, halved the amplitude of spontaneous miniature EPPs (MEPPs) and steepened the tetanic run-down of EPPs evoked at 10 Hz by increasing the quantum content of the first EPP of the train while having no effect on quantum content of plateau EPPs. With stimulation at 1 Hz, there was little run-down and the quantum content of all EPPs was increased by TC. 3. The use of binomial statistics to analyse release indicated that after TC the increase in the quantum content of the first EPP in the train at 10 Hz was due to an increase in n and that during the run-down there was a decrease in p so that plateau EPP quantum content at 10 Hz was not different from control. 4. To elucidate a possible role of cholinoreceptors in the presynaptic effects of TC, studies were made on the effects of pancuronium or of alpha-bungarotoxin (BTX), with concentrations and exposure times where they had postsynaptic effects equal to 0.15 microM-TC. The run-down of EPPs was unaffected by BTX, while pancuronium steepened it to a lesser extent than TC. 5. The anticholinesterase, ecothiopate, decreased the quantum content of plateau EPPs only at high frequencies of stimulation (50 Hz) and did not affect the presynaptic effects of TC at 10 Hz. 6. At concentrations which reduced MEPP amplitude, atropine (10 microM) or hexamethonium (50 microM) had no effect on EPP run-down. 7. These results indicate that TC could have presynaptic effects via a presynaptic acetylcholine receptor, but that such a receptor may not have the same binding specificities as the postsynaptic receptor.

Dissociation of the end-plate potential run-down and the tetanic fade from the postsynaptic inhibition of acetylcholine receptor by alpha-neurotoxins

The effects of so-called postsynaptic snake alpha-neurotoxins (alpha-bungarotoxin, cobratoxin, erabutoxin b) on the wanings of tetanic contraction (tetanic fade) and the run-down of end-plate potentials during stimulation at 100 Hz were studied, respectively, in intact and cut mouse phrenic nerve-diaphragm preparations. No tetanic fade was evident with high concentrations of toxins until the complete failure of contractile response whereas the tetanic fade was evident after prolonged incubation with lower concentrations of toxins. The proportion of junctions exhibiting end-plate potential run-down increased progressively during toxin incubation. However, depression of end-plate potential amplitude by the toxins was not necessarily accompanied by run-down. The tetanic fade and the run-down became more pronounced for a time shortly after washout of toxins despite the restoration of single twitches and end-plate potential amplitudes, indicating the presynaptic origin of these effects induced by alpha-neurotoxins. We demonstrated that the functions of the pre- and postsynaptic acetylcholine receptors can be dissociated by using the different kinetics of toxin-receptor interactions. The results also implicate that a positive feedback enhancement of transmitter release operates via the presynaptic acetylcholine receptor in the neuromuscular junction in normal physiological conditions during repetitive pulses.

Examination of the mechanisms involved in tetanic fade produced by vecuronium and related analogues in the rat diaphragm

The effects of vecuronium (Org NC45), Org 7678 and Org 7684 were examined on twitches and tetani recorded from rat isolated diaphragms. Org 7678 and Org 7684 exhibited approximately one tenth of the neuromuscular blocking potency of vecuronium. At concentrations producing equivalent amounts of twitch block, Org 7684 produced significantly less tetanic fade than did vecuronium or Org 7678. In cut muscles both vecuronium and Org 7684 reduced the endplate current (e.p.c.) amplitude (Ie.p.c.), reduced e.p.c. decay time constant (tau e.p.c.), and increased the e.p.c. train rundown. The effects of vecuronium were not voltage-dependent and vecuronium did not change tau noise. The effect of Org 7684 on Ie.p.c. and tau e.p.c. became greater with hyperpolarization, but the effect on e.p.c. train rundown was not voltage-dependent. It is concluded that both vecuronium and Org 7684 produce e.p.c. train rundown and tetanic fade by a prejunctional mechanism. However, whereas postjunctionally vecuronium blocks only the acetylcholine receptor, Org 7684 blocks both the receptor and its associated ion channel.

A regenerating release of acetylcholine from mouse motor nerve terminals treated with anticholinesterase agents

It was found by intracellular recording with glass microelectrodes that train stimulation (50-200 Hz) of the phrenic nerve of intact or cut mouse diaphragm induced an accumulative depolarization of the endplate and triggered after a few pulses an 'all-or-none' regenerative depolarization lasting for 300-900 ms when acetylcholinesterase was inhibited by neostigmine or diisopropylfluorophosphate. This depolarization was associated with a noise of the membrane potential and a failure of the end plate potential. Low Ca2+ prolonged whereas high Ca2+ shortened the duration of regenerative depolarization which needed no further stimulation once triggered. d-Tubocurarine abolished the depolarization while restoring the end plate potential. A regenerative release of acetylcholine due to an activation of presynaptic cholinoceptors is speculated.