Effects of tacrine, velnacrine (HP029), suronacrine (HP128), and 3,4-diaminopyridine on skeletal neuromuscular transmission in vitro

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

We measured effects of therapeutic concentrations of donepezil (10 nM to 1 μM) on AChE enzymic activity, muscle force responses to repetitive stimulation, and spontaneous and evoked endplate potentials (EPPs) recorded intracellularly from flexor digitorum brevis muscles from CD01 or C57BlWldS mice.

KEY RESULTS

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

CONCLUSION AND IMPLICATIONS

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

Schwann cell nodal membrane disruption triggers bystander axonal degeneration in a Guillain-Barré syndrome mouse model

In Guillain-Barré syndrome (GBS), both axonal and demyelinating variants can be mediated by complement-fixing anti-GM1 ganglioside autoantibodies that target peripheral nerve axonal and Schwann cell (SC) membranes, respectively. Critically, the extent of axonal degeneration in both variants dictates long-term outcome. The differing pathomechanisms underlying direct axonal injury and the secondary bystander axonal degeneration following SC injury are unresolved. To investigate this, we generated glycosyltransferase-disrupted transgenic mice that express GM1 ganglioside either exclusively in neurons [GalNAcT-/--Tg(neuronal)] or glia [GalNAcT-/--Tg(glial)], thereby allowing anti-GM1 antibodies to solely target GM1 in either axonal or SC membranes, respectively. Myelinated-axon integrity in distal motor nerves was studied in transgenic mice exposed to anti-GM1 antibody and complement in ex vivo and in vivo injury paradigms. Axonal targeting induced catastrophic acute axonal disruption, as expected. When mice with GM1 in SC membranes were targeted, acute disruption of perisynaptic glia and SC membranes at nodes of Ranvier (NoRs) occurred. Following glial injury, axonal disruption at NoRs also developed subacutely, progressing to secondary axonal degeneration. These models differentiate the distinctly different axonopathic pathways under axonal and glial membrane targeting conditions, and provide insights into primary and secondary axonal injury, currently a major unsolved area in GBS research.

Antagonistic postsynaptic and presynaptic actions of cyclohexanol on neuromuscular synaptic transmission and function

Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Some survivors develop a severe, acute or delayed myasthenic syndrome. In animal models, similar myasthenia has been associated with increasing plasma concentration of one insecticide solvent metabolite, cyclohexanol. We investigated possible mechanisms using voltage and current recordings from mouse neuromuscular junctions (NMJs) and transfected human cell lines. Cyclohexanol (10-25 mM) reduced endplate potential (EPP) amplitudes by 10-40% and enhanced depression during repetitive (2-20 Hz) stimulation by up to 60%. EPP decay was prolonged more than twofold. Miniature EPPs were attenuated by more than 50%. Cyclohexanol inhibited whole-cell currents recorded from CN21 cells expressing human postjunctional acetylcholine receptors (hnAChR) with an IC₅₀ of 3.74 mM. Cyclohexanol (10-20 mM) also caused prolonged episodes of reduced-current, multi-channel bursting in outside-out patch recordings from hnAChRs expressed in transfected HEK293T cells, reducing charge transfer by more than 50%. Molecular modelling indicated cyclohexanol binding (-6 kcal/mol) to a previously identified alcohol binding site on nicotinic AChR α-subunits. Cyclohexanol also increased quantal content of evoked transmitter release by ∼50%. In perineurial recordings, cyclohexanol selectively inhibited presynaptic K⁺ currents. Modelling indicated cyclohexanol binding (-3.8 kcal/mol) to voltage-sensitive K⁺ channels at the same site as tetraethylammonium (TEA). TEA (10 mM) blocked K⁺ channels more effectively than cyclohexanol but EPPs were more prolonged in 20 mM cyclohexanol. The results explain the pattern of neuromuscular dysfunction following ingestion of organophosphorus insecticides containing cyclohexanol precursors and suggest that cyclohexanol may facilitate investigation of mechanisms regulating synaptic strength at NMJs. KEY POINTS: Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Survivors may develop a severe myasthenic syndrome or paralysis, associated with increased plasma levels of cyclohexanol, an insecticide solvent metabolite. Analysis of synaptic transmission at neuromuscular junctions in isolated mouse skeletal muscle, using isometric tension recording and microelectrode recording of endplate voltages and currents, showed that cyclohexanol reduced postsynaptic sensitivity to acetylcholine neurotransmitter (reduced quantal size) while simultaneously enhancing evoked transmitter release (increased quantal content). Patch recording from transfected cell lines, together with molecular modelling, indicated that cyclohexanol causes selective, allosteric antagonism of postsynaptic nicotinic acetylcholine receptors and block of presynaptic K⁺ -channel function. The data provide insight into the cellular and molecular mechanisms of neuromuscular weakness following intentional ingestion of agricultural organophosphorus insecticides. Our findings also extend understanding of the effects of alcohols on synaptic transmission and homeostatic synaptic function.

Glial Sulfatides and Neuronal Complex Gangliosides Are Functionally Interdependent in Maintaining Myelinating Axon Integrity

Sulfatides and gangliosides are raft-associated glycolipids essential for maintaining myelinated nerve integrity. Mice deficient in sulfatide (cerebroside sulfotransferase knock-out, CST-/-) or complex gangliosides (β-1,4-N-acetylegalactosaminyltransferase1 knock-out, GalNAc-T-/-) display prominent disorganization of proteins at the node of Ranvier (NoR) in early life and age-dependent neurodegeneration. Loss of neuronal rather than glial complex gangliosides underpins the GalNAc-T-/- phenotype, as shown by neuron- or glial-specific rescue, whereas sulfatide is principally expressed and functional in glial membranes. The similarities in NoR phenotype of CST-/-, GalNAc-T-/-, and axo-glial protein-deficient mice suggests that these glycolipids stabilize membrane proteins including neurofascin155 (NF155) and myelin-associated glycoprotein (MAG) at axo-glial junctions. To assess the functional interactions between sulfatide and gangliosides, CST-/- and GalNAc-T-/- genotypes were interbred. CST-/-× GalNAc-T-/- mice develop normally to postnatal day 10 (P10), but all die between P20 and P25, coinciding with peak myelination. Ultrastructural, immunohistological, and biochemical analysis of either sex revealed widespread axonal degeneration and disruption to the axo-glial junction at the NoR. In addition to sulfatide-dependent loss of NF155, CST-/- × GalNAc-T-/- mice exhibited a major reduction in MAG protein levels in CNS myelin compared with WT and single-lipid-deficient mice. The CST-/- × GalNAc-T-/- phenotype was fully restored to that of CST-/- mice by neuron-specific expression of complex gangliosides, but not by their glial-specific expression nor by the global expression of a-series gangliosides. These data indicate that sulfatide and complex b-series gangliosides on the glial and neuronal membranes, respectively, act in concert to promote NF155 and MAG in maintaining the stable axo-glial interactions essential for normal nerve function.SIGNIFICANCE STATEMENT Sulfatides and complex gangliosides are membrane glycolipids with important roles in maintaining nervous system integrity. Node of Ranvier maintenance in particular requires stable compartmentalization of multiple membrane proteins. The axo-glial adhesion molecules neurofascin155 (NF155) and myelin-associated glycoprotein (MAG) require membrane microdomains containing either sulfatides or complex gangliosides to localize and function effectively. The cooperative roles of these microdomains and associated proteins are unknown. Here, we show vital interdependent roles for sulfatides and complex gangliosides because double (but not single) deficiency causes a rapidly lethal phenotype at an early age. These findings suggest that sulfatides and complex gangliosides on opposing axo-glial membranes are responsible for essential tethering of the axo-glial junction proteins NF155 and MAG, which interact to maintain the nodal complex.

Effect of Donepezil, Tacrine, Galantamine and Rivastigmine on Acetylcholinesterase Inhibition in Dugesia tigrina

Dugesia tigrina is a non-parasitic platyhelminth, which has been recently utilized in pharmacological models, regarding the nervous system, as it presents a wide sensitivity to drugs. Our trials aimed to propose a model for an in vivo screening of substances with inhibitory activity of the enzyme acetylcholinesterase. Trials were performed with four drugs commercialized in Brazil: donepezil, tacrine, galantamine and rivastigmine, utilized in the control of Alzheimer's disease, to inhibit the activity of acetylcholinesterase. We tested five concentrations of the drugs, with an exposure of 24 h, and the mortality and the inhibition of acetylcholinesterase planarian seizure-like activity (pSLA) and planarian locomotor velocity (pLMV) were measured. Galantamine showed high anticholinesterasic activity when compared to the other drugs, with a reduction of 0.05 μmol·min(-1) and 63% of convulsant activity, presenting screw-like movement and hypokinesia, with pLMV of 65 crossed lines during 5 min. Our results showed for the first time the anticholinesterasic and convulsant effect, in addition to the decrease in locomotion induced by those drugs in a model of invertebrates. The experimental model proposed is simple and low cost and could be utilized in the screening of substances with anticholinesterasic action.

Update on treatment options for Lambert-Eaton myasthenic syndrome: focus on use of amifampridine

In Lambert-Eaton myasthenic syndrome (LEMS), antibodies against presynaptic voltage-gated calcium channels reduce the quantal release of acetylcholine, causing muscle weakness and autonomic dysfunction. More than half of the affected patients have associated small cell lung cancer, and thorough screening for an underlying malignancy is crucial. The mainstay of treatment for LEMS is symptomatic but immunotherapy is needed in more severely affected patients. Symptomatic therapies aim at increasing the concentration of acetylcholine at the muscle endplate. While acetylcholinesterase inhibitors were the first drugs to be used for the amelioration of symptoms, 3,4-diaminopyridine (3,4-DAP, amifampridine) has been shown to be more effective. 3,4-DAP blocks presynaptic potassium channels, thereby prolonging the action potential and increasing presynaptic calcium concentrations. This then results in increased quantal release of acetylcholine. The efficacy of 3,4-DAP for increasing muscle strength and resting compound muscle action potentials has been demonstrated by four placebo-controlled trials. Side effects are usually mild, and the most frequently reported are paresthesias. The most common serious adverse events are epileptic seizures. 3,4-DAP is currently the treatment of choice in patients with Lambert-Eaton myasthenic syndrome.

Anti-GD1a antibodies activate complement and calpain to injure distal motor nodes of Ranvier in mice

The motor axonal variant of Guillain-Barré syndrome is associated with anti-GD1a immunoglobulin antibodies, which are believed to be the pathogenic factor. In previous studies we have demonstrated the motor terminal to be a vulnerable site. Here we show both in vivo and ex vivo, that nodes of Ranvier in intramuscular motor nerve bundles are also targeted by anti-GD1a antibody in a gradient-dependent manner, with greatest vulnerability at distal nodes. Complement deposition is associated with prominent nodal injury as monitored with electrophysiological recordings and fluorescence microscopy. Complete loss of nodal protein staining, including voltage-gated sodium channels and ankyrin G, occurs and is completely protected by both complement and calpain inhibition, although the latter provides no protection against electrophysiological dysfunction. In ex vivo motor and sensory nerve trunk preparations, antibody deposits are only observed in experimentally desheathed nerves, which are thereby rendered susceptible to complement-dependent morphological disruption, nodal protein loss and reduced electrical activity of the axon. These studies provide a detailed mechanism by which loss of axonal conduction can occur in a distal dominant pattern as observed in a proportion of patients with motor axonal Guillain-Barré syndrome, and also provide an explanation for the occurrence of rapid recovery from complete paralysis and electrophysiological in-excitability. The study also identifies therapeutic approaches in which nodal architecture can be preserved.

Effects of bis(7)-tacrine on spontaneous synaptic activity and on the nicotinic ACh receptor of Torpedo electric organ

Bis(7)-tacrine is a potent acetylcholinesterase inhibitor in which two tacrine molecules are linked by a heptylene chain. We tested the effects of bis(7)-tacrine on the spontaneous synaptic activity. Miniature endplate potentials (MEPPs) were recorded extracellularly on slices of electric organ of Torpedo marmorata. Bis(7)-tacrine, at a concentration of 100 nM, increased the magnitudes that describe MEPPs: amplitude, area, rise time, rate of rise, and half-width. We also tested the effect of bis(7)-tacrine on nicotinic acetylcholine receptors by analyzing the currents elicited by acetylcholine (100 microM) in Torpedo electric organ membranes transplanted in Xenopus laevis oocytes. Bis(7)-tacrine inhibited the acetylcholine-induced currents in a reversible manner (IC(50) = 162 nM). The inhibition of nicotinic acetylcholine receptors was not voltage dependent, and bis(7)-tacrine increased the desensitization of nicotinic acetylcholine receptors. The Hill coefficient for bis(7)-tacrine was -0.72 +/- 0.02, indicating that bis(7)-tacrine binds to the nicotinic acetylcholine receptor in a molecular ratio of 1:1, but does not affect the binding of alpha-bungarotoxin with the nicotinic acetylcholine receptor. In conclusion, bis(7)-tacrine greatly increases the spontaneous quantal release from peripheral cholinergic terminals at a much lower concentration than tacrine. Bis(7)-tacrine also blocks acetylcholine-induced currents of Torpedo electric organ, although the mechanism is different from that of tacrine: bis(7)-tacrine enhances desensitization, whereas tacrine reduces it.

The pharmacology of novel acetylcholinesterase inhibitors, (+/-)-huprines Y and X, on the Torpedo electric organ

The effects of the tacrine-huperzine A hybrid acetylcholinesterase inhibitors, (+/-)-12-amino-3-chloro-9-methyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride ((+/-)-huprine Y) and (+/-)-12-amino-3-chloro-9-ethyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride ((+/-)-huprine X), were tested on spontaneous synaptic activity by measuring the amplitude, the rise time, the rate of rise, the half-width and the area or the electrical charge of the miniature endplate potentials (m.e.p.ps) recorded extracellularly on Torpedo electric organ fragments. (+/-)-Huprine Y and (+/-)-huprine X at a concentration of 500 nM increased all the m.e.p.p. variables analyzed. The effect of (+/-)-huprine Y was smaller than that of (+/-)-huprine X for all the variables except for the rate of rise where there was no significant difference. The effects of these drugs were also tested on nicotinic receptors by analyzing the currents elicited by acetylcholine (100 microM) in Xenopus laevis oocytes, transplanted with membranes from Torpedo electric organ. Both drugs inhibited the currents in a reversible manner, (+/-)-huprine Y (IC(50)=452 nM) being more effective than (+/-)-huprine X (IC(50)=4865 nM). The Hill coefficient was 0.5 for both drugs. The inhibition of the nicotinic receptor was voltage-dependent and decreased at depolarizing potentials, and there was no significant difference in the effects between (+/-)-huprine Y and (+/-)-huprine X at concentrations near to their IC(50) values. At depolarizing potentials between -20 and +15 mV, these drugs did not have any detectable effect on the blockade of the nicotinic receptor. Both huprines increased the desensitization of the nicotinic receptors since the current closed quickly in the presence of the drugs, and there was no significant difference in this effect between (+/-)-huprine Y (500 nM) and (+/-)-huprine X (5 microM). We conclude that (+/-)-huprine Y and (+/-)-huprine X increase the level of acetylcholine in the synaptic cleft more effectively than tacrine. The interaction of (+/-)-huprine X with nicotinic receptors is weaker than that of (+/-)-huprine Y, suggesting that (+/-)-huprine X would be more specific to maintain the extracellular acetylcholine concentration.

Cadmium modulates diabetes-induced alterations in murine neuromuscular junction

Skeletal muscle function is compromised in diabetes mellitus and exposure to heavy metals may further complicate neuromuscular impairments. The present study investigated the effects of cadmium on diabetes induced dorsiflexor muscle dysfunction in C57 BL adult male mice. Forty mice were divided randomly into 2 groups (n=20 each). One group served as control and the other was injected once with i.p. streptozotocin (STZ) solution (60 mg/kg) to induce experimental diabetes. Each group was then divided into two sub-groups (n=10) of which one received 5 mM cadmium. Utilizing intracellular recording method, resting membrane potential (RMP) and miniature endplate potentials (MEPPs) were measured in dorsiflexor muscle obtained from urethane-anaesthetized (2 mg/g, i.p.) four weeks diabetic and matched control mice. Comparative analyses of isometric contractile characteristics of in situ dorsiflexor muscle were also conducted in both groups. In control mice, flexor muscle exposure to 5 mM cadmium for 10 min resulted in significant reduction in MEPPs frequencies and isometric twitch tensions without affecting RMP. In STZ-diabetic mice, the same exposure did not modify resting membrane potential and further decreased MEPPs frequencies and isometric twitch tensions. Current results indicated that cadmium probably via a Ca2+ antagonist and chelating activity at nerve terminals exacerbates diabetes complications.

Tacrine, a reversible acetylcholinesterase inhibitor, induces myopathy

Tacrine, an acetylcholinesterase (AChE) inhibitor that has been used in the treatment of Alzheimer's disease, increases available acetylcholine (Ach) levels in the synaptic cleft thereby enhancing the activity of cholinergic pathways. However, excessive stimulation of nicotinic receptors at the neuromuscular junction results in muscle deterioration. We tested whether reversible AChE inhibitors such as tacrine may induce similar effects. In the present study, tacrine administration (7.5 mg/kg twice daily) to rats produces a 20 and 30-fold increase in the number of degenerating cells in leg and diaphragm muscle, respectively, as compared to control. This myopathy is significantly decreased by co-administration of tacrine with the nitric oxide (NO) synthase inhibitor L-NAME. These results show that tacrine can induce myopathy which may be mediated by increased NO production.

Effects of CI-1002 and CI-1017 on spontaneous synaptic activity and on the nicotinic acetylcholine receptor of Torpedo electric organ

The effect of azepino[2,1-b]quinazoline 1,3-dichloro-6,7,8,9,10, 12-hexahydro-, mono-hydrochloride (CI-1002), a tacrine derivative, and 1-azabicyclo[2.2.1]heptan-3-one, O-[3-(methoxyphenyl)-2-propynyl]oxime [R-(Z)]-2-butenedioate (CI-1017), a muscarinic M(1) receptor agonist, on spontaneous synaptic activity was investigated by measuring amplitude, rise time, velocity of rising, half-width, and electrical charge of miniature endplate potentials (m.e.p.p.) recorded extracellularly in Torpedo electric organ fragments. The effect of CI-1002 and CI-1017 on the nicotinic acetylcholine receptor was investigated by measuring the current induced by acetylcholine in Xenopus laevis oocytes transplanted with membranes from Torpedo electric organ. CI-1002, at a concentration of 1 microM, altered the m.e.p.p. by increasing the amplitude (from 1.08+/-0.01 to 2.76+/-0.03 mV), rise time (from 0. 700+/-0.006 to 1.02+/-0.01 ms), rising rate (from 1.79+/-0.02 to 3. 45+/-0.05 mV/ms), half-width (from 0.990+/-0.008 to 2.40+/-0.02 ms), and electrical charge (from 304+/-4 to 784+/-11 mV s). CI-1017, at a concentration of 1 microM, altered the m.e.p.p. by decreasing the amplitude (from 1.08+/-0.01 to 0.650+/-0.007 mV), rise time (from 0. 700+/-0.006 to 0.530+/-0.007 ms), rising rate (from 1.79+/-0.02 to 1. 53+/-0.02 mV/ms), half-width (from 0.990+/-0.008 to 0.670+/-0.007 ms), and electrical charge (from 304+/-4 to 75+/-1 mV s). CI-1002 inhibited the acetylcholine-induced current of nicotinic acetylcholine receptors with an IC(50) of 3.4+/-0.3 microM. CI-1017 inhibited the acetylcholine-induced current of nicotinic acetylcholine receptors with an IC(50) of 0.8+/-0.1 microM. These results indicate that, although both drugs interacted negatively with nicotinic acetylcholine receptors, CI-1002 overcame this inhibition by recruiting more acetylcholine to build a quantum.

Tacrine inhibits nicotinic secretory and current responses in adrenal chromaffin cells

Tacrine enhanced acetylcholine-induced catecholamine secretion with a concentration of up to 10 microM, but inhibited it at over 10 microM in perfused adrenal glands. Qualitatively the same result was obtained with physostigmine. Both tacrine and physostigmine only inhibited the secretory responses to carbachol and/or nicotine in perfused glands and dispersed chromaffin cells. Acetylcholinesterase activity of adrenal homogenates was inhibited by tacrine and physostigmine in a concentration-dependent manner. In whole-cell patch-clamp experiments, tacrine and physostigmine caused reversible inhibition of nicotine-evoked inward currents with a dose range similar to that for the inhibitory action on the secretory response. These results suggest that the enhancing effect of tacrine and physostigmine on acetylcholine-induced catecholamine secretion results from the prevention of enzymatic hydrolysis of acetylcholine in adrenal glands and that the inhibitory effect is due to the inhibition of nicotinic receptor-mediated membrane currents in adrenal chromaffin cells.

The prejunctional inhibitory effect of suramin on neuromuscular transmission in vitro

The P2 purinoceptor antagonist suramin reverses skeletal muscle paralysis evoked by non-depolarizing neuromuscular blocking agents in vitro and in vivo. To further study the action of suramin on neuromuscular transmission, (miniature) endplate potentials ((m.)e.p.ps), motor nerve terminal currents and the release of radiolabeled acetylcholine was measured in isolated nerve-muscle preparations. In preparations paralysed by low Ca2+/high Mg2+ conditions, suramin (10 microM-1 mM) induced a concentration-dependent decrease in quantal content of the e.p.ps without affecting m.e.p.ps. Suramin reversed neuromuscular block by d-tubocurarine in these preparations. In erabutoxin paralysed preparations, suramin (40 microM-1 mM) inhibited the motor nerve terminal currents related to Ca2+ influx concentration-dependently, but did not affect Na+ currents. Suramin-induced inhibition of Ca2+ currents was not antagonized by ATP gamma S. Suramin (300 microM) reduced [14C]acetylcholine outflow in non-paralysed rat phrenic nerve-hemidiaphragm preparations by 32%. As suramin did not chelate Ca2+, these results indicate that suramin inhibits neuromuscular transmission by blocking prejunctional Ca2+ channels, thereby decreasing acetylcholine release upon nerve stimulation.

Modification of ionic currents underlying action potentials in mouse nerve terminals by the thiol-oxidizing agent diamide

The effect of diamide, a thiol-oxidizing agent, was tested using electrophysiological techniques to determine whether its ability to alter neuromuscular transmission in vitro could be attributed to alterations of ion channels controlling neuronal excitability and/or acetylcholine release. In mouse triangularis sterni preparations, diamide transiently increased the evoked release of acetylcholine and then blocked release. Extracellular recording of perineural waveforms associated with neuronal action potentials at motor nerve terminals showed that diamide reduced the waveforms associated with the delayed rectifier K+ current, a Ca2+ current and a Ca(2+)-activated K+ current (IK,Ca). Inhibition of quantal transmitter release was not associated with failure of action potentials to invade nerve terminals. Thus, diamide modifies the ionic currents underlying the nerve terminal action potential, some of these changes probably account for the complex effects of diamide on quantal transmission.

Treatment of stable chronic demyelinating polyneuropathy with 3,4-diaminopyridine

OBJECTIVE

To determine whether 3,4-diaminopyridine (3,4-DAP) would improve clinical or electrophysiologic function in patients with stable chronic demyelinating polyneuropathy.

DESIGN

We conducted a prospective, randomized, placebo-controlled, blinded, crossover study of 3,4-DAP in 34 patients with demyelinating polyneuropathy.

MATERIAL AND METHODS

Of the 17 men and 17 women, who were 21 to 80 years of age, 27 had hereditary motor and sensory neuropathy type I and 7 had acquired demyelinating polyneuropathy. Treatment consisted of stepped doses of 3,4-DAP (increasing to 20 mg four times daily) or placebo for 4 days. Pretreatment and posttreatment determination of the Neurologic Disability Score (NDS); isometric muscle strength testing; median, ulnar, and peroneal nerve conduction studies; and measurement of serum 3,4-DAP were performed. Quantitative computer-assisted sensory examinations were done in five patients.

RESULTS

The results for the final day of treatment with 3,4-DAP or placebo and the differences between pretreatment and posttreatment findings for total NDS, sensory NDS, isometric muscle strength testing, compound muscle action potential amplitude, sensory nerve action potential amplitude, motor and sensory conduction velocities, and vibration and cold detection thresholds did not vary significantly. A small improvement of 4 points in the motor NDS (P < 0.05) was found. Five patients with electrophysiologic conduction block had no significant reduction in the degree of block.

CONCLUSION

Because no improvement was noted in most measurements of neurologic function, despite use of high doses of drug, 3,4-DAP is unlikely to be beneficial in the treatment of stable chronic demyelinating polyneuropathy.

Ammodytoxin A acceptor in bovine brain synaptic membranes

Ammodytoxin A, the presynaptic neurotoxin from Vipera ammodytes ammodytes venom, was found to bind specifically and with high affinity to bovine cortex synaptic membrane preparation. The detected ammodytoxin A high-affinity binding was characterized by equilibrium binding analysis which revealed a single high-affinity binding site with Kd 4.13 nM and Bmax 6.67 pmoles/mg of membrane protein. 125I-ammodytoxin A was covalently cross-linked to its neuronal acceptor using a chemical cross-linking technique. As revealed by subsequent SDS-PAGE analysis and autoradiography, 125I-ammodytoxin A specifically attached to membrane components with apparent mol. wts 53,000-56,000. Besides by the native ammodytoxin A, the binding of radioiodinated ammodytoxin A to the neuronal acceptor was highly attenuated, also by other two iso-neurotoxins from V. a. ammodytes venom, ammodytoxins B and C, and neurotoxin crotoxin B from the venom of the South American rattlesnake (Crotalus durissus terrificus). Vipera berus berus phospholipase A2 was a weaker inhibitor, whereas nontoxic phospholipase A2, ammodytoxin I2 and myotoxic phospholipase A2 homologue, ammodytin L, both from V. a. ammodytes venom as well, were very weak inhibitors. No inhibitory effect on 125I-ammodytoxin A specific binding at all was, however, obtained with alpha-dendrotoxin, beta-bungarotoxin and crotoxin A, respectively. Treatment of synaptic membranes with proteinase K and Staphylococcus aureus V-8 proteinase, a combination of PNGase F and neuroaminidase, heat or acid lowered the 125I-ammodytoxin A specific binding to various extents but never completely abolished it. The ammodytoxin A binding site in bovine synaptic membranes is thus most likely a combination of membrane glycoprotein acceptor and membrane phospholipids. As ammodytoxin A reduced the second negative component of the perineural waveform, measured on mouse triangularis sterni preparation, which is very likely a result of an inhibition of a fraction of the terminal K+ currents, the ammodytoxin A acceptor could well be connected with K+ channels.

Structure-activity studies on scorpion toxins that block potassium channels

Scorpion venoms contain toxins that block different types of potassium channels. Some of these toxins have affinity for high conductance Ca(2+)-activated K+ channels and for dendrotoxin-sensitive voltage-dependent K+ channels. The structural features that determine the specificity of binding to different channel types are not known. We investigated this using natural and synthetic scorpion toxins. We have tested the effects of charybdotoxin (CTX) and two homologues (Lqh 15-1 and Lqh 18-2), iberiotoxin (IbTX), and kaliotoxin (KTX) from the scorpions Leiurus quinquestriatus hebreus, Buthus tamulus and Androctonus mauretanicus mauretanicus, respectively, and synthetic variants of CTX, namely CTX2-37, CTX3-37, CTX4-37, and CTX7-37, on a Ca(2+)-activated K+ current (IK-Ca) at a mammalian motor nerve terminal, and on the binding of a radiolabelled dendrotoxin, 125I-DpI, to voltage-dependent K+ channels on rat brain synaptosomal membranes. The native toxins contain 37-38 amino acid residues, they are over 30% identical in sequence (CTX and IbTX are 68% identical), and they have similar three-dimensional conformations. All toxins, except IbTX, displaced 125I-DpI from its synaptosomal binding sites: Lqh 18-2 (Ki = 0.25 nM), KTX (Ki = 2.1 nM), CTX (Ki = 3.8 nM), CTX2-37, (Ki = 30 nM), Lqg 15-1 (Ki = 50 nM), CTX3-37 (Ki = 60 nM), CTX4-37 (Ki = 50 nM), CTX7-37 (Ki = 105 nM). IbTX had no effect at 3 microM. When variants of CTX with deletions at the N-terminal portion were tested for their activity on IK-Ca on motor nerve terminals in mouse triangularis sterni nerve-muscle preparations, CTX3-37 and CTX4-37 were ineffective at 100 nM; and CTX7-37 was ineffective at up to 1 microM. IbTX and CTX (100 nM) completely blocked IK-Ca, but KTX (100 nM) did not affect the nerve terminal IK-Ca. Different residues appear to be important for interactions of the toxins with different K+ channels. IbTX did not displace dendrotoxin binding, but it did block IK-Ca, whereas KTX was as active as CTX against dendrotoxin binding but it did not affect the IK-Ca of the motor nerve terminals. The N-terminal section of the toxins appears to be particularly involved in block of IK-Ca at the motor nerve terminal: it is truncated in the inactive synthetic CTX variants; and it is positively charged at lysine-6 in KTX (which is inactive), but negatively charged in IbTX and neutral in CTX.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuromuscular effects of some potassium channel blocking toxins from the venom of the scorpion Leiurus quinquestriatus hebreus

The scorpion venom Leiurus quinquestriatus hebreus was fractionated by chromatography in order to isolate toxins that affected binding of radiolabelled dendrotoxin to K+ channel proteins on synaptosomal membranes and that facilitated acetylcholine release in chick biventer cervicis nerve-muscle preparations. In addition to the previously characterized charybdotoxin, three toxins were isolated: 14-2, 15-1 and 18-2. Toxin 14-2 has a blocked N-terminus and because of low quantities, it has not been sequenced; 15-1 is a newly sequenced toxin of 36 residues with some overall homology to charybdotoxin and noxiustoxin; 18-2 is identical to charybdotoxin-2. The apparent Ki against dendrotoxin binding were: charybdotoxin, 3.8 nM; 14-2, 150 nM; 15-1, 50 nM; and 18-2, 0.25 nM. Toxin 14-2 (75 nM-1.5 microM) had a presynaptic facilitatory effect on neuromuscular preparations. Toxin 15-1 augmented responses to direct muscle stimulation, probably because it blocked Ca(2+)-activated K+ currents in muscle fibres. Toxin 18-2 (charybdotoxin-2) had a potent presynaptic facilitatory action, with less effect on direct muscle stimulation. This contrasts with the relatively weak neuromuscular effects of the highly homologous charybdotoxin. On a Ca(2+)-activated K+ current in mouse motor nerve endings, charybdotoxin and toxin 18-2 produced maximal block at around 100 nM, whereas 15-1 was inactive at 300 nM. Charybdotoxin can increase quantal content, but this is more likely to result from block of voltage-dependent K+ channels than Ca(2+)-activated channels: the increase in transmitter release occurred in conditions in which little IKCa would be present; higher concentration of charybdotoxin and longer exposure times were required to increase transmitter release than those needed to block IKCa, and the facilitatory effects of charybdotoxin and toxin 18-2 correlated more with their effects on dendrotoxin binding than on block of IKCa.

Tacrine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in Alzheimer's disease

Tacrine is a centrally acting cholinesterase inhibitor with additional pharmacological activity on monoamine levels and ion channels. It has been postulated that some or all of these additional properties may also be relevant to the mode of action of the drug. There are wide interindividual variations in pharmacological and clinical response to tacrine, possibly related to interindividual variation in bioavailability. Tacrine appears to improve cognitive function and behavioural deficits in a proportion of patients with Alzheimer's disease, at dosages of 80 to 160 mg/day. In the best designed trials, 30 to 51% of evaluable patients showed an improvement of at least 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale, versus 16 to 25% of placebo recipients. A similar proportion of tacrine recipients were judged to have improved when global assessment scales were used. There was a significant dose-response relationship up to 160 mg/day. However, large numbers of patients were withdrawn during the trials, many because of tacrine-associated increases in transaminase levels. Elevated liver enzyme levels occurred in about 50% of tacrine recipients (reaching clinical significance in about 25%). Cholinergic symptoms also occurred more often in tacrine recipients than in those receiving placebo. A gradual increase in tacrine dosage, at 6-week intervals, is recommended when initiating therapy, and weekly serum transaminase monitoring is required for 6 weeks after each dosage increase. Despite the limitations implied by the low proportion of responders and high incidence of hepatic adverse effects associated with therapy, tacrine appears to make a measurable difference in both cognitive and behavioural function in a proportion of patients with Alzheimer's disease--a welcome advance in an area previously devoid of acceptable treatment options.

Velnacrine in Alzheimer's Disease : An Initial Appraisal of its Clinical Potential

Velnacrine is an hydroxylated derivative of the acetylcholinesterase inhibitor tacrine. The ability of velnacrine to increase cholinergic neurotransmission in vitro provides the rationale for its investigation as a potential treatment in patients with Alzheimer's disease, who are known to have reduced acetylcholine levels in the central nervous system. Single doses of velnacrine (100 or 150mg) attenuated cognitive impairment induced by central cholinergic blockade in healthy volunteers, and memory improved significantly in a small number of patients with Alzheimer's disease administered a 75mg dose.Evidence of efficacy for velnacrine is limited to results of briefly reported placebo-controlled studies. When administered in dosages of up to 225 mg/day for 6 weeks, velnacrine appeared to confer modest benefit in about one-third of 423 patients with Alzheimer's disease enrolled in a US dose-finding trial. Velnacrine 150 mg/day for 10 days was also considered superior to placebo in a small European trial involving 35 patients, notably in its effects on language, praxis and memory. Fuller results are anticipated from a 6-month investigation demonstrating efficacy for velnacrine 150 or 225 mg/day at 12-week interim analysis. Of interest is the finding from this trial that caregiver time assessed at 24 weeks was shorter for velnacrine compared with placebo recipients.The development of elevated plasma hepatic enzyme levels leading to treatment discontinuation in 27% of participants in the US trial, combined with the appearance of neutropenia in a few patients, has cast doubt over the tolerability profile of velnacrine. Ongoing investigations are endeavouring to identify the mechanism of the hepatotoxic effect, to establish whether a dose-response relationship exists, and to define possible subpopulations that may respond to velnacrine and those who may be at particular risk of developing hepatotoxicity. Other reported adverse events severe enough to cause treatment withdrawal have included rash, nausea, diarrhoea, headache and dizziness/fainting.In summary, questions surrounding the tolerability and efficacy of velnacrine must be resolved before its early promise as a treatment in Alzheimer's disease can be realised. Nonetheless, given the limited therapeutic options presently, available, the drug may yet prove to be of value in at least some patients with Alzheimer's disease.

Effects of scorpion (Buthus tamulus) venom on neuromuscular transmission in vitro

The effects of venom from the Indian red scorpion Buthus tamulus (BT) on neuromuscular transmission have been investigated by means of twitch tension and electrophysical recording techniques using isolated skeletal muscle preparations. On chick biventer cervicis preparations, BT (1-3 micrograms/ml) augmented the twitch responses to indirect, but not direct, muscle stimulation. Higher concentrations caused a transient augmentation followed by a large contracture and then a reduction in twitch height. BT at the concentrations tested caused little change in postjunctional sensitivity as assessed by responses to exogenous acetylcholine, carbachol and KCl. Tubocurarine abolished the prolonged contracture induced by BT (10 micrograms/ml) in the presence or absence of nerve stimulation. On mouse hemidiaphragm preparations, BT (3-10 micrograms/ml) increased the twitch responses to indirect stimulation but caused little change in directly stimulated preparations. On mouse triangularis sterni preparations, BT (3-10 micrograms/ml) increased quantal content of the evoked end-plate potentials (epps) by about 70%, without markedly affecting the time course and amplitude of miniature epps. BT also caused repetitive epps in response to single shock nerve stimulation. Extracellular recording of nerve terminal current waveforms in triangularis sterni preparations revealed that BT (10-30 micrograms/ml) slightly reduced the amplitude of the waveform. Subsequently, BT induced repetitive firing of nerve endings in response to single shock stimulation, and eventually markedly prolonged the time course of the nerve terminal waveform. The effects caused by BT were different from those caused by iberiotoxin, the blocker of Ca(2+)-activated K+ currents, isolated from BT. The effects were similar to those caused by ATX-II, a toxin that delays inactivation of Na+ channels. However, BT and ATX-II behaved differently in the presence of K+ channel blockers, 3,4-diaminopyridine (DAP) and tetraethylammonium (TEA). These results confirm that Buthus tamulus venom acts mainly prejunctionally to increase the release of acetylcholine. The effect of BT on the perineural waveforms suggests that some of its actions may be due to effects on Na+ channels at or near the nerve terminals; however, additional effects of K+ channels are likely.

Comparative effects of velnacrine, tacrine and physostigmine on the twitch responses in the rat phrenic-hemidiaphragm preparation

1. Cholinesterase inhibitors potentiated twitch responses induced by nerve stimulation, with physostigmine more potent than tacrine and velnacrine. However, at higher concentrations, tacrine decreased twitch responses in a concentration-dependent manner. 2. Tacrine strongly depressed directly-induced twitch responses, whereas the other drugs had minimal effects. 3. Physostigmine was the most potent drug in reversal of tubocurarine-induced blockade, but its antagonism index was similar to those obtained with tacrine and velnacrine at higher concentrations. When evaluating their ability to reverse neomycin-induced blockade, all drugs exhibited a similar effect. 4. It is concluded that tacrine and velnacrine are less potent than physostigmine in potentiating skeletal neuromuscular transmission. Additionally, the blocking effects of tacrine could limit its therapeutic efficacy.

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.

Reversal by cysteine of the cadmium-induced block of skeletal neuromuscular transmission in vitro

1. Neuromuscular transmission in isolated nerve-muscle preparations was blocked by exposure to Cd2+ for less than 30 min or more than 2 h. The abilities of cysteine, Ca2+ or 3,4-diaminopyridine (3,4-DAP) to reverse the blockade induced by Cd2+ were studied. 2. On the mouse hemidiaphragm preparation, exposure to Cd2+ (10 microM) for 10 to 20 min induced a blockade which was easily reversed by increasing the extracellular Ca2+ concentration (5-10 mM) or by 3,4-DAP (100 microM). Exposure to Cd2+ (3-10 microM) for over 2 h led to a blockade which was not reversed by Ca2+ (5-15 mM) or 3,4-DAP (100 microM). Cysteine (1 mM) was able to reverse completely the blockade induced by both brief and prolonged exposures to Cd2+. 3. In chick biventer cervicis preparations, Cd2+ (100 microM) decreased the twitch height of indirectly stimulated preparations without affecting responses to exogenously applied acetylcholine, carbachol or KCl. Cysteine (1-3 mM) had no appreciable effect on twitch responses to indirect stimulation or to exogenously applied agonists but fully reversed the blockade induced by Cd2+ (100 microM). 4. In mouse triangularis sterni preparations, Cd2+ (1-30 microM) depressed the evoked quantal release of acetylcholine. Concentrations of Cd2+ which completely blocked endplate potentials (e.p.ps) were without significant effect on miniature endplate potential (m.e.p.p.) amplitude and frequency or time constant of decay. Cysteine (1-10 mM) alone had no effect on e.p.ps or m.e.p.ps, but completely reversed the blockade induced by Cd2+.6. In addition to the competitive blocking action of Cd2+ at the prejunctional Ca2+ channels, long exposure to Cd2+ leads to a blockade that is not competitive. This probably involves binding of Cd2+" at an extracellular thiol site on, or close to, voltage-operated Ca2+' channels.

Apparent block of K+ currents in mouse motor nerve terminals by tetrodotoxin, mu-conotoxin and reduced external sodium

1. In mouse triangularis sterni nerve-muscle preparations, reduced extracellular Na+ concentrations and low concentrations of the Na+ channel blocking toxins tetrodotoxin (TTX, 18-36 nM) and mu-conotoxin GIIIB (0.4-2.0 microM) selectively decreased the amplitude of the component of perineural waveforms associated with nerve terminal K+ currents, without affecting the main Na+ spike. 2. Intracellular recording of endplate potentials (e.p.ps) and miniature endplate potentials (m.e.p.ps) from triangularis sterni preparations revealed that TTX and mu-conotoxin GIIIB depressed the evoked quantal release of acetylcholine without significant effects on m.e.p.p. amplitude, frequency or time constant of decay. 3. The apparent block of K+ current by low concentrations of TTX and mu-conotoxin is probably not a direct effect on K+ channels but results from a decrease in the passive depolarization of nerve terminals following blockade of a small proportion of axonal Na+ channels.

The effects of vesamicol on trains of endplate currents and on focally recorded nerve terminal currents at mammalian neuromuscular junctions

1. The effects of vesamicol, an inhibitor of vesicular acetylcholine (ACh) storage, were studied on trains of endplate currents (e.p.cs) in the cut rat hemidiaphragm nerve-muscle preparation and on trains of focally recorded nerve terminal current waveforms in the mouse triangularis sterni nerve-muscle preparation. 2. In the rat, 0.1 and 1 microM (-)-vesamicol produced an enhancement of the rundown of e.p.c. amplitudes during trains of high frequency (50 Hz) nerve stimulation. However, 1 microM (+)-vesamicol had no effect on the rundown of e.p.c. amplitudes. 3. In the mouse, high concentrations of (-)-vesamicol (10-100 microM) produced a concentration- and stimulation-dependent decrease in the amplitude of the second negative-going deflection of focally recorded nerve terminal current waveforms. 4. At 1 mM, (-)-vesamicol produced a stimulation-independent decrease in the amplitude of the first negative-going deflection of the nerve terminal current waveforms, an increase in signal delay and evidence of nerve conduction failure. These all indicate a local anaesthetic-like block of nodal Na(+)-channels. 5. In contrast to its effects on trains of e.p.cs, the effects of vesamicol on the nerve terminal current waveforms were not stereoselective, the (+)-isomer being equipotent with the (-)-isomer. 6. Low concentrations of the Na(+)-channel blocking toxin, tetrodotoxin (15-60 nM), produced similar changes in the focally recorded nerve terminal current waveforms to those seen with vesamicol. 7. It is concluded that the stereoselective rundown of e.p.c. amplitudes produced by (-)-vesamicol is due to an effect, either direct or indirect, on ACh mobilization within motor nerve terminals. Furthermore, in mammalian species, the inhibitory effects of vesamicol on nodal Na+-channels which are seen at high concentrations do not contribute to the principal neuromuscular effects of the compound.