N,O-di and N,N,O-tri ( 3 H) acetyl -bungarotoxins as specific labelling agents of cholinergic receptors

Neuromuscular activity of Micrurus laticollaris (Squamata: Elapidae) venom in vitro

In this work, we have examined the neuromuscular activity of Micrurus laticollaris (Mexican coral snake) venom (MLV) in vertebrate isolated nerve-muscle preparations. In chick biventer cervicis preparations, the MLV induced an irreversible concentration- and time-dependent (1–30 µg/mL) neuromuscular blockade, with 50% blockade occurring between 8 and 30 min. Muscle contractures evoked by exogenous acetylcholine were completely abolished by MLV, whereas those of KCl were also significantly altered (86% ± 11%, 53% ± 11%, 89% ± 5% and 89% ± 7% for one, three, 10 and 30 µg of venom/mL, respectively; n = 4; p < 0.05). In mouse phrenic nerve-diaphragm preparations, MLV (1–10 µg/mL) promoted a slight increase in the amplitude of twitch-tension (3 µg/mL), followed by neuromuscular blockade (n = 4); the highest concentration caused complete inhibition of the twitches (time for 50% blockade = 26 ± 3 min), without exhibiting a previous neuromuscular facilitation. The venom (3 µg/mL) induced a biphasic modulation in the frequency of miniature end-plate potentials (MEPPs)/min, causing a significant increase after 15 min, followed by a decrease after 60 min (from 17 ± 1.4 (basal) to 28 ± 2.5 (t₁₅) and 12 ± 2 (t₆₀)). The membrane resting potential of mouse diaphragm preparations pre-exposed or not to d-tubocurarine (5 µg/mL) was also significantly less negative with MLV (10 µg/mL). Together, these results indicate that M. laticollaris venom induces neuromuscular blockade by a combination of pre- and post-synaptic activities.

The neuromuscular activity of Micrurus pyrrhocryptus venom and its neutralization by commercial and specific coral snake antivenoms

The neuromuscular activity ofMicrurus pyrrochryptus venom was studied in chick biventer cervicis (BC) and mouse phrenic nerve-diaphragm (PND) preparations. The venom (0.5-50μg/ml) caused irreversible, time- and concentration-dependent blockade, with BC being more sensitive than PND (50% blockade with 10μg/ml in 22±;3min and 62±4min, respectively; mean±SEM, n=6; p<0.05). In BC preparations, venom (0.5μg/ml) progressively abolished ACh-induced contractures, whereas contractures to exogenous KCl and muscle twitches in curarized preparations were unaffected. The venom neither altered creatine kinase release (venom: 25.8±1.75IU/l vs control: 24.3±2.2IU/l, n=6, after 120min), nor it caused significant muscle damage (50μg of venom/ml vs control: 3.5±0.8% vs 1.1±0.7% for PND; 4.3±1.5% vs 1.2±0.5% for BC, n=5). The venom had low PLA(2) activity. Neurotoxicity was effectively neutralized by commercial Micrurus antivenom and specific antivenom. These findings indicate that M. pyrrhocryptus venom acts postsynaptically on nicotinic receptors, with no significant myotoxicity.

Contribution of a snake venom toxin to myasthenia gravis: the discovery of alpha-bungarotoxin in Taiwan

Myasthenia gravis (MG) is now recognized as an autoimmune disorder in which antibodies to acetylcholine (ACh) receptor lead to impairment of neuromuscular transmission. The discovery of alpha-bungarotoxin by Chang and Lee in 1963 has played a crucial role in establishing the new concept of MG. However, isolation of bungarotoxins from the venom of Taiwan banded krait, Bungarus multicinctus, was accomplished in the poorly funded and under equipped laboratory of the Department of Pharmacology, National Taiwan University, during the post-WWII period of economic depression and research isolation. Because alpha-bungarotoxin binds specifically and irreversibly with the muscle type nicotinic ACh receptor, it was used to localize ACh receptor and to isolate the ACh receptor protein, opening up a new era of receptor studies. It was also used to produce an antibody to ACh receptor and eventually an experimental autoimmune model of MG and clinical confirmation. The discovery of alpha-bungarotoxin has been considered the most important contribution to neurosciences from Taiwan.

Looking back on the discovery of alpha-bungarotoxin

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

Acetylcholine receptors at mature and aged mouse neuromuscular junctions

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

Muscarinic and nicotinic cholinergic binding sites in the terminal abdominal ganglion of the cricket (Acheta domesticus)

Cricket (Acheta domesticus) terminal abdominal ganglia (TG) contain high concentrations (approximately 2 pmol/mg protein) of muscarinic and nicotinic cholinergic binding sites, based on the capacity of TG to bind specifically the labelled ligands L-[3H]quinuclidinyl benzilate ([3H]QNB) and [125I]alpha-bungarotoxin ([125I]alpha-BGT) with high affinity. For both ligands, binding is saturable and reversible. Competitive displacement experiments indicate that the [3H]QNB and [125I]alpha-BGT binding sites probably represent pharmacologically distinct classes of putative TG acetylcholine receptors (AChRs). Results from physiological recording and autoradiographic localization experiments demonstrate that a portion of the putative nicotinic AChRs is localized in synaptic regions of the well-characterized cercal sensory-giant interneuron pathway in the TG, where they are likely to serve as functional synaptic AChRs. Unlike nicotinic ligands, muscarinic agents do not appear to be pharmacologically active in this pathway. Therefore, in the insect CNS, putative muscarinic and nicotinic AChRs coexist at high density, but can be pharmacologically distinguished from one another on the basis of criteria derived from both ligand binding and physiological methods.

Properties of junctional acetylcholine receptors that appear rapidly after denervation

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

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

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

Effects on muscle of a toxin from Indian cobra (Naja naja naja) venom

The mode of action of a purified toxin from Naja naja naja (Indian cobra) venom was investigated in frog rectus abdominis muscle, chick biventer cervicis muscle, cat tibialis anterior muscle (close-arterial) and in both innervated and denervated rat diaphragm muscle preparations. The toxin inhibited the acetylcholine responses of rectus abdominis muscle. The inhibition was antagonized by neostigmine and increasing concentrations of acetylcholine, suggesting a competitive binding of the toxin to cholinergic receptors. The toxin, even at high doses, did not produce depolarizing contractures in chronically denervated diaphragm, biventer cervicis muscle and rectus muscle preparations. In both cat tibialis anterior and denervated diaphragm muscles, the toxin abolished the acetylcholine sensitivity of the muscles at a faster rate than its effects on muscle contraction, suggesting a preferred action on the motor end-plate. A well-maintained tetanic contraction and very poor post-tetanic potentiation was observed in all preparations treated with toxin, indicating an atypical Wedensky inhibition. Anti-curare agents, such as K+ and Ca2+, were ineffective in antagonizing the curare-like neuromuscular block in phrenic nerve-diaphragm preparations. A frequency-independent neuromuscular block observed in these nerve-muscle preparations was suggestive of the absence of a possible presynaptic effect. These results demonstrate that although the neurotoxin in some cases can imitate d-tubocurarine, its neuromuscular blocking activity is different from that of curare in many respects.

Elapid alpha-toxins have no effect on the cholinergic responses of bivalve myocardia

Elapid alpha-toxins (alpha-bungarotoxin, alpha-cobratoxin and crude Bungarus caeruleus venom) do not affect the myocardial nicotinic ACh receptors of the following bivalve molluscs: Mercenaria mercenaria, Chione cancellata, Mya arenaria, Mytilus edulis, Rangia cuneata and Crassostrea virginica.

Regulation of acetylcholine receptor levels by a cholinergic agonist in mouse muscle cell cultures

The effects of continuous exposure to carbamylcholine (CbCho) on regulation and stabilization of acetylcholine receptors (AcChoR) were studied in cell cultures of G8, a continuous mouse muscle cell line. Exposure of cultures to 10-100 muM CbCho for 24-48 hr produced a 30-50% reduction in (125)I-labeled alpha-bungarotoxin binding. CbCho was not found to alter cell morphology, protein metabolism, or amino acid incorporation. Electrophysiological experiments demonstrated a 75% reduction in the maximum sensitivity of the myotubes to iontophoretic application of acetylcholine (AcCho). The reduction in AcCho sensitivity appeared to represent a true loss of functional receptors because there were no changes in the passive electrical properties of the cells or in the AcCho reversal potential and because receptor desensitization appeared not to be involved. Tetrodotoxin had no effect on receptor levels, either alone or in combination with CbCho. Receptor degradation in control cells could be described kinetically as a first-order process with a half-time of 19.2 hr; turnover rate in receptors remaining after prolonged exposure to CbCho was indistinguishable from that in control cells. We conclude that a receptor-active ligand can exert negative control over AcChoR levels and that prolonged exposure to an AcCho analog is not sufficient to induce a stable population of receptors in these cells.

Cytochemical localization of acetylcholine receptors at the neuromuscular junction by means of horseradish peroxidase-labeled alpha-bungarotoxin

The cytochemical localization of acetylcholine (ACh) receptors at the neuromuscular junction was investigated with a procedure utilizing alpha-bungarotoxin (alpha-BtX) labeled directly with horseradish peroxidase (HRP). Following incubation of tissues in the conjugate and reaction for peroxidase, activity was observed on the junctional folds of the motor endplate. A uniform layer of reaction product approximately 15 nm thick occurred over the apical portions of the junctional folds. Membranes at the bases of the synaptic cleft showed only small amounts of reaction product. Non-junctional regions of the muscle fiber were unreactive. Activity was also observed in the membrane of the axon facing the muscle surface, often including the axolemma overlying the "active zones" of the nerve terminal. Such presynaptic activity was still evident on nerve terminals disjuncted from the synapse by enzymatic treatment prior to incubation in the conjugate. This localization indicates the possible presence of presynaptic ACh receptors within the axolemma. In muscle denervated for 7-12 days, motor endplates were reactive and parajunctional sarcolemma showed slight activity, but most extrajunctional regions contained no obvious accumulations of reaction product. Activity at all sites was prevented by preincubation of tissues in native alpha-BTX prior to incubation in the conjugate and reaction for HRP. This procedure represents a simple and convenient method for the high resolution localization of ACh receptors.

Effect of denervation on the permeability of acetylcholine-activated channels to organic cations

Experiments were perfomed on chronically denervated frog sartorius muscles to determine the permeability of the acetylcholine-activated channels to organic cations. The membrane voltage response to bath-applied acetylcholine was measured with the moving electrode when the muscles were bathed in normal Ringer and in Ringer in which all of the Na+ had been replaced with an organic cation. The magnitude of the maximum voltage response was used to estimate the permeability of the channel to the organic cation. These results were compared with those which have been reported for innervated frog sartorius muscles (Maeno, Edwards, and Anraku, 1977). It is concluded that the permeability to a wide range of organic cations is virtually identical for the extrajunctional channels which develop following denervation and the channels with are localized at the junctional region of innervated muscles.

Appearance of new acetylcholine receptors on the baby chick biventer cervicis and denervated rat diaphragm muscles after blockade with alpha-bungarotoxin

1. The recovery of contractile responses and appearance of new alpha-bungarotoxin-binding sites were studied in the baby chick biventer cervicis and the rat diaphragm muscles after saturating the existing acetylcholine (ACh) receptors (AChR) with alpha-bungarotoxin in vitro.2. Washout of alpha-bungarotoxin restored gradually the response to exogenous ACh attaining about 30% recovery in 3 hr either in the chick muscle or in the denervated rat diaphragm. No recovery was obtained, however, for the response to nerve stimulation.3. The recovery of ACh-response was abolished by decreasing the bath temperature to 9 degrees C during the washout of the toxin whereas the recovery was not reduced in the presence of cycloheximide.4. The half-life of [(3)H]acetyl alpha-bungarotoxin bound specifically on the existing AChRs, junctional and extrajunctional receptors combined, was 16 hr in the chick muscle. That on the extrajunctional AChR was estimated to be 8 hr.5. New toxin-binding sites were found to be incorporated on the membrane of extrajunctional site rapidly after treatment with alpha-bungarotoxin in the chick and the denervated rat muscles along the muscle fibres but not in the innervated rat diaphragm. Treatment with (+)-tubocurarine, ACh or decamethonium did not cause an appreciable increase of the toxin-binding sites.6. The appearance of new binding sites was progressive during 5 hr at a rate of 24 sites/mum(2).hr in the chick muscle and 42 sites/mum(2).hr in the rat diaphragm denervated for 7 days. The existing extrajunctional AChR were about 50/mum(2) and 192/mum(2), respectively.7. ACh effectively antagonized the binding of alpha-bungarotoxin with the new sites whereas (+)-tubocurarine was less effective than its effect on the existing AChR.8. The new toxin-binding sites appeared to have a reduced capacity to evoke ACh response.9. The incorporation of new binding sites was reduced by lowering of the temperature, treatment with dinitrophenol, high K(+), high Ca(2+) and by the stimulation of either nerve or muscle. Cycloheximide, ACh, decrease of [Na(+)](o) and increase of [Mg(2+)](o) were without effect.10. It is suggested that binding of the extrajunctional AChRs with alpha-bungarotoxin cause a change of membrane architecture and trigger the incorporation of cytoplasmic AChR-precursor or hidden AChR into the membrane.

Further evidence that extrinsic acetylcholine acts preferentially on extrajunctional receptors in the chick biventer cervicis muscle

The specificity of action of extrinsic acetylcholine on extrajunctional and junctional receptors in the chick biventer cervicis muscle was studied by determining its ability to protect the responses evoked by acetylcholine and by tetanic nerve stimulation from the blockade by alpha-bungarotoxin, an irreversible binding agent of acetylcholine receptors. At concentrations of 50-100 mug/ml, acetylcholine caused a desensitization to extrinsic acetylcholine but not to nerve stimulation and protected only the contractile response to extrinsic acetylcholine from the toxin blockade whereas neither the response to tetanic nerve stimulation nor the endplate potentials were protected. For the protection of the latter, higher concentrations of acetylcholine were needed. In the presence of physostigmine, a concentration of acetylcholine as low as 10 mug/ml protected the endplate potentials from the toxin blockade. By contrast, d-tubocurarine protected the tetanic contraction and the endplate potentials induced by nerve stimulation at a concentration which produced the same protection of acetylcholine-induced contraction as that produced by 50-100 mug/ml acetylcholine. These results indicate that in contrast to d-tubocurarine, extrinsic acetylcholine at low concentrations acts preferentially on the extrajunctional receptors in the absence of an anticholinesterase.

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

1. Acetylcholine receptors in the end-plate and non-end-plate areas of the rat diaphragm, after treating the animal with hemicholinium-3, alpha- or beta-bungarotoxin in vivo, were studied by their specific binding of labelled alpha-bungarotoxin. 2. Subcutaneous injection of maximum tolerable doses of hemicholinium-3 (50 mug/kg) twice daily for 7 days increased the number of extrajunctional receptors along the whole length of muscle fibre, the approximate density of receptor on muscle membrane being increased from 6/mum2 in normal diaphragm to 38/mum2. Junctional receptors were also increased in number from 2-2 x 10(7) to 2-8 x 10(7) per end-plate. 3. Five days after denervation, there were approximately 153/mum2 extrajunctional receptors and the number of receptors on the end-plate was increased by 220%. 4. Intrathoracic injection of beta-bungarotoxin (50 mug/kg) also increased the density of extrajunctional receptors to approximately 104/mum2, and the number of end-plate receptors by 140% in 5 days. The neuromuscular block was extensive and prolonged. 5. [3H]Diacetyl alpha-bungarotoxin (150 mug/kg) injected into thoracic cavity caused complete neuromuscular blockade for 12 hr. At 24 hr, the synaptic transmission was restored in 80% of the junctions with less than 10% end-plate receptors freed, whereas the safety factor for transmission in normal diaphragm was 3-5. Extrajunctional receptors appeared to increase within 24 hr. This increase continued despite the restoration of neuromuscular transmission, and the receptor density at 5 days was approximately 5l/mum2. The number of junctional receptors, however, was not increased. Repeated injection of the toxin gave the same result. 6. It is concluded that the numbers of junctional and extrajunctional acetylcholine receptors are regulated in different ways, and the possible role of acetylcholine is discussed.

Acetylcholine receptor turnover in membranes of developing muscle fibers

[125I mono-iodo-alpha-bungarotoxin is used as a specific marker in a description of acetylcholine receptor metabolism. It is concluded that acetylcholine receptors in the surface membranes of chick and rat myotubes developing in cell cultures have a half-life of 22-24 h. Alpha-bungarotoxin (bound to a receptor which is removed from the membrane) is degraded to monoiodotyrosine which appears in the medium. Several observations are consistent with a model in which receptors or alpha-bungarotoxin-receptor complexes are internalized and then degraded: (a) the rate of appearance of iodotyrosine does not reach its maximal rate until 90 min after alpha-bungarotoxin is bound to the surface receptors; (b) 2,4-dinitrophenol, reduced temperature, and cell disruption all inhibit the degradation process. The degradation of surface receptors is not coupled to the process by which receptors are incorporated into the membrane. Evidence suggest that receptors are incorporated into the surface membrane from a presynthesized set of receptors containing about 10% as many alpha-bungarotoxin binding sites as does the surface. Additionally, a third set of acetylcholine receptors is described containing about 30% as amny binding sites as does the surface. These "hidden" recptors are not precursors yet are not readily accessible for binding of extracellular alpha-bungarotoxin. These findings are discussed in relation to both plasma membrane biosynthesis and control of chemosensitivity in developing and denervated skeletal muscle.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between &lsquo;curari&rsquo; and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

The binding of tetrodotoxin and alpha-bungarotoxin to normal and denervated mammalian muscle

1. The binding of [(3)H]tetrodotoxin and [(125)I]iodo-alpha-bungarotoxin to innervated and denervated rat diaphragm muscle has been measured.2. A saturable component of tetrodotoxin binding, which was inhibited by saxitoxin, was detected in addition to considerable non-saturable binding. The saturable component had an equilibrium constant of K = 6.1 nM (limits 4.7-7.8 nM) and binding capacity M = 2.5 f-mole/mg wet wt. (limits 2.1-2.8 f-mole/mg).3. If the saturable component consisted of one-to-one binding of tetrodotoxin to sodium channels, the density of sodium channels would be about 21/mum(2) of surface membrane, a figure similar to that found in other excitable membranes.4. After denervation the specific tetrodotoxin binding, as measured by the ratio M/K, fell by a factor of 2.8. This change appeared to be due to a fall in binding capacity rather than a decrease in affinity.5. After denervation the maximum rate of rise of the action potential fell by 27% and became partially resistant to tetrodotoxin. The maximum rate of rise was at first reduced by tetrodotoxin in similar concentrations to those affecting normal muscle, but even large concentrations which completely blocked normal muscle only reduced the maximum rate of rise by a factor of about 2.6. Detubulation with glycerol did not appreciably affect the tetrodotoxin sensitivity of normal or denervated muscle.7. Tetrodotoxin resistance was not observed after denervation of the frog sartorius muscle.8. [(125)I]iodo-alpha-bungarotoxin binding amounted to 3.8 +/- 0.7 f-mole/mg in innervated muscle and 44.5 +/- 2.1 f-mole/mg in denervated muscle. Most of the uptake was inhibitable by (+)-tubocurarine.9. The increase in the labelled bungarotoxin binding is much larger than the specific tetrodotoxin binding of innervated muscle, which renders implausible the possibility that the acetylcholine receptors which appear after denervation are related to the tetrodotoxin resistant-sodium channels.

Fluorescent staining of acetylcholine receptors in vertebrate skeletal muscle

1. alpha-Bungarotoxin was labelled with fluorescent dyes and used as a stain for visualizing the distribution of acetylcholine receptors in vertebrate skeletal muscle fibres.2. Dye-toxin conjugates had the same pharmacological properties as native toxin, but their potencies were lower.3. Fluorescent staining was examined in teased muscle fibres. The stain was found to be confined to the neuromuscular junction and associated with the subsynaptic membrane.4. Staining intensity was reduced by curare and even more so by carbachol, but not by atropine or neostigmine. Pre-treatment of muscles with unlabelled alpha-bungarotoxin entirely prevented staining.5. The staining at amphibian neuromuscular junctions was characterized by a pattern of intense transverse bands occurring at intervals of approximately 0.5-1 mum, with fluorescence of lower intensity between them. Fluorescent staining was not detected on adjacent, extrasynaptic, muscle membrane. In side views the staining appeared as a fine line with small protuberances occurring at the same intervals as the intense bands seen face-on. These results indicate that acetylcholine receptors are associated with the entire subsynaptic membrane, including the membrane of the junctional folds and that their density changes abruptly at the border between synaptic and extrasynaptic muscle membrane.

Neuromuscular junction in myasthenia gravis: decreased acetylcholine receptors

The number of acetylcholine receptors was determined in the neuromuscular junctions of eight patients with typical myasthenia gravis and in five controls, by means of (125)1-labeled alpha-bungarotoxin binding. The junctional acetylcholine receptors were reduced in the myasthenic muscles as compared with the controls. This reduction in receptors may account for the defect in neuromuscular transmission in myasthenia gravis.

Influence of chronic neostigmine treatment on the number of acetylcholine receptors and the release of acetylcholine from the rat diaphragm

1. Rats were treated twice daily for 7 days with neostigmine and the diaphragm was isolated for study of its acetylcholine content, release upon nerve stimulation and the number of receptors in the end-plate.2. While the content of total acetylcholine was unchanged, the release of acetylcholine on stimulation with trains of 500 pulses at 100 Hz every 20 sec was reduced by about 50%. Five days after the end of neostigmine treatment the release of acetylcholine recovered to normal.3. The total number of acetylcholine receptors in the end-plate as measured from the binding of N, O-di[(3)H]acetyl alpha-bungarotoxin was reduced from 2.1 x 10(7) to 1.2 x 10(7) per end-plate.4. The above-mentioned changes are not due to acute pharmacological effects of neostigmine, nor to an immediate effect of cholinesterase inhibition but presumably due to chronic accumulation of acetylcholine at the neuromuscular junction.