The effects of in vitro application of purified botulinum neurotoxin at mouse motor nerve terminals

Neuromuscular recovery from botulism involves multiple forms of compensatory plasticity

Introduction

Botulinum neurotoxin (BoNT) causes neuroparalytic disease and death by blocking neuromuscular transmission. There are no specific therapies for clinical botulism and the only treatment option is supportive care until neuromuscular function spontaneously recovers, which can take weeks or months after exposure. The highly specialized neuromuscular junction (NMJ) between phrenic motor neurons and diaphragm muscle fibers is the main clinical target of BoNT. Due to the difficulty in eliciting respiratory paralysis without a high mortality rate, few studies have characterized the neurophysiological mechanisms involved in diaphragm recovery from intoxication. Here, we develop a mouse model of botulism that involves partial paralysis of respiratory muscles with low mortality rates, allowing for longitudinal analysis of recovery.

Methods and results

Mice challenged by systemic administration of 0.7 LD50 BoNT/A developed physiological signs of botulism, such as respiratory depression and reduced voluntary running activity, that persisted for an average of 8-12 d. Studies in isolated hemidiaphragm preparations from intoxicated mice revealed profound reductions in nerve-elicited, tetanic and twitch muscle contraction strengths that recovered to baseline 21 d after intoxication. Despite apparent functional recovery, neurophysiological parameters remained depressed for 28 d, including end plate potential (EPP) amplitude, EPP success rate, quantal content (QC), and miniature EPP (mEPP) frequency. However, QC recovered more quickly than mEPP frequency, which could explain the discrepancy between muscle function studies and neurophysiological recordings. Hypothesizing that differential modulation of voltage-gated calcium channels (VGCC) contributed to the uncoupling of QC from mEPP frequency, pharmacological inhibition studies were used to study the contributions of different VGCCs to neurophysiological function. We found that N-type VGCC and P/Q-type VGCC partially restored QC but not mEPP frequency during recovery from paralysis, potentially explaining the accelerated recovery of evoked release versus spontaneous release. We identified additional changes that presumably compensate for reduced acetylcholine release during recovery, including increased depolarization of muscle fiber resting membrane potential and increased quantal size.

Discussion

In addition to identifying multiple forms of compensatory plasticity that occur in response to reduced NMJ function, it is expected that insights into the molecular mechanisms involved in recovery from neuromuscular paralysis will support new host-targeted treatments for multiple neuromuscular diseases.

Antidotal treatment of botulism in rats by continuous infusion with 3,4-diaminopyridine

Botulinum neurotoxins (BoNTs) are highly potent, select agent toxins that inhibit neurotransmitter release at motor nerve terminals, causing muscle paralysis and death by asphyxiation. Other than post-exposure prophylaxis with antitoxin, the only treatment option for symptomatic botulism is intubation and supportive care until recovery, which can require weeks or longer. In previous studies, we reported the FDA-approved drug 3,4-diaminopyridine (3,4-DAP) reverses early botulism symptoms and prolongs survival in lethally intoxicated mice. However, the symptomatic benefits of 3,4-DAP are limited by its rapid clearance. Here we investigated whether 3,4-DAP could sustain symptomatic benefits throughout the full course of respiratory paralysis in lethally intoxicated rats. First, we confirmed serial injections of 3,4-DAP stabilized toxic signs and prolonged survival in rats challenged with 2.5 LD50 BoNT/A. Rebound of toxic signs and death occurred within hours after the final 3,4-DAP treatment, consistent with the short half-life of 3,4-DAP in rats. Based on these data, we next investigated whether the therapeutic benefits of 3,4-DAP could be sustained throughout the course of botulism by continuous infusion. To ensure administration of 3,4-DAP at clinically relevant doses, three infusion dose rates (0.5, 1.0 and 1.5 mg/kg∙h) were identified that produced steady-state serum levels of 3,4-DAP consistent with clinical dosing. We then compared dose-dependent effects of 3,4-DAP on toxic signs and survival in rats intoxicated with 2.5 LD50 BoNT/A. In contrast to saline vehicle, which resulted in 100% mortality, infusion of 3,4-DAP at ≥ 1.0 mg/kg∙h from 1 to 14 d after intoxication produced 94.4% survival and full resolution of toxic signs, without rebound of toxic signs after infusion was stopped. In contrast, withdrawal of 3,4-DAP infusion at 5 d resulted in re-emergence of toxic sign and death within 12 h, confirming antidotal outcomes require sustained 3,4-DAP treatment for longer than 5 d after intoxication. We exploited this novel survival model of lethal botulism to explore neurophysiological parameters of diaphragm paralysis and recovery. While neurotransmission was nearly eliminated at 5 d, neurotransmission was significantly improved at 21 d in 3,4-DAP-infused survivors, although still depressed compared to naïve rats. 3,4-DAP is the first small molecule to reverse systemic paralysis and promote survival in animal models of botulism, thereby meeting a critical treatment need that is not addressed by post-exposure prophylaxis with conventional antitoxin. These data contribute to a growing body of evidence supporting the use of 3,4-DAP to treat clinical botulism.

3,4-diaminopyridine reverses paralysis in botulinum neurotoxin-intoxicated diaphragms through two functionally distinct mechanisms

Botulinum neurotoxins (BoNTs) are exceedingly potent neurological poisons that prevent neurotransmitter release from peripheral nerve terminals by cleaving presynaptic proteins required for synaptic vesicle fusion. The ensuing neuromuscular paralysis causes death by asphyxiation. Although no antidotal treatments exist to block toxin activity within the nerve terminal, aminopyridine antagonists of voltage-gated potassium channels have been proposed as symptomatic treatments for botulism toxemia. However, clinical evaluation of aminopyridines as symptomatic treatments for botulism has been inconclusive, in part because mechanisms responsible for reversal of paralysis in BoNT-poisoned nerve terminals are not understood. Here we measured the effects of 3,4-diaminopyridine (DAP) on phrenic nerve-elicited diaphragm contraction and end-plate potentials at various times after intoxication with BoNT serotypes A, B, or E. We found that DAP-mediated increases in quantal content promote neurotransmission from intoxicated nerve terminals through two functionally distinguishable mechanisms. First, DAP increases the probability of neurotransmission at non-intoxicated release sites. This mechanism is serotype-independent, becomes less effective as nerve terminals become progressively impaired, and remains susceptible to ongoing intoxication. Second, DAP elicits persistent production of toxin-resistant endplate potentials from nerve terminals fully intoxicated by BoNT/A, but not serotypes B or E. Since this effect appears specific to BoNT/A intoxication, we propose that DAP treatment enables BoNT/A-cleaved SNAP-25 to productively engage in fusogenic release by increasing the opportunity for low-efficiency fusion events. These findings have important implications for DAP as a botulism therapeutic by defining conditions under which DAP may be clinically effective in reversing botulism symptoms.

In vitro and ex vivo screening of candidate therapeutics to restore neurotransmission in nerve terminals intoxicated by botulinum neurotoxin serotype A1

Botulinum neurotoxins (BoNTs) are exceedingly potent neurological poisons that block cholinergic release in the peripheral nervous system and cause death by asphyxiation. While post-exposure prophylaxis can effectively eliminate toxin in the bloodstream, there are no clinically effective treatments to prevent or reverse disease once BoNT has entered the neuron. To address the need for post-symptomatic countermeasures, we designed and developed an in vitro assay based on whole-cell, patch-clamp electrophysiological monitoring of miniature excitatory post-synaptic currents in synaptically active murine embryonic stem cell-derived neurons. This synaptic function-based assay was used to assess the efficacy of rationally selected drugs to restore neurotransmission in neurons comprehensively intoxicated by BoNT/A. Based on clinical reports suggesting that elevated Ca²⁺ signaling promotes symptomatic relief from botulism, we identified seven candidate drugs that modulate presynaptic Ca²⁺ signaling and assessed their ability to reverse BoNT/A-induced synaptic blockade. The most effective drugs from the screen were found to phasically agonize voltage-gated calcium channel (VGCC) activity. Lead candidates were then applied to ex vivo studies in BoNT/A-paralyzing mouse phrenic nerve-hemidiaphragm (PND) preparations. Treatment of PNDs with VGCC agonists after paralytic onset transiently potentiated nerve-elicited muscle contraction and delayed progression to neuromuscular failure. Collectively, this study suggests that Ca²⁺-modulating drugs represent a novel symptomatic treatment for neuromuscular paralysis following BoNT/A poisoning.

Botulinum Neurotoxins Serotypes A and B induce paralysis of mouse striated and smooth muscles with different potencies

To address the scarcity of direct comparison of botulinum neurotoxin serotypes activity on smooth versus striatal muscle, we have studied the action of BoNT/A1 and BoNT/B1 on ex vivo preparations of both muscle types. We have set up and characterized a model of neurogenic contractions in the isolated mouse bladder, and used this model to explore the effects of the two serotypes on contractions evoked by electrical field stimulation. Both toxins were also tested in the mouse phrenic nerve hemidiaphragm assay, to compare their potency in smooth versus striated muscle. The characterization of the model of neurogenic contractions in the isolated mouse bladder indicates that about half of the activity is driven by purinergic signaling, and about half by cholinergic signaling. Furthermore, we find that BoNT/B1 is more potent than BoNT/A1 in inhibiting activity in the mouse detrusor smooth muscle preparation, but that both toxins have comparable potency on the striated muscle activity of the phrenic nerve hemidiaphragm model. We also show that these findings are mouse strain independent. In conclusion, the established mouse bladder detrusor smooth muscle model is able to discriminate between different botulinum neurotoxin serotypes and could be a useful preclinical tool to explore the pathophysiology of bladder overactivity, as well as the effects of new therapeutic candidates. It is interesting to note that the high proportion of purinergic transmission driving detrusor contractions in this model is similar to that seen in neurodetrusor overactivity disease, making this model relevant with regard to pathophysiological interest.

Synaptic abnormalities of mice lacking toll-like receptor (TLR)-9

Motor, sensory, and autonomic abnormalities are reported for toll-like receptor 9 (TLR9) knock-out (KO) mice. However, a physiological role of TLR9 in the nervous system is largely unknown. Since altered synaptic transmission can contribute to sensory and motor abnormalities, we evaluated neuromuscular junction (NMJ) function and morphology of TLR9 KO mice. Triangularis sterni nerve-muscle preparations were dissected from TLR9 KO and age-matched control mice. Two-electrode voltage clamp of the motor endplate revealed that the amplitude and frequency of miniature end plate currents (mEPCs) for TLR9 KO NMJs were significantly greater than control. In contrast, mean endplate current (EPC, 1Hz) amplitude was equivalent to control. The ratio of mean EPC to mean mEPC amplitude indicated a decline of quantal content (m) for TLR9 KO NMJs. Furthermore, m declined more rapidly than control in response to 50-Hz stimulus trains. A rightward shift of the mEPC amplitude distribution suggested formation of vesicles containing larger amounts of acetylcholine (ACh). Staining with rhodamine α-bungarotoxin revealed a significant decline of endplate size in TLR9 KO mice. This alteration may result from ACh-induced decline of acetylcholine receptor (AChR) expression resulting from increased frequency and amplitude of mEPCs. At the same time, excessive spontaneous vesicular ACh release may initiate retrograde suppression of excitation-secretion coupling. These data suggest a novel role of TLR9 in the development of the NMJ.

Function Suggests Nano-Structure: Quantitative Structural Support for SNARE-Mediated Pore Formation

Granule secretory content is released in either basal or calcium-activated complete exocytosis mode. A vital element in these processes is the establishment of a fusion pore between the granule membrane and the plasma membrane, initiated by the formation of a circular rosette docking arrangement of SNARE protein complexes. The controversially disputed number of SNARE complexes needed for granule priming leading to the formation of the fusion pore, is granule-size dependent and varies between secretion modes. Resorting to a statistical mechanics approach that views SNARE complexes and Ca(2+) ions as interacting particles, we have developed a relationship that links secretion rate to SNARE rosette size, Ca(2+) concentration and Ca(2+) ion cooperativity. Data are presented and discussed which suggest this SNARE-dependent generalization of existing narrow-range biophysical models that correlate secretion rate with Ca(2+) concentration and maximal Ca(2+) ion cooperativity. Evidence from dozens of examples in the literature advocate for this relation, which holds through the entire biological range. The coalescence of so many areas of diverse research methodologies has greatly augmented our understanding of so many different sequences of granule life cycle. Accordingly, these new tools may become valuable in a variety of electrophysiological experiments.

Importance of being Nernst: Synaptic activity and functional relevance in stem cell-derived neurons

Functional synaptogenesis and network emergence are signature endpoints of neurogenesis. These behaviors provide higher-order confirmation that biochemical and cellular processes necessary for neurotransmitter release, post-synaptic detection and network propagation of neuronal activity have been properly expressed and coordinated among cells. The development of synaptic neurotransmission can therefore be considered a defining property of neurons. Although dissociated primary neuron cultures readily form functioning synapses and network behaviors in vitro, continuously cultured neurogenic cell lines have historically failed to meet these criteria. Therefore, in vitro-derived neuron models that develop synaptic transmission are critically needed for a wide array of studies, including molecular neuroscience, developmental neurogenesis, disease research and neurotoxicology. Over the last decade, neurons derived from various stem cell lines have shown varying ability to develop into functionally mature neurons. In this review, we will discuss the neurogenic potential of various stem cells populations, addressing strengths and weaknesses of each, with particular attention to the emergence of functional behaviors. We will propose methods to functionally characterize new stem cell-derived neuron (SCN) platforms to improve their reliability as physiological relevant models. Finally, we will review how synaptically active SCNs can be applied to accelerate research in a variety of areas. Ultimately, emphasizing the critical importance of synaptic activity and network responses as a marker of neuronal maturation is anticipated to result in in vitro findings that better translate to efficacious clinical treatments.

Chapter 3: Molecular basis for the therapeutic effectiveness of botulinum neurotoxin type A

The utility of botulinum neurotoxin type A (BoNT/A) for treating overactive muscles and endocrine glands is attributable to a unique conflation of properties honed to exploit and inactivate synaptic transmission. Specific, high-affinity coincident binding to gangliosides plus an intraluminal loop of synaptic vesicle protein 2 (SV2) by the heavy chain (HC) of BoNT/A confers selectivity for presynaptic nerve terminals and subsequent uptake by endocytosis. Upon vesicle acidification, the HC forms a channel for transmembrane transfer of the light chain to the cytosol, as observed by single channel recordings. The light chain is a Zn(2+) -dependent endoprotease that cleaves and inactivates SNAP-25, thereby blocking exocytotic release of transmitters, a discovery that revealed the pivotal role of the latter in synaptic vesicle fusion. A di-leucine motif in BoNT/A light chain stabilizes this protease, contributing to its longevity inside nerves. The ubiquity of SV2 and SNAP-25 has prompted re-evaluation of the nerve types susceptible to BoNT/A. In urology, there is emerging evidence that BoNT/A blocks neuropeptide release from afferent nerves, exocytosis of acetylcholine and purines from efferent nerves, and possibly ATP release from the urothelium. Suppression by BoNT/A of the surface expression of nociceptor channels on bladder afferents might also contribute to its improvement of urological sensory symptoms.

Measurement of botulinum types A, B and E neurotoxicity using the phrenic nerve-hemidiaphragm: improved precision with in-bred mice

Botulinum neurotoxins induce a prolonged muscle paralysis by specifically blocking the release of neuronal transmitters from peripheral nerve junctions. The current method for assessing the potency of botulinum toxin and antitoxins is the mouse LD50 assay. The mouse phrenic nerve-diaphragm assay is an in vitro assay that closely mimics in vivo respiratory paralysis. In this study, we have further improved the assay by using gelatin as a non-frothing alternative to albumin and investigated the effects of botulinum toxin serotypes A, B and E on phrenic nerve-hemidiaphragms from out-bred MF1 and in-bred Balb/c mice. Improved reproducibility was found with in-bred mice. Balb/c mice were also found to be much less sensitive to type B toxin perhaps indicating differences in the expression of receptor components. Hemidiaphragm preparations from Balb/c mice were approximately 7 times more sensitive to type A toxin and 7-12 times more sensitive to type E toxin relative to type B toxin. These findings indicate that when fully optimised the mouse nerve-diaphragm preparation can provide a functional in vitro model for accurate and reproducible assessment of toxin activity.

The structure and mode of action of different botulinum toxins

The seven serotypes (A-G) of botulinum neurotoxin (BoNT) are proteins produced by Clostridium botulinum and have multifunctional abilities: (i) they target cholinergic nerve endings via binding to ecto-acceptors (ii) they undergo endocytosis/translocation and (iii) their light chains act intraneuronally to block acetylcholine release. The fundamental process of quantal transmitter release occurs by Ca2+-regulated exocytosis involving sensitive factor attachment protein-25 (SNAP-25), syntaxin and synaptobrevin. Proteolytic cleavage by BoNT-A of nine amino acids from the C-terminal of SNAP-25 disables its function, causing prolonged muscle weakness. This unique combination of activities underlies the effectiveness of BoNT-A haemagglutinin complex in treating human conditions resulting from hyperactivity at peripheral cholinergic nerve endings. In vivo imaging and immunomicroscopy of murine muscles injected with type A toxin revealed that the extended duration of action results from the longevity of its protease, persistence of the cleaved SNAP-25 and a protracted time course for the remodelling of treated nerve-muscle synapses. In addition, an application in pain management has been indicated by the ability of BoNT to inhibit neuropeptide release from nociceptors, thereby blocking central and peripheral pain sensitization processes. The widespread cellular distribution of SNAP-25 and the diversity of the toxin's neuronal acceptors are being exploited for other therapeutic applications.

Identification of the major steps in botulinum toxin action

Botulinum toxin is a uniquely potent substance synthesized by the organisms Clostridium botulinum, Clostridium baratii, and Clostridium butyricum. This toxin, which acts preferentially on peripheral cholinergic nerve endings to block acetylcholine release, is both an agent that causes disease (i.e., botulism) as well as an agent that can be used to treat disease (e.g., dystonia). The ability of botulinum toxin to produce its effects is largely dependent on its ability to penetrate cellular and intracellular membranes. Thus, toxin that is ingested or inhaled can bind to epithelial cells and be transported to the general circulation. Toxin that reaches peripheral nerve endings binds to the cell surface then penetrates the plasma membrane by receptor-mediated endocytosis and the endosome membrane by pH-induced translocation. Internalized toxin acts in the cytosol as a metalloendoprotease to cleave polypeptides that are essential for exocytosis. This review seeks to identify and characterize all major steps in toxin action, from initial absorption to eventual paralysis of cholinergic transmission.

Treatment of migraine headache with botulinum toxin type A

Migraine is a common headache disorder with profound implications on patients' quality of life and the overall health care system. Traditional treatment options have been less than optimal and many migraine patients lack confidence in over-the-counter and prescribed medications. BTX-A has shown promise as an efficacious, well-tolerated, long-lasting preventive therapy. Completed placebo-controlled trials showed that BTX-A injections for migraine resulted in fewer headaches, reduced headache severity and duration, reduced migraine-associated symptoms, and reduced use of migraine medications. Because the administration of BTX-A is nonsystemic, reported adverse events have been rare and mild. Larger trials are currently underway to further evaluate BTX-A efficacy and to determine optimal dosing and injection sites. Based on the collective experience of clinicians in neurology, facial plastic surgery, and otolaryngology, as well as supporting evidence from completed and ongoing clinical trials and theorized mechanism of action, an effective BTX-A approach for treatment of migraine is emerging. With further refinement to its use as prophylactic therapy, BTX-A can potentially be a primary option for candidate migraine sufferers and prescribing clinicians.

Retention of cleaved synaptosome-associated protein of 25 kDa (SNAP-25) in neuromuscular junctions: a new hypothesis to explain persistence of botulinum A poisoning

Botulinum neurotoxins can block neurotransmitter release for several months. The molecular mechanism of these toxins' action is known, but the persistence of neuromuscular paralysis that they cause is unexplained. At frog neuromuscular junctions, application of botulinum toxin type A caused paralysis and reduced the C-terminus immunoreactivity of SNAP-25, but not that of the remaining N-terminus fragment. Botulinum toxin type C caused paralysis and reduced syntaxin immunoreactivity without affecting that of SNAP-25. Co-application of botulinum A and C reduced syntaxin immunoreactivity, and that of both C and N termini of SNAP-25. Application of hydroxylamine to de-palmitoylate SNAP-25 resulted in a slight reduction of the immunoreactivity of SNAP-25 N terminus, while it had no effect on immunoreactivity of botulinum A cleaved SNAP-25. In contrast, application of hydroxylamine to nerve terminals where syntaxin had been cleaved by botulinum C caused a considerable reduction in SNAP-25 N-terminus immunoreactivity. Hence the retention of immunoreactive SNAP-25 at the neuromuscular junction depends on its interactions with syntaxin and plasma membrane. Persistence of cleaved SNAP-25 in nerve terminals may prevent insertion of new SNAP-25 molecules, thereby contributing to the longevity of botulinum A effects.Key words: SNAP receptor, neurotoxin, dystonia, botulism, torticollis.

Tetanus toxin inhibits spontaneous quantal release and cleaves VAMP/synaptobrevin

Tetanus toxin decreased the frequency of spontaneous events at the electric organ of Torpedo marmorata. This reduction was up to 70% in poisoned electric organ. According to distribution analysis of miniature end plate currents, only a subpopulation of events which have small amplitudes were recorded after poisoning. Furthermore, isolated cholinergic nerve terminals showed a decrease in VAMP/synaptobrevin when poisoned with tetanus toxin under similar conditions. The relationship between the two effects of the toxin, i.e. inhibition of vesicle exocytosis and peptidase activity on synaptobrevin, is discussed.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Botulinum toxin type A inhibits Ca(2+)-dependent transport of acetylcholine in reconstituted giant liposomes made from presynaptic membranes from cholinergic nerve terminals

Giant liposomes were made from a mixture of asolectin phospholipid vesicles and presynaptic plasma membranes isolated from Torpedo cholinergic nerve endings. Acetylcholine filled giant liposomes were able to release neurotransmitter upon stimulation by the Ca2+ ionophore A23187 and Ca2+. Botulinum neurotoxin type A inhibited this Ca(2+)-dependent acetylcholine transport. Additionally, Botulinum toxin type A decreased membrane fluidity of liposomes. These results suggest that Botulinum toxin can interact directly with components of the presynaptic plasma membrane and inhibit acetylcholine translocation. Furthermore, since the reconstituted liposomes do not have synaptic vesicle components, the observed effects may account for the action of Botulinum toxin on the non-quantal release of acetylcholine from motor nerve terminals.

Inhibition of norepinephrine secretion from digitonin permeabilized PC12 cells by botulinum type D toxin

Botulinum type D neurotoxin inhibited Ca(2+)-evoked norepinephrine secretion in digitonin permeabilized PC12 cells. Inhibition by the toxin required prior incubation with dithiothreitol (DTT). The inhibition was dependent on both concentration and incubation times of the toxin, and was affected by Ca2+ concentration. With less than 0.7 microM Ca2+ almost complete inhibition was observed; however, above 0.7 microM, Ca2+ stimulated additional norepinephrine release in a dose-dependent manner.

Botulinum toxin treatment of spasmodic torticollis

We reviewed the efficacy and adverse effects of repeated botulinum toxin injections into hyperactive neck muscles of 107 successive patients with spasmodic torticollis. They received 510 injection treatments over a median period of 15 months (range 3-42 months). One patient failed to benefit at all, but 101 (95%) patients reported considerable (moderate or excellent) benefit from at least one treatment. On a global subjective response rating, 93% of 429 treatments resulted in some improvement and 76% in moderate or excellent improvement. Pain reduction followed 89% of 190 treatments with moderate or excellent reduction after 66%. Median duration of benefit was 9 weeks. All torticollis types responded equally well and injections into two (or more) involved neck muscles were more effective than injection into a single muscle. The most frequent adverse effect was dysphagia, occurring after 44% of all treatments, but this was severe after only 2%. Antibodies to botulinum toxin were detected in the serum of three out of the five patients in whom loss of treatment efficacy occurred. We conclude that botulinum toxin treatment is the most effective available therapy for spasmodic torticollis and practical advice is provided for anyone wishing to set up the technique.

Botulinum Toxin Treatment of Spasmodic Torticollis

We reviewed the efficacy and adverse effects of repeated botulinum toxin injections into hyperactive neck muscles of 107 successive patients with spasmodic torticollis. They received 510 injection treatments over a median period of 15 months (range 3–42 months). One patient failed to benefit at all, but 101 (95%) patients reported considerable (moderate or excellent) benefit from at least one treatment. On a global subjective response rating, 93% of 429 treatments resulted in some improvement and 76% in moderate or excellent improvement. Pain reduction followed 89% of 190 treatments with moderate or excellent reduction after 66%. Median duration of benefit was 9 weeks. All torticollis types responded equally well and injections into two (or more) involved neck muscles were more effective than injection into a single muscle. The most frequent adverse effect was dysphagia, occurring after 44% of all treatments, but this was severe after only 2%. Antibodies to botulinum toxin were detected in the serum of three out of the five patients in whom loss of treatment efficacy occurred. We conclude that botulinum toxin treatment is the most effective available therapy for spasmodic torticollis and practical advice is provided for anyone wishing to set up the technique.

Miniature endplate potentials induced by ammonium chloride, hypertonic shock, and botulinum toxin

Intracellular recordings were made at the neuromuscular junction (NMJ) of the mouse diaphragm to study alteration of miniature endplate potential (MEPP) amplitude and rise time after different treatments. Following either hyperosmotic shock or 3 to 5 min of incubation in 10 to 50 mM ammonium chloride (NH4Cl) (replacing NaCl, a treatment which is known to raise intracellular pH) MEPP frequencies increased and the amplitudes of MEPPs decreased. These treatments as well as type A botulinum toxin (BoTx) gradually prolonged the rising phase of some MEPPs, which increased their time-to-peak (slow-MEPPs; Vautrin and Kriebel: Neuroscience 41:71-88, 1991) and increased eventually their amplitude. Fasciculation after hyperosmotic shock or during NH4Cl challenge was blocked by D-tubocurarine and was due to large slow-MEPPs that reached threshold for the muscle fiber action potential. The development of fasciculation provided the time course for the development of giant-MEPPs. Increased frequency of giant MEPP is accompanied by a block of the nerve-evoked muscle contraction. Effects of BoTx on spontaneous release were functionally antagonized either by NH4Cl or hyperosmotic shock. NH4Cl delayed BoTx blockage of bell-MEPPs. Data suggest that BoTx alters the formation of transmitter packets gradually but similarly to other treatments which increase incidence of skew-MEPPs.

Tetrahydroaminoacridine (tacrine) stimulates neurosecretion at mammalian motor endplates

1. Tacrine (20 microM) induced, like 4-aminoquinoline (4-AQ, 200 microM), the appearance of a population of miniature endplate potentials (m.e.p.ps) with more than twice the normal amplitude or time-to-peak. The times-to-peak of nerve impulse-evoked endplate potentials were not similarly affected. 2. Cholinesterase inhibition by edrophonium (25 microM) did not prevent tacrine or 4-AQ from inducing this population of m.e.p.ps. 3. Nerve-muscle preparations in which the normal calcium-sensitive quantal release of acetylcholine had been blocked by botulinum neurotoxin type A also responded to tacrine by an increase in the frequency of giant or slow m.e.p.ps. 4. Reduction of the temperature from 30 degrees to 14 degrees C reduced the frequency of giant or slow m.e.p.ps induced either by tacrine or by 4-AQ. A similar effect was obtained by colchicine (5 mM). This supports the idea that proximo-distal axonal transport is required for the secretory activity. 5. The neurosecretion evoked by tacrine could explain the therapeutic effects of the drug claimed in the treatment of Alzheimer's type of dementia.

Botulinum type F neurotoxin. Large-scale purification and characterization of its binding to rat cerebrocortical synaptosomes

1. A large-scale purification procedure has been developed for Clostridium botulinum type F neurotoxin. Commencing with 160 litres of bacterial culture, 101 mg of purified type F neurotoxin with a specific toxicity of 2 x 10(7) mouse LD50 (median lethal dose).mg-1 were obtained. 2. Purified type F neurotoxin was labelled to high specific radioactivity (900-1360 Ci/mmol) without loss of biological activity using a chloramine-T procedure. Of the two neurotoxin subunits, the heavy chain was preferentially radiolabelled. 3. Radiolabelled type F neurotoxin displayed specific saturable binding to rat synaptosomes. At least two pools of acceptors were evident: a low content of high-affinity acceptors sites [KD approximately 0.15 nM; Bmax (maximal binding) 20 fmol/mg] and a larger pool of lower-affinity sites (KD greater than 20 nM; Bmax greater than 700 fmol/mg). Both pools of acceptors were sensitive to trypsin and neuraminidase treatment, which suggests that protein and sialic acid residues are components of the synaptosomal acceptors. 4. Experiments investigating competition among botulinum neurotoxin types A, B, E and F for acceptors on rat brain synaptosomes showed that type F neurotoxin binds to acceptor molecules which are completely distinct from those of the other three neurotoxins.

The effects of L-vesamicol, an inhibitor of vesicular acetylcholine uptake, on two populations of miniature endplate currents at the snake neuromuscular junction

The actions of the active L-isomer of vesamicol, an inhibitor of the vesicular storage of acetylcholine, has been studied on spontaneous and evoked acetylcholine release at the snake neuromuscular junction. Miniature endplate currents and endplate currents were recorded from cut muscle fibres of the garter snake, Thamnophis sirtalis. In controls, prolonged periods of high frequency nerve stimulation produced a bimodal distribution of miniature endplate current amplitudes. The stimulation induced "small-mode" miniature endplate currents had a mean amplitude of around 40-55% of the pre-stimulation miniature endplate current. Relative to the normal-sized post-stimulation miniature endplate current, the proportion and, to a lesser extent, amplitude of the small-mode miniature endplate currents was related to both the frequency and duration of nerve stimulation and to the extracellular calcium ion concentration. In unstimulated preparations, L-vesamicol (2-5 microM) did not affect either endplate current quantal content or miniature endplate current amplitude or frequency. However, at these doses, the mean amplitude of the stimulation-induced, small-mode miniature endplate current was reduced by L-vesamicol in a concentration-dependent manner such that they were not visible at the highest dose. L-Vesamicol had no affect on the mean or coefficient of variance of amplitude of the larger, normal-sized miniature endplate current. Additionally, the stimulation-induced increase in overall miniature endplate current frequency seen in controls was abolished by 5 microM L-vesamicol. After prolonged 10 Hz nerve stimulation endplate current amplitude was markedly reduced in both controls (by 94%) and in the presence of 5 microM L-vesamicol (by 98%). Analysis of endplate current amplitude variance showed that in control the decrease was due to reductions in both quantal content and quantal size while in L-vesamicol the decrease was due entirely to a change in quantal content with no change in quantal size. Thus, we have observed that L-vesamicol selectively reduces the amplitude of a population of stimulation-induced small-mode quanta both as miniature endplate currents and as constituents of endplate currents. We suggest that these quanta are derived from a highly active, readily releasable pool. An action of L-vesamicol on this labile pool is consistent with previous observations on its ability to inhibit the vesicular storage of acetylcholine.

Characterization of the actions of botulinum neurotoxin type E at the rat neuromuscular junction

Botulinum neurotoxin (BoTx) serotype E blocks spontaneous and evoked quantal release of acetylcholine at the rat neuromuscular junction. Increasing extracellular Ca2+ to 8 mmol l-1 or substituting Ca2+ with La3+ (0.1 and 1.0 mmol l-1) or depolarizing the nerve terminals by 20 mmol l-1 K+ markedly increases miniature end-plate potential frequency in normal muscle, but in BoTx-E poisoned preparations none of these ions, with the exception of 1 mmol l-1 La3+, was able to restore spontaneous quantal transmitter release to levels recorded at unpoisoned junctions. In absolute values the enhancement with La3+ was much less than that reported at normal junctions. Nerve stimulation in the presence of 3,4-diaminopyridine (10-20 mumol l-1) and high calcium (8 mmol l-1) evoked multiquantal end-plate potentials and muscle twitches. We conclude that the neuromuscular block produced by BoTx serotype E is similar to that previously described for BoTx serotype A but differs from that produced by BoTx serotypes B, D and F in not causing desynchronization of nerve impulse-evoked transmitter release. 3,4-Diaminopyridine might be useful in the treatment of poisoning by BoTx serotype E since it markedly enhanced synchronous transmitter release from poisoned motor nerve terminals.

Effects of botulinum neurotoxin and Lambert-Eaton myasthenic syndrome IgG at mouse nerve terminals

The interaction between two presynaptically acting agents, Lambert-Eaton myasthenic syndrome (LEMS) immunoglobulin G (IgG) and purified botulinum neurotoxin (BoNT) type A, was studied. Intracellular microelectrode recordings were carried out on mouse muscles after injection with LEMS IgG. BoNT was either injected before recordings were made or applied in vitro. The time course of the in vitro actions of BoNT on miniature end-plate potential and end-plate potential parameters were not affected by pretreatment with LEMS IgG. After in vivo injection of BoNT, end-plate potential quantal content was reduced to less than 2% of control values, whether or not LEMS IgG had also been previously given. Quantitative electron-microscope autoradiographical analysis showed that neither the binding of 125I-BoNT to acceptors on the nerve terminal membrane nor the pattern of its internalisation were affected by pretreatment with LEMS IgG. We conclude that the effects of BoNT are not affected by LEMS IgG, suggesting different presynaptic binding sites for the two agents.

Involvement of the constituent chains of botulinum neurotoxins A and B in the blockade of neurotransmitter release

1. The abilities of botulinum neurotoxins, types A and B (single and two-chain forms) to inactivate an intraneuronal component required for transmitter release were quantified in a phrenic-nerve-diaphragm preparation, cerebrocortical synaptosomes or the buccal ganglion of Aplysia californica and compared with the mouse toxicity assay. 2. Homogeneous preparations of the individually renatured polypeptide chains of both toxin types showed low residual toxicity in the whole animal and had no effect on neurotransmission in all three systems, when tested singly. 3. Mixtures of individually renatured heavy chain, from type A or B, and either light chain proved very effective in blocking the evoked release of acetylcholine when bath-applied to the buccal ganglion of Aplysia whilst they were relatively inactive on mammalian nerve terminals, indicating a less efficient uptake of the polypeptides in the latter. 4. When renatured together, the homologous, but not the heterologous, chains of each toxin type yielded toxic, disulphide-linked two-chain species. 5. A role for the heavy chain alone in acceptor recognition and membrane translocation was implicated by the blockade of acetylcholine release produced when light chain was applied to a ganglion of Aplysia previously bathed in heavy chain and washed extensively. No blockade was observed when the order of application of the two chains was reversed. 6. These findings are discussed in the context of the intracellular requirement for both the constituent toxin chains for toxicity, and in the apparent need for these chains to be linked via a disulphide bond for uptake in rodents but not in Aplysia.

Neurotransmitter release is blocked intracellularly by botulinum neurotoxin, and this requires uptake of both toxin polypeptides by a process mediated by the larger chain

Botulinum neurotoxins (types A and B), which are microbial proteins consisting of two disulfide-linked chains, inhibit specifically and with high potency the release of acetylcholine from peripheral nerve terminals. As a prerequisite for a long-term development of effective treatments for botulism, the internalization and inhibitory action of the toxin and its constituent chains were examined by electrophysiological methods at identified synapses in Aplysia preparations that allow both intracellular and bath application of the neurotoxins. Intracellular recordings from cholinergic cells of the buccal ganglion demonstrated that extra- or intracellular application of low doses of botulinum neurotoxin results in a specific blockade of evoked transmitter release, without changing the quantal size; an intraneuronal site of action has thus been established. In contrast, release from noncholinergic neurons of cerebral ganglion was prevented by the neurotoxin only after injection into the cell. Purified preparations of the individual renatured chains, shown to be nontoxic in a mouse bioassay, failed to affect acetylcholine release when applied extra- or intracellularly. However, inhibition of release was observed after intracellular administration of both chains or when the light chain was injected and the heavy chain was bath-applied. These findings show that both chains are required on the cytosolic side of the neuronal plasma membrane for expression of toxicity and that the cholinergic specificity of the neurotoxin is attributable to its heavy chain, which mediates targeting and subsequent neuronal uptake.

Characterization of the inhibitory action of botulinum neurotoxin type A on the release of several transmitters from rat cerebrocortical synaptosomes

Under optimised conditions for intoxication, botulinum neurotoxin type A was shown to inhibit approximately 90% of Ca2+-dependent K+-evoked release of [3H]acetylcholine, [3H]noradrenaline, and [3H]dopamine from rat cerebrocortical synaptosomes; cholinergic terminals were most susceptible. In each case, the dose-response curve for the neurotoxin was extended, with about 50% of evoked release being inhibited at approximately 10 nM whereas 200 nM was required for the maximal blockade. This may suggest some heterogeneity in the release process. The action of the toxin was time and temperature dependent and appeared to involve binding and sequestration steps prior to blockade of release. The neurotoxin failed to exert any effect on synaptosomal integrity or on Ca2+-independent release of the transmitters tested; it produced only minimal changes in neurotransmitter uptake although small secondary effects were detected with cholinergic terminals. Blockade by the neurotoxin of Ca2+-dependent resting release of transmitter was apparent; Sr2+, Ba2+, or high concentrations of Ca2+ restored the resting release of 3H-catecholamine but not [3H]acetylcholine. Interestingly, none of the latter conditions or 4-aminopyridine could reverse the toxin-induced blockade of evoked release. This lack of specificity in its action on synaptosomes, and other published findings, lead to the conclusion that toxin-sensitive component(s) exist in all nerve terminals that are concerned with transmitter release.