Inhibition by botulinum toxin of depolarization-evoked release of (14C)acetylcholine from synaptosomes in vitro

Neuronal selectivity of botulinum neurotoxins

Botulinum neurotoxins (BoNTs) are highly potent toxins responsible for a severe disease, called botulism. They are also efficient therapeutic tools with an increasing number of indications ranging from neuromuscular dysfunction to hypersecretion syndrome, pain release, depression as well as cosmetic application. BoNTs are known to mainly target the motor-neurons terminals and to induce flaccid paralysis. BoNTs recognize a specific double receptor on neuronal cells consisting of gangliosides and synaptic vesicle protein, SV2 or synaptotagmin. Using cultured neuronal cells, BoNTs have been established blocking the release of a wide variety of neurotransmitters. However, BoNTs are more potent in motor-neurons than in the other neuronal cell types. In in vivo models, BoNT/A impairs the cholinergic neuronal transmission at the motor-neurons but also at neurons controlling secretions and smooth muscle neurons, and blocks several neuronal pathways including excitatory, inhibitory, and sensitive neurons. However, only a few reports investigated the neuronal selectivity of BoNTs in vivo. In the intestinal wall, BoNT/A and BoNT/B target mainly the cholinergic neurons and to a lower extent the other non-cholinergic neurons including serotonergic, glutamatergic, GABAergic, and VIP-neurons. The in vivo effects induced by BoNTs on the non-cholinergic neurons remain to be precisely investigated. We report here a literature review of the neuronal selectivity of BoNTs.

A highly sensitive and simply operated protease sensor toward point-of-care testing

The sensor is based on (i) low nonspecific adsorption and (ii) electrochemical–chemical redox cycling.

Effect of botulinum toxin A on the autonomic nervous system of the rat lower urinary tract

PURPOSE

The magnitude and duration of the effects of botulinum toxin A on acetylcholine (ACh) and norepinephrine release from the bladder and urethra of rats were measured using a radiochemical method.

MATERIALS AND METHODS

Saline (sham treatment) or botulinum toxin A was injected into the bladder (50 microl.) or urethra (30 microl.) in separate groups of animals. The release of 3H-norepinephrine or 14C-choline was measured at 2 time points after injection (5 or 30 days).

RESULTS

The fractional release of ACh in botulinum toxin A treated animals was significantly inhibited at higher frequencies of electrical field stimulation (20 Hz.) but not at lower frequencies (2 Hz.) 5 days after injection. However, ACh release recovered to sham injected values 30 days after toxin injection. No significant differences in the fractional release of norepinephrine from sham injected or botulinum toxin A bladders were observed. In contrast, norepinephrine release from the urethra was inhibited by botulinum toxin A for at least 30 days after injection. Similar to its effect on transmitter release in the bladder, botulinum toxin A inhibited norepinephrine release in the urethra at high (20 Hz.) but not at low (4 Hz.) electrical stimulation frequencies.

CONCLUSIONS

These data indicate that the clinical effects of botulinum toxin A on the lower urinary tract may vary depending on the site of injection and level of nerve activity.

Synaptophysin is phosphorylated in rat cortical synaptosomes treated with botulinum toxin A

Phosphorylation and dephosphorylation of neuronal proteins have been implicated in regulation of synaptic transmission. Studies were performed to determine if synaptophysin was phosphorylated or dephosphorylated during exposure of synaptosomes to botulinum toxin A (BoTX/A). Cholinergic-enriched synaptosomes were preincubated in the presence of 3H-choline to label newly synthesized acetylcholine (3H-ACh). This was followed by incubation with low or high potassium to stimulate release of newly synthesized 3H-ACh. BoTX/A inhibited total Ach release by 15-19% and inhibited release of newly synthesized 3H-ACh by 35%. A 165% increase in synaptophysin phosphorylation occurred in a dose-dependent manner over a range of doses (0.2 nM, 2 nM, 20 nM, 100 nM) of BoTX/A. When 4-Aminopyridine was added to synaptosomes that were BoTX/A treated, synaptophysin was dephosphorylated to control levels. Synaptosomes incubated with BoTX/A exhibited an inhibition of potassium stimulated ACh release and an increase in synaptophysin phosphorylation. Synaptophysin phosphorylation may be involved in inhibition of acetylcholine release.

Pharmacologic characterization of botulinum toxin for basic science and medicine

The use of Botulinum neurotoxin (BoNT) is increasing in both clinical and basic science. Clinically, intramuscular injection of nanogram quantities of BoNT is fast becoming the treatment of choice for a spectrum of disorders including movement disorders such as torticollis, blepharospasm, Meige Disease, and hemifacial spasm (Borodic et al., 1991, 1994a; Jankovic and Brin, 1991; Clarke, 1992). Neuroscientists are using BoNTs as tools to develop a better understanding of the mechanisms underlying the neurotransmitter release process. Consequently, our ability to accurately and reliably quantify the biologic activity of botulinum toxin has become more important than ever. The accurate measurement of the pharmacologic activity of BoNTs has become somewhat problematic with the most significant problems occurring with the clinical use of the toxins. The biologic activity of BoNTs has been measured using a variety of techniques including assessment of whole animal responses to in vitro effects on neurotransmitter release. The purpose of this review is to examine the approaches employed to characterize, quantify and investigate the actions of the BoNTs and to provide a guide to aid investigators in determining which of these methods is most appropriate for their particular application or use.

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.

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.

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.

Release of acetylcholine from tissue slices of the rat nucleus basalis magnocellularis

We investigated the release of acetylcholine (ACh) from tissue slices obtained from the nucleus basalis magnocellularis (nbM) of the rat brain. Potassium (35 mM) depolarization produced a 10- to 12-fold increase in the release of endogenous ACh above spontaneous release. Potassium-evoked ACh release was Ca2+ dependent. Injection of the excitotoxin quinolinic acid into the nbM produced a 72.8 +/- 13.0% decrease in spontaneous ACh release and a 60.4 +/- 8.2% decrease in potassium-evoked release. A fourfold increase in ACh release was observed following perfusion of the tissue with 1 mM 3,4-diaminopyridine (3,4-DAP) whereas 10 mM 3,4-DAP caused a sevenfold increase. The increase in ACh release caused by 3,4-DAP was inhibited by tetrodotoxin. Tissue slices accumulated [3H]choline by high-affinity choline uptake and this could be inhibited by hemicholinium-3. These results indicate that ACh can be released from tissue slices of the nbM by a calcium-dependent process and that a part of this release appears to be from the cholinergic neurons of the nbM.

Botulinum toxin type A blocks the morphological changes induced by chemical stimulation on the presynaptic membrane of Torpedo synaptosomes

The action of botulinum neurotoxin on acetylcholine release, and on the structural changes at the presynaptic membrane associated with the transmitter release, was studied by using a subcellular fraction of cholinergic nerve terminals (synaptosomes) isolated from the Torpedo electric organ. Acetylcholine and ATP release were continuously monitored by chemiluminescent methods. To catch the membrane morphological changes, the quick-freezing method was applied. Our results show that botulinum neurotoxin inhibits the release of acetylcholine from these isolated nerve terminals in a dose-dependent manner, whereas ATP release is not affected. The maximal inhibition (70%) is achieved at neurotoxin concentrations as low as 125 pM with an incubation time of 6 min. This effect is not linked to an alteration of the integrity of the synaptosomes since, after poisoning by botulinum neurotoxin type A, they show a nonmodified occluded lactate dehydrogenase activity. Moreover, membrane potential is not altered by the toxin with respect to the control, either in resting condition or after potassium depolarization. In addition to acetylcholine release inhibition, botulinum neurotoxin blocks the rearrangement of the presynaptic intramembrane particles induced by potassium stimulation. The action of botulinum neurotoxin suggests that the intramembrane particle rearrangement is related to the acetylcholine secretion induced by potassium stimulation in synaptosomes isolated from the electric organ of Torpedo marmorata.

Energy metabolism and quantal acetylcholine release: effects of botulinum toxin, 1-fluoro-2,4-dinitrobenzene, and diamide in the Torpedo electric organ

In the Torpedo electric organ, a modified nerve-muscle system, type A botulinum toxin blocked the release of acetylcholine (ACh) quanta, both neurally evoked and spontaneous. At the same time, the toxin increased the release of a class of small miniature potentials (the subminiature potentials), reduced the ATP and more the creatine phosphate content of the tissue, and impaired the activity of creatine kinase (CK). Thus, we compared this pattern of changes with those provoked by 1-fluoro-2,4-dinitrobenzene (FDNB), an efficient inhibitor of CK. As expected, FDNB rapidly inactivated CK, which resulted in a profound depletion of ATP whereas the stores of creatine phosphate were preserved. In addition, FDNB caused conspicuous morphological alterations of nerve endings and ACh depletion. This agent also suppressed evoked and spontaneous quantal release whereas the occurrence of subminature potentials was markedly increased. Diamide, a penetrating thiol oxidizing substance, provoked first a transient rise in quantal ACh release and then blockade of transmission with, again, production of a large number of subminiature potentials. Creatine phosphate was depleted in the tissue by diamide, the ATP content reduced, and CK activity partly inhibited. The morphology of nerve terminals did not show obvious changes with either diamide or botulinum toxin at the stage of transmission failure. Although the three poisons acted by different mechanisms, this resulted in a rather similar pattern of physiological changes: failure of quantal release and enhancement of subquantal release. These results and experiments on synaptosomes indicated that CK inhibition was probably a crucial mechanism for FDNB but not for diamide or botulinum intoxication.(ABSTRACT TRUNCATED AT 250 WORDS)

Tetanus and botulinum toxins block the release of acetylcholine from slices of rat striatum and from the isolated electric organ of Torpedo at different concentrations

Tetanus toxin, like botulinum toxin type A, blocks cholinergic synaptic transmission at the central and peripheral nervous systems. Nevertheless, the diseases induced by the two toxins are different since tetanus toxin induces a spastic paralysis and botulinum toxin elicits a flaccid paralysis. Thus, we have investigated the sensitivity of a central and a peripheral cholinergic synapse to these two toxins. We have studied the action of both poison on the release of acetylcholine from slices of the rat striatum and from the isolated electric organ of Torpedo, which is homologous to the neuromuscular junction. Acetylcholine release from the rat striatum was continuously monitored by a chemiluminescent method. The secretion of acetylcholine from the electric organ was estimated both by measuring the amplitude of the evoked electrical discharge from stacks of electroplaques, and by continuously monitoring the neurotransmitter release from isolated nerve terminals. Tetanus toxin blocks the electrical discharge of electric organ prisms, and also impairs the release of acetylcholine from the Torpedo electric organ nerve endings. Our results on acetylcholine release show that tetanus toxin is more potent than botulinum toxin type A at the central cholinergic synapse (tetanus/botulinum toxins potency ratio about 100-200) whereas botulinum toxin is the most potent at the peripheral cholinergic synapse (botulinum/tetanus toxins potency ratio about 100).

Relation of acetylcholine release to Ca2+ uptake and intraterminal Ca2+ concentration in guinea-pig cortex synaptosomes

[14C]Acetylcholine (ACh) release and parallel alterations in 45Ca2+ uptake and intrasynaptosomal free CA2+ concentration ([Ca2+]i) were measured in guinea-pig brain cortex synaptosomes. Depolarization by high K+ concentrations caused a rapid transient increase in Ca2+ uptake, terminating within 60 s (rate constant = 0.060 s-1; t1/2 = 11.6 s). This resulted in a rapid increase (within 1 s) in [Ca2+1]i, which then fell to a maintained but still-elevated plateau level (t1/2 for the decline was 15 s). Peaks of [Ca2+]i showed a sigmoidal dependence on depolarization, contrasting with the simple linear dependence of plateau levels of [Ca2+]i. The K+-evoked ACh release also had two phases: a fast initial increase (t1/2 = 11.3 s), which terminated within 60 s, was followed by a slow additional increase during sustained depolarizations of up to 10 min. Depolarization by veratridine led to a slow gradual increase in Ca2+ uptake (t1/2 = 130 s) over a 10-min incubation period, whereas an elevated plateau level of [Ca2+]i was achieved within 2 min (without a rapid peak elevation). The Ca2+-dependent fraction of the veratridine-evoked ACh release correlated with the increase in [Ca2+]i rather than with Ca2+ uptake. Using two different methods of depolarization partially circumvented the time limitations imposed by a buffering Ca2+ indicator and we suggest that, in the main, ACh is released in bursts associated with [Ca2+]i transients.

Specificity of ethylcholine mustard aziridinium as an irreversible inhibitor of choline transport in cholinergic and noncholinergic tissue

The sensitivity of choline transport to inhibition by ethylcholine mustard aziridinium (ECMA) was studied in several tissues. Choline transport was found to be inhibited irreversibly by ECMA in guinea pig and rat synaptosomes but not inhibited in erythrocytes or kidney slices. If this finding can be extended to other tissues ECMA sensitivity may provide a simple criterion for identifying the choline carrier associated with cholinergic tissue.

Botulinum toxin inhibits quantal acetylcholine release and energy metabolism in the Torpedo electric organ

1. Type A Botulinum toxin (BoTX) blocked nerve-electroplaque transmission in small fragments of Torpedo marmorata electric organ incubated in vitro. The effect was observed either with the crystalline toxin complex (associated with haemagglutinin) or with the purified neurotoxin (molecular weight approximately 150,000). 2. The quantal content of the evoked post-synaptic response was reduced by BoTX but the quantum size remained unchanged till complete blockade of the evoked response. 3. Spontaneous electroplaque potentials were composed of two populations: one with a bell-shaped amplitude distribution (miniature potentials or quanta) and a population of small events with a skewed distribution (subminiatures). In BoTX-poisoned tissue, the bell-distributed miniatures progressively disappeared, but the subminiatures kept on occurring. Occasionally, larger spontaneous potentials with a slow time course were recorded; they were also BoTX resistant. 4. A biochemical assay showed that evoked acetylcholine (ACh) release was impaired by BoTX. During the period when evoked transmission was blocked, spontaneous ACh release transiently increased. 5. At the time of transmission blockade, there was no significant change of ACh content, of ACh turnover, of ACh repartition in the vesicle-bound and free compartments, or of the number of synaptic vesicles. 6. The amount of ATP was reduced to 50% by BoTX, and that of creatine phosphate (CrP) to less than 20%. The ATP-CrP-converting enzyme, creatine kinase, was inhibited in BoTX-poisoned tissue. 7. Thus, the electrophysiological effects of BoTX are very similar at the nerve-electroplaque and the neuromuscular junctions. The present work suggests in addition that suppression of quantal release by BoTX is related to marked alterations of the energy metabolism in the tissue.

Effect of adenosine, adenosine derivatives, and caffeine on acetylcholine release from brain synaptosomes: interaction with muscarinic autoregulatory mechanisms

Synaptosomes, prepared from rat cerebral cortex and hippocampus, were preincubated with [methyl-3H]choline. The effect of adenosine, cyclohexyladenosine, N-ethylcarboxamide adenosine, 2'-deoxyadenosine, and oxotremorine on K+-evoked 3H efflux was investigated. High-voltage electrophoretic separation showed that in the presence of physostigmine, the K+-evoked 3H efflux from hippocampal synaptosomes was 90% [3H]acetylcholine and 10% [3H]choline. Adenosine (30 microM) and oxotremorine (100 microM) both decreased [3H]acetylcholine release from hippocampal synaptosomes. The effect was inversely proportional to the KCl concentration and disappeared at a KCl concentration of 50 mM. Cyclohexyladenosine was approximately 3,000 times more active than adenosine, whereas N-ethylcarboxamide adenosine and 2'-deoxyadenosine were inactive. This indicates that A1 adenosine receptors were involved in the inhibitory effect. Caffeine antagonized the adenosine effect, and at a concentration of 100 microM, it stimulated [3H]acetylcholine efflux. The inhibitory effect of oxotremorine was as great in cortical as in hippocampal synaptosomes. In contrast, adenosine was much less active in cortical than in hippocampal synaptosomes. When inhibitory concentrations of adenosine and oxotremorine were added together into the incubation medium, the effect of adenosine on [3H]acetylcholine release was consistently reduced. An interaction between muscarinic and A1 adenosine presynaptic receptors at a common site modulating acetylcholine release can be assumed.

Botulinum A neurotoxin unlike tetanus toxin acts via a neuraminidase sensitive structure

The binding and effects of tetanus and botulinum A neurotoxins were studied on mouse spinal cord cultures treated with neuraminidase. In untreated cultures both neurotoxins blocked synaptic transmission. Treatment of the cell cultures with neuraminidase, 25 mU/ml for 24 hr, decreased the potency of botulinum A neurotoxin. At 7 X 10(-11) M no toxin effect on inhibitory or excitatory synapses was observed, whereas at higher concentrations of the toxin the concentration-response curve was shifted to the right by a factor of about 30. Surprisingly, the action of tetanus toxin over a large concentration range was unaffected by pretreatment of the neurones with the enzyme. Accordingly, neurones treated with neuraminidase failed to bind 125I-botulinum A neurotoxin, whereas labelled tetanus toxin was still fixed by cell bodies, as well as by neurites, as shown by histoautoradiography. Chromatographic extraction of gangliosides from cultures prelabelled with 14C-glucosamine showed a dramatic loss in the contents of polysialogangliosides following treatment with neuraminidase. Our results indicate that neuraminidase-sensitive structures might be important for the action of botulinum A neurotoxin. The effect of tetanus toxin appears to be mediated by a different site which is insensitive to neuraminidase.

Botulinum A neurotoxin inhibits non-cholinergic synaptic transmission in mouse spinal cord neurons in culture

The effects of botulinum A neurotoxin and tetanus toxin were studied in cultured mouse spinal cord neurons. In approximately 60% of the neurons (n = 150), botulinum A neurotoxin caused paroxysmal depolarizing events. In two cells hyperpolarizing shifts were observed. The pattern of the burst-like activity varied in shape and frequency in individual cells. Between the paroxysmal events, ongoing synaptic activity could be recorded. The other 40% of the treated neurons did not develop a characteristic pattern of bursts, but there was a decrease in frequency of synaptically generated events. In contrast to botulinum A neurotoxin, tetanus toxin invariably produced well organized paroxysmal events without any synaptic activity between them. At later stages botulinum A neurotoxin and tetanus toxin blocked inhibitory and excitatory postsynaptic potentials in all neurons studied. These results have demonstrated, for the first time using electrophysiological techniques, that botulinum A neurotoxin blocks both excitatory and inhibitory synaptic transmission in the mammalian central nervous system. There are however differences between these effects of botulinum A neurotoxin and the actions of tetanus toxin on these cells. It is suggested that at the femtomolar range tetanus toxin blocks selectively central inhibitory systems and botulinum A neurotoxin the motor endplate. At the picomolar range both toxins affect many if not all, transmitter systems.

Kinetics of irreversible inhibition of choline transport in synaptosomes by ethylcholine mustard aziridinium

Ethylcholine mustard aziridinium (ECMA) inhibits choline transport in synaptosomes at a half-maximal concentration of about 20 microM. The rate of inhibition falls off rapidly after 10 min and the concentration dependency reaches a plateau at about 100 microM. The inhibition is not removed by washing the synaptosomes, and choline and hemicholinium-3 protect the carrier against attack by the mustard. Choline efflux, particularly that stimulated by choline in the medium (transactivation) is also inhibited by the aziridinium compound. Similarly choline influx activated by preloaded internal choline is inhibited by ECMA. The mustard can enter the synaptosomes in an active form but most of the carrier is alkylated when facing the outside. Prior depolarization of the synaptosomes causes an increase in the rate of inhibition by ECMA which is proportionally about the same as the increase in choline influx also caused by depolarization. At low ECMA concentrations the rate of inhibition is that of a first-order reaction with the carrier but at high ECMA concentrations the translocation of the carrier to the outward-facing conformation controls the rate of inhibition. Using a model of choline transport with some simplifying assumptions it is possible to estimate the amount of carrier; cholinergic synaptosomes carry about six times the concentration of carrier found in noncholinergic ones. In noncholinergic synaptosomes the carrier faces predominately out, the reverse in cholinergic ones. The rate constant of carrier translocation is increased by combination with choline some six- to sevenfold to about 3.5 min-1. The rate constant of ECMA attack on the carrier is about 440 M-1 sec-1.

Release of acetylcholine from rat brain synaptosomes by various agents in the absence of external calcium ions

The relationship between 86Rb+ distribution across synaptosomal membrane and [14C]acetylcholine (ACh) release have been studied in a rat brain cortex synaptosomal preparation using K+, ouabain and veratridine depolarization. Decrease in membrane potential, approximated from the 86Rb+ distribution, is accompanied by an increase in [14C]ACh release, but the extent of the increase at a certain depolarization is dependent on how the depolarization is induced. A substantial depolarization by K+ is necessary to enhance ACh release, as compared to ouabain and veratridine where only a slight depolarization is accompanied by an increase in ACh release. In Ca2+-free, EGTA-containing medium ouabain and veratridine can also increase [14C]ACh release. The relationship between membrane potential and ACh release is very similar in the presence of ouabain and veratridine both in Ca2+-containing and Ca2+-free medium. The effect of ouabain and veratridine on the Na-K exchange pump is different; ouabain can completely abolish Na-K-ATPase activity and 86Rb+ uptake of synaptosomes, whereas veratridine does not seem to influence the activity of the pump. m-Chloro-carbonylcianid phenyl hydrazon (50-500 nM) increases [14C]ACh release in a concentration-dependent manner without a considerable change of membrane potential or Na-K pump activity. The Ca2+ ionophore A 23187 induces a substantial increase in [14C]ACh release in the absence of external Ca2+. In this case neither Na-K pump activity nor membrane potential of synaptosomes is changed. A possible role of intracellular Ca2+ mobilization as a consequence of increased intracellular Na+ concentration in some depolarization-induced transmitter release is discussed.

Measurement of intrasynaptosomal free calcium by using the fluorescent indicator quin-2

The recently synthesized calcium indicator quin -2 was incorporated into synaptosomes from guinea-pig cerebral cortex following uptake and internal hydrolysis of quin -2 tetra-acetoxymethyl ester. Incubation in physiological media containing 1 mM- or 2 mM-CaCl2 led to equilibrium cytosolic ionized calcium concentrations of 85 +/- 10 nM and 205 +/- 5 nM respectively (mean +/- S.E.M. from eight and eighteen preparations respectively). Cytosolic Ca2+ was elevated following increases in external Ca2+ concentration, plasma membrane depolarization, mitochondrial inhibition, calcium ionophore addition or replacement of external sodium by lithium. Preliminary experiments were performed to assess changes in cytosolic Ca2+ accompanying the release of the neurotransmitter acetylcholine.

Characteristics of neuronal release of ATP

Recent studies have described a transmitter-like release of ATP in brain. Once released, extraneuronal ATP is rapidly metabolized to adenosine by ecto-ATPase and nucleotidase. Adenosine, through actions at specific receptors, inhibits neuronal firing in the brain. ATP shares these inhibitory actions, presumably by forming adenosine extraneuronally. Caffeine and theophylline probably exert CNS stimulation by antagonizing adenosine's inhibitory actions in the brain. Extracellular ATP occasionally excites quiescent neurons in the cortex. A possible role for ATP as a sensory neurotransmitter is suggested by its excitatory actions on a subpopulation of dorsal horn cells. ATP release has also been described from sensory nerves in the periphery, motor nerves, nerves of the myenteric plexus, bladder, vas deferens, and from adrenal chromaffin cells and platelets. The possibility that ATP might function as a transmitter, co-transmitter or modulator in the peripheral nervous system is discussed.

Taxol stabilizes synaptosomal microtubules without inhibiting acetylcholine release

Synaptosomes assemble an equatorial coil of microtubules during incubation at 37 degrees C. Stimulation of synaptosomes by veratridine or A23187 for 5 min results in disassembly of the microtubules. The role of microtubule disassembly in neurotransmitter release was investigated using the microtubule-stabilizing drug taxol. Taxol stimulates microtubule assembly in synaptosomes and prevents microtubule disassembly caused by A23187. However, taxol has no effect on the release of [3H]acetylcholine triggered by veratridine or A23187. These results suggest that microtubule turnover is not necessary for neurotransmitter release.

Amelioration of age-related neurochemical and behavioral deficits by 3,4-diaminopyridine

Alterations of the cholinergic system may be responsible for age-related changes in behavior. The in vitro calcium dependent release of acetylcholine and tight rope test performance decline in parallel during senescence. 3,4-Diaminopyridine, which enhances calcium influx by nerve terminals, diminishes these age-related alterations. In aged mice (30 month old), 3,4-diaminopyridine increases the calcium dependent release of acetylcholine (260%) and improves tight rope test performance (428%). These results support the hypothesis that alterations in calcium homeostasis underlie some of the cholinergic and behavioral deficits that accompany senescence.

The independency of choline transport and acetylcholine synthesis

The coupling of choline transport to acetylcholine synthesis has been investigated by measurement of the isotopic dilution of a pulse of [3H]choline during its incorporation into the recently synthesised acetylcholine of cerebral cortex synaptosomes. Recently synthesised acetylcholine was identified as that containing 14C-labelled precursors introduced by a preincubation before the pulse. When [14C]glucose was used to label acetyl-CoA coupling ratios (calculated as the inverse of the dilution of extracellular [3H]choline during its incorporation into [3H]acetylcholine) of about 0.05-0.2 were found at a choline concentration of 1 microM, rising to 0.5 at choline concentrations of 10-50 microM. Experiments using [14C]choline as a precursor gave similar results, and it was shown that the isotopic dilution did not occur extrasynaptosomally and was not affected by low glucose concentrations. Coupling ratios were always less than unity and rose as the choline concentration increased. It is concluded that choline transported into the nerve terminal has no privileged access to choline acetyltransferase. The results can be explained by a rate-controlling transport of choline into the terminal followed by its rapid acetylation rather than any linkage or coupling of the two processes.

Clostridium botulinum type C toxin: a sketch of the molecule

The purification and crystallization of type C botulinum toxin along with its physical characteristics are described. The shape of Clostridium botulinum type C toxin molecule is globular like a pressed ball with a 7.4 nm diameter and a 4.3 nm thickness. The molecular volume is approximately 185 nl and the molecular weight is 141000. The toxin molecule is composed of two parts, which are separable under appropriate conditions. These parts have some differences in the electrophoretic properties, amino acid distribution, immunological, and functional characteristics. The toxin molecule can be reconstituted by association of S-S bond between the two chains. The expression of the toxicity requires that the fragments of the polypeptide chain carrying the necessary information be functionally organized for the proper development of the specific tertiary structure for active conformation.

Uptake of horseradish peroxidase at isolated nerve terminals in relationship to transmitter release; biochemical results

The uptake of horseradish peroxidase (HRP) into isolated nerve terminals (synaptosomes) has been studied by using a spectrophotometric method to determine the enzyme activity. HRP is rapidly taken up by synaptosomes, it is not removed by multiple washes in iso-osmotic medium but is lost if the particles are ruptured by hypo-osmotic conditions. The uptake is not affected by metabolic poisons, is reduced at lower temperatures and is not associated with any significant release of cytoplasmic lactate dehydrogenase suggesting an endocytotic mechanism. Intra-synaptosomal HRP can be released by a process that is similar to uptake and is also not accompanied by any loss of synaptosomal lactate dehydrogenase indicating exocytosis. Depolarization of synaptosomes (by high potassium concentrations) was found to release [14C]ACh but to have no effect on HRP uptake either simultaneously or after recovery in a non-depolarizing medium. Absence of Ca2+ prevented depolarization evoked release of [14C]ACh but had no effect on the uptake of HRP. The release of HRP was not increased by depolarization even though [14C]choline taken up during the same period was released as [14C]ACh. It is concluded that the endo-exocytotic cycle that transports HRP across the synaptosomal membrane is unrelated to transmitter release. A discrete vesicular localization of HRP reaction product was only occasionally seen in the EM nor could consistent differences resulting from depolarization be observed. However, the ultrastructural localization was found to be unreliable because glutaraldehyde fixation irreversibly inactivated 80--90% of the HRP even when it was sequestered within synaptosomes and the insoluble reaction product precipitated from a supersaturated solution onto membranes.

Botulinum toxin type A: kinetics of calcium dependent paralysis of the neuromuscular junction and antagonism by drugs and animal toxins

The effect of botulinum Toxin (BoTx), which blocks the mechanism of release of acetylcholine at neuromuscular junctions and induces paralysis of muscles stimulated by nerves, is known to be Ca2+-dependent. Amplitude of muscular contractions evoked by nerve impulse was studied in BoTx poisoned preparations. The present report notes that an increase in Ca2+ concentration in vitro delays paralysis of muscular contractions of the frog evoked by nerve impulse. The restorative effect of different drugs on this paralysis has been tested: 4-aminopyridine, ATXII (toxin isolated and purified from the sea anemone Anemonia sulcata tentacles) and a crude venom isolated from the scorpion Androctonus australis antagonize the BotX induced paralysis at physiological concentrations of Ca2+ (Cao2+ = 2 mM), whereas the restorative effect observed with tetra-ethylammonium or guanidine occurs at higher concentrations of Ca2+ (Cao2+ = 4 mM), as in mammals. ATXII restores in vivo the activity of a BoTx paralysed muscle of guinea pig and this effect is more efficient if the interval between the injection of BoTx and ATXII is shortened. These results on the frog and guinea pig are in agreement with those obtained on other biological preparations by several investigators. Moreover it is suggested that the antagonism of BoTx induced paralysis is a consequence of the increase in Ca2+ at the nerve ending. The efficiency of 4-aminopyridine and animal toxins is explained by an action on the nerve ending, by increasing Ca2+ from an interval compartment of the cell, whereas antagonism produced by guanidine and tetraethylammonium involves uptake of Ca2+ from the external medium. The bathing medium must be at a higher concentration of Ca2+ than usual. This explains the differences in antagonism obtained by these drugs and toxins in vitro and in vivo.

Localization of sites for 125I-labelled botulinum neurotoxin at murine neuromuscular junction and its binding to rat brain synaptosomes

Botulinum neurotoxin, purified to homogeneity from Clostridium botulinum (Type A), was found to be highly neurotoxic (greater than 8 X 10(7) mouse LD50/mg protein). Labelling of this pure neurotoxin with 125I-iodine to high specific radioactivity was achieved without appreciable loss of biological activity. This was used to demonstrate saturable binding sites for this toxin at the neuromuscular junction, following in vivo administration into mice. A demonstrable inhibitory effect of the neurotoxin on release of acetylcholine from rat cerebrocortical synaptosomes indicates that it affects synapses in the central nervous system. Kinetic studies on the binding of 125I-labelled neurotoxin to brain synaptosomes yielded an association rate constant of 2.3 x 10(5)M-1s-1; dissociation plots were biphasic and the predominant species showed a rate constant of 1.2 X 10(-4)s-1. The saturable binding component is heat-sensitive and inactivated by trypsin. Preliminary studies showed that botulinum neurotoxin associates with plasma membrane fractions of synaptosomes and that binding does not result in any gross structural changes, at least in the majority of the toxin molecules.

Binding to mouse brain synaptosomes of Clostridium botulinum type E derivative toxin before and after tryptic activation

Clostridium botulinum type E125 I-labelled derivative toxin, both the partially active (intact) and the fully active (nicked) forms, bound to mouse brain synaptosomes. They gave the same KD and Bmax values in binding to synaptosomes, although the nicked form possessed a mouse lethal potency about 30 times higher than that of the intact form. These results may indicate that the binding site of the derivative toxin is not modified by tryptic activation and that the binding to synaptosomes is independent of the blockade of acetylcholine release.

Autoregulation of acetylcholine release in isolated hippocampal nerve endings

The existence of presynaptic autoreceptors modulating acetylcholine release from central cholinergic nerve endings was investigated by using rat hippocampal synaptosomes in a superfusion system. The presence of exogenous acetylcholine, carbachol or oxotremorine in the superfusion fluid produced a dose-dependent inhibition of the release of [3H]acetylcholine elicited by 15 mM KCl in synaptosomes prelabeled with tritiated choline. The inhibition was counteracted by atropine. Another well known muscarinic agonist, bethanechol, had no effect on [3H]acetylcholine release. Our results indicate that central cholinergic nerve terminals possess autoreceptors of the muscarinic type for the control of acetylcholine release. Moreover, differences seem to exist between pre-and postsynaptic muscarinic receptors in the central nervous system.

Tetanus toxin and botulinum A toxin inhibit acetylcholine release from but not calcium uptake into brain tissue

Slices or particles from rat forebrain cortex were preloaded with [3H]choline, and the release of [3H]acetylcholine was evoked with potassium ions in a superfusion system. Release depended on the presence of calcium. 1. Incubation of the preloaded tissue preparation for 2 h with tetanus or botulinum A toxin did not change the [3H]acetylcholine content or the ratio [3H]acetylcholine/[3H]choline. Tetanus toxin diminished, dependent on dose and time, the release of [3H]acetylcholine evoked by 25 mM K+. It was about ten times more potent than botulinum A toxin. The effect of botulinum toxin was due to its neurotoxin content. Raising the potassium concentration partially overcame the inhibition by the toxins. Hemicholinium-3, applied to preloaded slices, left the subsequent [3H]acetylcholine release unchanged. Pretreatment of particles with neuraminidase diminished the content of long-chain gangliosides to the detection limit. Such particles remained fully sensitive to tetanus toxin, and at least partially sensitive to botulinum A toxin. 2. The potassium or sea anemone toxin II stimulated uptake of 45Ca2+ into cortex synaptosomes or particles was not inhibited by either toxin. Both toxins appear to impede the Ca2+-dependent mobilization of an easily releasable acetylcholine pool, without inhibiting the transmembranal calcium fluxes.

The effect of acetylcholine release on choline fluxes in isolated synaptic terminals

As in intact tissues, choline influx into synaptosomes is enhanced after a period of depolarization induced release of acetylcholine. The activation of uptake is dependent on the presence of Ca2+ and inhibited by high Mg2+ concentrations in the medium during depolarization. Choline transport in erythrocytes was not activated by prior treatment with potassium. The permeability constant of the synaptosome membrane to choline was found to be 2.7 x 10(-8) cm . s-1 and to acetylcholine 1.8 x 10(-8) cm . s-1. Choline influx has been studied after pre-loading synaptosomes with choline. Different radiolabels were used to measure efflux of preloaded choline and influx simultaneously. Isotopic dilution in flux studies was estimated and corrected for. Influx was stimulated by high internal concentrations of choline, and efflux similarly stimulated by high outside concentrations of choline. The maximal influx and efflux at saturating opposite concentrations of choline were equal with a value of about 500 pmol . min-1 per mg synaptosomal protein. A reciprocating carrier would explain the equality of the maximal influx and efflux. Acetylcholine competes with choline for binding to the carrier but is itself hardly transported. Increased acetylcholine concentrations were shown to inhibit both choline influx and efflux from the trans position. Raising intrasynaptosomal acetylcholine concentrations by pre-loading abolished the stimulation of influx by prior depolarization. It is proposed that high concentrations of acetylcholine immobilize the carrier on the inside of the synaptic membrane. The stimulation of choline influx consequent upon depolarization is caused by release of ACh which results in relief of this immobilisation. The enhanced supply of choline achieved by this mechanism is likely to be important in maintaining stores of the acetylcholine in vivo.