Complete purification of beta-bungarotoxin. Characterization of its action and that of tityustoxin on synaptosomal accumulation and release of acetylcholine

An in vitro potency assay using nicotinic acetylcholine receptor binding works well with antivenoms against Bungarus candidus and Naja naja

In order to facilitate/expedite the production of effective and affordable snake antivenoms, a novel in vitro potency assay was previously developed. The assay is based on an antiserum’s ability to bind to postsynaptic neurotoxin (PSNT) and thereby inhibit the PSNT binding to the nicotinic acetylcholine receptor (nAChR). The assay was shown to work well with antiserum against Thai Naja kaouthia which produces predominantly the lethal PSNTs. In this work, the assay is demonstrated to work well with antiserum/antivenom against Bungarus candidus (BC), which also produces lethal presynaptic neurotoxins, as well as antivenom against Sri Lankan Naja naja (NN), which produces an abundance of cytotoxins. The in vitro and in vivo median effective ratios (ER₅₀s) for various batches of antisera against BC showed a correlation (R²) of 0.8922 (p < 0.001) while the corresponding value for the anti-NN antivenom was R² = 0.7898 (p < 0.01). These results, together with the known toxin profiles of various genera of elapids, suggest that this in vitro assay could be used with antisera against other species of Bungarus and Naja and possibly other neurotoxic snake venoms worldwide. The assay should significantly save numerous lives of mice and accelerate production of life-saving antivenoms.

Tityustoxin-induced release of ATP from rat brain cortical synaptosomes

Tityustoxin, a scorpion toxin that alters the Na+ channel activity, induces release of ATP from rat brain cortical synaptosomes. The effect of tityustoxin is dependent on its concentration and incubation time. Continuously or cumulative release of ATP evoked by tityustoxin was calcium-dependent and interestingly only partially inhibited by tetrodotoxin. We suggest that tityustoxin mainly releases ATP from the vesicular pool but other pools may also be involved.

K+ channel sub-types in rat brain: characteristic locations revealed using beta-bungarotoxin, alpha- and delta-dendrotoxins

Sub-types of fast-activating, voltage-dependent K+ channels were localized in rat brain using the specific probe, alpha-dendrotoxin, in conjunction with the putative K+ channel ligands, delta-dendrotoxin and beta-bungarotoxin. Sheet-film autoradiography of brain sections labelled with radioiodinated delta-dendrotoxin showed that its acceptors occur in most synapse-rich and gray matter regions, and nerve tracts; all of this labelling was abolished by alpha-dendrotoxin or its homologue, toxin I. Other structurally related peptides from mamba snake venom, beta- and gamma-dendrotoxin, were much less effective in preventing delta-dendrotoxin labelling. In common with the sites for alpha-dendrotoxin and beta-bungarotoxin, delta-dendrotoxin acceptors were enriched in cerebral cortex, thalamus and molecular layer of both the cerebellum and dentate gyrus of the hippocampus. However, delta-dendrotoxin failed to show significant binding to the Purkinje cell layer of the cerebellar cortex and stratum lacunosum moleculare of the hippocampal formation, areas labelled prominently by the other two probes. Evidence of this apparent heterogeneity in the toxin-binding proteins was consolidated by the observed inability of delta-dendrotoxin to inhibit 125I-labelled alpha-dendrotoxin or beta-bungarotoxin binding to these specified regions. Thus, delta-dendrotoxin, like beta-bungarotoxin, discriminates between sub-types of alpha-dendrotoxin acceptors but in different fashions. Whilst beta-bungarotoxin interacts preferentially with a sub-population in synaptic areas, delta-dendrotoxin distinguished sub-types in certain synaptic and gray matter regions and, in this, resembles mast cell degranulating peptide, a ligand known to block an alpha-dendrotoxin-sensitive K+ current.

Distribution in the rat central nervous system of acceptor sub-types for dendrotoxin, a K+ channel probe

Dendrotoxin, a snake polypeptide that facilitates the release of neurotransmitters, is a putative ligand for certain voltage-dependent, rapidly-activating K+ channels. Using a 125I-labelled derivative, the location of high-affinity acceptors for this toxin in the rat central nervous system was established by quantitative sheet film autoradiography. A widespread distribution of binding sites was observed, with high densities of acceptors being found in most gray matter regions and along nerve tracts. Heterogeneity in these acceptors was deduced from their differential interaction with beta-bungarotoxin, another probe that perturbs transmitter release. Whereas the latter blocked the majority of dendrotoxin sites in gray matter areas, it competed much less efficaciously for the acceptors in white matter. These collective findings demonstrate the occurrence of dendrotoxin acceptor sub-types which display characteristic distributions in the central nervous system. Notably, this heterogeneity can be related to electrophysiological evidence for the presence in neurons of multiple, dendrotoxin-sensitive, K+ conductances, though some of these remain to be shown directly in brain preparations.

Dendrotoxin, 4-aminopyridine, and beta-bungarotoxin act at common loci but by two distinct mechanisms to induce Ca2+-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes

The release of endogenous glutamate from guinea-pig cerebrocortical synaptosomes evoked by dendrotoxin, beta-bungarotoxin, and 4-aminopyridine is compared. Dendrotoxin and 4-aminopyridine cause Ca2+-dependent release, representing a partial depletion of the KCl-releasable transmitter pool. The decrease in the plasma membrane potential caused by 4-aminopyridine or dendrotoxin and the evoked release of glutamate from a transmitter pool accord with the inhibitory action of these agents on certain K+ conductances. In contrast, the massive release of glutamate evoked by beta-bungarotoxin is produced in the presence of Ca2+ but not of Sr2+, a result consistent with a generalised permeabilisation of synaptosomal plasma membranes. Although dendrotoxin inhibits the binding of beta-bungarotoxin and the resultant synaptosomal lysis, demonstration of a direct effect of beta-bungarotoxin binding per se on K+ permeability is impractical owing to its phospholipase A2 activity.

Distribution of acceptors for beta-bungarotoxin in the central nervous system of the rat

High-affinity acceptors for 125I-beta-bungarotoxin have been identified on rat brain cryostat sections and mapped quantitatively using 3H-sensitive sheet film autoradiography. A unique distribution of acceptors has thus been observed; the toxin sites are particularly enriched in grey matter areas and synaptic regions, consistent with the pharmacological action of beta-bungarotoxin. As the binding was abolished by dendrotoxin, a related polypeptide known to inhibit fast-activating K+ conductances, the occurrence of beta-bungarotoxin acceptors may indicate the location of certain voltage-sensitive K+ channels. The overall distribution is, however, distinct from that of any other ion channel described.

Mast cell degranulating peptide and dendrotoxin selectively inhibit a fast-activating potassium current and bind to common neuronal proteins

Dendrotoxin and mast cell degranulating peptide are highly potent convulsant polypeptides from mamba snake and bee venoms, respectively. Electrophysiological techniques and binding assays were used to study their interaction with fast-activating, voltage-dependent potassium channels in rat neurons. Intracellular recordings in sensory ganglion cells showed that mast cell degranulating peptide blocks the same slowly inactivating potassium current as dendrotoxin but with lower potency, the respective IC50 values in sensory A neurons of nodose ganglion being 2.1 nM and 37 nM. In contrast, the transient potassium current (IA) in superior cervical ganglion neurons was unaffected by either toxin, highlighting the heterogeneity of these potassium channels and the selective action of the toxins. Using biologically active 125I-labelled derivatives of dendrotoxin and beta-bungarotoxin (a related snake protein), the binding of mast cell degranulating peptide to two subtypes of high-affinity acceptors in rat cerebrocortical synaptosomal preparations was examined. Mast cell degranulating peptide antagonized the specific binding of both radioiodinated toxins to each of the acceptor species, in the membrane-bound state; additionally, [125I]dendrotoxin binding in detergent-solubilized extracts was, likewise, blocked by mast cell degranulating peptide. Notably, the observed inhibitory constants (KI) for mast cell degranulating peptide were appreciably larger than for dendrotoxin, consistent with their different efficacies in blocking the potassium conductances. It is concluded that the specific interaction of this apian polypeptide with dendrotoxin acceptors must underlie its selective action on potassium conductances, emphasizing a functional relationship between these membrane acceptors and the potassium channel variants, sensitive to both dendrotoxin and mast cell degranulating peptide.

Studies on the subunit structure of textilotoxin, a potent neurotoxin from the venom of the Australian common brown snake (Pseudonaja textilis)

Textilotoxin is a presynaptic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis. It has the highest lethality and is structurally the most complex of any known snake venom neurotoxin. It was resolved into its five non-covalently linked subunits in a single step by reverse-phase HPLC. Two of the subunits were identical. The N-terminal amino-acid sequence and amino-acid composition of each subunit were determined. Subunit A was the only one found to possess phospholipase A activity. Separation of textilotoxin into its subunits was reversible and reformed textilotoxin had the same Mr and lethality in mice as the native toxin. Experiments with various unnatural combinations of subunits have led to interesting variations in lethality and Mr of the resulting complexes.

Some biochemical characteristics and cell membrane actions of a toxic phospholipase A2 isolated from the venom of the pit viper Agkistrodon halys (Pallas)

A toxic component (AgTx) from the venom of Agkistrodon halys (Pallas) was isolated using DEAE-cellulose DE11 and CM-Sephadex C50 column chromatography and finally purified to homogeneity by FPLC on a MonoQ column. The toxin is a neutral (pI 6.9) single chain polypeptide with a mol. wt of 14,000 and an amino acid composition (123 residues) roughly similar to that of notexin. AgTx was found to have phospholipase A2 activity which was dependent on calcium and stimulated by sodium deoxycholate. The toxin caused efflux of 2-deoxy-(1-3H)-glucose-6-phosphate (a cell membrane integrity probe) as well as of [3H]acetylcholine from rat brain synaptosomes. No cell membrane damage was induced by AgTx on cultured N1E 115 neuroblastoma cells and chick myotube cultures. The LD50 ws 150 micrograms/kg (i.p.) in mice. The main symptom observed was respiratory paralysis. The results obtained show that AgTx can be classified as a toxic phospholipase A2 with a presynaptic site of action.

Uncoupling of cholinergic synaptic vesicles by the presynaptic toxin beta-bungarotoxin

The effect of the presynaptic neurotoxin beta-bungarotoxin (beta-BuTx) on the acetylcholine (ACh) storage system of synaptic vesicles isolated from the electric organ of Torpedo californica was studied. The toxin can totally inhibit active transport of [3H]ACh by the vesicles in a Ca2+-, time-, and concentration-dependent manner. Correlated with these effects is a 50-60% stimulation of the vesicle proton-pumping ATPase activity. The beta-BuTx-mediated transport inhibition and ATPase stimulation are antagonized by delipidated bovine serum albumin, not reversed by excess EGTA, and not mimicked by other cationic proteins or soybean or pancreatic trypsin inhibitors. The behavior is consistent with phospholipase A2 (PLA2)-dependent damage to the vesicle membrane caused by beta-BuTx, which results in uncoupling of the ATPase and ACh transporter systems. The nonneurotoxic Naja naja venom PLA2 causes similar effects, except that it is slightly more potent on a molar basis. About 100-fold more beta-BuTx is required to effect lysis of synaptic vesicles than to uncouple them. ATP is a strong inhibitor of beta-BuTx- but not of N. naja PLA2-mediated uncoupling. The observations suggest that a component of beta-BuTx toxicity in the cholinergic terminal might involve attack on synaptic vesicles or vesicle-like structures and that a nucleotide-like factor might modulate the toxicity.

Involvement of neuronal acceptors for dendrotoxin in its convulsive action in rat brain

Dendrotoxin, a snake-venom polypeptide, is a potent convulsant that facilitates transmitter release apparently by inhibition of voltage-sensitive K+ channels responsible for A-currents. A biologically active 125I-iodinated derivative of this toxin was prepared and used to characterize kinetically homogeneous non-interacting high-affinity acceptors in synaptic membranes from rat cerebral cortex and hippocampus. Binding of radiolabelled toxin from Dendroaspis angusticeps to its membrane acceptor protein was inhibitable by homologous polypeptides from other mamba snakes; most importantly, their rank order of potency was identical with that for their central neurotoxicities in rats, furnishing evidence for involvement of this binding component in the convulsive symptoms observed. Beta-Bungarotoxin, a presynaptically acting neurotoxin whose action on neurotransmitter release at the neuromuscular junction and effects on brain synaptosomes are antagonized by dendrotoxin, was only able to inhibit the binding of the 125I-labelled toxin with low efficacy, although dendrotoxin apparently interacts avidly with the acceptor sites for beta-bungarotoxin. This weak interaction of beta-bungarotoxin with the acceptor was not attributable to its phospholipolytic action. Other neurotoxins and ion-channel antagonists failed to affect the binding of dendrotoxin. The findings presented here, together with recent electrophysiological data, favour the interpretation that dendrotoxin binds to a membrane protein comprising, or closely associated with, this one group of voltage-dependent K+ channels.

Two acceptor sub-types for dendrotoxin in chick synaptic membranes distinguishable by beta-bungarotoxin

Using chick synaptic membranes, proteinaceous acceptors were characterized for dendrotoxin, a polypeptide from Dendroaspis angusticeps with convulsant activity due to its facilitation of transmitter release, resulting from inhibition of A-current K+ channels in brain. Both equilibrium and kinetic measurements of radioiodinated toxin binding showed that two populations of membraneous acceptors were discernible with different affinities (Kd approximately 0.5 nM and 15 nM; Bmax approximately 90 and 400 fmol/mg protein). Only the high-affinity component interacted avidly with beta-bungarotoxin, an inhibitory presynaptic neurotoxin whose lighter chain is homologous to dendrotoxin. Facilitatory homologues of dendrotoxin from Dendroaspis species antagonised its binding to both acceptor sub-types in proportion to their central neurotoxicities, whereas various other toxins (crotoxin, apamin), trypsin inhibitors and lectins proved ineffective. Cross-linking of toxin specifically bound to its membrane acceptors, using bis-imido esters followed by electrophoretic analysis in the presence of sodium dodecyl sulphate, revealed a polypeptide with Mr of 75,000 together with lesser amounts of a 69,000-Mr component. Notably, the covalent labelling of each of these bands was inhibited partially by low concentrations of beta-bungarotoxin, indicating that they are derived from both acceptor species. The demonstrated existence of an acceptor form shared by dendrotoxin and beta-bungarotoxin, together with another sub-type selective for dendrotoxin, is discussed in relation to the known pharmacological interactions of these toxins which exert opposite effects on transmitter release.

The effect of calmodulin inhibitors on the release of acetylcholine and protein phosphorylation induced by tityustoxin, K+ and ouabain

In this paper we describe our studies of the effects of calmodulin inhibitors on the relationship between the release of acetylcholine (ACh) and protein phosphorylation induced by tityustoxin (TsTX), K+ and ouabain. We observed that in cortical slices of brain, the TsTX induced release of ACh was inhibited in the presence of the calmodulin inhibitors trifluoperazine, W-7 and prenylamine, but not by trifluoperazine sulphoxide. The release of ACh elicited by ouabain and K+ (33 mM) was also inhibited by trifluoperazine and chlorpromazine. Studies on protein phosphorylation with crude or purified synaptosome fractions showed that TsTX and K+ (33 mM) increased the incorporation of 32P into a protein with the same apparent molecular weight of synapsin I. Surprisingly, ouabain did not stimulate phosphorylation of this protein. Trifluoperazine, at the same concentrations that inhibited the release of ACh, blocked the stimulatory effect of tityustoxin and K+ on protein phosphorylation. On the other hand, tetrodotoxin inhibited the effect of tityustoxin on protein phosphorylation but had no effect on phosphorylation evoked by K+. Our results demonstrate for the first time that, at least for ouabain, the release of neurotransmitter is not dependent on protein phosphorylation.

The mechanism of action of beta-bungarotoxin at the presynaptic plasma membrane

The beta-bungarotoxin-induced depolarization of the synaptosomal plasma membrane monitored by the efflux of 86Rb+ is potentiated by raising the albumin in the incubation, is Ca2+-dependent and is due neither to inhibition of the (Na+ + K+)-dependent ATPase nor to activation of the voltage-dependent Na+ channel. Occupancy of the beta-bungarotoxin-binding site by dendrotoxin inhibits partially the action of beta-bungarotoxin. The efflux of 86Rb+ is parallelled by a release of lactate dehydrogenase from the synaptosome, and the two processes are maximal with 2 nM-toxin. Digitonin induces a release of 86Rb+ and lactate dehydrogenase closely similar to that seen with beta-bungarotoxin. It is concluded that the toxicity of beta-bungarotoxin for mammalian nerve terminals can be largely accounted for by specific site-directed phospholipase A2-induced permeabilization of the plasma membrane.

Botulinum neurotoxin type B. Its purification, radioiodination and interaction with rat-brain synaptosomal membranes

Neurotoxin from Clostridium botulinum type B was purified to homogeneity by by affinity and ion-exchange chromatography; specific neurotoxicity of this protein (Mr of approximately equal to 155 000) following trypsinisation attained a level of 2 X 10(8) mouse LD50 units/mg protein. 125I-iodination of the toxin to high specific radioactivities (19-63 TBq/mmol) yielded typically greater than 65% of its original toxicity; dodecyl sulphate gel electrophoresis under reducing conditions, after trypsinisation, showed that the larger polypeptide (Mr of approximately equal to 101 000) was labelled preferentially. Saturable binding of the 125I-labelled neurotoxin to rat cerebrocortical synaptosomes was observed and Scatchard analysis showed a low content of acceptors with high affinity (Kd = 0.3-0.5 nM;Bmax approximately equal to 30-60 fmol/mg protein, together with a much larger population of weak-affinity sites. No significant differences in binding affinity were seen in competition experiments using native or fully activated (trypsinized) neurotoxin, indicating that chain cleavage is not essential for acceptor-toxin interaction. Type A botulinum neurotoxin showed a limited capacity to inhibit the synaptosomal binding of labelled type B toxin, even at high concentrations (1 muM), and other neurotoxins were without effect, emphasising the acceptor selectivity. Near-complete loss of specific toxin binding was produced by preincubation of synaptosomes with neuraminidase whereas inhibition of the low-affinity sites with wheat-germ agglutinin was less pronounced; such inactivation was prevented by inclusion of selective inhibitors (2,3-dehydro-2-deoxy-N-acetylneuraminic acid and N-acetylglucosamine, respectively). These observations implicate N-acetylneuraminic acid and, possibly, other sugar moieties as constituents of the toxin acceptors. Trypsinisation of synaptosomes gave incomplete inhibition of binding when assayed with 1 nM or 10 nM 125I-iodinated toxin. Detailed analysis of the actions of neuraminidase, trypsin and heat treatment on the concentration dependence of toxin binding suggest the existence of at least two distinguishable populations of sites that contain N-acetylneuraminic acid, with a protein component being associated with the acceptors of lower affinity. These findings are discussed in relation to those previously reported for type A neurotoxin and to the possible physiological significance of such membrane acceptors.

Actions of beta-bungarotoxin on spontaneous release of transmitter at muscle end-plates treated with botulinum toxin

Rat leg muscles were injected subcutaneously with sublethal doses of type A botulinum neurotoxin, and the extensor digitorum longus muscle removed three days later. Intracellular microelectrode recordings were then made of miniature end-plate potentials (mepps). The mepp frequency was reduced by botulinum toxin, while mepp rise times were slowed. Mepp amplitude distributions became characteristically skew. beta-Bungarotoxin (140 nM) was applied to normal muscles in vitro and recordings were made 10-30 min later. The main effect was an increase in mepp frequency during this period. Mepp rise times were unaffected. When beta-bungarotoxin was applied in vitro to muscles treated with botulinum toxin there was also an increase in mepp frequency, although to a value less than in normal muscles. The mepp rise times were speeded up to normal values. The mepp amplitude and rise time distributions showed no obvious evidence for the addition of a second component to the distribution. The data appear to support the hypothesis that the sites for spontaneous release in botulinised muscle may be located at or near the usual release sites at the active zones.

Antagonistic action of uranyl nitrate on presynaptic neurotoxins from snake venoms

Uranyl ion (UO2+2) antagonized the neuromuscular blocking action and phospholipase A2 activity of neurotoxins which act presynaptically [beta-bungarotoxin (beta-BuTX) and crotoxin] but did not affect the action of alpha-bungarotoxin and tetrodotoxin. On the basis of the kinetic analysis of the UO2+2 and strontium ion (Sr2+) antagonism of muscle paralysis induced by beta-bungarotoxin, it was found that they inhibited both the binding of the toxin and the steps following binding that brought about the neuromuscular blocking action of beta-bungarotoxin. Uranyl ion was about 50 times more potent than Sr2+ in antagonizing beta-bungarotoxin. High Ca2+ (10 mM) abolished but low Ca2+ (0.25-1.25 mM) medium enhanced the antagonizing action of UO2+2 and Sr2+. In low Ca2+ medium, UO2+2 markedly potentiated the amplitude of the twitch, subsequent addition of beta-bungarotoxin produced three phases of effects on the twitches, e.g. an initial depression, followed by the second facilitation and finally a rapid depression of twitches; however, approx. 70 min after beta-bungarotoxin the small twitches reached a steady state which persisted for more than 350 min. Therefore, it is evident that UO2+2 is the most potent antagonist of beta-bungarotoxin so far tested.

Bioenergetic actions of beta-bungarotoxin, dendrotoxin and bee-venom phospholipase A2 on guinea-pig synaptosomes

Low concentrations of beta-bungarotoxin or bee-venom phospholipase A2 cause a progressive Ca2+-dependent increase in the proton permeability of the mitochondria within the synaptosomal cytosol, manifested as an increase in oligomycin-insensitive respiration and a partial depolarization of the mitochondrial membrane potential. This uncoupling appears to be a consequence of fatty acids liberated by phospholipase A2 activity at the plasma membrane, since it can be mimicked by the addition of oleate-albumin complexes, in which case there is no requirement for external Ca2+. Dendrotoxin does not affect the mitochondrial proton permeability in situ, but protects partially against the uncoupling action of beta-bungarotoxin. In contrast, this effect of bee-venom phospholipase A2 is unaffected by dendrotoxin. beta-Bungarotoxin, but not bee-venom phospholipase A2, induces a slow progressive depolarization of the plasma membrane. The action of beta-bungarotoxin at the plasma membrane appears not to be related to fatty acid production, since it is augmented rather than inhibited by raising albumin concentrations in the medium. It is concluded that beta-bungarotoxin has at least two actions on intact synaptosomes, both of which may involve interaction at the plasma membrane with a site common to dendrotoxin: first, a mitochondrial uncoupling mediated by fatty acids and, secondly, a depolarization at the plasma membrane.

Influence of divalent cations on the phospholipase-independent action of beta-bungarotoxin at frog neuromuscular junctions

The influence of different divalent cations on the phospholipase-independent inhibition of transmitter release caused by beta-bungarotoxin (beta-BuTx), has been investigated by measuring the frequency of spontaneous miniature end-plate potentials (m.e.p.p.s) at frog neuromuscular junctions. After adding the toxin to normal calcium Ringer solution the m.e.p.p. frequency fell quickly to very low values. This was followed by an increase in frequency characterized by bursts of m.e.p.p.s. The temperature had a negligible effect on the speed of the first inhibition. In Ringer solutions where calcium had been substituted by other divalent cations (5 mM) in the presence of ethyleneglycol-bis-(beta-aminoethylether)N, N'-tetraacetic acid (EGTA, 1 mM), this beta-BuTx-induced decrease in m.e.p.p. frequency was markedly slower. The potency of cations in promoting the initial phase of toxin action was in the sequence: calcium greater than magnesium greater than strontium congruent to cobalt greater than manganese. This phospholipase-independent inhibition of transmitter release followed approximately first-order kinetics, suggesting that it depends mainly on toxin concentration. In the absence of any divalent cations in the Ringer solution beta-BuTx had practically no effect on m.e.p.p. frequency. It appears that beta-BuTx requires divalent cations in order to bind to motor nerve terminals and exert its initial inhibitory action on spontaneous release of transmitter quanta.

In vivo effects of beta-bungarotoxin on the acetylcholine system in different brain areas of the rat

The in vivo effects of beta-bungarotoxin (beta-BT) on the acetylcholine (ACh) system were studied in the whole cerebrum and in different brain regions. The effect of beta-BT on cerebral ACh and choline (Ch) contents was time-dependent. The results show that a single intracerebroventricular injection of 1 microgram toxin increased both the ACh and Ch contents in the cortex, hippocampus, and cerebellum, while in the striatum the ACh level was decreased. Ten nanograms of toxin injected into the lateral ventricle twice, on the first and third days, led to a reduced ACh level 2 days after the last treatment. In animals treated with the same dose three times, on the first, third, and fifth days, and sacrificed 2 days after the last injection, the choline acetyltransferase and acetylcholinesterase activities were reduced and the number of muscarinic acetylcholine receptors was decreased. A biphasic effect of the toxin was therefore demonstrated. It is suggested that in the first phase of the toxin effect the increased levels of ACh and Ch may be due to the inhibition of neuronal transmission, while in the second phase, when the elements of the ACh system are reduced, the neuronal degenerating effect of beta-BT plays a significant role.

Preparation of neurotoxic 3H-beta-bungarotoxin: demonstration of saturable binding to brain synapses and its inhibition by toxin I

1. Homogeneous beta-bungarotoxin, isolated from the venom of Bungarus multicinctus was radiolabelled with N-succinimidyl-[2.3-(3) H]propionate. Stable, di-propionylated material was obtained which was tritiated on both subunits and had a specific radioactivity of 102 Ci/mmol. 2. After separation from unlabelled toxin by isoelectric focussing, it was shown to exhibit significant biological activity in both the peripheral and central nervous systems but had negligible phospholipase A2 activity towards lecithin or cerebrocortical synaptosomes. 3. The labeled neurotoxin binds specifically to a single class of non-interacting sites of high affinity (Kd = 0.6 nM) on rat cerebral cortex synaptosomes; the content of sites is about 150 fmol/mg protein. This binding was inhibited by unlabelled beta-bungarotoxin with a potency which indicates that tritiation does not alter the affinity significantly. 4. The association of toxin with its binding component and its dissociation were monophasic; rate constants observed were 7.8 x 10(5) M-1 s-1 and 5.6 x 10(-4) s-1 at 37 C, respectively. 5. beta-Bungarotoxin whose phospholipase activity had been inactivated with p-bromophenacyl bromide inhibited to some extent the binding of tritiated toxin but with low efficacy. Taipoxin and phospholipase A2 from bee venom, but not Naja melanoleuca, inhibited the synaptosomal binding of toxin with low potencies in the presence, but not the absence, of Ca2+. 6. Toxin I, a single-chain protein from Dendroaspis polylepis known to potentiate transmitter release at chick neuromuscular junction, completely inhibited the binding of 3H-beta-bungarotoxin with a Ki of 0.07 nM; this explains its ability to antagonise the neuroparalytic action of beta-bungarotoxin. Other pure presynaptic neurotoxins, alpha-latrotoxin and botulinum neurotoxin failed to antagonise the observed binding; likewise tityustoxin, which is known to affect sodium channels, had no effect on 3H-beta-bungarotoxin binding. 7. Trypsinization of synaptosomes completely destroyed the binding activity, suggesting that the binding component is a protein; the functional role of the latter is discussed in relation to the specificity of toxin binding.

Biochemical and electrophysiological demonstrations of the actions of beta-bungarotoxin on synapses in brain

Homogeneous beta-bungarotoxin interacts irreversibly with rat olfactory cortex and produced permanent inhibition of neurotransmission (half-time of blockade for 230 nM toxin in 25 min). Binding occurs in the absence of divalent cations, but the rate of synaptic blockade is increased by Ca2+, which activates the intrinsic phospholipase A2 activity of the toxin. Other observable actions of the toxin, seen with rat cerebrocortical synaptosomes, are an increase in the release of acetylcholine, glutamate and gamma-aminobutyrate and impairment of transmitter uptake, which are all insensitive to tetrodotoxin. Inactivation of the toxin's phospholipase activity by chemical modification with p-bromophenacyl bromide diminishes the observed concomitant efflux of the neurotransmitters and lactate dehydrogenase. Collectively, the results support the idea that the toxin binds specifically and irreversibly to component(s) on nerve terminals and this together with the resultant phospholipolysis leads eventually to synaptic blockade. Such a proposal would account for the unique toxicity of the protein relative to phospholipase A2 enzymes.

Preferential action of beta-bungarotoxin at nerve terminal regions in the hippocampus

The action of beta-bungarotoxin on the transverse slice of rat hippocampus has been studied in vitro. The toxin (230 nM) initially impaired neurotransmission in the major subdivisions of the slice with a half time for blockade of about 10 min. Intracellular recordings revealed no reduction in pyramidal cell sensitivity to putative neurotransmitters, suggesting a primary action of the toxin upon transmitter release. More protracted effects of beta-bungarotoxin included a reduction of neuronal excitability, particularly in the terminal regions of hippocampal fibre pathways, but these proceeded at a much slower rate than the action on synaptic transmission. It is concluded that the toxin binds to some component present at terminal regions to mediate its preferential effect in the hippocampus.

Electrophysiological analysis of the presynaptic action of beta-bungarotoxin in the central nervous system

In view of previously reported actions of beta-bungarotoxin (beta-BuTX) on central brain synaptosomes, the effects on this toxin on the electrical activity of two brain slice preparations have been examined in vitro. beta-BuTX initially suppresses the synaptic component of the field responses to electrical stimulation in olfactory cortex and hippocampal slices. Intracellular recordings demonstrate that this synaptic depression occurs without detectable reduction in the sensitivity of the postsynaptic neuron to putative neurotransmitters. Following longer exposure to the toxin, reduced neuronal excitability is observed both pre- and post-synaptically. Elimination of the phospholipase A2 activity of beta-BuTX, by chemical modification or removing the Ca2+ necessary for enzymic activity, greatly reduces but does not totally eradicate the toxin's ability to block neurotransmission in the olfactory cortex. In the absence of enzymic activity beta-BuTX has no obvious effect on axonal conduction. Pure phospholipases A2, such as that from Naja melanoleuca mimic the transmission-blocking action of beta-BuTX, but with lower potency and without the effects on fibre excitability. Collectively, these results are taken as evidence that beta-BuTX initially suppresses transmitter release, a notion supported by the observed loss of spontaneous synaptic activity in hippocampal cells. Prolonged exposure to the toxin induces apparently less specific effects on neuronal excitability which are dependent on phospholipase A2 activity and are discussed with reference to the selective action of beta-BuTX on hippocampal fibre systems which possess release sites.

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.

The effects of alpha- and beta-neurotoxins from the venoms of various snakes on transmission in autonomic ganglia

We have previously shown that certain commercially available lots of alpha-bungarotoxin block transmission in ciliary and choroid neurons of both pigeon and chicken ciliary ganglia at a concentration of 10 microgram/ml (1.2 microM). The blockade is antagonized by pre-incubation with 100 microM tubocurarine. Further evidence that this blockade is produced by a postsynaptic action, as one would expect of an alpha-neurotoxin, are our findings that: (a) exposure to the toxin prevents the depolarization of ganglion cells normally seen in response to the cholinergic agonist, carbachol; and (b) the blocking activity of the toxin is removed by treatment with membranes purified from Torpedo electric organ containing an excess of alpha-neurotoxin binding sites. A high affinity binding site for [125I]alpha-bungarotoxin was characterized in the chicken ciliary ganglion. However, since it is labelled equally well by lots of alpha-bungarotoxin which block transmission and those that do not, this site does not appear to be involved in the blockade of transmission. alpha-Cobratoxin (from Naja naja siamensis), the alpha-neurotoxin L.s. III (from Laticauda semifasciata) and certain lots of alpha-bungarotoxin produce a partial blockade of transmission in ciliary neurons of the pigeon ciliary ganglion at a concentration of 10 microgram/ml (1.2 microM), but have no effect on transmission in choroid neurons. Two other alpha-neurotoxins from Laticauda semifasciata, erabutoxin a and erabutoxin b, have no effect on transmission in either cell population at this concentration. None of the alpha-neurotoxins tested had any effect on transmission in either the rat superior cervical ganglion or the rat pelvic ganglion at concentrations up to 100 microgram/ml (12 microM). Collagenase treatment of these ganglia, in an attempt to increase access of the toxins to ganglion cells, did not alter these negative results. beta-Bungarotoxin (0.5 microgram/ml, 0.02 microM) produces a complex blockade of transmission in both avian ciliary ganglia and rat superior cervical ganglia. Unlike the action of alpha-bungarotoxin, the blockade of ciliary ganglion transmission by beta-bungarotoxin is irreversible and is not prevented by pretreatment with tubocurarine.

Tritiation of alpha-bungarotoxin with N-succinimidyl [2,3-3H]propionate. A useful reagent for labelling proteins

Mono[3H]propionyl-alpha-bungarotoxin, prepared with N-succinimidyl [2,3-3H]propionate (sp. radioactivity 50 Ci/mmol) and purified to homogeneity by electrofocusing, retains its biological activity and stability. Rate constants for its binding to acetylcholine receptor were 4.4-fold lower than for unlabelled toxin; no dissociation was detectable. Analysis of enzymic digests of toxin showed 3H is located mainly of entirely in epsilon-propionyl-lysine.

A further study of the phospholipase-independent action of beta-bungarotoxin at frog end-plates

1. The effect of beta-bungarotoxin (beta-BuTx) at the frog neuromuscular junction has been investigated further in order to distinguish more clearly between phospholipase- independent and phospholipase-dependent actions on transmitter release. 2. Inhibition of the enzymatic activity, by substitution of strontium for calcium, allowed determination of the dose-response curve of the early rapid decrease in transmitter release caused by the toxin. In the presence of strontium ions there was, however, still about 7% residual enzymatic activity, and electrophysiological evidence of it could be seen in room-temperature experiments at high concentrations of beta-BuTx. This residual enzymatic activity could be suppressed by lowering the temperature to 5 degrees C. 3. In normal calcium-Ringer solution beta-BuTx produced the typical triphasic effect on the amplitude of end-plate potentials (e.p.p.s). Lowering the temperature markedly delayed an then diminished the secondary transient increase. There was, however, comparatively little temperature influence on the first rapid decrease in e.p.p. amplitude. Enzymatic assays confirmed the temperature dependence of the toxin's phospholipase activity on model phospholipid substrates. 4. The kinetics of the phospholipase-independent action of beta-BuTx were examined in strontium-Ringer compared to calcium-Ringer solution, as well as in calcium-Ringer at different temperatures. Both the time to onset of inhibition and the time to 50% inhibition of the e.p.p., during the first phase of toxin action, are temperature-dependent and briefer in calcium than in strontium-Ringer solution. It is suggested that calcium is more effective than strontium in promoting this phospholipase- independent interaction of beta-BuTx with the nerve terminal membrane.