Changes in spontaneous and evoked release of transmitter induced by the crotoxin complex and its component phospholipase A2 at the frog neuromuscular junction

Varespladib (LY315920) prevents neuromuscular blockage and myotoxicity induced by crotoxin on mouse neuromuscular preparations

Varespladib (LY315920) is a synthetic phospholipase A₂ (PLA₂) inhibitor that has been demonstrating antiophidic potential against snake venoms that present PLA₂ neurotoxins. In this study, we evaluate the capacity of Varespladib to inhibit the neuromuscular effects of crotoxin (CTX), the main toxic component of Crotalus durissus terrificus snake venom, and its PLA₂ subunit (CB). We performed a myographic study to compare the neuromuscular effects of CTX or CB and the mixture of these substances plus Varespladib in mice phrenic nerve-diaphragm muscle preparations. CTX (5 μg/mL), CB (20 μg/mL), or toxin-inhibitor mixtures pre-incubated with different concentration ratios of Varespladib (1:0.25; 1:0.5; 1:1; w/w) were added to the preparations and maintained throughout the experimentation period. Myotoxicity was assessed by light microscopic analysis of diaphragm muscle after myographic study. CTX and CB blocked the nerve-evoked twitches, and only CTX induced histological alterations in diaphragm muscle. Pre-incubation with Varespladib abolished the muscle-paralyzing activity of CTX and CB, and also the muscle-damaging activity of CTX. These findings emphasize the clinical potential of Varespladib in mitigating the toxic effects of C. d. terrificus snakebites and as a research tool to advance the knowledge of the mechanism of action of snake toxins.

Crotalus Neutralizing Factor (CNF) inhibits the toxic effects of Crotoxin at mouse neuromuscular preparations

Crotalus Neutralizing Factor (CNF) was the first phospholipase A₂ inhibitor isolated from the plasma of the South American rattlesnake (Crotalus durissus terrificus). Previous biochemical and biophysical studies demonstrate an interaction of CNF with Crotoxin (CTX), the main toxic component in the venom of these snakes. CTX promotes the blockade of neuromuscular transmission by a sum of neurotoxic and myotoxic activities. However, the ability of CNF to inhibit these activities has not been shown until the present study. We performed a myographic study to compare the neuromuscular effects of CTX and the mixture CTX plus CNF in mice phrenic nerve-diaphragm muscle preparations. CTX (5 μg/mL) alone, or pre-incubated with CNF (5, 20 or 50 μg/mL) for 15 min was added to the preparations and maintained throughout the experimentation period. Myotoxicity was assessed by light microscopic analysis of diaphragm muscle after myographic study. CTX (5 μg/mL) blocked both indirectly and directly evoked twitches in neuromuscular preparations. In addition, CTX induced histological alterations in diaphragm muscle. Pre-incubation with CNF (50 μg/mL) abolished both the muscle-paralyzing and muscle-damaging activities of CTX. Therefore, the present study confirms, through functional studies, the antiophidic potential of CNF.

Crotoxin-Induced Mice Lung Impairment: Role of Nicotinic Acetylcholine Receptors and COX-Derived Prostanoids

Respiratory compromise in Crotalus durissus terrificus (C.d.t.) snakebite is an important pathological condition. Considering that crotoxin (CTX), a phospholipase A₂ from C.d.t. venom, is the main component of the venom, the present work investigated the toxin effects on respiratory failure. Lung mechanics, morphology and soluble markers were evaluated from Swiss male mice, and mechanism determined using drugs/inhibitors of eicosanoids biosynthesis pathway and autonomic nervous system. Acute respiratory failure was observed, with an early phase (within 2 h) characterized by enhanced presence of eicosanoids, including prostaglandin E2, that accounted for the increased vascular permeability in the lung. The alterations of early phase were inhibited by indomethacin. The late phase (peaked 12 h) was marked by neutrophil infiltration, presence of pro-inflammatory cytokines/chemokines, and morphological alterations characterized by alveolar septal thickening and bronchoconstriction. In addition, lung mechanical function was impaired, with decreased lung compliance and inspiratory capacity. Hexamethonium, a nicotinic acetylcholine receptor antagonist, hampered late phase damages indicating that CTX-induced lung impairment could be associated with cholinergic transmission. The findings reported herein highlight the impact of CTX on respiratory compromise, and introduce the use of nicotinic blockers and prostanoids biosynthesis inhibitors as possible symptomatic therapy to Crotalus durissus terrificus snakebite.

Biological and Structural Characterization of Crotoxin and New Isoform of Crotoxin B PLA2 (F6a) from Crotalus durissus collilineatus Snake Venom

A new crotoxin B isoform PLA(2) (F6a), from Crotalus durissus collilineatus was purified from by one step reverse phase HPLC chromatography using mu-Bondapack C-18 column analytic. The new crotoxin B isoform PLA(2) (F6a), complex crotoxin, the catalytic subunit crotoxin B isoform PLA(2) (F6a) and two crotapotin isoforms (F3 and F4), were isolated from the venom of Crotalus durissus collilineatus. The crotapotins isoforms F3 and F4 had similar chemical properties, the two proteins different in their ability to inhibit of isoforms of PLA(2) (F6 and F6a). The molecular masses estimated by MALDI-TOF mass spectrometry were: crotoxin B: 14,943.14 Da, crotapotin F3: 8,693.24 Da, and crotapotin F4: 9 314.56 Da. The new crotoxin B isoform PLA(2) (F6a) contained 122 amino acid residues and a pI of 8.58. Its amino acid sequence presents high identity with those of other PLA(2)s, particularly in the calcium binding loop and active site helix 3. It also presents similarities in the C-terminal region with other myotoxic PLA(2)s. The new crotoxin B isoform PLA(2) (F6a) contained 122 amino acid residues, with a primary structure of HLLQFNKMIK FETRRNAIPP YAFYGCYCGW GGRGRPKDAT DRCCFVHDCC YGKLAKCNTK WDFYRYSLKS GYITCGKGTW CEEQICECDR VAAECLRRSL STYRYGYMIY PDSRCRGPSE TC. A neuromuscular blocking activity was induced by crotoxin and new crotoxin B isoform PLA(2) (F6a) in the isolated mouse phrenic nerve diaphragm and the biventer cervicis chick nerve-muscle preparation. Whole crotoxin was devoid of cytolytic activity upon myoblasts and myotubes in vitro, whereas new crotoxin B isoform PLA(2) (F6a) was clearly cytotoxic to these cells.

Neurotoxic and myotoxic actions of crotoxin-like and Crotalus durissus cascavella whole venom in the chick biventer cervicis preparation

Crotoxin from Crotalus durissus cascavella venom was purified by a combination of molecular exclusion chromatography (Superdex 75 column) and HPLC molecular exclusion (Protein Pack 300SW column). Neurotoxic and myotoxic effects from C. durissus cascavella whole venom and its main fraction, the crotoxin-like, were studied in the chick biventer cervicis (CBC) nerve-muscle preparation. Both venom and its crotoxin showed significant (p < 0.05) blockade of neuromuscular transmission at concentrations as low as 0.2-1, 5 and 25 microg/ml, but no significant effect has been shown with a concentration of 0.04 microg/ml (n = 5 each). The time required to produce 50% neuromuscular blockade with the venom and its crotoxin was 53.6+/-8.2 and 65.9+/-4.9 min (0.2 microg/ml), 29.7+/-1.9 and 34.3+/-1.9 min (1 microg/ml), 24.8+/-1.6 and 21.1+/-1.5 min (5 microg/ml), 20.9+/-3.7 and 20.1+/-1.4 min (25 microg/ml), respectively. The addition to the incubation bath of acetylcholine (55 and 110 microM) or KCl (20.1 mM), either before or after the venom or the crotoxin induced contracture in the presence of a total blockade, in all the concentrations used. Morphological analysis showed that the damage caused by C. durissus cascavella venom is stronger than that caused by crotoxin. The myonecrotic picture was more marked at higher venom and crotoxin doses (1, 5 or 25 microg/ml). Only at 25 microg/ml concentrations of the venom and crotoxin, marked muscle fiber changes were detected. We concluded that the crotoxin-like and the whole venom from C. durissus cascavella possess a preponderant and quite potent neurotoxic action in this preparation, and a myotoxic action which is observed only at higher doses.

The pharmacological effect of Bothrops neuwiedii pauloensis (jararaca-pintada) snake venom on avian neuromuscular transmission

The neuromuscular effects of Bothrops neuwiedii pauloensis (jararaca-pintada) venom were studied on isolated chick biventer cervicis nerve-muscle preparations. Venom concentrations of 5-50 micro g/ml produced an initial inhibition and a secondary increase of indirectly evoked twitches followed by a progressive concentration-dependent and irreversible neuromuscular blockade. At venom concentrations of 1-20 micro g/ml, the responses to 13.4 mM KCl were inhibited whereas those to 110 micro M acetylcholine alone and cumulative concentrations of 1 micro M to 10 mM were unaffected. At venom concentrations higher than 50 micro g/ml, there was pronounced muscle contracture with inhibition of the responses to acetylcholine, KCl and direct stimulation. At 20-24 degrees C, the venom (50 g/ml) produced only partial neuromuscular blockade (30.7 +/- 8.0%, N = 3) after 120 min and the initial inhibition and the secondary increase of the twitch responses caused by the venom were prolonged and pronounced and the response to KCl was unchanged. These results indicate that B.n. pauloensis venom is neurotoxic, acting primarily at presynaptic sites, and that enzyme activity may be involved in this pharmacological action.

Analysis of fatty acids released by crotoxin in rat brain synaptosomes

Crotoxin, the main toxin of Crotalus durissus terrificus venom, exerts its lethal effect by blocking neurotransmission at the neuromuscular junction level through a triphasic mechanism. This effect seems to depend on its phospholipasic activity, suggesting that the mechanism of neurotransmission blockage may be related to fatty acids release in specific sites of the nervous terminal. In this work, we purified the fatty acids released by crotoxin's activity and this outline was compared with other phospholipases A(2), including CB, a subunit of crotoxin. Our results show a higher release of palmitate and arachidonate by crotoxin when compared to other phospholipases A(2). Since palmitate has a role in protein acylation processes and arachidonate participates in signal transduction events, these mechanisms may be related to the neurotoxic actions of crotoxin.

Nucleotide sequence of crotamine isoform precursors from a single South American rattlesnake (Crotalus durissus terrificus)

A cDNA phage library was constructed from venom glands of a single adult specimen of crotamine-plus Crotalus durissus terrificus (South American rattlesnake) captured in a known region. Fifteen crotamine positive clones were isolated using a PCR-based screening protocol and sequenced. These complete cDNAs clones were grouped for maximal alignment into six distinct nucleotide sequences. The crotamine cDNAs, with 340-360 bases, encompass open reading frame of 198 nucleotides with 5' and 3' untranslated regions of variable size, signal peptide sequence, one crotamine isoform message, and putative poly(A+) signal. Of these six different crotamine cDNA precursors, two predict the identical amino acid sequence previously described by Laure (1975), and the other four a crotamine isoform precursor where the Leucine residue at position 19 is replaced by isoleucine by a single base change. On the other hand, nucleotide variation was observed in the 5' and 3' untranslated regions, with one interesting variant containing an 18 base pair deletion at the 5' untranslated region which results in the usual ATG initiator being replaced by the rarely used GUG start codon. Comparison by Northern blot analysis of poly(A+) RNA from venom glands of a crotamine-plus specimen to total and poly(A+) RNA from a crotamine-minus snake indicated that crotamine transcripts were not expressed in the crotamine-minus specimen.

Effect of Bothrops insularis venom on the mouse and chick nerve-muscle preparation

The effects of Bothrops insularis venom were examined in vivo in mice and chicks and in vitro using the mouse phrenic nerve diaphragm and chick biventer cervicis muscle preparations. Incubation of the indirectly or directly stimulated mouse preparation with B. insularis venom (20-80 micrograms/ml) produced an initial increase in twitch tension followed by irreversible blockade. With direct stimulation in the presence of D-tubocurarine, no increase in twitch tension was observed prior to the onset of blockade. A venom-induced effect on presynaptic activity was suggested by the marked increase in the frequency of the mepps recorded in vitro 5-15 min after venom addition. A direct muscular effect was shown by the dose- and time-dependent reduction in the resting membrane potential of the diaphragm. Chick preparations were more sensitive than those of the mouse. In the isolated chick biventer cervicis muscle preparation, B. insularis venom induced a contracture and a dose-dependent block of responses to indirect stimulation. At low venom concentrations (1-5 micrograms/ml), no significant release of creatine kinase (CK) was observed from this preparation. However, a dose-dependent release of CK was detected at higher doses (10-80 micrograms/ml). For morphological studies, B. insularis venom was injected into the chick left pectoralis muscle. At low doses (0.4 microgram), only an inflammatory reaction was present, while at high doses (20-80 micrograms) increasing numbers of necrotic fibers were observed as well as occlusive thrombosis and hemorrhage. The muscular effect, also observed on the incubated muscle, points to a direct myolytic action of the whole venom.

Crotoxin, a phospholipase A2 neurotoxin from snake venom, interacts with epithelial mammary cells, is internalized and induces secretion

Prolactin (PRL) induces liberation of arachidonic acid (AA) from phospholipids of lactating mammary epithelial cells and stimulates casein secretion. In order to investigate the possible involvement of phospholipase A2 (PLA2) activity in the hormonal control of casein secretion by PRL, we examined the effects of crotoxin, a PLA2 neurotoxin from snake venom, on mammary epithelial cells. Crotoxin is made of two subunits: a basic PLA2 with low toxicity (component B, CB) and an acidic, non-toxic and enzymatically inactive component A (CA) which enhances the pharmacological action of CB. While CA is inactive, the PLA2 subunit (CB) induces an accumulation of secretory products in the lumen of mammary acini, an extensive development of the Golgi apparatus. The secretion of newly synthesized casein is increased in the presence of CB and this effect is inhibited by nordihydroguaiaretic acid (NDGA) and caffeic acid, two inhibitors of the lipoxygenase pathway which also prevent stimulation of secretion by PRL. Further, CB transiently induces the release of radiolabelled AA from mammary tissues previously labelled with [14C]AA, the highest release being observed between 15 s and 5 min of contact with CB and CA. Immunofluorescence labelling by anti-CB antibodies of epithelial mammary tissues previously incubated with CA, CB or a combination of CA and CB indicates that CB binds to epithelial cells and is internalized, at least in part, and that CA enhances both CB binding and its internalization. These observations emphasize the involvement of PLA2 in the control of casein secretion and suggest that PLA2 acts intracellularly.

Multiplicity of acidic subunit isoforms of crotoxin, the phospholipase A2 neurotoxin from Crotalus durissus terrificus venom, results from posttranslational modifications

Crotoxin, the major toxin of the venom of the South American rattlesnake, Crotalus durissus terrificus, is made of two subunits: component B, a basic and weakly toxic phospholipase A2, and component A, an acidic and nontoxic protein that enhances the lethal potency of component B. Crotoxin is a mixture of isoforms that results from the association of several isoforms of its two subunits. In the present investigation, we have purified four component A isoforms that, when associated with the same purified component B isoform, produced different crotoxin isoforms, all having the same specific enzymatic activity and the same lethal potency. We further determined by Edman degradation the polypeptide sequences of these four component A isoforms. They are made of three disulfide-linked polypeptide chains (alpha, beta, and gamma) that correspond to three different regions of a phospholipase A2 precursor. We observed that the polypeptide sequences of the various component A isoforms all agree with the sequence of an unique precursor. The differences between the isoforms result first by differences in the length of the various chains alpha and beta, indicating that component A isoforms are generated from the proteolytic cleavage of the component A precursor at very close sites, possibly by the combined actions of endopeptidases and exopeptidases, and second by the possible cyclization of the alpha-NH2 of the N-terminal glutamine residue of chains beta and gamma. These observations indicate that the component A isoforms are the consequence of different posttranslational events occurring on an unique precursor, rather than the expression of different genes.

The regulation of quantal size

Quantal size can be altered experimentally by numerous treatments that seem to lack any common thread. The observations may seem haphazard and senseless unless clear distinctions are made from the outset. Some treatments shift the size of the entire population of quanta. These quanta are released by nerve stimulation. Other treatments add quanta of abnormal size or shape--monstrosities--to the population (4.0). Usually, perhaps even invariably, the monstrosities are not released by nerve stimulation. 6.1. POPULATION SIZE INCREASES. 6.1.1. Quantal size must be regulated. The size of the entire quantal population can be experimentally shifted to a larger size, with the mean rising two- or even four-fold. Before these observations, it was reasonable to suppose that quantal size was relatively fixed, with little room for maneuver. A logical picture is that synaptic vesicles have a maximum transmitter capacity, and usually they are filled to the brim. This picture is wrong. The quantity of transmitter packaged in the quantum must be regulated by the neuron, so depending on circumstances, quantal size can be increased or decreased. Figure 18 makes the case for regulation more strongly than words. We are beginning to identify some of the signals for up and down regulation, and the first steps have been made in discovering the signal transduction pathways, but we are far from a true understanding. This is hardly surprising, because our information about how transmitter molecules are assembled into quantal packages is still imperfect. Until we understand the engine, it may be difficult to picture the accelerator or the brake. 6.1.2. Signals that up regulate size. Stimulation of the presynaptic neuron increases quantal size at the NMJ, at synapses in autonomic ganglia and in hippocampus. The stimulus parameters necessary to elicit the quantal size increase have not been explored sufficiently in any of these cases, and all deserve further investigation. At both frog and mouse NMJs quantal size is roughly doubled following exposure to hypertonic solutions, which elevate the rate of spontaneous quantal release. This discovery, coupled with the increases caused by tetanic stimulation, suggested that the signal for up regulation is a period of greatly enhanced quantal output. The size increase takes about 15 min in hypertonic solution in mouse and about 60 min in frog. Highly hypertonic solutions do not increase the rate of quantal release in frog; they also do not increase quantal size. This supported the idea that quantal release rate is the signal for up regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning and nucleotide sequences of crotamine genes

A cDNA library containing snake toxin genes was constructed in bacteriophage lambda by using mRNA isolated from the glands of the South American rattlesnake, Crotalus durissus terrificus. The first high-density screening of 400,000 plaques for crotamine-containing genes yielded over 800 positives when a labeled cDNA probe with sequence homology to crotamine was used. Four of these clones with insert sizes from 270 to 400 base pairs were chosen and their inserts subcloned into pGEM-3Z and sequenced. Nucleotide sequence analysis of the cloned cDNAs predicted the existence of multiple variants of the crotamine toxin. The different forms, identified from the DNA sequences, displayed discrepancies in amino acid sequence for crotamine when compared with previously published reports. Direct amino acid sequencing of commercially purified crotamine and CNBr fragments thereof confirmed the structures predicted by the nucleic acid sequences.

Binding of divalent and trivalent cations with crotoxin and with its phospholipase and its non-catalytic subunits: effects on enzymatic activity and on the interaction of phospholipase component with phospholipids

We have studied the interaction of divalent and trivalent with a potent phospholipase A(2) neurotoxin, crotoxin, from Crotalus durissus terrificus venom. The pharmacological action of crotoxin requires dissociation of its catalytic subunit (component B) and of its non-enzymatic chaperone subunit (component A), then the binding of the phospholipase subunit to target sites on cellular membranes and finally phospholipid hydrolysis. In this report, we show that the phospholipase A(2) activity of crotoxin and of component B required Ca2+ and that other divalent cations (Sr2+, Cd2+ and Ba2+) and trivalent lanthanide ions are inhibitors. The lowest phospholipase A(2) activity was observed in the presence of Ba2+, which proved to be a competitive inhibitor of Ca2+. The binding of divalent cations and trivalent lanthanide ions to crotoxin and to its subunits has been examined by equilibrium dialysis and by spectrofluorimetric methods. We found that crotoxin binds two divalent cations per mole with different affinities; the site presenting the highest affinity (K(d) in the mM range) in involved in the activation (or inhibition) of the phospholipase A(2) activity and must therefore be located on component B, the other site (K(d) higher than 10 mM) is probably localized on component A and does not play any role in the catalytic activity of crotoxin. We also observed that crotoxin component B binds to vesicular and micellar phospholipids, even in the absence of divalent cations. The affinity of this interaction either does not change or else increases by an order of magnitude in the presence of divalent cations.

Giant miniature end-plate potentials at the untreated and emetine-treated frog neuromuscular junction

1. Intracellular recordings from the cutaneous pectoris muscle fibres of the frog showed that giant miniature end-plate potentials (gMEPPs) occurred in untreated preparations. Emetine (10 microM), after a 15-20 min delay, increased the frequency of gMEPPs. In both cases gMEPPs disappeared in the presence of (+)-tubocurarine. 2. There was no correlation between the frequency of gMEPPs and the frequency of normal MEPPs (nMEPPs) in untreated or emetine-treated fibres. 3. Tetraphenylborate (TPB, 50 microM) applied to muscles pre-treated with emetine caused the frequency and amplitude of both nMEPPs and gMEPPs to decrease gradually. All MEPPs disappeared in about 10 min. 4. Chloride permeability was modified by changing the pH of the Ringer solution. Changing the pH from 7.2 to 8.2 or 6.2, which in both cases caused marked increases in nMEPP frequency, had no significant effect on gMEPP frequency in untreated muscles. 5. Decreasing the pH from 7.2 to 6.2 blocked the ability of emetine to increase gMEPP frequency. Increasing the pH from 7.2 to 8.2 had no significant effect on emetine ability to increase gMEPP frequency. 6. Treatment with the Cl- channel blocker SITS (0.5 mM) had no effect on gMEPPs in untreated muscle or on the ability of emetine to increase the frequency of these potentials. 7. The Ca2+ sensitivity of gMEPPs in untreated or emetine-treated muscles was tested by treatments which are known to alter intracellular Ca2+. Raising extracellular Ca2+ (10 mM), treatment with Mn2+ (10 mM), Mg2+ (10 mM), K+ (7.5 mM), hypotonic solution, ouabain (0.2 mM) or ethanol (0.5 M), although causing profound changes in nMEPP frequency had no significant effect on gMEPP frequency in untreated fibres or on the ability of emetine to increase gMEPP frequency. 8. It is concluded that at the frog neuromuscular junction generation of the normally occurring or emetine-induced gMEPPs is independent of Ca2+ and does not seem to be influenced by changing membrane C1- permeability.

Neutralization of lethal potency and inhibition of enzymatic activity of a phospholipase A2 neurotoxin, crotoxin, by non-precipitating antibodies (Fab)

Rabbit antibodies were prepared against both purified catalytic (component-B) and purified non-catalytic (component-A) subunits of crotoxin, the major phospholipase A2 neurotoxin from the South American rattlesnake. They cross-react with crotoxin-like toxins from the venom of several Crotalus species as well as with single-chain phospholipase A2 neurotoxins from Crotalid and Viperid venoms (agkistrodontoxin and ammodytoxin A) but not from Elapid venoms (notexin). Immunological cross-reactions of anti-component-A and anti-component-B sera with crotoxin and with its isolated components A and B showed that component-A exposes determinants of low immunogenicity which are present on component-B, whereas the major antigenic determinants of component-B are not present on component-A. Anti-component-B antibodies, but not anti-component-A antibodies, neutralize the lethal potency of crotoxin and inhibit its enzymatic activity. Furthermore, non-precipitating anti-component-B Fab fragments were as potent as antibodies, indicating that crotoxin neutralization results from the binding of the antibodies to the catalytic subunit, rather than the formation of an immunoprecipitate.

The importance of phospholipase A2 in the early induction by crotoxin of biphasic changes in endplate potentials at the frog neuromuscular junction

In Ca2+-Ringer, crotoxin induced a rapid fall in amplitude of endplate potentials followed by a marked secondary rise but, in Ca2+-free, Sr2+-EGTA Ringer, crotoxin failed to elicit either response. In Ca2+-washout experiments, the onset of giant miniature endplate potentials indicated that binding of crotoxin occurred in Sr2+-EGTA Ringer. Facilitation of endplate potential amplitude due to a closely spaced twin impulse was increased to 162% of control near the peak of the crotoxin-induced secondary phase. These findings are consistent with the phospholipase A2 subunit acting specifically at the active zone.

Early induction by crotoxin of biphasic frequency changes and giant miniature endplate potentials in frog muscle

1. Following the addition of crotoxin (250 nM) at the frog neuromuscular junction, there was an initial fall in frequency of miniature endplate potentials (m.e.p.ps), followed by a secondary rise which was characterized by the appearance of large spontaneous potentials (giants, g.m.e.p.ps) and an occasional large potential of the burst type. 2. In the presence of 2-(4-phenylpiperidino)cyclohexanol (AH5183, vesicamol), an inhibitor of vesicular acetylcholine uptake, the frequency of g.m.e.p.ps induced by crotoxin was reduced. 3. The characteristic changes in m.e.p.p. frequency and amplitude distribution were absent with crotoxin in Sr-EGTA Ringer. In the presence of high concentrations of Mn (3.6 or 5.4 mM with 0.9 mM Ca), the crotoxin-induced initial fall and the onset of the secondary rise in m.e.p.p. and g.m.e.p.p. frequencies were slower. The timing of these phases was unaffected by Ca concentrations ranging from 6.3 to 0.9 mM. 4. High concentrations of Mn ions partially inhibited the phospholipase A2 activity of crotoxin on artificial phospholipid membranes. This also supports the involvement of the Ca-dependent phospholipase A2 subunit in both phases of the physiological action of the toxin. 5. G.m.e.p.ps were associated with a moderate increase in m.e.p.p. frequency (2-3 s-1) and were of a time-course similar to that of m.e.p.ps. They persisted after washing with medium lacking Ca ions and in the presence of Ca-Mn Ringer that blocked evoked responses. 6. It is concluded that crotoxin, acting through its phospholipase A2 subunit, produces specific disturbances of synaptic exocytosis and vesicle formation in the axolemma of the motor nerve terminal which lead to biphasic changes in m.e.p.p. frequency and the onset of large spontaneous potentials.

Emetine increases giant miniature endplate potential population at the frog neuromuscular junction

Intracellular recordings from 90 endplates in untreated cutaneous pectoris muscles of the frog showed that giant miniature endplate potentials (gMEPPs) occurred at a low frequency of 0.017 +/- 0.002 s-1. These spontaneous potentials are several times larger in amplitude and have time to peak twice longer than the normal MEPPs (nMEPPs). The frequency of gMEPPs was not significantly influenced by the presence or absence of tetrodotoxin, calcium or high Mg2+. The frequency of gMEPPs was not dependent on the frequency of nMEPPs nor was it influenced by prior prolonged motor nerve stimulation. After a delay of 10-20 min, the antiamebic drug, emetine, (10 microM) increased the frequency of gMEPPs by about 10-fold without affecting nMEPP frequency. The emetine-induced gMEPPs were very similar in characteristics to gMEPPs occurring in untreated muscles. The ability of emetine to increase the gMEPP frequency was not affected by the absence of extracellular calcium. The results indicate that emetine enhances the frequency of the calcium-independent gMEPPs that are normally seen in untreated muscles.

Effects of crotoxin on autonomic neuromuscular transmission in the guinea-pig myenteric plexus and vas deferens

The effects of crotoxin, the neurotoxic complex from the venom of the South American rattlesnake Crotalus durissus terrificus on mammalian autonomic neuromuscular transmission, have been investigated. In the longitudinal muscle of the guinea-pig ileum, crotoxin induced a dose-dependent contraction which was followed by relaxation, in spite of the continued presence of the toxin. The contractile response was inhibited by indomethacin, tetrodotoxin, verapamil or nifedipine, but was unaffected by atropine, propranolol, mepyramine or methysergide. In addition, crotoxin caused a presynaptic inhibition of the electrically-evoked twitch of the longitudinal muscle of the guinea-pig ileum. In the guinea-pig vas deferens crotoxin also caused an inhibition of the response to field stimulation. The inhibition was reversible after washing and the preparation remained insensitive to further doses of the toxin. The inhibitory effects of crotoxin were not mediated by noradrenaline and were not due to a non-specific smooth muscle depression, because it was not associated with any reduction in motor responses to acetylcholine, ATP, bradykinin or substance P. Pre-incubation of the guinea-pig vas deferens with indomethacin blocked the inhibitory effects of the toxin. This suggests that the presynaptic activity of crotoxin in the vas deferens might be mediated by prostaglandins.

Extracellular chloride replacement by isethionate induces abnormal spontaneous release of transmitter at the frog neuromuscular junction

1 Replacement of chloride by isethionate in Ringer solution bathing frog skeletal muscle fibres induces, after a delay of about 30 min, marked mechanical activity which was blocked by tubocurarine. This effect is reversed by washing out the isethionate. 2 Miniature end plate potentials (m.e.p.ps) and giant potentials (potentials greater than or equal to 2 X modal value) were recorded intracellularly in normal Ringer and isethionate Ringer solution. 3 The frequency of m.e.p.ps was unaltered by isethionate. The proportion of giant potentials increased from 3% in normal Ringer to 24.5% in isethionate Ringer after 90 min. This effect is usually reversible if the exposure to isethionate does not exceed 2 h. 4 The giant potentials were large enough to initiate trains of action potentials and still occurred in the presence of tetrodotoxin or Ca2+-free Ringer. Isethionate produced no change in the tau D of miniature endplate currents. 5 Chloride replacement by propionate produced no change in the proportion of giant potentials. 6 It is suggested that the isethionate anion can induce giant potentials and the possible mechanism of action is discussed.

A comparison of the effects of acute and chronic cholinesterase inactivation on spontaneous transmitter release

The irreversible inhibitor of acetylcholinesterase (AChE), paraoxon, when given in vivo to rats in a single injection (0.23 mg/kg s.c.) raised the miniature endplate potential (MEPP) frequency to values greater than 3 times control levels in 34% of the fibers in the rat phrenic nerve-hemidiaphragm preparation. The elevated MEPP frequencies were observed in areas of extensive muscle twitching and were associated with high frequencies of giant MEPPs. Following 3 daily injections of paraoxon; the overall MEPP frequency was reduced below control levels, the frequency of giant MEPPs returned to normal, and a greater percentage of fibers showed no spontaneous activity. This depressant effect of chronic AChE inhibition on the overall MEPP frequency diminished during 1-2 weeks of daily paraoxon treatment (0.12 mg/kg s.c. paraoxon/day). After one week of recovery from 14 daily paraoxon injections (0.12 mg/kg, 1 injection/day), the original response to a single injection (0.23 mg/kg) was restored. In an attempt to determine whether paraoxon exerts its effects on spontaneous release by depolarizing the presynaptic terminal, the effect of increases in the potassium concentration on the MEPP and giant potential frequency were examined in control (saline injection) preparations, and preparations treated with 1 or 3 daily injections of paraoxon. The results suggest that paraoxon does not act by reducing the presynaptic membrane potential, but may interact more directly with the mechanism(s) responsible for regulating the release of MEPPs and giant MEPPs.

4-aminoquinoline-induced 'giant' miniature endplate potentials at mammalian neuromuscular junctions

4-Aminoquinoline (4-AQ) in concentrations around 200 micrometers induces, within minutes of its application to isolated mouse or rat neuromuscular junctions, the appearance of a population of miniature endplate potentials (m.e.p.ps) with a larger than normal amplitude, so-called giant m.e.p.ps (g.m.e.p.ps). With amplitudes 2-12 times the modal value of m.e.p.p. amplitude, the population of g.m.e.p.ps varied between 15 and 45% of the total population of m.e.p.ps. There was no increase in the frequency of m.e.p.ps but a positive correlation between the frequency of g.m.e.p.ps and the total frequency of m.e.p.ps. In many instances the rise time and decay time of g.m.e.p.ps were prolonged compared to normal. Elevated extracellular calcium concentrations increased the frequency of m.e.p.ps but had no effect on g.m.e.p.p. frequency. High extracellular potassium concentrations markedly increased m.e.p.p. frequency but failed to influence g.m.e.p.p. frequency. Similar observations were made with ethanol 0.1 M, ouabain 200 micrometers or black widow spider venom. Botulinum toxin type A markedly reduced total m.e.p.p. frequency but 4-AQ still induced g.m.e.p.ps. Nerve stimulation failed to release quanta corresponding to the g.m.e.p.ps. G.m.e.p.ps seemed to originate from quantal acetylcholine release from the nerve terminal since they were abolished by surgical denervation and by the addition of d-tubocurarine to the medium. Blockade of voltage-sensitive calcium or sodium channels by, respectively, manganese ions or tetrodotoxin failed to affect the appearance and the frequency of g.m.e.p.ps. The electrophysiological findings and a statistical analysis of the characteristics of the m.e.p.ps indicate that they belong to two populations. One population is accelerated by the depolarization-release coupling mechanism responsible for evoked transmitter release and is characterized by an amplitude distribution and a process in time that indicate that they correspond to releases occurring at 'active zones' in the nerve terminal. The second population of m.e.p.ps is uninfluenced by nerve terminal depolarization and transmembrane calcium fluxes. This population apparently originates from sites dispersed in the nerve terminal membrane and outside the 'active zones'. 4-AQ increases the frequency of this second m.e.p.p. population without affecting the first population.

Synergism of the two subunits of crotoxin

Crotoxin, a potent neurotoxin from the venom of Crotalus durissus terrificus, is composed of an acidic subunit which is non-toxic and enzymatically inactive and a basic subunit which possesses a phospholipase activity and low toxicity. It is shown that crotoxin very efficiently blocks the cholinergic post-synaptic response of the isolated electroplaque from Electrophorus electricus and of cellular microsacs fromTorpedo marmorata. This post-synpatic effects was investigated by studying the binding of crotoxin and its isolated subunits to acetylcholine receptor-rich membranes from Torpedo and by analysing the relationship between its catalytic activity and its pharmacological effects on Electrophorus electroplaque. The mechanism of action of crotoxin could be divided into two distinct steps: a quasi irreversible binding step, which has no blocking action by itself, and a catalytic step, which irreversibly inhibits the postsynaptic response. These results suggest that the non-enzymatic subunit of crotoxin enhances the pharmacological efficiency of the phospholipase by preventing its adsorption to non-saturable binding sites, restricting its binding to specific critical target sites. These sites are distinct from the cholinergic receptor sites, but probably closely related to them, as suggested by their approximately equal number and by the fact that after crotoxin action the receptor appears to be blocked in a molecular form very similar to the desensitized state. The mechanism proposed explains in simple terms the synergistic action of the two subunits of crotoxin at the level of the cholinergic receptor-ionophore assembly.

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.