kappa-Bungarotoxin: binding of a neuronal nicotinic receptor antagonist to chick optic lobe and skeletal muscle

Structural and functional characterization of a novel homodimeric three-finger neurotoxin from the venom of Ophiophagus hannah (king cobra)

Snake venoms are a mixture of pharmacologically active proteins and polypeptides that have led to the development of molecular probes and therapeutic agents. Here, we describe the structural and functional characterization of a novel neurotoxin, haditoxin, from the venom of Ophiophagus hannah (King cobra). Haditoxin exhibited novel pharmacology with antagonism toward muscle (alphabetagammadelta) and neuronal (alpha(7), alpha(3)beta(2), and alpha(4)beta(2)) nicotinic acetylcholine receptors (nAChRs) with highest affinity for alpha(7)-nAChRs. The high resolution (1.5 A) crystal structure revealed haditoxin to be a homodimer, like kappa-neurotoxins, which target neuronal alpha(3)beta(2)- and alpha(4)beta(2)-nAChRs. Interestingly however, the monomeric subunits of haditoxin were composed of a three-finger protein fold typical of curaremimetic short-chain alpha-neurotoxins. Biochemical studies confirmed that it existed as a non-covalent dimer species in solution. Its structural similarity to short-chain alpha-neurotoxins and kappa-neurotoxins notwithstanding, haditoxin exhibited unique blockade of alpha(7)-nAChRs (IC(50) 180 nm), which is recognized by neither short-chain alpha-neurotoxins nor kappa-neurotoxins. This is the first report of a dimeric short-chain alpha-neurotoxin interacting with neuronal alpha(7)-nAChRs as well as the first homodimeric three-finger toxin to interact with muscle nAChRs.

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Presynaptic nicotinic receptors and the modulation of transmitter release

Nicotine is increasingly recognized to promote transmitter release in the brain by a direct action on presynaptic terminals. Pharmacological evidence indicates that this action is mediated by nicotinic receptors. From their sensitivity to mecamylamine, neosurugatoxin and neuronal bungarotoxin these presynaptic receptors can be distinguished from alpha-bungarotoxin-sensitive muscle-type nicotinic receptors, and can be correlated with [3H] nicotine binding sites in the brain. The release of many transmitters in different brain regions is susceptible to stimulation by nicotine, but this effect is not ubiquitous. However, lesioning and subcellular fractionation studies suggest that the majority of brain nicotine receptors are located presynaptically, so that a direct influence of nicotine on transmitter release assumes considerable importance. Although the sensitivity of presynaptic receptors is such that they are likely to be partially activated by doses of nicotine obtained by smoking, the desensitization-induced up-regulation of nicotinic binding sites that follows chronic nicotine treatment raises questions about their functional status during tobacco usage. Chronic administration of the agonist (+)anatoxin-a also up-regulated [3H] nicotine binding sites, and led to increased nicotine-evoked transmitter release in vitro. This could have implications for the involvement of these receptors during withdrawal.

Snake postsynaptic neurotoxins: gene structure, phylogeny and applications in research and therapy

Snake venoms are complex mixtures of biologically active polypeptides that target a variety of vital physiological functions in mammals. alpha-Neurotoxins, toxins that cause paralysis by binding to the nicotinic receptors at the postsynaptic region of the neuromuscular junction have been widely studied in terms of their structure-function relationships as well as gene structure, organization and expression. In this review, we describe the structure of alpha-neurotoxin genes and discuss their evolutionary relationships. Almost all members of neurotoxins have been found to exhibit a common evolutionary origin. The importance of alpha-neurotoxins in therapy and research has also been discussed to highlight their potential applications especially in the area of drug discovery.

Only snake curaremimetic toxins with a fifth disulfide bond have high affinity for the neuronal alpha7 nicotinic receptor

Long chain and short chain curaremimetic toxins from snakes possess 66-74 residues with five disulfide bonds and 60-62 residues with four disulfide bonds, respectively. Despite their structural differences all of these toxins bind with high affinity to the peripheral nicotinic acetylcholine receptors (AChR). Binding experiments have now revealed that long chain toxins only, like the neuronal kappa-bungarotoxin, have a high affinity for a chimeric form of the neuronal alpha7 receptor, with Kd values ranging from about 1 to 12 nM. In contrast, all other toxins bind to the chimeric alpha7 receptor with a low affinity, with Kd values ranging between 3 and 22 microM. These results are supported by electrophysiological recordings on both the wild-type and chimeric alpha7 receptors. Amino acid sequence analyses have suggested that high affinities for the neuronal receptor are associated with the presence of the fifth disulfide at the tip of the toxin second loop. In agreement with this conclusion, we show that a long chain toxin whose fifth disulfide is reduced and then dithiopyridylated has a low affinity (Kd = 12 microM) for the neuronal alpha7 receptor, whereas it retains a high affinity (Kd = 0.35 nM) for the peripheral AChR. Thus, a long chain curaremimetic toxin having a reduced fifth disulfide bond behaves like a short chain toxin toward both the peripheral and neuronal AChR. Therefore, functional classification of toxins that bind to AChRs should probably be done by considering their activities on both peripheral and neuronal receptors.

Nicotinic acetylcholine receptors in separate brain regions exhibit different affinities for methyllycaconitine

The family of nicotinic acetylcholine receptors contains numerous subtypes. Since the subunit compositions of most native neuronal nicotinic receptors are unknown, an important method for distinguishing subtypes of functional neuronal receptors is based on pharmacological criteria, such as affinity for snake toxins. We have now examined the affinities of native chick nicotinic receptors for methyllycaconitine, a toxin purified from Delphinium. We find that methyllycaconitine is a potent antagonist at central nicotinic receptors located on Edinger-Westphal neurons, producing nearly complete functional blockade of nicotinic responses at 10 nM. In marked contrast, methyllycaconitine is 1000-fold less potent at blocking nicotinic responses in the lateral spiriform nucleus. Methyllycaconitine inhibits kappa-bungarotoxin-sensitive nicotinic receptors in ciliary ganglia at 0.5-1.0 microM. Radioligand binding studies also reveal heterogeneity in the affinity of the toxin for nicotinic receptors. Methyllycaconitine binds most avidly to [125I] alpha-bungarotoxin sites in brain (Ki = 5.4 nM), and is 200-fold less potent at muscle nicotinic receptors (IC50 = 1.1 microM). The least potent binding of the toxin is to [3H]nicotine sites in brain (Ki = 3.7 microM). Methyllycaconitine is thus a useful pharmacological tool for distinguishing certain subtypes of native nicotinic receptors. The relatively low affinity of the toxin for nicotinic receptors in the lateral spiriform nucleus is consistent with the known properties of these receptors, which include a high affinity for [3H]nicotine and a lack of sensitivity to alpha- and kappa-bungarotoxin. On the basis of high affinity for methyllycaconitine and insensitivity to alpha-bungarotoxin, the nicotinic receptors in the Edinger-Westphal nucleus are unlike any previously described nicotinic receptor subtype.

alpha-Bungarotoxin, kappa-bungarotoxin, alpha-cobratoxin and erabutoxin-b do not affect [3H]acetylcholine release from the rat isolated left hemidiaphragm

Endplate preparations of the rat left hemidiaphragm were incubated with [3H]choline to label neuronal transmitter stores. Nerve evoked release of newly-synthesized [3H]acetylcholine was measured in the absence of cholinesterase inhibitors to investigate whether snake venom neurotoxins by blocking presynaptic nicotinic autoreceptors affect evoked transmitter release. Contractions of the indirectly stimulated hemidiaphragm were recorded to characterize the blocking effect of alpha-neurotoxins at the post-synaptic nicotinic receptors. Neither the long chain neurotoxins alpha-cobratoxin (1 microgram ml-1) and alpha-bungarotoxin (5 microgram ml-1) nor the short chain neurotoxin erabutoxin-b (0.1, 1 and 10 micrograms ml-1) affected the nerve-evoked release of [3H]acetylcholine. kappa-Bungarotoxin (1 and 5 micrograms ml-1), a toxin preferentially blocking neuronal nicotinic receptors, did also not affect evoked [3H]acetylcholine release, whereas (+)-tubocurarine (1 microM) under identical conditions reduced the release by about 50%. alpha-Bungarotoxin, alpha-cobratoxin and erabutoxin-b concentration-dependently (0.01-0.6 micrograms ml-1) inhibited nerve-evoked contractions of the hemidiaphragm. All neurotoxins except erabutoxin-b enhanced the basal tritium efflux immediately when applied to the endplate preparation or to a non-innervated muscle strip labelled with [3H]choline. This effect was attributed to an enhanced efflux of [3H]phosphorylcholine, whereas the efflux of [3H]choline and [3H]acetylcholine was not affected. It is concluded that the alpha-neurotoxins and kappa-bungarotoxin do not block presynaptic nicotinic receptors of motor nerves. These nicotinic autoreceptors differ from nicotinic receptors localized at the muscle membrane and at autonomic ganglia.

Structure of beta 2-bungarotoxin: potassium channel binding by Kunitz modules and targeted phospholipase action

BACKGROUND

beta-bungarotoxin is a heterodimeric neurotoxin consisting of a phospholipase subunit linked by a disulfide bond to a K+ channel binding subunit which is a member of the Kunitz protease inhibitor superfamily. Toxicity, characterized by blockage of neural transmission, is achieved by the lipolytic action of the phospholipase targeted to the presynaptic membrane by the Kunitz module.

RESULTS

The crystal structure at 2.45 A resolution suggests that the ion channel binding region of the Kunitz subunit is at the opposite end of the module from the loop typically involved in protease binding. Analysis of the phospholipase subunit reveals a partially occluded substrate-binding surface and reduced hydrophobicity.

CONCLUSIONS

Molecular recognition by this Kunitz module appears to diverge considerably from more conventional superfamily members. The ion channel binding region identified here may mimic the regulatory interaction of endogenous neuropeptides. Adaptations of the phospholipase subunit make it uniquely suited to targeting and explain the remarkable ability of the toxin to avoid binding to non-target membranes. Insight into the mechanism of beta-bungarotoxin gained here may lead to the development of therapeutic strategies against not only pathological cells, but also enveloped viruses.

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.

Characterization of nicotinic receptor-mediated [3H]dopamine release from synaptosomes prepared from mouse striatum

This study establishes that presynaptic nicotinic receptors modulate dopamine release in the mouse striatum. Nicotinic agonists elicit a dose-dependent increase in the release of [3H]dopamine from synaptosomes prepared from mouse striatum. At low concentrations, this release is Ca2+ dependent, whereas at higher concentrations Ca(2+)-independent, mecamylamine-insensitive release was also observed. The Ca(2+)-dependent nicotine-evoked release was not blocked by alpha-bungarotoxin but was effectively blocked by neuronal bungarotoxin as well as several other nicotinic receptor antagonists. The relationship between potency for stimulation of release for agonists and potency for inhibition of release for antagonists was compared to the affinity of these compounds for the [3H]nicotine binding site. The overall correlation between release and binding potency was not high, but the drugs may be classified into separate groups, each of which has a high correlation with binding. This finding suggests either that more than one nicotinic receptor regulates dopamine release or that not all agonists interact with the same receptor in an identical fashion.

Synthesis and expression in Escherichia coli of a gene for kappa-bungarotoxin

A gene which codes for the 66-residue polypeptide of kappa-bungarotoxin has been chemically synthesized by linking together 3 synthetic double-stranded oligonucleotides in a bacterial plasmid. The synthesis incorporated six unique silent restriction sites spaced throughout the gene for use in cassette mutagenesis. Direct expression of the kappa-bungarotoxin polypeptide by itself in Escherichia coli failed to result in a stable product. The toxin polypeptide was stabilized and expressed in E. coli as part of a fusion protein with rat intestinal fatty acid binding protein under control of the nalidixic acid inducible recA promoter. Two fusion protein constructs were prepared that differed only in the cleavage site between the fatty acid binding protein and the toxin polypeptide. One contained a factor Xa cleavage site, and the other, since the toxin itself is devoid of methionine, contained a methionyl residue that served as a cyanogen bromide cleavage site. The fusion proteins were isolated by ion-exchange chromatography and reverse-phase HPLC. The construct containing the factor Xa cleavage site could not be cleaved under nondenaturing conditions. On the other hand, kappa-bungarotoxin was efficiently cleaved from the methionyl fusion protein with CNBr. The toxin polypeptide was isolated by reverse-phase HPLC and ion-exchange chromatography and produced a complete and specific blockade of neuronal nicotinic acetylcholine receptors in chick ciliary ganglia which was indistinguishable from that produced by a comparable amount of venom-purified kappa-bungarotoxin.

Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed alpha 2, alpha 3, alpha 4 ...) and structural (termed beta 2, beta 3, beta 4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the alpha 4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as beta 2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that alpha 3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while alpha 4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that alpha 3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina.

Nicotinic acetylcholine receptor subtypes in human frontal cortex: changes in Alzheimer's disease

Molecular genetic and pharmacological studies have suggested that several subtypes of nicotinic acetylcholine receptors exist in the mammalian and avian brain. Combining 3H-(-)-nicotine, 125I-alpha-bungarotoxin, and 125I-kappa-bungarotoxin as ligands, we report here the first evidence for the existence in human frontal cortex of at least three different subtypes of nicotinic receptors. Autoradiographic analysis shows that specific 125I-kappa-bungarotoxin binding sites are concentrated mainly in several cortical layers. We also show that kappa-bungarotoxin, but not alpha-bungarotoxin decreases the evoked release of 3H-acetylcholine in rat cortical slices, indicating a likely presynaptic localization for some of the alpha-bungarotoxin-insensitive kappa-bungarotoxin sites in mammalian brain. The brains of patients with Alzheimer's disease show marked decreases in Bmax values for low-affinity 125I-kappa-bungarotoxin sites and both high- and low-affinity 3H-nicotine sites, whereas 125I-alpha-bungarotoxin sites are not significantly different in number from age-matched control brains. We conclude that Alzheimer's disease does not affect all subtypes of nicotinic receptors in the frontal cortex to the same extent.

Intracellular recording in avian brain of a nicotinic response that is insensitive to K-bungarotoxin

We examined nicotinic acetylcholine receptors in the avian brain using a combination of autoradiographic and intracellular electrophysiological techniques. We found that the lateral spiriform nucleus (SPL) in the mesencephalon has a very high density of 3H-nicotine binding sites but no detectable 125I-K-bungarotoxin (125I-K-BuTx) or 125I-alpha-bungarotoxin (125I-alpha-BuTx) bindings sites. Intracellular recordings in brain slices revealed that SPL neurons depolarize in response to nicotine and carbachol (in the presence of atropine). These depolarizations were blocked by the classic nicotinic antagonists d-tubocurarine and dihydro-beta-erythroidine. As predicted for nicotinic receptors with a high affinity for nicotine, neither K-BuTx nor alpha-BuTx blocked these nicotinic responses. Thus, although the existence of high-affinity 3H-nicotine binding sites has been known for some time, we now report the in situ detection of a functional nicotinic receptor that has a high affinity for nicotine and is K-BuTx-insensitive.

Neurotoxins distinguish between different neuronal nicotinic acetylcholine receptor subunit combinations

Neuronal and muscle nicotinic acetylcholine receptor subunit combinations expressed in Xenopus oocytes were tested for sensitivity to various neurotoxins. Extensive blockade of the alpha 3 beta 2 neuronal subunit combination was achieved by 10 nM neuronal bungarotoxin. Partial blockade of the alpha 4 beta 2 neuronal and alpha 1 beta 1 gamma delta muscle subunit combinations was caused by 1,000 nM neuronal bungarotoxin. The alpha 2 beta 2 neuronal subunit combination was insensitive to 1,000 nM neuronal bungarotoxin. Nearly complete blockade of all neuronal subunit combinations resulted from incubation with 2 nM neosurugatoxin, whereas 200 nM neosurugatoxin was required for partial blockade of the alpha 1 beta 1 gamma delta muscle subunit combination. The alpha 2 beta 2 and alpha 3 beta 2 neuronal subunit combinations were partially blocked by 10,000 nM lophotoxin analog-1, whereas complete blockade of the alpha 4 beta 2 neuronal and alpha 1 beta 1 gamma delta muscle subunit combinations resulted from incubation with this concentration of lophotoxin analog-1. The alpha 1 beta 1 gamma delta muscle subunit combination was blocked by the alpha-conotoxins G1A and M1 at concentrations of 100 nM. All of the neuronal subunit combinations were insensitive to 10,000 nM of both alpha-conotoxins. Thus, neosurugatoxin and the alpha-conotoxins distinguish between muscle and neuronal subunit combinations, whereas neuronal bungarotoxin and lophotoxin analog-1 distinguish between different neuronal subunit combinations on the basis of differing alpha subunits.

Brain alpha-bungarotoxin binding protein cDNAs and MAbs reveal subtypes of this branch of the ligand-gated ion channel gene superfamily

alpha-Bungarotoxin (alpha Bgt) is a potent, high-affinity antagonist for nicotinic acetylcholine receptors (AChRs) from muscle, but not for AChRs from neurons. Both muscle and neuronal AChRs are thought to be formed from multiple homologous subunits aligned around a central cation channel whose opening is regulated by ACh binding. In contrast, the exact structure and function of high-affinity alpha Bgt binding proteins (alpha BgtBPs) found in avian and mammalian neurons remain unknown. Here we show that cDNA clones encoding alpha BgtBP alpha 1 and alpha 2 subunits define alpha BgtBPs as members of a gene family within the ligand-gated ion channel gene superfamily, but distinct from the gene families of AChRs from muscles and nerves. Subunit-specific monoclonal antibodies raised against bacterially expressed alpha BgtBP alpha 1 and alpha 2 subunit fragments reveal the existence of at least two different alpha BgtBP subtypes in embryonic day 18 chicken brains. More than 75% of all alpha BgtBPs have the alpha 1 subunit, but no alpha 2 subunit, and a minor alpha BgtBP subtype (approximately 15%) has both the alpha 1 and alpha 2 subunits.

Kappa 2-bungarotoxin and kappa 3-bungarotoxin: two new neuronal nicotinic receptor antagonists isolated from the venom of Bungarus multicinctus

Neuronal nicotinic acetylcholine receptors are recognized with high affinity by two snake venom kappa-neurotoxins, kappa-bungarotoxin and kappa-flavitoxin. Native and radiolabeled kappa-neurotoxins have been used to localize and quantitate neuronal nicotinic receptors in a variety of species. We now report the identification of two new kappa-neurotoxins. kappa 2-Bungarotoxin and kappa 3-bungarotoxin were purified from the venom of Bungarus multicinctus collected in the province of Guangdong, China. kappa-Bungarotoxin has as yet not been found in this venom, although it is the only kappa-neurotoxin to be isolated thus far from Taiwanese Bungarus multicinctus. The geographical separation of Guangdong and Taiwan might account for this evolutionary divergence within the species. Both of the new kappa-neurotoxins are potent antagonists of nicotinic transmission in the chick ciliary ganglion. kappa 3-Bungarotoxin, the least potent of the kappa-neurotoxins, produces a complete blockage of nicotinic transmission in 60 min at 250 nM. Protection experiments using the short-acting nicotinic antagonists dihydro-beta-erythroidine and (+)-tubocurarine demonstrate that kappa 2-bungarotoxin blocks transmission by binding to the acetylcholine recognition sites of neuronal nicotinic receptors. The isoelectric point of kappa 2-bungarotoxin (pI = 8.9) is similar to that of kappa-bungarotoxin and kappa-flavitoxin, but kappa 3-bungarotoxin is considerably more basic, with pI greater than 11. Partial amino acid sequences are reported for both kappa 2-bungarotoxin and kappa 3-bungarotoxin. These sequences show a high degree of homology (approximately 80%) with other kappa-neurotoxins, and allow the determination of the critical differences between the kappa-neurotoxins and the structurally related alpha-neurotoxins. For example, all 4 kappa-neurotoxins lack a tryptophanyl residue which is invariant and important for function in the alpha-neurotoxins. The kappa-neurotoxins also differ from the alpha-neurotoxins by having an invariant prolinyl residue at a critical sequence position. Heterodimers were detected consisting of one subunit each of kappa 2-bungarotoxin and kappa 3-bungarotoxin. These heterodimers, which form between any combination of two kappa-neurotoxins, appear to be physiologically active and confirm that a further distinction between kappa-neurotoxins and alpha-neurotoxins is the strong tendency of the former to self-associate in solution. The present results help to establish the definition of 'kappa-neurotoxin'. These snake toxins are now being used by a number of laboratories in physiological and biochemical experiments on neuronal nicotinic receptors from a variety of species.(ABSTRACT TRUNCATED AT 400 WORDS)

Kappa-neurotoxins: heterodimer formation between different neuronal nicotinic receptor antagonists

The kappa-neurotoxins are a family of snake venom polypeptides that are competitive antagonists of acetylcholine at a variety of neuronal nicotinic receptors. We have previously determined that kappa-bungarotoxin, purified from the venom of Bungarus multicinctus, exists in solution entirely as a dimer of identical subunits. We now report that the three other known kappa-neurotoxins, namely, kappa 2-bungarotoxin and kappa 3-bungarotoxin from Bungarus multicinctus and kappa-flavitoxin from Bungarus flaviceps, also self-aggregate in solution. Furthermore, when two different kappa-neurotoxins are mixed, a heterodimer species spontaneously forms and reaches an equilibrium with the two homodimers after which 40-50% of the protein exists as the heterodimer. A cation-exchange high-pressure liquid chromatography procedure is described which readily separates kappa-neurotoxin heterodimers from the homodimers. Sedimentation equilibria experiments give an Mr = 15,500 +/- 1000 for kappa-flavitoxin and an Mr = 14,500 +/- 700 for a mixture of kappa-bungarotoxin and kappa-flavitoxin. Since the subunit molecular weights of kappa-bungarotoxin and kappa-flavitoxin are respectively 7313 and 7242, self-aggregation of these toxins in solution results in a preponderance of kappa-neurotoxin dimers. The stoichiometry of the heterodimer formed by kappa-bungarotoxin and kappa-flavitoxin is 1:1, as determined by amino acid sequence analysis. After isolation, the kappa-neurotoxin heterodimer partially dissociates and again reaches equilibrium with the homodimers, a process which requires 2-4 h at 23 degrees C. The ability to self-aggregate to form heterodimers and homodimers thus appears to be a common property of the kappa-neurotoxins.(ABSTRACT TRUNCATED AT 250 WORDS)

Unusual amino acid sequence of fasciatoxin, a weak reversibly acting neurotoxin in the venom of the banded krait, Bungarus fasciatus

A weak reversibly acting neurotoxin, fasciatoxin, was found in the venom of Bungarus fasciatus. The sequencing was completed by manual and automated Edman analyses of the reduced and carboxymethylated protein and of the peptides obtained from enzyme digestions. It is composed of 63 amino acid residues with four disulphide bonds and a unique sequence at the C-terminal end. According to the criteria set by Ryden, Gabel & Eaker [(1973) Int. J. Pept. Protein Res. 5, 261-273], fasciatoxin lacks all of the five functionally invariant residues of neurotoxins. The hydropathy index indicates that fasciatoxin is devoid of a strong hydrophilicity domain for binding to the receptor site. Structural comparison with some typical neurotoxins also reveals the uniqueness of fasciatoxin in that the extent of similarity is only about 30%.

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon

Choline acetyltransferase, a specific marker for cholinergic neurons, has been immunohistochemically localized in the mesencephalon and in the caudal diencephalon of the chicken. A complete series of transverse sections through the mesencephalon is presented. In the diencephalon, cholinergic fibers were found in the stria medullaris, the fasciculus retroflexus, and the ventral portion of the supraoptic decussation. The nucleus triangularis and the nucleus geniculatus lateralis, pars ventralis also contained cholinergic fibers. Small cholinergic cell bodies were found in the medial habenula. In the pretectum, cholinergic fibers innervated the nucleus lentiformis mesencephali and the tectal gray. The nucleus spiriformis lateralis also contained cholinergic fibers, while most of the cell bodies in the nucleus spiriformis medialis were cholinergic. In the mesencephalon, labelled fibers were found in the nucleus intercollicularis and in all layers of the optic tectum except the stratum opticum. The highest density of tectal cholinergic fibers was in the stratum griseum et fibrosum superficiale (SGFS), layer f. Radial cells located in SGFS, layer i were also cholinergic. In the isthmic nuclei, cholinergic fibers were found in the pars magnocellularis, while the pars parvicellularis and the nucleus semilunaris contained labelled cells. The oculomotor, Edinger-Westphal, trochlear, and trigeminal motor nuclei all had cholinergic cell bodies. Cholinergic axons were present in the oculomotor and trochlear nerves. In the tegmentum, cell bodies were labelled in the nucleus mesencephalicus profundus, pars ventralis, while the nucleus interpeduncularis had dense cholinergic innervation. Our localization of cholinergic cell bodies and fibers has been compared with earlier autoradiographic and anatomical studies to help define cholinergic systems in the avian brain. For example, the results indicate that the chicken may have a cholinergic habenulointerpeduncular system similar to that reported in the rat. Establishing the cholinergic systems within the avian midbrain is important for designing future neurophysiological and pharmacological studies of cholinergic transmission in this region.

Control of transmitter release from the motor nerve by presynaptic nicotinic and muscarinic autoreceptors

Until recently, release studies have failed to indicate the existence of autoreceptors on motor nerves. Ignaz Wessler now reports on a refinement of the technique - the measurement of newly synthesized [3H]acetylcholine released from the phrenic nerve - which provides clear evidence in support of release-modulating autoreceptors. Presynaptic nicotinic receptors mediate a positive feedback mechanism, can rapidly be desensitized and appear to differ in their pharmacological profile from the postsynaptic receptors. In addition, inhibitory and facilitatory muscarinic receptors appear to be involved in the presynaptic control of transmitter release from the phrenic nerve.