Synthesis of new, highly radioactive tetrodotoxin derivatives and their binding properties to the sodium channel

Metabolically regulated spiking could serve neuronal energy homeostasis and protect from reactive oxygen species

So-called spontaneous activity is a central hallmark of most nervous systems. Such non-causal firing is contrary to the tenet of spikes as a means of communication, and its purpose remains unclear. We propose that self-initiated firing can serve as a release valve to protect neurons from the toxic conditions arising in mitochondria from lower-than-baseline energy consumption. To demonstrate the viability of our hypothesis, we built a set of models that incorporate recent experimental results indicating homeostatic control of metabolic products-Adenosine triphosphate (ATP), adenosine diphosphate (ADP), and reactive oxygen species (ROS)-by changes in firing. We explore the relationship of metabolic cost of spiking with its effect on the temporal patterning of spikes and reproduce experimentally observed changes in intrinsic firing in the fruitfly dorsal fan-shaped body neuron in a model with ROS-modulated potassium channels. We also show that metabolic spiking homeostasis can produce indefinitely sustained avalanche dynamics in cortical circuits. Our theory can account for key features of neuronal activity observed in many studies ranging from ion channel function all the way to resting state dynamics. We finish with a set of experimental predictions that would confirm an integrated, crucial role for metabolically regulated spiking and firmly link metabolic homeostasis and neuronal function.

Hydrogen Peroxide Gates a Voltage-Dependent Cation Current in Aplysia Neuroendocrine Cells

Nonselective cation channels promote persistent spiking in many neurons from a diversity of animals. In the hermaphroditic marine-snail, Aplysia californica, synaptic input to the neuroendocrine bag cell neurons triggers various cation channels, causing an ∼30 min afterdischarge of action potentials and the secretion of egg-laying hormone. During the afterdischarge, protein kinase C is also activated, which in turn elevates hydrogen peroxide (H₂O₂), likely by stimulating nicotinamide adenine dinucleotide phosphate oxidase. The present study investigated whether H₂O₂ regulates cation channels to drive the afterdischarge. In single, cultured bag cell neurons, H₂O₂ elicited a prolonged, concentration- and voltage-dependent inward current, associated with an increase in membrane conductance and a reversal potential of ∼+30 mV. Compared with normal saline, the presence of Ca²⁺-free, Na⁺-free, or Na⁺/Ca²⁺-free extracellular saline, lowered the current amplitude and left-shifted the reversal potential, consistent with a nonselective cationic conductance. Preventing H₂O₂ reduction with the glutathione peroxidase inhibitor, mercaptosuccinate, enhanced the H₂O₂-induced current, while boosting glutathione production with its precursor, N-acetylcysteine, or adding the reducing agent, dithiothreitol, lessened the response. Moreover, the current generated by the alkylating agent, N-ethylmaleimide, occluded the effect of H₂O₂ The H₂O₂-induced current was inhibited by tetrodotoxin as well as the cation channel blockers, 9-phenanthrol and clotrimazole. In current-clamp, H₂O₂ stimulated burst firing, but this was attenuated or prevented altogether by the channel blockers. Finally, H₂O₂ evoked an afterdischarge from whole bag cell neuron clusters recorded ex vivo by sharp-electrode. H₂O₂ may regulate a cation channel to influence long-term changes in activity and ultimately reproduction.SIGNIFICANCE STATEMENT Hydrogen peroxide (H₂O₂) is often studied in a pathological context, such as ischemia or inflammation. However, H₂O₂ also physiologically modulates synaptic transmission and gates certain transient receptor potential channels. That stated, the effect of H₂O₂ on neuronal excitability remains less well defined. Here, we examine how H₂O₂ influences Aplysia bag cell neurons, which elicit ovulation by releasing hormones during an afterdischarge. These neuroendocrine cells are uniquely identifiable and amenable to recording as individual cultured neurons or a cluster from the nervous system. In both culture and the cluster, H₂O₂ evokes prolonged, afterdischarge-like bursting by gating a nonselective voltage-dependent cationic current. Thus, H₂O₂, which is generated in response to afterdischarge-associated second messengers, may prompt the firing necessary for hormone secretion and procreation.

The molecular mystique of tetrodotoxin

In many respects tetrodotoxin (TTX) is the quintessential natural toxin. It is unequivocally toxic to mammals with LD(50) values for mice in the range of 10 μg/kg (intraperitoneal), 16 μg/kg (subcutaneous), and 332 μg/kg (oral) (Kao, 1966). Its biothreat status is recognized by its listing as a "Select Agent" by the US Department of Health and Human Services which includes regulated agents "determined to have the potential to pose a severe threat to both human and animal health" (http://www.selectagents.gov/). It has a well-defined cellular target (i.e., NaV channels) and pharmacological mode of action (i.e., block of nerve and muscle action potentials), and it is an indispensable chemical tool in neuroscience. It is widely distributed in marine and terrestrial ecosystems where it plays a role in the chemical ecology of predator-prey relationships and drives evolutionary selection of TTX-resistance (Hanifin, 2010; Williams, 2010; Zimmer and Ferrer, 2007). Lastly, TTX has acquired a certain mystique in scientific lore attributable to many fascinating aspects of its natural history and molecular interactions as presented in selected summary below. Additional information may be found in other excellent reviews (Fozzard and Lipkind, 2010; Kao, 1966; Lee and Ruben, 2008; Narahashi, 2001, 2008).

11-Oxo-tetrodotoxin and a specifically labelled 3H-tetrodotoxin

Tetrodotoxin was oxidized to a hydrated aldehyde, 11-oxo-tetrodotoxin, which shares the specificity of tetrodotoxin for the Na+ channel of the isolated voltage-clamped frog skeletal muscle fiber, but is four to five times more potent. It binds to the solubilized Na+ channel of eel electroplax with a similarly higher potency, because of an equilibrium dissociation constant about 0.25, and a dissociation rate constant 2.4 times slower than those for tetrodotoxin. 11-Oxo-tetrodotoxin can be reduced to regenerate a tetrodotoxin, which is chemically and biologically indistinguishable from the original tetrodotoxin. By reducing with tritiated sodium borohydride, a 3H marker can be inserted regiospecifically to yield 11-[3H]-tetrodotoxin. Because it has a defined specific activity of > 2.5 Ci/mmole, and a 3H marker which does not exchange with solvent proton, 11-[3H]-tetrodotoxin is an ideal tracer for tetrodotoxin. It may enable studies of problems which require higher signals and/or better stability of the marker than those obtainable from currently available tracer Na(+)-channel ligands.

Photolabeled sites with a tetrodotoxin derivative in the domain III and IV of the electroplax sodium channel

Forty three percent of the labeled sites, at least, in the electroplax sodium channel with a photoactivable tetrodotoxin derivative were identified by probing protease-digested labeled fragments with several sequence-directed antibodies. They are located in the loop between segments S5 and S6 of domain IV, as well as the region containing transmembrane segment S6 and adjacent extracellular and cytoplasmic sequences in domain III. No photolabeled fragments were detected in the corresponding region of domain I. These results suggest that C-11 of tetrodotoxin where the photoreactive moiety is attached orients to the region between S5 and S6 in domain III and IV. Probable orientation of the tetrodotoxin molecule in sodium channels is considered by taking together with the recent report of the site-directed mutagenesis.

Expression of sodium channel subtypes during development in rat skeletal muscle

This study contrasts the developmental patterns of expression of 2 subtypes of the voltage-dependent sodium channel in rat muscle that are differentiated by their immunoreactivity with monoclonal antibodies raised to the purified muscle sodium channel protein. One subtype is found in the transverse tubular (T) system of slow twitch fibers as well as the plasma membrane of fast and slow twitch fibers in the anterior tibial and soleus muscles. The second is present in the plasma membrane in all fibers of both muscles. The transverse tubular subtype exhibits 2 immunocytochemical staining patterns within muscle fibers, reticular and homogeneous, which may represent labeling of the developing T tubular system and of a cytoplasmic pool of alpha subunits of the sodium channel respectively. The reticular pattern eventually disappears in fast twitch fibers but persists into the adult stage in slow twitch fibers. The homogeneous pattern is also seen with antibodies to the plasma membrane subtype and disappears in early development as immunoreactivity to both subtypes gradually appears in the surface membrane. A reticular pattern is never seen with the plasma membrane subtype. The factors that modulate the expression of these subtypes is unknown.

A novel sodium channel inhibitor from Conus geographus: purification, structure, and pharmacological properties

A novel toxin, tentatively named conotoxin GS (CGS), has been isolated from a marine snail, Conus geographus. CGS was found to exist as a single polypeptide chain, consisting of 34 amino acid residues, cross-linked by three disulfide bonds. Its amino acid sequence was shown to be Ala-Cys-Ser-Gly-Arg-Gly-Ser-Arg-Cys-Hyp-Hyp-Gln-Cys-Cys-Met-Gly-Leu-Arg- Cys-Gly - Arg-Gly-Asn-Pro-Gln-Lys-Cys-Ile-Gly-Ala-His-Gla-Asp-Val. In competition experiments, CGS inhibited the bindings of [3H]Lys-tetrodotoxin ([3H]Lys-TTX) and [3H]propionylconotoxin GIIIA to Electrophorus electricus electroplax membranes, with Ki values of 34 nM and 24 nM, respectively. The toxin inhibited the binding of [3H]Lys-TTX (1 nM) to rat skeletal muscle homogenates with an IC50 value of 880 nM but showed very little effect on this binding to the rat brain P2 fraction at 10 microM. These binding studies indicate that CGS belongs to the same group of Na channel inhibitors as TTX, STX (saxitoxin), and mu-conotoxins. Although CGS, like the mu-conotoxins, is a pharmacological probe for distinguishing between neuronal and muscle Na channel subtypes, the homology in the sequences of CGS and mu-conotoxins is very limited.

Distribution of voltage-dependent Na+ channels identified by high-affinity receptors for tetrodotoxin and saxitoxin in rat and human brains: quantitative autoradiographic analysis

The localization of a putative voltage-dependent Na+ channel in adult rat and human brain was studied by light microscopic quantitative autoradiography using a tritiated derivative of tetrodotoxin ([3H]enTTX) and tritiated saxitoxin [( 3H]STX). Equilibrium binding experiments in the whole rat brain gave dissociation constants of 7.0 nM ([3H]enTTX) and 5.0 nM ([3H]STX). The dissociation constant for the binding of [3H]STX in the different human brain regions was near 1.5 nM. Autoradiograms demonstrated a heterogeneous distribution of toxin binding sites in the brain with a very good correlation of the mapping of tetrodotoxin and saxitoxin receptors. With the exception of a few regions, the same type of cartography was observed for human and rat brain structures. If toxin receptors were present in all brain regions, their density was particularly important in cerebral cortex, hippocampus, lateral septum and molecular layer of cerebellar cortex. Conversely, the medulla oblongata contained only low amounts of binding sites.

Identification in mammalian brain of an endogenous substance with Na+ channel blocking activities similar to those of tetrodotoxin

A substance with Na+ channel blocking activities has been isolated from pig brain after extraction and purification on sulfopropyl-Sephadex C-25, reversed-phase and carboxymethyl Synchropak high pressure liquid chromatography columns. The peptidic material i) displaces [3H]ethylenediamine tetrodotoxin ([3H]en-TTX) from its binding sites on rat brain membranes, (ii) it blocks 22Na+ influx induced by veratridine and sea anemone toxin on neuroblastoma and embryonic chick heart cells in culture, (iii) it specifically decreases the height of the action potential generated in frog sciatic nerve, and (iv) it blocks the fast Na+ current in voltage-clamped neuroblastoma cells. These properties are similar to those of tetrodotoxin while the endogenous factor is a peptide that is destroyed by proteases. These results suggest the presence in pig brain of a potent Na+ channel modulation activity.

Autoradiographic analysis in rat brain of the postnatal ontogeny of voltage-dependent Na+ channels, Ca2+-dependent K+ channels and slow Ca2+ channels identified as receptors for tetrodotoxin, apamin and (-)-desmethoxyverapamil

The postnatal development of the distribution of 3 different ionic channel proteins in rat brain was studied using light microscopic autoradiography. [3H]Ethylenediaminetetrodotoxin, [125I]apamin and (-)-[3H]desmethoxyverapamil were used to label one class of voltage-dependent Na+ channel proteins, one class of Ca2+-dependent K+ channel proteins, and the slow Ca2+ channel protein, respectively. Ca2+-dependent K+ channel proteins are detected very early in the germinative zone. They are associated to neuronal somas during their migration and their maturation. In hippocampus and cerebral cortex, apamin binding sites are already present at birth and their density increases to day 20 postnatal when the adult localization is established. Slow Ca2+ channel protein development occurs later in CNS ontogenesis. The development of slow Ca2+ channels seems to follow the development of dendrites. Density of these channel proteins increases regularly until adult age. At the resolution level of this analysis, Na+ channel proteins are absent in diencephalon at birth. Their appearance and their increase in density are strictly correlated to the synaptogenesis in particular in cerebral and cerebellar cortex and hippocampus. Although cerebellum, neocortex and hippocampus have been particularly analyzed, other brain structures have also been examined.

Na+ channels as sites of action of the cardioactive agent DPI 201-106 with agonist and antagonist enantiomers

This paper shows the interaction of the cardiotonic agent 4-[3-(4-diphenylmethyl-1-piperazinyl)-2-hydroxypropoxy]-1H-indole- 2-carbonitrile (DPI 201-106) and its optic enantiomers R-DPI (205-429) and S-DPI (205-430) with the Na+ channel of a variety of excitable cells. Voltage-clamp experiments show that DPI 201-106 acts on neuroblastoma cells and rat cardiac cells. S-DPI (205-430) increases the peak Na+ current, slows down the kinetics of Na+ channel inactivation, and is cardiotonic on heart cells. Conversely, R-DPI (205-429) reduces the peak Na+ current and blocks Na+ channel activity and cardiac contractions. Binding experiments using radioactively labeled toxins indicate that DPI 201-106 and its enantiomers do not interact with sites already identified for tetrodotoxin or sea anemone and scorpion toxins. DPI 201-106 and its enantiomers inhibit binding of a 3H-labeled batrachotoxin derivative, [3H]batrachotoxinin A 20-alpha-benzoate, to brain membranes. The dissociation constant of the complex formed between the Na+ channel and both R-DPI and S-DPI is Kd congruent to 100 nM. 22Na+ uptake experiments using different cell types have shown that R and S enantiomers of DPI 201-106 are active on the different Na+ channel subtypes with similar IC50 values. These results are discussed in relation with the cardiotonic properties of DPI 201-106 that are not accompanied by cardiotoxic effects.

Distribution of tritiated tetrodotoxin administered intraperitoneally to pufferfish

Tetrodotoxin was recoil-tritiated by the 3He(n,p)3H reaction and purified by gel filtration. The [3H]tetrodotoxin gave only one spot in both cellulose acetate strip electrophoresis and thin layer chromatography. The specific toxicity of tetrodotoxin did not decrease during the recoil tritiation and the [3H]tetrodotoxin showed a specific radioactivity of 25 x 10(-6) Ci/mmole. In spite of the low specific radioactivity, the [3H]tetrodotoxin was able to be used to investigate the anatomical distribution of tetrodotoxin in pufferfish. When intraperitoneally injected into 'torafugu' puffer, [3H]tetrodotoxin accumulated in most tissues, the level being highest in the skin, followed by the liver, intestines and muscle. With time, the [3H]tetrodotoxin radioactivity level in the injected pufferfish decreased in most tissues, except for skin and gallbladder. Based on these results, the metabolism of tetrodotoxin in pufferfish is discussed.

Mevinolin, an inhibitor of cholesterol biosynthesis, drastically depresses Ca2+ channel activity and uncouples excitation from contraction in cardiac cells in culture

Mevinolin (MK803), a potent inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) (Ki, 30 X 10(-9) M), depressed de novo synthesis of cholesterol in 11-day chicken embryonic cardiac cells cultured in lipoprotein-deficient serum (LPDS). Cardiac cells exposed to different concentrations of mevinolin for 1-3 days presented different electrophysiological and mechanical properties: The resting membrane potential, the rate of increase, and the shape of the action potential and contractile properties were changed at concentrations as low as 0.1 microM mevinolin. At a concentration of 1 microM mevinolin, the cardiac cells became quiescent and electrical stimulation induced action potentials of short duration without contraction. Isoproterenol and Bay K8644 were unable to restore excitability and contraction. Although the number of receptors for the tritiated Ca2+ channel blocker nitrendipine was the same in control and in mevinolin-treated cells, voltage-clamp data on isolated cardiac cells and 45Ca2+ flux experiments on monolayers showed that most of the slow Ca2+ channel activity was lost in mevinolin-treated cells. These results suggest that the disappearance of Ca2+ channel activity is most probably at the origin of the loss of cardiac contractility.

Developmental appearance of sodium channel subtypes in rat skeletal muscle cultures

22Na influx was measured in the established muscle cell line L-6 and in primary rat skeletal muscle cultures following activation of sodium channels by veratridine and sea anemone toxin II. Inhibition of the activated channels by tetrodotoxin (TTX) was analyzed with computer-assisted fits to one- or two-site binding models. In L-6 cultures, two inhibitable sodium channel populations were resolved at all ages in culture: a TTX-sensitive (K = 0.6-5.0 X 10(-8) M) and an insensitive population (Ki = 3.3-4.9 X 10(-6) M). In primary rat muscle cultures, the sensitivity of the toxin-stimulated channels to TTX changed with time in culture. In 4-day-old cultures, a single sodium channel population was detected using TTX (Ki = 2.4 X 10(-7)M). A single population was also found in 6-day-old cultures (Ki = 5.3 X 10(-7) M). By day 7 in culture, the inhibition of 22Na influx by TTX could be resolved into two components with high- and low-affinity sites for the toxin (Ki = 1.3 X 10(-9) M and 9.6 X 10(-7) M). We conclude that a single, toxin-activated sodium channel population with low affinity for TTX exists at early stages, whereas a second, high-affinity population evolves with time in primary rat muscle cultures. The expression of a high-affinity site apparently does not require ongoing neuronal involvement and may reflect an intrinsic property of the muscle cells.

The voltage-dependent sodium channel is co-localized with the acetylcholine receptor at the vertebrate neuromuscular junction

Isolated motor endplates from mouse intercostal muscles can be obtained after subcellular fractionation. On these motor endplates, localization of the nicotinic receptor and of the voltage-dependent Na+ channel coincides as demonstrated by double labeling with rhodamine alpha-bungarotoxin and a specific anti-Na+ channel monoclonal antibody. High density of Na+ channel at the motor endplate is confirmed by the enrichment in TTX binding sites as compared to the crude homogenate. In contrast isolated motor endplates are almost completely devoid of Ca2+ channel antagonist binding sites.

[3H]-tetrodotoxin binding in neuronal and non-neuronal spinal cord cultures

The binding of [3H]-tetrodotoxin [TTX], a sodium channel blocker, was studied in dissociated spinal cord cultures derived from fetal mice. A comparison was made between cultures that consisted of a mixture of neurons and non-neuronal background cells (N + BG) with those that were comprised of only background cells (BG). Specific binding of 1 nM [3H]-TTX was studied in 28-day-old cultures. The IC50 for TTX displacement of [3H]-TTX binding was 10 nM for (N + BG) cultures and 15 nM for (BG). The binding of [3H]-TTX to (N + BG) cultures was approximately 9-fold greater than that observed for the (BG) cultures. During development from day 6 to day 28, the binding of [3H]-TTX in (N + BG) cultures increased about 10-fold per dish or about 30% as expressed as fmol bound per mg protein. Nitrendipine did not displace [3H]-TTX in day 6 (N + BG) cultures, although previous studies indicated that TTX displaced [3H]-nitrendipine binding in developing spinal cord cultures.

Sodium pathway markers in normal and kindled frog brains

The present report evaluates Na,K-ATPase activity as well as Na channel levels in the frog telencephalon after kindling, i.e. the acquisition of an epileptic focus through localized low-voltage electrical stimulation of one hemisphere. K-dependent phosphatase activity and binding of tritiated ouabain were measured, revealing no change in Na,K-ATPase activity 14 h after the last seizure. Na channels were measured by binding assays using a tritiated ethylenediamine tetrodotoxin derivative. Na channels were reduced in kindled brain as compared to controls.

Tetrodotoxin-sensitive sodium channels in a continuously cultured cell line derived from the adult rat cerebellum

A method to establish continuously cultured cell lines from adult cerebellar cortex is reported. Clones (prepared by this procedure) were isolated from cerebellar established cultures at the 25th passage and after 15 months in vitro. One clone (UCHCC1) was maintained in culture and studied while the others were frozen. The cerebellar cell line UCHCC1 retained a neuronal-like morphology; the addition of dimethylsulfoxide (DMSO) to the culture medium elicited a reproducible morphological 'differentiation' event, characterized mainly by process extension. In 'differentiated' cells, veratridine significantly increased the uptake of 22Na. Such enhanced uptake was blocked by tetrodotoxin (TTX) with a half-maximal inhibitory concentration of 0.9 nM. Binding of an [3H]ethylenediamine derivative of TTX ([3H]en-TTX) to the microsomal fraction prepared from same DMSO-treated cells, showed a single class of receptors with a maximal binding (Bmax) of 173 fmol/mg protein and a Kd of 1.1 nM. Thyroid UCHT1 cells and 'undifferentiated' (cultured without DMSO) cerebellar cells, did not show significant effects of veratridine on 22Na-uptake, or [3H]en-TTX binding. The 'differentiated' nerve-like properties, induced by appropriate environmental manipulation, demonstrate the usefulness of cerebellar UCHCC1 cells as a model system for the developing central neuron. On the other hand, the novel transforming procedure opens new possibilities for obtaining permanent cell lines from other regions of the adult CNS.

Blockade of [3H]lysine-tetrodotoxin binding to sodium channel proteins by conotoxin GIII

Conotoxin GIII from Conus geographus inhibited the binding of [3H]lysine-tetrodotoxin (4 nM) to electroplax membranes from Electrophorus electricus and to the rat brain P2 fraction with IC50 values of 13 nM and 7.9 microM, respectively. This inhibition observed with electroplax membranes was irreversible. These and physiological findings (Life Sci., 21 (1977) 1759-1770 suggest that conotoxin GIII inhibits Na channel activation by its interaction with the tetrodotoxin binding site of the Na channel. The differences in structures related to the activation of Na channels between the eel electroplax and the rat brain are indicated.

Ion pathways in transverse tubules. Quantification of receptors in membranes isolated from frog and rabbit skeletal muscle

The presence of four cation pathways in membrane vesicles isolated from transverse tubules of frog and rabbit skeletal muscle was studied by measuring binding of specific blockers. Transverse tubules purified from frog muscle have a maximal binding capacity for [3H]nitrendipine (a marker for voltage-dependent calcium channels) of 130 pmol/mg of protein; this binding is strongly dependent on temperature and, at 37 degrees C, on the presence of diltiazem. Receptors for [3H]ethylenediamine tetrodotoxin (a marker for voltage-dependent sodium channels) and for 125I-labeled alpha-bungarotoxin (a marker for acetylcholine-mediated channels) showed maximal binding values of about 5 pmol/mg. The number of sodium-pumping sites in the isolated tubule vesicles, inferred from [3H]ouabain binding, was 215 pmol/mg. The high purity of this preparation makes feasible the use of these values as a criterion to judge the degree of purity of isolated preparations, and it allows investigation of transverse tubule contamination in other muscle membrane fractions.

Axonal transport of the voltage-dependent Na+ channel protein identified by its tetrodotoxin binding site in rat sciatic nerves

Na+ channels levels were measured in different segments of rat vagus and sciatic nerves by in vitro binding using a tritiated ethylene-diamine tetrodotoxin derivative ([3H]en-TTX). Binding sites were found to accumulate on both sides of a ligature tied on the sciatic nerve indicating an anterograde and retrograde axoplasmic transport of Na+ channels. Accumulation of Na+ channels at the ligature was time-dependent and appeared to occur through fast axoplasmic transport mechanisms. This accumulation on both sides of a ligature was also visualized by autoradiographic studies in longitudinal sections of sciatic nerves using [3H]en-TTX.

Cooperativity of tetrodotoxin action in the frog node of Ranvier

The steady state effects and rates of action of tetrodotoxin (TTX) on sodium current were studied in the voltage clamped frog node of Ranvier. Inactivation of the sodium current was separated into fast and slow phases. Both phases were assumed to correspond to two different currents (fast and slow) flowing through fast and slow channels (Benoit et al. 1985). The dose-response curve of the steady state effect of tetrodotoxin on the fast current was sigmoid. An analysis of this effect in double logarithmic coordinates gave a Hill coefficient of 1.74. The rates of tetrodotoxin action on the fast current were determined by the reversible reduction of the peak current recorded at a potential (+20 mV) at which the slow current was relatively small. After an initial delay, the onset of TTX effect followed an exponential function of time whose constant decreased with increasing tetrodotoxin concentrations. Expressed as the time corresponding to a reduction of 2% of the current, the delay (delta t2%) increased from about 100 ms with 300 nM-TTX to about 30 s with 1 nM-TTX. When tetrodotoxin was removed, the offset developed quasi-instantaneously and followed an exponential function of time whose constant was independent of the toxin concentration. Both steady state and rates of tetrodotoxin effects could be fitted well if one assumed that the block of one fast channel occurred after binding of two TTX molecules to two cooperative sites.

The voltage-dependent Na+ channel of insect nervous system identified by receptor sites for tetrodotoxin, and scorpion and sea anemone toxins

Receptor sites for some of the most important toxins known to be specific for voltage-sensitive Na+ channel in the mammalian nervous system have been identified in a purified membrane preparation of house fly brain. Very high affinities have been found for the association of tetrodotoxin or tetrodotoxin derivatives with the insect Na+ channel (Kd = 0.03 - 0.08 nM). The gamma toxin from the Brazilian scorpion Tityus serrulatus forms a complex with the Na+ channel having a Kd of 6.1 pM. The Kd value for toxin II from the sea anemone Anemonia sulcata is 0.12 microM. These results show a high degree of conservation of the pharmacological properties of the brain Na+ channels between insects and mammals.

A monoclonal immunotoxin acting on the Na+ channel, with properties similar to those of a scorpion toxin

We describe the properties of a monoclonal antibody against the Na+ channel. The antibody, 72.38, competitively inhibited (Ki = 1.5 X 10(-9) M) the binding of an 125I-labeled toxin from the Brazilian scorpion Tityus serrulatus (125I-TiTX gamma) to Na+ channels of rat brain membranes. No significant inhibition of binding of a number of other Na+ channel toxins was observed. The inhibition of 125I-TiTX gamma binding also was observed with the solubilized Na+ channel from rat brain membranes (Ki = 2 X 10(-9) M). Antibody 72.38 antagonized 125I-TiTX gamma binding to Na+ channels from different animal species (fish, avian, and mammalian) and from different tissues (electroplax, brain, heart, and muscle). Moreover, 72.38 has been used for immunofluorescence labeling of Na+ channels in rat sciatic nodes of Ranvier and cultured dorsal root ganglion cells. Electrophysiological experiments on rat muscle cells fully confirmed the similarity between TiTX gamma and 72.38 seen in binding experiments. Both produce slow oscillations of the membrane potential accompanied by bursts of action potentials which are due to a selective action on the Na+ channel. TiTX gamma and 72.38 are without effect on the ion selectivity of the Na+ channel, but they both drastically change the voltage-dependence of activation and inactivation of the Na+ channel.

Autoradiographic localization of tetrodotoxin-sensitive Na+ channels in rat brain

The localization of tetrodotoxin-sensitive Na+ channels in rat brain was studied using a tritiated derivative of tetrodotoxin. The autoradiographic distribution of a tritiated ethylenediamine derivative of tetrodotoxin [( 3H]en-TTX) binding showed a high concentration of sites in cortical layers, hippocampus, globus pallidus, substantia nigra and the molecular layer of the cerebellar cortex. Lower levels were found principally in the striatum and median forebrain bundle. The white matter was not labelled. The characteristic distribution of tetrodotoxin-sensitive Na+ channels was compared to that of nitrendipine-sensitive Ca2+ channels and to that of apamin-sensitive Ca2+-dependent K+ channels.

Different functional states of tetrodotoxin sensitive and tetrodotoxin resistant Na+ channels occur during the in vitro development of rat skeletal muscle

There are three stages of differentiation of voltage dependent Na+ channels during the in vitro development of rat skeletal muscle. Myoblasts which are less than 60 h old in culture have Na+ channels which normally do not give rise to action potentials but do so after treatment of the cells with very low concentrations of sea anemone toxin. These Na+ channels revealed by sea anemone toxin are resistant to TTX. Myoblasts prior to fusion are electrically excitable (Vmax = 10 V/s). Electrically activated Na+ channels are only blocked by high concentrations of TTX. Titration of TTX resistant Na+ channels with a tritiated derivative of TTX indicates a dissociation constant of the TTX-Na+ channel complex of 50 nM. Myotubes have both high and low affinity binding sites for TTX (Frelin et al. 1983). Action potentials (Vmax = 100-200 V/s) are only inhibited at high concentrations of TTX. Experiments with rat myoballs indicate that only Na+ channels with a low affinity binding site for TTX are functional in voltage-clamp studies. The K0.5 value for TTX inhibition of the peak Na+ current is observed at 70 nM. Spontaneous contractions of myotubes are blocked by TTX with a K0.5 value of 100 nM, suggesting that TTX resistant Na+ channels are also the ones responsible for the spontaneous contractions in rat myotubes in culture. 22Na+ flux studies after activation of the Na+ channel with neurotoxins have been carried out at the different stages of differentiation. Toxin activated Na+ channels have the same high affinity for sea anemone toxins at all stages of development; likewise, the sensitivity for TTX is the same.(ABSTRACT TRUNCATED AT 250 WORDS)

A competitive displacement assay to detect saxitoxin and tetrodotoxin

An assay is described which detects saxitoxin (STX) and tetrodotoxin (TTX) by their competitive displacement of [3H]saxitoxin from its receptor in rat brain membranes. The assay has a sensitivity of 0.15 ng STX/ml and 0.8 ng TTX/ml for buffer samples. The assay was also applied to detection of these toxins in unextracted human plasma and found to have a sensitivity of 0.5 ng STX/ml and 0.6 ng TTX/ml. The competitive displacement assay appears to be the most sensitive procedure yet for detection of STX and TTX.

Phosphate derivatives of thiamine and Na+ channel in conducting membranes

The results show that thiamine derivatives are copurified with the specific proteins forming the Na+ channel in conducting membranes. Therefore, thiamine derivatives could well play a specific role in the molecular aspects of bioelectrogenesis , an interpretation that could help explain the neurological symptoms observed in human pathology as well as in animals experimentally rendered deficient in vitamin B1.

Characterization, solubilization, affinity labeling and purification of the cardiac Na+ channel using Tityus toxin gamma

Saturable, high-affinity binding of iodinated toxin gamma from Tityus serrulatus scorpion venom (TiTx gamma) to Na+ channel receptor was identified in sarcolemma membrane of chick heart. A binding capacity of 450-600 fmol/mg of protein was found similar to that of tetrodotoxin-binding component. The enrichment of these membrane-bound toxin binding sites follows that of other sarcolemma markers. Kinetic data and displacement of 125I-TiTx gamma from its binding sites by unlabeled TiTx gamma gave an equilibrium dissociation constant (Kd) of 1-3 pM. The gating component and the selectivity filter of the voltage-sensitive Na+ channel, identified as binding sites of TiTx gamma and of tetrodotoxin respectively, have been efficiently solubilized with Nonidet P-40. Purification was achieved by ion-exchange chromatography on DEAE-Sephadex A-25, affinity chromatography on wheat-germ-agglutinin-Sepharose and sucrose density gradient centrifugation. An enrichment of 1400-fold from the original detergent extract was measured for both toxin binding sites (1120-1230 pmol/mg of protein). Sodium dodecyl sulfate gel electrophoresis reveals a single large polypeptide component of Mr230000-270000. The purified material exhibits an apparent sedimentation coefficient of 8.8S. Covalent cross-linking of 125I-TiTx gamma to its membrane-embedded cardiac receptor shows that the cross-linked material, solubilized and purified by the same procedure comprises a single polypeptide chain of the same Mr of 230000-270000. Furthermore, as seen for Electrophorus electricus electroplax and rat brain, the tetrodotoxin-binding component and the TiTx gamma-binding component are carried by the same polypeptide chain. The functional Na+ channel might be an oligomer of this subunit of Mr23000-270000.

High-yield synthesis of a [3H]ethylenediamine ditetrodotoxin derivative

[3H]Tetrodotoxin [( 3H]TTX) and a [3H]ethylenediamine derivative of TTX are the most widely used ligands for the study of the Na channel. The former ligand presents a low specific radioactivity (1 Ci/mmol) while the latter is highly labeled (30 Ci/mmol). However, its two-step synthesis, i.e., mild oxidation followed by coupling of [3H]ethylenediamine, has been described with a low overall yield of 1.7%. In this work, more favorable experimental conditions are defined for the limiting reaction, i.e., the oxidation step, using [14C]testosterone as a model molecule. Applied to the oxidation of tetrodotoxin, this procedure produces yield values of 30-50%, as determined by high-performance liquid chromatography. Moreover, two oxidized TTX molecules appear to be covalently linked to [3H]ethylenediamine, yielding a new labeled tetrodotoxin derivative with a specific radioactivity of 45 Ci/mmol and a dissociation constant of 0.6 nM for electroplax membranes.

Binding of [3H]ethylenediamine di-tetrodotoxin to its solubilized receptor from excitable tissues. Binding measurements by rapid gel-filtration and receptor stabilization by phosphatidylcholine

The molecular study of bioelectrogenesis requires the purification of the membrane proteins involved in the Na-channel electrical activity. This complex biological structure contains various binding sites for different classes of neurotoxins. Labelled forms of the blocking agent, tetrodotoxin, are used to identified and quantified the solubilized membrane proteins during the purification. Such a specific probe was synthetized in our laboratory and this work reports the experimental set-up of the binding technique. A fast-gel-filtration method has been optimized with respect to column design, centrifugation time and speed and, delay between sample application and column centrifugation.

Paranodal dysmyelination and increase in tetrodotoxin binding sites in the sciatic nerve of the motor end-plate disease (med/med) mouse during postnatal development

Motor end-plate disease (med), in the mouse, is a hereditary neuromuscular defect, caused by a single gene mutation and characterized by a progressive muscle weakness. +Med/+med mice die 21-23 days after birth and the neurobiological abnormalities already reported are nerve terminal sprouting and swelling and neurotransmission failures. We studied +med/+med mice at preclinical (9-11 days after birth) as well as at clinically recognized stages of the disease. The nonmyelinated gaps of the nodes of Ranvier in the +med/+med sciatic nerve are found to be significantly widened in +med/+med animals compared to control littermates, even in the preclinical stage, although the nodes of Ranvier are not yet ultrastructurally mature. The maximal binding capacity for [3H]ethylene-diamine tetrodotoxin, expressed in femtomoles per milligram of protein, is significantly increased in +med/+med sciatic nerves. Thus, Na+ channels, which are known to be located mainly at the nodes of Ranvier in normal myelinated axons, are increased in number in +med/+med mice even before the disease becomes clinically established. Both the ultrastructural and biochemical developmental abnormalities of the node of Ranvier rapidly approach their maximal expression as the behavioral signs develop. Such nerve abnormalities may be closely related to the physiological impairment of nerve impulse conduction which leads to the pathophysiological expression of motor end-plate disease.

Tityus gamma toxin, a high affinity effector of the Na+ channel in muscle, with a selectivity for channels in the surface membrane

Toxin gamma from the venom of Tityus serrulatus scorpion produces a partial block of the surface Na+ channel in frog muscle. This block occurs with no change in the voltage-dependence or in the kinetics of the remaining surface Na+ current. The partial blockade of Na+ channel activity occurs with no change in tubular Na+ currents nor in twitch tension. The maximum effect of the toxin is attained at concentrations as low as 3 X 10(-10) M. Hyperpolarization to potentials more negative than the resting potential (E = -90 mV) reduces or abolishes the effect of the toxin. Radioiodinated toxin gamma binds to frog muscle membranes with a very high affinity corresponding to a dissociation constant of about 1 X 10(-11) M. Data obtained with both rabbit and frog muscle indicate that toxin gamma is specific for Na+ channels in surface membranes. Toxin gamma does not seem to bind to Na+ channels in T-tubule membranes. The biochemical data are in good agreement with electrophysiological studies and data on contraction. There is one Tityus gamma toxin binding site per tetrodotoxin binding site in surface membranes. Competition experiments have confirmed that Tityus gamma toxin binds to a new toxin receptor site on the Na+ channel structure. This site is the same that the toxin II from Centruroides suffusus binding site, but this toxin has 100 times less affinity for the Na+ channel than Tityus gamma toxin.

Affinity labelling of the tetrodotoxin-binding component of the Na+ channel

Tetrodotoxin-binding sites were covalently labelled with a highly tritiated derivative of tetrodotoxin. Cross-linking experiments, using dissucinimidyl suberate, on partially purified tetrodotoxin-binding component from electroplax of Electrophorus electricus, revealed covalent labelling of a single polypeptide chain of MW 270,000.

Purification of binding protein for Tityus gamma toxin identified with the gating component of the voltage-sensitive Na+ channel

The gating component associated with the voltage-sensitive Na+ channel from electroplax membranes of Electrophorus electricus has been purified by using toxin gamma from the venom of the scorpion Tityus serrulatus serrulatus. The toxin-binding site was efficiently solubilized with Lubrol PX, resulting in an extract of high initial specific activity. Purification was achieved by adsorption of the toxin-binding component to DEAE-Sephadex A-25 followed by desorption at high ionic strength and chromatography on either wheat germ agglutinin-Ultrogel or Sepharose 6B. Maximal final specific activities were at least 42% of the specific activity expected for a pure toxin-binding component. The purified material exhibited a Stokes radius of 85 A, and sodium dodecyl sulfate/polyacrylamide gel electrophoresis demonstrated a single polypeptide component of Mr 270,000. Furthermore, tetrodotoxin binding activity and Tityus gamma toxin binding activity copurified, suggesting that the selectivity filter and the gating component of the Na+ channel are carried by the same polypeptide chain.

Complete monitoring of the purification of the plasma membrane from rabbit skeletal muscle

A fast and reproducible purification procedure for rabbit skeletal muscle plasma membrane is described. Each step was monitored by determination of tetrodotoxin, ouabain and insulin receptors. A ouabain-sensitive K+-stimulated and a Ca2+-dependent phosphatases, probably identical to, respectively the (Na+-K+) and Ca2+-ATPases, were also evaluated. All plasma membrane receptors and the ouabain-sensitive activity accumulated in the lightest fraction separated by sucrose gradient centrifugation (peak at 18% sucrose; purification from crude homogenate, 30-fold).