The sodium channel in non-impulsive cells. Interaction with specific neurotoxins

Effects of veratridine on sodium currents and fluxes

Veratridine causes Na+ channels to stay open during a sustained membrane depolarization by abolishing inactivation. The consequential Na+ influx, either by itself or by causing a maintained depolarization, leads to many secondary effects such as increasing pump activity, Ca2+ influx, and in turn exocytosis. If the membrane is voltage clamped in the presence of the alkaloid, a lasting depolarizing impulse induces, following the "normal" transient current, another much more slowly developing Na+ current that reaches a constant level after a few seconds. Repolarization then is followed by an inward tail current that slowly subsides. Development of these slow currents is enhanced by additional treatment with agents that inhibit inactivation. Most of these phenomena can be satisfactorily explained by assuming that Na+ channels must open before veratridine binds to them, and that the slow current changes reflect the kinetics of binding and unbinding. It is unclear, however, where the alkaloid stays when it is not bound. Although the effect sets in promptly, once this pool is filled, access to it from outside must be impeded since in most preparations veratridine can only partially be washed out. Cooling acts as if the available concentration is reduced, but this reversible "reduction" takes much longer to develop than the cold-induced changes in kinetics. Several authors assume that the binding site, site 2, is accessed from the lipid phase of the membrane. Considerations of this kind are often based on experiments with batrachotoxin, the widely used site-2 ligand which has a much higher affinity and acts as a full agonist in contrast to the partial agonist veratridine. Batrachotoxin thus lends itself to binding studies using radiolabeled derivatives. Such experiments may eventually lead to the characterization of neurotoxin site 2; the first promising steps have been taken. Modern techniques of molecular biology will almost certainly be successful, and one hopes for point-mutated channels with distinctly different reactions also to veratridine. A considerable amount of research is still required to clarify the structural basis for the numerous allosteric interactions with other sites, the mechanism of the very large potential shift of activation, the reduced single-channel conductance and selectivity, and the chemical nature of the different affinities of the site-2 toxins. Note Added in Proof. A report on point mutations with effects on neurotoxin site 2 (see Sect. 8) has just appeared: Wang S-Y, Wang GK (1988) Point mutations in segment I-S6 render voltage-gated Na+ channels resistant to batrachotoxin. Proc Natl Acad USA 95:2653-2658. In microliter muscle Na+ channels expressed in mammalian cells, mutation Asn434Lys leads to complete, Asn434Ala to partial insensitivity to 5 mM batrachotoxin. (Asn434 corresponds to Asn419 of Trainer et al. 1996). The mutant channel displays almost normal current kinetics and in the presence of veratridine little, if any, slow tail current. However, veratridine inhibits peak Na+ currents in the mutant which may point to a complex structure of site 2.

Veratridine-enhanced persistent sodium current induces bursting in CA1 pyramidal neurons

The mechanism of veratridine-induced bursting activity was studied in rat hippocampal CA1 pyramidal neurons. Veratridine (0.1-0.3 microM) induces bursting in previously normal pyramidal neurons. The current-voltage curves of untreated neurons show a slight deviation from the linear Ohmic relation; this deviation is known as the "depolarizing rectification". Veratridine markedly accentuates the depolarizing rectification so that a zero slope or negative slope appears in the current-voltage curve of these neurons. Both the veratridine-induced bursting activity and negative slope resistance are blocked by small concentrations of tetrodotoxin or by raising the calcium concentration of the superfusion medium. Under single-electrode voltage clamping, a subthreshold persistent (slowly inactivating) sodium current, which can be recorded in untreated neurons, is found to be enhanced in the veratridine-treated neurons. This current is thought to be responsible for the slow depolarizing phase of bursting activity and the development of negative slope resistance in the current-voltage relationship. The present results demonstrate that veratridine enhances the slowly inactivating sodium current, leading to the development of negative slope resistance and induction of bursting in rat hippocampal CA1 pyramidal neurons.

Endogenous bursting due to altered sodium channel function in rat hippocampal CA1 neurons

Intracellular recordings were obtained from pyramidal neurons in the rat hippocampal CA1 area in order to investigate membrane mechanisms involved in veratridine-induced epileptiform activity. Veratridine (0.03-0.2 microM) caused no changes in the passive membrane parameters including the resting potential, input resistance, and time constant. In the presence of small doses (0.03-0.1 microM) of veratridine, a single stimulus caused a relatively slow, large, synaptic-independent potential called the slow depolarizing after-potential (SDAP). When the hippocampal slice was treated with higher doses of veratridine (over 0.1 microM), bursting, or seizure-like activity (SLA) occurred in response to a brief super threshold intracellular stimulation. The duration of SLA bursting could be as long as ten seconds depending on the amplitude of SDAP, and was independent of the stimulus strength or duration. The frequency and configuration of SLA were sensitive to changes in membrane potential caused by applied DC current. At 0.3 microM or higher, veratridine induced spontaneous rhythmic bursting that was also sensitive to membrane potential changes. The evoked or spontaneous bursting is characterized by being: (1) independent of synaptic transmission in that it persisted after complete blockade of evoked synaptic potential with kynurenic acid (0.5 mM), (2) sensitive to selective inhibition by low doses of the specific sodium channel blockers tetrodotoxin (TTX) or cocaine with no apparent influence on the evoked action potential. These results indicate that endogenous SLA bursting can be induced in hippocampal CA1 pyramidal neurons when certain properties of sodium channels are altered by veratridine.

Veratridine blocks voltage-gated potassium current in human T lymphocytes and in mouse neuroblastoma cells

(i) Effects of veratridine on ionic conductances of human peripheral blood T lymphocytes have been investigated using the whole-cell patch-clamp technique. (ii) Veratridine reduces the net outward current evoked by membrane depolarizations. The reduction originates from block of a 4-aminopyridine-sensitive, voltage-gated K+ current. (iii) Human T lymphocytes do not appear to express voltage-gated Na+ channels, since inward currents are observed neither in control nor in veratridine- and bretylium-exposed lymphocytes. (iv) The effect of veratridine consists of an increase in the rate of decay of the voltage-gated K+ current and a reduction of the peak current amplitude. Both effects depend on veratridine concentration. Half-maximum block occurs at 97 microM and the time constant of decay is reduced by 50% at 54 microM of veratridine. (v) Possible mechanisms of veratridine action are discussed. The increased rate of K+ current decay is most likely due to open channel block. The decrease of current amplitude may involve an additional mechanism. (vi) In cultured mouse neuroblastoma N1E-115 cells, veratridine blocks a component of voltage-gated K+ current, in addition to its effect on voltage-gated Na+ current. This result shows that the novel effect of veratridine is not confined to lymphocytes.

Micromolar concentrations of veratridine activate sodium channels in embryonic cockroach neurones in culture

The mode of action of the alkaloid veratridine has been reinvestigated on cultured cockroach neurones, which are normally inexcitable and do not have a detectable fast sodium current. The whole-cell and cell-attached configurations of the patch-clamp technique were used to record the macroscopic and single channel currents, respectively. Concentrations of veratridine ranging from 10(-8) to 10(-5) M were found to induce a small tetrodotoxin (TTX)-sensitive inward current, which peaked around +10 mV and reversed around +55 mV. This current exhibited a pronounced plateau and was insensitive to changes in the holding potential. Bath application of veratridine induced typical TTX-sensitive inwardly-directed single-channel activity, falling into two (apparently coupled) categories of events: first, relatively large events (1 pA at a hyperpolarized potential of -125 mV relative to rest) of short duration and, second, small bursting events (0.4 pA under similar conditions) of slightly longer duration. Pipette application of similar concentrations of veratridine had similar effects in that two categories of events were observed: first, bursts of large events with multiple conductance states and, second, small events of very long duration. The current/voltage relationship of these events was linear for the voltage range studied and the (extrapolated) reversal potential approximated +110 mV. These results support the hypothesis that veratridine, in small concentrations, induces a slow voltage-dependent activation of TTX-sensitive sodium channels, independent of the fast activating and inactivating sodium channels involved in action potential generation.

Functions and distribution of voltage-gated sodium and potassium channels in mammalian Schwann cells

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.

The sodium channels of the neuroblastoma x glioma 108 CC 15 hybrid cell change their sensitivity for volatile and local anesthetics upon continuous passage

We have studied the ion flux through the sodium channels of low passage number (less than 50 p.) and high passage number (greater than 150 p.) neuroblastoma x glioma hybrid cells using [14C] guanidinium and specific neurotoxins to induce channel opening and closing. The sodium channels of low passage number hybrid cells could be opened by veratridine alone, suggesting the presence of voltage dependent channels in agreement with electrophysiological studies reported in the literature. The sodium channels of the high passage number hybrid cells, however, needed the synergistic action of veratridine and scorpion toxin for activation suggesting that these channels are "silent". The [14C] guanidinium ion flux through the sodium channels of the high passage number hybrid cells was inhibited by significantly lower concentrations of the volatile anesthetics (halothane, isoflurane and enflurane) and the local anesthetics (tetracaine and bupivacaine) than the comparable flux through the sodium channels of the low passage number hybrid cells.

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.

Tetrodotoxin-sensitive and tetrodotoxin-resistant Na+ channels differ in their sensitivity to Cd2+ and Zn2+

The sensitivity of Na+ channels to inhibition by Cd2+ and Zn2+ was studied in 22Na+ uptake experiments after stabilization of an open conformation of the Na+ channels with different neurotoxins and in voltage clamp experiments. Six different cell types of neuronal, cardiac or skeletal muscle origin were surveyed. Three cell types possess Na+ channels that are highly sensitive to tetrodotoxin (TTX) (Kd = 1-5 nM) and three possess Na+ channels that are resistant to TTX (Kd = 0.3-1 microM). The 22Na+ uptake experiments using veratridine or batrachotoxin to activate Na+ channels indicated that TTX-resistant Na+ channels are more sensitive to the inhibitory action of Cd2+ (IC50(Cd2+) = 0.2 mM) and of Zn2+ (IC50(Zn2+) = 50 microM) than TTX-sensitive Na+ channels (IC50(Cd2+) = 5 mM, IC50(Zn2+) = 2 mM). Electrophysiological experiments showed that high concentrations of Cd2+ (IC50 = 2 mM) are necessary to inhibit both TTX-sensitive and TTX-insensitive Na+ channels when the channels are activated by voltage steps. The results suggest that Cd2+ acts competitively with veratridine or batrachotoxin and that the difference in the effects of Cd2+ and Zn2+ on 22Na+ fluxes in TTX-sensitive and TTX-resistant cells is related to differences at the site of action of alkaloid neurotoxins.

Kinetics of veratridine action on Na channels of skeletal muscle

Veratridine bath-applied to frog muscle makes inactivation of INa incomplete during a depolarizing voltage-clamp pulse and leads to a persistent veratridine-induced Na tail current. During repetitive depolarizations, the size of successive tail currents grows to a plateau and then gradually decreases. When pulsing is stopped, the tail current declines to zero with a time constant of approximately 3 s. Higher rates of stimulation result in a faster build-up of the tail current and a larger maximum value. I propose that veratridine binds only to open channels and, when bound, prevents normal fast inactivation and rapid shutting of the channel on return to rest. Veratridine-modified channels are also subject to a "slow" inactivation during long depolarizations or extended pulse trains. At rest, veratridine unbinds with a time constant of approximately 3 s. Three tests confirm these hypotheses: (a) the time course of the development of veratridine-induced tail currents parallels a running time integral of gNa during the pulse; (b) inactivating prepulses reduce the ability to evoke tails, and the voltage dependence of this reduction parallels the voltage dependence of h infinity; (c) chloramine-T, N-bromoacetamide, and scorpion toxin, agents that decrease inactivation in Na channels, each greatly enhance the tail currents and alter the time course of the appearance of the tails as predicted by the hypothesis. Veratridine-modified channels shut during hyperpolarizations from -90 mV and reopen on repolarization to -90 mV, a process that resembles normal activation gating. Veratridine appears to bind more rapidly during larger depolarizations.

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Halothane inhibits the neurotoxin stimulated [14C]guanidinium influx through 'silent' sodium channels in rat glioma C6 cells

We have investigated the effect of pharmacological agents on [14C]guanidinium ion influx through sodium channels in C6 rat glioma and N18 mouse neuroblastoma cells. The sodium channels of the N18 cells can be activated by aconitine alone, indicating that they are voltage-dependent channels. In contrast, sodium channels in the C6 cells require the synergistic action of aconitine and scorpion toxin for activation and are therefore characterized as so-called silent channels. The general anesthetic halothane used at clinical concentrations, specifically inhibited the ion flux through the silent sodium channel of C6 rat glioma cells. The voltage-dependent channels of the N18 cells were insensitive to halothane at the concentrations tested.

The effects of the Anemonia sulcata toxin (ATX II) on membrane currents of isolated mammalian myocytes

The effects of Anemonia sulcata toxin (ATX II) on action potentials and membrane currents were studied in single myocytes isolated from guinea-pig or bovine ventricles. Addition of ATX II (2-20 nM) prolonged the action potential duration without a significant change in resting membrane potential. Concentrations of 40 nM-ATX II or more induced after-depolarizations and triggered automaticity. The effects were reversible after washing or upon addition of 60 microM-tetrodotoxin (TTX). 5 mM-Ni did not modify the effects. The single patch-electrode voltage-clamp technique of Hamill, Marty, Neher, Sakmann & Sigworth (1981) was applied to record membrane currents in response to 8.4 S long depolarizations starting from a holding potential of -90 mV. Currents flowing later than 5 ms after the depolarizing step were analysed. The fast events could not be considered because of insufficient voltage homogeneity. After 2 min of exposure to ATX II (20 nM) the changes in net membrane currents were measured. The difference between the currents in the presence of ATX II and during control was defined as the 'ATX-II-induced current' (iATX). After 4 min of wash iATX disappeared. Within 10 S of exposure to 60 microM-TTX, iATX was blocked completely. At potentials positive to -60 mV, iATX was inwardly directed and decayed slowly but incompletely during the 8.4 S long depolarizing pulse. The rate of decay was faster during clamp pulses to more positive potentials. A high amplitude noise was superimposed on the current trace; its amplitude decreased with more positive potentials. We analysed the voltage dependence of iATX with 'isochronous' current-voltage relations. The 0.1 S isochrone of iATX was characterized by a 'threshold' for negative currents at -60 mV, a branch with a negative slope (k = -7 mV, potential of half-maximal activation (V0.5) = -38 mV, bovine cells) leading to a maximum inward current at -20 mV, and an ascending branch which led to an apparent reversal potential (Erev) around +40 mV. The values measured in guinea-pig myocytes were similar though not identical (k = -5.5 mV, V0.5 = -30 mV, maximum of inward current at -5 mV, Erev = +50 mV). Erev shifted to less positive potentials in later isochrones. Holding the membrane at -45 mV prevented the induction of extra current by ATX II. When the holding potential was then changed to -85 mV, iATX developed within some 2 min. Returning the holding potential to -45 mV blocked iATX with a similar slow time course.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

High-affinity neurotensin binding sites in differentiated neuroblastoma N1E115 cells

This paper describes the interaction of neurotensin with mouse neuroblastoma N1E115 cells. Neurotensin binding sites are undetectable in nondifferentiated neuroblastoma cells. They appear during cell differentiation in the presence of a low serum concentration and dimethyl sulfoxide, and reach a maximal level after 50-60 h of incubation under these conditions. The binding of monoiodo[Trp11]neurotensin to homogenates of differentiated N1E115 cells is specific, saturable, and reversible. The interaction is characterized by a dissociation constant of 150 pM and a maximal binding capacity of 9 fmol/mg of protein at 0 degrees C, pH 7.5. These binding parameters, as well as the specificity toward a series of neurotensin analogues, are similar for neurotensin receptors in N1E115 cells and for the high-affinity binding sites that had been previously characterized in rat brain synaptic membranes by means of the same radiolabeled ligand. The presence of high-affinity binding sites for neurotensin in the neuroblastoma N1E115 provides a useful model to study the cellular responses that are generated by the association of neurotensin to its receptor in electrically excitable cells.

Sodium-channels in non-excitable glioma cells, shown by the influence of veratridine, scorpion toxin, and tetrodotoxin on membrane potential and on ion transport

Veratridine induces membrane potential oscillations in non-excitable glioma cells, which are not affected by ouabain (2 mM) or by D600 (0.1 mM). In the presence of veratridine, scorpion toxin causes depolarization of the glioma cells to a positive value of the membrane potential. These effects of veratridine and of scorpion toxin are observed in Na+ but not in choline medium and are inhibited by tetrodotoxin. The response of the glioma cells to bradykinin has also been studied during these experiments. Previously bradykinin has been shown in these cells to induce a hyperpolarizing response caused by an increase in K+ conductance. This response to bradykinin can still be seen during the veratridine-induced oscillations of the membrane potential. In the glioma cells the uptake of guanidinium, a substitute for Na+, is enhanced by veratridine plus scorpion toxin. This stimulation is tetrodotoxin-sensitive. However, in the excitable neuroblastoma X glioma hybrid cells studied for comparison, veratridine causes membrane potential oscillations accompanied at the rising phase by one action potential or a train of action potentials. The results demonstrate that in non-excitable glioma cells tetrodotoxin-sensitive Na+ channels can be activated by veratridine and by scorpion toxin.

Extraneuronal saxitoxin binding sites in rabbit myelinated nerve

The changes in binding of 3H-labeled saxitoxin (STX) to rabbit sciatic nerve during axonal regeneration (after nerve crush) and during axonal degeneration (after nerve section) were measured and compared with the corresponding changes in the sciatic nerves of other mammals (rat, guinea pig, and cat). In the rabbit and rat, regeneration after nerve crush is associated with a 2- to 4-fold increase in STX binding capacity, consistent with the known corresponding increase in the number of nodes of Ranvier in regenerating nerve. Furthermore, consistent with the disappearance of nodes that occurs with Wallerian degeneration, nerve section leads to a disappearance of all, or most, of the STX binding in rat and guinea pig nerve, similar to that previously found for cat nerve. However, in the rabbit, nerve section leads to a large maintained increase in STX binding. Intraneural injection of diphtheria toxin, which is known to damage Schwann cells and which causes an increase in STX binding in intact nerves, abolishes the binding in cut nerves. It is suggested that the increased binding in cut nerves is to nonneuronal sites situated on the surface membrane of the Schwann cells, which have greatly proliferated in number as axonal degeneration has progressed. The reason for the difference between rabbits and other species and the possibility that the binding sites of rabbit Schwann cells represent functional sodium channels remain to be investigated.

Isolation of minax toxins from the venom of the scorpion Buthus minax and their metabolic effects

Two neurotoxins, minax toxins 1 and 2, were isolated from venom of the scorpion Buthus minax from the Sudan. Molecular weights of 7000 and 6800 and 66 and 62 amino acids were found for minax toxins 1 and 2, respectively. Both toxins contain four disulfide bonds, 1 mol each of phenylalanine, histidine, and tryptophan, no free sulfhydryl groups, and no methionine. Both minax toxins 1 and 2 are basic polypeptides with isoelectric points of 8.2 and 9.0, respectively. There is a significant increase in the calcium content of rat hearts envenomated with minax toxins 1 and 2 or crude venom. This confirms earlier electron microscopic findings of calcium deposits in the heart following scorpion envenomation. There is a concomitant decrease in the calcium and phosphorus content of rat serum following envenomation. It seems that neither scorpion toxins nor scorpion venoms affect the mineral metabolism of the bone. The present investigation indicates that scorpion toxins have not only a neurotoxic action but also broader biological effects such as mineral metabolism.

Differential action of tetrodotoxin on identified leech neurons

1. In leech segmental ganglia, the maximum rate of depolarization of action potentials was found to depend largely on Na in the Retzius (R) cell, the mechanosensory P, N and T cells and an identifiable neuron of unknown function, the X cell. 2. Tetrodotoxin (TTX) 15 100 mumol/l had little or no effect on R and X cells. In contrast, membrane excitation in N, P and T cells was depressed in dose- and use-dependent fashion. 3. The data imply the existence of two kinds of Na channels in normal, fully differentiated leech neurons. Correlation of such differences should lead to a better understanding of how particular neurons perform different functions.

Modification of single Na+ channels by batrachotoxin

The modifications in the properties of voltage-gated Na+ channels caused by batrachotoxin were studied by using the patch clamp method for measuring single channel currents from excised membranes of N1E-115 neuroblastoma cells. The toxin-modified open state of the Na+ channel has a decreased conductance in comparison to that of normal Na+ channels. The lifetime of the modified open state is drastically prolonged, and channels now continue to open during a maintained depolarization so that the probability of a channel being open becomes constant. Modified and normal open states of Na+ channels coexist in batrachotoxin-exposed membrane patches. Unlike the normal condition, Na+ channels exposed to batrachotoxin open spontaneously at large negative potentials. These spontaneous openings apparently cause the toxin-induced increase in Na+ permeability which, in turn, causes membrane depolarization.

Na+ channels with binding sites of high and low affinity for tetrodotoxin in different excitable and non-excitable cells

The properties of interaction of tetrodotoxin with its receptor site on the voltage-sensitive Na+ channel were analysed in two ways: (a) by titrating Na+ channels with a tetrodotoxin derivative, [3H]ethylenediamine-tetrodotoxin; (b) by studying the physiological properties of interaction of the toxin with its receptor from 22Na flux measurements. Using a variety of cell types in culture, three different kinds of situations were observed. 1. Cells like N1E 115 neuroblastoma, CCl 39 fibroblasts, embryonic chick cardiomyocytes and chick skeletal myotubes only have one family of Na+ channels with high-affinity binding sites (in the nanomolar range) for tetrodotoxin. These Na+ channels are the same ones as those that are activated by the alkaloid and polypeptide toxins that accelerate 22Na+ influx. 2. C9 cells have Na+ channels with low-affinity binding sites for tetrodotoxin. These Na+ channels are also the ones that are activated by alkaloid and polypeptide toxins (the median inhibitory concentration for tetrodotoxin inhibition of 22Na+ influx through these Na+ channels in 300 nM). 3. Rat myotubes that have differentiated in culture in the absence of neuronal influence have both high-affinity binding sites (in the nanomolar range) detected with the tritiated tetrodotoxin derivative and low-affinity binding sites (on the micromolar range) detected by 22Na+ flux experiments. Only low-affinity binding sites correspond to Na+ channels that can be activated with alkaloid and polypeptide toxins.

Sodium channels induced by depolarization of the Xenopus laevis oocyte

An electrically gated Na+ channel can be made to appear in the membrane of the Xenopus laevis oocyte by simple depolarization. This membrane normally responds passively to imposed transmembrane currents with resting potentials around -60 mV, but when it is held depolarized to more than about +30 mV it becomes possible to obtain long-lasting regenerative depolarizations up to +80 mV; these depolarizations can last as long as 20 min. This potential is due to an "induction" of a Na+-dependent channel that is electrically gated open and closed. Its threshold for opening is about -20 mV and it is selective for Na+ over Cs+ and choline+ but is blocked by relatively small quantities of Li+. When a long voltage clamp step to a positive potential under ENa (+70 to +90 mV) is applied, an inward current is observed for many minutes, implying that this channel does not have an inactivation mechanism. The inward Na+ current is blocked by 0.50 mM tetrodotoxin. When the membrane is held at or near resting potential, the excitability will disappear with time, but it can be made to reappear by again depolarizing the membrane.

Developmental properties of the fast Na+ channel in embryonic cardiac cells using neurotoxins

This paper describes how neurotoxins specific of the fast Na+ channel are used to study its differentiation in embryonic cardiac cells during heart ontogenesis. Structural and functional differentiation of the fast Na+ channel have been followed using both electrophysiological and biochemical techniques.

Differentiation of the fast Na+ channel in embryonic heart cells: interaction of the channel with neurotoxins

The sensitivity of embryonic cardiac cells to tetrodotoxin (TTX) increases with age. At the early embryonic stage, the maximum upstroke velocity is not affected by the presence of TTX. In the course of both in ovo and in vitro development, this velocity reaches an adult-like value of 90-120 V/sec, which is decreased in the presence of TTX to 5-10 V/sec. The differentiation of the Na+ channel has been followed by using three types of specific toxins: (i) TTX or a tritiated derivative of it, (ii) a polypeptide toxin extracted from sea anemone, and (iii) the alkaloidic toxins veratridine and batrachotoxin. Electrophysiological, including voltage-clamp experiments, and biochemical studies have shown (i) that the TTX receptor and the fast Na+ channel machinery exist even when action potentials are insensitive to TTX--the channel is then in a nonfunctional or silent form that is revealed (or chemically activated) by both the alkaloids and the polypeptide toxin--and (ii) that the total number of Na+ channels increases during development by a factor of 4 or 5. In monolayers of cardiac cells insensitive to TTX in which all Na+ channels are in a nonfunctional form, the rate of degradation of the TTX receptor follows first-order kinetics with a half-time of 9 hr. In aggregates fully sensitive to TTX, the number of TTX receptors remains perfectly stable 24 hr after blockade of protein synthesis.

Tetrodotoxin receptors in membrane fragments: purification from Electrophorus electricus electroplax and binding properties

A tetrodotoxin receptor-rich preparation of membrane fragments from the electric organ of Electrophorus electricus is described. The specific binding of neurotoxins and freeze-fracture electron microscopy are used as tools to identify and to characterize membrane fractions. Freeze-fracture electron micrographs of the electric organ demonstrate a high density of membrane particles in the extrasynaptic regions. Density gradient fractions show a broad distribution of [3H]tetrodotoxin, [3H]saxitoxin and 125I-labelled bungarotoxin binding in the range of 1.04--1.15 g/ml sucrose densities, with specific neurotoxin binding up to approx. 5 pmol/mg protein. Carrier-free column electrophoresis of density gradient fractions yields a subfraction with tetrodotoxin and alpha-neurotoxin binding up to 30 pmol/mg protein. The major part of the membrane fragments forms vesicles, which are separated by lectin chromatography into an outside-out and inside-out population. The latter represents at least 50% of the material of a density gradient fraction. For the association of tetrodotoxin, a bimolecular kinetic constant kf greater than or equal to 3.10(5) M-1.s-1 is determined. The dissociation constant is k'b = 2.5.10(-2)s-1. These data are in agreement with a thermodynamic dissociation constant of Kd = 20 nM as determined earlier for E. electricus membrane fragments by equilibrium methods (Grünhagen, H.H., Rack, M., Stämpfli, R., Fasold, H. and Reiter, P. (1981) Arch. Biochem. Biophys. 206, in the press). However, these association kinetics of tetrodotoxin binding in vitro are significantly different from kinetics determined electrophysiologically in Rana (Wagner, H.H. and Ulbricht, W. (1975) Pflügers Arch. 359, 297--315) or Xenopus (Schwarz, J.R., Ulbricht, W. and Wagner, H.H. (1973) J. Physiol. 233, 167--194).

Transition temperatures of the electrical activity of ion channels in the nerve membrane

The temperature dependence of some of the electrical characteristics of neuronal membranes from Aplysia giant neurons and crustacean and cuttlefish giant axons has been analyzed. Arrhenius plots for the maximum rate of depolarization of (V+max) or repolarization (V-max) of the action potential, for the resting membrane conductance, and for the speed of propagation of the action potential, exhibited clear breaks at characteristic temperatures between 17 and 20 degrees C. Lobster giant axons and frog nodes of Ranvier were voltage-clamped at different temperatures between 5 and 30 degrees C. Arrhenius plots for relaxation times related to the opening and closing processes affecting the Na+ and K+ channels were linear. No 'transition' temperature was detected. However, clear-cut changes in (Formula: see text) Na+ and K+ currents, were consistantly observed around 18 degrees C. Values for (Formula: see text) plateaued above 18 degrees C, then decreased gradually as a function of reduced temperature. Variations in temperature between 1 and 30 degrees C did not alter the binding properties of [3H]tetrodotoxin to a purified crab axonal membrane. Pharmacological properties of the Na+ channel are sensitive to temperature. The temperature-dependent effect of veratridine has been studied and indicates a change in properties of the Na+ channel below 20 degrees C. These results support the possibility that the fluidity of membrane lipids in the ionic channel microenvironment may influence the degree to which the channel can open.

Toxin-induced K+ efflux through the Na+ channel of neuroblastoma cells

Neurotoxins which modify the gating system of the Na+ channel in neuroblastoma cells and increase the initial rate of 22Na+ influx through this channel also give rise to the efflux of 86Rb+ and 42K+. These effluxes are inhibited by tetrodotoxin and are dependent on the presence in the extracellular medium of cations permeable to the Na+ channel. These stimulated effluxes are not due to membrane depolarization or increases in the intracellular content of Na+ and Ca2+ which occur subsequent to the action of neurotoxins. The relationships of 22Na+ influx and 42K+ (or 86Rb+) effluxes to both the concentration of neurotoxins and the concentration of external permeant cations strongly suggest that the open form of the Na+ channel stabilized by neurotoxins permits an efflux of K+ ions. Our results indicate that for the efflux of each K+ ion there is a corresponding influx of two Na+ ions into the Na+ channel.

Interaction of pyrethroids with the Na+ channel in mammalian neuronal cells in culture

The interaction of a series of pyrethroids with the Na+ channel of mouse neuroblastoma cells has been followed using both an electrophysiological and a 22Na+ influx approach. By themselves, pyrethroids do not stimulate 22Na+ entry through the Na+ channel (or the stimulation they give is too small to be analyzed). However, they stimulate 22Na+ entry when used in conjunction with other toxins specific for the gating system of the channel. These include batrachotoxin, veratridine, dihydrograyanotoxin II or polypeptide toxins like sea anemone and scorpion toxins. This stimulatory effect is fully inhibited by tetrodotoxin with a dissociation constant of 1.6 nM for the tetrodotoxin-receptor complex. Half-maximum saturation of the pyrethroid receptor on the Na+ channel is observed in the micromolar range for the most active pyrethroids, Decis and RU 15525. The synergism observed between the effect of pyrethroids on 22Na+ influx on the one hand, and the effects of sea anemone toxin II, Androctonus scorpion toxin II, batrachotoxin, veratridine and dihydrograyanotoxin II on the other, indicates that the binding component for pyrethroids on the Na+ channel is distinct from the other toxin receptors. It is also distinct from the tetrodotoxin receptor. Some of the pyrethroids used in this study bind to the Na+ channel but are unable to stimulate 22Na+ entry. These inactive compounds behave as are unable to stimulate 22Na+ entry. These inactive compounds behave as antagonists of the active pyrethroids. An electrophysiological approach has shown that pyrethroids by themselves are active on the Na+ channel of mammalian neurones, and essentially confirm the conclusions made from 22Na+ flux measurements. Pyrethroids are also active on C9 cells in which Na+ channels are 'silent', that is, not activatable by electrical stimulation. Pyrethroids chemically activate the silent Na+ channel in a manner similar to that with veratridine, batrachotoxin, or polypeptide toxins, which are known to slow down the inactivation process of a functional Na+ channel.

Identification of a tetrodotoxin-sensitive Na+ channel in a variety in fibroblast lines

The action potential Na+ ionophore of excitable cells can be activated either by alkaloid compounds such as veratridine, or by small polypeptide toxins extracted from scorpion venom or sea anemone. One of the main features of this Na+ channel is that it is blocked by tetrodotoxin (TTX). However, we report here that during analysis of Na+ influx in resting fibroblasts, we found that a variety of fibroblast lines also possess a TTX receptor. Veratridine and sea anemone toxin act synergistically to stimulate Na+ influx 7 to 10-fold in hamster and rat fibroblasts. As in excitable cells, this toxin-stimulated Na+ influx is blocked by TTX. Addition of serum to hamster fibroblasts arrested in G0 stimulates Na+ influx three-fold. Observations that TTX does not prevent serum-activated Na+ influx, initiation of DNA synthesis and cell proliferation suggest that the fast Na+ channel which we have identified in fibroblasts is not involved in growth control.

Properties of the interaction of the sodium channel with permeant monovalent cations

The use of sea anemone toxin, veratridine and scorpion toxin which specifically interact with the gating system of the sodium channel and maintain the channel in an open conformation has permitted a study of the mechanism of transport of monovalent cations through the selectivity filter of this channel. The initial rate of 22Na+ influx through the tetrodotoxin-sensitive Na+ channels of excitable cells is dependent upon the external concentrations of Na+ and Na+-substitutes with the following properties. (a) It is saturable at high Na+ concentrations and increases with the external Na+ concentration in a cooperative manner (nH = 1.6). (b) At low external Na+ concentrations (1 mM), it is activated and then inhibited by increasing external concentrations of monovalent cations such as Li+, guanidinium, hydrazinium, hydroxylamine and K+. The activating effect of these cations disappears at higher external Na+ concentrations (10 mM). The experimental data are consistent with a model involving at least two allosteric cation-binding sites per Na+ channel. The binding of monovalent cations to Na+ sites is characterized by a high positive homotropic cooperativity. Most of the work describes the properties of the Na+ channel in neuroblastoma cells. The mechanism has also been shown to be valid for excitable cells of other types and origins.

Binding of sea anemone toxin to receptor sites associated with gating system of sodium channel in synaptic nerve endings in vitro

Iodination of toxin II from the sea anemone Anemonia sulcata gives a labeled monoiododerivative that retains 80% of the original neurotoxicity. This derivative binds specifically to rat brain synaptosomes at 20 degrees C and pH 7.4 with a second-order rate constant of association ka = 4.6 x 10(4) M-1 sec-1 and a first-order rate constant of dissociation kd = 1.1 x 10(-2) sec-1. The binding occurs on the Na+ channel at a binding site distinct from that of other gating system toxins like batrachotoxin, veratridine, grayanotoxin, aconitine, and pyrethroids. The maximal binding capacity Bmax is 3.2 pmol/mg of protein (i.e., about two sea anemone toxin binding sites per tetrodotoxin binding site) and the Kd is 240 nM for the monoiododerivative and 150 nM for the native toxin. Corresponding binding parameters for the association of a 125I-labeled derivative of toxin II from the scorpion Androctonus australis Hector are Bmax = 0.3 pmol/mg of protein and Kd = 1 nM, whereas the Kd of the unmodified scorpion toxin is 0.6 nM. Competition experiments involving scorpion toxins, sea anemone toxins, and synaptosomes demonstrate that, although the sea anemone toxin is able to displace the scorpion toxin bound to synaptosomes, the scorpion toxin does not displace the sea anemone toxin. The sea anemone toxin but not the scorpion toxin binds to depolarized synaptosomes. Differences between binding properties of the two polypeptide toxins are analyzed in the discussion.