Properties of maintained sodium current induced by a toxin from Androctonus scorpion in frog node of Ranvier

Analysis of the effect of the scorpion toxin AaH-II on action potential generation in the axon initial segment

The toxin AaH-II, from the scorpion Androctonus australis Hector venom, is a 64 amino acid peptide that targets voltage-gated Na+ channels (VGNCs) and slows their inactivation. While at macroscopic cellular level AaH-II prolongs the action potential (AP), a functional analysis of the effect of the toxin in the axon initial segment (AIS), where VGNCs are highly expressed, was never performed so far. Here, we report an original analysis of the effect of AaH-II on the AP generation in the AIS of neocortical layer-5 pyramidal neurons from mouse brain slices. After determining that AaH-II does not discriminate between Nav1.2 and Nav1.6, i.e. between the two VGNC isoforms expressed in this neuron, we established that 7 nM was the smallest toxin concentration producing a minimal detectable deformation of the somatic AP after local delivery of the toxin. Using membrane potential imaging, we found that, at this minimal concentration, AaH-II substantially widened the AP in the AIS. Using ultrafast Na+ imaging, we found that local application of 7 nM AaH-II caused a large increase in the slower component of the Na+ influx in the AIS. Finally, using ultrafast Ca2+ imaging, we observed that 7 nM AaH-II produces a spurious slow Ca2+ influx via Ca2+-permeable VGNCs. Molecules targeting VGNCs, including peptides, are proposed as potential therapeutic tools. Thus, the present analysis in the AIS can be considered a general proof-of-principle on how high-resolution imaging techniques can disclose drug effects that cannot be observed when tested at the macroscopic level.

A Multiscale Approach to Axon and Nerve Stimulation Modeling: A Review

Electrical nerve fiber stimulation is a technique widely used in prosthetics and rehabilitation, and its study from a computational point of view can be a useful instrument to support experimental tests. In the last years, there was an increasing interest in computational modeling of neural cells and numerical simulations on nerve fibers stimulation because of its usefulness in forecasting the effect of electrical current stimuli delivered to tissues through implanted electrodes, in the design of optimal stimulus waveforms based on the specific application (i.e., inducing limb movements, sensory feedback or physiological function restoring), and in the evaluation of the current stimuli properties according to the characteristics of the nerves surrounding tissue. Therefore, a review study on the main modeling and computational frameworks adopted to investigate peripheral nerve stimulation is an important instrument to support and drive future research works. To this aim, this paper deals with mathematical models of neural cells with a detailed description of ion channels and numerical simulations using finite element methods to describe the dynamics of electrical stimulation by implanted electrodes in peripheral nerve fibers. In particular, we evaluate different nerve cell models considering different ion channels present in neurons and provide a guideline on multiscale numerical simulations of electrical nerve fibers stimulation.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

The Boltzmann equation in molecular biology

In the 1870's, Ludwig Boltzmann proposed a simple equation that was based on the notion of atoms and molecules and that defined the probability of finding a molecule in a given state. Several years later, the Boltzmann equation was developed and used to calculate the equilibrium potential of an ion species that is permeant through membrane channels and to describe conformational changes of biological molecules involved in different mechanisms including: open probability of ion channels, effect of molecular crowding on protein conformation, biochemical reactions and cell proliferation. The aim of this review is to trace the history of the developments of the Boltzmann equation that account for the behaviour of proteins involved in molecular biology and physiology.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Characterization of two Bunodosoma granulifera toxins active on cardiac sodium channels

1. Two sodium channel toxins, BgII and BgIII, have been isolated and purified from the sea anemone Bunodosoma granulifera. Combining different techniques, we have investigated the electrophysiological properties of these toxins. 2. We examined the effect of BgII and BgIII on rat ventricular strips. These toxins prolong action potentials with EC50 values of 60 and 660 nM and modify the resting potentials. 3. The effect on Na+ currents in rat cardiomyocytes was studied using the patch-clamp technique. BgII and BgIII slow the rapid inactivation process and increase the current density with EC50 values of 58 and 78 nM, respectively. 4. On the cloned hH1 cardiac Na+ channel expressed in Xenopus laevis oocytes, BgII and BgIII slow the inactivation process of Na+ currents (respective EC50 values of 0.38 and 7.8 microM), shift the steady-state activation and inactivation parameters to more positive potentials and the reversal potential to more negative potentials. 5. The amino acid sequences of these toxins are almost identical except for an asparagine at position 16 in BgII which is replaced by an aspartic acid in BgIII. In all experiments, BgII was more potent than BgIII suggesting that this conservative residue is important for the toxicity of sea anemone toxins. 6. We conclude that BgII and BgIII, generally known as neurotoxins, are also cardiotoxic and combine the classical effects of sea anemone Na+ channels toxins (slowing of inactivation kinetics, shift of steady-state activation and inactivation parameters) with a striking decrease on the ionic selectivity of Na+ channels.

The scorpion α-like toxin Lqh III specifically alters sodium channel inactivation in frog myelinated axons

The effects of 1-100 nM Lqh III, an alpha-like toxin isolated from the scorpion Leiurus quinquestriatus hebraeus, were assessed on the nodal membrane potential and ionic currents of single frog myelinated axons. In current-clamped axons, Lqh III increased the duration of action potentials without markedly affecting the peak amplitude and the resting membrane potential. The toxin was less effective when the resting membrane potential of axons was increasingly more positive. The Lqh III-induced increase in action potential duration was not due to the blockade of K(+) channels, since the toxin had no significant effect upon the K(+) current. In contrast, Lqh III inhibited the inactivation of a fraction of the Na(+) current, leading to a maintained late inward Na(+) current which represented about 45% of the peak Na(+) current, as observed during long-lasting depolarisations and in steady-state Na(+) current inactivation-voltage relationships when the pre-pulse potential was more positive than about -30mV. The activation kinetics of the late Na(+) current were well described by a single exponential whose time constant was 8.53+/-0.78 ms (n=3). Finally, Lqh III slowed the time-course of the remaining peak Na(+) current inactivation by altering initial amplitudes (to time zero of depolarisation) and time constants of its fast and slow phases. No significant additional effect was detected during the action of the toxin. In conclusion, we propose that, in frog myelinated axons, the effects of Lqh III are those typically attributed to classical scorpion alpha-toxins.

Nodal swelling produced by ciguatoxin-induced selective activation of sodium channels in myelinated nerve fibers

Ciguatoxin-1b, the major toxin involved in ciguatera fish poisoning, and D-mannitol were examined on frog nodes of Ranvier using confocal laser scanning microscopy and conventional current- and voltage-clamp techniques. During the action of 10 nM ciguatoxin-1b, an increase in nodal volume was observed as determined by digital image processing and three-dimensional reconstruction of axons. The increase was prevented by blocking Na+ channels with tetrodotoxin. Ciguatoxin-1b (10 nM) induced high frequency action potential discharges up to 70-100 Hz. Analysis of Na+ current revealed that the toxin modified a current fraction which was activated at resting membrane potential and failed to inactivate. Increasing the osmolality of the external solution by about 50% with D-mannitol restored the nodal volume to its control value and suppressed spontaneous action potentials. In addition, D-mannitol affected unmodified and ciguatoxin-1b-treated Na+ currents in a similar manner causing a reduction of maximum conductance, negative shifts of current reversal potential and modification of the voltage-dependence of current activation and inactivation. In conclusion, ciguatoxin-1b induced a tetrodotoxin-sensitive swelling of nodes of Ranvier and selectively affected the Na+ current of myelinated axons. It is proposed that ciguatoxin-1b, by modifying Na+ current, increased intracellular Na+ concentration which caused water influx and nodal swelling. This may explain some of the reported symptoms of ciguatera fish poisoning. D-mannitol, an agent used for ciguatera treatment, was found to reverse the effects of ciguatoxin-1b by reducing Na+ entry and increasing the efflux of water through its osmotic action. It is the first time that osmotic changes produced by the selective activation of ionic channels, i.e. Na+ channels, are reported.

Penicillin-induced triphasic modulation of GABAA receptor-operated chloride current in frog sensory neuron

Effects of penicillin-G (PCN) on GABA-evoked Cl- current (IGABA) were investigated in freshly dissociated frog sensory neurons by the use of the concentration-clamp technique combined with the suction-pipette method. Under conditions where the internal and external solutions allowed only Cl- permeability, PCN elicited triphasic modulation on IGABA, consisting of two modes of blockade on IGABA and a following rebound (rebound-like transient IGABA). Simultaneously applied PCN and GABA depressed IGABA immediately (phasic blockade), with the depressed IGABA slightly recovering in amplitude to achieve a stable level of blockade (tonic blockade). When a solution containing a mixture or PCN and GABA was quickly replaced by one containing GABA alone, a rebound-like transient Cl- current (IR) was evoked. Each component of the PCN actions on IGABA was PCN- and GABA-concentration-dependent. The reversal potential for each component of the PCN actions on IGABA was close to the chloride equilibrium potential (ECl) calculated using the Nernst equation. The current-voltage (I-V) relations for both the phasic and tonic blockade revealed inward rectification, while I-V curves for the control IGABA and the IR were outwardly rectified. The degree of IGABA-desensitization and the amplitude of the IR correlated well. The data suggest that partial removal of the GABAA receptor-desensitization may result in generation of the IR.

Voltage-dependent ionic currents in dissociated paratracheal ganglion cells of the rat

1. Conventional whole-cell voltage-clamp technique was used to study the electrophysiological and pharmacological properties of voltage-dependent Na+, K+ and Ca2+ channels in parasympathetic neurones enzymatically dissociated from the paratracheal ganglia of rat trachea. The voltage-dependent Na+, K+ and Ca2+ currents (INa, IK and ICa) were separated by the use of ion subtraction and pharmacological treatments. 2. INa was activated by a step depolarization more positive than -50 mV and fully activated at positive potentials more than +10 mV. The inactivation phase of INa consisted of fast and slow exponential components (tau if and tau is, respectively). The tau if and tau is were voltage dependent and decreased with a more positive step pulse. 3. The time course for recovery of INa from the complete inactivation exhibited two exponential processes. 4. The reversal potential of INa was equal to the Na+ equilibrium potential (ENa) and resembled a simple Na+ electrode depending only on external Na+ concentration. 5. Tetrodotoxin (TTX) reduced INa without affecting the current kinetics in a concentration-dependent manner, and the concentration of half-maximal inhibition (IC50) was 6 x 10(-9) M. There was no TTX-resistant component of INa in any of the cells tested. 6. Scorpion toxin increased the peak amplitude of INa and prolonged the inactivation phase in a time- and concentration-dependent manner. In the presence of toxin, both tau is and the fractional contribution of the slow current component to total INa increased concentration dependently. 7. High-threshold (L-type) ICa was activated by a step depolarization more positive than -30 mV and reached a peak at near 0 mV in the external solution with 2.5 mM Ca2+. The current was inactivated to only a small extent (< 10%) during 100 ms of depolarizing step pulse. There was no low-threshold (T-type) ICa in this preparation. 8. The maximum ICa in individual current-voltage (I-V) relationships was saturated by an increase in extracellular Ca2+ concentration ([Ca2+]o). The I-V relationships were also shifted along the voltage axis to the more positive potential with increasing [Ca2+]o. 9. The inactivation process of the L-type ICa was dependent on Ca2+ influxes (ICa-dependent inactivation). 10. Relative maximum peak currents of divalent cations passing through the L-type Ca2+ channels were in the order of IBa > ICa > ISr. 11. Organic and inorganic Ca2+ antagonists blocked the ICa in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Tetrodotoxin-sensitive calcium-conducting channels in the rat hippocampal CA1 region

1. Tetrodotoxin (TTX)-sensitive Ca2+ conducting channels which produce a transient inward current were investigated in pyramidal neurones freshly dissociated from the dorsal part of rat hippocampal CA1 region by the use of the suction-pipette technique, which allows for intracellular perfusion under a single-electrode voltage clamp. 2. In all cells superfused with Na(+)- and K(+)-free external solution containing 10 mM-Ca2+ and 10(-5) M-La3+, a transient inward Ca2+ current was evoked by a step depolarization to potentials more positive than about -50 mV from a holding potential (VH) of -100 mV. This current was inhibited by either removing the extracellular Ca2+ or adding TTX (termed as 'TTX-ICa'). 3. Activation and inactivation processes of the TTX-ICa were highly potential dependent at 20-22 degrees C, and the latter was fitted by a double exponential function. The time to peak of the current decreased from 5.0 to 2.3 ms at a test potential change from -50 to 0 mV. The time constants of the current decay decreased from 2.8 to 2.2 ms for fast component (tau if) and from 16.0 to 8.2 ms for slow component (tau is) at a potential change from -35 to -10 mV. 4. The TTX-ICa was activated at threshold potential of about -55 mV and reached full activation at -30 mV. The steady-state inactivation of TTX-ICa could be fitted by a Boltzmann equation with a slope factor of 6.0 mV and a half-inactivation voltage of -72.5 mV. 5. Biphasic recovery (reactivation) from the complete inactivation of TTX-ICa was observed. The time constant of the major component (78.8 to 91.6% of total) of the reactivation was 13.1 ms, and that of the minor one was 120 to 240 ms. Therefore, TTX-ICa remained fairly constant at a train of stimulation up to 3 Hz. However, the inhibition of current amplitude occurred as the repetitive stimulation increased more than 10 Hz, and considerable tonic inhibition occurred with increasing stimulation frequency. 6. When the peak amplitudes in the individual current-voltage (I-V) relationships of TTX-ICa at various extracellular Ca2+ concentrations ([Ca2+]o) were plotted as a function of [Ca2+]o, the current amplitude increased linearly without showing any saturation. 7. The ratio of peak amplitude in the individual I-V relationships of Ca2+, Sr2+ and Ba2+ currents passing through the TTX-sensitive Ca2+ conducting channel was 1:0.33:0.05, although the current kinetics were much the same.(ABSTRACT TRUNCATED AT 400 WORDS)

Novel Ca2+ currents in mammalian CNS neurons

Voltage-dependent Ca2+ currents (ICa) in neurons can be classified into T-, N- and L-types. In the CA1 pyramidal neurons freshly dissociated from rat hippocampus we found an additional tetrodotoxin (TTX)-sensitive Ca2+ current (termed 'TTX-ICa'). The TTX-ICa showed a heterogeneous distribution, preferentially in the dorsal site of CA1 region. Activation and inactivation processes of the TTX-ICa were highly potential-dependent, and the latter was fitted by a double exponential function. The TTX-ICa was activated at a threshold potential of about -55 mV and reached full activation at -30 mV. The steady-state inactivation of TTX-ICa could be fitted by a Boltzmann equation with a slope factor of 6.0 mV and a half-inactivation voltage of -72.5 mV. When the peak amplitudes of TTX-ICa were plotted as a function of extracellular Ca2+ concentration ([Ca2+]o), the current amplitude increased linearly without showing any saturation. The ratio of peak amplitude in the individual I-V relationships of Ca2+, Sr2+ and Ba2+ currents passing through the TTX-sensitive Ca(2+)-conducting channel was 1:0.33:0.05, although the current kinetics were much the same. TTX inhibited the TTX-ICa in time- and concentration-dependent manner without affecting the current kinetics. Lignocaine inhibited the TTX-ICa in a second in a concentration-dependent manner, with accelerating the inactivation process. The concentrations of half-inhibition (IC50) were 3.5 x 10(-9) M for TTX and 3.6 x 10(-4) M for lignocaine. Scorpion toxin prolonged the inactivation phase of TTX-ICa in a time- and concentration-dependent manner. In the toxin-treated neurons, both the slow time constant of inactivation (tau is) and its functional contribution to the total current increased with increasing the toxin concentration.

Voltage-sensitive Na+ channels in mammalian peripheral nerves detected using scorpion toxins

The localization of voltage-sensitive sodium channels was investigated in mouse, rat and rabbit sciatic nerves using iodinated alpha- and beta-Scorpion toxins (ScTx) as specific probes. Saturable specific binding for a beta-ScTx was detected in mouse sciatic nerve homogenates (Kd = 90 pM, binding site capacity = 90 fmol mg-1 protein). LM autoradiographic studies demonstrated that the two types of ScTx stained the Ranvier nodes of the myelinated fibres, and also showed a clear but weaker labelling of the unmyelinated Remak bundles. In the sciatic nerve, which is widely considered as a model 'myelinated nerve', the nodal membrane represented only a small fraction of the total axonal membranes (0.2% and 0.05% for mouse and rabbit sciatic nerves respectively). Therefore, despite their high channel density, nodal membranes contribute only a small proportion of the total labelling by beta-ScTx (15% and 2.3% for mouse and rabbit sciatic nerves respectively), with the major contribution to labelling arising from unmyelinated axons. The distribution of specific binding sites for a beta-Scorpion toxin was then analysed in cross-sections of rabbit sciatic nerve at the EM level. The quantitative analysis of autoradiograms involved three methods, the 50% probability circle method, and two cross-fire analyses using either systematically distributed hypothetical sources or hypothetical sources only located on the plasma membranes of axons and of Schwann cells associated with unmyelinated Remak bundles. No specific beta-Scorpion toxin binding sites were detected at the plasma membrane of Schwann cells from either myelinated fibres or unmyelinated bundles, or at the internodal surface of myelinated axons. Sites were only detected at the surface of unmyelinated axons and at nodal axolemma. Their density in unmyelinated axons was found to be in the range of 1-6 per micron2 of plasma membrane surface area by combining quantitative EM autoradiography and stereological measurements.

Modification of electrophysiological and pharmacological properties of K channels in neuroblastoma cells induced by the oxidant chloramine-T

The effects of chloramine-T (CL-T) on voltage-dependent potassium channels in neuroblastoma cells were analysed using the whole-cell current recording technique. CL-T irreversibly decreased the peak whole-cell K current, considerably slowed its inactivation and shifted its activation-voltage curve towards positive voltages by 6 mV. Under control conditions, the inactivation of the whole-cell K current could be described by the sum of two exponentials, F and S, whose time constants at +50 mV were tau F = 1.00 +/- 0.15 S and tau S = 5.72 +/- 0.47 S respectively. After CL-T, it could be described by the sum of two (S1 and S2) or three (F, S1 and S2) exponentials whose time constants at +50 mV were: tau F = 0.81 +/- 0.22 S, tau S1 = 6.46 +/- 0.60 S and tau S2 = 48.56 +/- 3.64 S. Under control conditions, F and S inactivating components of the whole-cell K current were blocked by 4-aminopyridine, with a Hill coefficient of 1 and apparent dissociation constants of 0.04 and 0.7 mM respectively. After CL-T, both S1 and S2 components were equally blocked by 4-aminopyridine with a Hill coefficient of 0.25, being reduced to 64% of their control values by 10 mM. CL-T is known to slow the inactivation of sodium channels and to oxidize sulphydryl amino acids and unsaturated lipids. It is concluded that the inactivation gates of voltage-dependent sodium and potassium channels are either constituted of the same amino acid residues or are controlled by unsaturated lipid surrounding or bound to the channel proteins.

The inactivation of sodium channels in the node of Ranvier and its chemical modification

The many experimental studies reported demonstrate the complexity of what is termed inactivation, the decrease of current flow through sodium channels at maintained depolarization. Even at the normal resting potential of, say, -70 mV for a frog node of Ranvier, ca. 20% of the channels are closed and inactivated, i.e., incapable of passing current on a sudden depolarization, in contrast to the remaining 80% of closed but resting channels. The term inactivation has thus evolved from bulk current ("macroscopic") phenomena and is applied to channels although its single-channel ("microscopic") basis is not entirely clear and may even vary among preparations. It is conceivable that the macroscopic phenomenon may have more than a single microscopic cause; this point will probably not be settled until a physical description of the conformational states of the channel macromolecule becomes available. At any rate, channel transition into an inactivated closed state can be easily affected by numerous reagents of highly diverse chemical nature and, most likely, different primary sites of action as already suggested by the sidedness of effective application, e.g., iodate and endopeptidases to the inside, polypeptide toxins to the outside. But also the search for a common denominator, a secondary target of all these treatments, has not been very successful as demonstrated by the experiments with group-specific reagents. Since modification of inactivation is often accompanied by shifts in the voltage dependence of gating parameters, a target could be the "voltage sensor" of the channel, charged and/or dipolar components of the channel macromolecule that, by being moved in the electric field, somehow induce gating and whose movement is measured as gating current (e.g, Hille, 1984). The fraction of open channels as a function of membrane potential, F(E), may serve as an indicator. It may be simply shifted (to more negative potentials) as by veratridine (Leibowitz et al., 1987) or flattened (reduction of gating charge?) and shifted (in the positive direction) as by Anemonia sulcata toxin II (Ulbricht and Schmidtmayer, 1981) or chloramine-T (Drews, 1987). On the other hand, the steady-state inactivation curve is shifted to more negative potentials by the toxin (Ulbricht and Schmidtmayer, 1981), but to more positive potentials by chloramine-T (Wang, 1984a; Schmidtmayer, 1985). Obviously, modifiers may affect activation and inactivation quite differently, a result that touches on the question as to what extent inactivation derives its potential dependence from activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in Na channel properties of frog and rat skeletal muscles induced by the AaH II toxin from the scorpion Androctonus australis

The effects of the mammal toxin II isolated from the venom of the scorpion Androctonus australis Hector (AaH II) were studied under current and voltage clamp conditions in frog (semitendinosus) and rat (fast e.d.l. and slow soleus) skeletal twitch muscle fibres. In both species, AaH II induced a dose-dependent prolongation of the action potential (AP) leading at saturating concentration to APs with long plateaus of about 1.5 s in frog and 5 s in rat e.d.l. and soleus fibres. The concentrations to induce 50% of the maximal effect (K0.5) were 9.1 x 10(-9) M in the frog and 1.4 x 10(-9) M in the rat. AaH II increased the time constants of inactivation of the peak Na current and induced a maintained Na current that was greater in rat e.d.l. and soleus (31.6% of peak current amplitude at -30 mV; K0.5 = 0.8 x 10(-9) M) than in frog (16.5%; K0.5 = 15.5 x 10(-9) M) muscles. Peak and maintained Na currents were TTX-sensitive and had identical threshold and reversal potentials. The half-maximum maintained permeability occurred at a potential 20 mV more positive than the peak permeability. Recovery from inactivation and steady-state inactivation of the inactivating Na current remained unchanged. The maintained current deactivated with normal fast kinetics. The action of the toxin reversed poorly on washout but could be largely removed by conditioning depolarizations more positive than the reversal potential of the Na current. Our results suggest that, in vertebrate skeletal muscle fibres, AaH II affects all the Na channels and are consistent with the hypothesis that the maintained current originates from a reopening of previously inactivated Na channels.

Voltage and temperature dependence of normal and chemically modified inactivation of sodium channels. Quantitative description by a cyclic three-state model

(1) Voltage clamp experiments were done on single myelinated nerve fibres of the frog, Rana esculenta, with 10 mM TEA in the external solution to block potassium channels. (2) The potential dependence of normal sodium current inactivation was studied over a potential range of V = -50 mV to 80 mV. At depolarizations (V greater than or equal to 30 mV) inactivation is diphasic. The relative contribution of the fast phase increases from 0.4 at V = 30 mV to 0.96 at V = 80 mV. At resting potential (V = 0 mV) recovery from inactivation shows a sigmoidal time course. At strong hyperpolarization (V = -50 mV) recovery is diphasic with a predominant fast phase. (3) For a quantitative description of these findings a cyclic three-state model of inactivation, with one open and two closed states, is formulated. The potential dependence of the rate constants is determined and calculations from this model are compared with the experimental data. (4) To test the availability of the proposed model, normal inactivation was modified by treatment with 0.6 mM chloramine-T. This substance causes inactivation to become slow and incomplete; the potential dependence of steady-state inactivation becomes non-monotonic. All these effects are explained as quantitative changes of the rate constants in the cyclic inactivation model. (5) The influence of temperature on normal inactivation was studied at a range of 8-20 C. Both time constants as well as the two inactivation components are temperature-dependent. For a quantitative description of temperature effects by the cyclic model, the activation enthalpies of the rate constants are evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)

Scorpion toxin prolongs an inactivation phase of the voltage-dependent sodium current in rat isolated single hippocampal neurons

The effects of scorpion toxin on the voltage-dependent sodium current (INa) of CA1 pyramidal neurons isolated from rat hippocampus were studied under the single-electrode voltage-clamp condition using a 'concentration-clamp' technique. The toxin increased the peak amplitude of INa and prolonged its inactivation phase in a time- and dose-dependent manner. Inactivation phase of INa proceeded with two exponential components in the absence (control) and presence of the toxin. In the toxin-treated neurons, both the time constant of slow component and its fractional contribution to the total current increased dose-dependently while the fractional contribution of the fast one decreased in a dose-dependent fashion without changing its time constant. Actions of scorpion toxin on the sodium channels of hippocampal pyramidal neurons were essentially similar to those of peripheral preparations. Therefore, it can be concluded that the sodium channels of mammalian brain neurons have structures and functions similar to peripheral channels.

The action of pyrethroids on sodium channels in myelinated nerve fibres and spinal ganglion cells of the frog

The interaction of pyrethroids with the voltage-dependent sodium channel was studied in voltage-clamped nodes of Ranvier and isolated spinal ganglion neurons of the clawed frog, Xenopus laevis. In the node, pyrethroids prolonged the sodium tail current associated with a step repolarization of the membrane. It was found that the amplitude of the slow, pyrethroid-induced, sodium tail current (PIT) first increased and then decreased as a function of the duration of membrane depolarization (to -5 mV). This decrease of the PIT amplitude was absent when depolarizations to the sodium equilibrium potential (+40 mV) were used. Measurements of changes in sodium reversal potential indicated that sodium ion depletion in the perinodal space is largely responsible for the inactivation of the pyrethroid-modified sodium current. Inactivation is not completely abolished by pyrethroid treatment since the probability of channel opening, measured in membrane patches excised from spinal ganglion cells, decreased slowly during prolonged depolarization. Analysis of unitary currents indicated that both activation and inactivation are retarded by pyrethroids. The arrival of sodium channels in the pyrethroid-modified open state followed a time course that was slower than both activation and inactivation of unmodified sodium channels. Our findings indicate that sodium channels are modified when in the closed resting state and that both opening and closing kinetics are delayed by pyrethroids.

Effects of toxin II from the scorpion Androctonus australis Hector on sodium current in neuroblastoma cells and their modulation by oleic acid

The effects of toxin II (AaH II) isolated from the scorpion Androctonus australis Hector on sodium current in neuroblastoma X glioma NG 108-15 hybrid cells were analysed under patch clamp conditions in the whole cell configuration. AaH II (70 nM) induced a maintained sodium current, as well as increasing both fast and slow inactivation time constants and the amplitude of the peak current. This latter effect occurred via a shift of the activation-voltage curve towards negative voltage values by about 9 mV. Oleic acid (5 microM), which had no effect on INa under control conditions, decreased the AaH II-induced maintained current. It also reversed, or prevented the increase of the peak current induced by AaH II. However, it neither prevented nor modified the AaH II-induced increase in inactivation time constants. The binding of the toxin to its specific site and the number of binding sites for AaH II were not significantly modified by oleic acid. The oleic acid-induced effects could not be related to the activation of protein kinase C since PMA, a potent activator of this enzyme, did not produce oleic acid-like effects. From these results, it is concluded that AaH II has several independent effects on sodium channels, some of which could be modulated by the lipid environment of sodium channels in the membrane.

Interactions of guanidinium ions with sodium channels in frog myelinated nerve fibre

1. The effects of external guanidinium ions on fast and slow inactivating currents flowing through sodium channels of the frog myelinated nerve fibre (Benoit, Corbier & Dubois, 1985) were analysed under voltage-clamp conditions. 2. When external sodium ions were partially replaced by guanidinium ions, the fast inactivating current was preferentially reduced and was absent in a solution containing guanidinium ions as the only external permeant cations. The inactivation time constants of both fast and slow currents were not significantly modified by the replacement of sodium ions by guanidinium ions. 3. Substitution of guanidinium ions for all sodium ions shifted the steady-state inactivation curve of the slow inactivating current towards positive voltages. 4. The voltage dependence of the activation of fast and slow inactivating currents was shifted towards positive voltages by guanidinium ions. Moreover, the activation-voltage curve of the slow inactivating current, which was biphasic under control conditions, was monophasic when guanidinium ions were substituted for all sodium ions. 5. Whereas the slow inactivating current could be carried by guanidinium ions, these cations were not only impermeant through the sodium channels which give rise to the fast inactivating current but also blocked this type of channel with an apparent dissociation constant of 49 mM. 6. It is concluded that guanidinium appears to be an efficient tool for further separating the two types of inactivating current and studying the properties of the slow inactivating current. These results are consistent with the suggestion that there are two types of sodium channels, fast and slow, with guanidinium ions being permeant only through the slow ones.