Hydrogen currents through aconitine-modified sodium channels in nerve fiber membranes

Ionic currents in nodal membrane treated with aconitine were measured under voltage clamp conditions when nodes were bathed in Na-free solutions. At pH lower than 4.6 inward ionic currents were detected which had kinetics and voltage range of activation analogous to those of aconitine-modified sodium channels at low pH. These currents were blocked by benzocaine (2 mM). Experiments with various concentrations of Ca2+, tris+, TEA+, choline+ ions showed that these ions are essentially impermeable both at normal and acidic pH. It is concluded that the inward currents observed are carried by H+ (or H3O+) ions through aconitine-modified sodium channels. From reversal potential measurements relative permeability (PH/PNa) of sodium channels is estimated to be 1059 +/- 88. The results suggest that the aconitine-modified channel is a rather wide water-filled pore and the rate of H+ passing through the channel is limited by its binding to an acidic group.     

Mediobasal hypothalamic neurons are excited by the iontophoretic application of sodium

In the course of studies on the responsiveness of mediobasal hypothalamic neurons to the iontophoretic application of cortisol, it was found that positive currents applied to a sodium chloride (1 M) barrel alone, but not to a choline chloride (1 M) barrel, frequently increased the firing of these neurons. Subsequently, systematic examination demonstrated that out of 102 MBH neurons 52 (51%) increased their firing by at least 30% with application of NaCl, using currents no greater than 10 nA. No such effect was obtained in response to Na application from a dilute solution (0.05 or 0.1 M). When glutamate was absent from the electrodes, the incidence of Na+ sensitivity fell to 17%, despite the routine use of backing currents to the glutamate barrel. K+ ions were more active than Na+ ions in producing excitation. When Na+ sensitivity was found, however, Na+ effects were produced by currents greater than K+ currents producing equivalent excitation. Like glutamate, K+ ions were capable of greatly enhancing responses to Na+. Comparison was made between cortisol and Na+ sensitivity in 70 MBH neurons; 28 cells responded to both, and 24 of them were inhibited by cortisol. Thus Na+ sensitivity is a frequent characteristic of MBH neurons inhibited by cortisol, and was present in 83% of cortisol-sensitive cells in this region. Iontophoresis of Na+ is commonly used as a control in pharmacological studies of the nervous system. Even more common is the case of concentrated NaCl solutions for recording. These procedures may not be as inert as previously thought, particularly in the hypothalamus.     

Cobalt-sensitive and dihydropyridine-insensitive stimulation of dopamine release from tuberoinfundibular neurons by high extracellular concentrations of barium ions

Recently, it has been demonstrated that Ca2+ entrance into the neuronal cytoplasm can occur upon the activation of 3 different types of specific voltage-dependent channels which can be characterized according to the following criteria: (1) voltage threshold for activation; (2) tendency to inactivation; (3) bivalent cation permeability; and (4) drug sensitivity. In this study we investigated, in tuberoinfundibular dopaminergic (TIDA) hypothalamic neurons, the biochemical and pharmacological properties of Ca2+ channels, by comparing the effects of high extracellular concentrations of Ba2+ and Ca2+ ions on [3H]dopamine (DA) release from TIDA neurons. The results obtained show that extracellular Ba2+ ion concentrations dose-dependently (10-20 mM) stimulated [3H]DA release from superfused TIDA neurons and that this effect was prevented by Co2+ ions (2 mM). In addition, superfusion of TIDA neurons with a concentration of Ca2+ ions equimolar to that of Ba2+ ions (20 mM) failed to modify [3H]DA release. The fact that tetraethylammonium (10 mM), a blocker of K+ currents in excitable cells, did not mimick the stimulatory action of Ba2+ ions on [3H]DA release, seems to exclude that the effect of Ba2+ ions was dependent on the inhibition of K+ channels in TIDA neurons. The omission of Ca2+ ions from the extracellular medium did not prevent the stimulatory effect on [3H]DA release elicited by elevated concentrations of Ba2+ ions, but rather reinforced this effect. Finally, nitrendipine (50 microM) did not modify the stimulatory effect of high extracellular Ba2+ ions on [3H]DA release from TIDA neurons.     

Calcium activates two types of potassium channels in rat hippocampal neurons in culture

Several calcium-dependent potassium currents can contribute to the electrophysiological properties of neurons. In hippocampal pyramidal cells, 2 afterhyperpolarizations (AHPs) are mediated by different calcium-activated potassium currents. First, a rapidly activated current contributes to action-potential repolarization and the fast AHP following individual action potentials. In addition, a slowly developing current underlies the slow AHP, which occurs after a burst of action potentials and contributes substantially to the spike-frequency accommodation observed in these cells during a prolonged depolarizing current pulse. In order to investigate the single Ca2(+)-dependent channels that might underlie these currents, we performed patch-clamp experiments on hippocampal neurons in primary culture. When excised inside-out patches were exposed to 1 microM Ca2+, 2 types of channel activity were observed. In symmetrical bathing solutions containing 140 mM K+, the channels had conductances of 19 pS and 220 pS, and both were permeable mainly to potassium ions. The properties of these 2 channels differed in a number of ways. At negative membrane potentials, the small-conductance channels were more sensitive to Ca2+ than the large channels. At positive potentials, the small-conductance channels displayed a flickery block by Mg2+ ions on the cytoplasmic face of the membrane. Low concentrations of tetraethylammonium (TEA) on the extracellular face of the membrane specifically caused an apparent reduction of the large-channel conductance. The properties of the large- and small-conductance channels are in accord with those of the fast and slow AHP, respectively.     

2 selective filters in the calcium channel of the somatic membrane of mollusk neurons

The modification of inward currents in the somatic membrane of mollusc neurons produced by EDTA and other CA-chelating agents was investigated. The results obtained indicate the presence of two selective filters in the calcium channel of this membrane. The first one is located near the outer mouth of the calcium channel. It binds divalent cations in the following sequence--pKCa: pKSr: pKBa: pKMg=6.6 : 5.5 : 4.8 : 4.2. This outer filter controls channel selectivity according to the magnitude of cation charge and presumably contains several carboxylic groups. The second selective filter is located inside the channel and controls its permeability for cations with the same charge. It is assumed that the structure of the inner selective filter resembles very much the structure postulated by Hille for the selective filter of sodium channel and that it contains only one carboxylic group. The investigation of the effect of Ca2+ and Cd2+ ions on sodium currents of the same membrane has shown that the fast sodium current is not blocked by these ions and the observed decrease of its amplitude is connected with the change of the membrane surface potential and corresponding change of near-membrane concentration of carrier ions. On the basis of these experiments, it is suggested that the selectivity filter of sodium channel of this membrane does not contain a carboxylic group.     

Calcium inhibits willardiine-induced responses in kainate receptor GluR6(Q)/KA-2

A number of studies have demonstrated that willardiine [(S)-1-(2-amino-2-carboxyethyl) pyrimidine-2,4-dione] is a useful agonist for the activation of AMPA/kainate receptors. Here we examine the effect of extracellular calcium on currents evoked by willardiine in HEK 293 cells expressing the GluR6(Q)/KA-2 kainate receptor subunits. At a concentration of 1.8 mM, Ca2+ inhibited the currents induced by 100 microM willardiine by approximately 50%. When extracellular Na+ ions were replaced with Ca2+ ions there were no measurable inward currents. We conclude that Ca2+ inhibition of the willardiine-induced response is concentration dependent.     

Electroregulated sodium channels of the somatic membrane of sympathetic neurons

Electrically-operated sodium channels in the somatic membrane of isolated neurons from the rat superior cervical ganglion have been studied by means of intracellular dialysis technique under voltage clamp conditions. It was shown that in this preparation sodium currents can be carried by two independent systems of sodium channels. The mathematical analysis of voltage-dependent TTX-sensitive fast sodium currents was performed by the Hodgkin-Huxley formalism; their kinetic properties were compared with those described in other objects. TTX-sensitive sodium channels in the somatic membrane of sympathetic neurons were found to be highly selective for Na+ ions. Kinetic and voltage-dependent characteristics of slow TTX-resistant sodium current were also described. This component of the sodium current was observed only in a few neurons (not more than 2%).     

Hydrogen ion currents through sodium channels in myelinated nerve fiber membranes

Ionic currents in the nodal membrane of myelinated frog nerve fibre were measured under voltage clamp conditions when the Ranvier node was bathed in solutions containing impermeant cations instead Na. At pH lower than 4.0 small (less than 0.1 nA) currents were detected which rose to peak and then decayed more slowly. Kinetics and voltage range of activation of these currents were similar to those of usual sodium currents at low pH. These currents were reversibly blocked by benzocaine (1 mM). All this permitted identifying them as currents through sodium channels. Experiments in which concentrations of substituting cations (tris+, choline+), Ca2+ and H+ ions were varied showed that the inward currents observed are carried by hydrogen (or hydronium) ions. According to reversal potential measurements the relative permeability of the channels (PH/PNa) is equal to 203 +/- 14 on the average. It is concluded that the energy barriers for H+ in sodium channel are much lower than for Na+, but their passage through the channel is slow because of binding to an acidic group in the channel.     

The effect of intracellular cAMP injections on stationary membrane conductance and voltage- and time-dependent ionic currents in identified snail neurons

3'5'-cAMP injected iontophoretically into identified neurons of the snail, Helix pomatia, induced depolarization of the membrane. Clamping the membrane potential revealed the appearance of a simultaneous inward transmembrane current ('cAMP-current') followed by a weaker outward current. External application of theophylline increased the amplitude of cAMP-current. Imidazole had an opposite effect on this current. Tolbutamide and lowering of temperature largely reduced its rate of rise and, correspondingly, its amplitude. Simultaneous removal of Na+ ions from external solution and addition of Cd2+ ions resulted in complete disappearance of the inward cAMP-current. Analysis of current-voltage characteristics of the cAMP-current at varying external concentrations of Na+, K+, Cl and Ca2+-ions has shown that the cAMP-current is due to an increase of membrane conductance to Na+, K+ and Ca2+ ions; a late component of the cAMP-current is associated with an increase of potassium conductance of the neuronal membrane induced, probably by Ca2+, influx. Besides the induction of stationary currents, cAMP injection also decreased the voltage- and time-dependent calcium currents reducing the maximum calcium conductance. After the end of injection, calcium currents restored their initial value in 1-2 min. An analogous decrease of the calcium current could be evoked by prolonged external application of theophylline. Possible mechanisms of intracellular effects of cAMP on electrical characteristics of the neuronal membrane are discussed.     

Cobalt-dependent potentiation of net inward current density in Helix aspersa neurons.

A low concentration of transition metal ions Co2+ and Ni2+ increases the inward current density in neurons from the land snail Helix aspersa. The currents were measured using a single electrode voltage-clamp/internal perfusion method under conditions in which the external Na+ was replaced by Tris+, the predominant external current carrying cation was Ca2+, and the internal perfusate contained 120 mM Cs+/0 K+; 30 mM tetraethylammonium (TEA) was added externally to block K+ current. In the presence of Co2+ (3 mM) or Ni2+ (0.5 mM) inward Ca2+ currents were stimulated normally by voltage-dependent activation of Ca2+ channels. There was a 5-10% decrease in the rate of rise of the inward current. The principal effect of Co2+ and Ni2+ in increasing the current density seems to be a decrease in the rate at which the inward currents decline during a depolarizing voltage pulse. The results may be due to a decrease in a voltage-dependent or Ca(2+)-dependent outward current and/or an inhibition of Ca2+ channel inactivation. Outward current under these conditions (zero internal K+) was significant and most likely due to Cs+ efflux through the voltage-activated or Ca(2+)-activated nonspecific cation channels. Co2+ is an extremely effective blocker of this outward current. These results are not an artifact of internal perfusion or the special ionic conditions. Intracellular recording of unperfused neurons in normal Helix Ringer's solution showed that the Ca(2+)-dependent action potential duration was increased significantly by low concentrations of Co2+. This result is consistant with the Co(2+)-dependent increase in inward (depolarizing) current seen in voltage-clamp experiments.     

Ionic conductances underlying the activity of interneurons that control heartbeat in the medicinal leech

Electrical properties of interneurons that control heartbeat in the leech (HN cells) were studied using intracellular recording and stimulation in isolated ganglia bathed by salines of various ionic compositions. Substitution of Na+ ions in the bath by Tris stopped the spontaneous firing of HN cells and led to their gradual hyperpolarization by 15-20 mV. In the absence of Na+, HN neurons produced long-lasting regenerative plateau potentials with thresholds near -55 mV and peaks near -30 mV that were accompanied by an increase in membrane conductance. Elevation of Ca2+ concentration enhanced plateaus, as did replacement of Ca2+ by Ba2+. Plateaus were formed when Sr2+ replaced Ca2+, but were blocked by addition of Mg2+ or Co2+ to the bath, Co2+ being effective at lower concentrations than Mg2+. Hyperpolarization of HN neurons with injected currents revealed a time-dependent change in membrane potential, whereby initial maximum hyperpolarization was followed by a "sag" in potential towards more depolarized values. The sag showed dual voltage dependence, being diminished when HN neurons were hyperpolarized or depolarized outside the normal range of oscillation. The sag was found to depend on the presence of Na+ ions and to be blocked by Cs+ but not by Ba2+. This time-dependent change in membrane potential counters hyperpolarizations of HN neuron membrane potential and may contribute to the escape of these neurons from synaptic inhibition.     

Protein Kinase C Facilitation of Acetylcholine Release at the Rat Neuromuscular Junction

Protein kinase C (PKC) is a Ca²⁺-dependent enzyme involved in synaptic transmission, which can be experimentally activated by the phorbol ester, phorbol 12-myristate-13-acetate (TPA). We studied the effects of TPA application on acetylcholine (ACh) release at the rat neuromuscular junction by means of the focal recording technique; possible effects of TPA at the postsynaptic site had been ruled out in preliminary studies. In extracellular solutions containing 2 mM Ca²⁺ and at the stimulation frequency of 0.1 Hz, TPA increased endplate current (EPC) amplitude. In non-stimulated preparations spontaneous current frequency was increased at a similar rate. The similar time course of TPA action on evoked and spontaneous currents suggests that an increased presynaptic Ca²⁺ efficacy can be considered to be the probable mechanism of action. The interactions of PKC with ACh release were further investigated. In 0.1 mM Ca²⁺ extracellular solutions, TPA enhanced evoked currents only at stimulation frequencies (e.g. 40 Hz) that were themselves capable of inducing facilitation. This facilitation is classically associated with presynaptic Ca²⁺ accumulation, indicating that PKC interacts synergistically with Ca²⁺ to facilitate ACh release. In particular, since mean quantum size and release probability remained almost unchanged during TPA facilitation, it was concluded that PKC acted by enlarging the immediately available store. Interestingly, TPA also increased the presynaptic currents that were observed to be largely brought about by Ca²⁺-dependent K⁺ currents: evidence was obtained to suggest that increases in these currents provide negative feedback against excess release activation rather than being an expression of enhanced Ca²⁺ influx.     

Calcium-dependent action potentials and associated inward currents in guinea-pig neocortical neurons in vitro

Calcium-dependent potential changes and inward currents were studied in guinea-pig neocortical neurons maintained in vitro. Under conditions of reduced outward potassium current, induced by external application of tetraethylammonium ions or internal application of caesium ions, regenerative Ca2+-dependent action potentials could be elicited. Strontium and barium ions could substitute for calcium as the charge carrier but not magnesium; cadmium blocked the calcium spikes. In caesium-loaded neurons, in the presence of tetrodotoxin and tetraethylammonium, inward currents were recorded when the membrane potential was step-depolarized to potentials more positive than -50 mV. These currents were blocked by cadmium. It is concluded that guinea-pig neocortical neurons are capable of generating a calcium action potential via the activation of a slow inward current.     

Three types of voltage-dependent calcium current in cultured rat hippocampal neurons

Voltage-dependent calcium (Ca2+) currents in cultured rat hippocampal neurons were studied with the whole-cell recording mode of the patch-clamp technique. On the basis of the voltage-dependence of activation, kinetics of inactivation and pharmacology, 3 types of Ca2+ currents were distinguished. The low-threshold Ca2+ current (Il) was activated at -60 mV, and completely inactivated during a 100-ms depolarization to -40 mV (time constant: tau = 16 +/- 1 ms). The high-threshold currents (Ih), which were activated at -20 mV, could be separated into two types. The high-threshold, fast inactivating current (Ih,f) decayed quickly during a maintained depolarization (tau = 33 +/- 3 ms at 0 mV), whereas the high-threshold, slowly inactivating current (Ih,s) decayed with a much slower time constant (tau = 505 +/- 42 ms at 0 mV). The inactivations of Ih,f and Ih,s exhibited different time- and voltage-dependencies. Nickel ions (Ni2+, 25 microM) markedly suppressed Il, but little affected Ih. Cadmium ions (Cd2+, 10 microM) almost completely suppressed Ih, but left a small amount of Il. Lanthanum ions (La3+, 10 microM) almost completely suppressed both Il and Ih. Ih,s was sensitive to block by the dihydropyridine antagonist nicardipine (10 microM).     

Steady growth cone currents revealed by a novel circularly vibrating probe: a possible mechanism underlying neurite growth

The rate and direction of neurite growth have been shown in a number of studies to be determined by the distribution of adhesive sites on the growth cone. Recent evidence showing that the application of extrinsic electric fields can redistribute membrane molecules and alter both the rate and direction of neurite growth have raised the question whether endogenous electric fields might be produced by steady currents in growth cones. To investigate this question, we have devised a novel circularly vibrating microprobe capable of measuring current densities in the range of 5 nA/cm2 (near the theorectical limit of sensitivity), with a spatial resolution of 2 micron. The design of this device and the development of a novel algorithm for computing current vectors on-line is described. Using this probe we have found that cultured goldfish retinal ganglion cell growth cones generate steady inward currents at their tips. The measured currents, in the range of 10-100 nA/cm2, appear to flow into the filopodia at their tips and back outward near the junctures of the filopodia and the growth cone. The currents appear to be produced only during active growth. Ion substitution experiments support the conclusion that the majority of this current is carried by Ca2+ ions, which we postulate flow through a population of activated voltage-sensitive Ca2+ channels located on the filopodial tips. Calculation of the transmembrane current density (4 X 10(-6) nA/cm2) leads to an estimate of channel density (10 channels/micron2) in close agreement with the measured density of Ca2+ channels in other systems. The assumption that calcium channel proteins are conveyed to nerve terminals by active transport, whereas sodium channel proteins are conveyed passively by a slower somatofugal diffusion process [Strichartz et al, 1984], would explain why developing neurons tend to display Ca2+-sensitive electrogenesis at their growing tips, and Na+-sensitive action potentials later in development. In order to gain some insight into the possible role of these steady growth currents, we estimated the membrane depolarization and axial voltage gradient they produce. It is likely that the currents produce sufficient membrane depolarization (approximately equal to 4 mV) to cause autogenous activation of ion channel permeabilities. Similarly, the axial voltage gradient (approximately equal to 4 mV/cm) would be expected to move intracytoplasmic vesicles by electrophoresis at a rate (20-40 microns/hr) very close to that at which the filopodia are observed to grow.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neonatal Rat Cerebellar Granule and Purkinje Neurons in Culture Express Different GABAA Receptors

We have established a culture system for microexplants of rat cerebellar cortical tissue in which cells develop morphologically, express type-A receptors for the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and form GABAergic synaptic connections. Criteria of cell size and shape allow reliable identification of granule and Purkinje neurons, criteria confirmed by studies of the binding of antibodies to calbindin D28K and GABA. Both granule and Purkinje neurons express GABA_A receptors, but granule neurons fall into two classes in terms of their sensitivity. Granule neurons which do not show spontaneous synaptic currents are relatively insensitive to GABA, while granule neurons with synaptic currents are much more sensitive. The responses of Purkinje neurons to applications of 1 μM GABA are relatively insensitive to Zn²⁺ ions (10 μM), and are potentiated by chlordiazepoxide (100 μM) and La³⁺ ions (100 μM). Responses of innervated granule neurons, on the other hand, are blocked more strongly by Zn²⁺ ions, are less affected by chlordiazepoxide and are equally potentiated by La³⁺ ions. Hence these cultures provide a source of identifiable, functionally innervated cells which express distinct types of GABA_A receptors.     

Calcium and calcium-activated potassium currents of motor nerve endings in the frog

Evoked electrical responses of nerve endings were recorded in experiments on frog cutaneous-pectoris muscle under visual control. During superfusion by Ca2+-free solution with tubocurarine a late inward current was found in the evoked response of nerve endings recorded by CaCl2 filled electrode. With addition of the external solution of 4-aminopyridine the late current was changed into an outward one. This outward current depended on Ca2+ concentration in the electrode, decreased after local increase of K+ concentration and was abolished by Co2+ ions. Local ionophoretic application of tetraethyl-ammonium eliminated the outward current and revealed a strong and prolonged inward current. Similar currents were recorded by microelectrodes filled with SrCl2, BaCl2 and MgCl2. It is concluded that the late inward current is carried through potential-dependent calcium channels, while the late outward one--through Ca2+-activated K+ channels. The ratio of calcium and Ca2+-activated potassium currents in motor nerve endings and their role in the transmitter release are discussed.     

Activation of calcium channel currents in rat sensory neurons by large depolarizations: effect of Guanine nucleotides and (-)-baclofen

Calcium channel currents have been recorded from cultured rat sensory neurons at clamp potentials of between -30 and +120 mV. At large depolarizing potentials between +50 and +120 mV, the current was outward. This outward current was shown to be largely due to ions passing through calcium channels, because it was substantially although generally incompletely blocked by Cd2+ (1 mM) and omega-conotoxin (1 microM). Internal GTP-gamma-S (100 microM) and to a lesser extent GTP (1 mM) reduced the amplitude and slowed the activation of the outward, as well as the inward calcium channel current. Baclofen (100 microM) reversibly inhibited both the inward and outward currents. These results suggest that the effect of baclofen and G protein activation on calcium channel currents is not due to a shift in the voltage-dependence of channel availability.     

Gadolinium block of calcium channels: influence of bicarbonate

The selectivity of block of voltage-activated barium (Ba⁺) currents by lanthanide ions was studied in a rat dorsal root ganglion (DRG) cell line (F11-B9), rat and frog peripheral neurons, and rat cardiac myocytes using the whole-cell patch clamp technique. Gadolinium (Gd³⁺) produced a dose-dependent and complete inhibition of whole-cell Ba²⁺ current in all cells studied, including cells expressing identified dihydropyridine-sensitive L-type currents and ω-conotoxin-sensitive N-type currents. Like Gd³⁺, lutetium (Lu³⁺) and lanthanum (La³⁺) blocked all Ba²⁺ current with little selectivity for different components of the whole-cell current. Gd³⁺ block of Ba²⁺ currents was incomplete, however, when sodium bicarbonate (5–22.6 mM) was added to the standard HEPES-buffered external Ba⁺ solution. In rat DRG neurons and F11-B9 cells, a fraction of the whole-cell Ba²⁺ current recorded in the presence of bicarbonate was resistant to block by saturating concentrations of Gd³⁺ (50–100 μM). The resistant current inactivated more rapidly than the original current giving the appearance that, under these conditions, Gd³⁺ block is more selective for the slowly inactivating component of the whole-cell current. Bicarbonate modification of Gd³⁺ block occurred both before and after ω-conotoxin block of N-type currents in rat DRG neurons, suggesting that even in the presence of bicarbonate, Gd³⁺ block was not selective for N-type currents.