Intracellular electrolyte concentrations in rat sympathetic neurones measured with an electron microprobe

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

The nicotinic activation of the denervated sympathetic neuron of the rat

Nicotinic responses to endogenous acetylcholine and to exogenously applied agonists have been studied in the intact or denervated rat sympathetic neuron in vitro, by using the two-microelectrode voltage-clamp technique. Preganglionic denervation resulted in progressive decrease of the synaptic current (excitatory postsynaptic current, EPSC) amplitude, which disappeared within 24 h. These effects were accompanied by changes in ion selectivity of the nicotinic channel (nAChR). The extrapolated EPSC null potential (equilibrium potential for acetylcholine action, E(Syn)) shifted from a mean value of -15.9+/-0.7 mV, in control, to -7.4+/-1.6 mV, in denervated neurons, indicating a decrease of the permeability ratio for the main components of the synaptic current (P(K)/P(Na)) from 1.56 to 1.07. The overall properties of AChRs were investigated by applying dimethylphenylpiperazinium or cytisine and by examining the effects of endogenous ACh, diffusing within the ganglion after preganglionic tetanization in the presence of neostigmine. The null potentials of these macrocurrents (equilibrium potential for dimethylphenylpiperazinium action, E(DMPP); and equilibrium potential for diffusing acetylcholine, E(ACh), respectively) were evaluated by applying voltage ramps and from current-voltage plots. In normal neurons, E(Syn) (-15.9+/-0.7 mV) was significantly different from E(DMPP) (-26.1+/-1.0) and E(ACh) (-31.1+/-3.3); following denervation, nerve-evoked currents displayed marked shifts in their null potentials (E(Syn)=-7.4+/-1.6 mV), whereas the amplitude and null potential of the agonist-evoked macrocurrents were unaffected by denervation and its duration (E(DMPP)=-26.6+/-1.2 mV). It is suggested that two populations of nicotinic receptors, synaptic and extrasynaptic, are present on the neuron surface, and that only the synaptic type displays sensitivity to denervation.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

Voltage- and activity-dependent chloride conductance controls the resting status of the intact rat sympathetic neuron

Remarkable activity dependence was uncovered in the chloride conductance that operates in the subthreshold region of membrane potential, by using the two-microelectrode voltage-clamp technique in the mature and intact rat sympathetic neuron. Both direct and synaptic neuron tetanization (15 Hz, 10-s duration to saturate the response) resulted in a long-lasting (not less than 15 min) increase of cell input conductance (+70-150% 10 min after tetanus), accompanied by the onset of an inward current with the same time course. Both processes developed with similar properties in the postganglionic neuron when presynaptic stimulation was performed under current- or voltage-clamp conditions and were unaffected by external calcium on direct stimulation. The posttetanic effects were sustained by gCl increase because both conductance and current modifications were blocked by 0.5 mM Anthracene-9-carboxylic acid (a chloride channel blocker) but were unaffected by TEACl or cesium chloride treatments. The chloride channel properties were modified by stimulation: their voltage sensitivity and rate of closure in response to hyperpolarization strongly increased. The voltage dependence of the three major conductances governing the cell subthreshold status (gCl, gK, and gL) was evaluated over the -40/-110 mV membrane potential range in unstimulated neurons and compared with previous results in stimulated neurons. A drastic difference between the voltage-conductance profiles was observed, exclusively sustained by gCl increase. The chloride channel thus hosts an intrinsic mechanism, a memory of previous neuron activity, which makes the chloride current a likely candidate for natural controller of the balance between opposite resting currents and thus of membrane potential level.

Adaptation and temporal decorrelation by single neurons in the primary visual cortex

Limiting redundancy in the real-world sensory inputs is of obvious benefit for efficient neural coding, but little is known about how this may be accomplished by biophysical neural mechanisms. One possible cellular mechanism is through adaptation to relatively constant inputs. Recent investigations in primary visual (V1) cortical neurons have demonstrated that adaptation to prolonged changes in stimulus contrast is mediated in part through intrinsic ionic currents, a Ca2+-activated K+ current (IKCa) and especially a Na+-activated K+ current (IKNa). The present study was designed to test the hypothesis that the activation of adaptation ionic currents may provide a cellular mechanism for temporal decorrelation in V1. A conductance-based neuron model was simulated, which included an IKCa and an IKNa. We show that the model neuron reproduces the adaptive behavior of V1 neurons in response to high contrast inputs. When the stimulus is stochastic with 1/f 2 or 1/f-type temporal correlations, these autocorrelations are greatly reduced in the output spike train of the model neuron. The IKCa is effective at reducing positive temporal correlations at approximately 100-ms time scale, while a slower adaptation mediated by IKNa is effective in reducing temporal correlations over the range of 1-20 s. Intracellular injection of stochastic currents into layer 2/3 and 4 (pyramidal and stellate) neurons in ferret primary visual cortical slices revealed neuronal responses that exhibited temporal decorrelation in similarity with the model. Enhancing the slow afterhyperpolarization resulted in a strengthening of the decorrelation effect. These results demonstrate the intrinsic membrane properties of neocortical neurons provide a mechanism for decorrelation of sensory inputs.

The identification of the sympathetic neurons innervating the hamster submandibular gland and their electrophysiological membrane properties

The neuron innervating the hamster submandibular (SM) gland was identified in the superior cervical ganglion (SCG) in vitro by recording the antidromic response using the intracellular recording technique. After the cellular response was recorded, methylene blue was injected iontophoretically into the neuron from the recording electrode, and the location of the cell soma was determined. The salivatory neurons of the SM gland were in the small- to medium-sized group of the entire cell population of the SCG. The cell size was 36.3 x 24.4 microm (mean, n=45). The postganglionic fibers were entirely unmyelinated (mean: 0.34 m/sec at 28-30 degrees C, n=141). Eighty-seven percent of the cells were distributed in the central one-third of area between the external carotid nerve origin and the caudal pole in the SCG. The resting membrane potential, membrane input resistance, membrane time constant and membrane input capacitance of the salivatory neuron were as follows: -49.2+/-7.6 mV (n=102), 52.9+/-23.6 Mohms (n=71), 8.0+/-3.4 msec (n=71) and 147+/-50 pF (n=71). Fast- and slow-excitatory postsynaptic potentials (EPSPs) were evoked, but not slow-inhibitory postsynaptic potentials (IPSPs). The fast EPSP was 13.1+/-5.7 mV in amplitude and 46.2+/-17.1 msec in duration (n=35). The slow EPSP (20 Hz, 5 sec) was 6.9+/-11 .9 mV in amplitude and 101+/-43 sec in duration (n=16). The directly-evoked spike was 63.0+/-11.9 mV in amplitude and 5.9+/-1.3 msec in duration (n=54). The spike after-hyperpolarization (AHP) was 12.5+/-3.5 mV in amplitude and 353+/-161 msec in duration. Na+ and Ca+ channels were involved in the spike generation. The voltage-dependent K+ channels (delayed rectifier), A channels and rapidly Ca2+-activated K+ channels (BK channels) regulated the spike-falling phase. The delayed rectifiers, A channels, and BK and SK (slowly Ca2+-activated) channels were involved in generation of spike-AHP. Muscarine suppressed the Ca2+ component of spike via muscarinic receptors.

Cellular mechanisms of long-lasting adaptation in visual cortical neurons in vitro

The cellular mechanisms of spike-frequency adaptation during prolonged discharges and of the slow afterhyperpolarization (AHP) that follows, as occur in vivo with contrast adaptation, were investigated with intracellular recordings of cortical neurons in slices of ferret primary visual cortex. Intracellular injection of 2 Hz sinusoidal or constant currents for 20 sec resulted in a slow (tau = 1-10 sec) spike-frequency adaptation, the degree of which varied widely among neurons. Reducing either [Ca(2+)](o) or [Na(+)](o) reduced the rate of spike-frequency adaptation. After the prolonged discharge was a slow (12-75 sec) AHP that was associated with an increase in membrane conductance and a rightward shift in the discharge frequency versus injected current relationship. The reversal potential of the slow AHP was sensitive to changes in [K(+)](o), indicating that it was mediated by a K(+) current. Blockade of transmembrane Ca(2+) conductances did not reduce the slow AHP. In contrast, reductions of [Na(+)](o) reduced the slow AHP, even in the presence of pronounced Ca(2+) spikes. We suggest that the activation of Na(+)-activated and Ca(2+)-activated K(+) currents plays an important role in prolonged spike-frequency adaptation and therefore may contribute to contrast adaptation and other forms of adaptation in the visual system in vivo.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Participation of a chloride conductance in the subthreshold behavior of the rat sympathetic neuron

The presence of a novel voltage-dependent chloride current, active in the subthreshold range of membrane potential, was detected in the mature and intact rat sympathetic neuron in vitro by using the two-microelectrode voltage-clamp technique. Hyperpolarizing voltage steps applied to a neuron held at -40/-50 mV elicited inward currents, whose initial magnitude displayed a linear instantaneous current-voltage (I-V) relationship; afterward, the currents decayed exponentially with a single voltage-dependent time constant (63.5 s at -40 mV; 10.8 s at -130 mV). The cell input conductance decreased during the command step with the same time course as the current. On returning to the holding potential, the ensuing outward currents were accompanied by a slow increase in input conductance toward the initial values; the inward charge movement during the transient ON response (a mean of 76 nC in 8 neurons stepped from -50 to -90 mV) was completely balanced by outward charge displacement during the OFF response. The chloride movements accompanying voltage modifications were studied by estimating the chloride equilibrium potential (E(Cl)) at different holding potentials from the reversal of GABA evoked currents. [Cl(-)](i) was strongly affected by membrane potential, and at steady state it was systematically higher than expected from passive ion distribution. The transient current was blocked by substitution of isethionate for chloride and by Cl(-) channel blockers (9AC and DIDS). It proved insensitive to K(+) channel blockers, external Cd(2+), intracellular Ca(2+) chelators [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)] and reduction of [Na(+)](e). It is concluded that membrane potential shifts elicit a chloride current that reflects readjustment of [Cl(-)](i). The cell input conductance was measured over the -40/-120-mV voltage range, in control medium, and under conditions in which either the chloride or the potassium current was blocked. A mix of chloride, potassium, and leakage conductances was detected at all potentials. The leakage component was voltage independent and constant at approximately 14 nS. Conversely, gCl decreased with hyperpolarization (80 nS at -40 mV, undetectable below -110 mV), whereas gK displayed a maximum at -80 mV (55.3 nS). Thus the ratio gCl/gK continuously varied with membrane polarization (2.72 at -50 mV; 0.33 at -110 mV). These data were forced in a model of the three current components here described, which accurately simulates the behavior observed in the "resting" neuron during membrane migrations in the subthreshold potential range, thereby confirming that active K and Cl conductances contribute to the genesis of membrane potential and possibly to the control of neuronal excitability.

Na+-activated K+ channels in small dorsal root ganglion neurones of rat

1. Whole-cell Na+-activated K+ (KNa) channel currents and single KNa channels were studied with the patch-clamp method in small (20-25 micrometer) dorsal root ganglion (DRG) neurones in slices of rat dorsal root ganglia. 2. The whole-cell KNa channel current was identified as an additional K+-selective leakage current which appeared after cell perfusion with internal solutions containing different [Na+]. The concentration for half-maximal activation of KNa channel current was 39 mM and the Hill coefficient was 3.5. At [Na+]i above 12 mM, KNa channel current dominated the unspecific leakage current. The ratio of maximum KNa channel current to unspecific leakage current was 45. 3. KNa channel current was not activated by internal Li+. It was suppressed by external 20 mM Cs+ but not by 10 mM tetraethylammonium. 4. Single KNa channels with a conductance of 142 pS in 155 mM external K+ (K+o)-85 mM internal K+ (K+i) solutions were observed at a high density of about 2 channels micrometer-2. 5. In two-electrode experiments, a direct correlation was seen between development of whole- cell KNa channel current and activation of single KNa channels during perfusion of the neurone with Na+-containing internal solution. 6. Under current-clamp conditions, KNa channels did not contribute to the action potential. However, internal perfusion of the neurone with Na+ shifted the resting potential towards the equilibrium potential for K+ (EK). Varying external [K+] indicated that in neurones perfused with Na+-containing internal solution the resting potential followed the EK values predicted by the Nernst equation over a broader voltage range than in neurones perfused with Na+-free solution. 7. It is concluded that the function of KNa channels has no links to firing behaviour but that the channels could be involved in setting or stabilizing the resting potential in small DRG neurones.

Calcium-activated chloride current in normal mouse sympathetic ganglion cells

1. In rat sympathetic ganglion cells, axotomy induces the appearance of a depolarizing after-potential (ADP) produced by a calcium-activated chloride current. Here we report that this current is also present in normal sympathetic neurones from the mouse. 2. In an in vitro preparation of the superior cervical ganglion, an ADP was observed after spike firing in 50% of the cells studied with single-electrode current- and voltage-clamp techniques. 3. When the cells were voltage clamped at -50 mV in the presence of tetrodotoxin (TTX) and tetraethylammonium chloride (TEA), depolarizing jumps evoked inward calcium currents which were contaminated by outward chloride currents, followed by slowly decaying inward chloride tail currents. 4. The ADP and the inward tail currents disappeared when calcium was removed from the extracellular solution or when cadmium was added. 5. The reversal potential for the inward tail current was approximately -24 mV and was displaced in agreement with the Nernst equation for chloride when the extracellular NaCl was replaced by sucrose or sodium isethionate. The chloride channel blocker anthracene-9-carboxylic acid (9AC) inhibited both the ADP and the tail current. 6. Using intracellular injection of neurobiotin, we found that cells with shorter dendrites had larger ADPs. In axotomized ganglia practically all cells showed very pronounced ADPs. 7. We conclude that normal mouse sympathetic ganglion cells have a calcium-activated chloride current that generates an ADP. The channels responsible for this current are probably located in the dendrites.

Potential-dependent block of human delayed rectifier K+ channels by internal Na+

The delayed rectifier K+ currents in differentiated human SH-SY5Y neuroblastoma cells were characterized with tight-seal recording techniques. Activation and inactivation parameters were measured. At high positive potentials, the current showed a marked rectification, causing a region of negative slope conductance in the current vs. potential curve. The rectification depended markedly on the pipette Na+ concentration. Without Na+, no rectification was observed, whereas with high Na+ (20-60 mM), a marked rectification was always observed. Tail current measurements showed a fast ( < 400 microseconds) block of K+ currents in the presence of internal Na+. With 60 mM Na+ in the pipette 8% of the K+ current was blocked at 0 mV, 27% at +20 mV, and 82% at +100 mV. Similar degrees of block were often seen with 30 mM Na+ in the pipette. The submembrane Na+ concentration in intact cells was estimated, on the basis of the reversal of Na+ current, to be approximately 15 mM. Single-channel K+ currents, in the cell-attached configuration, showed a conductance of approximately 20 pS at 40-60 mV above rest but showed rectification at high potentials.

Na(+)-activated K+ channels localized in the nodal region of myelinated axons of Xenopus

1. A potassium channel activated by internal Na+ ions (K+Na channel) was identified in peripheral myelinated axons of Xenopus laevis using the cell-attached and excised configurations of the patch clamp technique. 2. The single-channel conductance for the main open state was 88 pS with [K+]o = 105 mM and pS with [K+]o = 2.5 mM ([K+]i = 105 mM). The channel was selectively permeable to K+ over Na+ ions. A characteristic feature of the K+Na channel was the frequent occurrence of subconductance states. 3. The open probability of the channel was strongly dependent on the concentration of Na+ ions at the inner side of the membrane. The half-maximal activating Na+ concentration and the Hill coefficient were 33 mM and 2.9, respectively. The open probability of the channel showed only weak potential dependence. 4. The K+Na channel was relatively insensitive to external tetraethylammonium (TEA+) in comparison with voltage-dependent axonal K+ channels; the half-maximal inhibitory concentration (IC50) was 21.3 mM (at -90 mV). In contrast, the channel was blocked by low concentrations of external Ba2+ and Cs+ ions, with IC50 values of 0.7 and 1.1 mM, respectively (at -90 mV). The block by Ba2+ and Cs+ was more pronounced at negative than at positive membrane potentials. 5. A comparison of the number of K+Na channels in nodal and paranodal patches from the same axon revealed that the channel density was about 10-fold higher at the node of Ranvier than at the paranode. Moreover, a correlation between the number of K+Na channels and voltage-dependent Na+ channels in the same patches was found, suggesting co-localization of both channel types. 6. As weakly potential-dependent ('leakage') channels, axonal K+Na channels may be involved in setting the resting potential of vertebrate axons. Simulations of Na+ ion diffusion suggest two possible mechanisms of activation of K+Na channels: the local increase of Na+ concentration in a cluster of Na+ channels during a single action potential or the accumulation in the intracellular axonal compartment during a train of action potentials.

Calcium-dependent chloride current induced by axotomy in rat sympathetic neurons

1. Seven to ten days after sectioning their axons, rat sympathetic neurons were studied using intracellular recording techniques in an in vitro preparation of the superior cervical ganglion. 2. In 75% of axotomized cells, an after-depolarization (ADP) was observed following spike firing or depolarization with intracellular current pulses. Discontinuous single-electrode voltage-clamp techniques were employed to study the ADP. When the membrane potential was clamped at the resting level just after an action potential, a slow inward current was recorded in cells that showed an ADP. 3. In the presence of TTX and TEA, inward peaks and outward currents were recorded during depolarizing voltage jumps, followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. The inward peak and tail currents activated between -10 and -20 mV and reached maximum amplitudes around 0 mV. With depolarizing jumps to between +40 and +50 mV, net outward currents were recorded during the depolarizing jumps but inward tail currents were still activated. 4. In the presence of the Ca2+ channel blocker cadmium, or when Ca2+ was substituted by Mg2+, the ADP disappeared. In voltage-clamped cells, cadmium blocked the inward tail currents. The reversal potential for the inward tail current was approximately -15 mV. Substitution of the extracellular NaCl by sucrose or sodium isethionate increased the amplitude of the inward tail current, and displaced its equilibrium potential to more positive values. Changes in extracellular [K+] did not appreciably affect the inward tail current amplitude or equilibrium potential. Niflumic acid, a blocker of chloride channels activated by Ca2+, almost completely blocked the tail current. 5. No ADPs were observed in non-axotomized neurons, and when depolarizing pulses were applied while in voltage clamp no inward tail currents were evoked in these normal cells. 6. It is concluded that axotomy of sympathetic ganglion cells produces the appearance of a Ca(2+)-dependent chloride current responsible for the ADP observed following spike firing.

A large, sustained Na(+)- and voltage-dependent K+ current in spinal neurons of the frog embryo

1. Neurons from the Xenopus embryo spinal cord were dissociated and conventional patch clamp techniques were used to record the whole-cell currents in the presence of tetrodotoxin (TTX). 2. The outward currents of the acutely isolated spinal neurons were rapidly reduced to about half their control value by substitution of extracellular Na+ with N-methyl-D-glucamine, lysine or choline. 3. The use of Li+ as a Na+ substitute partially reduced the outward currents. 4. The reversal potential of the Na(+)-sensitive current was close to the K+ equilibrium potential and could be altered by changing extracellular K+. The Na(+)-sensitive current was therefore a K+ current. 5. The Na(+)-sensitive K+ current was voltage dependent and activated in a sustained manner and appeared very similar to the delayed rectifier present in these neurons. 6. While the Na(+)-sensitive current increased with voltage as might be expected for an outward current, at very positive potentials it progressively decreased in amplitude. The voltage range over which this decrease was present moved closer to zero as the levels of intracellular Na+ were increased. The tail currents evoked by positive test potentials did not correspondingly decrease in amplitude, suggesting that channel block was rapidly relieved by stepping back to the holding potential. 7. Intracellular perfusion of the patch pipette with solutions containing varying amounts of Na+ (0-20 mM) showed that the K+ currents could be increased in a dose-dependent manner by raising intracellular Na+. The current had an EC50 for Na+ of 7.3 mM and a Hill coefficient of 4.6. 8. Single channel recordings from isolated inside-out patches revealed a channel that gated more frequently when the bathing levels of Na+ were elevated from 3 to 12 or 50 mM. Xenopus spinal cord neurons therefore possess a current that is not only voltage dependent but is also sensitive to internal Na+. 9. Xenopus spinal neurons possess a transient Na+ current (blocked by the inclusion of TTX) and a leak channel permeable to Na+. The inward leakage of Na+ appeared to provide the Na+ necessary for the gating of the Na(+)-dependent channel. 10. Blocking the Na(+)-K+ exchange pumps by removing extracellular K+, reduced the effect of removal of external Na+, suggesting that the Na(+)-K+ exchange pumps could be important in controlling the submembrane Na+.(ABSTRACT TRUNCATED AT 400 WORDS)

A five-conductance model of the action potential in the rat sympathetic neurone

The origin of the action potential in neurones has yet to be answered satisfactorily for most cells. We present here a five-conductance model of the somatic membrane of the mature and intact sympathetic neurone studied in situ in the isolated rat superior cervical ganglion under two-electrode voltage-clamp conditions. The neural membrane hosts five separate types of voltage-dependent ionic conductances, which have been isolated at 37 degrees C by using simple manipulations such as conditioning-test protocols and external ionic pharmacological treatments. The total current could be separated into two distinct inward components: (1) the sodium current, INa, and (2) the calcium current, ICa; and three outward components: (1) the delayed rectifier, IKV, (2) the transient IA, and (3) the calcium-dependent IKCa. Each current has been kinetically characterized in the framework of the Hodgkin-Huxley scheme used for the squid giant axon. Continuous mathematical functions are now available for the activation and inactivation (where present) gating mechanisms of each current which, together with the maximum conductance values measured in the experiments, allow for a satisfactory reconstruction of the individual current tracings over a wide range of membrane voltage. The results obtained are integrated in a full mathematical model which, by describing the electrical behaviour of the neurone under current-clamp conditions, leads to a quantitative understanding of the physiological firing pattern. While, as expected, the fast inward current carried by Na+ contributes to the depolarizing phase of the action potential, the spike falling phase is more complex than previous explanations. IKCa, with a minor contribution from IKV, repolarizes the neurone only under conditions of low cell internal negativity. Their role becomes less pronounced in the voltage range negative to -60 mV, where membrane repolarization allows IA to deinactivate. In the spike arising from these voltage levels the membrane repolarization is mainly sustained by IA, which proves to be the only current sufficiently fast and large enough to recharge the membrane capacitor at the speed observed during activity. Different modes of firing coexist in the same neurone and the switching from one to another is fast and governed by the membrane potential level, which makes the selection between the different voltage-dependent channel systems. The neurone thus seems to be prepared to operate within a wide voltage range; the results presented indicate the basic factors underlying the different discrete behaviours.

Disruption of cellular elements and water in neurotoxicity: studies using electron probe X-ray microanalysis

Regulation of elements and water in nerve cells is a complex, multifaceted process which appears to be vulnerable to neurotoxic events. However, much of our knowledge concerning the potential role of elements in nerve cell injury is limited by the relatively gross level of corresponding analyses. If we are to confirm and understand the proposed role, more precise and detailed information is needed. As indicated in this commentary, research employing electron probe microanalysis and digital X-ray imaging has begun to provide this necessary information. Recent EPMA studies of nerve and glial cells in the peripheral and central nervous systems have shown that each cell type and their corresponding morphologic compartments exhibit unique distributions of elements and water. The use of microprobe analysis has allowed us to document precisely how elements and water redistribute in morphological compartments of damaged nerve cells. Accumulating evidence from EPMA studies suggests that, rather than being an epiphenomenon, intracellular changes in diffusible elements might mediate the functional and structural consequences of neurotoxic insult. It is also evident from this research that elements other than Ca might play a pertinent role in the injury response and that changes in intraneuronal elemental composition might develop according to a specific temporal pattern, e.g., transection-induced sequential alterations in axonal K, Na, Cl, and Ca. Therefore, rather than conducting end-point studies, longitudinal investigations are necessary to define the sequential pattern of elemental perturbation associated with a given neurotoxic event. Such research can also help identify the role of individual elements in the injury response. Future microprobe studies should be combined with measurements of ion levels (e.g., using fura-2 or ion selective electrodes) to provide a comprehensive and dynamic view of elemental deregulation. In addition, parallel biochemical studies should be performed to determine mechanisms of elemental disruption and possible biochemical and metabolic consequences of this disruption. Although evidence presented in this commentary suggests that each type of neurotoxic event produces a characteristic pattern of decompartmentalization, further work is necessary to confirm this possibility. Finally, based on a presumed involvement of elements in nerve injury, efforts are currently underway in several laboratories to develop appropriate pharmacological therapies for certain chemical- and trauma-induced neuropathological conditions (Dretchen et al., 1986; El-Fawal et al., 1989; Beattie et al., 1989).(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium current activated by intracellular sodium in quail trigeminal ganglion neurons

Whole-cell voltage clamp and single-channel recordings were performed on cultured trigeminal ganglion neurons from quail embryos in order to study a sodium-activated potassium current (KNa). When KNa was activated by a step depolarization in voltage clamp, there was a proportionality between KNa and INa at all voltages between the threshold of INa and ENa. Single-channel recordings indicated that KNa could be activated already by 12 mM intracellular sodium and was almost fully activated at 50 mM sodium. 100 mM lithium, 100 mM choline, or 5 microM calcium did not activate KNa. The relationship between the probability for the channel to be open (Po) vs. the sodium concentration and the relationship of KNa open time-distributions vs. the sodium concentration suggest that two to three sodium ions bind cooperatively before KNa channels open. KNa channels were sensitive to depolarization; at 12 mM sodium, a 42-mV depolarization caused an e-fold increase in Po. Under physiological conditions, the conductance of the KNa channel was 50 pS. This conductance increased to 174 pS when the intra- and extracellular potassium concentrations were 75 and 150 mM, respectively.

Chloride enters glial cells and photoreceptors in response to light stimulation in the retina of the honey bee drone

Double-barrelled ion-selective microelectrodes were used to measure free [Cl-] in photoreceptors, extracellular space, and glial cells in superfused slices of drone retina. Tests indicated that with normal superfusate the intracellular electrode signal was due essentially to Cl- and not to some other interfering anion. The results indicate that Cl- is more concentrated in both photoreceptors and glial cells than would be predicted for a passive electrochemical distribution. When the photoreceptors were stimulated by a standard train of 20 ms flashes, 1/s for 90 s, their intracellular free [Cl-] (Cli) rose by 8 +/- 1 mM. At the end of stimulation Cli usually continued to rise for up to a further 2 min and then returned toward the baseline over about 10 min. During light stimulation Cli in the glia rose. The magnitude of the increase was 5.1 +/- 0.4 mM, about half the increase in Ki. In some extracellular recording sites, light stimulation caused [Cl-] to increase and in others to decrease. The mean change was -0.7 mM, SD 6.5 mM. The Cl- that entered the photoreceptors and the glia was presumably made available by the shrinking of the extracellular space. When the cells were depolarized by increasing [K+] in the superfusate from 7.5 mM to 18 mM, Cli increased. The half-time of the change in Cli was longer than the half-time of the depolarization by 10-30 s in the glia and 50-250s in the photoreceptors. During superfusion with 0 Cl- Ringer's solution, the light-induced rise in extracellular [K+] was greater by a factor of 1.4-2.7, and the clearance after the end of the stimulation was slower. The rate of increase in glial Ki during light stimulation fell; the rate of increase of glial Ki caused by superfusion with raised [K+] (in the absence of Cl-) fell more. We conclude that when extracellular [K+] is increased, entry of Cl- into the glia is necessary for part, but not all, of the net uptake of K+. During light stimulation, the observed movement of CL- into glia contributes to homeostasis of extracellular [K+], and the cell swelling associated with movement of Cl- into both glia and photoreceptors contributes to homeostasis of extracellular [Na+].

Distribution of elements in rat peripheral axons and nerve cell bodies determined by x-ray microprobe analysis

X-ray microprobe analysis was used to determine concentrations (millimoles of element per kilogram dry weight) of Na, P, Cl, K, and Ca in cellular compartments of frozen, unfixed sections of rat sciatic and tibial nerves and dorsal root ganglion (DRG). Five compartments were examined in peripheral nerve (axoplasm, mitochondria, myelin, extraaxonal space, and Schwann cell cytoplasm), and four were analyzed in DRG nerve cell bodies (cytoplasm, mitochondria, nucleus, and nucleolus). Each morphological compartment exhibited characteristic concentrations of elements. The extraaxonal space contained high concentrations of Na, Cl, and Ca, whereas intraaxonal compartments exhibited lower concentrations of these elements but relatively high K contents. Nerve axoplasm and axonal mitochondria had similar elemental profiles, and both compartments displayed proximodistal gradients of decreasing levels of K, Cl, and, to some extent, Na. Myelin had a selectively high P concentration with low levels of other elements. The elemental concentrations of Schwann cell cytoplasm and DRG were similar, but both were different from that of axoplasm, in that K and Cl were markedly lower whereas P was higher. DRG cell nuclei contained substantially higher K levels than cytoplasm. The subcellular distribution of elements was clearly shown by color-coded images generated by computer-directed digital x-ray imaging. The results of this study demonstrate characteristic elemental distributions for each anatomical compartment, which doubtless reflect nerve cell structure and function.

The role of ultrastructural investigations in neurotoxicology

The ways in which ultrastructural approaches have been applied to the investigation of xenobiotic-induced toxicity of the nervous system have been briefly reviewed. These approaches have been grouped in 3 broad areas, viz. morphology, function and composition. Firstly, morphological approaches permit the visualisation of changes in intercellular relationships, the identification of the subcellular target(s) of a xenobiotic substance and the discrimination between what may appear ostensibly to be identical cellular responses to one or more chemically distinct toxins. Secondly, functional approaches using, e.g. cytochemistry, ion precipitation, immunocytochemistry and autoradiography provide indications of metabolic state, the identity or the intra- or extracellular location of the "reactive species". Thirdly, those approaches, viz. electronprobe X-ray microanalysis and electron energy loss spectroscopy which provide information of the elemental composition of cells and tissues permit an assessment of the subcellular distribution and compartmentalisation of endogenous substances and toxic or therapeutic xenobiotics. In concert, ultrastructural approaches possess the ability to contribute unique information on the effects of exposure of cells of the nervous system to toxic substances and so direct further investigation towards an understanding of the mechanism of action.

Extracellular volume in the bovine corneal epithelium

Three different substances, [14C]inulin, [14C]polyethylenglycol (PEG, molecular weight 4000) and [3H]sucrose have been examined for suitability as markers for determination of the extracellular volume (ECV) in the bovine corneal epithelium and stroma. In both tissues, sucrose was the best marker reaching equilibrium of distribution within 90 min. Inulin and 4000 PEG have been found to be unacceptable because of a slow rate of penetration of the ECV. In addition, both inulin and 4000 PEG gave considerably lower values for the ECV than sucrose. Using the value for ECV determined with sucrose as a marker in the bovine corneal epithelium, i.e. 21.6 +/- 1.4% of tissue water, and the results obtained previously by the chemical analysis of this tissue (Midelfart 1987) the intracellular concentrations of 34.4 +/- 8.6 mmol Na/kg H2O and 154.3 +/- 11.1 mmol K/kg H2O were determined. These results were compared with the results obtained by other methods in corneae from other species.

Single calcium-activated potassium channels recorded from cultured rat sympathetic neurones

1. The properties of single Ca2+-activated K+ channels in cultured rat superior cervical ganglionic neurones were studied in cell-attached and excised patches using the patch-clamp technique. 2. In cell-attached patches using an external K+ concentration ([K+]o) of 150 mM, approximately equal to the internal [K+], the channel slope conductance was approximately 200 pS and independent of membrane voltage between -50 and +50 mV. Using [K+]o of 4.7 mM (providing a near physiological K+ gradient), the I-V relationship was non-linear with a slope conductance of approximately 120 pS at 0 mV. 3. The channel was selective for K+ over Cs+ and Na+ which were impermeant from either side of the membrane. Both Na+ and Cs+ also blocked the movement of K+ through the channel. Cs+ was active on either side of the membrane, whereas Na+ apparently blocked the channel only when applied to the cytoplasmic side. 4. The channel was activated by increasing the Ca2+ concentration on the inside of the membrane ([Ca2+]i). The channel was virtually inactive when [Ca2+]i = 0.01 microM. Depolarizing the patch at a constant [Ca2+]i usually further increased the opening probability. 5. The gating properties of the channel were studied using cell-attached patches. At potentials more negative than the resting membrane potential, the open-time distribution was described by a single exponential. On depolarization, two exponentials were required. The closed-time distribution was fitted by three exponentials. 6. Depolarization of the patch caused the long mean open lifetime to increase whilst the short mean open and closed lifetimes were unaffected. Both the intermediate and long mean closed lifetimes decreased with depolarization from -60 to +60 mV. 7. In cell-attached patches, the long mean open lifetimes were usually smaller than those observed in excised patches at depolarized potentials (greater than 0 mV). 8. A fourth closed state, possibly representing an inactivated form of the channel, was infrequently observed. A 50% substate of the full single-channel current was also observed occasionally. This substate was always associated with openings to the full current state. 9. The channel was blockable by external tetraethylammonium (25 microM-1 mM), Ba2+ (1-10 mM), and quinine (10-200 microM). External d-tubocurarine (25-100 microM) also blocked this IC channel. However it was insensitive to apamin (100-300 nM), muscarine (10 microM) and 4-aminopyridine (1-3 mM). The channel was also blocked by internal tetraethylammonium (5-10 mM) or Ba2+ (0.3-1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

A quantitative description of the sodium current in the rat sympathetic neurone

The somata of rat sympathetic neurones were voltage-clamped in vitro at 27 degrees C using separate intracellular voltage and current micro-electrodes. Na currents were isolated from other current contributions by using: Cd to block the Ca current (ICa) and the related Ca-dependent K current (IK(Ca)), and external tetraethylammonium to suppress the delayed rectifier current (IK(V) ). The holding potential was maintained at -50 mV to inactivate the fast transient K current (IA) when the IA contamination was unacceptable. Step depolarizations beyond -30 mV activated a fast, transient inward current carried by Na ions; it was suppressed by tetrodotoxin and was absent in Na-free solution. Once activated, INa declined exponentially to zero with a voltage-dependent time constant. The underlying conductance, gNa, showed a sigmoidal activation between -30 and +10 mV, with half-activation at -21.1 mV and a maximal value (mean gNa) of 4.44 microS per neurone. The steady-state inactivation level, h infinity, varied with membrane potential, ranging from complete inactivation at -30 mV to minimal inactivation at about -90 mV with a midpoint at -56.2 mV. Double-pulse experiments showed that development and removal of inactivation followed a single-exponential time course; tau h was markedly voltage-dependent and ranged from 46 ms at -50 mV to 2.5 ms at -100 mV. Besides the fast inactivation, the Na conductance showed a slow component of inactivation. The steady-state value, s infinity, was maximal at -80 mV and minimal at -40 mV. The removal of slow inactivation is a two-time-constant process, the first with a time constant in the order of hundreds of milliseconds and the second with a time constant of seconds. Slow inactivation onset appeared to be a faster process than its removal. When slow inactivation was fully removed the peak INa increased by a factor of 1.8. INa was well described by assuming it to be proportional to m3h. The temperature dependence of peak INa, tau m and tau h was studied in the temperature range 17-27 degrees C and found similar to that reported for other preparations. The Q10 of these parameters allowed the reconstruction of the INa kinetic properties at 37 degrees C.

Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Non-nicotinic slow synaptic currents were recorded from voltage-clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d-tubocurarine and (usually) 1 microM-neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage-dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.

An intracellular analysis of gamma-aminobutyric-acid-associated ion movements in rat sympathetic neurones

Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.

Calcium-dependent inward currents in voltage-clamped guinea-pig olfactory cortex neurones

Guinea-pig olfactory cortex neurones in vitro (23 degrees C--25 degrees C) were voltage clamped by means of a single microelectrode sample-and-hold technique. In most Cs+-loaded neurones (in the presence of tetrodotoxin), membrane depolarization beyond -60 mV elicited inward currents, which had rapid activation kinetics. The steady-state current-voltage relationship was N-shaped with a region of negative slope conductance between - 50 mV and - 20 mV. The rate of inactivation varied according to the holding potential and the command potential. The inward currents were maintained when external Ca2+ was replaced by Ba2+, and were blocked by Cd2+, suggesting that Ca2+ was the principal charge carrier. The results demonstrate the existence of calcium current in olfactory cortex neurones.

Two components of muscarine-sensitive membrane current in rat sympathetic neurones

Membrane currents induced by muscarine (Imus) were recorded in voltage-clamped neurones in isolated rat superior cervical ganglia. Two components of Imus were regularly recorded: an inward current resulting from inhibition of the outward K+ current, IM; and an outward current attributable to the reduction of a steady inward current. The presence of these two components caused a 'cross-over' in the current-voltage curves at -50 +/- 3 mV in neurones impaled with KCl-filled micro-electrodes or at -63 +/- 4 mV in neurones impaled with K-acetate-filled electrodes. Both components of Imus were prevented by atropine. Both persisted in Krebs solution containing tetrodotoxin (1 microM), Cd2+ (200 microM) or 0 Ca2+. When IM was inhibited by external Ba2+ or internal Cs+ only the outward component of Imus could be detected. This component reversed at +3 +/- 2 mV in cells impaled with CsCl-filled electrodes or at -20 +/- 3 mV in cells impaled with Cs-acetate-filled electrodes. The reversal potentials agreed with those for the currents induced by gamma-aminobutyric acid (+4 +/- 2 mV and -16 +/- 3 mV with CsCl and Cs acetate electrodes respectively). Replacement of external NaCl with Na acetate (so reducing external Cl- concentration ( [Cl-]o) from 155 to 22 mM) shifted the reversal potential for Imus by +25 and +14.5 mV in two cells impaled with CsCl-filled electrodes. A tenfold reduction of external [Na+] (by glucosamine replacement) did not significantly alter the reversal potential for Imus in KCl or CsCl-impaled cells. Under conditions where IM is already inhibited, the residual outward component of Imus can lead to hyperpolarization and inhibition of neuronal activity in unclamped cells. We conclude that both inward and outward components of Imus result from direct activation of muscarinic receptors on the ganglion cells. The inward component results from IM inhibition. We suggest that the outward component results from inhibition of another, voltage-independent current IX which largely comprises a Cl- current. The inward component induces membrane depolarization and an increased excitability; the outward component can lead to hyperpolarization and reduced excitability.