The effect of temperature on Na currents in rat myelinated nerve fibres

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

Human pain ratings to electrical sinusoids increase with cooling through a cold-induced increase in C-fibre excitability

ABSTRACT

Low-frequency sinusoidal current applied to human skin evokes local axon reflex flare and burning pain, indicative of C-fibre activation. Because topical cooling works well as a local analgesic, we examined the effect of cooling on human pain ratings to sinusoidal and rectangular profiles of constant current stimulation. Unexpectedly, pain ratings increased upon cooling the skin from 32 to 18°C. To explore this paradoxical observation, the effects of cooling on C-fibre responses to stimulation with sinusoidal and rectangular current profiles were determined in ex vivo segments of mouse sural and pig saphenous nerve. As expected by thermodynamics, the absolute value of electrical charge required to activate C-fibre axons increased with cooling from 32°C to 20°C, irrespective of the stimulus profile used. However, for sinusoidal stimulus profiles, cooling enabled a more effective integration of low-intensity currents over tens of milliseconds resulting in a delayed initiation of action potentials. Our findings indicate that the paradoxical cooling-induced enhancement of electrically evoked pain in people can be explained by an enhancement of C-fibre responsiveness to slow depolarization at lower temperatures. This property may contribute to symptoms of enhanced cold sensitivity, especially cold allodynia, associated with many forms of neuropathic pain.

Macromolecular rate theory explains the temperature dependence of membrane conductance kinetics

The factor Q10 is used in neuroscience to adjust reaction rates of voltage-activated membrane conductances to different temperatures and is widely assumed to be constant. By performing an analysis of published data of the reaction rates of sodium, potassium, and calcium membrane conductances, we demonstrate that 1) Q10 is temperature dependent, 2) this relationship is similar across conductances, and 3) there is a strong effect at low temperatures (<15°C). We show that macromolecular rate theory (MMRT) explains this temperature dependency. MMRT predicts the existence of optimal temperatures at which reaction rates decrease as temperature increases, a phenomenon that we also found in the published data sets. We tested the consequences of using MMRT-adjusted reaction rates in the Hodgkin-Huxley model of the squid's giant axon. The MMRT-adjusted model reproduces the temperature dependence of the rising and falling times of the action potential. Furthermore, the model also reproduces these properties for different squid species that live in different climates. In a second example, we compare spiking patterns of biophysical models based on human pyramidal neurons from the Allen Cell Types database at room and physiological temperatures. The original models, calibrated at 34°C, failed to generate realistic spikes at room temperature in more than half of the tested models, while the MMRT produces realistic spiking in all conditions. In another example, we show that using the MMRT correction in hippocampal pyramidal cell models results in 100% differences in voltage responses. Finally, we show that the shape of the Q10 function results in systematic errors in predicting reaction rates. We propose that the optimal temperature could be a thermodynamical barrier to avoid over excitation in neurons. While this study is centered on membrane conductances, our results have important consequences for all biochemical reactions involved in cell signaling.

A microfabricated nerve-on-a-chip platform for rapid assessment of neural conduction in explanted peripheral nerve fibers

Peripheral nerves are anisotropic and heterogeneous neural tissues. Their complex physiology restricts realistic in vitro models, and high resolution and selective probing of axonal activity. Here, we present a nerve-on-a-chip platform that enables rapid extracellular recording and axonal tracking of action potentials collected from tens of myelinated fibers. The platform consists of microfabricated stimulation and recording microchannel electrode arrays. First, we identify conduction velocities of action potentials traveling through the microchannel and propose a robust data-sorting algorithm using velocity selective recording. We optimize channel geometry and electrode spacing to enhance the algorithm reliability. Second, we demonstrate selective heat-induced neuro-inhibition of peripheral nerve activity upon local illumination of a conjugated polymer (P3HT) blended with a fullerene derivative (PCBM) coated on the floor of the microchannel. We demonstrate the nerve-on-a-chip platform is a versatile tool to optimize the design of implantable peripheral nerve interfaces and test selective neuromodulation techniques ex vivo.

Peripheral nerves have a complex physiology and it is therefore difficult to measure axonal activity in vitro. Here the authors make a nerve-on-a-chip platform to align peripheral nerves and permit measurement of conduction amplitude and velocity along several axons in a single experiment.

Cooling reverses pathological bifurcations to spontaneous firing caused by mild traumatic injury

Mild traumatic injury can modify the key sodium (Na⁺) current underlying the excitability of neurons. It causes the activation and inactivation properties of this current to become shifted to more negative trans-membrane voltages. This so-called coupled left shift (CLS) leads to a chronic influx of Na⁺ into the cell that eventually causes spontaneous or "ectopic" firing along the axon, even in the absence of stimuli. The bifurcations underlying this enhanced excitability have been worked out in full ionic models of this effect. Here, we present computational evidence that increased temperature T can exacerbate this pathological state. Conversely, and perhaps of clinical relevance, mild cooling is shown to move the naturally quiescent cell further away from the threshold of ectopic behavior. The origin of this stabilization-by-cooling effect is analyzed by knocking in and knocking out, one at a time, various processes thought to be T-dependent. The T-dependence of the Na⁺ current, quantified by its Q _10-Na factor, has the biggest impact on the threshold, followed by Q _10-pump of the sodium-potassium exchanger. Below the ectopic boundary, the steady state for the gating variables and the resting potential are not modified by temperature, since our model separately tallies the Na⁺ and K⁺ ions including their separate leaks through the pump. When only the gating kinetics are considered, cooling is detrimental, but in the full T-dependent model, it is beneficial because the other processes dominate. Cooling decreases the pump's activity, and since the pump hyperpolarizes, less hyperpolarization should lead to more excitability and ectopic behavior. But actually the opposite happens in the full model because decreased pump activity leads to smaller gradients of Na⁺ and K⁺, which in turn decreases the driving force of the Na⁺ current.

Basal Ganglia Neuromodulation Over Multiple Temporal and Structural Scales-Simulations of Direct Pathway MSNs Investigate the Fast Onset of Dopaminergic Effects and Predict the Role of Kv4.2

The basal ganglia are involved in the motivational and habitual control of motor and cognitive behaviors. Striatum, the largest basal ganglia input stage, integrates cortical and thalamic inputs in functionally segregated cortico-basal ganglia-thalamic loops, and in addition the basal ganglia output nuclei control targets in the brainstem. Striatal function depends on the balance between the direct pathway medium spiny neurons (D1-MSNs) that express D1 dopamine receptors and the indirect pathway MSNs that express D2 dopamine receptors. The striatal microstructure is also divided into striosomes and matrix compartments, based on the differential expression of several proteins. Dopaminergic afferents from the midbrain and local cholinergic interneurons play crucial roles for basal ganglia function, and striatal signaling via the striosomes in turn regulates the midbrain dopaminergic system directly and via the lateral habenula. Consequently, abnormal functions of the basal ganglia neuromodulatory system underlie many neurological and psychiatric disorders. Neuromodulation acts on multiple structural levels, ranging from the subcellular level to behavior, both in health and disease. For example, neuromodulation affects membrane excitability and controls synaptic plasticity and thus learning in the basal ganglia. However, it is not clear on what time scales these different effects are implemented. Phosphorylation of ion channels and the resulting membrane effects are typically studied over minutes while it has been shown that neuromodulation can affect behavior within a few hundred milliseconds. So how do these seemingly contradictory effects fit together? Here we first briefly review neuromodulation of the basal ganglia, with a focus on dopamine. We furthermore use biophysically detailed multi-compartmental models to integrate experimental data regarding dopaminergic effects on individual membrane conductances with the aim to explain the resulting cellular level dopaminergic effects. In particular we predict dopaminergic effects on Kv4.2 in D1-MSNs. Finally, we also explore dynamical aspects of the onset of neuromodulation effects in multi-scale computational models combining biochemical signaling cascades and multi-compartmental neuron models.

Human Nav1.6 Channels Generate Larger Resurgent Currents than Human Nav1.1 Channels, but the Navβ4 Peptide Does Not Protect Either Isoform from Use-Dependent Reduction

Voltage-gated sodium channels are responsible for the initiation and propagation of action potentials (APs). Two brain isoforms, Nav1.1 and Nav1.6, have very distinct cellular and subcellular expression. Specifically, Nav1.1 is predominantly expressed in the soma and proximal axon initial segment of fast-spiking GABAergic neurons, while Nav1.6 is found at the distal axon initial segment and nodes of Ranvier of both fast-spiking GABAergic and excitatory neurons. Interestingly, an auxiliary voltage-gated sodium channel subunit, Navβ4, is also enriched in the axon initial segment of fast-spiking GABAergic neurons. The C-terminal tail of Navβ4 is thought to mediate resurgent sodium current, an atypical current that occurs immediately following the action potential and is predicted to enhance excitability. To better understand the contribution of Nav1.1, Nav1.6 and Navβ4 to high frequency firing, we compared the properties of these two channel isoforms in the presence and absence of a peptide corresponding to part of the C-terminal tail of Navβ4. We used whole-cell patch clamp recordings to examine the biophysical properties of these two channel isoforms in HEK293T cells and found several differences between human Nav1.1 and Nav1.6 currents. Nav1.1 channels exhibited slower closed-state inactivation but faster open-state inactivation than Nav1.6 channels. We also observed a greater propensity of Nav1.6 to generate resurgent currents, most likely due to its slower kinetics of open-state inactivation, compared to Nav1.1. These two isoforms also showed differential responses to slow and fast AP waveforms, which were altered by the Navβ4 peptide. Although the Navβ4 peptide substantially increased the rate of recovery from apparent inactivation, Navβ4 peptide did not protect either channel isoform from undergoing use-dependent reduction with 10 Hz step-pulse stimulation or trains of slow or fast AP waveforms. Overall, these two channels have distinct biophysical properties that may differentially contribute to regulating neuronal excitability.

Relation between symptoms and pathophysiology in inflammatory neuropathies: Controversies and hypotheses

This review attempts to explain the symptoms experienced by patients with inflammatory neuropathies by pathophysiological events. The emphasis is not on the primary events that may cause a particular illness but on downstream events taking place in peripheral nerves or muscles. Symptoms that will be discussed include sensory predominance, motor predominance, activity-induced weakness, heat paresis, and cold paresis. Each symptom is associated with, but not limited to, particular neuropathies.

Temperature differentially facilitates spontaneous but not evoked glutamate release from cranial visceral primary afferents

Temperature is fundamentally important to all biological functions including synaptic glutamate release. Vagal afferents from the solitary tract (ST) synapse on second order neurons in the nucleus of the solitary tract, and glutamate release at this first central synapse controls autonomic reflex function. Expression of the temperature-sensitive Transient Receptor Potential Vanilloid Type 1 receptor separates ST afferents into C-fibers (TRPV1+) and A-fibers (TRPV1-). Action potential-evoked glutamate release is similar between C- and A-fiber afferents, but TRPV1 expression facilitates a second form of synaptic glutamate release in C-fibers by promoting substantially more spontaneous glutamate release. The influence of temperature on different forms of glutamate release is not well understood. Here we tested how temperature impacts the generation of evoked and spontaneous release of glutamate and its relation to TRPV1 expression. In horizontal brainstem slices of rats, activation of ST primary afferents generated synchronous evoked glutamate release (ST-eEPSCs) at constant latency whose amplitude reflects the probability of evoked glutamate release. The frequency of spontaneous EPSCs in these same neurons measured the probability of spontaneous glutamate release. We measured both forms of glutamate from each neuron during ramp changes in bath temperature of 4-5 °C. Spontaneous glutamate release from TRPV1+ closely tracked with these thermal changes indicating changes in the probability of spontaneous glutamate release. In the same neurons, temperature changed axon conduction registered as latency shifts but ST-eEPSC amplitudes were constant and independent of TRPV1 expression. These data indicate that TRPV1-operated glutamate release is independent of action potential-evoked glutamate release in the same neurons. Together, these support the hypothesis that evoked and spontaneous glutamate release originate from two pools of vesicles that are independently modulated and are distinct processes.

A combination of topical antiseptics for the treatment of sore throat blocks voltage-gated neuronal sodium channels

Amylmetacresol and dichloro-benzylalcohol are ingredients of lozenges used for the treatment of sore throat. In a former in vitro study, a local anaesthetic-like effect of these substances has been described. Since amylmetacresol and dichloro-benzylalcohol are co-administered in over-the-counter lozenges, the intention of this study is to evaluate the in vitro effects of the combination of these compounds on the voltage-gated sodium channel. We analysed the block of inward sodium currents induced by the combination of amylmetacresol, dichloro-benzylalcohol and the local anaesthetic lidocaine. Tonic and use-dependent block and effects on the inactivated channel state of the neuronal sodium channel were examined. Therefore, the α-subunit of the voltage-gated NaV1.2 sodium channel was heterologously expressed in HEK 293 cells in vitro. Inward sodium currents were investigated in the whole-cell configuration of the patch-clamp technique. The combination of amylmetacresol and dichloro-benzylalcohol and the combination of amylmetacresol and lidocaine induced a block of resting and inactivated sodium channels both displaying a pronounced block at the inactivated channel state. In addition, the combination of all three compounds also resulted in a voltage-dependent block of inward sodium currents. While use-dependent block by co-application of amylmetacresol and dichloro-benzylalcohol was moderate (<20 %), lidocaine and amylmetacresol induced a robust use-dependent block (up to 50 %). This study demonstrates local anaesthetic-like effects of a combination of amylmetacresol and dichloro-benzylalcohol as established ingredients of lozenges. In the presence of amylmetacresol, dichloro-benzylalcohol and lidocaine, a prominent block of inward sodium currents is apparent.

Pathophysiology of immune-mediated demyelinating neuropathies--Part II: Neurology

In the second part of this review we deal with the clinical aspects of immune-mediated demyelinating neuropathies. We describe the relationship between pathophysiology and symptoms and discuss the pathophysiology of specific disease entities, including Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, anti-myelin-associated glycoprotein neuropathy, and POEMS syndrome.

Integration of cortical and pallidal inputs in the basal ganglia-recipient thalamus of singing birds

The basal ganglia-recipient thalamus receives inhibitory inputs from the pallidum and excitatory inputs from cortex, but it is unclear how these inputs interact during behavior. We recorded simultaneously from thalamic neurons and their putative synaptically connected pallidal inputs in singing zebra finches. We find, first, that each pallidal spike produces an extremely brief (∼5 ms) pulse of inhibition that completely suppresses thalamic spiking. As a result, thalamic spikes are entrained to pallidal spikes with submillisecond precision. Second, we find that the number of thalamic spikes that discharge within a single pallidal interspike interval (ISI) depends linearly on the duration of that interval but does not depend on pallidal activity prior to the interval. In a detailed biophysical model, our results were not easily explained by the postinhibitory "rebound" mechanism previously observed in anesthetized birds and in brain slices, nor could most of our data be characterized as "gating" of excitatory transmission by inhibitory pallidal input. Instead, we propose a novel "entrainment" mechanism of pallidothalamic transmission that highlights the importance of an excitatory conductance that drives spiking, interacting with brief pulses of pallidal inhibition. Building on our recent finding that cortical inputs can drive syllable-locked rate modulations in thalamic neurons during singing, we report here that excitatory inputs affect thalamic spiking in two ways: by shortening the latency of a thalamic spike after a pallidal spike and by increasing thalamic firing rates within individual pallidal ISIs. We present a unifying biophysical model that can reproduce all known modes of pallidothalamic transmission--rebound, gating, and entrainment--depending on the amount of excitation the thalamic neuron receives.

Temperature dependence of rapidly adapting mechanically activated currents in rat dorsal root ganglion neurons

Rapidly adapting mechanically activated channels (RA) are expressed on somatosensory neurons and thought to play a role in mechanical transduction. Because mechanical sensations can be significantly affected by temperatures, we examined thermal sensitivity of RA currents in cultured dorsal root ganglion (DRG) neurons to see if RA channel activity is highly temperature-dependent. RA currents were evoked from DRG neurons by membrane displacements and recorded by the whole-cell patch-clamp recording technique. We found that RA currents were significantly enhanced by warming temperatures from 22 to 32 °C and reduced by cooling temperatures from 24 to 14 °C. RA channel activation exhibited steep temperature-dependence with a large temperature coefficient (Q10>5) and a high activation energy (Ea>30 kcal/mol). We further showed that RA channel activation by mechanical stimulation led to membrane depolarization, which could result in action potential firing at 22 °C or 32 °C but not at 14 °C. Taken together, our results provide the measurements of thermal dynamics and activation energy of RA channels, and suggest that a high energy barrier is present for RA channels to open. These findings are in agreement with temperature sensitivity of mechanical sensations in mammals.

A hot topic: temperature sensitive sodium channelopathies

Perturbations to body temperature affect almost all cellular processes and, within certain limits, results in minimal effects on overall physiology. Genetic mutations to ion channels, or channelopathies, can shift the fine homeostatic balance resulting in a decreased threshold to temperature induced disturbances. This review summarizes the functional consequences of currently identified voltage-gated sodium (NaV) channelopathies that lead to disorders with a temperature sensitive phenotype. A comprehensive knowledge of the relationships between genotype and environment is not only important for understanding the etiology of disease, but also for developing safe and effective treatment paradigms.

Tumor necrosis factor-α downregulates sodium current in skeletal muscle by protein kinase C activation: involvement in critical illness polyneuromyopathy

Sepsis is involved in the decrease of membrane excitability of skeletal muscle, leading to polyneuromyopathy. This effect is mediated by alterations of the properties of voltage-gated sodium channels (Na(V)), but the exact mechanism is still unknown. The aim of the present study was to check whether tumor necrosis factor (TNF-α), a cytokine released during sepsis, exerts a rapid effect on Na(V). Sodium current (I(Na)) was recorded by macropatch clamp in skeletal muscle fibers isolated from rat peroneus longus muscle, in control conditions and after TNF-α addition. Analyses of dose-effect and time-effect relationships were carried out. Effect of chelerythrine, a PKC inhibitor, was also studied to determine the way of action of TNF-α. TNF-α induced a reversible dose- and time-dependent inhibition of I(Na). A maximum inhibition of 75% of the control current was observed. A shift toward more negative potentials of activation and inactivation curves of I(Na) was also noticed. These effects were prevented by chelerythrine pretreatment. TNF-α is a cytokine released in the early stages of sepsis. Besides a possible transcriptional role, i.e., modification of the channel type and/or number, we demonstrated the existence of a rapid, posttranscriptional inhibition of Na(V) by TNF-α. The downregulation of the sodium current could be mediated by a PKC-induced phosphorylation of the sodium channel, thus leading to a significant decrease in muscle excitability.

Endotoxin reduces availability of voltage-gated human skeletal muscle sodium channels at depolarized membrane potentials

OBJECTIVE

Critical illness myopathy is a common cause for difficulties in weaning from the respirator and prolonged rehabilitation of patients recovering from sepsis. Several studies have shown that the primary cause of acute generalized muscle weakness is loss of muscle membrane excitability. This study was designed to investigate a potential direct interaction of lipopolysaccharides from Escherichia coli with voltage-gated human skeletal muscle sodium channels (NaV1.4) in vitro.

DESIGN

In vitro laboratory investigation.

SETTING

University laboratory.

SUBJECTS

NaV1.4 sodium channel alpha-subunits stably expressed in human embryonic kidney (HEK293) cells.

INTERVENTIONS

We investigated the effect of lipopolysaccharide on voltage-dependent sodium channel gating by using two distinct modes of application: 1) acute perfusion (pharmacologic lipopolysaccharide concentrations between 5 ng/mL and 50 microg/mL) in order to establish a concentration-effect relationship; and 2) incubation with a clinically relevant concentration of lipopolysaccharide (300 pg/mL).

MEASUREMENTS AND MAIN RESULTS

Lipopolysaccharide did not alter the kinetics of sodium current activation or inactivation when depolarizations were started from hyperpolarized holding potentials. However, when either fast or slow inactivation was induced by membrane depolarization before the test pulse, lipopolysaccharide reversibly reduced channel availability during the test pulse at concentrations of > or = 50 ng/mL revealed by a maximum hyperpolarizing shift of -25 mV in the voltage dependence of fast and slow inactivation, respectively. Incubation with a lipopolysaccharide concentration of 300 pg/mL for 1 hr reproduced the effects on slow but not on fast inactivation. After 20 hrs of low-dose lipopolysaccharide, the peak sodium current was significantly reduced.

CONCLUSIONS

Our results show that lipopolysaccharide interacts with voltage-gated sodium channels, reducing channel availability at depolarized membrane potentials during acute application, independent of the membrane potential after chronic exposure. These effects may contribute to reduced muscle membrane excitability in sepsis.

Computer simulation of wild-type and mutant human cardiac Na+ current

Long QT syndrome (LQTS) and Brugada syndrome (BrS) are inherited diseases predisposing to ventricular arrhythmias and sudden death. Genetic studies linked LQTS and BrS to mutations in genes encoding for cardiac ion channels. Recently, two novel missense mutations at the same codon in the gene encoding the cardiac Na+ channel (SCN5A) have been identified: Y1795C (causing the LQTS phenotype) and Y1795H (causing the BrS phenotype). Functional studies in HEK293 cells showed that both mutations alter the inactivation of Na+ current and cause a sustained Na+ current upon depolarisation. In this paper, a nine state Markov model was used to simulate the Na+ current in wild-type Na+ cardiac channel and the current alterations observed in Y1795C and Y1795H mutant channels. The model includes three distinct closed states, a conducting open state and five inactivation states (one fast-, two intermediate- and two closed-inactivation). Transition rates between these states were identified on the basis of previously published voltage-clamp experiments. The model was able to reproduce the experimental Na+ current in mutant channels just by altering the assignment of model parameters with respect to wild-type case. Parameter assignment was validated by performing action potential clamp experiments and comparing experimental and simulated I(Na) current. The Markov model was subsequently introduced in the Luo-Rudy model of ventricular myocyte to investigate "in silico" the consequences on the ventricular cell action potential of the two mutations. Coherently with their phenotypes, the Y1795C mutation prolongs the action potential, while the Y1795H mutation causes only negligible changes in action potential morphology.

Temperature dependence of erythromelalgia mutation L858F in sodium channel Nav1.7

Background

The disabling chronic pain syndrome erythromelalgia (also termed erythermalgia) is characterized by attacks of burning pain in the extremities induced by warmth. Pharmacological treatment is often ineffective, but the pain can be alleviated by cooling of the limbs. Inherited erythromelalgia has recently been linked to mutations in the gene SCN9A, which encodes the voltage-gated sodium channel Nav1.7. Nav1.7 is preferentially expressed in most nociceptive DRG neurons and in sympathetic ganglion neurons. It has recently been shown that several disease-causing erythromelalgia mutations alter channel-gating behavior in a manner that increases DRG neuron excitability.

Results

Here we tested the effects of temperature on gating properties of wild type Nav1.7 and mutant L858F channels. Whole-cell voltage-clamp measurements on wild type or L858F channels expressed in HEK293 cells revealed that cooling decreases current density, slows deactivation and increases ramp currents for both mutant and wild type channels. However, cooling differentially shifts the midpoint of steady-state activation in a depolarizing direction for L858F but not for wild type channels.

Conclusion

The cooling-dependent shift of the activation midpoint of L858F to more positive potentials brings the threshold of activation of the mutant channels closer to that of wild type Nav1.7 at lower temperatures, and is likely to contribute to the alleviation of painful symptoms upon cooling in affected limbs in patients with this erythromelalgia mutation.

In silico assessment of Y1795C and Y1795H SCN5A mutations: implication for inherited arrhythmogenic syndromes

The effects of two SCN5A mutations (Y1795C, Y1795H), previously identified in one Long QT syndrome type 3 (LQT3) and one Brugada syndrome (BrS) families, were investigated by means of numerical modeling of ventricular action potential (AP). A Markov model capable of reproducing a wild-type as well as a mutant sodium current ( INa) was identified and was included into the Luo-Rudy ventricular cell model for action potential (AP) simulation. The characteristics of endocardial, midmyocardial, and epicardial cells were reproduced by differentiating the transient outward current ( ITO) and the ratio of slow delayed rectifier potassium ( IKs) to rapid delayed rectifier current ( IKr). Administration of flecainide and mexiletine was simulated by appropriately modifying INa, calcium current ( ICa), ITO, and IKr. Y1795C prolonged AP in a rate-dependent manner, and early afterdepolarizations (EADs) appeared during bradycardia in epicardial and midmyocardial cells; flecainide and mexiletine shortened AP and abolished EADs. Y1795H resulted in minimal changes in the APs; flecainide but not mexiletine induced APs heterogeneity across the ventricular wall that accounts for the ST segment elevation induced by flecainide in Y1795H carriers. The AP abnormalities induced by Y1795H and Y1795C can explain the clinically observed surface ECG phenotype. For the first time by modeling the effects of flecainide and mexiletine, we are able to gather mechanistic insights on the response to drugs administration observed in affected patients.

Temperature dependence of human ether-à-go-go-related gene K+ currents

The function of voltage-gated human ether-à-go-gorelated gene ( hERG) K+ channels is critical for both normal cardiac repolarization and suppression of arrhythmias initiated by premature excitation. These important functions are facilitated by their unusual kinetics that combine relatively slow activation and deactivation with rapid and voltage-dependent inactivation and recovery from inactivation. The thermodynamics of these unusual features were examined by exploring the effect of temperature on the activation and inactivation processes of hERG channels expressed in Chinese hamster ovary cells. Increased temperature shifted the voltage dependence of activation in the hyperpolarizing direction but that of inactivation in the depolarizing direction. This increases the relative occupancy of the open state and contributes to the marked temperature sensitivity of hERG current magnitude observed during action potential voltage clamps. The rates of activation and deactivation also increase with higher temperatures, but less markedly than do the rates of inactivation and recovery from inactivation. Our results also emphasize that one cannot extrapolate results obtained at room temperature to 37°C by using a single temperature scale factor.

Application of infrared light for in vivo neural stimulation

A novel method for damage-free, artifact-free stimulation of neural tissue using pulsed, low-energy infrared laser light is presented. Optical stimulation elicits compound nerve and muscle potentials similar to responses obtained with conventional electrical neural stimulation in a rat sciatic nerve model. Stimulation and damage thresholds were determined as a function of wavelength using a tunable free electron laser source (lambda = 2 to 10 microm) and a solid state holmium:YAG laser (lambda = 2.12 microm). Threshold radiant exposure required for stimulation varies with wavelength from 0.312 Jcm2 (lambda = 3 microm) to 1.22 Jcm2 (lambda = 2.1 microm). Histological analysis indicates no discernable thermal damage with suprathreshold stimulation. The largest damage/stimulation threshold ratios (>6) were at wavelengths corresponding to valleys in the IR spectrum of soft tissue absorption (4 and 2.1 microm). Furthermore, optical stimulation can be used to generate a spatially selective response in small fascicles of the sciatic nerve that has significant advantages (e.g., noncontact, spatial resolution, lack of stimulation artifact) over conventional electrical methods in diagnostic and therapeutic procedures in neuroscience, neurology, and neurosurgery.

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.

Heat Stress–Mediated Plasticity in a Locust Looming-Sensitive Visual Interneuron

Neural circuits are strongly affected by temperature and failure ensues at extremes. However, detrimental effects of high temperature on neural pathways can be mitigated by prior exposure to high, but sublethal temperatures (heat shock). Using the migratory locust, Locusta migratoria, we investigated the effects of heat shock on the thermosensitivity of a visual interneuron [the descending contralateral movement detector (DCMD)]. Activity in the DCMD was elicited using a looming stimulus and the response was recorded from the axon using intracellular and extracellular methods. The thoracic region was perfused with temperature-controlled saline and measurements were taken at 5° intervals starting at 25°C. Activity in DCMD was decreased in control animals with increased temperature, whereas heat-shocked animals had a potentiated response such that the peak firing frequency was increased. Significant differences were also found in the thermosensitivity of the action potential properties between control and heat-shocked animals. Heat shock also had a potentiating effect on the amplitude of the afterdepolarization. The concurrent increase in peak firing frequency and maintenance of action potential properties after heat shock could enhance the reliability with which DCMD initiates visually guided behaviors at high temperature.

Hyperthermia increases sensitivity of pulmonary C-fibre afferents in rats

This study was carried out to investigate whether an increase in tissue temperature alters the excitability of vagal pulmonary C-fibres. Single-unit afferent activities of 88 C-fibres were recorded in anaesthetized and artificially ventilated rats when the intrathoracic temperature (T(it)) was maintained at three different levels by isolated perfusion of the thoracic chamber with saline: control (C: approximately 36 degrees C), medium (M: approximately 38.5 degrees C) and high (H: approximately 41 degrees C), each for 3 min with 30 min recovery. Our results showed: (1) The baseline fibre activity (FA) of pulmonary C-fibres did not change significantly at M, but increased drastically (>5-fold) at H. (2) The C-fibre response to right-atrial injection of capsaicin (0.5 microg kg(-1)) was markedly elevated at H (deltaFA = 5.94 +/- 1.65 impulses s(-1) at C and 13.13 +/- 2.98 impulses s(-1) at H; P < 0.05), but not at M. Similar increases in the C-fibre responses to other chemical stimulants (e.g. adenosine, etc.) were found at H; all the enhanced responses returned to control in 30 min. (3) The C-fibre response to lung inflation was also significantly potentiated at H. In sharp contrast, there was no detectable change in either the baseline activity or the responses to lung inflation and deflation in 10 rapidly adapting pulmonary receptors and 10 slowly adapting pulmonary receptors at either M or H. (4) The enhanced C-fibre sensitivity was not altered by pretreatment with indomethacin or capsazepine, a selective antagonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor, but was significantly attenuated by ruthenium red that is known to be an effective blocker of all TRPV channels. (5) The response of pulmonary C-fibres to a progressive increase in T(it) in a ramp pattern further showed that baseline FA started to increase when T(it) exceeded 39.2 degrees C. In conclusion, a pronounced increase in the baseline activity and excitability of pulmonary C-fibres is induced by intrathoracic hyperthermia, and this enhanced sensitivity probably involves activation of temperature-sensitive ion channel(s), presumably one or more of the TRPV receptors, expressed on the C-fibre endings.

Effects of temperature on slow and fast inactivation of rat skeletal muscle Na(+) channels

Patch-clamp studies of mammalian skeletal muscle Na(+) channels are commonly done at subphysiological temperatures, usually room temperature. However, at subphysiological temperatures, most Na(+) channels are inactivated at the cell resting potential. This study examined the effects of temperature on fast and slow inactivation of Na(+) channels to determine if temperature changed the fraction of Na(+) channels that were excitable at resting potential. The loose patch voltage clamp recorded Na(+) currents (I(Na)) in vitro at 19, 25, 31, and 37 degrees C from the sarcolemma of rat type IIb fast-twitch omohyoid skeletal muscle fibers. Temperature affected the fraction of Na(+) channels that were excitable at the resting potential. At 19 degrees C, only 30% of channels were excitable at the resting potential. In contrast, at 37 degrees C, 93% of Na(+) channels were excitable at the resting potential. Temperature did not alter the resting potential or the voltage dependencies of activation or fast inactivation. I(Na) available at the resting potential increased with temperature because the steady-state voltage dependence of slow inactivation shifted in a depolarizing direction with increasing temperature. The membrane potential at which half of the Na(+) channels were in the slow inactivated state was shifted by +16 mV at 37 degrees C compared with 19 degrees C. Consequently, the low availability of excitable Na(+) channels at subphysiological temperatures resulted from channels being in the slow, inactivated state at the resting potential.

Temperature dependence of excitability indices of human cutaneous afferents

The temperature dependence of different indices of axonal excitability (threshold, latency, refractoriness, supernormality, strength-duration time constant, and rheobase) was studied for cutaneous afferents of 8 healthy human volunteers using threshold tracking. Cooling from approximately 32 - approximately 22 degrees C dramatically increased the threshold for a conditioned potential evoked during the relatively refractory period (average increase 573%) but had little effect on the threshold for unconditioned potentials (increased by 4% with 0.1-ms test stimuli), strength-duration time constant (increased by 18%), or rheobase (decreased by 12%). Cooling increased the latency of the unconditioned test potential by 41%, but this slowing was small compared with the effect of cooling on the latency slowing attributable to refractoriness. This measure of refractoriness was initially 0.17 ms at a conditioning-test interval of 2 ms, and increased with cooling to 1.30 ms at the same interval. With cooling, refractoriness was both greater at any one conditioning-test interval and longer in duration, extending into intervals normally associated with supernormality. It is concluded that, although cooling affects all excitability indices to some extent, the most prominent feature is the increase in refractoriness. By contrast, strength-duration time constant is influenced little by temperature.

Kinetic parameters of the ionic currents in myelinated axons: characterization of temperature effects in a hibernator and a nonhibernator

Na+ and K+ currents were measured by the patch-clamp method in the paranodal region of single sciatic nerve fibres of rats and of warm-adapted and cold-adapted golden hamsters. Kinetic parameters and temperature dependence of the Na+ currents were determined. The time constant for activation (about 0.2 ms for rats and hamsters) as well as the time constant for inactivation (about 1.6 ms for rats and hamsters) at 15 degrees C and at -35 mV compared well with single fibre voltage-clamp data from the rat. Differences amongst the three groups of animals were not significant. The temperature coefficient, Q10, for the activation and the inactivation time constant as well as for the time-to-peak of the Na+ current ranged between 2.3 and 3.1. No data have previously been published on the temperature dependence of the delayed-rectifier K channels of mammalian nerve fibres. Most of the K+ current was carried by intermediate (KI) and fast (KF) K channels. Dendrotoxin block indicated that "approximate"55% of the K+ current was due to KI channels, with no significant difference amongst the three groups of animals tested. The Arrhenius plot of the time constant of K+ current activation, "tau"n, yielded a mean Q10 of 3.3 at -40 mV (4. 0 at + 60 mV). No significant differences of the channel kinetics between rats, warm-adapted hamsters and cold-adapted hamsters were detected. We observed, however, a significant decrease of the Na channel density in the paranodal region of cold-adapted hamsters.

Block by 4-aminopyridine of a Kv1.2 delayed rectifier K+ current expressed in Xenopus oocytes

1. The blocking action of 4-aminopyridine (4-AP) on a delayed rectifier Kv1.2 K+ channel expressed in oocytes was investigated at room temperature (22 degrees C) and physiological temperature (34 degrees C) using the double-electrode voltage clamp and patch clamp techniques. 2. At room temperature, 4-AP (100 microM) inhibition occurred only after activation of current. The rate of onset of block was dependent upon the length of time current was activated by a depolarizing step. Similarly, removal of block required current activation. The degree of steady-state block by 4-AP was not reduced by increasingly more depolarized step potentials. The degree of steady-state block also did not change over the duration of a 1 s step. 3. When channels were nearly fully inactivated, 4-AP produced no additional block of a subsequent depolarizing step, suggesting that 4-AP did not bind when channels were in the inactivated state. In single channel experiments, 4-AP decreased the mean open time in a dose-dependent manner but did not alter the single-channel current amplitude. 4. At 34 degrees C the I-V relationship and inactivation curve shifted to more negative potentials. Increasing the temperature to 34 degrees C did not alter the degree of block by 4-AP, although the rate of onset of block was greatly enhanced. 5. Results suggest that 4-AP binds to the open state of the Kv1.2 channel and is trapped when the channel closes. 4-AP cannot bind when the channel is closed or inactivated prior to the addition of the drug. C-type inactivation and 4-AP binding to the channel are mutually exclusive. A model for the proposed mechanism of action of 4-AP on the Kv1.2 channel is proposed based on experimental data.

Sodium channel mutations in paramyotonia congenita uncouple inactivation from activation

Mutations in the adult human skeletal muscle Na+ channel alpha subunit cause the disease paramyotonia congenita. Two paramyotonia congenita mutations, R1448H and R1448C, substitute histidine and cysteine for arginine in the S4 segment of domain 4. These mutations, expressed in a cell line, have only small effects on the activation of Na+ currents, but mutant channels inactivate more slowly with less voltage dependence than wild-type channels and exhibit an enhanced rate of recovery from inactivation. Increase of extracellular pH made the rate of inactivation of R1448H similar to that of R1448C, suggesting that this residue has an extracellular location and that its charge is important for normal inactivation. Analysis of single-channel data reveals that mutant channels inactivate normally from closed states, but poorly from the open state. The data suggest a critical role for the S4 helix of domain 4 in coupling between activation and inactivation.

Modeling axon membranes for functional electrical stimulation

Four models are discussed which can be used to predict the behavior of warm-blooded axons, when excited by electric fields. Up to now, most results were obtained with the Frankenhaeuser-Huxley model, but nearly all of them are wrong in time scale and the cathodic block phenomenon was not observable because the temperature dependence of the gating mechanism has been neglected. However, in the corrected form this model reacts with similar excitability as the two other myelinated nerve models which consider that the potassium current is negligible in mammalian axon membranes. Strength-duration relations for cathodic and anodic excitations, as well as for cathodic blockade, are presented. Paradoxically, the "warm" squid model of Hodgkin and Huxley is the only one which reflects phenomena known from stimulations of the (myelinated) acoustic nerve by cochlear implants.

Effects of laser-induced hyperthermia treatment on ionic permeability of myelinated nerve

The effect of laser-induced hyperthermia on the ionic permeability of nerve membranes was studied using the nodes of Ranvier in amphibian myelinated nerve as a model. To effect a photothermal modification of nerve membrane functions, controlled laser irradiation consisting of a 5-sec thermal pulse was applied to the nodal membrane, increasing the temperature to a maximum of 48-58 degrees C at the node. Major electrophysiological changes observed in the nodal membrane following laser-induced hyperthermia were a differential reduction of the sodium and potassium permeability, an increase in the leakage current, and a negative shift on the potential axis of the steady-state Na inactivation. There was no significant change in the kinetics of ion channel activation and inactivation for treatments below 56 degrees C. The results suggest that a primary photothermal damage mechanism at temperatures below 56 degrees C could be a reduction in the number of active Na channels in the node, rather than a change in individual channel kinetics, or in the properties of the lipid bilayer of intervening nerve membrane. A differential heat sensitivity between the noninactivated and the inactivated Na channels is also suggested. For the treatments of 56 degrees C and above, a significant increase of membrane leakage current suggests an irreversible thermal damage to the lipid bilayer.

Thermal influences on nervous system function

The various effects of temperature change are only partially predictable. Temporal measures relevant to membrane activity, action potentials, synaptic transmission, and evoked potentials are all consistently increased with cooling and decreased by warming. However, the various measures of amplitude at different levels, and even within similar preparations, are contradictory: Some laboratories report increased amplitudes with cooling and others report decreased amplitudes under similar conditions. Emphasis is given to identifying factors that may resolve the differences. These include: (a) the rate of temperature change, (b) sites of cooling, stimulation and recording, (c) stimulus characteristics, and (d) fundamental differences in temperature sensitivities of different neural tissue. Other factors that may affect the ability to predict thermal influences on neural function from existing formulations are: relative ion permeabilities, metabolic ion pumps, the resting potential at the onset of cooling, and an animal's acclimated temperature at sacrifice.

Blockade of sodium channels by Bistramide A in voltage-clamped frog skeletal muscle fibres

The effect of Bistramide A, a toxin isolated from Bistratum lissoclinum Sluiter (Urochordata), on the peak sodium current (INa) of frog skeletal muscle fibres was studied with the double sucrose gap voltage clamp technique. External or internal application of Bistramide A inhibited INa without alteration of the kinetic parameters of the current nor of the apparent reversal potential for Na. The steady-state activation curve of INa was unchanged while the steady-state inactivation curve of INa was shifted towards more negative membrane potentials. Dose-response curves indicated an apparent dissociation constant for Bistramide A of 3.3 microM and a Hill coefficient of 1.2 which suggested a one to one relation between the toxin and Na channel. The inhibition of INa occurred at rest, and was more important at more positive holding potentials. Bistramide A exhibited only a weak frequency-dependent effect. The toxin did not interact with the use-dependent effect of lidocaine. It mainly blocked Na channels at more depolarized holding potentials. The toxin blocked Na channels when it was internally applyed and when the inactivation gating system has been previously destroyed by internal diffusion of iodate. The data suggest that Bistramide A inhibited the Na channel both at rest and in the inactivated state and occupied a site which was not located on the inactivation gate.

Differential impact of hypothermia and pentobarbital on brain-stem auditory evoked responses

Two experiments were conducted to determine the effects of hypothermia and pentobarbital anesthesia, alone and in combination, on the brain-stem auditory evoked responses (BAERs) of rats. In experiment I, unanesthetized rats were cooled to colonic temperatures 0.5 and 1.0 degrees C below normal. In experiment II, 2 groups of rats were cooled and tested at 37.5, 36.0, 34.5 and 31.5 degrees C. One group was anesthetized during testing and the other group was awake. The rat BAER was sensitive to cooling of 1 degree C or less. Peak latencies were prolonged and peak-to-peak amplitudes were increased by hypothermia alone. The effect on amplitude may be related to the time course of temperature change or to stimulus level. Pentobarbital significantly affected both latencies and amplitudes over and above the effects of cooling. The specific effects of pentobarbital differed by BAER peak and by temperature. The findings point up the importance of the potential confound of anesthetic drugs in most of the evoked potential literature on hypothermia and, for the first time, quantify the complex interactions between pentobarbital and temperature which affect the BAER wave form.

Temperature sensitivity of Ca currents in chick sensory neurones

We have investigated the effects of temperature on the Ca currents of chick sensory neurones. Raising the temperature from 17 to 37 degrees C, caused low-threshold (LVA, T) and high-threshold (HVA, L and N) Ca currents to show a marked amplitude increase and a drastic acceleration of their activation-inactivation gatings. Compared to HVA channels, the LVA type showed a weaker temperature sensitivity. Its average Q10 values were closer to those of other voltage-operated ion channels: 1.7 (permeability), 1.9 (activation) and 2.2 (inactivation). Alternatively, the activation kinetics and peak permeability of HVA Ca channels showed maximal Q10 values of about 5 and 2.8, respectively. HVA channel deactivation was less sensitive to temperature (Q10 1.8). Inactivation of these channels was slow and monoexponential between 17 and 22 degrees C, but faster and double exponential above 30 degrees C, uncovering a fast temperature-sensitive decaying phase. The size and rate of decay of this component decreased with increasing membrane depolarizations and persisted at holding potentials positive to -80 mV, suggesting the involvement of temperature-sensitive Ca-mediated processes in the mechanism of HVA channel inactivation. Our data are consistent with the view that heating from 17 to 37 degrees C causes both an increased probability of Ca channels to open and a drastic acceleration of their activation-inactivation kinetics.

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)

Temperature dependence of gating current in myelinated nerve fibers

Asymmetrical displacement currents and Na currents of single myelinated nerve fibers of Xenopus laevis were studied in the temperature range from 5 to 24 degrees C. The time constant of the on-response at E = 4 mV, tau on, was strongly temperature dependent, whereas the amount of displaced charge at E = 39 mV, Qon, was only slightly temperature dependent. The mean Q10 for tau on-1 was 2.54, the mean Q10 for Qon was 1.07. The time constant of charge immobilization, tau i, at E = 4 mV varied significantly (alpha = 0.001) with temperature. The mean Q10 for tau i-1 was 2.71 +/- 0.38. The time constants of immobilization of gating charge and of fast inactivation of Na permeability were similar in the temperature range from 6 to 22 degrees C. The Qoff/Qon ratio for E = 4 mV pulses of 0.5 msec duration decreased with increasing temperature. The temperature dependence of the time constant of the off-response could not be described by a single Q10 value, since the Q10 depended on the duration of the test pulse. Increasing temperature shifted Qon (E) curves to more negative potentials by 0.51 mV K-1, but shifted PNa (E) curves and h infinity (E) curves to more positive potentials by 0.43 and 0.57 mV K-1, respectively. h infinity (E = -70 mV) increased monotonously with increasing temperature. The present data indicate that considerable entropy changes may occur when the Na channel molecule passes from closed through open to inactivated states.

The sodium current underlying action potentials in guinea pig hippocampal CA1 neurons

Neurons were acutely dissociated from the CA1 region of hippocampal slices from guinea pigs. Whole-cell recording techniques were used to record and control membrane potential. When the electrode contained KF, the average resting potential was about -40 mV and action potentials in cells at -80 mV (current-clamped) had an amplitude greater than 100 mV. Cells were voltage-clamped at 22-24 degrees C with electrodes containing CsF. Inward currents generated with depolarizing voltage pulses reversed close to the sodium equilibrium potential and could be completely blocked with tetrodotoxin (1 microM). The amplitude of these sodium currents was maximal at about -20 mV and the amplitude of the tail currents was linear with potential, which indicates that the channels were ohmic. The sodium conductance increased with depolarization in a range from -60 to 0 mV with an average half-maximum at about -40 mV. The decay of the currents was not exponential at potentials more positive than -20 mV. The time to peak and half-decay time of the currents varied with potential and temperature. Half of the channels were inactivated at a potential of -75 mV and inactivation was essentially complete at -40 to -30 mV. Recovery from inactivation was not exponential and the rate varied with potential. At lower temperatures, the amplitude of sodium currents decreased, their time course became longer, and half-maximal inactivation shifted to more negative potentials. In a small fraction of cells studied, sodium currents were much more rapid but the voltage dependence of activation and inactivation was very similar.

Contrasting thermal sensitivity of spontaneously active A- and C-fibers in experimental nerve-end neuromas

Injured afferent A- and C-fibers ending in experimental neuromas in the rat sciatic nerve generate a substantial spontaneous discharge. We show that for individual axons the rate and percent incidence of spontaneous discharge are sensitive to neuroma temperature. Within the range of 14-43 degrees C, firing rate of all of the myelinated fibers examined increased as temperature rose, and decreased as temperature fell. For fibers with a tonic rhythmic discharge pattern, Q10 averaged 1.64 at 34-42 degrees C. Some fibers that were initially silent began to fire as the neuroma was warmed, and some fibers active at baseline temperature fell silent when the neuroma was cooled. Unmyelinated fibers behaved quite differently, showing either no response to temperature changes (44% of fibers sampled), or an increase in discharge rate upon cooling (56%). These effects are probably not secondary to vascular changes, but rather reflect thermal sensitivity of the ectopic neuroma impulse generator sites. This thermal sensitivity may account for the aggravation of phantom limb pain and other neuralgias during cold weather (i.e., post-traumatic cold intolerance).

Na currents and action potentials in rat myelinated nerve fibres at 20 and 37 degrees C

(1) Action potentials and membrane currents were recorded in single myelinated rat nerve fibres at 20 and 37 degrees C. Three experiments were also performed in single cat nerve fibres. (2) K currents were blocked by internal CsCl and external TEA. The steady state and kinetic parameters of Na activation and inactivation were determined at both temperatures. (3) When the temperature was raised from 20 to 37 degrees C, steady state Na activation, m infinity (V), and inactivation, h infinity (V), did not change significantly. (4) The time constant of Na activation, tau m, was determined within the potential range of -40 to 125 mV at 20 degrees C and V = 40-60 mV at 37 degrees C. The temperature coefficient, Q10, of tau m was 2.2. (5) The decay in the Na current was described by two exponentials at both temperatures. The amplitude of the slow phase was 1-10%. The time constant of the fast phase of Na inactivation, tau h1, was determined at both temperatures within the potential range of -50 mV to 125 mV. The Q10 of tau h1 was 2.9 and did not depend on potential. (6) The Na equilibrium potential was 152 mV at 20 degrees C and 144 mV at 37 degrees C. The leakage conductance was 24 nS at 20 degrees C and 43 nS at 37 degrees C. These differences were interpreted as signs of fibre deterioration at higher temperature. (7) The results from the current and voltage clamp experiments performed in the cat nerve were essentially the same as those in the rat nerve fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature dependence of Na currents in rabbit and frog muscle membranes

The effect of temperature (0-22 degrees C) on the kinetics of Na channel conductance was determined in voltage-clamped rabbit and frog skeletal muscle fibers using the triple-Vaseline-gap technique. The Hodgkin-Huxley model was used to extract kinetic parameters; the time course of the conductance change during step depolarization followed m3h kinetics. Arrhenius plots of activation time constants (tau m), determined at both moderate (-10 to -20 mV) and high (+100 mV) depolarizations, were linear in both types of muscle. In rabbit muscle, Arrhenius plots of the inactivation time constant (tau h) were markedly nonlinear at +100 mV, but much less so at -20 mV. The reverse situation was found in frog muscle. The contrast between the highly nonlinear Arrhenius plot of tau h at +100 mV in rabbit muscle, compared with that of frog muscle, was interpreted as revealing an intrinsic nonlinearity in the temperature dependence of mammalian muscle Na inactivation. These results are consistent with the notion that mammalian cell membranes undergo thermotropic membrane phase transitions that alter lipid-channel interactions in the 0-22 degrees C range. Furthermore, the observation that Na channel activation appears to be resistant to this effect suggests that the gating mechanisms that govern activation and inactivation reside in physically distinct regions of the channel.