Potassium and the excitability properties of normal human motor axons in vivo.

Hyperkalemia is an important cause of membrane depolarization in renal failure. A recent theoretical model of axonal excitability explains the effects of potassium on threshold electrotonus, but predicts changes in superexcitability in the opposite direction to those observed. To resolve this contradiction we assessed the relationship between serum potassium and motor axon excitability properties in 38 volunteers with normal potassium levels. Most threshold electrotonus measures were strongly correlated with potassium, and superexcitability decreased at higher potassium levels (P = 0.016), contrary to the existing model. Improved modelling of potassium effects was achieved by making the potassium currents obey the constant-field theory, and by making the potassium permeabilities proportional to external potassium, as has been observed in vitro. This new model also accounted well for the changes in superexcitability and other excitability measures previously reported in renal failure. These results demonstrate the importance of taking potassium levels into account when assessing axonal membrane dysfunction by excitability testing, and provide evidence that potassium currents are activated by external potassium in vivo.

Neuroprotection through excitability and mTOR required in ALS motoneurons to delay disease and extend survival.

Delaying clinical disease onset would greatly reduce neurodegenerative disease burden, but the mechanisms influencing early preclinical progression are poorly understood. Here, we show that in mouse models of familial motoneuron (MN) disease, SOD1 mutants specifically render vulnerable MNs dependent on endogenous neuroprotection signaling involving excitability and mammalian target of rapamycin (mTOR). The most vulnerable low-excitability FF MNs already exhibited evidence of pathology and endogenous neuroprotection recruitment early postnatally. Enhancing MN excitability promoted MN neuroprotection and reversed misfolded SOD1 (misfSOD1) accumulation and MN pathology, whereas reducing MN excitability augmented misfSOD1 accumulation and accelerated disease. Inhibiting metabotropic cholinergic signaling onto MNs reduced ER stress, but enhanced misfSOD1 accumulation and prevented mTOR activation in alpha-MNs. Modulating excitability and/or alpha-MN mTOR activity had comparable effects on the progression rates of motor dysfunction, denervation, and death. Therefore, excitability and mTOR are key endogenous neuroprotection mechanisms in motoneurons to counteract clinically important disease progression in ALS.

Cellular hyper-excitability caused by mutations that alter the activation process of voltage-gated sodium channels.

Voltage-gated sodium channels (Nav) are widely expressed as macro-molecular complexes in both excitable and non-excitable tissues. In excitable tissues, the upstroke of the action potential is the result of the passage of a large and rapid influx of sodium ions through these channels. NaV dysfunction has been associated with an increasingly wide range of neurological, muscular and cardiac disorders. The purpose of this review is to summarize the recently identified sodium channel mutations that are linked to hyper-excitability phenotypes and associated with the alteration of the activation process of voltage gated sodium channels. Indeed, several clinical manifestations that demonstrate an alteration of tissue excitability were recently shown to be strongly associated with the presence of mutations that affect the activation process of the Nav. These emerging genotype-phenotype correlations have expanded the clinical spectrum of sodium channelopathies to include disorders which feature a hyper-excitability phenotype that may or may not be associated with a cardiomyopathy. The p.I141V mutation in SCN4A and SCN5A, as well as its homologous p.I136V mutation in SCN9A, are interesting examples of mutations that have been linked to inherited hyperexcitability myotonia, exercise-induced polymorphic ventricular arrhythmias and erythromelalgia, respectively. Regardless of which sodium channel isoform is investigated, the substitution of the isoleucine to valine in the locus 141 induces similar modifications in the biophysical properties of the Nav by shifting the voltage-dependence of steady state activation toward more negative potentials.

Effect of β-amyloid block of the fast-inactivating K+ channel on intracellular Ca2+ and excitability in a modeled neuron

β-Amyloid peptide (Aβ), one of the primary protein components of senile plaques found in Alzheimer disease, is believed to be toxic to neurons by a mechanism that may involve loss of intracellular calcium regulation. We have previously shown that Aβ blocks the fast-inactivating potassium (A) current. In this work, we show, through the use of a mathematical model, that the Aβ-mediated block of the A current could result in increased intracellular calcium levels and increased membrane excitability, both of which have been observed in vitro upon acute exposure to Aβ. Simulation results are compared with experimental data from the literature; the simulations quantitatively capture the observed concentration dependence of the neuronal response and the level of increase in intracellular calcium.

Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus.

We recorded in the CA1 region from hippocampal slices of prion protein (PrP) gene knockout mice to investigate whether the loss of the normal form of prion protein (PrPC) affects neuronal excitability as well as synaptic transmission in the central nervous system. No deficit in synaptic inhibition was found using field potential recordings because (i) responses induced by stimulation in stratum radiatum consisted of a single population spike in PrP gene knockout mice similar to that recorded from control mice and (ii) the plot of field excitatory postsynaptic potential slope versus the population spike amplitude showed no difference between the two groups of mice. Intracellular recordings also failed to detect any difference in cell excitability and the reversal potential for inhibitory postsynaptic potentials. Analysis of the kinetics of inhibitory postsynaptic current revealed no modification. Finally, we examined whether synaptic plasticity was altered and found no difference in long-term potentiation between control and PrP gene knockout mice. On the basis of our findings, we propose that the loss of the normal form of prion protein does not alter the physiology of the CA1 region of the hippocampus.

Free, long-chain, polyunsaturated fatty acids reduce membrane electrical excitability in neonatal rat cardiac myocytes.

Because previous studies showed that polyunsaturated fatty acids can reduce the contraction rate of spontaneously beating heart cells and have antiarrhythmic effects, we examined the effects of the fatty acids on the electrophysiology of the cardiac cycle in isolated neonatal rat cardiac myocytes. Exposure of cardiomyocytes to 10 microM eicosapentaenoic acid for 2-5 min markedly increased the strength of the depolarizing current required to elicit an action potential (from 18.0 +/- 2.4 pA to 26.8 +/- 2.7 pA, P < 0.01) and the cycle length of excitability (from 525 ms to 1225 ms, delta = 700 +/- 212, P < 0.05). These changes were due to an increase in the threshold for action potential (from -52 mV to -43 mV, delta = 9 +/- 3, P < 0.05) and a more negative resting membrane potential (from -52 mV to -57 mV, delta = 5 +/- 1, P < 0.05). There was a progressive prolongation of intervals between spontaneous action potentials and a slowed rate of phase 4 depolarization. Other polyunsaturated fatty acids--including docosahexaenoic acid, linolenic acid, linoleic acid, arachidonic acid, and its nonmetabolizable analog eicosatetraynoic acid, but neither the monounsaturated oleic acid nor the saturated stearic acid--had similar effects. The effects of the fatty acids could be reversed by washing with fatty acid-free bovine serum albumin. These results show that free polyunsaturated fatty acids can reduce membrane electrical excitability of heart cells and provide an electrophysiological basis for the antiarrhythmic effects of these fatty acids.

Neuroprotectant effects of iso-osmolar D-mannitol to prevent Pacific ciguatoxin-1 induced alterations in neuronal excitability: A comparison with other osmotic agents and free radical scavengers

The basis for the neuroprotectant effect of D-mannitol in reducing the sensory neurological disturbances seen in ciguatera poisoning, is unclear. Pacific ciguatoxin-1 (P-CTX-1), at a concentration 10 nM, caused a statistically significant swelling of rat sensory dorsal root ganglia (DRG) neurons that was reversed by hyperosmolar 50 MM D-mannitol. However, using electron paramagnetic resonance (EPR) spectroscopy, it was found that P-CTX-1 failed to generate hydroxyl free radicals at concentrations of toxin that caused profound effects on neuronal excitability. Whole-cell patch-clamp recordings from DRG neurons revealed that both hyper- and iso-osmolar 50 MM D-mannitol prevented the membrane depolarisation and repetitive firing of action potentials induced by P-CTX-1. In addition, both hyper- and iso-osmolar 50 MM D-mannitol prevented the hyperpolarising shift in steady-state inactivation and the rise in leakage current through tetrodotoxin (TTX)-sensitive Na-v channels, as well as the increased rate of recovery from inactivation of TTX-resistant Nav channels induced by P-CTX-1. D-Mannitol also reduced, but did not prevent, the inhibition of peak TTX-sensitive and TTX-resistant I-Na amplitude by P-CTX-1. Additional experiments using hyper- and isoosmolar D-sorbitol, hyperosmolar sucrose and the free radical scavenging agents Trolox (R) and L-ascorbic acid showed that these agents, unlike D-mannitol, failed to prevent the effects of P-CTX-1 on spike electrogenesis and Na-v channel gating. These selective actions of D-mannitol indicate that it does not act purely as an osmotic agent to reduce swelling of nerves, but involves a more complex action dependent on the Nav channel subtype, possibly to alter or reduce toxin association. (c) 2005 Elsevier Ltd. All rights reserved.

Block of voltage-gated potassium channels by Pacific ciguatoxin-1 contributes to increased neuronal excitability in rat sensory neurons

The present study investigated the actions of the polyether marine toxin Pacific ciguatoxin-1 (P-CTX-1) on neuronal excitability in rat dorsal root ganglion (DRG) neurons using patch-clamp recording techniques. Under current-clamp conditions, bath application of 2-20 nM P-CTX-1 caused a rapid, concentration-dependent depolarization of the resting membrane potential in neurons expressing tetrodotoxin (TTX)-sensitive voltage-gated sodium (Na-v,.) channels. This action was completely suppressed by the addition of 200 nM TTX to the external solution, indicating that this effect was mediated through TTX-sensitive Na-v channels. In addition, P-CTX-1 also prolonged action potential and afterhyperpolarization (AHP) duration. In a subpopulation of neurons, P-CTX-1 also produced tonic action potential firing, an effect that was not accompanied by significant oscillation of the resting membrane potential. Conversely, in neurons expressing TTX-resistant Na-v currents, P-CTX-1 failed to alter any parameter of neuronal excitability examined in this study. Under voltage-clamp conditions in rat DRG neurons, P-CTX-1 inhibited both delayed-rectifier and 'A-type' potassium currents in a dose-dependent manner, actions that Occurred in the absence of alterations to the voltage dependence of activation. These actions appear to underlie the prolongation of the action potential and AHP. and contribute to repetitive firing. These data indicate that a block of potassium channels contributes to the increase in neuronal excitability, associated with a modulation of Na-v. channel gating, observed clinically in response to ciguatera poisoning. (c) 2004 Elsevier Inc. All rights reserved.

Visual feedback alters the variations in corticospinal excitability that arise from rhythmic movements of the opposite limb

Augmented visual feedback can have a profound bearing on the stability of bimanual coordination. Indeed, this has been used to render tractable the study of patterns of coordination that cannot otherwise be produced in a stable fashion. In previous investigations (Carson et al. 1999), we have shown that rhythmic movements, brought about by the contraction of muscles on one side of the body, lead to phase-locked changes in the excitability of homologous motor pathways of the opposite limb. The present study was conducted to assess whether these changes are influenced by the presence of visual feedback of the moving limb. Eight participants performed rhythmic flexion-extension movements of the left wrist to the beat of a metronome (1.5 Hz). In 50% of trials, visual feedback of wrist displacement was provided in relation to a target amplitude, defined by the mean movement amplitude generated during the immediately preceding no feedback trial. Motor potentials (MEPs) were evoked in the quiescent muscles of the right limb by magnetic stimulation of the left motor cortex. Consistent with our previous observations, MEP amplitudes were modulated during the movement cycle of the opposite limb. The extent of this modulation was, however, smaller in the presence of visual feedback of the moving limb (FCR omega(2) =0.41; ECR omega(2)=0.29) than in trials in which there was no visual feedback (FCR omega(2)=0.51; ECR omega(2)=0.48). In addition, the relationship between the level of FCR activation and the excitability of the homologous corticospinal pathway of the opposite limb was sensitive to the vision condition; the degree of correlation between the two variables was larger when there was no visual feedback of the moving limb. The results of the present study support the view that increases in the stability of bimanual coordination brought about by augmented feedback may be mediated by changes in the crossed modulation of excitability in homologous motor pathways.

Mechanisms of altered cortical excitability in photosensitive epilepsy

Despite the multiplicity of approaches and techniques so far applied for identifying the pathophysiological mechanisms of photosensitive epilepsy, a generally agreed explanation of the phenomenon is still lacking. The present thesis reports on three interlinked original experimental studies conducted to explore the neurophysiological correlates and the phatophysiological mechanism of photosensitive epilepsy. In the first study I assessed the role of the habituation of the Visual Evoked Response test as a possible biomarker of epileptic visual sensitivity. The two subsequent studies were designed to address specific research questions emerging from the results of the first study. The findings of the three intertwined studies performed provide experimental evidence that photosensitivity is associated with changes in a number of electrophysiological measures suggestive of altered balance between excitatory and inhibitory cortical processes. Although a strong clinical association does exist between specific epileptic syndromes and visual sensitivity, results from this research indicate that photosensitivity trait seems to be the expression of specific pathophysiological mechanisms quite distinct from the “epileptic” phenotype. The habituation of Pattern Reversal Visual Evoked Potential (PR-VEP) appears as a reliable candidate endo-phenotype of visual sensitivity. Interpreting the findings of this study in the context of the broader literature on visual habituation we can hypothesise the existence of a shared neurophysiological background between photosensitive epilepsy and migraine. Future studies to elucidate the relationship between the proposed indices of cortical excitability and specific polymorphisms of excitatroy and inhibitory neurotransmission will need to be conducted to assess their potential role as biomarkers of photosensitivity.

Ethosuximide modifies network excitability in the rat entorhinal cortex via an increase in GABA release

Ethosuximide is the drug of choice for treating generalized absence seizures, but its mechanism of action is still a matter of debate. It has long been thought to act by disrupting a thalamic focus via blockade of T-type channels and, thus, generation of spike-wave activity in thalamocortical pathways. However, there is now good evidence that generalized absence seizures may be initiated at a cortical focus and that ethosuximide may target this focus. In the present study we have looked at the effect ethosuximide on glutamate and GABA release at synapses in the rat entorhinal cortex in vitro, using two experimental approaches. Whole-cell patch-clamp studies revealed an increase in spontaneous GABA release by ethosuximide concurrent with no change in glutamate release. This was reflected in studies that estimated global background inhibition and excitation from intracellularly recorded membrane potential fluctuations, where there was a substantial rise in the ratio of network inhibition to excitation, and a concurrent decrease in excitability of neurones embedded in this network. These studies suggest that, in addition to well-characterised effects on ion channels, ethosuximide may directly elevate synaptic inhibition in the cortex and that this could contribute to its anti-absence effects. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Transcranial magnetic stimulation reveals modulation of corticospinal excitability when observing actions with the intention to imitate

Studies using transcranial magnetic stimulation have demonstrated that action observation can modulate the activity of the corticospinal system. This has been attributed to the activity of an 'action observation network', whereby premotor cortex activity influences corticospinal excitability. Neuroimaging studies have demonstrated that the context in which participants observe actions (i.e. whether they simply attend to an action, or observe it with the intention to imitate) modulates action observation network activity. The study presented here examined whether the context in which actions were observed revealed similar modulatory effects on corticospinal excitability. Eight human participants observed a baseline stimulus (a fixation cross), observed actions in order to attend to them, or observed the same actions with the intention to imitate them. Whereas motor evoked potentials elicited from the first dorsal interosseus muscle of the hand were facilitated by attending to actions, observing the same actions in an imitative capacity led to no facilitation effect. Furthermore, no motor facilitation effects occurred in a control muscle. Electromyographic data collected when participants physically imitated the observed actions revealed that the activity of the first dorsal interosseus muscle increased significantly during action execution compared with rest. These data suggest that an inhibitory mechanism acts on the corticospinal system to prevent the immediate overt imitation of observed actions. These data provide novel insight into the properties of the human action observation network, demonstrating for the first time that observing actions with the intention to imitate them can modulate the effects of action observation on corticospinal excitability.

Diverse antiepileptic drugs increase the ratio of background synaptic inhibition to excitation and decrease neuronal excitability in neurones of the rat entorhinal cortex in vitro

Although most anti-epileptic drugs are considered to have a primary molecular target, it is clear that their actions are unlikely to be limited to effects on a single aspect of inhibitory synaptic transmission, excitatory transmission or voltage-gated ion channels. Systemically administered drugs can obviously simultaneously access all possible targets, so we have attempted to determine the overall effect of diverse agents on the balance between GABAergic inhibition, glutamatergic excitation and cellular excitability in neurones of the rat entorhinal cortex in vitro. We used an approach developed for estimating global background synaptic excitation and inhibition from fluctuations in membrane potential obtained by intracellular recordings. We have previously validated this approach in entorhinal cortical neurones [. Greenhill and Jones (2007a) Neuroscience 147:884-892]. Using this approach, we found that, despite their differing pharmacology, the drugs tested (phenytoin, lamotrigine, valproate, gabapentin, felbamate, tiagabine) were unified in their ability to increase the ratio of background GABAergic inhibition to glutamatergic excitation. This could occur as a result of decreased excitation concurrent with increased inhibition (phenytoin, lamotrigine, valproate), a decrease in excitation alone (gabapentin, felbamate), or even with a differential increase in both (tiagabine). Additionally, we found that the effects on global synaptic conductances agreed well with whole cell patch recordings of spontaneous glutamate and GABA release (our previous studies and further data presented here). The consistency with which the synaptic inhibition:excitation ratio was increased by the antiepileptic drugs tested was matched by an ability of all drugs to concurrently reduce intrinsic neuronal excitability. Thus, it seems possible that specific molecular targets among antiepileptic drugs are less important than the ability to increase the inhibition:excitation ratio and reduce overall neuronal and network excitability. © 2010 IBRO.