Sodium Channels and Therapy of Central Nervous System Diseases

Discovery of Novel Pyrimidine-Based Derivatives as Nav1.2 Inhibitors with Efficacy in Mouse Models of Epilepsy

Dysfunction of voltage-gated sodium channel Nav1.2 causes various epileptic disorders, and inhibition of the channel has emerged as an attractive therapeutic strategy. However, currently available Nav1.2 inhibitors exhibit low potency and limited structural diversity. In this study, a novel series of pyrimidine-based derivatives with Nav1.2 inhibitory activity were designed, synthesized, and evaluated. Compounds 14 and 35 exhibited potent activity against Nav1.2, boasting IC50 values of 120 and 65 nM, respectively. Compound 14 displayed favorable pharmacokinetics (F = 43%) following intraperitoneal injection and excellent brain penetration potency (B/P = 3.6). Compounds 14 and 35 exhibited robust antiepileptic activities in the maximal electroshock test, with ED50 values of 3.2 and 11.1 mg/kg, respectively. Compound 35 also demonstrated potent antiepileptic activity in a 6 Hz (32 mA) model, with an ED50 value of 18.5 mg/kg. Overall, compounds 14 and 35 are promising leads for the development of new small-molecule therapeutics for epilepsy.

Gaps in Understanding Mechanism and Lack of Treatments: Potential Use of a Nonhuman Primate Model of Oxaliplatin-Induced Neuropathic Pain

The antineoplastic agent oxaliplatin induces an acute hypersensitivity evoked by cold that has been suggested to be due to sensitized central and peripheral neurons. Rodent-based preclinical studies have suggested numerous treatments for the alleviation of oxaliplatin-induced neuropathic pain, but few have demonstrated robust clinical efficacy. One issue is that current understanding of the pathophysiology of oxaliplatin-induced neuropathic pain is primarily based on rodent models, which might not entirely recapitulate the clinical pathophysiology. In addition, there is currently no objective physiological marker for pain that could be utilized to objectively indicate treatment efficacy. Nonhuman primates are phylogenetically and neuroanatomically similar to humans; thus, disease mechanism in nonhuman primates could reflect that of clinical oxaliplatin-induced neuropathy. Cold-activated pain-related brain areas in oxaliplatin-treated macaques were attenuated with duloxetine, the only drug that has demonstrated clinical efficacy for chemotherapy-induced neuropathic pain. By contrast, drugs that have not demonstrated clinical efficacy in oxaliplatin-induced neuropathic pain did not reduce brain activation. Thus, a nonhuman primate model could greatly enhance understanding of clinical pathophysiology beyond what has been obtained with rodent models and, furthermore, brain activation could serve as an objective marker of pain and therapeutic efficacy.

Identification and electrophysiological evaluation of 2-methylbenzamide derivatives as Nav1.1 modulators

Voltage-gated sodium channels (Nav) are crucial to the initiation and propagation of action potentials (APs) in electrically excitable cells, and during the past decades they have received considerable attention due to their therapeutic potential. Here, we report for the first time the synthesis and the electrophysiological evaluation of 16 ligands based on a 2-methylbenzamide scaffold that have been identified as Nav1.1 modulators. Among these compounds, N,N'-(1,3-phenylene)bis(2-methylbenzamide) (3a) has been selected and evaluated in ex-vivo experiments in order to estimate the activation impact of such a compound profile. It appears that 3a increases the Nav1.1 channel activity although its overall impact remains moderate. Altogether, our preliminary results provide new insights into the development of small molecule activators targeting specifically Nav1.1 channels to design potential drugs for treating CNS diseases.

The anti-seizure drugs vinpocetine and carbamazepine, but not valproic acid, reduce inflammatory IL-1β and TNF-α expression in rat hippocampus

In the present study, the effects of the two classical anti-epileptic drugs, carbamazepine and valproic acid, and the non-classical anti-seizure drug vinpocetine were investigated on the expression of the pro-inflammatory cytokines IL-1β and TNF-α in the hippocampus of rats by PCR or western blot after the administration of one or seven doses. Next, the effects of the anti-seizure drugs were investigated on the rise in cytokine expression induced by lipopolysaccharides (LPS) inoculation in vivo. To validate our methods, the changes induced by the pro-convulsive agents 4-aminopyridine, pentylenetetrazole and pilocarpine were also tested. Finally, the effect of the anti-seizure drugs on seizures and on the concomitant rise in pro-inflammatory cytokine expression induced by 4-aminopyridine was explored. Results show that vinpocetine and carbamazepine reduced the expression of IL-1β and TNF-α from basal conditions, and the increase in both pro-inflammatory cytokines induced by LPS. In contrast, valproic acid failed to reduce both the expression of the cytokines from basal conditions and the rise in IL-1β and TNF-α expression induced by LPS. Tonic-clonic seizures induced either by 4-aminopyridine, pentylenetetrazole or pilocarpine increased the expression of IL-1β and TNF-α markedly. 4-aminopyridine-induced changes were reduced by all the tested anti-seizure drugs, although valproic acid was less effective. We conclude that the anti-seizure drugs, vinpocetine and carbamazepine, whose mechanisms of action involve a decrease in ion channels permeability, also reduce cerebral inflammation. The mechanism of action of anti-seizure drugs like vinpocetine and carbamazepine involves a decrease in Na(+) channels permeability. We here propose that this mechanism of action also involves a decrease in cerebral inflammation.

A highly predictive 3D-QSAR model for binding to the voltage-gated sodium channel: design of potent new ligands

A comprehensive comparative molecular field analysis (CoMFA) model for the binding of ligands to the neuronal voltage-gated sodium channel was generated based on 67 diverse compounds. Earlier published CoMFA models for this target provided μM ligands, but the improved model described here provided structurally novel compounds with low nM IC₅₀. For example, new compounds 94 and 95 had IC₅₀ values of 129 and 119 nM, respectively.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

In vivo neurochemical characterization of clothianidin induced striatal dopamine release

Clothianidin (CLO) is a neonicotinoid insecticide with selective action on nicotinic acetylcholine receptors. The aim of this study was to determine the neurochemical basis for CLO-induced striatal dopamine release using the microdialysis technique in freely moving and conscious rats. Intrastriatal administration of CLO (3.5mM), produced an increase in both spontaneous (2462 ± 627% with respect to basal values) and KCl-evoked (4672 ± 706% with respect to basal values) dopamine release. This effect was attenuated in Ca(2+)-free medium, and was prevented in reserpine pre-treated animals or in presence of tetrodotoxin (TTX). To investigate the involvement of dopamine transporter (DAT), the effect of CLO was observed in presence of nomifensine. The coadministration of CLO and nomifensine produced an additive effect on striatal dopamine release. The results suggest that the effect of CLO on striatal dopamine release is predominantly mediated by an exocytotic mechanism, Ca(2+), vesicular and TTX-dependent and not by a mechanism mediated by dopamine transporter.

The up-regulation of voltage-gated sodium channel Nav1.6 expression following fluid percussion traumatic brain injury in rats

BACKGROUND

The influx of Na and the depolarization mediated by voltage-gated sodium channels (VGSCs) is an early event in traumatic brain injury (TBI) induced cellular abnormalities and is therefore well positioned as an upstream target for pharmacologic modulation of the pathological responses to TBI. Alteration in the expression of the VGSC alpha-subunit has occurred in a variety of neuropathological states including focal cerebral ischemia, spinal injury, and epilepsy.

OBJECTIVE

In this study, changes in Nav1.6 mRNA and protein expression were investigated in rat hippocampus after TBI.

METHODS

Forty-eight adult male Sprague Dawley rats were randomly assigned to control or TBI groups. TBI was induced with a lateral fluid percussion device. Expression of mRNA and protein for Nav1.6 in the bilateral hippocampus was examined at 2, 12, 24, and 72 hours after injury by real-time quantitative polymerase chain reaction and Western blot. Immunofluorescence was performed to localize the expression of Nav1.6 protein in the hippocampus.

RESULTS

Expression of >Nav1.6 mRNA was significantly up-regulated in the bilateral hippocampus at 2 and 12 hours post-TBI. Significant up-regulation of Nav1.6 protein was identified in the ipsilateral hippocampus from 2 to 72 hours post-TBI and in the contralateral hippocampus from 2 to 24 hours post-TBI. Expression of Nav1.6 occurred predominantly in neurons in the hippocampus.

CONCLUSION

Results of the study showed significant up-regulation of mRNA and protein for Nav1.6 in rat hippocampal neurons after TBI.

Inhibitory gating modulation of small conductance Ca2+-activated K+ channels by the synthetic compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reduces afterhyperpolarizing current in hippocampal CA1 neurons

SK channels are small conductance Ca(2+)-activated K(+) channels important for the control of neuronal excitability, the fine tuning of firing patterns, and the regulation of synaptic mechanisms. The classic SK channel pharmacology has largely focused on the peptide apamin, which acts extracellularly by a pore-blocking mechanism. 1-Ethyl-2-benzimidazolinone (1-EBIO) and 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) have been identified as positive gating modulators that increase the apparent Ca(2+) sensitivity of SK channels. In the present study, we describe inhibitory gating modulation as a novel principle for selective inhibition of SK channels. In whole-cell patch-clamp experiments, the compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reversibly inhibited recombinant SK3-mediated currents (human SK3 and rat SK3) with potencies around 100 nM. However, in contrast to known pore blockers, NS8593 did not inhibit (125)I-apamin binding. Using excised patches, it was demonstrated that NS8593 decreased the Ca(2+) sensitivity by shifting the activation curve for Ca(2+) to the right, only slightly affecting the maximal Ca(2+)-activated SK current. NS8593 inhibited all the SK1-3 subtypes Ca(2+)-dependently (K(d) = 0.42, 0.60, and 0.73 microM, respectively, at 0.5 microM Ca(2+)), whereas the compound did not affect the Ca(2+)-activated K(+) channels of intermediate and large conductance (hIK and hBK channels, respectively). The site of action was accessible from both sides of the membrane, and the NS8593-mediated inhibition was prevented in the presence of a high concentration of the positive modulator NS309. NS8593 was further tested on mouse CA1 neurons in hippocampal slices and shown to inhibit the apaminand tubocurarine-sensitive SK-mediated afterhyperpolarizing current, at a concentration of 3 microM.

The role of quetiapine in the treatment of bipolar disorder

Through a review of randomised, controlled trials, this article evaluates the efficacy and tolerability of quetiapine in the acute and maintenance phases of bipolar disorder. In trials involving mania patients, quetiapine was found to be effective as adjunctive therapy in combination with lithium or valproate, significantly superior to placebo, and equal to lithium or haloperidol as monotherapy. With regard to the prevention of relapses in bipolar disorder, quetiapine seems to differ from other atypical antipsychotics in its characteristics as a mood stabiliser, which are associated with a promising efficacy in the treatment of bipolar depressive episodes. However, further larger controlled long-term prospective studies are needed to confirm the efficacy of quetiapine for the prevention of relapses in bipolar disorder. Quetiapine seems to have a satisfactory safety and tolerability profile, with a low prevalence of extrapyramidal symptom-related adverse events, treatment-emergent depression and weight gain. Sedation is the main side effect of treatment with quetiapine.

The differential effects of volatile anesthetics on electrophysiological and biochemical changes during and recovery after hypoxia in rat hippocampal slice CA1 pyramidal cells

Two volatile agents, isoflurane and sevoflurane have similar anesthetic properties but different potencies; this allows the discrimination between anesthetic potency and other properties on the protective mechanisms of volatile anesthesia. Two times the minimal alveolar concentration of an anesthetic is approximately the maximally used clinical concentration of that agent; this concentration is 2% for isoflurane and 4% for sevoflurane. We measured the effects of isoflurane and sevoflurane on cornus ammonis 1 (CA1) pyramidal cells in rat hippocampal slices subjected to 10 min of hypoxia (95% nitrogen 5% carbon dioxide) and 60 min of recovery. Anesthetic was delivered to the gas phase using a calibrated vaporizer for each agent. At equipotent anesthetic concentrations, sevoflurane (4%) but not isoflurane (2%), enhanced the initial hyperpolarization (6.7 vs. 3.4 mV), delayed the hypoxic rapid depolarization (521 vs. 294 s) and reduced peak hypoxic cytosolic calcium concentration (203 vs. 278 nM). While both agents reduced the final membrane potential at 10 min of hypoxia compared with controls, 4% sevoflurane had a significantly greater effect than 2% isoflurane (-24.4 vs. -3.5 mV). The effect of these concentrations of isoflurane and sevoflurane was not different for sodium, potassium or ATP concentrations at 10 min of hypoxia, the only difference at 5 min of hypoxia was that ATP was better maintained with 4% sevoflurane (2.2 vs. 1.3 nmol/mg). If the same absolute concentration (4%) of isoflurane and sevoflurane is compared then the cellular changes during hypoxia are similar for both agents and they both improve recovery. We conclude that an anesthetic's absolute concentration and not its anesthetic potency correlates with improved recovery of CA1 pyramidal neurons. The mechanisms of sevoflurane-induced protection include delaying and attenuating the depolarization and the increase of cytosolic calcium and delaying the fall in ATP during hypoxia.

Voltage-gated cation channel modulators for the treatment of stroke

Neuronal voltage-gated cation channels regulate the transmembrane flux of calcium, sodium and potassium. Neuronal ischaemia occurring during acute ischaemic stroke results in the breakdown in the normal function of these ion channels, contributing to a series of pathological events leading to cell death. A dramatic increase in the intracellular concentration of calcium during neuronal ischaemia plays a particularly important role in the neurotoxic cascade resulting in stroke-related acute neurodegeneration. One approach to provide therapeutic benefit following ischaemic stroke has been to target neuronal voltage-gated cation channels, and particularly blockers of calcium and sodium channels, for post-stroke neuroprotection. A recent development has been the identification of openers of large-conductance calcium- and voltage-dependent potassium channels (maxi-K channels), which hyperpolarize ischaemic neurons, reduce excitatory amino acid release, and reduce ischaemic calcium entry. Thus far, targeting these voltage-gated cation channels has not yet yielded significant clinical benefit. The reasons for this may involve the lack of small-molecule blockers of many neuronal members of these ion channel families and the design of preclinical stroke models, which do not adequately emulate the clinical condition and hence lack sufficient rigor to predict efficacy in human stroke. Furthermore, there may be a need for changes in clinical trial designs to optimise the selection of patients and the course of drug treatment to protect neurons during all periods of potential neuronal sensitivity to neuro-protectants. Clinical trials may also have to be powered to detect small effect sizes or be focused on patients more likely to respond to a particular therapy. The development of future solutions to these problems should result in an improved probability of success for the treatment of stroke.

Nefopam blocks voltage-sensitive sodium channels and modulates glutamatergic transmission in rodents

In order to specify the nature of interactions between the analgesic compound nefopam and the glutamatergic system, we examined the effects of nefopam on binding of specific ligands on the three main subtypes ionotropic glutamate receptors: N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), or quisqualic acid (QA) and kainic acid (KA) in rat brain membrane preparations. Functionally, we investigated the effects of nefopam against the seizures induced by agonists of these excitatory glutamate receptors in mice. Since the synaptic release of glutamate mainly depends upon the activation of membrane voltage-sensitive sodium channels (VSSCs), the nature of interactions between nefopam and these ionic channels was studied by evaluating the effects of nefopam on binding of 3H-batrachotoxinin, a specific ligand of the VSSCs in rat brain membrane preparations. The functional counterpart of the binding of nefopam on VSSCs was evaluated by its effects on the 22Na uptake-stimulated by veratridine on human neuroblastoma cells and in the maximal electroshock test in mice. Nefopam showed no affinity for the subtypes of ionotropic glutamate receptors up to 100 microM. On the other hand, nefopam was effective against NMDA, QA and KA induced clonic seizures in mice. Nefopam displaced 3H-batrachotoxinin and inhibited the uptake of 22Na in the micromolar range and it protected mice against electroshock induced seizures. Nefopam may block the VSSCs activity: consequently, at the presynaptic level, this effect led to a reduction of glutamate release and at the postsynaptic level, it led to a decrease of the neuronal excitability following activation of the glutamate receptors.

Bunazosin hydrochloride reduces glutamate-induced neurotoxicity in rat primary retinal cultures

To study neuroprotective effects of bunazosin hydrochloride which is an alpha(1)-adrenoceptor antagonist used as an ocular hypotensive drug compared to other alpha(1)-adrenoceptor antagonists, and its mechanism of action. We evaluated the neuroprotective effects of bunazosin hydrochloride or seven other alpha(1)-adrenoceptor antagonists against glutamate-induced cell death in rat primary retinal cultures. We also evaluated the binding inhibition of bunazosin hydrochloride for 24 different receptors/channels and its effects on the Na(+) influx into cells induced by veratridine or glutamate. Bunazosin hydrochloride significantly inhibited glutamate-induced cell death at concentrations of 1 and 10 microM. Cells were also protected when treated with some alpha(1)-adrenoceptor antagonists, but not by the others. Bunazosin hydrochloride showed a high inhibition for Na(+) channels and inhibited the Na(+) influx induced by veratridine or glutamate. These findings indicate that in retinal cultures bunazosin hydrochloride has a neuroprotective effect against glutamate-induced cell death and that the inhibition of Na(+) channels by bunazosin hydrochloride may be partly responsible for this effect.

Neuroprotection assessment by topographic electroencephalographic analysis: effects of a sodium channel blocker to reduce polymorphic delta activity following ischaemic brain injury in rats

The spatiotemporal electroencephalogram (EEG) pathology associated with brain injury was studied using high-resolution, 10-electrode cortical EEG mapping in a rat model of middle cerebral artery occlusion (MCAo). Using this model we evaluated the ability of the novel sodium channel blocker and neuroprotective agent RS100642 to resolve injury-induced EEG abnormalities as a measure of neurophysiological recovery from brain injury. The middle cerebral artery (MCA) was occluded for 1 h during which a dramatic loss of EEG power was measured over the injured cortex with near complete recovery upon reperfusion of blood to the MCA region in all rats. The resultant progression of the MCAo/reperfusion injury (6-72 h) included the appearance of diffuse polymorphic delta activity (PDA), as visually indicated by the presence of high-amplitude slow-waves recorded from both brain hemispheres. PDA was associated with large increases in EEG power, particularly evident in outer 'peri-infarct' regions of the ipsilateral parietal cortex as visualized using topographic EEG mapping. Post-injury treatment with RS100642 (1.0 mg/kg, i.v.) significantly reduced the PDA activity and attenuated the increase in EEG power throughout the course of the injury. These effects were associated with a reduction in brain infarct volume and improved neurological function. These methods of EEG analysis may be helpful tools to evaluate the physiological recovery of the brain from injury in humans following treatment with an experimental neuroprotective compound.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Differential interaction of anaesthetics and antiepileptic drugs with neuronal Na+ channels, Ca2+ channels, and GABA(A) receptors

BACKGROUND

Current theories favour multiple agent-specific neuronal actions for both general anaesthetics and antiepileptic drugs, but the pharmacological properties that distinguish them are poorly understood. We compared the interactions of representative agents from each class on their putative targets using well-characterized radioligand binding assays.

METHODS

Synaptosomes or membranes prepared from rat cerebral cortex were used to analyse drug effects on [(35)S]t-butyl bicyclophosphorothionate ([(35)S]TBPS) binding to the picrotoxinin site of GABA(A) receptors, [(3)H]batrachotoxinin A 20-alpha benzoate ([(3)H]BTX-B) binding to site 2 of voltage-gated Na(+) channels, (+)-[methyl-(3)H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-methoxycarboxyl-2,6-dimethyl-3-pyridinecarboxylate ([(3)H]PN200-110; isradipine) binding to L-type Ca(2+) channels, and [cyclohexyl-2,3-(3)H](N)glibenclamide ([(3)H]GB) binding to K(ATP) channels.

RESULTS

I.V. anaesthetics other than ketamine preferentially inhibited [(35)S]TBPS binding (etomidate approximately equal alphaxalone > propofol > thiopental > pentobarbital). Volatile anaesthetics inhibited both [(35)S]TBPS and [(3)H]BTX-B binding with comparable potencies (halothane approximately equal isoflurane approximately equal enflurane). Antiepileptic drugs preferentially antagonized either [(35)S]TBPS (diazepam > phenobarbital) or [(3)H]BTX-B (phenytoin > carbamazepine) binding. Local anaesthetics (lidocaine, tertracaine) selectively antagonized [(3)H]BTX-B binding. None of the drugs tested were potent antagonists of [(3)H]PN200-110 or [(3)H]GB binding.

CONCLUSIONS

Comparative radioligand binding assays identified distinct classes of general anaesthetic and antiepileptic drugs based on their relative specificities for a defined target set. I.V. anaesthetics interacted preferentially with GABA(A) receptors, while volatile anaesthetics were essentially equipotent at Na(+) channels and GABA(A) receptors. Antiepileptic drugs could be classified by preferential actions at either Na(+) channels or GABA(A) receptors. Anaesthetics and antiepileptic drugs have agent-specific effects on radioligand binding. Both general anaesthetics and antiepileptic drugs interact with Na(+) channels and GABA(A) receptors at therapeutic concentrations, in most cases with little selectivity.

Voltage-gated sodium channels in epilepsy

Animal experiments, and particularly functional investigations on human chronically epileptic tissue as well as genetic studies in epilepsy patients and their families strongly suggest that some forms of epilepsy may share a pathogenetic mechanism: an alteration of voltage-gated sodium channels. This review summarizes recent data on changes of sodium channel expression, molecular structure and function associated with epilepsy, as well as on the interaction of new and established antiepileptic drugs with sodium currents. Although it remains to be determined precisely how and to what extent altered sodium-channel functions play a role in different epilepsy syndromes, future promising therapy approaches may include drugs modulating sodium currents, and particularly substances changing their inactivation characteristics.

Regulation of voltage-dependent sodium channel expression in adrenal chromaffin cells: involvement of multiple calcium signaling pathways

The density and electrical activity of cell surface voltage-dependent Na(+) channels are key determinants regulating the neuronal plasticity including development, differentiation, and regeneration. Abnormalities of Na(+) channels are associated with various neurological diseases. In this paper, we review the regulatory mechanisms of cell surface Na(+) channel expression mediated by Ca(2+) signaling pathways in cultured bovine adrenal chromaffin cells. Sustained, but not transient, elevation of intracellular Ca(2+) concentration reduced the number of cell surface Na(+) channels. The reduction of Na(+) channels was suppressed by an inhibitor of calpain, a Ca(2+)-dependent protease, and by an inhibitor of protein kinase C (PKC). The activation of conventional PKC-alpha and novel PKC-epsilon reduced cell surface Na(+) channels by the acceleration of internalization of the channels and by the increased degradation of Na(+) channel alpha-subunit mRNA, respectively. On the contrary, the activation of PKC-epsilon increased Na(+) channel beta(1)-subunit mRNA level. The inhibition of calcineurin, a Ca(2+)/calmodulin-dependent protein phosphatase 2B, by immunosuppressants upregulated cell surface Na(+) channels by both stimulating externalization and inhibiting internalization of the channels without changing Na(+) channel alpha- and beta(1)-subunit mRNA levels. Thus, the signal transduction pathways mediated by intracellular Ca(2+) modulate cell surface Na(+) channel expression via multiple Ca(2+)-dependent events, and the changes in the intracellular vesicular trafficking are the important mechanisms in the regulation of Na(+) channel expression.

Solution structure of mu-conotoxin PIIIA, a preferential inhibitor of persistent tetrodotoxin-sensitive sodium channels

Mu-conotoxins are peptide inhibitors of voltage-sensitive sodium channels (VSSCs). Synthetic forms of mu-conotoxins PIIIA and PIIIA-(2-22) were found to inhibit tetrodotoxin (TTX)-sensitive VSSC current but had little effect on TTX-resistant VSSC current in sensory ganglion neurons. In rat brain neurons, these peptides preferentially inhibited the persistent over the transient VSSC current. Radioligand binding assays revealed that PIIIA, PIIIA-(2-22), and mu-conotoxins GIIIB discriminated among TTX-sensitive VSSCs in rat brain, that these and GIIIC discriminated among the corresponding VSSCs in human brain, and GIIIA had low affinity for neuronal VSSCs. (1)H NMR studies found that PIIIA adopts two conformations in solution due to cis/trans isomerization at hydroxyproline 8. The major trans conformation results in a three-dimensional structure that is significantly different from the previously identified conformation of mu-conotoxins GIIIA and GIIIB that selectively target TTX-sensitive muscle VSSCs. Comparison of the structures and activity of PIIIA to muscle-selective mu-conotoxins provides an insight into the structural requirements for inhibition of different TTX-sensitive sodium channels by mu-conotoxins.

Neuroprotective effects of the sodium channel blocker RS100642 and attenuation of ischemia-induced brain seizures in the rat

Seizurogenic activity develops in many patients following brain injury and may be involved in the pathophysiological effects of brain trauma and stroke. We have evaluated the effects of the use-dependent sodium channel blocker RS100642, an analog of mexiletine, as a neuroprotectant and anti-seizure agent in a rat model of transient middle cerebral artery occlusion (MCAo). Post-injury treatment with RS100642 (0.01-5.0 mg/kg) dose-dependently reduced brain infarction, improved functional recovery of electroencephalographic (EEG) power, and improved neurological outcome following 2 h of MCAo and 24 h recovery. This effect was more potent and offered a larger reduction of brain infarct volume than a maximal neuroprotective dose of mexiletine (10.0 mg/kg). Furthermore, brain seizure activity recorded following 1 h MCAo and 72 h of recovery in injured rats was either completely blocked (30 min pre-MCAo treatment) or significantly reduced (30 min post-MCAo treatment) with RS100642 (1.0 mg/kg) treatment resulting in greater than 60% reduction of core brain infarct. These results indicate that brain seizure activity during MCAo likely contributes to the pathophysiology of brain injury and that RS100642 may be an effective neuroprotective treatment not only to decrease brain injury but also to reduce the pathological EEG associated with focal ischemia.

The novel anticonvulsant BIA 2-093 inhibits transmitter release during opening of voltage-gated sodium channels: a comparison with carbamazepine and oxcarbazepine

(S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide (BIA 2-093) is endowed with high anticonvulsant activity and shares with carbamazepine (CBZ) and oxcarbazepine (OXC) the capability to inhibit voltage-gated sodium channels (VGSC). The present study was aimed to compare the effects of BIA 2-093, CBZ and OXC on the release of glutamate, aspartate, gamma-aminobutyric acid (GABA) and dopamine from striatal slices induced by the VGSC opener veratrine. The release of glutamate, aspartate, GABA and aspartate by veratrine from rat striatal slices was a concentration and time dependent process. All the three dibenzazepine carboxamide derivatives, BIA 2-093, CBZ and OXC inhibited in a concentration dependent manner (from 30 to 300 microM) the veratrine-induced release of glutamate, aspartate, GABA and dopamine. CBZ, OXC and BIA 2-093 were endowed with similar potencies in inhibiting veratrine-induced transmitter release. It is concluded that BIA 2-093, CBZ and OXC inhibit veratrine-induced transmitter release, which is in agreement with their capability to block VGSC. This property may be of importance for the anticonvulsant effects of BIA 2-093.

Quantitative functional imaging of the brain: towards mapping neuronal activity by BOLD fMRI

Quantitative magnetic resonance imaging (MRI) and spectroscopy (MRS) measurements of energy metabolism (i.e. cerebral metabolic rate of oxygen consumption, CMR(O2)), blood circulation (i.e. cerebral blood flow, CBF, and volume, CBV), and functional MRI (fMRI) signal over a wide range of neuronal activity and pharmacological treatments are used to interpret the neurophysiologic basis of blood oxygenation level dependent (BOLD) image-contrast at 7 T in glutamatergic neurons of rat cerebral cortex. Multi-modal MRI and MRS measurements of CMR(O2), CBF, CBV and BOLD signal (both gradient-echo and spin-echo) are used to interpret the neuroenergetic basis of BOLD image-contrast. Since each parameter that can influence the BOLD image-contrast is measured quantitatively and separately, multi-modal measurements of changes in CMR(O2), CBF, CBV, BOLD fMRI signal allow calibration and validation of the BOLD image-contrast. Good agreement between changes in CMR(O2) calculated from BOLD theory and measured by (13)C MRS, reveals that BOLD fMRI signal-changes at 7 T are closely linked with alterations in neuronal glucose oxidation, both for activation and deactivation paradigms. To determine the neurochemical basis of BOLD, pharmacological treatment with lamotrigine, which is a neuronal voltage-dependent Na(+) channel blocker and neurotransmitter glutamate release inhibitor, is used in a rat forepaw stimulation model. Attenuation of the functional changes in CBF and BOLD with lamotrigine reveals that the fMRI signal is associated with release of glutamate from neurons, which is consistent with a link between neurotransmitter cycling and energy metabolism. Comparisons of CMR(O2) and CBF over a wide dynamic range of neuronal activity provide insight into the regulation of energy metabolism and oxygen delivery in the cerebral cortex. The current results reveal the energetic and physiologic components of the BOLD fMRI signal and indicate the required steps towards mapping neuronal activity quantitatively by fMRI at steady-state. Consequences of these results from rat brain for similar calibrated BOLD fMRI studies in the human brain are discussed.

Effect of the New Antiepileptic Drug Levetiracetam in an Animal Model of Mania

The new antiepileptic drug levetiracetam (LEV, Keppra) was evaluated in a putative animal model for mania, namely, dexamphetamine-chlordiazepoxide mixture-induced hyperactivity in rats submitted to a Y-shaped maze test. Lithium chloride, sodium valproate, and carbamazepine, all clinically effective drugs in the treatment of acute mania, were used as comparators. The results indicate that the clinical references significantly attenuated the mixture-induced hyperactivity, thus confirming the sensitivity and pharmacological validity of this model. LEV also significantly attenuated the mixture-induced hyperactivity at doses within the range of those reported to be active in epilepsy models. ucb L060, the R-enantiomer of LEV, was without effect, thus indicating that the "antimanic" activity of LEV is stereospecific. These results suggest a potential for LEV in the treatment of mania and possibly in the management of bipolar disorder.

In vivo pharmacology of BIIR 561 CL, a novel combined antagonist of AMPA receptors and voltage-dependent Na(+) channels

Glutamate receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype and voltage-gated Na(+) channels are associated with diseases of the central nervous system characterized by neuronal over-excitation as in epilepsy or cerebral ischaemia. In animal models, AMPA receptor antagonists and Na(+) channel blockers provide protection in these conditions. Dimethyl-[2-[2-(3-phenyl-[1,2,4]oxadiazol-5-yl)-phenoxyl]-ethyl]-amine hydrochloride (BIIR 561 CL) combines both, AMPA receptor - and Na(+) channel blocking properties in one molecule. Here, BIIR 561 CL was investigated in vivo. BIIR 561 CL protected mice against AMPA-induced toxicity with an ED(50) value of 4.5 mg kg(-1) following subcutaneous (s.c.) administration. A 0.1% solution of BIIR 561 CL provided local anaesthesia in the corneal reflex test in rabbits. In mice, the compound prevented tonic seizures in the maximal electroshock (MES) model with an ED(50) value of 3.0 mg kg(-1) s.c. In amygdala-kindled rats, BIIR 561 CL inhibited seizures at doses of 3 and 11 mg kg(-1) following intraperitoneal (i.p.) injection. The data show that the combination of blocking AMPA receptor- and voltage-gated Na(+) channels in one molecule induces effective protection in animal models of neuronal over-excitation.

Interaction of the novel anticonvulsant, BIA 2-093, with voltage-gated sodium channels: comparison with carbamazepine

PURPOSE

BIA 2-093 [(S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide] is endowed with an anticonvulsant potency similar to that of carbamazepine (CBZ), but produces less cognitive and motor impairment. This study evaluated whether voltage-gated sodium channels (VGSCs) are a primary locus for the action of BIA 2-093.

METHODS

We used the whole-cell voltage-clamp technique in the mouse neuroblastoma cell line N1E-115 to investigate the effects of BIA 2-093 and CBZ on VGSCs, displacement of [3H]-batrachotoxinin A 20-alpha-benzoate ([3H]-BTX), and [3H]-saxitoxin to define their relative potency to bind to rat brain sodium channels, and inhibition of uptake of 22Na by rat brain cortical synaptosomes stimulated by veratridine as a measure of sodium entry.

RESULTS

The inhibitory potencies of BIA 2-093 and CBZ increased as the holding potential was made less negative (-100, -90, -80, and -70 mV) with median inhibitory concentration (IC50) values (in microM) of, respectively, 4,337, 618, 238, and 139 for BIA 2-093, and 1,506, 594, 194, and 101 for CBZ. BIA 2-093 displayed a similar potency in displacing [3H]-BTX (IC50 values, 222 vs. 361 microM; p > 0.05) and inhibiting the uptake of 22Na (IC50 values, 36 vs. 138 microM; p > 0.05). Both drugs failed to displace [3H]-saxitoxin in concentrations up to 300 microM.

CONCLUSIONS

BIA 2-093, like CBZ, inhibits sodium currents in a voltage-dependent way by an interaction predominantly with the inactivated state of the channel and interacts with neurotoxin receptor site 2, but not with receptor site 1. BIA 2-093 displayed a potency blocking VGSCs similar to that of CBZ.

Inhibition of voltage-dependent sodium channels suppresses the functional magnetic resonance imaging response to forepaw somatosensory activation in the rodent

Results of recent studies suggest that the glutamate-glutamine neurotransmitter cycle between neurons and astrocytes plays a major role in the generation of the functional imaging signal. In the current study, the authors tested the hypothesis that activation of voltage-dependent Na(+) channels is involved in the blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) responses during somatosensory activation. The BOLD fMRI and cerebral blood flow (CBF) experiments were performed at 7 Tesla on alpha-chloralose-anesthetized rats undergoing forepaw stimulation before and for successive times after application of lamotrigine, a neuronal voltage-dependent Na+ channel blocker and glutamate release inhibitor. The BOLD fMRI signal changes in response to forepaw stimulation decreased in a time-dependent manner from 6.7% +/- 0.7% before lamotrigine injection to 3.0% +/- 2.5% between 60 and 105 minutes after lamotrigine treatment. After lamotrigine treatment, the fractional increase in CBF during forepaw stimulation was an order of magnitude less than that observed before the treatment. Lamotrigine had no effect on baseline CBF in the somatosensory cortex in the absence of stimulation. These results strongly suggest that activation of voltage-dependent Na+ channels is involved in the BOLD fMRI responses during somatosensory activation of the rat cortex.

Cytoplasmic polyamines as permeant blockers and modulators of the voltage-gated sodium channel

We report that voltage-gated Na+ channels (Na(V)) from rat muscle (mu1) expressed in HEK293 cells exhibit anomalous rectification of whole-cell outward current under conditions of symmetrical Na+. This behavior gradually fades with time after membrane break-in, as if a diffusible blocking substance in the cytoplasm is slowly diluted by the pipette solution. The degree of such block and rectification is markedly altered by various mutations of the conserved Lys(III) residue in Domain III of the Na(V) channel selectivity filter (DEKA locus), a principal determinant of inorganic ion selectivity and organic cation permeation. Using whole-cell and macropatch recording techniques, we show that two ubiquitous polyamines, spermine and spermidine, are potent voltage-dependent cytoplasmic blockers of mu1 Na(V) current that exhibit relief of block at high positive voltage, a phenomenon that is also enhanced by certain mutations of the Lys(III) residue. In addition, we find that polyamines alter the apparent rate of macroscopic inactivation and exhibit a use-dependent blocking phenomenon reminiscent of the action of local anesthetics. In the presence of a physiological Na+/K+ gradient, spermine also inhibits inward Na(V) current and shifts the voltage dependence of activation and inactivation. Similarities between the endogenous blocking phenomenon observed in whole cells and polyamine block characterized in excised patches suggest that polyamines or related metabolites may function as endogenous modulators of Na(V) channel activity.

Voltage-gated sodium channels as therapeutic targets

Voltage-gated sodium channels (VGSCs) play a central role in the generation and propagation of action potentials in neurons and other cells. VGSC modulators have their origins in empirical pharmacology and are being used as local anaesthetics, antiarrhythmics, analgesics and antiepileptics, and for other disorders. However, the identification of a multigene family of VGSCs, along with tools to study the different subtypes in pathophysiology, is now providing a rational basis for selective intervention. Recent advances have addressed the technical challenges of expressing and assaying these complex proteins, enabling the correlation of empirical pharmacology to subtypes and the screening of individual subtypes for novel inhibitors with increased potency and selectivity.

Vinpocetine protects from aminoglycoside antibiotic-induced hearing loss in guinea pig in vivo

The principal objective of this study is to explore the hypothesis that a blockade of Na(+) channels can prevent some of the mechanisms involved in ototoxicity. For this purpose, the potential action of the voltage sensitive Na(+) channel antagonist, vinpocetine, on the ototoxicity induced by the representative aminoglycoside antibiotic, amikacin, in guinea pigs was tested for almost half a year. Amikacin (450 mg/kg) administered daily (i.m.) for 5 days increases the thresholds of the auditory brainstem response (ABR) to the two frequencies tested (4 and 8 kHz). These threshold increases are permanent or at least long-lived, as after 40 days they are already established and are maintained until the end of the experiment (160 days after the antibiotic administration). Amikacin decreases the amplitude of ABR waves, particularly P1, and after 160 days increases the latency of ABR waves, particularly at the higher frequency tested (8 kHz). When the above amikacin regimen is followed by a daily (i.p.) vinpocetine (2 mg/kg) administration for 13 days the increase in ABR threshold and latency caused by amikacin alone is prevented. Moreover, the animals treated with amikacin alone show a decreased weight gain and a remarkable increased mortality in comparison with the group of animals post-treated with vinpocetine. We hope that the multiple beneficial effects exerted by the Na(+) channel blocker, vinpocetine, against aminoglycoside antibiotics-induced side effects could help to solve the serious limitations of the use of this type of antibiotic.

Overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene induces cell proliferation and phenotypic transformation in mouse fibroblasts

Na(+)/K(+)/Cl(-) cotransporter activity is stimulated in early G(1) phase of the cell cycle and this stimulation was shown to be an essential event in fibroblast cell proliferation. In order to elucidate further the role of the Na(+)/K(+)/Cl(-) cotransporter in cell proliferation, we overexpressed the gene encoding the Na(+)/K(+)/Cl(-) cotransporter in mouse fibroblasts, and analyzed cellular phenotypic changes. Mouse Balb/c 3T3 cells were stably transfected with the cDNA of the shark rectal gland Na(+)/K(+)/Cl(-) cotransporter gene (NKCC1), and expressed in a mammalian vector under the cytomegalovirus promoter (Balb/c-NKCC1 cells). The transfected cells exhibited up to 10-fold greater bumetanide-sensitive Rb(+) influx compared to the control cells. The Balb/c-NKCC1 cells have acquired a typical transformation phenotype indicated by: (1) Loss of contact inhibition exhibited by growth to a higher cell density in confluent cultures, and formation of cell foci; (2) proliferation in low serum concentrations; and (3) formation of cell colonies in soft agar. The control cells transfected with the NKCC1 gene inserted in the opposite orientation in the vector retained their normal phenotype. Furthermore, the two specific inhibitors of the Na(+)/K(+)/Cl(-) cotransporter activity; bumetanide and furosemide inhibited the clonogenic efficiency in the NKCC1 transfected cells. These control experiments indicate that the apparent transformation phenotype acquired by the Balb/c-NKCC1 cells was not merely associated with the process of transfection and selecting for the neomycin-resistant clones, but rather with the overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene. In order to ascertain that the regulated and normal expression of the Na(+)/K(+)/Cl(-) cotransporter control cell proliferation, the effect of bumetanide a specific inhibitor of the cotransporter, was tested on Balb/c 3T3 cell proliferation, induced by fibroblasts growth factor (FGF) and fetal calf serum (FCS). Bumetanide inhibited synchronized Balb/c 3T3 cell exit from the G(0)/G(1) arrest and entering S-phase. The inhibition was reversible, as removal of bumetanide completely released cell proliferation. Taken together, these results propose that the NKCC1 gene is involved in the control of normal cell proliferation, while its overexpression results in apparent cell transformation, in a manner similar to some protooncogenes.

Effects of anticonvulsants on veratridine- and KCl-evoked glutamate release from rat cortical synaptosomes

We compared the effects of three conventional (phenytoin, carbamazepine and phenobarbital) and three novel (BW 1003C87, lamotrigine and riluzole) anticonvulsants on evoked glutamate release from rat cortical synaptosomes. Glutamate release was evoked by either 20 microM veratridine (which requires both Na+ and Ca2+ channel activation) or 30 mM KCl (which requires Ca2+ channel, but not Na+ channel, activation) to assess the involvement of Na+ and/or Ca2+ channels in the presynaptic actions of these anticonvulsants. All six compounds inhibited veratridine-evoked glutamate release; BW 1003C87 (IC50 = 2.0 microM) was the most potent and phenobarbital (IC50 = 3.2 mM) was the least potent inhibitor. Only phenobarbital (IC50 = 6.4 mM), riluzole (IC50 > 50 microM) and phenytoin (IC50 > 800 microM) significantly inhibited KCl-evoked glutamate release. These results suggest that therapeutic concentrations of BW 1003C87, lamotrigine, phenytoin, carbamazepine and riluzole, but not phenobarbital, inhibit synaptic glutamate release by preferentially blocking presynaptic Na+ channels. Presynaptic Na+ channel blockade with inhibition of the release of glutamate, and possibly other transmitters, may contribute to their anticonvulsant and/or neuroprotective effects.

Effect of small changes in temperature on CA1 pyramidal cells from rat hippocampal slices during hypoxia: implications about the mechanism of hypothermic protection against neuronal damage

Small reductions in temperature have been shown to improve neurologic recovery after ischemia. We have examined the effect of temperature on biochemical and physiological changes during hypoxia using rat hippocampal slices as a model system. The postsynaptic population spike recorded from the CA1 pyramidal cell region of slices subjected to 7 min of hypoxia with hypothermia (34 degrees C) recovered to 73% of its prehypoxic level; slices subjected to the same period of hypoxia at 37 degrees C did not recover. After 7 min of hypoxia ATP fell to 48% of its prehypoxic concentration at 34 degrees C and 30% at 37 degrees C. Potassium fell to 86% during 7 min of hypoxia with hypothermia, this compares to a fall to 58% at 37 degrees C. The increase in sodium after 7 min of hypoxia was also attenuated by hypothermia (133% vs. 163% of its prehypoxic concentration). When the hypoxic period was shortened to 3 min (37 degrees C) the population spike recovered to 94%. If the temperature was increased to 40 degrees C there was only 7% recovery of the population spike after 3 min of hypoxia. With hyperthermia (40 degrees C), ATP fell to 33% after 3 min of hypoxia, this compares to 81% at normothermia. Potassium fell to 76% after 3 min of hypoxia with hyperthermia, this compares to 91% at 37 degrees C. Sodium concentrations increased with hyperthermia before hypoxia, at 3 min of hypoxia there was no significant difference between the hyperthermic and normothermic tissue; there was a large increase in sodium with hyperthermia after 5 min of hypoxia (209% vs. 146%). We conclude that the improved recovery after hypothermic hypoxia is at least in part due to the attenuated changes in ATP, potassium and sodium during hypoxia and that the worsened recovery with hyperthermia is due to an exacerbation of the change in ATP, potassium and sodium concentrations during hypoxia.

Neuroprotective action of a novel compound--M50463--in primary cultured neurons

The neuroprotective effects of a novel synthetic compound, M50463, have been determined by using embryonic rat neocortical neurons in various culture conditions. M50463 was initially characterized as a potent specific ligand for a voltage-dependent sodium channel by radioligand binding studies. In fact, M50463 inhibited neuronal cell death induced by veratrine and inhibited an increase of the intracellular calcium level in neurons evoked by veratrine. In addition to such expected effects, M50463 had the ability to prevent glutamate neurotoxicity, to promote the neuronal survival in serum-deprived medium and to prevent nitric oxide-induced neurotoxicity. These results suggested that M50463 is not a simple sodium channel blocker, but a neuroprotective agent which has some crucial mechanism of action on neuronal death occurring in various situations, and it is a novel, innovative candidate for neuroprotective therapy for various neurodegenerative disorders.

Sodium channels as molecular targets for antiepileptic drugs

Voltage-gated sodium channels mediate regenerative inward currents that are responsible for the initial depolarization of action potentials in brain neurons. Many of the most widely used antiepileptic drugs, as well as a number of promising new compounds suppress the abnormal neuronal excitability associated with seizures by means of complex voltage- and frequency-dependent inhibition of ionic currents through sodium channels. Over the past decade, advances in molecular biology have led to important new insights into the molecular structure of the sodium channel and have shed light on the relationship between channel structure and channel function. In this review, we examine how our current knowledge of sodium channel structure-function relationships contributes to our understanding of the action of anticonvulsant sodium channel blockers.