Review

Voltage-gated sodium channels as therapeutic targets in epilepsy and other neurological disorders

Massimo Mantegazza, Giulia Curia, Giuseppe Biagini, David S Ragsdale, Massimo Avoli

Voltage-gated sodium channels (VGSCs) are key mediators of intrinsic neuronal and muscle excitability. Abnormal Lancet Neurol 2010; 9: 413–24 VGSC activity is central to the pathophysiology of epileptic seizures, and many of the most widely used antiepileptic Dipartimento di drugs, including phenytoin, carbamazepine, and lamotrigine, are inhibitors of VGSC function. These antiepileptic Neurofi siopatologia, drugs might also be effi cacious in the treatment of other nervous system disorders, such as migraine, multiple Fondazione Istituto Neurologico C Besta, Milano, sclerosis, neurodegenerative diseases, and neuropathic pain. In this Review, we summarise the structure and function Italy (M Mantegazza PhD); of VGSCs and their involvement in the pathophysiology of several neurological disorders. We also describe the IN2M-IPMC Nice-Sophia biophysical and molecular bases for the mechanisms of action of antiepileptic VGSC blockers and discuss the effi cacy Antipolis, Valbonne, France of these drugs in the treat ment of epileptic and non-epileptic disorders. Overall, clinical and experimental data indicate (M Mantegazza); Montreal Neurological Institute and that these drugs are effi cacious for a range of diseases, and that the development of drugs with enhanced selectivity for Department of Neurology and specifi c VGSC isoforms might be an eff ective and novel approach for the treatment of several neurological diseases. Neurosurgery, McGill University, Montreal, Quebec, Canada

Introduction of seconds, which is referred to as persistent Na+ current (G Curia PhD, D S Ragsdale PhD, M Avoli MD); Dipartimento di 3 Many of the most common neurological disorders, such (INaP; fi gure 1C). In recordings from brain neurons, INaP Scienze Biomediche, Università as epilepsy, migraine, neurodegenera tive diseases, and amounts for typically less than 1% of the maximal di Modena e Reggio Emilia, neuropathic pain, involve abnormalities of neuronal transient current (eg, compare panels B and C in Modena, Italy (G Biagini MD); excitability. There is a growing body of data that implicates fi gure 1);2 nevertheless, this current has important eff ects Dipartimento di Medicina Sperimentale, Sapienza abnormal expression and function of voltage-gated on neuronal function, including amplifi cation of synaptic Università di Roma, Roma, Italy sodium channels (VGSCs) in these disorders. Pharma- potentials, generation of subthreshold oscillations, (M Avoli) cological inhibitors of VGSCs have been used for decades facilitation of repetitive fi ring, and maintenance of Correspondence to: to treat epileptic seizures, the most common disease of prolonged depolarised plateau potentials.3,4 Therefore, Massimo Avoli, neuronal excitability, and it is becoming increasingly even small modifi cations of I amplitude can 3801 University Street, NaP Room 794, Montreal, Quebec, evident that these antiepileptic VGSC blockers might substantially alter neuronal properties. INaP undergoes H3A 2B4, Canada also be effi cacious against a broad range of neurological modulation by intracellular factors, which is likely to be [email protected] disorders. In this Review, we summarise the emerging important for fi ne-tuning neuronal excitability.5,6 Of evidence for a central role of VGSCs in the patho- particular importance, neurological disorders such as physiology of epilepsy, migraine, neuro degeneration, epilepsy and neuropathic pain are associated with INaP and neuropathic pain, and examine the effi cacy of amplitudes several times larger than those typically antiepileptic VGSC blockers in the treatment of these observed under normal physiological conditions. These neurological diseases. We also outline future develop- relatively large persistent currents are thought to ments that might extend the therapeutic use of contribute to the pathophysiological hyperexcitability and compounds that target VGSCs. cytoplasmic Na+ loading that are associated with these disorders as we explain below.4,7,8 Biophysical and molecular properties of VGSCs Brain VGSCs comprise a central α-subunit of 260 kDa Most neuroscientists and neurologists are familiar with and two auxiliary β-subunits of about 35 kDa (β1–β4) the textbook description of VGSC function1 (fi gure 1A). that modulate the properties of the α-subunit and are VGSCs are closed at resting membrane potentials implicated in its sub cellular targeting9 (fi gure 2). The characteristic of quiescent neurons. In response to α-subunit, which forms the ion-conducting pore and the membrane depolarisation, they open within a few channel gate for activation and inactivation, consists of hundred microseconds (a process termed activation), four domains, designated I–IV (or D1–D4), each with six resulting in an inward sodium ion (Na+) current, and α-helical transmembrane segments, referred to as then convert within a few milliseconds to a non- S1–S6. Pore loops between S5 and S6 in each of the four conducting inactivated state through a process called fast domains form the selectivity fi lter of the channel. Each inactivation. Transient Na+ infl ux through thousands of pore loop contributes a single amino acid (aspartate rapidly opening and inactivating VGSCs results in the from domain I, glutamate from domain II, lysine from familiar transient macroscopic Na+ current detected in domain III, and alanine from domain IV), which whole-cell voltage clamp studies (fi gure 1B). This together form a narrow ring that is mainly responsible transient current gives rise to the depolarising phase of for conferring Na+ selectivity. The four S6 segments the action potential in neurons and other excitable cells. form the cytoplasmic end of the pore, which binds In many neurons, inactivation of Na+ current is various types of therapeutically important pore-blocking incomplete, resulting in a small slowly-inactivating compounds, including local anaesthetics, antiar- current, with kinetics of inactivation in the order of tens rhythmic drugs, and antiepileptic drugs (AEDs). The www.thelancet.com/neurology Vol 9 April 2010 413 Review

A

Closed Open Inactivated

α ββ

Na+ B Transient current C Persistent current

300 pA 25 pA

5 ms 50 ms

Figure 1: Functional states of the voltage-gated Na+ channel (A) At hyperpolarised membrane potentials, the channel is in a closed state. In response to depolarisation, the channel briefl y opens, resulting in an inward Na+ current, and then converts to a non-conducting inactivated state. (B) A whole-cell Na+ transient current, generated by depolarisations from a holding potential of –80 mV to potentials of –60 mV to –5 mV, in 5 mV increments. (C) Traces from a diff erent neuron at a higher gain (note the diff erence in the scale bars between + panels B and C), showing the small persistent Na currents (INaP) remaining at the end of membrane depolarisations, after fast inactivation is complete. Panels B and C are reprinted from Magistretti and colleagues,2 with permission from the Society for Neuroscience. α=α-subunit. β=β-subunit.

S4 segments in each of the four domains contain homoeostasis.9 Expression of the diff erent subtypes is regularly spaced, positively charged amino acid residues developmentally regulated and is cell specifi c and tissue and serve as voltage-sensors, coupling membrane specifi c. The main subtypes expressed in adult brain

depolarisation to channel activation. The intracellular neurons are Nav1.6, found at axonal initial segments, loop between domains III and IV forms the fast- nodes of Ranvier, somata, and dendrites of projection

inactivation gate that occludes the cytoplasmic end of neurons; Nav1.2, found in unmyelinated axons and in the pore when the channel inactivates (fi gure 1A). The myelinated axons early in development (before being

C-terminal cytoplasmic domain is important for setting replaced by Nav1.6); and Nav1.1, localised in neuronal 14 some of the properties of fast inactivation and contains somata. In rodents, Nav1.3 is expressed primarily in binding sites for interacting proteins.15 In addition to embryonic neurons; however, expression in the human fast inactivation, a distinct process called slow CNS remains comparatively high into adulthood.17 The

inactivation develops during prolonged depolarising recent identifi cation of the Lys354Gln mutation in Nav1.3 plateaus and during high frequency repetitive fi ring. in a patient with epilepsy (see below)18 is consistent with The kinetics of onset and recovery of slow inactivation expression of this subtype in human neurons for an

are about four orders of magnitude slower than those of extended period after embryogenesis. Nav1.4 is the main

fast inactivation. Slow inactivation does not depend on subtype in adult skeletal muscle, whereas Nav1.5 is the fast inactivation gate formed by the intracellular expressed in cardiac muscle, and it is also expressed in

loop be tween domains III and IV, but instead mainly some neurons. Nav1.7, Nav1.8, and Nav1.9, are found in involves rearrangements of the pore of the channel.16 peripheral primary sensory aff erents and have important Nine α-subunit subtypes have been cloned and roles in transmission of nociceptive signals from the

functionally expressed. These subtypes are designated periphery. VGSC subtypes Nav1.1–Nav1.4, Nav1.6, and

Nav1.1–Nav1.9 for the proteins and SCN1A–SCN5A and Nav1.7 are inhibited by nanomolar concentrations of SCN8A–SCN11A for the genes; SCN6A/SCN7A codifi es tetrodotoxin, whereas Nav1.5 requires micromolar + the related protein Nax, which might be involved in Na concentra tions to be blocked and Nav1.8 and Nav1.9 have

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A B β-subunit α-subunit

β1–4 I II III IV IIIS6 Phe1579 NH 2 IVS6 Leu1280

Tyr1586 S–S N+ + + + + + + + + + + + +

COOH

IIS6 Asn434 IS6

NH2 COOH

Figure 2: Membrane topology of voltage-gated Na+ channel (A) Transmembrane segments are shown as cylinders. The main pore-forming and voltage-sensing α-subunit comprises four domains (labelled I–IV), each with six transmembrane segments. The β-subunits have a single transmembrane segment, a short intracellular domain and a single, extracellular immunoglobulin-like loop; β1 and β3 have non-covalent interactions with the α-subunit, whereas β2 and β4 are covalently linked to it with disulfi de bridges. Site-directed mutagenesis studies10–12 have identifi ed residues (yellow circles) in transmembrane segments IS6, IIIS6, and IVS6, which are important for binding of local anaesthetic and antiepileptic sodium channel blockers. (B) The typical structure of sodium channel blockers consists of a positively charged nitrogen moiety at one end and an aromatic ring at the other end. Molecular modelling of the drug binding site13 suggests that the positively charged amine interacts strongly with a phenylalanine in domain IV (Phe1579 in the Nav1.4 channel used for the modelling analysis) and, to a lesser extent, with a leucine in domain III (Leu1280 in Nav1.4), whereas the aromatic group interacts with a tyrosine in domain IV (Tyr1586) and an asparagine in domain I (Asn434). half maximal eff ective concentrations for tetrodotoxin in A growing body of experimental evidence indicates that the 40–60 mM range. abnormal expression or function of VGSCs might have a VGSCs are also found in microglia, where they might role in the pathophysiology of both acquired and inherited contribute to phagocytic activity,19 and in oligodendrocytes epilepsy. Altered concentrations of mRNA and protein and astrocytes, where their functional roles are poorly for α-subunits Nav1.1, Nav1.2, Nav1.3, and Nav1.6 and understood, but might include regulation of cytoplasmic β-subunits have been reported in animals24–27 and in Na+ homoeostasis.20 Glial cells do not normally generate human brain tissue28 in acquired epilepsy. This abnormal action potentials; however, action potential-like events channel expression might be involved in the process of have been recorded in glial precursor cells and astrocytes.20 epileptogenesis or in the maintenance of the epileptic 21 Káradótter and colleagues described a subset of state. Furthermore, signifi cantly increased INaP (2–5-fold oligodendrocyte precursor cells, which receive excitatory above control amplitudes) was observed in models of and inhibitory synaptic inputs from neurons, generate temporal lobe epilepsy and in neurons obtained from the action potentials when depolarised, and are particularly resected temporal lobe of patients with epilepsy.26,29,30 Data sensitive to excitotoxicity. Substantial upregulation of Na+ from transgenic mice that expressed an incompletely currents has been reported in astrocytes from human inactivating Nav1.2 mutant indicated that increased INaP is epileptic tissue, suggesting that patho physiological glial suffi cient to cause chronic seizures.31 excitability might contribute to the spread of seizures.22 The most convincing data that support a role for VGSCs in epileptogenesis comprise the identifi cation of several VGSCs and epileptic seizures hundred mutations in VGSC genes leading to inherited Epilepsy is a disorder of neuronal excitability, character- epileptic syndromes.32–35 Epileptic syndromes linked to ised by episodes of excessive synchronised neuronal VGSC mutations range in severity from relatively mild activity. Electroencephalographic recordings from disorders such as benign neonatal-infantile familial patients with partial epileptic disorders reveal two types seizures,36,37 simple febrile seizures,38 and generalised of abnormal activity: interictal events, which are short epilepsy with febrile seizures plus (GEFS+)39 to the epileptic asymptomatic episodes recurring periodically between encephalopathy termed severe myoclonic epilepsy of seizures, and ictal discharges, which are more prolonged infancy (SMEI), also known as Dravet’s syndrome.40,41 abnormalities in neuronal activity associated with Several other Na+ channelopathies fi t along this disease behavioural manifestations.23 Both ictal and interictal spectrum, including borderline SMEI, intractable discharges are characterised by sustained fi ring of childhood epilepsy with generalised tonic-clonic seizures, Na+-dependent action potentials riding on a slow and possibly other epileptic encephalopathies.33,34 Although depolarised potential, mainly generated by synaptic a few epileptogenic VGSC mutations lie within SCN2A 23 32,36,37 ligand-gated cation currents (fi gure 3A, upper trace). (the gene encoding Nav1.2), in SCN1B (encoding the www.thelancet.com/neurology Vol 9 April 2010 415 Review

A Field potential

20 mV 4 s

Intracellular

CSD

20 mV 4 s Extracellular K B 92·00 10 s + concentration (mM)

8·00

3·25

CSD

Figure 3: Field potential, intracellular recordings, and K+ activities during epileptiform discharges and CSDs generated in vitro (A) Simultaneous fi eld potential and intracellular sharp-electrode recordings obtained from the deep layers of the entorhinal cortex in an adult rat brain slice during continuous bath application of the K+ channel blocker 4-aminopyridine (50 μM). Traces in the upper panel are a continuous recording and show an ictal discharge and an interictal event (arrow), whereas traces in the lower panel show interictal discharges (arrows) as well as an ictal discharge leading to an episode of CSD. Note that fast, Na+-dependent action potentials are generated during both interictal and ictal events, while the CSD is characterised by large amplitude, negative-going fi led potential shift that is associated with sustained depolarisation, during which blockade of the action potential occurs. (B) Extracellular K+ recording from a human neocortical slice maintained in vitro and perfused with a Mg²+-free medium, which evokes epileptiform activity and spreading depression in the slice. Under these experimental conditions, interictal events and CSD were recorded. Note that extracellular K+ concentrations transiently increase up to around 8 mM during each interictal event (arrow) and attain values around 90 mM during the CSD. CSD=cortical spreading depression.

32,42–44 45 auxiliary β1-subunit), and possibly in SCN9A (Nav1.7) Recent data from transgenic mice have indicated that a 18 For more on SCN1A variants and SCN3A (Nav1.3), most are in SCN1A (encoding the GEFS+ mutant characterised by a gain of function in see http://www.molgen.ua.ac. 32–35 Nav1.1 α-subtype ). heterologous expression systems induces a loss of func- be/SCN1AMutations/ Nav1.1 mutations that cause GEFS+ are missense tion in neurons and, notably, that the functional eff ects of For more on Na 1.1 mutations v mutations, whereas those that give rise to the much more the mutation depend on the neuronal subtype in which the see http://www.scn1a.info/ severe disorder SMEI can either be missense ones or mutant is expressed.49 Some missense mutations cause result in trun cated channels that are predicted to be non- loss of function because of folding defects, which can be functional. The mutations are distributed throughout the rescued by interactions with accessory proteins or VGSC Nav1.1 α-subunit, without any clear hotspots or obvious blockers, and this rescue eff ect might also be cell-type relations to domains known to give rise to diff erent specifi c.50,51 It is puzzling that loss-of-function mutations in aspects of channel function (see fi gure 2 in Harkin et a VGSC lead to epilepsy, a disorder characterised by brain 34 al ). When expressed in heterologous cells, missense hyperexcitability; however, data from Nav1.1 knockout 52,53 Nav1.1 mutants result in loss of function or gain of mice and SMEI Nav1.1 knock-in mice (Arg1407X 54 function, often compromised inactivation, and increased mutation) indicate that Nav1.1 is the predominant isoform 46,47 INaP (for an extended review of the heterogeneity of the in at least some types of inhibitory interneurons. Hence, functional eff ects, readers are referred to Avanzini and reduced excitability of inhibitory neurons and compromised 35 colleagues ). However, several lines of evidence point to network inhibition is presumably a major factor in Nav1.1-

a loss of function as the main eff ect of Nav1.1 mutations. related genetic epilepsies.

In fact, more than half of the identifi ed SMEI mutants Some epilepsy mutations of Nav1.2 cause a gain of are predicted to be non-functional32–35 and a careful review function in transfected cells,37,55 and benign neonatal-

of the data highlights that the eff ect of most Nav1.1 infantile familial seizure, a Nav1.2-related epilepsy, shows missense mutations, which have been functionally spontaneous remission during the infantile period,36 + 48 characterised, leads to reduced Na currents. consistent with the transient expression of Nav1.2 in

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myelinated axons of excitatory neurons early during the loss of myelin within the CNS white matter.8 In these postnatal development.9 Mutations of the β1-subunit cases, factors such as nitric oxide, infl ammation-induced induce loss of function of β1, reducing its modulation of ischaemia, and impairment of mitochondrial function α-subunit functions and its interactions with other cause reduced energy production, leading to decreased proteins,35,42–44 although how this eff ect relates to Na+/K+-ATPase pump activity, membrane depolarisation, hyperexcitability is not yet clear. activation of persistent inward Na+ current (probably

mediated mainly by Nav1.6), and abnormal accumulation VGSCs and other neurological disorders of Na+ inside the axon. Intracellular Na+ overload drives Migraine the Na+/Ca²+ exchanger to import calcium ion (Ca²+) into

In addition to causing epilepsy, mutations of Nav1.1 are axons, triggering a pathogenic loop that causes further also associated with familial hemiplegic migraine mitochondrial damage and activation of proteases, type 3,56–59 a severe autosomal dominant inherited subtype lipases, and nitric oxide synthase, ultimately leading to of migraine with visual aura and hemiparesis during axonal injury. Similar Na+ overload might be involved in attacks. Data from studies in patients with migraine the hypoxia-induced neuronal damage caused by oxygen indicate that the aura coincides with cortical spreading deprivation in clinical disorders such as stroke and depression (CSD), a wave of neuronal depolarisation that apnoea,7 and in degeneration associated with traumatic spreads across the cerebral cortex and generates transient brain injury.63 intense fi ring followed by a long-lasting suppression of In mouse models of amyotropic lateral sclerosis, INaP is activity (fi gure 3A).60 CSD is accompanied by increases in increased in both cortical and spinal motor neurons, potassium ion (K+) concentrations that are several fold resulting in hyperexcitability.64,65 This fi nding is consis tent larger than those seen during epileptic discharges61 with the early signs of cortical hyperexcitability observed (fi gure 3B). Results from experiments in animals have in patients.66 Therefore, in patients with amyotropic lateral shown that CSD stimulates trigeminovascular aff erents sclerosis, hyperexcitability with consequently increased from the meninges, thus activating brain areas involved glutamate release and, alternatively or concomitantly, Na+ 62 in the perception of pain. The head ache is probably overload caused by increased INaP might have a major role caused by the activation of this pain pathway. in neurodegeneration. Familial hemiplegic migraine type 3 mutations of VGSCs (in particular Nav1.1, Nav1.5, and Nav1.6) are

Nav1.1 have been studied by heterologous expression in upregulated in activated mi croglia and macrophages in cultured human embryonic cell lines and, similar to models of autoimmune and infl ammatory disorders and epileptogenic mutations, the reported eff ects vary from epilepsy, and contribute to phagocytic functions and gain of function to complete loss of function.58,59 The migration of these cells.19,67 Blockade of VGSCs (by pathogenic mechanisms of familial hemiplegic migraine selective blockers or genetic mutations) in mi croglia type 3 are not completely understood, but a current activated in vitro or in animal models of multiple hypothesis states that VGSC mutations cause cortical sclerosis, decreases migration of microglia, ameliorates hyperexcitability, enhanced neurotransmitter release, the infl ammatory cell infi ltrate, reduces phagocytic and accumulation of extracellular K+, leading to CSD and functions, and attenuates the release of pro-infl ammatory thus to migraine61 (fi gure 3). Some patients with familial cytokines.19,67 Thus, blockade of VGSCs might ameliorate hemiplegic migraine type 3 present with seizures,56,61 neuroinfl ammatory disorders not only through inhibition consistent with the view that migraine and epilepsy share of Na+ overload, but also via anti-infl ammatory a common underlying pathophysiology. Nevertheless, mechanisms. seizures occur independently from migraine attacks, and patients with Nav1.1 mutations might have only epilepsy Neuropathic pain or migraine, indicating that a combination of factors Adult dorsal root ganglion neurons express at least fi ve 56,61 68 contribute to the disease phenotypes. VGSC subtypes (Nav1.1 and Nav1.6–Nav1.9). Nav1.7 has recently received intense interest in the pain fi eld. Gain- Neurodegeneration of-function mutations in this subtype cause two diff erent Neuronal loss can be associated with acute events such episodic pain syndromes: inherited erythromelalgia 69,70 71 as severe seizures, stroke, circulatory arrest, and apnoea. and paroxysmal extreme pain disorder. Loss of Nav1.7 Several lines of experimental evidence indicate that function results in congenital indiff erence to pain,72 a abnormal Na+ infl ux and Na+ loading might be involved disorder in which aff ected individuals feel no pain, in the neurodegeneration associated with these events;7,8 despite otherwise normal sensory, motor, and cognitive hence, VGSC blockers might be eff ective in preventing function. neuronal injury caused by decreased oxygen supply. Other VGSC subtypes are involved in neuropathic pain VGSCs are also thought to have a role in the neurode- resulting from chronic injury to sensory neurons. Among generation and infl ammation that occur in multiple the changes observed in nociceptive sensory neurons sclerosis and in other demyelinating diseases, in which after injury are upregulation of the embryonic subtype degeneration of axons and their cell bodies accompanies Nav1.3 in the somata, redistribution of tetrodotoxin-re- www.thelancet.com/neurology Vol 9 April 2010 417 Review

sistant sensory neuron-specifi c subtypes Nav1.8 and Nav1.9 with much higher affi nity (in the low micromolar from the somata to the peripheral axon at the site of the range).1,77 Voltage-dependent and frequency-dependent lesion, and upregulation of the accessory subunits β2 and inhibition suggests a basis for the ability of phenytoin to β3.68,73 These modifi cations contribute to the spontaneous suppress seizures while having minimum eff ects on fi ring of nociceptive neurons, often at pathologically high cognition. According to this hypothesis, phenytoin only frequency and from ectopic sites. In central neuropathic weakly suppresses Na+ currents during the periods pain, nociceptive neurons in the spinal cord and thalamus between seizures, in which neurons depolarise only

also show hyperexcitability caused partly by Nav1.3 upregu- transiently and fi re single or short bursts of action 74 lation. Nav1.7–Nav1.9 are also upregulated in dorsal root potentials. Conversely, during seizures, neurons have ganglion neurons in models of infl am matory pain, thus prolonged discharges of action potentials riding on contributing to the development and maintenance of sustained depolarising episodes (fi gure 3A), the optimum chronic infl ammatory pain.75,76 condition for phenytoin inhibition of VGSC activity. Therefore, phenytoin is thought to selectively inhibit VGSC blockers as AEDs abnormal epileptiform activity while having minimum The widely used AEDs phenytoin and carbamazepine eff ects on physiological function. inhibit VGSCs at therapeutic concentrations, and this Carbamazepine, similar to phenytoin, inhibits VGSCs attenuation of Na+ current is thought to be the main in a voltage-dependent and frequency-dependent manner mechanism of their therapeutic effi cacy.77 These drugs at clinically relevant concentrations and is eff ective are eff ective in the maximal electroshock seizure test, a against partial and generalised tonic-clonic seizures but model of tonic-clonic seizures that assesses the ability of not against absence seizures. Compared with phenytoin, AEDs to suppress hindlimb fl exion/extension induced in carbamazepine has 3-fold lower affi nity for depolarised normal rodents by electrical stimuli delivered through channels but binds to them at a 5-fold faster rate.77 Hence, corneal electrodes. In contrast, these drugs are ineff ective this drug might be more eff ective than phenytoin at in pentetrazol-treated rodents, a model used to identify inhibiting seizures characterised by relatively brief drugs that are effi cacious for absence seizures. Consistent depolarising shifts; this could explain why some patients with these observations, phenytoin and carbamazepine respond better to phenytoin but others are more show effi cacy for the treatment of partial and generalised eff ectively treated with carbamazepine. tonic-clonic seizures in human beings, but they are not eff ective against absence seizures. Several other AEDs AEDs that act on Na+ channels and other molecular inhibit VGSCs to varying degrees, but also act on targets additional molecular targets in the brain. Compared with The AEDs reviewed in this section inhibit VGSCs, but phenytoin and carbamazepine, these drugs are clinically also act on other mo lecular targets in the brain. Compared eff ective against diff erent types of seizures. In the next with phenytoin and carbamazepine, these drugs have sections, we describe the mechanism of action of diff erent or broader therapeutic profi les. For example, phenytoin and carbamazepine and discuss other AEDs valproate, an AED introduced in the 1970s, has an that have complex mechanisms of action. exceptionally broad range of anticonvulsant effi cacy both in animals and in clinical practice. This drug is eff ective Phenytoin and carbamazepine against partial and generalised tonic-clonic seizures, Phenytoin, the prototypic VGSC-specifi c AED, was used absence sei zures, and myoclonic seizures.79 The to suppress epileptic activity without substantially molecular basis of valproate action remain unclear, but interfering with normal cognitive function—a crucial its wide scope of clinical usefulness suggests that it might advance in anti-seizure pharmacotherapy.77 Phenytoin is act on several brain targets. Although increased GABA a weak blocker of VGSCs at hyperpolarised membrane turnover might be of particular importance in valproate’s potentials (fi gure 4A, left traces) and low rates of channel ability to control seizures, experimental evidence suggests activation, but its inhibitory action is greatly enhanced by that inhibition of VGSCs is a possible additional sustained membrane depolarisation (fi gure 4A, right explanation for its action.79 traces) and during high frequency channel activity Lamotrigine is another eff ective treatment not only for (fi gure 4B). Voltage-dependent and frequency-dependent partial and generalised tonic-clonic seizures, but also for inhibition is currently explained by a modulated receptor the management of Lennox-Gastaut syndrome and for model, fi rst developed to describe the action of local absence attacks in primary generalised epilepsies.77 As anaesthetic drugs (fi gure 4C).1 According to this model, with phenytoin, the well established action of lamotrigine closed VGSCs, which predominate at hyperpolarised is voltage-dependent and frequency-dependent blockade membrane potentials, have a low affi nity for phenytoin of VGSCs. However, its eff ectiveness cannot be accounted (dissociation constant >>100 μM, the solubility limit of for solely by VGSC inhibition, as the VGSC blockers phenytoin), whereas inactivated channel states, which phenytoin and carbamazepine are not eff ective against are prevalent at depolarised holding potentials and absence seizures. Indeed, lamo trigine has an eff ect on during high frequency channel activation, bind phenytoin the excitability of pyramidal neuron dendrites through a

418 www.thelancet.com/neurology Vol 9 April 2010 Review

A B

–130 mV –65 mV 1·0

0·8 2 nA 1 ms 0·6

0·4 Normalised Normalised current

0·2

Phenytoin 0 0 12345678910 Time (s) 1 ms Control

C Closed Inactivated

Low-affinity receptor High-affinity receptor Antiepileptic drug

Figure 4: Voltage-dependent and frequency-dependent inhibition of voltage-gated Na+ channels by Na+ channel blockers (A) Inhibition of Na+ currents by phenytoin is dependent on membrane holding potential. The top traces show currents evoked by depolarisation to 0 mV, with control (a standard physiological saline solution) and with 50 μM phenytoin. In each of the four pairs of current traces, the larger trace is the control. The traces on the left were elicited from a holding potential of –130 mV, whereas the traces on the right were from a holding potential of –65 mV. The control current at –65 mV is smaller, due to greater inactivation induced by the more depolarised holding potential. Moreover, phenytoin is a much more eff ective blocker at the more depolarised holding potential. This is more evident in the bottom traces, in which the amplitudes have been scaled so that the control currents are the same size. (B) Phenytoin block builds up during repetitive channel activation. The graph shows normalised current amplitudes of whole-cell Na+ currents over the course of depolarising pulses to 0 mV, from a holding potential of –85 mV, applied at 2 Hz. Red dots indicate discrete measurements. Dashed line indicates a baseline to monitor decline in amplitude. (C) Voltage-dependent and frequency-dependent inhibition of Na+ channels by phenytoin and other antiepileptic or local anaesthetic drugs with a similar mode of action (green diamond) are qualitatively explained by a modulated receptor model. In this model, the drug binds with higher affi nity to inactivated channel states than to resting channel states. Biophysical and molecular data suggest that the modulated receptor site is within the inner vestibule of the ion-conducting pore. Panels A and B are reprinted from Ragsdale et al,78 with permission from the American Society for Pharmacology and Experimental Therapeutics. direct action on the hyperpolarisation-activated cation Topiramate and lamotrigine were introduced in clinical current.80 Additionally, this drug inhibits N-type and practice during the 1990s. Topiramate is eff ective in P-type high-voltage-activated Ca²+ channels81 and patients presenting with partial seizures and has been enhances K+ repolarising currents.82 considered for the treatment of primary generalised www.thelancet.com/neurology Vol 9 April 2010 419 Review

tonic-clonic seizures.83 Topiramate is characterised by a prevent migraine and are a further resource for migraine phenytoin-like profi le in maximal electroshock seizure therapy; clinical data also suggest that lamotrigine and and pentetrazol tests, and it presumably acts on VGSCs, carbamazepine can reduce aura and migraine attacks.95 thus depressing sustained repetitive fi ring and voltage- The mechanism of action of these AEDs in the treatment gated Na+ currents.83 However, topi ramate antiepileptic of migraine is not fully understood, but it might involve eff ects presumably rely on additional mechanisms, interrupting the pathogenic cycle of migraine by including the interaction with excitatory amino acid inhibiting the abnormal cortical excitability that leads to receptor-mediated transmission.84,85 CSD or modifying nociceptive signalling in trigeminal Inhibition of VGSCs might also be a mechanism of fi bres and in central pain pathways. action for ethosuximide, the AED of fi rst choice for patients with absence seizures but not for patients with other types Neurodegenerative disorders and multiple sclerosis of generalised epilepsies. Absence attacks are characterised As stated above, abnormal Na+ infl ux and Na+ loading is by 3 Hz spike-and-wave activity associated with high involved in neuronal loss induced by severe seizures, frequency (200–500 Hz) bursts of action potentials in stroke, circulatory arrest, and apnoea. Accordingly, thalamocortical neurons evoked by the activation of T-type administration of tetrodotoxin, a potent and highly + 86 low-threshold Ca² current, IT. Data from early studies selective blocker of VGSCs, before or during anoxia, have indicated that therapeutic concentrations of attenuates neurodegeneration in animal models of + ethosuximide reduce IT in thalamic cells maintained in hypoxia both in vitro and in vivo, showing that Na infl ux vitro.87 However, this evidence was later challenged by through VGSCs is involved in hypoxia-induced neuronal Leresche and colleagues,88 who re ported that ethosuximide injury.7 Tetrodotoxin is not a useful therapeutic com- + + reduced INaP and Ca² -activated K currents without aff ecting pound, because its high affi nity for neuronal and skeletal transient Na+ currents or low-threshold and high-threshold muscle VGSCs makes it extremely toxic; however, several Ca²+ currents in rat and cat thalamic neurons. clinically relevant VGSC blockers have shown neuro- Finally, two molecules, riluzole and lacosamide, might protective eff ects in animal models of epilepsy and in be atypical in our list of AEDs. Riluzole—originally models of anoxia or hypoxia. Phenytoin, carbamazepine, developed as an AED but approved for treatment of lamotrigine, topiramate, and the antiarrhythmic VGSC patients with amyotropic lateral sclerosis—is generally blocker fl ecainide reduce ischaemic damage induced by thought to be an antiglutamatergic drug; however, this permanent middle cerebral artery occlusion or global drug has also been characterised as a classic VGSC ischaemia in rats or in in vitro models.96 Valproate or blocker.89 Lacosamide—eff ective in patients with topiramate treatment after chemical or electrical induction uncontrolled partial seizures and inhibitory of seizure of status epilepticus reduced neurodegeneration in the activity in several in vivo and in vitro models of rat hippocampus, although did not prevent the subsequent epilepsy90—also inhibits VGSCs, at least partly by development of spontaneous seizures.97 These data are selectively enhancing channel slow inactivation.91 Voltage- intriguing but whether these drugs provide signifi cant clamp experiments on cultured cortical cells have also benefi cial eff ects in patients is still not known. revealed that lacosamide decreases the frequency of Administration of phenytoin, lamotrigine, or fl ecainide inhibitory and excitatory postsynaptic currents without signifi cantly decreased axon degeneration and improved infl uencing membrane passive properties or several neurological status in the experimental autoimmune ligand-gated mechanisms.92 Thus, lacosamide might se- encephalomyelitis rodent model of multiple sclerosis.8,98 lectively target VGSCs, but with a diff erent mechanism These data are consistent with the hypothesis that blocking from that typical of phenytoin and car bamazepine. VGSCs prevents neurodegeneration induced by cytoplasmic Na+ overload. These blockers might also act VGSC blockers and treatment of other through the inhibition of VGSCs involved in activation neurological disorders and phagocytic functions of microglia, which might VGSC blockers might be clinically eff ective in several contribute to their benefi cial eff ects in animal models of neurological disorders, including migraine, neuro- multiple sclerosis. These results have provided the degeneration, and neuropathic pain;93,94 their potential rationale for the initiation of clinical trials with phenytoin, uses in the treatment of these disorders is described in topiramate, lamotrigine, and riluzole in patients with the next sections. multiple sclerosis.8,98 Disappointingly, in a recent phase 2 trial, lamotrigine did not show signifi cant benefi cial eff ects Migraine in patients with secondary progressive multiple A wide range of drugs are used to treat migraine, sclerosis.99,100 Furthermore, withdrawal of phenytoin in a including β-adrenergic receptor antagonists, Ca²+ channel mouse experimental autoimmune encepha lomyelitis blockers, serotonin antagonists, tricyclic antidepressants, model resulted in increased infl ammatory infi ltrate, monoamine oxidase inhibitors, and non-steroidal anti- worsening of symptoms, and high incidence of mortality, infl ammatory drugs. Recent evidence from controlled leading to suspension of one of the clinical trials.8 Thus, clinical trials suggests that valproate and topiramate whether VGSC blockers will provide a safe and eff ective

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new strategy for the treatment of multiple sclerosis is not for decades to treat epilepsy and some pain syndromes. clear at present. As highlighted in this Review, these drugs have more As mentioned above, riluzole is used to delay the recently been used to treat patients with migraine and progression of motor neuron degen eration in amyotropic might also be used in other disorders. However, current lateral sclerosis. This compound inhibits INaP at VGSC blockers have little discrimination between various therapeutic concen trations in slices of rat neocortex,101 VGSC subtypes, and thus the development of selective consistent with the upregulation of INaP observed in blockers might increase their clinical usefulness. For 64,65 animal models of amyotropic lateral sclerosis. This example, a drug that selectively inhibits the Nav1.7 VGSC, evidence suggests that Na+-mediated hyperexcitability which seems to be crucially and specifi cally involved in and excitotoxicity might have important uses in the nociception, would presumably act as a powerful pathogenic mechanism of the disease.66 analgesic, with few side-eff ects. Moreover a VGSC blocker

that selectively targets Nav1.6 would probably be an Neuropathic pain eff ective AED, as indicated by the amelioration of the Abnormal hyperexcitability in nociceptive pathways is phenotype of SMEI mouse models108 and by the resistance now widely accepted to contribute to various forms of to the initiation and development of kindling epilepto- 26 neuropathic pain. VGSC blockers, including the AEDs genesis obtained by impairing Nav1.6 function. carbamazepine, phenytoin, and lamotrigine, might be Interestingly, so far VGSC openers have been considered effi cacious in the treatment of neuropathic pain only as toxic compounds;109 however, a selective opener associated with painful peripheral neuropathies, such as could have clinical applications in some disorders. For 102,103 90,104 diabetic neuropathy. As recently reviewed, example, a VGSC opener selective for Nav1.1, whose loss experiments with lacosamide indicate that this AED is of function can cause GEFS+, SMEI, and other genetic effi cacious in animal models of tumour-induced and epileptic syndromes, might be a particularly eff ective chemotherapy-induced cancer pain, osteoarthritis pain, AED for these syndromes. Similarly, compounds able to painful diabetic neuropathy, and spinal cord or trigeminal selectively increase Nav1.1 expression levels would nerve injury. Lacosamide was approved by the US Food probably be even more effi cacious in these cases, but little and Drug Administration in 2008 as an adjunctive has been done to develop drugs with this mechanism of treatment for partial seizures; however, this drug is not action. Furthermore, VGSC blockers that selectively target approved for neuropathic pain at present. INaP could be eff ective to limit some types of pathological High doses of carbamazepine are also eff ective in excitability and neurodegeneration. Riluzole has some 101 ameliorating symptoms of patients with paroxysmal specifi city for INaP, as does ranolazine, which inhibits the extreme pain disorder.105 In contrast, inherited cardiac persistent Na+ current and has been approved for erythromelalgia is unresponsive to carbamazepine or other treating angina pectoris, but has not been tested yet in VGSC blockers.106 The reason for the diff erent effi cacy of neurological diseases. Additionally, compounds able to carbamazepine in paroxysmal extreme pain disorder target diff erent functional properties of VGSCs might be versus inherited erythromelalgia might be explained by eff ective in disorders that do not respond to VGSC the fact that paroxysmal extreme pain disorder involves blockers (eg, in inherited erythro melalgia, in which changes in VGSC inactivation, a process that is modulated mutations modify activation properties). by VGSC blockers, whereas inherited erythromelalgia The range of VGSC pathologies will probably continue involves negative shifts in the voltage dependence of to expand, as shown by the identifi cation of a loss-of- activation, which are not ameliorated by these drugs. function mutation of Nav1.6 in a patient with cerebellar Consistent with this hypothesis, in a recent investigation, atrophy, behavioural defi cits, and ataxia;110 thus, novel patients with inherited erythromelalgia in whom the valine selective VGSC modulators are likely to be increasingly at position 400 in Nav1.7 (SCN9A) is mutated to methionine attractive for personalised treatment of various (Val400Met) also present with a modifi ed fast inactivation neurological disorders. The new generation of VGSC of Na+ current and actually their symptoms improve when treated with carbamazepine.107 Search strategy and selection criteria Conclusions and future directions References for this Review were identifi ed through searches An emerging theme that unifi es many supposedly diverse of PubMed with the search terms “amyotrophic lateral neurological disorders is altered neuronal excitabil ity, sclerosis”, “antiepileptic drugs”, “epilepsy”, “infl ammation”, caused by abnormal expression and function of “migraine”, “multiple sclerosis”, “mutation”, membrane ion channels. VGSCs, as the main “neurodegenerative”, “pain”, “sodium channels”, and “stroke” determinants of intrinsic neuronal excitability, are from June, 1963, up to October, 2009. Further references implicated in many of these inherited and acquired were identifi ed from those cited in articles. The fi nal reference channelopathies and, thus, they are particularly appealing list was generated from papers that were relevant to the targets for pharmacological intervention. VGSC blockers, topics covered in the Review. including AEDs and local anaesthetics, have been used www.thelancet.com/neurology Vol 9 April 2010 421 Review

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