Description

	• Classical roles of ion channels in the nervous and neuromuscular systems are signal propagation along a cell surface and signal translation (by release of transmitters) into the cytoplasm of the cell.
	• Voltage-gated K+, Na+, and Ca+ ion channels play a role in the pathophysiology of neurologic diseases are targets for therapy as well.
	• Several other ion channels are involved on pathomechanism of various neurologic disorders.
	• Genetic mutations of ion channels may be associated with neurogenetic syndromes.

Voltage-gated channels. The stimulus for gating is the change of voltage across the membrane in which the channels are embedded. Obvious candidates for voltage sensors are charged residues in the membrane-embedded domain of the channel protein. Voltage-gated potassium (K+), sodium (Na+), and calcium (Ca2+) channels are members of a related gene family and are fractionally autonomous in voltage-dependent activation. Purification, molecular cloning, and determination of primary structures of primary subunits of voltage-gated channels have provided a molecular template for probing the relationship between their structure and function.

Potassium channels. Potassium channels act like electrical switches to regulate cellular excitability. They can be compared to electrical rectifiers that conduct potassium better in one direction than in another. Potassium channels have an important role to play in the physiology and pharmacology of the cardiovascular and nervous systems. The molecular diversity of potassium-selective channels is mirrored by the broad spectrum of physiological functions subserved by these proteins. There are more subtypes of potassium channels than any other ion channel. Studies from the Caenorhabditis elegans genome project indicate that more than 70 distinct potassium channel genes exist. The best-characterized potassium channels are members of one of the two so-called super families. Superfamily 1 includes the A-channel (KA), the delayed rectifier (Kv), and the large conductance Ca-sensitive channel (KCa). Superfamily 2 includes the inward rectifier (KIR) and ATP-sensitive channels.

Another subtype is hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. They are expressed in the brain and have a pacemaker function, ie, regulation of excitability and rhythmicity. Disturbance of the regulation of HCN channels may contribute to epilepsy, chronic pain, and other neurologic disorders (24).

Ether-à-go-go1 (Eag1) is a member of the voltage-gated K+ channel family mainly distributed in the CNS and cancer cells. EAG1 channels are expressed in dopaminergic neurons, indicating that the channel may play a role in Parkinson disease and other types of neurodegenerative diseases in which the oxidative damage is an etiological factor (18). Research into EAG1 channel structure and the mechanisms of channel regulation by endogenous factors widely expressed in the human body, particularly in the nervous systems, may provide valuable information for drug development targeting the EAG1 channel.

Sodium channels. Open sodium channels in biological membranes are selective for Na+ ions and have only minor permeability for other anions and cations. They are classified into types 1, 2, 3, µ1, H1, and PN3. Of these types, 1, 2, and 3 are predominantly expressed in the nervous system, and type µ1 is expressed in the skeletal muscle. The voltage-gated sodium channels play an essential role in nerve tissue, where they are responsible for the rapidly conducting nerve impulse. A single gene on human chromosome 19 encodes the beta-subunit expressed in the brain, heart, and skeletal muscle. Although the alpha-subunit, alone, forms the functional channel, the muscle beta-subunit binds to the perimeter, perhaps by interaction of its transmembrane helix with one or more of the repeat domains. There are multiple types of sodium channels in the skeletal muscle, based on sensitivity to toxins and antibodies.

Resurgent currents in sodium channels have been associated with multiple channelopathies, including paroxysmal extreme pain disorder mutation in the human peripheral neuronal sodium channel Nav1.7 and paramyotonia congenita mutation in the human skeletal muscle sodium channel Nav1.4 (20). Loss of function of the gene SCN9A, encoding sodium channel Nav1.7, causes not only congenital insensitivity to pain in humans but also loss of smell in both mice and humans (45).

A monoclonal antibody that targets the voltage sensor of Nav1.7 not only inhibits Nav1.7 with high selectivity, but also effectively suppresses inflammatory and neuropathic pain in mice (23). It also inhibits acute and chronic itch despite well-documented differences in pain and itch modulation, indicating that Nav1.7 plays a key role in the transmission of pain as well as itch in the spinal cord. These studies show that the antibody has a therapeutic potential for suppressing pain as well as itch.

Calcium channels. Voltage-dependent calcium channels open in response to membrane depolarization and allow the passage of calcium through the channel's pore without significant flux of any other ion. The resulting entry of calcium into a cell triggers a variety of intracellular processes, including muscle contraction and secretion of neurotransmitters. Although the sodium and potassium pumps are mainly involved in the action potential, calcium channels also play an important part in the nerve impulse. These channels are normally closed at the resting membrane potential and opened reversibly by depolarization to more positive potentials. The opening and closing occurs within a few milliseconds, in most instances. Calcium channels are also subject to inactivation during prolonged depolarizations. Some instances of calcium channel inactivation are secondary to intracellular calcium accumulation; others are a direct effect of membrane depolarization. Calcium channel subtypes with different electrophysiological and pharmacological properties are related to differences in alpha1-subunit structure. A classification of calcium channels is shown in Table 1.

Table 1. Classification of Calcium Ion Channels

Type: L
	Properties: High-voltage activated; slow inactivation Location: Smooth muscles, endocrine cells Principal function: Smooth muscle contraction; endocrine secretion Blockers: Dihydropyridines
Type: N
	Properties: High-voltage activated; moderate rate of inactivation Location: Only in neurons Principal function: Neurotransmitter disease Blockers: Omega-conotoxins
Type: P/Q
	Properties: Moderately high-voltage activated; nonactivating Location: Some CNS neurons (Purkinje cells) Principal function: Unknown Blockers: Omega-agatoxin and omega-conotoxins
Type: T
	Properties: Low-voltage activated; inactivation slower than L or N types Location: Heart neurons Principal function: Influences pacemaker activity in the heart Blockers: Mibefradil

Intracellular calcium concentration in neurons can be transiently increased by 2 mechanisms: (1) calcium influx through calcium-permeable channels in the plasma membrane or (2) the mobilization of calcium from intracellular calcium stores. Neurotransmitters can trigger both mechanisms. Glutamate activates ionotropic glutamate-receptor channels, which induce cell depolarization and, hence, calcium influx through voltage-sensitive calcium channels. Thus, a close functional interaction exists between glutamate receptors, intracellular calcium stores, and calcium-sensitive ion channels in the cell membrane.

Chloride channels. Chloride channels belong to the group of anion channels, which contribute to the regulation of membrane potential and cell signaling. Chloride channels mediate passive transport of chloride ions across the lipid bilayer by forming an aqueous diffusion pore. They are present in plasma membranes, as well as in intracellular organelles. In the intracellular compartment, chloride channels are often found in conjunction with cation transport systems, where chloride serves as a counter ion to the transported cation. There is no satisfactory classification of chloride channels. Many different types of chloride channels have been found by electrophysiological studies and are subject to various types of regulation. Most chloride channels, like other ion channels, show some dependence of open-probability on the membrane voltage. Three well-defined, structural classes of plasma membrane chloride channels exist. Voltage-gated (swelling-activated, volume-dependent) chloride channels are activated by an increase in cell volume, but it is likely that other mediators are also involved. Physiological concentrations of Ca2+ and Mg2+ can modulate volume-regulated Cl-currents in epithelial tissues. ClC-1 is the major skeletal muscle chloride channel that is essential for muscle excitability.

The volume-regulated anion channel (VRAC), a member of the chloride/anion family of channels, plays a role in volume regulation of cells, including those in the CNS. VRAC also plays a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules, eg, excitatory amino acid neurotransmitters glutamate and aspartate. This excitotoxic process has an impact on both normal brain functioning, such as astrocyte-neuron signaling, and neuropathology by promoting the death of neuronal cells in stroke and traumatic brain injury (30). VRAC is now identified as containing leucine-rich repeat-containing 8 (LRRC8) proteins, which will have an impact on further progress in neuroscience research. Information on the LRRC8 proteins’ sequences will enable use of site-directed mutagenesis to gain knowledge of the mechanisms of cell volume–dependent and cell volume–independent VRAC gating.

Acetylcholine receptor channel. Acetylcholine carries chemical messages across all human nerve-muscle synapses. The acetylcholine receptor channel is a molecule-sized valve that opens and closes to regulate the flow of electricity in nerve and muscle cells. More than 40 different channelopathies have been identified, with representative disorders from every major class of ion channel and affecting all electrically excitable tissues, including the nervous system, muscles, and the heart. Approximately 16 genes encoding subunits of mammalian nicotinic acetylcholine receptors have been identified. Mutations of nicotinic receptor genes can cause Mendelian disorders, most importantly congenital myasthenic syndromes, multiple pterygium syndromes, nocturnal frontal lobe epilepsies, and neuropsychiatric phenotypes in 15q13.3 deletion syndrome (37).

Aquaporins. These are transmembrane proteins that selectively allow the passage of water through the plasma membrane. In the brain, they are involved in the production of cerebrospinal fluid and the control of water movement at the blood-brain barrier. They have been implicated in the formation of brain edema. Further research on aquaporins may lead to targeted therapies for cerebral edema. In intractable epilepsy, expression of aquaporin ion channels increases in astrocytes, but not in neurons or oligodendrocytes. Whether this is the cause or an effect of seizures remains to be determined.

Acid-sensing ion channels. Because pH regulates all important biochemical reactions in the brain, a reduction of pH can disturb brain function. Acidosis is associated with neurologic disorders, such as cerebral ischemia and seizures. Acid-sensing ion channels (ASICs), mainly expressed in the nervous system, are proton-gated cation channels that sense extracellular pH reduction. ASICs belong to the degenerin/epithelial sodium channel superfamily because of their sodium permeability.

Activation of ASICs is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, pain, ischemia, and multiple sclerosis, so their potential as therapeutic targets is increasing (34). ASICs have also been implicated in multiple disease conditions, including ischemic brain injury, multiple sclerosis, Alzheimer disease, and drug addiction (36).

Aberrant activation of ASICs is associated with extracellular acidification in atherosclerosis-prone regions of arterial walls, which is pro-atherosclerotic by exerting detrimental effect on macrophages, endothelial cell dysfunction, and vascular smooth muscle cell proliferation (47). A better understanding of ASICs in these processes may provide promising new therapeutic targets for treatment and prevention of cerebrovascular atherosclerotic diseases.

Gap junctions. These are connections that allow molecules and ions to flow between cells. Junctions are composed of two hemichannels that bridge the intercellular space.

Ligand-gated and transmitter-gated ion channels. Ligand-gated and transmitter-gated ion channels form a class of multi-subunit membrane-spanning receptors that are essential for rapid signal transduction. The property that defines this class is that the transmitter molecule itself operates the opening and closing of the channel by binding to a site on the receptor. Gating refers to conformational change in ion channels that is triggered by ligand binding. In the nervous system, voltage-gated ion channels mediate action potentials and trigger transmitter release, whereas ligand-gated channels are responsible for chemical signaling mediated by classical fast-acting neurotransmitters. Neurotransmitters also trigger slower transmitter responses that are mediated by G protein-coupled receptors. Ligand-activated ion channels are preferably termed transmitter-gated ion channels and divided into two categories: (1) extracellularly activated and (2) intracellularly activated. A simplified and brief list is shown in Table 2.

Table 2. Ligand- and Transmitter-Gated Ion Channels

Ligand	Ion selectivity
Extracellularly activated:
GABAA Glycine Acetylcholine (nicotinic, muscle type) Acetylcholine (nicotinic, neuronal) 5-HT3 Glutamate: non-NMDA Glutamate: NMDA ATP	Cl-, HCO3- Cl-, HCO3- Na+, K+, Ca2+ Na+, K+, Ca2+ Na+, K+ Na+, K+, (Ca2+)a Na+, K+, Ca2+ Ca2+, Na+, K+
Intracellularly activated:
cGMP (photoreceptors) cAMP (olfactory neurons) IP3 Ca2+ (ryanodine receptor)	Na+, K+ Na+, K+ Ca2+

Structure of ion channels. Ion channels are macromolecular proteins that span the cell membrane. The cell membrane is generally considered to have two essential components: (1) a lipid bilayer and (2) a protein; it also has 3 general functions:

(1) Controlled maintenance of water and electrolytes, as well as nonelectrolytes between two separate aqueous compartments. This is accomplished by a series of solute leaks, ion transporters, and ion pumps.

(2) Signal transduction

(3) Surface for molecular interactions

The lipid bilayer is an effective barrier for ion permeation; hydrophilic solutes permeate bilayers poorly, but their flux across the membrane can be dramatically increased by ion channel proteins.

Studies of the cell membrane indicate that in the liquid crystalline state (fluid), the cell membrane is approximately 50 angstrom thick and is composed of a wide variety of different phospholipids and cholesterol. The ionic channels originally found by studies of excitable membranes have now been detected in virtually all membranes in the cell, including the endoplasmic reticulum, liposomes, and vacuoles.

Molecular structure of ion channels. Most ion channel proteins are composed of individual subunits or groups of subunits, with each subunit containing six hydrophobic transmembrane regions S1 to S6. Sodium and calcium channels consist of a single alpha subunit, containing four repeats of the six transmembrane-spanning motifs. Potassium channels consist of four subunits, each containing the six transmembrane-spanning motifs.

Physiology of ion channels. Membrane potentials are the voltages that occur across cell membranes. These are conventionally expressed as the voltage of the inside of the cell relative to the extracellular fluid and are measured in millivolts. In the resting stage, most animal cells have negative membrane potentials that range from negative 30 mV to negative 90 mV. The term “depolarization” indicates a reduction in the membrane potential values so that the inside of the cell becomes less negative, as occurs in the upward stroke of the action potential, eg, during flow of positively charged sodium from the extracellular space to the inside of the cell. The downstroke, or the return of the membrane potential to its resting value, is called “repolarization,” which can occur during outflow of positively charged ions such as potassium through potassium channels. Outflow of negatively charged ions such as chloride will depolarize the cell.

The knowledge of equilibrium potential for an ion in a cell is important for determining the way in which a channel permeable to that ion will affect the membrane potential of the cell. The direction and magnitude of the electrochemical gradient can be calculated for the ion, and the direction, as well as the rate of ion flow through an open channel, can be predicted. The current carried by an ion flow through a single channel is depicted by the symbol “i.” The average current flowing through a channel depends on the open probability of the channel and the amount of current flowing through it during the open state.

Voltage-gated ion channels are responsible for generation of electrical signals in neurons and other excitable cells. The basic principle of channel conformation is that the channel conformation, closed or open, is regulated by the membrane potential. This implies that the voltage sensor must link the voltage to these conformational changes. The obvious candidates are electrical charges embedded in the cell membrane. All channels studied so far display discrete open and closed states with transition between these states attributable to the conformational changes of the channel proteins. The ion channels open only during excitation of nerve and muscle cells, implying that they are closed in the nonexcitable state.

Role of ion channels in the nervous system. Classical roles of ion channels in the nervous and neuromuscular systems are signal propagation along a cell surface and signal translation (by release of transmitters) into the cytoplasm of the cell. Ion channels discriminate between calcium, potassium, and sodium, although these ions are similar in size. It is generally accepted that ions bind selectively to the channels. Neurotransmitter-gated channels are of fundamental importance in cell signaling but are less ion-selective than voltage-gated channels.

The membrane of the myelinated axon of the nervous system contains the following physiologically active molecules:

• Voltage-sensitive Na+ channels. • The K+IR channels modulate axonal excitability. • Na+/K+-ATPase and Ca2+-ATPase maintain transmembrane gradients of Na+, K+, and Ca2+.

Disturbances of the fluid and electrolytes are associated with pathologic processes in the brain. Glial cells have a capacity to sense transmitter-mediated activity in the CNS. Voltage-gated ionic channels in astrocytes may have a possible role in ionic homeostasis. Swelling of astrocytes leads to marked changes in the membrane transport properties, which would seriously affect astrocyte regulation of the ionic environments of the nervous tissue. Disturbance of ion homeostasis induced by brain injury leads to depolarization and intracellular Na+ accumulation that triggers inappropriate Ca2+ influx and transmitter release, which can further overactivate a variety of Ca2+-sensitive enzyme systems, resulting in permanent injury to axon, myelin, and glia (42).

Oxidative modification KCNB1 (Kv2.1), member 1 subfamily B of the voltage-gated K+ channel, is among the mechanisms of neuronal vulnerability affecting several conditions associated with oxidative stress that range from normal aging to neurodegenerative disorders. Oxidation of KCNB1 channels is reported to be exacerbated in the postmortem brains of Alzheimer disease compared to age-matched controls (44).

Kir4.1, a weakly inwardly rectifying K+ channel, is expressed exclusively in glial cells with highest expression in astrocytes. Functions of Kir4.1 include K+ homeostasis, maintenance of the astrocyte resting membrane potential, high astrocyte K+ conductance, astrocyte cell volume regulation, and facilitation of glutamate uptake (33). Increased expression of Kir4.1 is associated with epilepsy, Alzheimer disease, amyotrophic lateral sclerosis, and spinal cerebellar ataxia. In mouse models, Kir4.1 expression is decreased in astrocytes that express mutant huntingtin, indicating possible relation with striatal medium spiny neuron dysfunction seen in Huntington disease (41). Mutations of the Kir4.1 gene (KCNJ10) cause autosomal recessive disorder, SeSAME/EAST syndrome (seizures, sensorineural deafness, ataxia, mental retardation and electrolyte imbalance/epilepsy, ataxia, sensorineural deafness and tubulopathy) in human patients (05).

Transient receptor potential ion channels transduce thermal, chemical, and mechanical stimuli into inward currents as the first step for eliciting thermal and pain sensations.

Hyperpolarization-activated cation channels (h-channels) are key regulators of neuronal excitation and inhibition and are affected by seizures.

Nicotinic acetylcholine receptors (nAChRs), widely expressed throughout the central and peripheral nervous systems, form ligand-gated ion channels involved in fast synaptic transmission. Various subunits of nAChRs play crucial roles in modulating a wide range of higher cognitive functions by mediating presynaptic, postsynaptic, and extrasynaptic signaling.

Genomics and ion channels. Genomics is in the center stage of molecular biology and will have a tremendous impact on the practice of medicine. Vertebrate genomes encode several hundred ion channel subtypes, each of which has evolved to perform a highly selective electrical “task” within specific cells. These “tasks” can be discerned by examining those cellular conditions that gate (open and close) the channel. Each phase of an “excitability cycle” is associated with specific channel subtypes, and recruitment of the “appropriate” components (from the possibilities encoded by the genome) is precisely specified by the gene expression program for that developmental cell-type. The cell-specific excitability cycle model enables the building-up of a logical hierarchy for cell function from the ionic or molecular level to the cell-type and organism levels. Thus, it may form a useful “frameworking” tool for correlating different descriptions of genome structure, gene expression pattern, and molecular or cell function (phenotype).

All these activities are gathering momentum, as the sequencing of the human genome project has been completed. Molecular cloning and functional expression studies have revealed that many genes underlie ion channels. The axon initial segment, with a high density of voltage-gated ion channels on the axonal membrane, regulates action potentials and neuronal excitability (06). Mutations in genes related to axon initial segment and altered channel expression are involved in the pathogenesis of several neurologic disorders. New ion channels are also being discovered. It is expected that mutations of several genes will be correlated with several diseases and will form the basis of rational drug discovery for these diseases.

Regulation of ion channels and receptors in the CNS. The primary amine Agmatine, an amine synthesized in the brain by decarboxylation of L-arginine, can regulate ion channel cascades and receptors that are related to the major CNS disorders. Agmatine is related to the regulation of cell differentiation, nitric oxide synthesis, relief of chronic pain, cerebral edema, and apoptotic cell death in experimental CNS disorders (03). It has a neuroprotective effect in ischemic stroke, traumatic brain injury, Alzheimer disease, and Parkinson disease.

Role of ion channels in pathophysiology of neurologic disorders. Ion channels and their transporters are involved in the pathomechanism of several disturbances of the nervous system. For example, activation of Na, K, and Cl ion co-transporters play a role in astrocyte swelling/brain edema in ischemia, trauma, and ammonia neurotoxicity. Channelopathies may be acquired as well as inherited. Maternal antibodies can cross the placenta and cause neonatal disease, and some neurodevelopmental conditions can be caused by maternal antibodies to specific neuronal and muscle ion channels.

Inherited neurologic channelopathies. Several channelopathies involving the nervous system have been described. Descriptions of the individual diseases are given in more detail in their respective MedLink Neurology clinical summaries. Non-neurologic diseases such as cystic fibrosis are related to ion channel defects. Cardiac ion channel disorders, such as the congenital long Q-T syndrome, may cause syncope, seizures, or sudden death due to cardiac arrhythmia.

Movement disorders due to mutations of the ion channel genes. Examples of these include the following:

	• Some epilepsy and paroxysmal movement disorders are channelopathies that may overlap and share some common pathophysiology.
	• A gene for familial paroxysmal choreoathetosis has been mapped to a region of chromosome 1p, where a cluster of potassium genes is located.

Idiopathic epilepsies as disorders of ion channels. Several paroxysmal disorders, including epilepsy, are due to ion channel abnormalities or channelopathies. Both familial and de novo mutations in neuronal voltage-gated and ligand-gated ion channel subunit genes have been identified in autosomal dominant epilepsies. Mutations in genes encoding beta subunits of voltage-gated Na channels are linked to paroxysmal diseases including epilepsy and are potential targets for antiepileptic drugs (07). Functional studies characterizing the molecular defects of the mutant channels point to a central role of GABAergic synaptic inhibition in the pathophysiology of idiopathic generalized epilepsies. A systematic analysis of 41 genes associated with epilepsy found a distinct correlation between functional alterations and phenotype was found in SCN1A, which shows that functional alterations are important for evaluating the pathogenicity of mutations (43).

Generalized epilepsies with Mendelian inheritance pattern are associated with the mutations in genes encoding ion channel proteins. Two human epilepsy syndromes, benign familial neonatal convulsions and generalized epilepsy with febrile seizures, represent K+ and Na+ channelopathies. KCNQ2/KCNQ3 K+ channels that are mutated in benign familial neonatal convulsions represent an important new target for antiepileptic drugs. Mutations in KCNQ3 and KCNQ5 can cause benign familial neonatal convulsions whereas KCNQ4 is associated with hereditary deafness. Some epilepsies, previously classified as idiopathic, have been shown to be channelopathies based on gene analysis in animal models of epilepsy and human familial epilepsies. This knowledge will provide a better understanding of epilepsy and enable the design of novel therapies.

Examples of epilepsy syndromes as channelopathies are as follows:

(1) Autosomal dominant frontal nocturnal epilepsy, an ACh receptor channelopathy due to mutation of CHRNA4, CHRNB2, and ENFL2 genes.

(2) Familial generalized epilepsy with febrile seizures, plus a GABA(A) receptor channelopathy, due to mutation of the GABRG2 gene.

(3) Genetic variations in the T-type calcium channel gene CACNA1H are found in patients with various generalized epilepsy syndromes; they contribute to susceptibility to epilepsy but are not the cause of epilepsy on their own.

(4) Mutations in nAChR genes are found in nocturnal frontal lobe epilepsy.

Hereditary muscle channelopathies. Release of intracellular calcium triggers generation and propagation of action potentials for mechanical contraction of skeletal muscle, which depends on the proper functioning of ion channels. Several muscle channelopathies caused by mutations in muscle ion channel genes have been identified.

Acquired or secondary neurologic channelopathies. Ion channels may be involved in several acquired disorders. Recognized causes include toxins and autoimmune phenomena. Some examples are:

Marine ciguatoxin toxin. This contaminant of fish is a potent sodium channel blocker that causes a rapid onset of numbness, intense paresthesias, and muscle weakness.

Toxic effects of drugs. The effect of toxins at the neuromuscular junction can be mediated by a rise of intracellular-free calcium within the presynaptic motor nerve terminals. This is due to the decreased efficiency of the organelles within the terminal to sequestrate calcium using an ATP-dependent pump. This mechanism has been offered as an explanation for distal motor axonopathy caused by cycloleucine, an inhibitor of adenosyltransferase. The resulting abnormalities in phospholipid composition of the axolemma impair the efficiency of the ion channels and pumps that are responsible for maintaining electrochemical gradients essential for the maintenance of the structural and functional integrity of the neuromuscular junction.

Drug-induced disturbances of sensory receptors may involve ion channel disturbances. Some examples of this are as follows:

• Drug action involving sodium channels: amiloride, spironolactone, lithium. • Drug action involving calcium channels: calcium channel blockers.

Drug addiction. Nicotinic AChR belongs to a family of proteins that form ligand-gated transmembrane ion channels. Several therapeutic agents and drugs of abuse, such as cocaine, inhibit the AChR and interfere with nervous system function. Some ligands bind to a regulatory site on the closed-channel conformation of the AChR with higher affinity than to the site on the open-channel form, resulting in inhibition of the receptor. Such AChR ion channel inhibitors are potential therapeutic agents for drug abuse.

Autoimmune channelopathies. Several other antibody-mediated neuromuscular disorders have been identified, each associated with an antibody against a ligand- or voltage-gated ion channel. Examples are myasthenia gravis, Lambert-Eaton syndrome, autoimmune autonomic ganglionopathy, and acquired neuromyotonia (Isaacs syndrome), which is a type of neuromyotonia caused by antibodies to peripheral nerve potassium channels.

Lambert-Eaton myasthenic syndrome (muscle weakness). This is due to reduced release of acetylcholine from motor nerve terminals. This syndrome has a presynaptic origin, whereas myasthenia gravis (another form of neuromuscular weakness) has a postsynaptic origin. The pathomechanism is a reduction of voltage-activated calcium channels by autoimmune-generated antibodies. The reduction involves N-type functional calcium channels that are sensitive to w-conotoxin and are involved in the control of neurotransmitter (such as acetylcholine) release from neurons. These patients have an exaggerated response to neuromuscular blocking agents, and the disease may be first recognized when there is prolonged apnea after use of neuromuscular blocking agents during surgery. Lambert-Eaton syndrome may also occur as an autoimmune paraneoplastic syndrome in patients with lung carcinoma. A high frequency of P/Q-type calcium-channel antibodies is found in these patients, indicating that voltage-gated calcium channels have a role in the pathogenesis of this disorder.

Chronic inflammatory demyelinating myelopathy. The neurophysiological abnormalities associated with demyelination can be explained by sodium channel dysfunction mediated by antibodies. Acute paralysis in Guillain-Barré syndrome may be related to a sodium channel-blocking factor in the cerebrospinal fluid.

Glioblastoma. Several ion channels that regulate membrane potential, cytosolic Ca2+ concentration, and cell volume in glioblastoma cells play significant roles in sustaining enhanced cell invasion and death evasion, which make surgery and accompanying therapies highly ineffective for glioblastoma. One of these channels, the volume-regulated anion channel (VRAC), which mediates the swelling-activated chloride current and is highly expressed in glioblastoma cells, is primarily involved in reestablishing the original cell volume that may be lost under several physiopathological conditions, but also in sustaining the shape and cell volume changes needed for cell migration and proliferation (04). The authors showed that two conditions normally occurring in pathological glioblastoma tissues, ie, high serum levels of VRAC expression and severe hypoxia, are both able to activate this channel to promote cell migration and resistance to cell death, both features that enhance glioblastoma malignancy. Targeting this channel or its intracellular regulators may represent an effective strategy to control the growth of glioblastoma.

Multiple sclerosis. This is a chronic inflammatory disease of the central nervous system that involves demyelination and axonal degeneration. In addition to demyelination, dysregulation of ion channel expression can further interfere with the neuronal signaling process. Moreover, neurodegeneration is accelerated by the altered expression of ion channels on denuded axons; therefore, therapeutic strategies should include remyelination-promoting approaches in addition to targeting the neuroinflammatory and degenerative aspects (13).

Dysregulated sodium channels can trigger calcium-mediated axonal injury via reverse sodium-calcium exchange and can explain cerebellar deficits in patients with multiple sclerosis. Pharmacological blockade of these channels might ameliorate the loss of axons.

Potassium channels play an important role in transmitting impulses to muscle and nerve cells, and these processes are inhibited in patients with multiple sclerosis. ATP-sensitive inward rectifying potassium channel KIR4.1 is a target of the autoantibody response in a subgroup of persons with multiple sclerosis (39). This autoantibody was found in almost half of the patients in this study and was absent in healthy patients.

The voltage-gated potassium channel Kv1.3 is highly expressed on inflammatory infiltrates in multiple sclerosis brain and provides the rationale for the use of specific Kv1.3 antagonists in management of this disease. Functional dysregulation of ion channels contributes to practically all pathophysiological steps of multiple sclerosis, particularly the detrimental redistribution of ion channels along axons, as well as neuronal excitotoxicity with imbalanced glutamate homeostasis during chronic CNS inflammation (38). Under demyelinating conditions, oligodendrocyte progenitor cells migrate to the site of lesions and differentiate into mature oligodendrocytes to remyelinate damaged axons, but this process fails due to impaired oligodendrocyte progenitor cell differentiation during the chronic phase of the disease (08). Moreover, oligodendrocyte progenitor cells are crucial players in neuroglial communication as they receive synaptic inputs from neurons and express ion channels and neurotransmitter/neuromodulator receptors that control their maturation.

The top five FDA-approved drugs for multiple sclerosis are based on voltage-gated channels, and their modulation ameliorates some of the symptoms. For example, fampridine, a broad-spectrum potassium channel blocker, is used for the treatment of multiple sclerosis patients. Ion channels in the nervous system are attractive future targets for developing new drugs for multiple sclerosis.

Stroke. In cerebral ischemia, oxygen/glucose deprivation opens hemichannels, or half gap junctions, in neurons, which disrupts levels of calcium and potassium in brain cells and is associated with ischemic neuronal death. It is possible that stroke therapies may be developed to block brain cell hemichannels from opening. Pannexin 1 and pannexin 2 form large-pore nonselective channels in the plasma membrane of cells, and their dysfunction in neurons contributes to ischemic brain damage in stroke (02).

Pathomechanism of disease due to mutated ion channel proteins. There are several ways in which changes occur in the properties or regulation of ion channels:

• Genetic mutations of the ion channel gene. • Abnormalities in the regulatory mechanism of the channels, such as phosphorylation. • Presence of toxic materials that block channels or prevent their resynthesis. • Development of autoimmune disease directed at the channel protein or its regulator, resulting in down-regulation or dysfunction of the channel, as in Lambert-Eaton syndrome.

Genetic mutations of the ion channels genes. Defective and malfunctioning proteins may be expressed due to improper coding. It is possible for a defect to occur in a subunit or a regulator shared by several types of channels. This explains the malfunction of more than one type of channel in a disease. Three mechanisms explain how mutated proteins give rise to diseases:

	• Gain of function in which the altered protein due to an ion channel defect augments a function, eg, hypokalemic periodic paralysis, familial hemiplegic migraine, and malignant hyperthermia.
	• Loss of function due to quantitative insufficiency of the protein to support normal cell function, eg, episodic ataxia type 2. The ataxic attacks are remarkably sensitive to acetazolamide, a carbonic anhydrase inhibitor that is considered to ameliorate ion channel function.
	• Dominant negative effects in which the mutant protein interferes with the activity of the normal protein. An example is spinocerebellar ataxia, in which abnormal alpha 1A subunits may interfere with the assembly of P/Q channels.

Channelopathies according to channel involved. More than one gene may regulate a function in a channel; thus, different genetic mutations may manifest with the same disorder. Disorders involving sodium, potassium, calcium, and chloride channels are listed here.

Potassium channelopathies. Potassium channel dysfunction has been implicated in a variety of genetic and acquired neurologic disorders that are collectively referred to as the potassium channelopathies. These include the following:

• Acquired neuromyotonia • Episodic ataxia type-1 • Hereditary deafness syndromes • Benign familial neonatal convulsions • Hypokalemic periodic paralysis • Reduction in Kv7.2/3 channel activity is essential for noise-induced tinnitus in mice and for tinnitus-specific neuronal hyperactivity (25).

Calcium channelopathies. These include the following:

	• Hypokalemic periodic paralysis
	• Malignant hyperthermia
	• Congenital myopathy with susceptibility to malignant hyperthermia
	• Familial hemiplegic migraine. The CACNA1A gene encoding the brain-specific P/Q type calcium channel has been cloned and mutations in this gene, located on chromosome 19p13, are involved in familial hemiplegic migraine.
	• Episodic ataxia type 2
	• Spinocerebellar ataxia (SCA-6). This is due to triple repeat of the CACNA1A gene, coding for the voltage-gated calcium channels type P/Q, expressed in the cerebellar Purkinje and granule cells.
	• Absence epilepsy can be associated with dysfunction of the brain P/Q-type voltage-gated calcium channel.

Sodium channelopathies. The SCN1A gene provides instructions for the alpha subunit of the sodium channel NaV1.1, which is primarily found in the brain, where it controls the flow of sodium ions into cells, release of neurotransmitters, and electrical signals.

Reductions in the expression of some previously active sodium channel genes have been found in peripheral neuropathies. A sodium channel gene that is normally silent in spinal sensory neurons is induced by nerve injury. These changes are thought to lead to hyperexcitability and might contribute to the hyperalgesia and allodynia that are observed in cases of neuropathic pain.

Several genes encoding voltage-gated sodium channels are found in the human genome. Mutations of the SNC4A gene are associated with the following:

	• Hyperkalemic periodic paralysis
	• Paramyotonia congenita
	• Potassium-aggravated myotonia
	• Genetic defects in genes encoding 2 pore-forming alpha subunits (SCN1A and SCN2A) and the accessory beta1 subunit (SCN1B) are responsible for a group of epilepsy syndromes with overlapping clinical characteristics but divergent clinical severity.
	• Familial hemiplegic migraine type 3 is caused by mutations in the sodium channel NaV1.1, which are also associated with generalized epilepsy, indicating that both disorders may share common molecular mechanisms.
	• Painful inherited neuropathy is associated with mutations in the SCN9A gene.
	• Mutations of Nav1.8 contribute to painful peripheral neuropathy (12).

Chloride channelopathies. Myotonia congenita (dominant and recessive forms) is associated with mutations of the CLCN1 sarcolemmal rectifying chloride channel. Impaired chloride transport can lead to disorders such as epilepsy, hyperekplexia, lysosomal storage disease, and deafness.

Ion channels and pain. Aberrant peripheral or central ion channel activity underlies or initiates many painful conditions. Understanding the basis of ion channel malfunction and its effect on neurotransmission may lead to new pain therapies.

Multiple sodium channels, with distinct electrophysiological properties, are encoded by distinct mRNAs within small dorsal root ganglion neurons, which include nociceptive cells. Several of these neuron-specific sodium channels now have been cloned and sequenced. Sodium channel expression in dorsal root ganglia neurons is dynamic, changing significantly after injury. Sodium channels within primary sensory neurons may play an important role in the pathophysiology of pain. The voltage-gated sodium channels may play a crucial role in neuropathic pain, but their role is mostly documented for mechanical static and dynamic allodynia, and either peripheral or central sodium channels may be involved. Familial pain syndromes may be due to mutations in the Na(V)1.7 channel. Gain-of-function mutations cause paroxysmal pain disorders due to hyperexcitability of sensory neurons, whereas loss-of-function mutations cause congenital indifference to pain due to attenuation of action potential firing.

Currently, research is focused on targeting ion channels involved in pain transduction, axonal conduction, and neurotransmitter release to provide targets for discovery of new analgesics. Knowledge of function of voltage‐gated ion channels, such as Na+ channels and K+ channels involved in neuronal excitability and Ca2+ channels responsible for transmitter release, has advanced through measurement of protein up‐regulation during pain states, knowledge of channel dysfunction in pain syndromes associated with human genetics, and development of transgenic knock‐down and knock‐in technology to support in vitro and in vivo models of pain (40).

Calcium release-activated calcium (CRAC) channels are activated in response to depletion of intracellular calcium stores and are highly permeable to Ca2+ compared to other cations. CRAC channels are expressed in the nervous system and are involved in pathological conditions, including pain (29).

Sodium and calcium channels. These are the main targets for anticonvulsant drugs in neuropathic pain. Several receptors and ion channels present in sensory neurons are now under investigation as potential new analgesic drug targets. These include the following:

• Voltage-gated sodium channels that are specific for sensory neurons • Novel sodium channels • Adenosine triphosphate-gated channels • Capsaicin-gated channels, such as vanilloid receptor 1 • Proton-gated channels, such as acid-sensing ion channel

The use of antisense oligonucleotides to target specific channel subtypes shows that the functional localization of the channel subtype sodium V1.8 after nerve injury is essential for persistent pain states. As sodium V1.8 expression is restricted to sensory neurons, this channel offers a highly specific and effective molecular target for the treatment of neuropathic pain.

Acid-sensing ion channels (ASICs). These ion channels can induce action potential triggering on sensory neurons after a moderate extracellular pH decrease. They participate in the hypersensitization of the nociceptive system in inflammatory pain. Inhibition of their expression is the mechanism of action of nonsteroidal anti-inflammatory drugs. Decreased extracellular pH and subsequent activation of ASICs may play a role in migraine pathophysiology indication efficacy of ASIC-blockers such as amiloride for management of migraine (11). ASIC1a also plays a prominent role in spinal cord neurons that modulate and transmit pain signals to the brain (10).

Transient receptor potential (TRP) ion channels in pain. TRP ion channels, a large superfamily of related cation channels, function as dedicated biological sensors that are essential in processes such as vision, taste, tactile sensation, and hearing. There are 6 different TRPs (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, and TRPA1), which are expressed in pain-sensing neurons and primary afferent nociceptors. The TRP family contains a novel group of nonselective cation channels that are distinct from classical voltage-gated ion channels. TRPs respond to a variety of stimuli, including changes to specific ligands, temperature, acid, salt concentration, and second messenger signaling. As such, TRPs act as multimodal signal integrators, representing approximately 20% of all ion channels found in the body. Various TRP channels are associated with different types of neuropathic pain. TRP channel modulators are in development for the treatment of neuropathic pain (28).

Clinical applications

	• Knowledge about the role of ion channels in neurologic disorders would be of practical use in the management of neurologic disorders.
	• Several ion channels serve as targets for CNS drugs.

Application of knowledge of ion channels could be both preventive and therapeutic. For example, if individual predisposition to malignant hyperthermia can be detected by genetic testing, exposure to anesthetics precipitating the disease can be avoided. Channelopathies due to gain of function might respond to drugs blocking the action of those channels, eg, calcium channel blockers. Molecular studies of mutated ion channels or channel subunits will also help to develop effective drug treatments for channelopathies.

Ion channels as targets for drug action. The voltage-gated potassium, sodium, and calcium channels control nerve impulse conduction and frequency; because of this, they are common targets for anesthetics. Neurotransmitter-gated ion channels, particularly receptors for GABA and glutamate, are modulated by most anesthetics at both synaptic and extrasynaptic sites. Ion channels are pharmacologic receptors with specific drug binding sites and are regulated by chronic drug action. The binding sites are usually multiple. SuperPain is a freely available database for pain-relieving compounds that bind to ion channels involved in the transmission of pain (17). Examples of drugs targeted to ion channels for the treatment of neurologic disorders are shown in Table 3.

Table 3. Ion Channels as Targets for Drugs Used in the Treatment of Neurologic Disorders

Ion channel or action	Drugs in clinical use	Indication	Investigational applications (if any)
Sodium channel blocking	Carbamazepine, phenytoin, fosphenytoin (prodrug of phenytoin), valproic acid	Anticonvulsant	Fosphenytoin has been tested as a neuroprotective in phase III trials but was not found to be effective.
	Cannabidiol	Seizures in Lennox-Gastaut syndrome and Dravet syndrome
	Bupivacaine, lidocaine, mepivacaine	Local anesthetics
Chloride channel blocking	Clonazepam, phenobarbital	Anticonvulsants
	Lorazepam	Hypnotics
	Diazepam	Muscle relaxants
Calcium channel blocking	Nimodipine: L-type calcium channel antagonists	Inhibition of calcium-dependent vasospasm associated with aneurysmal subarachnoid hemorrhage	Reduces infarct size after experimental cerebral ischemia. Clinical trials in stroke patients did not prove efficacy
	Flunarizine: T-type calcium channel antagonist	Prophylaxis of migraine	Neuroprotective effect. Reduces infarct size after experimental cerebral ischemia. Clinical trials in stroke patients did not prove efficacy
	Ziconotide: N-type calcium channel antagonist	Intrathecal ziconotide used for management of chronic pain	Phase 3 clinical trials for cerebral ischemia did not show efficacy.
	Pregabalin: Binds to alpha2delta-subunit of voltage-gated calcium channels	Antiepileptic used for neuropathic pain.	Tremor
Potassium channel opening	None	None	BMS-204352 has neuroprotective effect. Phase 3 clinical trial in stroke
	Flupirtine: Activation of a G-protein regulated inwardly rectifying K+ ion channel -- GIRK(35)	Novel analgesic approved for acute and chronic pain	Treatment of pain in fibromyalgia
Potassium channel blocking	Fampridine	Multiple sclerosis	Spinal cord injury.

Sodium channel blockers. Voltage-gated sodium channels are the primary targets of drugs used as local anesthetics, antiepileptics, and neuroprotectants (09). Drugs that modulate voltage-gated sodium channels have the potential to regulate synaptic plasticity through N-methyl-d-aspartate receptors (15). The best-known sodium channel blockers are established antiepileptic drugs: carbamazepine and phenytoin. Carbamazepine reduces currents through sodium channels. Phenytoin enhances active sodium extrusion and inhibits passive sodium entry leading to normalization of the sodium gradient and stabilization of the membrane. It may also inhibit transmitter release by reducing calcium-dependent phosphorylation of membrane proteins. It may also modulate GABA receptors. The antiepileptic effect of valproic acid may be mediated via multiple mechanisms, which include inhibition of glutamate and aspartate and reduction of excitability of neuronal membranes, through its influence on sodium and potassium channels. The mode of antiepileptic action of cannabidiol likely involves the following: (1) compound partitioning in lipid membranes, which alters membrane fluidity affecting gating; and (2) direct interactions with sodium and potassium channels (16). Lamotrigine acts on voltage-sensitive sodium channels to stabilize the neuronal membrane and inhibit the release of glutamate and aspartate. The exact mechanism of action of gabapentin is not understood. Experimental evidence indicates that gabapentin inhibits voltage-gated calcium channels. One explanation of its efficacy in relieving neuropathic pain is that it affects a calcium channel in the pain-transmitting nerve cells of the spinal cord.

A functional link has been shown between sodium V1.3 expression and neuronal hyperexcitability associated with central neuropathic pain. In this study, antisense oligonucleotide injection targeting the sodium V1.3 channel resulted in decreased production of the sodium V1.3 sodium channels, less hypersensitivity of the pain-signaling nerve cells within the spinal cord, and reduced pain-related behaviors.

Calcium channel antagonists (blockers). Dysfunction of calcium channels is implicated in a variety of clinical disorders, such as migraine, hemiplegia, and epilepsy. Therefore, calcium channels are of considerable pharmaceutical interest as targets for peptide drugs to treat these diseases (14). Calcium channels have been implicated indirectly in some neurologic disorders by the therapeutic effect of calcium antagonists. For example, calcium entry blockers bind to the L-type calcium channels and reduce the transport of calcium through these channels. The resulting effect is relaxation of smooth muscle in cerebral arterial vasospasm following subarachnoid hemorrhage. Neuronal calcium overload along with excessive extracellular glutamate produces neurotoxicity and is a common pathway for neuronal injury or death in neurodegenerative diseases. Resveratrol has a neuroprotective effect in neurodegenerative diseases by blocking different glutamate receptors and calcium ion channels (46). Hypokalemic periodic paralysis (HypoPP) type 2 is caused by mutations in the skeletal muscle voltage-gated sodium channel NaV1.4, and Hm-3 toxin from the crab spider inhibits gating pore currents through such mutant channels, which is a potentially useful basis for developing gating pore current inhibitors for HypoPP therapy (27).

Potassium channel openers. Potassium channel openers or activators have aroused considerable pharmaceutical interest during the last decade as smooth muscle relaxants. This characteristic property can be predicted from knowledge of their site and mechanism of action. Effects of K-openers on tissues can be antagonized by inhibitors of KATP-like glyburide. Patch clamp studies have shown that K-channel openers open KATP. Besides ATP, other factors that modify KATP channels include adenosine and neurotransmitters. The effect of K-channel openers on the skeletal muscle has been investigated in relatively fewer studies. Cromakalim increases channel opening probability in the presence of an inhibitory concentration of ATP. Most of the K-channel openers are in development for non-neurologic indications.

Potassium channel blockers. Potassium channel blocker, 4-aminopyridine, has been used for the treatment of episodic ataxia type 2 due to mutations in the CACNA1A gene and clinically characterized by recurrent attacks of vertigo, imbalance, and ataxia. It is also beneficial for interictal cerebellar ataxia in late-onset episodic ataxia type 2.

Chloride channel modulators. GABA antagonists are chloride channel blockers. This is the likely mechanism of action of barbiturates and benzodiazepines. As a benzodiazepine, the binding site for diazepam is linked to the GABA receptor, which has a major role in inhibitory functions in the CNS. Benzodiazepine binding sites are contained within the GABA receptor complex in CNS neurons. Activation and inhibition of GABA receptors by several pharmacological agents has been employed in the treatment of several neurologic and neuromuscular disorders.

Future. Advances in genomic technologies will enable personalized medicines, including those for patients with neurologic disorders. Demonstration of heterogeneity in diseases such as migraine may facilitate a rational individualized treatment with currently available drugs. Ion channels will provide targets for medicines to treat neurologic disorders for which no cure is available. Ion channels associated with more than one disorder might provide a clue to common factors in pathogenesis, as well as treatment, of more than one disorder with a single drug. Examples of epilepsy and neurodegenerative disorders are given to provide a glimpse into the impact of knowledge of ion channels on the future management of neurologic disorders.

There are specific alterations in the structure or function of ion channels in the epileptic brain. If these properties cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. These may yield important clues for future therapeutic approaches aimed at preventing epileptogenesis.

Development of genetic techniques for sensitizing neurons to optical stimulation has enabled selective and targeted control of neuronal activity with single-cell resolution by application of temporally focused laser pulses (01). This approach has clinical potential for treatment of neuropsychiatric disorders as an alternative to electrical stimulation.

Multiple neurotransmitter systems can be modulated via gated ion channels to produce therapies for enhancing cognition and memory in patients with neurodegenerative disorders. These efforts include the following:

• Gamma-aminobutyric acid subtype A receptor and benzodiazepine inverse agonists • Nicotinic acetylcholine receptor agonists • Serotonin subtype 3 receptor (5-HT3R) antagonists • Potassium (K+) M-channel inhibitors