Pathophysiology of Epilepsy

Number 2 in a SeriesVersion 2

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Pathophysiology of Epilepsy

IntroductionEpilepsy, a disease that has been in existence for ages, continues to affect approximately 50 million individuals worldwide, including about 2.7 million in the United States.1,2 The disease is often accompanied by neurobiologic, cognitive, psychological, and behavioral changes that may heighten susceptibility to seizures and affect quality of life. Anti-epileptic drugs (AEDs) are the primary option for the management of epilepsy. Although research over the years has led to significant advances in understanding the pathophysiology of epilepsy, the specific causes of several types of epilepsy are unknown,3 and there remains a great need for research on the neural mechanisms that potentially underlie drug resistance. This brochure aims to provide an overview of the pathophysiology of epilepsy.

Normal Neurologic FunctioningPrior to the discussion of the pathophysiology of epilepsy, a brief review of the anatomy and physiology of normal neurologic functioning (Figures 1-5) is presented.

The anatomy of the neuron, with an inset of an excitatory synapse, is shown in Figure 1. During synaptic transmission, neurotransmitters are released into the synaptic cleft in a Ca2+ dependent manner and bind to their corresponding receptors.4 Synaptic transmission is regulated through neurotransmitter turnover, which occurs via reuptake into the vesicles and enzymatic degradation.

Figure 1. Anatomy Of A Neuron And An Excitatory Synapse

Modified from WikiMedia and drawn by Miller Montealegre.

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Figure 2. The ionic compositions of the intracellular fluid (cytosol) and of the surrounding extracellular fluid. A− represents negatively charged proteins, which neutralize excess Na+ and K+ ions.4

Figure 3. (Left) GABAA receptor: Synaptic (phasic) GABA receptor with a view of the extracellular face showing the two recognition sites for GABA and the benzodiazepine recognition site; (Right) A typical extrasynaptic (tonic) GABAA receptor with two GABA recognition sites (from Meldrum 2007).6

Neuronal axons have a resting membrane potential of about -70 mV inside vs outside. Action potentials occur due to net positive inward ion fluxes, resulting in local changes in the membrane potential.4,5 Membrane potentials vary with the activation of either ligand- or voltage-gated ion channels, which are affected by changes in either the membrane potential or intracellular ion concentrations (Figure 2).4

GABA, the principal inhibitory neurotransmitter in the brain, binds postsynaptically to the ionotropic receptor, GABAA

(Figure 3), and presynaptically to the metabotropic receptor, GABAB.6

Reprinted with permission from Molecular Cell Biology, 4th ed., Lodish H, Berk A, Zipursky S, et al. 2000.

Reprinted with permission from Neurotherapeutics;4:18-61, Meldrum BS, Rogawski MA, Molecular targets for antiepileptic drug development. 2007.

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Figure 4. (A) One of the subunits of a complete receptor. The long N-terminal region forms the ligand-binding site, while the remainder of the protein spans the membrane either four times (left) or three times (right). (B) Assembly of either four or five subunits into a complete receptor. (C) A diversity of subunits come together to form functional ionotropic neurotransmitter receptors.

Glutamate, the principal excitatory neurotransmitter, binds to both ionotropic and metabotropic types of receptors. Glutamate acts on 3 classes of ionotropic receptors—n-methyl- D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainate (KA). AMPA receptors are the most abundant, followed by NMDA and KA receptors.7,8 These receptors contain subunits whose structure affects the biophysical properties of the receptor (Figure 4). AMPA receptors have lower glutamate affinity than NMDA receptors, but their faster kinetics account for the fast initial component of the excitatory postsynaptic potential (Figure 5).8

Figure 5. Dual component excitatory post-synaptic potential showing the fast initial AMPA component and the slow NMDA component.

Reprinted with permission from Neuroscience, 2nd ed., Purves D, Augustine GJ, Fitzpatrick D, et al., editors. http://www.ncbi.nlm.nih.gov/books/NBK10834/figure/A492/?report=objectonly © 2001

Redrawn with permission from Journal of Nutrition;130:1007S-15S, Meldrum BS, Glutamate as a neurotransmitter in the brain: review of physiology and pathology. 2000.

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Primary Physiologic Mechanisms Involved in Epilepsy To accurately portray the pathophysiology of epilepsy, it is necessary to differentiate epilepsy, which is a progressive neurologic disorder of the brain, from seizures themselves, which are distinct, transient occurrences caused by abnormal, excessive, or synchronous neuronal activity in the brain.9 Signs and symptoms of seizures may include warnings, such as visual or sensory auras, déjà vu, tingling fingers, altered awareness, and abnormal or convulsive movements. The pathophysiology underlying the epileptic process includes mechanisms involved in initiation of seizures (ictogenesis), as well as those involved in transforming the normal brain into a seizure-prone brain (epileptogenesis).9

Mechanisms of IctogenesisHyperexcitation is the key factor underlying ictogenesis (Figure 6). Excessive excitation may originate from individual neurons, the neuronal environment, or neuronal networks.3

• Excitability from individual neurons may arise from structural or functional changes in the

postsynaptic membrane; alterations in the type, number, and distribution of voltage- and ligand-gated ion channels; or biochemical modification of receptors that increase permeability to Ca2+, favoring development of the prolonged depolarization that precedes seizures10

• Excitability arising from the neuronal environment may result from both physiologic and structural changes. Physiologic changes include alterations in concentrations of ions, metabolic alterations, and in neurotransmitter levels. Structural changes affect both neurons and glia. Seizure- associated astrocytes reportedly are complex, arborized, highly branched processes with a stellate appearance and with a ratio of Na+ to K+ conductance that is 3-4 fold higher than that observed in normal astrocytes. Consequently, glial K+ buffering may be affected and may lead to epileptic activity.3,11 Extracellular Ca2+ concentration decreases by over 85% during a seizure, preceding the changes in K+ concentration by milliseconds. However, Ca2+ levels return to normal faster than K+ levels

• Alterations in the neuronal network may facilitate excitability through sprouting of the axons of the granule cells of the dentate gyrus or mossy fibers; loss of inhibitory neurons; loss of excitatory neurons needed to activate inhibitory neurons; or changes in neuronal firing properties due to channelopathies

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Mechanisms of Ictal-Interictal Transition and Epileptogenesis3 Both nonsynaptic and synaptic mechanisms that affect synchronicity, signal amplification, and spread of seizures play a role during ictal-interictal transition, promoting epileptogenesis (Figure 6).

Nonsynaptic MechanismsChanges in ionic concentrations observed during hyperexcitation—increased extracellular K+ or decreased extracellular Ca2+, for example—may be caused by decreases in extracellular size or volume. Failure of Na+-K+ pumps due to hypoxia or ischemia is known to promote epileptogenesis in animal models, and interference with Cl--K+ transport, which controls intracellular Cl and regulates GABA-activated inhibitory Cl currents, may lead to enhanced excitation. Excitability of synaptic terminals depends on the extent of depolarization and the amount of neurotransmitter released. Synchronization following abnormal bursts of spikes in the axonal branching of thalamocortical relay cells plays a key role in epileptogenesis. Ephaptic interactions that occur between neighboring neurons separated by small extracellular spaces also contribute to increased synchronization.

Figure 6. Summary of mechanisms involved in epilepsy, and the key molecular players.

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Synaptic MechanismsSynaptic pathophysiology of epilepsy and epileptic disorders primarily involves reduced GABAergic inhibition or enhanced glutamatergic excitation.

GABAGABA levels have been shown to be reduced in the cerebrospinal fluid (CSF) of patients with certain kinds of epilepsy, such as infantile spasms and untreated generalized tonic-clonic seizures, and in excised epileptic tissue from patients with drug-resistant epilepsy, suggesting that these patients have decreased inhibition.12 Dogs with epilepsy have been shown to have low CSF levels of GABA, and mice genetically susceptible to audiogenic seizures have a lower number of GABA receptors than non-seizure prone animals. Reduced [3H]-GABA binding to GABA receptors has been reported in human brain tissue, and low glutamic acid decarboxylase levels have been shown in kindled rats and in excised human epileptic tissue, suggestive of decreased GABAergic inhibition.3

GlutamateHippocampal recordings from conscious human brains have shown sustained increases in the levels of extracellular glutamate levels during and preceding seizures. GABA levels remain low in the epileptogenic hippocampus, but during seizures, GABA concentrations increase, although mostly in the non-epileptogenic hippocampus. This leads to a toxic increase in extracellular glutamate due to reduced inhibition in the epileptogenic areas.13

In human hippocampal epilepsy, densities of glutamate AMPA receptor subunits correlated with the locations of the densest aberrant mossy fibers. Increases in AMPA receptors in a KA model of epileptic rats preceded

mossy fiber ingrowth, and demonstrated a greater increase than the increase in presynaptic mossy fiber inputs14; KA receptors have also been shown to be involved in ongoing glutamatergic transmission in granule cells of chronic epileptic animals.15 Thus, while the role of NMDA receptors in epilepsy has been known for some time, there is now growing evidence of the role of AMPA and KA receptors in epilepsy.

Thalamocortical Network ExcitationGeneralized epilepsies are characterized by abnormally synchronized activity in large neuronal networks.16 In absence seizures, the 3-4 Hz spike-and-wave patterns are thought to be the result of high frequency thalamocortical oscillations.16 Thalamocortical oscillations are generated by the synaptic interplay of 3 structures—nucleus reticularis thalami (nRT), thalamocortical neurons (TCNs), and cortical pyramidal neurons (Figure 7). Both nRT and TCNs have an intrinsic ability to fire in bursts when their cell membrane is hyperpolarized, a process that is dependent on extracellular Ca2+ and the transient or T-type Ca2+ channels. TCNs fire action potentials in high-frequency and short-duration bursts. As a result, they play a key role in the pathophysiology of epilepsy.17

Figure 7. Panel A, B showing simplified thalamocortical network and spike wave complex.

Reprinted with permission from Khrosravani H, Zamponi GW, Physiol Rev, Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies, Vol. 86, No. 3, 941-966

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Role of glial cells in excitationAlthough most of the work on the pathogenesis of epilepsy has focused on neurons, glial cells are known to play a key role in buffering functions that maintain the uptake of K+ and glutamate; disrupting these functions may cause hyperexcitability. Recent evidence also suggests that glutamate release from glia can generate a paroxysmal depolarizing shift (PDS), the prolonged depolarization reflected in EEG recordings of interictal discharges. Even in the absence of synaptic interactions, astrocytic release of glutamate can trigger PDS-like events (Figure 8).18

Pathophysiology Underlying Specific Epileptic DisordersFor some, but not all, forms of epilepsy, the pathogenesis is at least partially understood.3 Insults to the brain, such as status epilepticus, traumatic brain injury, neonatal and adult hypoxia-ischemia, and encephalitis, as well as certain degenerative disorders, have been associated with epilepsy. Although specific mechanisms underlying each of these conditions are unclear, they may all trigger events leading to the structural and functional changes in the brain that can initiate ictogenesis and epileptogenesis.19 Etiologies of some known forms of epilepsies are elaborated below.

Monogenic mutationsMost familial epilepsies have complex modes of inheritance resulting from interaction of several genetic loci with environmental factors. However, some epileptic disorders, seen in only 1% of patients,3 are associated with single-gene mutations, many of which have been found in ion-channel proteins. Table 1 presents a list of known epileptic syndromes with single-gene mutations and the affected gene products.

Figure 8. Astrocytic release of glutamate can trigger PDS-like events.

Reprinted by permission from Macmillan Publishers Ltd: Nature Medicine, Rogawski MA. Astrocytes get in the act in epilepsy. Nat Med 2005;11:919-20, © 2005.

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TABLE 1. Epileptic Syndromes Associated With Single-Gene Mutations20

Syndrome Mutated Gene Gene Product

Generalized epilepsy with febrile seizures plus

Benign familial neonatal epilepsy

Autosomal dominant nocturnal frontal-lobe

epilepsy

Childhood absence epilepsy and febrileseizuresAutosomal dominant partial epilepsy with auditory features

SCN1B

SCN1A

SCN2A

GABRG2

KCNQ2

KCNQ3

CHRNA4

CHRNB2

GABRG2

LGI1

Sodium-channel subunit

Sodium-channel subunit

Sodium-channel subunit

GABAA receptor subunit

Potassium channel

Potassium channel

Neuronal nicotinic acetylcholine-receptor subunit

Neuronal nicotinic acetylcholine-receptor subunit

GABAA receptor subunit

Leucine-rich transmembrane protein

Epilepsy-associated neuronal migration Several developmental disorders of neuronal migration, with underlying genetic or intrauterine causes, are associated with epilepsy. Agyria or lack of gyri and sulci, and pachygyria (thick convolutions) are commonly associated with abnormalities in neuronal migration. Such cortical malformations, including microgyric cortices, have been associated with increases in postsynaptic glutamate receptors and decreases in GABA receptors, a condition that can promote epileptogenesis.3 Tuberous sclerosis, X-linked lissencephaly, and double cortex syndrome are other examples of developmental disorders associated with epilepsy and disordered neuronal migration.3

Autoimmune pathogenesisRasmussen’s encephalitis is a progressive degenerative disease affecting children. Patients have seizures that are typically resistant to AEDs. Progressive hemiparesis with dementia is a characteristic of this rare disease.17 Recent discovery of anti-GluR3 antibodies suggests that this disease may be the result of autoimmune pathogenesis.3

Pathophysiology and Antiepileptic Drugs Epilepsy interventions currently rely on AEDs, surgery, diet, and implantation of medical devices, such as those employed for vagus nerve stimulation (VNS). However, a large proportion of patients suffer a significant compromise in the quality of their lives due to drug resistance. Most AEDs target the basic mechanism underlying ictogenesis—that is, hyperexcitation.

The mechanism of action of AEDs may be conveniently organized into 3 major categories: modulation of voltage-gated ion channels, enhancement of synaptic inhibition, and inhibition of synaptic excitation.21 However, importantly, some AEDs work through complex complementary mechanisms involving more than one of the above.

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Modulation of voltage-gated ion channelsIon channels—Na+, Ca2+, and K+—shape the sub-threshold electrical activity of the neuron, regulate response to synaptic activity, and thus contribute to the PDS involved in seizure generation. Voltage-gated Na+ channels are responsible for the rising phase of the action potential, with fast current generating the action potential and persistent current contributing to burst discharges by enhancing after-depolarization potentials.6 Ca2+ channels can allow Ca2+ entry into the cell, thus causing depolarization and activating other ion channels. This process is especially important in the thalamus where neuronal T-type Ca2+ channels lead to burst firing and oscillatory activity, as in the sleep and wake cycle.21 Ca2+ channels come under 2 major groups—high-voltage activated (HVA) and low-voltage activated (LVA). HVA Ca2+ channels are responsible for Ca2+ entry and presynaptic release of neurotransmitters, while LVA channels trigger low-threshold spikes that in turn trigger burst firing mediated by Na+ channels. Burst firing is associated with the synchronicity observed in the thalamus, as in absence epilepsy. Thus, AEDs that block voltage-gated Ca2+ channels are an important target for AEDs.6,22 Some AEDs that inhibit Na+ channels also block T-type Ca2+ channels.21

Modulation of ligand-gated ion channelsConductance of ligand-gated channels is modulated by binding to neurotransmitters that regulate inhibition and excitation. AEDs can suppress epileptic activity by enhancing GABA-mediated inhibition or by suppressing glutamatergic excitation.

Enhancement of synaptic inhibition:Many of the existing AEDs aim to enhance GABAergic inhibition by interacting with fast ionotropic GABAA receptors or by modifying the activity of enzymes and transporters involved in GABA synthesis or reuptake.6

Suppression of synaptic excitation:Glutamatergic excitation may be influenced through action on NMDA, AMPA, or KA receptors. However, AMPA receptors are the most abundant ionotropic glutamate receptors that mediate synaptic signaling.7

Mechanisms Involved in Drug ResistanceAlthough the mechanisms underlying drug resistance are not yet clear, recent studies demonstrate that drug resistance in patients with epilepsy may be present and identifiable early.23 Two major hypotheses have been proposed based on the evidence—the target hypothesis and the transporter hypothesis.23

Target hypothesisThe target hypothesis attributes resistance to alterations in the cellular or molecular target of an AED, causing reduced sensitivity to the drug. For instance, the GABAA receptor subtype has been shown to be altered in patients with uncontrolled temporal lobe epilepsy. However, it is unclear whether the altered receptor structure would itself affect the action of the AED sufficiently to cause drug resistance.23

Transporter hypothesisThe transporter hypothesis tries to account for the fact that drug resistance often involves intolerability to multiple drugs with varying MOAs, suggesting that there must be an independent mechanism underlying drug resistance itself.23 An overexpression of certain active drug transporters belonging to the ATP-binding cassette (ABC) transporter superfamily has been implicated in drug resistance.24 In fact, 22 of the known 48 members of this transporter family are associated with drug resistance. Of these, the P-glycoprotein (P-gp), the multi-resistant proteins (MRP1–7), and the breast cancer resistant protein (BCRP) occur in the blood-brain barrier and cerebrospinal fluid-brain barrier and drive

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the flow of their substrates against their concentration gradients, resulting in lowered plasma levels of the drug despite adequate administration.24 In epileptogenic brain specimens of patients with uncontrolled epilepsy, high levels of P-gp and MRP have been illustrated in both vascular endothelial cells and brain parenchymal cells.24 However, due to the lack of control specimens, it is unclear if this overexpression of transporters exists before the onset of epilepsy, or if it is a consequence of the seizures or the treatment.24 Thus, although there is some evidence in support of the transporter hypothesis, it remains an area of active research.

ConclusionsMechanisms underlying epilepsy, ictogenesis, and epileptogenesis are complex and manifold depending on the specific type of epilepsy. The hallmark mechanisms common to most epilepsies are hyperexcitability and excessive synchronicity. Treatment paradigms are complicated by the complexity of the nervous system. For example, GABA, which is inhibitory in the mature brain, can be excitatory in the immature brain. There is a need for greater research into the mechanisms underlying drug resistance itself.

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