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Basic Mechanisms Underlying Seizures and Epilepsy American Epilepsy Society B-Slide 1.

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Presentation on theme: "Basic Mechanisms Underlying Seizures and Epilepsy American Epilepsy Society B-Slide 1."— Presentation transcript:

1 Basic Mechanisms Underlying Seizures and Epilepsy American Epilepsy Society B-Slide 1

2 Definitions  Seizure: the clinical manifestation of an abnormal and excessive synchronization of a population of cortical neurons  Epilepsy: a tendency toward recurrent seizures unprovoked by any systemic or acute neurologic insults  Epileptogenesis: sequence of events that converts a normal neuronal network into a hyperexcitable network B-Slide 2 Revised per Ruteckie 7/06 American Epilepsy Society 2008

3 Hippocampal Anatomy B-Slide 3 From Chang and Lowenstein, 2003 American Epilepsy Society 2008

4 Basic Mechanisms Underlying Seizures and Epilepsy  Feedback and feed-forward inhibition, illustrated via cartoon and schematic of simplified hippocampal circuit B-Slide 4 Babb TL, Brown WJ. Pathological Findings in Epilepsy. In: Engel J. Jr. Ed. Surgical Treatment of the Epilepsies. New York: Raven Press 1987: American Epilepsy Society 2008

5 Epilepsy—Basic Neurophysiology  Causes of Hyperexcitability : excitatory post synaptic potentials (EPSPs) inhibitory post synaptic potentials (IPSPs) changes in voltage gated ion channels alteration of local ion concentrations B-Slide 5 American Epilepsy Society 2008

6 Epilepsy—Basic Neurophysiology  Major Neurotransmitters in the brain : Glutamate GABA Acetylcholine Dopamine Serotonin Histamine Other modulators: neuropeptides, hormones B-Slide 6 American Epilepsy Society 2008

7 Epilepsy—Glutamate  The brain’s major excitatory neurotransmitter  Two groups of glutamate receptors Ionotropic—fast synaptic transmission Three subtypes – AMPA, kainate, NMDA Glutamate-gated cation channels Metabotropic—slow synaptic transmission G-protein coupled, regulation of second messengers (cAMP and phospholipase C) Modulation of synaptic activity  Modulation of glutamate receptors Glycine, polyamine sites, Zinc, redox site B-Slide 7 American Epilepsy Society 2008

8 B-Slide 8 Epilepsy—Glutamate Diagram of the various glutamate receptor subtypes and locations From Takumi et al, 1998 American Epilepsy Society 2008

9 Epilepsy—GABA  Major inhibitory neurotransmitter in the CNS  Two types of receptors GABA A —post-synaptic, specific recognition sites, linked to CI - channel GABA B —presynaptic autoreceptors that reduce transmitter release by decreasing calcium influx, postsynaptic coupled to G-proteins to increase K + current B-Slide 9 American Epilepsy Society 2008

10 Epilepsy—GABA Diagram of the GABA A receptor From Olsen and Sapp, 1995 B-Slide 10 GABA site Barbiturate site Benzodiazepine site site Steroid site Picrotoxin site American Epilepsy Society 2008

11 Cellular Mechanisms of Seizure Generation  Excitation (too much) Ionic—inward Na +, Ca ++ currents Neurotransmitter—glutamate, aspartate  Inhibition (too little) Ionic—inward CI -, outward K + currents Neurotransmitter—GABA B-Slide 11 American Epilepsy Society 2008

12 Normal CNS Function B-Slide 12 Excitation Inhibition glutamate, aspartate GABA Modified from White, 2001 American Epilepsy Society 2008

13 Hyperexcitability reflects both increased excitation and decreased inhibition B-Slide 13 Excitation Inhibition GABA glutamate, aspartate Modified from White, IGES, 2001 Modified from White, 2001 American Epilepsy Society 2008

14 Neuronal (Intrinsic) Factors Modifying Neuronal Excitability  Ion channel type, number, and distribution  Post-translational modification of channels (phosphorylation, etc).  Activation of second-messenger systems that affect channel function (e.g. G proteins)  Modulation of gene expression of ion channels B-Slide 14 American Epilepsy Society 2008

15 Epilepsy and Channelopathies  Inherited  Voltage-gated ion channel mutations  Ligand-gated ion channel (neurotransmitter receptor) mutations  Different mutations in the same gene can result in radically different types of seizures and epilepsy  Acquired  Auto-immune (anti-potassium channel antibodies)  Changes in channel expression after seizures B-Slide 15 American Epilepsy Society 2008

16 Ion Channel & Neurotransmitter Receptors Mutated in Epilepsy - I Voltage-gated Sodium Channel Gene Mutations SCN1A Generalized Epilepsy & Febrile Seizures Plus (GEFS+) type 2 Severe Myoclonic Epilepsy of Infancy (SMEI) SCN1B GEFS+ type 1 SCN2A1 GEFS+ Benign Familial Neonatal-Infantile Seizures (BFNIS) B-Slide 16 American Epilepsy Society 2008

17 Ion Channel & Neurotransmitter Receptors Mutated in Epilepsy - II Voltage-gated Chloride Channel Gene Mutations CLCN2A Juvenile Absence Epilepsy (JAE) Juvenile Myoclonic Epilepsy (JME) Epilepsy with Grand Mal upon Awakening (EGMA) B-Slide 17 American Epilepsy Society 2008

18 Ion Channel & Neurotransmitter Receptors Mutated in Epilepsy - III Voltage-gated Potassium Channel Mutations KCNQ2, KCNQ3 Benign Familial Neonatal Convulsions (BFNC) KCND2 Temporal Lobe Epilepsy (TLE) KCNMA1 Generalized Epilepsy with Paroxysmal Dyskinesia (GEPD) B-Slide 18 American Epilepsy Society 2008

19 Ion Channel & Neurotransmitter Receptors Mutated in Epilepsy - IV N eurotransmitter Receptor Mutations GABRG2 (GABA-receptor gamma-2 subunit) GEFS+ type 3 GABRA1 (GABA-receptor alpha-1 subunit) JME CHRNA4 (nicotinic acetylcholine receptor alpha-4 subunit) Autosomal Dominant Nocturnal Frontal Lobe Epilepsy (ADNFLE) type 1 CHRNB2 (nicotinic acetylcholine receptor beta-2 subunit) ADNFLE type 3 B-Slide 19 American Epilepsy Society 2008

20 Synaptic Factors Modifying Neuronal Excitability  Alterations in expression of transmitter gated ionotropic channels  Post-translational changes in neurotransmitter channels  Remodeling of synapse location or configuration (deafferentation, sprouting)  Changes in gap-junction synaptic function B-Slide 20 American Epilepsy Society 2008

21 Non-synaptic (Extrinsic) Factors Modifying Neuronal Excitability  Changes in extracellular ion concentration  Changes in extracellular space  Modulation of transmitter metabolism or uptake by glial cells B-Slide 21 American Epilepsy Society 2008

22 B-Slide 22 Mechanisms of Generating Hyperexcitable Networks  Excitatory axonal “sprouting”  Loss of inhibitory neurons  Loss of excitatory neurons “driving” inhibitory neurons  Change in neuronal firing properties (channelopathies) American Epilepsy Society 2008

23 Timm Stain Showing Mossy Fiber Sprouting B-Slide 23 Normal Rat Dentate Gyrus Epileptic Rat Dentate Gyrus Epileptic Human Dentate Gyrus Cavazos and Cross, 2006 Timm stain (black) for mossy fiber terminals containing zinc American Epilepsy Society 2008

24 Hippocampal Circuit Changes With Hippocampal Sclerosis B-Slide 24 Chang and Lowenstein, 2003 American Epilepsy Society 2008

25 Epileptogenesis B-Slide 25 Cavazos and Cross, 2006 American Epilepsy Society 2008

26 Electroencephalogram (EEG)  Graphical depiction of cortical electrical activity, usually recorded from the scalp.  Advantage of high temporal resolution but poor spatial resolution of cortical disorders.  EEG is the most important neurophysiological study for the diagnosis, prognosis, and treatment of epilepsy. B-Slide 26 American Epilepsy Society 2008

27 10/20 System of EEG Electrode Placement B-Slide 27 American Epilepsy Society 2008

28 Physiological Basis of the EEG B-Slide 28  Extracellular dipole generated by excitatory post-synaptic potential at apical dendrite of pyramidal cell American Epilepsy Society 2008

29 Physiological Basis of the EEG (cont.) B-Slide 29  Electrical field generated by similarly oriented pyramidal cells in cortex (layer 5) and detected by scalp electrode American Epilepsy Society 2008

30 Electroencephalogram (EEG)  Clinical applications Seizures/epilepsy Sleep Altered consciousness Focal and diffuse disturbances in cerebral functioning B-Slide 30 American Epilepsy Society 2008

31 EEG Frequencies  Gamma: Hz  Beta: Hz  Alpha: 8 to ≤ 13 Hz  Theta: 4 to under 8 Hz  Delta: < 4 Hz B-Slide 31 American Epilepsy Society 2008

32 B-Slide 32 EEG Frequencies Radtke, in Ebersole and Pedley, 2003 American Epilepsy Society 2008

33 B-Slide 33 EEG Frequencies Radtke, in Ebersole and Pedley, 2003 American Epilepsy Society 2008

34 EEG Abnormalities  Background activity abnormalities Slowing not consistent with behavioral state May be focal, lateralized, or generalized Significant asymmetry  Transient abnormalities / Discharges Spikes Sharp waves Spike and slow wave complexes May be focal, lateralized, or generalized B-Slide 34 American Epilepsy Society 2008

35 Focal seizure generation Seizure initiation burst of action potentials, i.e. paroxysmal depolarizing shift hypersynchronization of neighboring cells Propagation activation of nearby neurons loss of surrounding inhibition B-Slide 35 American Epilepsy Society 2008

36 Sharp Waves  An example of a left temporal lobe sharp wave (arrow) B-Slide 36 American Epilepsy Society 2008

37 The “Interictal Spike and Paroxysmal Depolarization Shift” B-Slide 37 Intracellular and extracellular events of the paroxysmal depolarizing shift underlying the interictal epileptiform spike detected by surface EEG Ayala et al., 1973 American Epilepsy Society 2008

38 Generalized Spike Wave Discharge B-Slide 38 American Epilepsy Society 2008

39 EEG: Absence Seizure B-Slide 39 American Epilepsy Society 2008

40 Circuitry Underlying Generalized Epilepsies B-Slide 40 McCormick and Contreras, 2001 American Epilepsy Society 2008

41 Causes of Acquired Epilepsy Severe head injury Cerebral hemorrhage Brain tumor CNS infection ? Early life febrile seizures B-Slide 41 American Epilepsy Society 2008

42 Development of acquired epilepsy B-Slide 42 American Epilepsy Society 2008

43 Development of acquired epilepsy B-Slide 43 American Epilepsy Society 2008

44 Possible Mechanism of Delayed Epileptogenesis  Kindling model: repeated subconvulsive stimuli resulting in electrical afterdischarges Eventually lead to stimulation-induced clinical seizures In some cases, lead to spontaneous seizures (epilepsy) Applicability to human epilepsy uncertain B-Slide 44 American Epilepsy Society 2008


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