Neuromodulation in Experimental Animal Models of Epilepsy Stefanie Dedeurwaerdere Promoter: Prof. Dr. Paul Boon Co-promoter: Prof. Dr. Walter Verraes Ghent University Neuromodulation in Experimental Animal Models of Epilepsy Stefanie Dedeurwaerdere Thesis submitted to fulfill the requirements for the degree of Doctor in Medical Sciences Promoter: Prof. Dr. Paul Boon Co-promoter: Prof. Dr. Walter Verraes Laboratory for Clinical and Experimental Neurophysiology, Department of Neurology Nothing in life is to be feared It is only to be understood Marie Curie Table of contents Chapter 1: General introduction 5 Chapter 2: General methodology 23 Chapter 3: Efficacy of levetiracetam 43 Dedeurwaerdere,S., Boon,P., De Smedt,T., Claeys,P., Raedt,R., Bosman,T., Van Hese,P., Van Maele,G. and Vonck,K. (2005) Chronic levetiracetam treatment early in life decreases epileptiform events in GAERS, but does not prevent the expression of spike and wave discharges during adulthood. Seizure 14(6), 403-411. Chapter 4: Efficacy of vagus nerve stimulation 61 Dedeurwaerdere,S., Vonck,K., Claeys,P., Van Hese,P., D'Have,M., Grisar,T., Naritoku,D. and Boon,P. (2004). Acute vagus nerve stimulation does not suppress spike and wave discharges in "Genetic Absence Epilepsy Rats from Strasbourg". Epilepsy Res 59, 191-198. Dedeurwaerdere,S., Vonck,K., Van Hese,P., Wadman,W., Boon,P. (2005) The acute and chronic effect of vagus nerve stimulation in "Genetic Absence Epilepsy Rats from Strasbourg" (GAERS). Epilepsia 46(Suppl 5), 94-97. Dedeurwaerdere,S., Gilby,K., Vonck,K., Delbeke,J., Boon,P. and McIntyre,D.C. Vagus nerve stimulation does affect memory in Fast rats, but has both pro-convulsive and anti-convulsive effects on amygdala kindled seizures. Submitted. Chapter 5: Mechanism of action of vagus nerve stimulation 103 Vonck,K., Van Laere,K., Dedeurwaerdere,S., Caemaert,J., De Reuck,J. and Boon,P. (2001) The mechanism of action of vagus nerve stimulation for refractory epilepsy: the current status. J Clin Neurophysiol 18, 394-401. Dedeurwaerdere,.S., Cornelissen,B., Van Laere,K., Vonck,K., Achten,E., Slegers,G. and Boon,P. Small animal positron emission tomography during vagus nerve stimulation in rats: a pilot study. Epilepsy Res In Press. Chapter 6: General discussion 131 Levetiracetam Vagus nerve stimulation Summary-Samenvatting-Résumé 175 Abbreviations 187 Dankwoord 198 Curriculum vitae 193 Chapter 1: General introduction General introduction Chapter 1 General introduction Epilepsy is a neurological disorder consisting of recurrent seizures, resulting from excessive, uncontrolled electrical activity in the brain. Despite the pharmacological development of new treatments, still one third of the epilepsy patients does not respond sufficiently to anti-epileptic drugs (AEDs). Those patients are called refractory patients. Hence, there is a constant impetus to search for other treatment strategies like epilepsy surgery, gamma knife surgery, vagus nerve stimulation and deep brain stimulation. Besides the ongoing research on the efficacy of anti-epileptic treatments in suppressing seizures (anti- seizure effect), we want to seek therapies that can lead to plastic changes in the epileptic network and in this way have a modulating effect. The impact of such therapies cannot be overlooked, because they may slow down processes underlying epilepsy, might prevent or even cure epilepsy. This thesis deals with the potential of neuromodulatory treatments for patients with refractory epilepsy. The treatments that are considered include the pharmacological therapy with levetiracetam and vagus nerve stimulation. The effect of these therapies was investigated in chronic models of epilepsy, which imitate human epilepsy. 1.1 A short introduction to epilepsy The word ‘epilepsy’ derives from the Greek verb έπιλαμβαυειυ (epilambanein), meaning ‘to be seized, to be overwhelmed by surprise’. Epilepsy is the most common serious brain disorder affecting 0.5-1% of the general population (Hauser, 1998), but still there is a lot of prejudice and misunderstanding about the disease. Epileptic seizures are characterized by a paroxysmal manifestation of highly synchronized abnormal neuronal activity of a part of the brain (partial) or the whole brain (generalized) resulting in various clinical symptoms which can manifest as motor, sensory, emotional or mixed phenomena possibly with alteration or loss of consciousness. Although 9% of the population experiences a seizure once in a lifetime, epilepsy is only diagnosed when seizures are recurring. The conversion from a normal neuronal network into a hyperexcitable epileptic network is called epileptogenesis, which consists of complex and dynamic processes. Several epilepsy syndromes exist, but they are all characterized by repetitive seizures. There are multiple causes for epilepsy and seizures can be classified according to their etiology into three categories: idiopathic (primary, without a known or suggested genetic origin), symptomatic (secondary, resulting from known origins e.g. tumors, lesions, infections, vascular causes) or cryptogenic (presumably symptomatic but currently of unknown specific etiology) (Engel and Pedley, 1999). Furthermore, the International League 7 General introduction Chapter 1 Against Epilepsy has classified epilepsies according to their seizures onset zone: focal (partial) or generalized. 1.2 Treatment of epilepsy Epilepsy treatment is indicated following two or more unprovoked epileptic seizures and is successful in the majority of the cases. Despite the pharmacological development of new treatments still, one third of the epilepsy patients do not respond sufficiently to AEDs and are called ‘refractory patients’ (Kwan and Brodie, 2000). This leads to a constant search for other treatment strategies such as epilepsy surgery, gamma knife surgery, ketogenic diet, deep brain stimulation, vagus nerve stimulation and transcranial magnetic stimulation but also the development of new AEDs with novel mechanisms of action. Pharmacological treatment is still the most important way to treat epilepsy and newly developed AEDs (e.g. levetiracetam) can provide seizure freedom in up to 7% of refractory patients (Fisher, 1993). AEDs are mainly administered as a long-term treatment to prevent the occurrence of seizures. During the last two or three decades, considerable advances have been made in our understanding of the mechanism of action (MOA) of AEDs (Table 1). This allowed a more rational approach to drug treatment. There are different ways by which AEDs can suppress epileptic seizures: i) decrease in neuronal excitation (glutamatergic system), ii) increase in neuronal inhibition (GABAergic system) or iii) by modulation of ion channels (modification of cellular excitability) (Bradford, 1995). The difficulty of treating epilepsy is that it can involve multifactorial causes and that the use of one approach is often insufficient. Table 1: Different AEDs with their mechanisms of action (Brodie and French, 2003). The original goal of pharmacological therapy for patients with epilepsy is to suppress seizures without side effects. In seizure free patients, seizures will mostly remain suppressed as long as the treatment continues, but may reoccur once therapy stops. Anti-epileptogenic 8 General introduction Chapter 1 treatment is therefore an attractive idea. However, for the rational development of anti- epileptogenic therapies there is need for a better understanding of epileptogenesis. Levetiracetam (LEV) is believed to belong to a novel class of AEDs having anti- epileptogenic properties and it was discovered by unconventional drug screening. LEV has a favorable pharmacokinetic profile with rapid absorption following oral administration, excellent bioavailability, quick achievement of steady-state concentrations, linear kinetics and a minimal plasma binding (Patsalos, 2000). The MOA of LEV differs from other AEDs and is as yet not fully elucidated. Epilepsy surgery is an invasive but often curative treatment option that aims at removing the ictal onset zone believed to be responsible for seizure occurrence (Figure 1). Depending on the underlying epilepsy syndrome, approximately 70% of the patients are rendered seizure free after resective epilepsy surgery (Engel, 1996). The risk-benefit-analysis for surgery must be individualized using a presurgical evaluation protocol and eventually a substantial number of patients have to be rejected (Boon et al., 1999). Figure 1: resection of epileptic tissue. A promising alternative is gamma knife surgery (GKS), which is based on the convergence of 201 gamma ray beams. Cortical structures can be targeted with GKS with a stereotaxic precision, without opening the skull and with a limited risk. A recent multicenter study demonstrates that GKS has positive outcomes in patients with epilepsy (Regis et al., 2004). The ketogenic diet is a high-fat, adequate protein, low carbohydrate diet that mimics the biochemical changes associated with starvation, which create ketosis. It appears to be a very effective treatment for epilepsy, particularly during childhood, but has many side effects. It must be prescribed thoughtfully, implemented carefully and monitored closely (Nordli, 2002). Different sites in the nervous system have been stimulated electrically in an attempt to treat seizures resulting in several types of neurostimulation (Figure 2). Deep brain stimulation (DBS) consists of continuous stimulation of specific deep brain regions at the seizure onset zone e.g. amygdala, hippocampus, thalamus