Abstract
Anti-Epileptogenesis: Electrophysiology, Diffusion Tensor Imaging and Behavior in a Genetic Absence Model
van Luijtelaar G, Mishra AM, Edelbroek P, Coman D, Frankenmolen N, Schaapsmeerders P, Covolato G, Danielson N, Niermann H, Janeczko K, Kiemeneij A, Burinov J, Bashyal C, Coquillette M, Lüttjohann A, Hyder F, Blumenfeld H, van Rijn CM. Neurobiol Dis 2013;60:126–138.
The beneficial effects of chronic and early pharmacological treatment with ethosuximide on epileptogenesis were studied in a genetic absence epilepsy model comorbid for depression. It was also investigated whether there is a critical treatment period and treatment length. Cortical excitability in the form of electrical evoked potentials, but also to cortico-thalamo-cortical network activity (spike-wave discharges, SWD and afterdischarges), white matter changes representing extra cortico-thalamic functions and depressive-like behavior were investigated. WAG/Rij rats received either ethosuximide for 2 months (post natal months 2–3 or 4–5), or ethosuximide for 4 months (2–5) in their drinking water, while control rats drank plain water. EEG measurements were made during treatment, and 6 days and 2 months post treatment. Behavioral test were also done 6 days post treatment. DTI was performed ex vivo post treatment. SWD were suppressed during treatment, and 6 days and 2 months post treatment in the 4 month treated group, as well as the duration of AD elicited by cortical electrical stimulation 6 days post treatment. Increased fractional anisotropy in corpus callosum and internal capsule on DTI was found, an increased P8 evoked potential amplitude and a decreased immobility in the forced swim test. Shorter treatments with ETX had no large effects on any parameter. Chronic ETX has widespread effects not only within but also outside the circuitry in which SWD are initiated and generated, including preventing epileptogenesis and reducing depressive-like symptoms. The treatment of patients before symptom onset might prevent many of the adverse consequences of chronic epilepsy.
Commentary
If there is any “sure thing” in epilepsy, the syndrome of childhood absence epilepsy (CAE) may come close to fitting the bill. The characteristic seizure semiology (staring spells), response to specific medications (ethosuximide or valproate), and prognosis (often outgrown by late adolescence) are about as close as we get to a well-defined syndrome with reproducible clinical features and response to specific treatment. CAE is a subtype of generalized genetic epilepsy. The underlying pathophysiology of CAE involves abnormal thalamocortical firing as a result of aberrant T-type calcium channels in thalamocortical and thalamic reticular neurons, giving rise to the responsiveness of absence seizures to the T-type calcium channel blocker, ethosuximide. However, emerging information suggests that many of these presumptions are overly simplified. For example, children with absence epilepsy often express comorbidities including cognitive and psychiatric impairments such as depression and attentional difficulties, albeit less severe than those present in most epilepsies (1). It is not known why seizures begin at a certain age. Despite the purportedly specific action of ethosuximide as a blocker of the T-type calcium channel in thalamic neurons, this drug does not always work. Finally, brain regions outside the thalamocortical axis likely also participate in abnormal rhythm generation and some microscopic structural changes have been documented in the brains of patients with CAE (2, 3).
Animal models of absence epilepsy may be helpful in sorting out some of these uncertainties. Several animal models recapitulate salient clinical and electrophysiological aspects of absence epilepsy, one of the most prominent being the WAG/Rij rat (Wistar Albino Glaxo rats from Rijswijk). The WAG/Rij rat strain has been studied in detail by investigators in the Netherlands (4) and elsewhere (5). This rat model has provided many insights into generalized epilepsy of the absence type. WAG/Rij rats exhibit age-related onset of spike-wave discharges (SWD) (7–8 Hz) that correlate with absence-like clinical spells and behavioral arrest. The SWD develop in an age-dependent fashion, emerging around 3 months of age and becoming fully established by 5 to 6 months of age when virtually all rats express SWD. However, unlike many children with CAE, WAG/Rij rats have persistent SWD and absence seizures throughout their adult life span. Previous studies have shown that seizures and SWD in WAG/Rij rats respond briskly to treatment with ethosuximide at a dose that blocks SWD but does not cause toxic effects; SWD were not decreased by drugs that are ineffective in CAE (5, 6). Of importance, SWD reduction persists after discontinuation of ethosuximide, suggesting that the drug is associated with an antiepileptogenic effect (5). Therefore, pretreatment at an age prior to SWD/seizure onset was hypothesized to avert this epileptogenic process.
In the present study, the authors use WAG/Rij rats to investigate the requisite timing and duration of early, chronic treatment with ethosuximide to alter epileptogenesis—the development of spike-wave discharges on EEG, clinical absence seizures, and depression, a well-recognized comorbidity in this model. They found that treating WAG/Rij rats with daily ethosuximide in their drinking water ameliorated SWD occurrence, but only when administered for the 4-month treatment period spanning 2 and 5 months of age. There was no reduction of SWD when ethosuximide was given for a shorter 2-month treatment period (i.e., from 2 to 3 months of age or from 4 to 5 months of age). Ethosuximide-treated rats had reduced cortical excitability as evidenced by briefer and fewer afterdischarges in response to cortical stimulation and more pronounced inhibitory components in electrical evoked potential recordings. Using diffusion tensor imaging, white matter changes were also found in ethosuximide-treated rats, which demonstrated increased fractional anisotropy in corpus callosum and the internal capsule, suggesting that drug treatment is associated with improved structural integrity in critical pathways. The same white matter pathways were shown to have decreased fractional anisotropy in untreated WAG/Rij rats (7).
Finally, ethosuximide treatment was shown to alleviate some of the signs of depression in WAG/Rij rats, a finding that deserves further comment. Depression is a frequent comorbidity of many epilepsy syndromes (8, 9), including absence epilepsy (10). The bidirectional occurrence of these two disorders—epilepsy and depression—suggests that shared pathophysiological mechanisms may exist (11). WAG/Rij rats develop depressive symptoms along with their absence seizures, as demonstrated on a well-validated experimental measure of depression—the forced swim test (FST). In the FST, rats are placed in a tub of water from which they cannot escape. Animals typically display two phases of behavior. In the first phase, the rat swims frantically around the tub but cannot escape. In the second phase, the rat seems to give up and becomes immobile, floating on the water with just enough movement to keep its head above water. The latency to the immobile phase is considered to be a measure of despair (a requisite component of depression). WAG/Rij rats display depression-like behavior with increased immobility in the FST (12). In the present study, WAG/Rij rats treated with ethosuximide had significantly lower immobility scores than vehicle-treated controls, suggesting that prevention of epileptogenesis is associated with reduction in comorbid depression. In that regard, this epilepsy model can serve to study the development, and potential avoidance, of concurrent depression.
These findings further validate the WAG/Rij rat as a model that shares many features of human absence epilepsy, now adding the ability to prevent the occurrence of the seizures and epilepsy comorbidities. Caution should be heeded, though, with respect to the authors’ suggestion that treating CAE patients before seizure onset might prevent the consequences of chronic epilepsy. While reasonable in principle, it is not feasible clinically to predict which children will develop CAE (even within an at-risk family), let alone treat them prophylactically. That being said, the WAG/Rij model remains poised to address several outstanding questions, such as the mechanisms that determine the critical period and discovery of specific treatments of comorbidities.