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Summary: Purpose: Potential antiepileptic drugs (AEDs) are typically screened on acute seizures in normal animals, such as those induced in the maximal electroshock and pentylenetet-razole models. As a proof-of-principle test, the present experiments used spontaneous epileptic seizures in kainate-treated rats to examine the efficacy of topiramate (TPM) with a repeated-measures, crossover protocol.
Methods: Kainic acid was administered in repeated low doses (5 mg/kg) every hour until each Sprague–Dawley rat experienced convulsive status epilepticus for >3 h. Six 1-month trials (n = 6–10 rats) assessed the effects of 0.3–100 mg/kg TPM on spontaneous seizures. Each trial involved six pairs of TPM and saline-control treatments administered as intraperitoneal injections on alternate days with a recovery day between each treatment day. Data analysis included a log transformation to compensate for the asymmetric distribution of values and the heterogeneous variances, which appeared to arise from clustering of seizures.
Results: A significant effect of TPM was observed for 12 h (i.e., two 6-h periods) after a 30-mg/kg injection, and full recovery from the drug effect was complete within 43 h. TPM exerted a significant effect at doses of 10, 30, and 100 mg/kg, and the effects of TPM (0.3–100 mg/kg) were dose dependent.
Conclusions: These data suggest that animal models with spontaneous seizures, such as kainate- and pilocarpine-treated rats, can be used efficiently for rapid testing of AEDs with a repeated-measures, crossover protocol. Furthermore, the results indicate that this design allows both dose–effect and time-course-of-recovery studies.
Traditional antiepileptic drug (AED) testing has used acute-seizure models based on chemical and electrical induction of seizures in otherwise normal animals. Efficacious AEDs may be ineffective in these models (1). Hypothetically, new AEDs that would be effective in pharmacoresistant epilepsy may be discovered by testing them in animal models with epileptic seizures, and these new AEDs may be ineffective in the acute-seizure models (1–3). Relatively little research has been conducted studying the effects of AEDs on spontaneous seizures in animals with injury-induced epilepsy. If epileptogenesis involves new mechanisms not present in the normal brain (e.g., altered receptor subunits or new circuits), then traditional AED testing in acute-seizure models may not identify effective versus ineffective drugs, because they are being tested on animals whose brains have not undergone the epileptogenic changes.
The National Institutes of Health (NIH)-sponsored “Models II Workshop” recommended that potential AEDs be tested on animals with chronic epilepsy (4). Although imperfect, these animals aim to “model” the condition of temporal lobe epilepsy. What is epileptogenic in these animal models and how these alterations may apply to human temporal lobe epilepsy is unknown. It has been hypothesized that the use of animals that have experienced epileptogenesis (and any changes that occur during this process) will more effectively detect new drugs (3,5–8). Chronic epilepsy models may be better able to predict the clinical success of experimental drugs because they produce spontaneous seizures, a chronic epileptic state, and histopathologic alterations qualitatively similar to the mesial temporal sclerosis observed in human temporal lobe epilepsy (9). Our laboratory recently generated evidence that chronic epilepsy models with spontaneous seizures, such as the kainate- and pilocarpine-induced epilepsy models, can be used to test the efficacy of AEDs (10). The present study in chronically epileptic rats attempted to develop an improved paradigm for testing AEDs that not only would provide dose–effect data, but also would allow time-course-of-recovery analyses.
Topiramate (TPM) is a broad-spectrum AED with multiple proposed uses and mechanisms of drug action (11–16). Possible mechanisms include antagonism of α-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)/kainate-type glutamate receptor–mediated inward currents (8,17), attenuation of voltage-dependent Na+ channels (18), negative modulation of L-type Ca2+ channels (19), augmentation of γ-aminobutyric acid (GABA)A receptor-mediated Cl− currents (20), activation of K+ conductance (21), and inhibition of carbonic anhydrase (22). The diverse mechanisms of drug action exhibited by TPM allow a variety of clinical uses of the drug. In these studies, we conducted a proof-of-principle experiment to determine if a repeated-measures, crossover protocol could be used to perform both dose–effect and time-course-of-recovery analyses for intraperitoneal injections of TPM.
The experimental design in our previous study in rats with pilocarpine-induced epilepsy proved useful for comparing the effects of different AEDs (10). The important conceptual and practical problem in this previous study, however, was that the three drugs—1-(3-trifluoromethylphenyl) piperazine (TFMPP), phenobarbital (PB), and fluoxetine—each affected the frequency of spontaneous seizures for substantially different periods at the doses tested (i.e., TFMPP for ∼6 h, PB for slightly less than a day, and fluoxetine for substantially more than a day). In the present study, we have designed a new protocol that would also enable us to evaluate the duration of the anticonvulsant effect. We used a different but similar chemoconvulsant model of chronic epilepsy (i.e., the kainate-treated rat). A repeated-measures, crossover protocol allows each animal to be used as its own control, in spite of differences in baseline seizure frequency. In the present study, single AED injections were alternated with single vehicle-control injections, compared with the previous study in which the animals received each treatment for 5 consecutive days (10). In addition to being able to determine the dose–effect relations, we also were able to estimate recovery from the drug effect.