Abstracts

Rationale: Preventing the expression of acquired epilepsy (AE) following a neuronal injury (anti-epileptogenesis) is a major goal in treating epilepsy. Alterations in neuronal calcium levels ([Ca2+]i)following status epilepticus (SE) and subsequent impaired Ca2+ homeostasis have been implicated in models of AE. Recently we demonstrated that carisbamate is neuroprotective and antiepileptogenic in an in vitro model of AE. However, whether the antiepileptogenic effect of carisbamate was mediated via its effects on Ca2+ plateau is unknown.Methods: SE was induced into hippocampal neuronal cultures (HNC) by exposing them to a solution containing no added MgCl2. For inducing AE, SE was continued for 3-h at the end of which, cultures were restored to 1 mM MgCl2 solution. Calcium transients from vehicle and carisbamate (100 ?M) treated HNCs were evaluated using fluorescent Ca2+ indicator Fura-2 AM. Ratio images were acquired using alternating 340/380 nm excitation wavelengths. Results: Sham-control neurons displayed [Ca2+]i ratios of 0.32 0.04, whereas low Mg2+ treated neurons displayed ratio values of 0.8 0.02 at the end of 3-h period. Treatment with carisbamate lowered [Ca2+]i ratio to baseline (0.35 0.03). An analysis of the decay in Ca2+ ratio revealed that by 5 min post-treatment, carisbamate-treated cells exhibited ratios that were approx. 50% of the peak ratio compared to an approx. 90% of the peak ratio observed in sham-treated neurons at the end of in vitro SE. In HNCs exposed to carisbamate after the low Mg2+ treatment, [Ca2+]i returned to the baseline observed in the sham controls within 30 min. Twenty-four hours after the SE, HNCs exhibited spontaneous recurrent seizures- an in vitro correlate of AE. Sham-control neurons displayed [Ca2+]i ratios of 0.30 0.03, whereas AE neurons displayed ratio values of 0.46 0.04. Treatment with carisbamate lowered [Ca2+]i ratio to near control levels (0.32 0.02). AE neurons are characterized by alterations in Ca2+ homeostasis which can be studied by the ability of these neurons to buffer a given load of Ca2+. When challenged with 50 ?M glutamate for 1 min, sham-control neurons were able to attain pre-glutamate levels in approx. 10-12 min. AE neurons did not reach baseline even after 40 min of post-glutamate exposure. In carisbamate treated AE neurons pre-glutamate levels were attained in approx. 15-min comparable to sham-control neurons. Conclusions: Elevations in [Ca2+]i ratios occurred immediately following SE and remained elevated during AE. This Ca2+ plateau underlies the development and expression of AE (epileptogenesis). Carisbamate lowered the [Ca2+]i ratio to baseline following in vitro SE. Since carisbamate prevented the development of the Ca2+ plateau following SE, this mechanism could underlie its antiepileptogenic action. In addition, the ability of carisbamate to lower elevated basal Ca2+ in AE neurons could contribute towards its antiepileptic effect. Our study proposes a novel mechanism of action for carisbamate and suggests that development of drugs targeting the Ca2+ plateau could provide potent antiepileptogenic agents.