The Effects of Topiramate on Epileptiform Discharges Induced by Mg(2+)- Free Medium and 4-Aminopyridine in Hippocampal Slices of Immature Rats

Park, So Hyun; Yeon, Yeong Heum; Lee, In Goo; Choi, Byung Joon; Kim, Young Hoon; Chung, Seung Yun; Whang, Kyung Tai

Abstract

PURPOSE: Topiramate(TPM), one of the newest antiepileptic drugs, has been prescribed not only to refractory partial seizures but to generalized tonic-clonic seizures. However, its action mechanisms are not well understood and the optimal dose of antiepileptic efficacy in animal seizure models is not determined yet. In order to elucidate the action mechanisms and the optimal concentration of TPM that suppresses epileptic discharges, we observed ictal and interictal discharges from immature rat hippocampal slices in Mg(2+)-free, and 4-aminopyridine(AP) added artificial CSF with various TPM concentrations.
METHODS
We divided Sprague-Dawley rats of 19 to 23 days old into 5 groups; namely, a control group(n=12) and 4 TPM groups according to the concentration of TPM, 6 (n=11), 20(n=7), 60(n=10), and 200(n=14) micrometer. The rats were anesthetized and their brains were taken, and soaked in artificial CSF(NaCl 125 mM; KCl 2.5 mM; NaH2PO4 2 mM; MgSO4 1.25 mM; NaHCO3 25 mM; CaCl2 2 mM, Glucose 10 mM, and pH 7.3-7.4). Then the brains were cut into 400 micrometer hippocampal slices by a vibratome. The slices of the control group were soaked in 200 micrometer 4-AP added Mg(2+)-free medium for 1 hour, and then extracellular recordings were performed in the hippocampal CA1 pyramidal region. The slices of TPM groups were soaked in solutions containing 6, 20, 60, 200 micrometer TPM, and then extracellular recordings were performed.
RESULTS
Interictal discharges were observed in the control group and 6, 20 micrometer groups but the frequency decreased as the concentration of TPM increased:90% in 60 micrometer group, and 35.7% in 200 micrometer group. And the amplitude of TPM groups was much smaller than that of the control group. The latency to the first interictal discharge after 4-AP addition was 52.7+/-7.5 sec in the control group, 290.2+/-78 sec in 60 micrometer group, and 568+/-113.1 sec in 200 micrometer group. Duration of the interictal discharge was 64.6+/-10.3 sec in the control group, but was prolonged to 141+/-38.1 sec in 60 micrometer group(P<0.05). Ictal discharges were observed in all of the control and 6 micrometer groups, but the frequency decreased as the concentration of TPM increased:55.6% in 60 micrometer, and 28.6% in 200 micrometer groups. The amplitude of the TPM groups was much smaller than that of the control group. The latency to ictal discharges after 4-AP addition was 141+/-15.2 sec in the control group, but increased as the concentration of TPM increased:431.8+/-57.4 sec in 60 micrometer, and 627.8+/-143.5 sec in 200 micrometer group(P<0.05). The duration of ictal discharges was 1,534.7+/-97.9 sec in the control group, but decreased as the concentration of TPM increased, the shortest in 60 micrometer group, 155.2+/-65.5 sec(P<0.05). Status epilepticus was seen in 58.3% of the control and 27.2% of 6 microM groups.
CONCLUSION
TPM suppresses the frequency, latency, and duration of epileptiform discharges induced by Mg(2+)-free, and 4-AP added artificial CSF in immature rat hippocampal slices, starting from 20 micrometer and reaching the maximal effect at over 60 micrometereter. This finding is presumably due to TPM enhancing of GABA receptor currents and/or K+ channel conductance in response to TPM.