Zonisamide (ZNS) is an antiepileptic drug developed in Japan. Various experimental studies have investigated the effects of ZNS. However, the mechanism of action of ZNS against limbic seizures and secondary generalization is not well-known. We studied ictal regional accumulation of ZNS in the rat brain during kainic acid (KA)-induced limbic status epilepticus.

Fourteen male Wistar rats underwent a stereotactic operation. For recording the electroencephalogram (EEG), electrodes were placed in the left amygdala (LA), left dorsal hippocampus, and over the left sensorimotor cortex. For microinjection, a stainless steel cannula was also inserted into the LA. Seven days after surgery, rats were anesthetized and a catheter was inserted into the femoral vein. The animals were immobilized and allowed to recover from anesthesia for at least two hours. In eight rats, 1.0μL (1.0μg) of KA was injected into the LA, and 1.0 μL of phosphate buffer solution was injected into the LA in six control rats. Sixty minutes after injection, 14C-ZNS was administered intravenously, and an autoradiographic study was done.

During limbic status epilepticus, only seizures in the sensorimotor cortex were markedly attenuated a few minutes after 14C-ZNS administration. Additionally, high uptake of 14C-ZNS was noted ipsilaterally in the sensorimotor cortex, parietal cortex and thalamus (lateral portion). In control rats, no EEG change was seen, and distribution of 14C-ZNS was rather homogeneous.

These results suggested that ZNS suppresses secondary generalization of limbic seizures by a direct effect on the cerebral cortex.