Abstracts

RATIONALE: We have demonstrated that a certain type of the hippocampal interneurons showed a long-lasting burst discharge prior to the initiation of the seizure discharge (Epilepsia, in preparation), and that the hippocampal pyramidal dendrites exhibited abnormal active properties such as dendritic-spiking and/or irregular activity during kindling (Kogure, Epilepsy Res., 1997). Recent studies also demonstrated that the hyperpolarization-dependent Na/Ca current occurred in the apical dendrites of the pyramidal cells (Tsubokawa et al., J. Physiol., 1999). To test a hypothesis that so-called H-current induced the seizure discharges, effects of cesium chloride which is a blocker to H-current on the electrically induced paroxysmal discharges (PAD) were examined in the rabbit hippocampal CA1 region. METHODS: Ten adult rabbits were used: They were anesthetized with pentobarbital sodium, immobilized with D-tubocurarine, and placed in a stereotaxic apparatus on artificial respiration. A pair of concentric injecting electrodes was introduced into each side of the hippocampal CA1 region and used for stimulating as well as recording hippocampal EEG. Each tube was connected to a microsyringe filled with 100 mM CsCl solution, while connected to it containing saline in control cases. The injection was made with a speed of 50 l/min for 1 min. The stimulus train for PAD was biphasic pulses (1 msec duration each) of 50 Hz for 1 sec. RESULTS: The electrode locations were identified in the stratum radiatum or lacunosum-moleculare of the CA1 region by the profile of hippocampal theta activity and by the histological result (a blue spot by ferricyanide reaction). In the control group (n=3), the threshold for PAD was measured before and after injecting saline: they were 112 b5 A and 107 b4 A, respectively. On the other hand, the threshold for PAD significantly (P<0.02) increased to a level of 263 b7 A after injecting CsCl (n=7), while remaining nearly the same level (115 b4 A) before the injection. CONCLUSIONS: We conclude that CsCl can increase the PAD threshold and also H-current plays an important role in the seizure generation. This work is supported by GASR(C) of The Ministry of ESSC, Japan.