Abstracts

RATIONALE: The hyperpolarization-activated current (Ih) is a voltage-gated non selective cation conductance activated by membrane hyperpolarization. It is well known that Ih channel is predominantly distributed in the apical dendrites of CA1 pyramidal cells in the rats and mice (Tsubokawa et al., J. Physiol., 517(1): 135-142, 1999), but it remains that the functional effect of Ih is still unclear. The present study was designed to reveal whether Ih at the apical dendrites of hippocampal CA1 pyramidal cells could contribute to the seizure generation or not.
METHODS: Thirty two 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 electrodes was implanted into each side of the hippocampal CA1 region and used for recording hippocampal EEG. One of them (anterior right electrode) was settled as the stimulating and injecting electrode, and its tip was manipulated to locate at the apical dendritic layer in accordance with the [theta] wave profile. Its tube was connected to the microsyringe filled with the various concentration of cesium chloride (CsCl: an Ih blocker) and/or 0.9% saline as a control. The injection was made with a speed of 50 [mu]l/min for 1 min. The stimulus train for paroxysmal discharge (PAD) was 1 ms pulses of 50 Hz for 1 s.
RESULTS: Analyses were done in the only case that the tip of the injecting electrode was histologically identified at the apical dendritic layer (from the stratum radiatum to the lacunosum-moleculare) of CA1 region. In the control group (n=7), the PAD threshold was measured before and after injecting saline: they were 183[plusminus]8 (mean[plusminus]S.E.) [mu]A and 190[plusminus]8 [mu]A, respectively. On the other hand, 1mM CsCl injection (n=5) could increase the threshold (146[plusminus]8 [mu]A vs 222[plusminus]9 [mu]A), and the significant increments (P[lt]0.01) were obtained both in 10mM (n=6: 132[plusminus]6 [mu]A vs 318[plusminus]10 [mu]A) and 100mM (n=7: 167[plusminus]7 [mu]A vs 316[plusminus]9 [mu]A) CsCl injecting groups. We also observed that the increment of PAD threshold had the time-dependency, which was approximately in proportion to the CsCl concentration.
CONCLUSIONS: Since CsCl injection into the apical dendritic layer where Ih channels were predominantly distributed could increase PAD threshold, it was concluded that Ih might play an important role in the hippocampal seizure generation.
Support: This work is supported by GASR(C) of The Ministry of ESSC, Japan.