大脳皮質のDendritic PotentialとAfterdischargeに対するGABAの作用

Effect of GABA upon the dendritic potential and the afterdischarge

　大脳皮質を直接刺激して得られる所謂dendritic potential（DP）はdendrite膜の活動電位^1）或いはシナップス電位^2）などといわれ未だ定説がない。我々^3）はDPの基本的波形である持続5〜10msの陰性電位についていろんな刺激条件（パルス幅，電位，電極配置）のもとに実験を行つてきた。更に継時刺激の実験^4）からかかるDPは一定の不応期をもつSpike様電位（Spike）と加重性を有する緩電位（SP）とから成つており，前者の刺激閾値は後者にくらべて高い。勿論SPのみは単独に記録することができるが，刺激を少しずつ強くするとSpikingを起して遂にはSpikeが得られる。かかる実験と嗅球皮質^5）の直接刺激で得られた結果からSpikeは表層に密に存在しているapical dendrite膜の活動電位であると云える。併しSPについては加重性を示す電位であると云う以外は発生部位についても分つていない。併しながらChangのrecovery process，或いはGrund festの不完全加重，Bishopの完全加重はSpikeとSPの基本的性質を夫々示すもので，DPに関する従来の諸説を統一的に解釈することが出来た。

　今回はかかるSpikeとSPに対するGABAの作用及び皮質の単一刺激によるAfterdischarge（AD）とrhythmic waveの出現について興味ある所見を得たので報告する。

　The effect of topical application of GABA (0.5~2%) to the cortical surface was tested upon the dendritic potential in the urethane-anesthetized rabbit.

　(1) GABA depressed completely the surface negative spike-like potential of apical dendrites leaving only a late positivity.

　But, as the later effect of GABA, there was a marked augmentation and prolongation of the spike-like potential.

　However, Such an enhancement subsequent to application of GABA has been accounted for by a change in the summating slow potential and also the Chang's second component.

　On the basis of threshold differences, manners of temporal and spatial summations, and also pharmacological selectivity, the spikelike potential (5~10ms) is assignable to the electrical activity in apical dendrites. However, the summating slow potential (10~20ms ) is an independent excitation process separate from that of spike-like potential.

　(2) Repetitive afterdischarge of 5~10c/s which follows after single shock on cortical surface was increased in amplitude after the application of GABA, and the polarity of the afterdischarge was always reversed to surface positive spike, whereas the normal afterdischarge was not necessarily so.

　Following the afterdischarge, rhythmic slow waves of 2~5c/s was recorded, and they appeared generally coincident with the augmentation of dendritic potential. Afterdischarge were not disappeared by the subcortical cutting, but the rhythmic slow waves were abolished compeletely.

　From these experimental facts, it is suggested that the afterdischarges are resulted from a selfre-exciting mechanism involving numerous neurons in the cortex which form short neuronal circuits. But rhythmic waves represent the activity of the corticothalamic reverberating circuit.

　GABA has been considered to be an "inhibitory" or a "depressant" factor of cortical activity. However, our results show that GABA does not seem to be a general depressant of neuronal activities.