2）海馬における長期増強

Long-term potentiation in hippocampus.

I．はじめに

　海馬の興奮性入力線維束を短時間（約1秒間），高頻度で刺激して，その線維束につながるシナプスを強く活動させると，その後そのシナプスの伝達が長時間（数週間）にわたって促進した状態を保つ。この現象は長期増強とよばれ，海馬体の記憶形成機能の基礎過程とみなされている。長期増強は脳の可塑性を急性実験において調べうる数少ないモデルとして，多くの研究室において徹底的に調べられている。これまでの知見について既に多くの総説があるので^10,14），ここでは主に最近のシナプス前起源説の展開について述べる。

Long-term potentiation (LTP) in the hippocampus has been regarded as a neuronal substrate of memory and learning. Although several lines of evidence suggest that LTP of commissural-associational fiber synapses is of postsynaptic origin, recent statistical treatment of facilitated synaptic events pro-vides evidence for a presynaptic hypothesis. We have shown (1) that the statistical evidence for the presynaptic hypothesis obtained by the whole-cell clump method was not due to technical artifact caused by dialysis of postsynaptic neurons, (2) that the possibility is excluded that LTP of the pre-synaptic origin at the first phase is gradually replaced by LTP of the postsynaptic origin in later phase, (3) LTP of mossy fiber synapses is alsc of presynaptic origin. For definitive conclusions, however, a rigorous quantal analysis of synaptic events before and after LTP is necessary.