Study on long-term potentiation of brain hippocampus

1973年首次发现的突触传递长时程增强被认为是人类学习记忆的主要模型。然而，其机制尚未阐明，尽管该现象的分子事件被认为是基础。有证据表明，兴奋突触后细胞的N-甲基-D-天冬氨酸（NMDA）谷氨酸受体可引起海马CA 1区的长时程增强，进而引起Ca 2+内流增加。细胞内Ca^2+浓度的升高导致Ca^2+依赖性蛋白激酶的激活。在Ca^2+依赖性酶中，CaM激酶II特别令人感兴趣，这有两个主要原因，1）它是突触后密度的主要成分，2）它是一种多聚体全酶，由几个催化亚基组成，其可在被钙和钙调蛋白激活后自磷酸化，从而在延长的时间段内保持活性。在已被提出的分子机制，有助于在海马的长时程增强的激活和自磷酸化的钙调蛋白激酶II。高频率而非低频率刺激两组CA 1传入纤维，可使该酶的非Ca^<2+>依赖性活性和总活性持续增加，并使非Ca^<2+>依赖性活性占总活性的比例增加。使用两种不同的CaM激酶II底物获得的效果，它被观察到在海马切片和海马器官型文化，它可以被阻断预孵育的切片与NMDA受体拮抗剂D-2-氨基-5-phosphonopentanoate。用calyculin A（一种磷酸酶抑制剂）处理切片，改变了酶的活性，但仍能诱导长时程增强，并且仍能观察到非Ca^2+依赖性CaM激酶II活性的进一步增加。

Long-term potentiation of synaptic transmission which was first discovered in 1973 is now considered to be a primary model of human learning and memory. However, its mechanism has not been elucidated, although the molecular events of the phenomenon are supposed to underlie. Several lines of evidence indicated that the stimulation of the N-methyl-D-aspartate (NMDA) glutamate receptor in the postsynaptic cell triggers the long-term potentiation in CAl area of hippocampus and in turn causes the increase in Ca^<2+> influx. The elevation of the intracellular Ca^<2+> concentration results in activation of Ca^<2+>-dependent protein kinases.Among Ca^<2+>-dependent enzymes, CaM kinase II is of particular interest for two major reasons, 1) it is a major component of postsynaptic densities and 2) it is a multimeric holoenzyme, composed of several catalytic subunits, which may be autophosphorylated following activation by calcium and calmodulin and thereby remain active for a prolonged period of time. Among the molecular mechanisms that have been proposed to contribute to long-term potentiation in hippocampus are the activation and autophosphorylation of CaM kinase II. High, but not low frequency stimulation applied to two groups of CAl afferents resulted in a long lasting increase in the Ca^<2+>-independent and total activities of the enzyme as well as an increase in the ratio of Ca^<2+>-independent to total activity. The effect was obtained using two different CaM kinase II substrates, it was observed in hippocampal slices and in hippocampal organotypic cultures, and it could be blocked by preincubation of slices with the NMDA receptor antagonist D-2-amino-5-phosphonopentanoate. Treatment of slices with calyculin A, a phosphatase inhibitor, modified the activity of the enzyme, but long term potentiation could still be induced and a further increase in Ca^<2+>-independent CaM kinase II activity still observed.