Smith Mark A, Ellis-Davies Graham C R, Magee Jeffrey C
Neuroscience Center, LSUHSC, 2020 Gravier Street, New Orleans, LA 70112,USA.
J Physiol. 2003 Apr 1;548(Pt 1):245-58. doi: 10.1113/jphysiol.2002.036376. Epub 2003 Feb 21.
Schaffer collateral axons form excitatory synapses that are distributed across much of the dendritic arborization of hippocampal CA1 pyramidal neurons. Remarkably, AMPA-receptor-mediated miniature EPSP amplitudes at the soma are relatively independent of synapse location, despite widely different degrees of dendritic filtering. A progressive increase with distance in synaptic conductance is thought to produce this amplitude normalization. In this study we examined the mechanism(s) responsible for spatial scaling by making whole-cell recordings from the apical dendrites of CA1 pyramidal neurons. We found no evidence to suggest that there is any location dependence to the range of cleft glutamate concentrations found at Schaffer collateral synapses. Furthermore, we observed that release probability (Pr), paired-pulse facilitation and the size of the readily releasable vesicular pool are not dependent on synapse location. Thus, there do not appear to be any changes in the fundamental presynaptic properties of Schaffer collateral synapses that could account for distance-dependent scaling. On the other hand, two-photon uncaging of 4-methoxy-7-nitroindolinyl-caged L-glutamate onto isolated dendritic spines shows that the number of postsynaptic AMPA receptors per spine increases with distance from the soma. We conclude, therefore, that the main synaptic mechanism involved in the production of distance-dependent scaling of Schaffer collateral synapses is an elevated postsynaptic AMPA receptor density.
谢弗侧支轴突形成兴奋性突触，这些突触分布在海马CA1锥体神经元树突分支的大部分区域。值得注意的是，尽管树突滤波程度差异很大，但胞体处由AMPA受体介导的微小兴奋性突触后电位（mEPSP）幅度相对独立于突触位置。突触电导随距离的逐渐增加被认为产生了这种幅度归一化。在本研究中，我们通过对CA1锥体神经元顶端树突进行全细胞记录，研究了负责空间缩放的机制。我们没有发现证据表明谢弗侧支突触处发现的裂隙谷氨酸浓度范围存在任何位置依赖性。此外，我们观察到释放概率（Pr）、双脉冲易化以及易释放囊泡池的大小并不依赖于突触位置。因此，谢弗侧支突触的基本突触前特性似乎没有任何变化可以解释距离依赖性缩放。另一方面，将4-甲氧基-7-硝基吲哚啉基笼化L-谷氨酸通过双光子解笼作用施加到分离的树突棘上，结果表明每个树突棘上的突触后AMPA受体数量随距胞体的距离增加而增加。因此，我们得出结论，谢弗侧支突触距离依赖性缩放产生的主要突触机制是突触后AMPA受体密度升高。
