Svoboda K, Denk W, Kleinfeld D, Tank D W
Biological Computation Research Department, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA.
Nature. 1997 Jan 9;385(6612):161-5. doi: 10.1038/385161a0.
The dendrites of mammalian pyramidal neurons contain a rich collection of active conductances that can support Na+ and Ca2+ action potentials (for a review see ref. 1). The presence, site of initiation, and direction of propagation of Na+ and Ca2+ action potentials are, however, controversial, and seem to be sensitive to resting membrane potential, ionic composition, and degree of channel inactivation, and depend on the intensity and pattern of synaptic stimulation. This makes it difficult to extrapolate from in vitro experiments to the situation in the intact brain. Here we show that two-photon excitation laser scanning microscopy can penetrate the highly scattering tissue of the intact brain. We used this property to measure sensory stimulus-induced dendritic [Ca2+] dynamics of layer 2/3 pyramidal neurons of the rat primary vibrissa (Sm1) cortex in vivo. Simultaneous recordings of intracellular voltage and dendritic [Ca2+] dynamics during whisker stimulation or current injection showed increases in [Ca2+] only in coincidence with Na+ action potentials. The amplitude of these [Ca2+] transients at a given location was approximately proportional to the number of Na+ action potentials in a short burst. The amplitude for a given number of action potentials was greatest in the proximal apical dendrite and declined steeply with increasing distance from the soma, with little Ca2+ accumulation in the most distal branches, in layer 1. This suggests that widespread Ca2+ action potentials were not generated, and any significant [Ca2+] increase depends on somatically triggered Na+ action potentials.
哺乳动物锥体神经元的树突含有丰富的主动电导，可支持钠离子（Na⁺）和钙离子（Ca²⁺）动作电位（综述见参考文献1）。然而，Na⁺和Ca²⁺动作电位的存在、起始位点和传播方向存在争议，似乎对静息膜电位、离子组成和通道失活程度敏感，并取决于突触刺激的强度和模式。这使得从体外实验推断完整大脑中的情况变得困难。在这里，我们表明双光子激发激光扫描显微镜可以穿透完整大脑的高度散射组织。我们利用这一特性在体内测量大鼠初级触须（Sm1）皮层第2/3层锥体神经元的感觉刺激诱导的树突[Ca²⁺]动力学。在触须刺激或电流注入期间同时记录细胞内电压和树突[Ca²⁺]动力学，结果显示只有在与Na⁺动作电位同时出现时[Ca²⁺]才会增加。在给定位置，这些[Ca²⁺]瞬变的幅度大约与短串Na⁺动作电位的数量成正比。对于给定数量的动作电位，幅度在近端顶端树突中最大，并随着距胞体距离的增加而急剧下降，在第1层的最远端分支中几乎没有Ca²⁺积累。这表明没有产生广泛的Ca²⁺动作电位，任何显著的[Ca²⁺]增加都取决于体细胞触发的Na⁺动作电位。
