Aponte Yexica, Bischofberger Josef, Jonas Peter
Physiological Institute I, University of Freiburg, Hermann-Herder-Str. 7, D-79104 Freiburg, Germany.
J Physiol. 2008 Apr 15;586(8):2061-75. doi: 10.1113/jphysiol.2007.147298. Epub 2008 Feb 14.
Fast-spiking parvalbumin-expressing basket cells (BCs) represent a major type of inhibitory interneuron in the hippocampus. These cells inhibit principal cells in a temporally precise manner and are involved in the generation of network oscillations. Although BCs show a unique expression profile of Ca(2+)-permeable receptors, Ca(2+)-binding proteins and Ca(2+)-dependent signalling molecules, physiological Ca(2+) signalling in these interneurons has not been investigated. To study action potential (AP)-induced dendritic Ca(2+) influx and buffering, we combined whole-cell patch-clamp recordings with ratiometric Ca(2+) imaging from the proximal apical dendrites of rigorously identified BCs in acute slices, using the high-affinity Ca(2+) indicator fura-2 or the low-affinity dye fura-FF. Single APs evoked dendritic Ca(2+) transients with small amplitude. Bursts of APs evoked Ca(2+) transients with amplitudes that increased linearly with AP number. Analysis of Ca(2+) transients under steady-state conditions with different fura-2 concentrations and during loading with 200 microm fura-2 indicated that the endogenous Ca(2+)-binding ratio was approximately 200 (kappa(S) = 202 +/- 26 for the loading experiments). The peak amplitude of the Ca(2+) transients measured directly with 100 microm fura-FF was 39 nm AP(-1). At approximately 23 degrees C, the decay time constant of the Ca(2+) transients was 390 ms, corresponding to an extrusion rate of approximately 600 s(-1). At 34 degrees C, the decay time constant was 203 ms and the corresponding extrusion rate was approximately 1100 s(-1). At both temperatures, continuous theta-burst activity with three to five APs per theta cycle, as occurs in vivo during exploration, led to a moderate increase in the global Ca(2+) concentration that was proportional to AP number, whereas more intense stimulation was required to reach micromolar Ca(2+) concentrations and to shift Ca(2+) signalling into a non-linear regime. In conclusion, dentate gyrus BCs show a high endogenous Ca(2+)-binding ratio, a small AP-induced dendritic Ca(2+) influx, and a relatively slow Ca(2+) extrusion. These specific buffering properties of BCs will sharpen the time course of local Ca(2+) signals, while prolonging the decay of global Ca(2+) signals.
快速发放的表达小白蛋白的篮状细胞（BCs）是海马中主要的抑制性中间神经元类型。这些细胞以时间精确的方式抑制主细胞，并参与网络振荡的产生。尽管BCs表现出钙通透性受体、钙结合蛋白和钙依赖性信号分子的独特表达谱，但这些中间神经元中的生理性钙信号尚未得到研究。为了研究动作电位（AP）诱导的树突钙内流和缓冲，我们将全细胞膜片钳记录与急性脑片中经过严格鉴定的BCs近端顶端树突的比率型钙成像相结合，使用高亲和力钙指示剂fura-2或低亲和力染料fura-FF。单个AP诱发幅度较小的树突钙瞬变。AP串诱发的钙瞬变幅度随AP数量呈线性增加。在不同fura-2浓度的稳态条件下以及在加载200微摩尔fura-2期间对钙瞬变进行分析表明，内源性钙结合比率约为200（加载实验中κ(S)=202±26）。用100微摩尔fura-FF直接测量的钙瞬变峰值幅度为39纳米/AP。在约23℃时，钙瞬变的衰减时间常数为390毫秒，对应于约600秒-1的外排速率。在34℃时，衰减时间常数为203毫秒，相应的外排速率约为1100秒-1。在这两个温度下，每个θ周期有三到五个AP的连续θ爆发活动，如在探索过程中体内发生的那样，导致全局钙浓度适度增加，该增加与AP数量成比例，而需要更强的刺激才能达到微摩尔钙浓度并将钙信号转变为非线性状态。总之，齿状回BCs表现出高内源性钙结合比率、小的AP诱导树突钙内流和相对缓慢的钙外排。BCs的这些特定缓冲特性将锐化局部钙信号的时间进程，同时延长全局钙信号的衰减。
