Kraushaar U, Jonas P
Physiologisches Institut der Universität Freiburg, Germany.
J Neurosci. 2000 Aug 1;20(15):5594-607. doi: 10.1523/JNEUROSCI.20-15-05594.2000.
We have examined factors that determine the strength and dynamics of GABAergic synapses between interneurons [dentate gyrus basket cells (BCs)] and principal neurons [dentate gyrus granule cells (GCs)] using paired recordings in rat hippocampal slices at 34 degrees C. Unitary IPSCs recorded from BC-GC pairs in high intracellular Cl(-) concentration showed a fast rise and a biexponential decay, with mean time constants of 2 and 9 msec. The mean quantal conductance change, determined directly at reduced extracellular Ca(2+)/Mg(2+) concentration ratios, was 1.7 nS. Quantal release at the BC-GC synapse occurred with short delay and was highly synchronized. Analysis of IPSC peak amplitudes and numbers of failures by multiple probability compound binomial analysis indicated that synaptic transmission at the BC-GC synapse involves three to seven release sites, each of which releases transmitter with high probability ( approximately 0.5 in 2 mm Ca(2+)/1 mm Mg(2+)). Unitary BC-GC IPSCs showed paired-pulse depression (PPD); maximal depression, measured for 10 msec intervals, was 37%, and recovery from depression occurred with a time constant of 2 sec. Paired-pulse depression was mainly presynaptic in origin but appeared to be independent of previous release. Synaptic transmission at the BC-GC synapse showed frequency-dependent depression, with half-maximal decrease at 5 Hz after a series of 1000 presynaptic action potentials. The relative stability of transmission at the BC-GC synapse is consistent with a model in which an activity-dependent gating mechanism reduces release probability and thereby prevents depletion of the releasable pool of synaptic vesicles. Thus several mechanisms converge on the generation of powerful and sustained transmission at interneuron-principal neuron synapses in hippocampal circuits.
我们在34℃的大鼠海马脑片中，通过配对记录研究了决定中间神经元[齿状回篮状细胞（BCs）]与主神经元[齿状回颗粒细胞（GCs）]之间GABA能突触强度和动力学的因素。在高细胞内氯离子浓度下从BC-GC对记录的单位IPSCs显示出快速上升和双指数衰减，平均时间常数分别为2毫秒和9毫秒。在降低的细胞外钙（2+）/镁（2+）浓度比下直接测定的平均量子电导变化为1.7纳秒。BC-GC突触处的量子释放延迟短且高度同步。通过多重概率复合二项式分析对IPSC峰值幅度和失败次数进行分析表明，BC-GC突触处的突触传递涉及三到七个释放位点，每个位点释放递质的概率都很高（在2毫摩尔钙（2+）/1毫摩尔镁（2+）中约为0.5）。单位BC-GC IPSCs表现出双脉冲抑制（PPD）；以10毫秒间隔测量的最大抑制率为37%，抑制恢复的时间常数为2秒。双脉冲抑制主要源于突触前，但似乎与先前的释放无关。BC-GC突触处的突触传递表现出频率依赖性抑制，在一系列1000个突触前动作电位后，5赫兹时出现半数最大下降。BC-GC突触处传递的相对稳定性与一种模型一致，在该模型中，一种活动依赖性门控机制降低了释放概率，从而防止了可释放突触小泡池的耗尽。因此，几种机制共同作用于海马回路中中间神经元-主神经元突触处强大且持续的传递的产生。
