Dual whole-cell voltage recordings were made from synaptically connected layer 5 (L5) pyramidal neurones in slices of the young (P14-P16) rat neocortex. The Ca2+ buffers BAPTA or EGTA were loaded into the presynaptic neurone via the pipette recording from the presynaptic neurone to examine their effect on the mean and the coefficient of variation (c.v.) of single fibre EPSP amplitudes, referred to as unitary EPSPs. 2. The fast Ca2+ buffer BAPTA reduced unitary EPSP amplitudes in a concentration dependent way. With 0.1 mM BAPTA in the pipette, the mean EPSP amplitude was reduced by 14 +/- 2.8% (mean +/- s.e.m., n = 7) compared with control pipette solution, whereas with 1.5 mM BAPTA, the mean EPSP amplitude was reduced by 72 +/- 1.5% (n = 5). The concentration of BAPTA that reduced mean EPSP amplitudes to one-half of control was close to 0.7 mM. 3. Saturation of BAPTA during evoked release was tested by comparing the effect of loading the presynaptic neurone with 0.1 mM BAPTA at 2 and 1 mM [Ca2+]o. Reducing [Ca2+]o from 2 to 1 mM, thereby reducing Ca2+ influx into the terminals, decreased the mean EPSP amplitude by 60 +/- 2.2% with control pipette solution and by 62 +/- 1.9% after loading with 0.1 mM BAPTA (n = 7). 4. The slow Ca2+ buffer EGTA at 1 mM reduced mean EPSP amplitudes by 15 +/- 2.5% (n = 5). With 10 mM EGTA mean EPSP amplitudes were reduced by 56 +/- 2.3 % (n = 4). 5. With both Ca2+ buffers, the reduction in mean EPSP amplitudes was associated with an increase in the c.v. of peak EPSP amplitudes, consistent with a reduction of the transmitter release probability as the major mechanism underlying the reduction of the EPSP amplitude. 6. The results suggest that in nerve terminals of thick tufted L5 pyramidal cells the endogenous mobile Ca2+ buffer is equivalent to less than 0.1 mM BAPTA and that at many release sites of pyramidal cell terminals the Ca2+ channel domains overlap, a situation comparable with that at large calyx-type terminals in the brainstem.
摘要
对幼年（P14 - P16）大鼠新皮质切片中通过突触连接的第5层（L5）锥体神经元进行双全细胞电压记录。将Ca²⁺缓冲剂BAPTA或EGTA通过记录突触前神经元的移液管加载到突触前神经元中，以研究它们对单纤维兴奋性突触后电位（EPSP）幅度（称为单位EPSP）的平均值和变异系数（c.v.）的影响。2. 快速Ca²⁺缓冲剂BAPTA以浓度依赖的方式降低单位EPSP幅度。移液管中含有0.1 mM BAPTA时，与对照移液管溶液相比，平均EPSP幅度降低了14±2.8%（平均值±标准误，n = 7），而含有1.5 mM BAPTA时，平均EPSP幅度降低了72±1.5%（n = 5）。将平均EPSP幅度降低至对照一半时的BAPTA浓度接近0.7 mM。3. 通过比较在2 mM和1 mM细胞外Ca²⁺浓度（[Ca²⁺]o）下用0.1 mM BAPTA加载突触前神经元的效果，测试诱发释放过程中BAPTA的饱和度。将[Ca²⁺]o从2 mM降低到1 mM，从而减少Ca²⁺流入终末，使用对照移液管溶液时平均EPSP幅度降低了60±2.2%，用0.1 mM BAPTA加载后降低了62±1.9%（n = 7）。4. 1 mM的慢Ca²⁺缓冲剂EGTA使平均EPSP幅度降低了15±2.5%（n = 5）。10 mM EGTA时平均EPSP幅度降低了56±2.3%（n = 4）。5. 使用这两种Ca²⁺缓冲剂时，平均EPSP幅度的降低与EPSP峰值幅度的c.v.增加相关，这与递质释放概率降低作为EPSP幅度降低的主要机制一致。6. 结果表明，在厚簇状L5锥体细胞的神经终末中，内源性可移动Ca²⁺缓冲剂相当于小于0.1 mM的BAPTA，并且在锥体细胞终末的许多释放位点，Ca²⁺通道结构域重叠，这种情况与脑干中大花萼型终末的情况类似。
