Yasuda H, Kinoshita S, Tsumoto T
Department of Neurophysiology, Biomedical Research Center, Osaka University Medical School, Suita City, Japan.
Neuroscience. 1998 Aug;85(4):1011-24. doi: 10.1016/s0306-4522(97)00671-4.
To examine the role of N-methyl-D-aspartate receptors in the synaptic input-induced increase in Ca2+ in layer II/III neurons of visual cortex, Ca2+ imaging with a relatively low-affinity indicator was carried out simultaneously with whole-cell recordings of synaptic activity in cortical slices of young rats. Electrical stimulation of layer IV elicited excitatory postsynaptic potentials that generated action potentials concomitantly with a marked rise of Ca2+ signal in apical dendrites of 24 pyramidal cell-like neurons. Dendritic regions about 50 microm from the soma (peak regions) consistently showed the strongest increase in signal. The application of an N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovalerate, reduced the Ca2+ signal in peak regions but did not in other regions in nine of the 15 neurons tested. In all the dendritic regions in which Ca2+ signal was measured, the magnitude of the increase in signal was related linearly to the number of action potentials, suggesting that voltage-dependent Ca2+ channels activated by action potentials may also be involved in the increase. In 25 of 33 neurons which were voltage-clamped at 80 mV and perfused with the Mg2+-free solution, layer IV stimulation could induce a local rise in Ca2+ signal in apical dendrites. This rise was blocked by the N-methyl-D-aspartate receptor antagonist almost completely. Such an antagonist-sensitive Ca2+ rise was still seen even when the membrane potential was held at +10 mV when voltage-dependent Ca2+ channels were expected not to be activated by excitatory synaptic inputs. These results suggest that N-methyl-D-aspartate receptors are involved in the synaptic input-induced rise in Ca2+ at postsynaptic sites in apical dendrites of layer II/III neurons, and the increase may be boosted through voltage-dependent Ca2+ channels activated by action potentials.
为研究N-甲基-D-天冬氨酸受体在视觉皮层II/III层神经元突触输入诱导的Ca2+增加中的作用，在幼鼠皮层切片中，使用亲和力相对较低的指示剂进行Ca2+成像，并同时对突触活动进行全细胞记录。对IV层进行电刺激可诱发兴奋性突触后电位，该电位伴随着24个锥体细胞样神经元顶树突中Ca2+信号的显著升高而产生动作电位。距胞体约50微米的树突区域（峰值区域）信号增强始终最为明显。在15个测试神经元中的9个中，应用N-甲基-D-天冬氨酸受体拮抗剂DL-2-氨基-5-磷酸戊酸可降低峰值区域的Ca2+信号，但对其他区域无影响。在所有测量Ca2+信号的树突区域中，信号增加的幅度与动作电位的数量呈线性相关，这表明动作电位激活的电压依赖性Ca2+通道可能也参与了信号增加过程。在33个神经元中的25个中，将其电压钳制在80 mV并灌注无Mg2+溶液，对IV层的刺激可诱导顶树突中Ca2+信号局部升高。这种升高几乎完全被N-甲基-D-天冬氨酸受体拮抗剂阻断。即使将膜电位保持在+10 mV（此时电压依赖性Ca2+通道预计不会被兴奋性突触输入激活），仍可观察到这种拮抗剂敏感的Ca2+升高。这些结果表明，N-甲基-D-天冬氨酸受体参与了II/III层神经元顶树突突触后位点突触输入诱导的Ca2+升高，并且这种升高可能通过动作电位激活的电压依赖性Ca2+通道而增强。
