Sun Min-Yu, Chisari Mariangela, Eisenman Lawrence N, Zorumski Charles F, Mennerick Steven J
Department of Psychiatry, Washington University School of Medicine, St. Louis, Missouri.
Department of Biomedical and Biotechnological Sciences, Section of Pharmacology, University of Catania, Catania, Italy.
J Neurophysiol. 2017 Jul 1;118(1):532-543. doi: 10.1152/jn.00106.2017. Epub 2017 Mar 29.
-methyl-d-aspartate receptors (NMDARs) govern synaptic plasticity, development, and neuronal response to insult. Prolonged activation of NMDARs such as during an insult may activate secondary currents or modulate Mg sensitivity, but the conditions under which these occur are not fully defined. We reexamined the effect of prolonged NMDAR activation in juvenile mouse hippocampal slices. NMDA (10 μM) elicited current with the expected negative-slope conductance in the presence of 1.2 mM Mg However, several minutes of continued NMDA exposure elicited additional inward current at -70 mV. A higher concentration of NMDA (100 µM) elicited the current more rapidly. The additional current was not dependent on Ca, network activity, or metabotropic NMDAR function and did not persist on agonist removal. Voltage ramps revealed no alteration of either reversal potential or NMDA-elicited conductance between -30 mV and +50 mV. The result was a more linear NMDA current-voltage relationship. The current linearization was also induced in interneurons and in mature dentate granule neurons but not immature dentate granule cells, dissociated cultured hippocampal neurons, or nucleated patches excised from CA1 pyramidal neurons. Comparative simulations of NMDA application to a CA1 pyramidal neuron and to a cultured neuron revealed that linearization can be explained by space-clamp errors arising from gradual recruitment of distal dendritic NMDARs. We conclude that persistent secondary currents do not strongly contribute to NMDAR responses in juvenile mouse hippocampus and careful discernment is needed to exclude contributions of clamp artifacts to apparent secondary currents. We report that upon sustained activation of NMDARs in juvenile mouse hippocampal neurons there is apparent loss of Mg block at negative membrane potentials. However, the phenomenon is explained by loss of dendritic voltage clamp, leading to a linear current-voltage relationship. Our results give a specific example of how spatial voltage errors in voltage-clamp recordings can readily be misinterpreted as biological modulation.
N-甲基-D-天冬氨酸受体（NMDARs）调控突触可塑性、发育以及神经元对损伤的反应。NMDARs的长时间激活，比如在损伤期间，可能会激活次级电流或调节镁敏感性，但这些情况发生的条件尚未完全明确。我们重新研究了在幼年小鼠海马切片中NMDARs长时间激活的影响。在存在1.2 mM镁的情况下，NMDA（10 μM）引发了具有预期负斜率电导的电流。然而，持续几分钟的NMDA暴露在-70 mV时引发了额外的内向电流。更高浓度的NMDA（100 µM）能更快地引发该电流。额外的电流不依赖于钙、网络活动或代谢型NMDAR功能，并且在去除激动剂后不会持续存在。电压斜坡显示在-30 mV至+50 mV之间，反转电位或NMDA引发的电导均无变化。结果是NMDA电流-电压关系更加线性。电流线性化在中间神经元和成熟齿状颗粒神经元中也会被诱导，但在未成熟齿状颗粒细胞、解离培养的海马神经元或从CA1锥体神经元切除的有核膜片中不会出现。将NMDA应用于CA1锥体神经元和培养神经元的比较模拟表明，线性化可以由远端树突状NMDARs的逐渐募集引起的空间钳制误差来解释。我们得出结论，持续性次级电流对幼年小鼠海马中的NMDAR反应贡献不大，需要仔细辨别以排除钳制伪迹对明显次级电流的影响。我们报告，在幼年小鼠海马神经元中NMDARs持续激活后，在负膜电位下镁阻滞明显丧失。然而，这种现象是由树突电压钳制的丧失所解释的，导致了线性电流-电压关系。我们的结果给出了一个具体例子，说明电压钳记录中的空间电压误差如何容易被误解为生物学调节。
