The allosteric glycine site of the N-methyl-D-aspartate receptor modulates GABAergic-mediated synaptic events in neonatal rat CA3 hippocampal neurons.

We report in this study that, in the presence of magnesium, bath application of micromolar concentrations of glycine have prominent effects on synaptic events and N-methyl-D-aspartate (NMDA) responses in neonatal but not in adult hippocampal slices. Intracellular recordings were made from 71 rat CA3 hippocampal neurons in neonatal slices. In keeping with our earlier study, during the first postnatal week, CA3 neurons exhibited giant depolarizing potentials (GDPs). These GDPs are mediated by gamma-aminobutyric acid (GABA) acting on type A GABA (GABAA) receptors and modulated presynaptically by NMDA receptors. In the majority of cells (18 out of 31), glycine (10-30 microM) increased the frequency of GDPs (from 0.14 to 0.29 Hz). This effect was mimicked by D-serine (10-20 microM) and blocked by the NMDA receptor antagonists D-(-)-2-amino-5-phosphonovalerate (50 microM) and DL-2-amino-7-phosphonoheptanoate (50 microM) and by the GABAA antagonist bicuculline (10 microM) but not by strychnine (1 microM). Subthreshold concentrations of glycine (or D-serine) and NMDA, when given together, enhanced synaptic noise and the frequency of GDPs. In the presence of tetrodotoxin (1 microM), glycine and D-serine (up to 50 microM) did not modify the NMDA-induced inward currents in CA3 pyramidal cells. However the reduction of NMDA-mediated currents by 7-chlorokynurenate (10-20 microM) was reversed by glycine and D-serine (100-200 microM). In contrast, glycine (up to 100 microM) had no effect on membrane potential, input resistance, or NMDA responses after postnatal day 10. It is concluded that GABA-mediated events are facilitated by glycine acting on presynaptically located NMDA receptors.

本研究表明，在镁离子存在时，以浴槽灌流方式给予微摩尔浓度的甘氨酸，可对新生大鼠海马脑片的突触事件及N-甲基-D-天冬氨酸（N-methyl-D-aspartate, NMDA）反应产生显著影响，但对成年大鼠海马脑片无此效应。本研究对71个新生大鼠海马CA3区神经元进行了细胞内记录。与本团队此前的研究一致，在出生后第一周，CA3神经元可记录到巨型去极化电位（giant depolarizing potentials, GDPs）。此类巨型去极化电位由作用于A型γ-氨基丁酸（gamma-aminobutyric acid type A, GABAA）受体的γ-氨基丁酸（gamma-aminobutyric acid, GABA）所介导，并由NMDA受体实现突触前调控。在31个受试细胞中的多数（18个），甘氨酸（10~30 μM）可使GDPs的频率从0.14 Hz提升至0.29 Hz。该效应可被D-丝氨酸（D-serine, 10~20 μM）模拟，并可被NMDA受体拮抗剂D-(-)-2-氨基-5-膦酰戊酸（50 μM）、DL-2-氨基-7-膦酰庚酸（50 μM）以及GABAA受体拮抗剂荷包牡丹碱（bicuculline, 10 μM）阻断，但不受士的宁（strychnine, 1 μM）的影响。当亚阈值浓度的甘氨酸（或D-丝氨酸）与NMDA联合给药时，可增强突触噪声并提升GDPs的频率。在河豚毒素（tetrodotoxin, 1 μM）存在的条件下，甘氨酸与D-丝氨酸（最高浓度可达50 μM）并不会改变CA3锥体神经元中NMDA诱导的内向电流。然而，7-氯犬尿酸（7-chlorokynurenate, 10~20 μM）对NMDA介导电流的抑制作用，可被甘氨酸与D-丝氨酸（100~200 μM）逆转。与之相反，在出生后第10天及之后，甘氨酸（最高浓度可达100 μM）对膜电位、输入电阻或NMDA反应均无影响。综上，本研究得出结论：甘氨酸通过作用于突触前定位的NMDA受体，可增强GABA介导的突触事件。