Vaaga Christopher E, Tovar Kenneth R, Westbrook Gary L
Neuroscience Graduate Program, Oregon Health and Science University, Portland, Oregon; and Vollum Institute, Oregon Health and Science University, Portland, Oregon
Vollum Institute, Oregon Health and Science University, Portland, Oregon.
J Neurophysiol. 2014 Sep 15;112(6):1241-5. doi: 10.1152/jn.00290.2014. Epub 2014 Jun 18.
Glutamate acts as the universal agonist at ionotropic glutamate receptors in part because of its high degree of conformational flexibility. Other amino acids and small peptides, however, can activate N-methyl-d-aspartate (NMDA) receptors, albeit usually with lower affinity and efficacy. Here, we examined the action of glycine-proline-glutamate (GPE), a naturally occurring tripeptide formed in the brain following cleavage of IGF-I. GPE is thought to have biological activity in the brain, but its mechanism of action remains unclear. With its flanking glutamate and glycine residues, GPE could bind to either the agonist or coagonist sites on NMDA receptors, however, this has not been directly tested. Using whole cell patch-clamp recordings in combination with rapid solution exchange, we examined both steady-state currents induced by GPE as well as the effects of GPE on synaptically evoked currents. High concentrations of GPE evoked inward currents, which were blocked either by NMDA receptor competitive antagonists or the voltage-dependent channel blocker Mg(2+). GPE also produced a slight attenuation in the NMDA- and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated excitatory postsynaptic currents without altering the paired-pulse ratio. Our results suggest that GPE can activate NMDA receptors but at concentrations well above the expected concentration of GPE in the brain. Therefore, it is unlikely that endogenous GPE interacts with glutamate receptors under normal conditions.
谷氨酸作为离子型谷氨酸受体的通用激动剂，部分原因在于其高度的构象灵活性。然而，其他氨基酸和小肽也能激活N-甲基-D-天冬氨酸（NMDA）受体，尽管通常亲和力和效能较低。在此，我们研究了甘氨酸-脯氨酸-谷氨酸（GPE）的作用，GPE是胰岛素样生长因子-I（IGF-I）裂解后在大脑中天然形成的三肽。人们认为GPE在大脑中具有生物活性，但其作用机制仍不清楚。由于其侧翼的谷氨酸和甘氨酸残基，GPE可能与NMDA受体上的激动剂或共激动剂位点结合，然而，这一点尚未得到直接验证。我们结合快速溶液交换，使用全细胞膜片钳记录技术，研究了GPE诱导的稳态电流以及GPE对突触诱发电流的影响。高浓度的GPE诱发内向电流，该电流被NMDA受体竞争性拮抗剂或电压依赖性通道阻滞剂Mg(2+)阻断。GPE还使NMDA和α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）介导的兴奋性突触后电流略有衰减，但不改变配对脉冲比率。我们的结果表明，GPE可以激活NMDA受体，但所需浓度远高于大脑中GPE的预期浓度。因此，内源性GPE在正常情况下不太可能与谷氨酸受体相互作用。
