Fossom L H, Basile A S, Skolnick P
Laboratory of Neuroscience, National Institute for Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Mol Pharmacol. 1995 Dec;48(6):981-7.
Partial agonists at the strychnine-insensitive glycine sites coupled to N-methyl-D-aspartate (NMDA) receptors reduce both glutamate-induced neurotoxicity in vitro and ischemia-induced neurodegeneration in vivo. Paradoxically, sustained exposure of cultured cerebellar granule cell neurons to glycinergic ligands, including glycine and the glycine partial agonists (+/-)-3-amino-1-hydroxy-2-pyrrolidone, 1-aminocyclopropanecarboxylic acid (ACPC), and D-cycloserine, attenuates the neuroprotective effects of (+/-)-3-amino-1-hydroxy-2-pyrrolidone and ACPC. In the present study, we investigated the mechanisms responsible for this attenuated neuroprotection. Three NMDA receptor-mediated responses were examined after sustained exposure to ACPC: glutamate-induced neurotoxicity, NMDA-stimulated increases in cGMP levels, and NMDA-stimulated increases in [Ca+2]i. Consistent with previous findings, coincubation with ACPC blocked glutamate-induced neurotoxicity, whereas sustained (24 hr) exposure to ACPC attenuated its protective effects. Moreover, sustained exposure to ACPC caused an apparent approximately 2-fold increase in the potency of both glutamate to act as neurotoxin and NMDA to stimulate cGMP formation. Sustained exposure to ACPC also increased NMDA-stimulated [Ca+2]i approximately 3-fold compared with control granule cell cultures but did not affect basal [Ca+2]i. This apparent increase in glutamate sensitivity may be attributable to a change in NMDA receptor subunit composition as sustained ACPC exposure resulted in a approximately 2.5-fold increase in NMDA receptor 2C RNA levels, without concomitant changes in the amounts of RNA encoding the NMDA receptor 2A, 2B, or 1 subunit. This is the first demonstration that sustained exposure to a glycinergic ligand can alter the expression of RNAs encoding NMDA receptor subunits. Because glycinergic ligands are potential clinical candidates, these results may have important implications for the treatment of neurodegenerative disorders.
与N-甲基-D-天冬氨酸（NMDA）受体偶联的对士的宁不敏感的甘氨酸位点上的部分激动剂，在体外可降低谷氨酸诱导的神经毒性，在体内可减轻缺血诱导的神经退行性变。矛盾的是，培养的小脑颗粒细胞神经元持续暴露于甘氨酸能配体，包括甘氨酸和甘氨酸部分激动剂（±）-3-氨基-1-羟基-2-吡咯烷酮、1-氨基环丙烷羧酸（ACPC）和D-环丝氨酸，会减弱（±）-3-氨基-1-羟基-2-吡咯烷酮和ACPC的神经保护作用。在本研究中，我们调查了这种减弱的神经保护作用的机制。在持续暴露于ACPC后，检测了三种NMDA受体介导的反应：谷氨酸诱导的神经毒性、NMDA刺激的cGMP水平升高以及NMDA刺激的[Ca+2]i升高。与先前的研究结果一致，与ACPC共同孵育可阻断谷氨酸诱导的神经毒性，而持续（24小时）暴露于ACPC会减弱其保护作用。此外，持续暴露于ACPC会使谷氨酸作为神经毒素的效力以及NMDA刺激cGMP形成的效力明显增加约2倍。与对照颗粒细胞培养物相比，持续暴露于ACPC还使NMDA刺激的[Ca+2]i增加约3倍，但不影响基础[Ca+2]i。谷氨酸敏感性的这种明显增加可能归因于NMDA受体亚基组成的变化，因为持续暴露于ACPC导致NMDA受体2C RNA水平增加约2.5倍，而编码NMDA受体2A、2B或1亚基的RNA量没有相应变化。这是首次证明持续暴露于甘氨酸能配体可改变编码NMDA受体亚基的RNA的表达。由于甘氨酸能配体是潜在的临床候选药物，这些结果可能对神经退行性疾病的治疗具有重要意义。
