Wroblewska B, Santi M R, Neale J H
Department of Biology, Georgetown University, Washington, DC 20057-1229, USA.
Glia. 1998 Oct;24(2):172-9. doi: 10.1002/(sici)1098-1136(199810)24:2<172::aid-glia2>3.0.co;2-6.
N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.
N-乙酰天门冬氨酰谷氨酸（NAAG）是哺乳动物神经系统中最普遍的肽。NAAG符合神经递质的传统标准，包括定位于突触小泡、去极化诱导释放、对某些N-甲基-D-天冬氨酸受体的低效激活，以及对cAMP偶联的代谢型谷氨酸受体（mGluR）的高度选择性激活，其效力接近谷氨酸。该肽以高浓度存在于培养的皮质神经胶质细胞中，并被细胞表面肽酶活性水解。在本研究中，我们测试了以下假设：NAAG选择性激活培养的大鼠小脑神经胶质细胞（主要是星形胶质细胞）上的一类代谢型受体。这些神经胶质细胞表达mGluR亚型1、3、4、5、6、7和8的mRNA。我们使用逆转录聚合酶链反应在这些细胞中未能检测到mGluR2的信息。小脑神经胶质细胞对NAAG、谷氨酸和反式-ACPD的反应是福斯高林刺激的cAMP形成减少。AP4是III组受体mGluR4、mGluR6、mGluR7和mGluR8的激动剂，对刺激的cAMP水平几乎没有影响。用低微摩尔浓度的NAAG处理可通过激活代谢型谷氨酸受体显著增加培养的星形胶质细胞对放射性胸苷的摄取。II组mGluRs的拮抗剂可阻止NAAG导致的cAMP减少和胸苷摄取增加。培养的小脑星形胶质细胞每毫克蛋白质表达20 pmol NAAG，该值比从小鼠皮质制备的混合神经胶质细胞培养物所表达的值至少低30倍。我们得出结论，小脑星形胶质细胞通过mGluR3受体对NAAG作出反应，并且该肽在从神经元或神经胶质细胞释放后可能选择性激活星形胶质细胞中的这种受体亚型。
