Ambrosini A, Bresciani L, Fracchia S, Brunello N, Racagni G
Center of Neuropharmacology, University of Milan, Italy.
Mol Pharmacol. 1995 May;47(5):1057-64.
The functional effects of G protein-linked glutamate receptor activation have been studied in mouse mesencephalic neurons in vitro. We have been able to identify two receptor classes, one linked to phosphoinositide hydrolysis and another that inhibits adenylate cyclase. The agonist (1S,3R)-aminocyclopentane-1,3-dicarboxylate (ACPD) affected the two responses with similar potency (EC50 = 2 and 7 microM, respectively). In contrast, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine selectively decreased adenylate cyclase activity (EC50 = 150 nM), without interfering with the phosphoinositide pathway. Activation of ion channel-linked glutamate receptors in mesencephalic neurons leads to cGMP formation. In this study, we demonstrate that cell pretreatment with ACPD or (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine prevented, in a dose-dependent fashion, N-methyl-D-aspartate (NMDA)-induced cGMP formation but not the kainate-stimulated response. The pharmacological profile suggests that receptors that are negatively coupled to adenylate cyclase are responsible for this effect. Coexposure of neurons to ACPD and Ba2+, a K+ channel blocker, counteracted the ACPD-induced blockade of NMDA receptors, suggesting that activation of K+ conductances could be involved in the post-transduction events triggered by metabotropic receptors in the mesencephalon. Neuronal treatment with NMDA for 10 min caused a reduction in mitochondrial activity. Direct inhibition of nitric oxide synthase with the inhibitor NG-nitro-L-arginine or removal of extracellular nitric oxide with reduced hemoglobin did not prevent this metabolic impairment, thus excluding a role for nitric oxide in this test for excitotoxicity. On the contrary, the mitochondrial function was maintained when neurons exposed to NMDA were preincubated with metabotropic receptor agonists. To summarize, our results suggest that metabotropic receptors that are negatively coupled to adenylate cyclase exert modulatory control specifically on NMDA receptor activity. This event could also contribute to the reduction of neurotoxic effects due to NMDA receptor hyperactivity.
G蛋白偶联型谷氨酸受体激活的功能效应已在体外培养的小鼠中脑神经元中进行了研究。我们已能够鉴定出两类受体，一类与磷酸肌醇水解相关，另一类则抑制腺苷酸环化酶。激动剂（1S,3R）-氨基环戊烷-1,3-二羧酸（ACPD）对这两种反应的影响效力相似（EC50分别为2和7微摩尔）。相比之下，（2S,3S,4S）-α-（羧基环丙基）甘氨酸选择性降低腺苷酸环化酶活性（EC50 = 150纳摩尔），而不干扰磷酸肌醇途径。中脑神经元中离子通道偶联型谷氨酸受体的激活会导致环鸟苷酸（cGMP）的形成。在本研究中，我们证明用ACPD或（2S,3S,4S）-α-（羧基环丙基）甘氨酸对细胞进行预处理，能以剂量依赖性方式阻止N-甲基-D-天冬氨酸（NMDA）诱导的cGMP形成，但不能阻止海人藻酸刺激的反应。药理学特征表明，与腺苷酸环化酶负偶联的受体介导了这一效应。神经元同时暴露于ACPD和钾通道阻滞剂Ba2+，可抵消ACPD诱导的NMDA受体阻断作用，这表明钾电导的激活可能参与了中脑代谢型受体触发的转导后事件。用NMDA处理神经元10分钟会导致线粒体活性降低。用抑制剂NG-硝基-L-精氨酸直接抑制一氧化氮合酶或用还原血红蛋白去除细胞外一氧化氮并不能阻止这种代谢损伤，因此排除了一氧化氮在该兴奋性毒性试验中的作用。相反，当暴露于NMDA的神经元用代谢型受体激动剂预孵育时，线粒体功能得以维持。总之，我们的结果表明，与腺苷酸环化酶负偶联的代谢型受体对NMDA受体活性具有特异性调节控制作用。这一事件也可能有助于减少因NMDA受体过度激活而产生的神经毒性作用。
