Miranda A F, Boegman R J, Beninger R J, Jhamandas K
Department of Pharmacology and Toxicology, Queen's University, Kingston, Ontario, Canada.
Neuroscience. 1997 Jun;78(4):967-75. doi: 10.1016/s0306-4522(96)00655-0.
Endogenous excitotoxins have been implicated in the degeneration of dopaminergic neurons in the substantia nigra compacta of patients with Parkinson's disease. One such agent quinolinic acid is an endogenous excitatory amino acid receptor agonist. This study examined whether an increased level of endogenous kynurenic acid, an excitatory amino acid receptor antagonist, can protect nigrostriatal dopamine neurons against quinolinic acid-induced excitotoxic damage. Nigral infusion of quinolinic acid (60 nmoles) or N-methyl-D- aspartate (15 nmoles) produced a significant depletion in striatal tyrosine hydroxylase activity, a biochemical marker for dopaminergic neurons. Three hours following the intraventricular infusion of nicotinylalanine (5.6 nmoles), an agent that inhibits kynureninase and kynurenine hydroxylase activity, when combined with kynurenine (450 mg/kg i.p.), the precursor of kynurenic acid, and probenecid (200 mg/kg i.p.), an inhibitor of organic acid transport, the kynurenic acid in the whole brain and substantia nigra was increased 3.3-fold and 1.5-fold respectively when compared to rats that received saline, probenecid and kynurenine. This elevation in endogenous kynurenic acid prevented the quinolinic acid-induced reduction in striatal tyrosine hydroxylase. However, 9 h following the administration of nicotinylalanine with kynurenine and probenecid, a time when whole brain kynurenic acid levels had decreased 12-fold, quinolinic acid injections produced a significant depletion in striatal tyrosine hydroxylase. Intranigral infusion of quinolinic acid in rats that received saline with kynurenine and probenecid resulted in a significant depletion of ipsilateral striatal tyrosine hydroxylase. Administration of nicotinylalanine in combination with kynurenine and probenecid also blocked N-methyl-D-aspartate-induced depletion of tyrosine hydroxylase. Tyrosine hydroxylase immunohistochemical assessment of the substantia nigra confirmed quinolinic acid-induced neuronal cell loss and the ability of nicotinylalanine in combination with kynurenine and probenecid to protect neurons from quinolinic acid-induced toxicity. The present study demonstrates that increases in endogenous kynurenic acid can prevent the loss of nigrostriatal dopaminergic neurons resulting from a focal infusion of quinolinic acid or N-methyl-D-aspartate. The strategy of neuronal protection by increasing the brain kynurenic acid may be useful in retarding cell loss in Parkinson's disease and other neurodegenerative diseases where excitotoxic mechanisms have been implicated.
内源性兴奋性毒素被认为与帕金森病患者黑质致密部多巴胺能神经元的变性有关。喹啉酸就是这样一种物质，它是一种内源性兴奋性氨基酸受体激动剂。本研究检测了内源性犬尿喹啉酸（一种兴奋性氨基酸受体拮抗剂）水平的升高是否能保护黑质纹状体多巴胺能神经元免受喹啉酸诱导的兴奋性毒性损伤。向黑质注射喹啉酸（60纳摩尔）或N-甲基-D-天冬氨酸（15纳摩尔）会导致纹状体酪氨酸羟化酶活性显著降低，酪氨酸羟化酶是多巴胺能神经元的一种生化标志物。脑室内注射烟酰丙氨酸（5.6纳摩尔），一种抑制犬尿氨酸酶和犬尿氨酸羟化酶活性的物质，3小时后，当与犬尿氨酸（450毫克/千克腹腔注射）（犬尿喹啉酸的前体）和丙磺舒（200毫克/千克腹腔注射）（一种有机酸转运抑制剂）联合使用时，与接受生理盐水、丙磺舒和犬尿氨酸的大鼠相比，全脑和黑质中的犬尿喹啉酸分别增加了3.3倍和1.5倍。内源性犬尿喹啉酸的这种升高阻止了喹啉酸诱导的纹状体酪氨酸羟化酶的降低。然而，在烟酰丙氨酸与犬尿氨酸和丙磺舒联合给药9小时后，此时全脑犬尿喹啉酸水平已下降了12倍，注射喹啉酸导致纹状体酪氨酸羟化酶显著降低。在接受生理盐水、犬尿氨酸和丙磺舒的大鼠中，向黑质内注射喹啉酸导致同侧纹状体酪氨酸羟化酶显著降低。烟酰丙氨酸与犬尿氨酸和丙磺舒联合给药也能阻止N-甲基-D-天冬氨酸诱导的酪氨酸羟化酶降低。对黑质进行酪氨酸羟化酶免疫组织化学评估证实了喹啉酸诱导的神经元细胞丢失，以及烟酰丙氨酸与犬尿氨酸和丙磺舒联合使用保护神经元免受喹啉酸诱导毒性的能力。本研究表明，内源性犬尿喹啉酸的增加可以防止因局部注射喹啉酸或N-甲基-D-天冬氨酸导致的黑质纹状体多巴胺能神经元丢失。通过增加脑内犬尿喹啉酸来保护神经元的策略可能有助于延缓帕金森病和其他涉及兴奋性毒性机制的神经退行性疾病中的细胞丢失。
