6-cyano-7-nitroquinoxaline-2-3-dione and 6-hydroxydopa

2,4,5-Trihydroxyphenylalanine (TOPA) oxidizes in solution to form a quinone derivative that is a non-NMDA glutamatergic agonist and neurotoxin. DOPA can autoxidize in physiological solutions to form small amounts of both TOPA and TOPA quinone. We report here that this conversion can be dramatically enhanced by iron (II) alone, but more so by iron (II) in the presence of hydrogen peroxide. This conversion is of sufficient magnitude that the resulting product can elicit non-NMDA, glutamate receptor-mediated electrical responses in cultured cortical neurons isolated from rat. This finding suggests that TOPA quinone may play a role in pathological processes involving abnormal iron metabolism in catecholaminergic neurons.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Ascorbic Acid; Cells, Cultured; Chromatography, High Pressure Liquid; Dihydroxyphenylalanine; Electrophysiology; Hydrogen Peroxide; Iron; Neurons; Neurotoxins; Oxidation-Reduction; Rats

The neurotoxic properties of 2,4,5-trihydroxyphenylalanine (TOPA; the 6-hydroxylated derivative of dopa) was investigated in cultures of central neurons. Application of solutions of TOPA to cerebellar granule cells resulted in a concentration- and time-dependent neuronal death, with prolonged (24 hr) exposure producing a clear left-handed shift in the dose-response relationship from the one observed with a 60-min exposure (LD50: 4 and 29 microM, respectively). This toxicity was largely blocked by the non-N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Solutions of TOPA were also toxic to mesencephalic neurons after acute or chronic exposure, displaying the same leftward shift in LD50. This latter preparation contained a minor population of dopaminergic, tyrosine hydroxylase immunopositive cells which were likewise sensitive to the excitotoxic effects of TOPA. Neurotoxic activity of TOPA appeared to depend upon its oxidation in solution, as judged using chemical analysis and reducing agents. The monosialoganglioside GM1 was effective in protecting against neurodegeneration induced by brief or prolonged exposure to solutions of TOPA. These results suggest that an abnormal production or accumulation of TOPA or its oxidation product(s) might be involved in excitotoxicity directed to areas of the brain with dopaminergic innervation, and in other brain areas in Parkinson's disease patients on long-term dopa therapy. The selective action of gangliosides in disrupting the pathological consequences of glutamate receptor activation proposes their use as chemoprophylactic agents for preventing or arresting the neuronal losses accompanying such situations.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cells, Cultured; Cerebellum; Dihydroxyphenylalanine; G(M1) Ganglioside; Mesencephalon; MPTP Poisoning; Oxidation-Reduction; Parkinson Disease; Quinoxalines; Rats; Rats, Sprague-Dawley

We have investigated the pharmacologic and neurotoxic properties of 2,4,5-trihydroxyphenylalanine [topa; the 6-hydroxylated derivative of 3,4-dihydroxyphenylalanine (dopa)] in central neurons. Application of solutions of topa to the chicken eyecup preparation results in glutamatergic responses mediated predominantly by non-N-methyl-D-aspartate receptors. Pharmacological activity depends upon oxidation in solution to a new compound. This compound is tentatively identified as topa quinone. Solutions of topa are toxic to cortical neurons in culture, and this toxicity is blocked by the non-N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. These results suggest that production or accumulation of topa or its oxidation products might be involved in excitotoxicity, especially in dopaminergic neurons and their projection targets.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cells, Cultured; Cerebral Cortex; Chickens; Dihydroxyphenylalanine; Dose-Response Relationship, Drug; Eye; In Vitro Techniques; Neurons; Neurotoxins; Ocular Physiological Phenomena; Quinoxalines; Rats

Despite several decades of research aimed at elucidating the mechanisms underlying neuronal degeneration in Parkinson's and Huntington's diseases, these mysteries remain unfathomed. The brain contains high concentrations of the putative transmitters, glutamate and aspartate, which have neurotoxic (excitotoxic) potential and are thought to cause neuronal degeneration in certain acute neurological disorders. However, no mechanism has been identified by which these diffusely distributed agents might cause the regionally selective patterns of neuronal degeneration characterizing Parkinson's and Huntington's diseases. Here we report that L-DOPA, the natural precursor to dopamine, is a weak excitotoxin and its ortho-hydroxylated derivative, 6-OH-DOPA, is a powerful excitotoxin. We propose that an excitotoxic process mediated by L-DOPA or an acidic derivative such as 6-OH-DOPA might be responsible for degeneration of nigral neurons in Parkinson's disease or striatal neurons in Huntington's disease.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Chick Embryo; Dibenzocycloheptenes; Dihydroxyphenylalanine; Dizocilpine Maleate; Hippocampus; Huntington Disease; Levodopa; Neurons; Parkinson Disease; Quinoxalines; Receptors, Neurotransmitter; Retina

Applications of solutions of 2,4,5-trihydroxyphenylalanine (TOPA or 6-hydroxyDOPA) to rat cortical neurons in culture monitored under whole-cell voltage clamp with patch electrodes resulted in currents which could be nearly completely blocked by the non-N-methyl-D-aspartate (non-NMDA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but only weakly antagonized by the NMDA antagonist D.L-2-amino-5-phosphonovalerate (APV). Thus, TOPA can generate glutamatergic responses by interacting preferentially with non-NMDA receptors in cortical neurons. As these results show that a product closely related to the catecholamine precursor 3,4-dihydroxyphenylalanine (DOPA) has glutamatergic agonist properties, it is conceivable that catecholamine-containing brain areas may be at special risk for excitotoxic damage under certain conditions.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cells, Cultured; Cerebral Cortex; Dihydroxyphenylalanine; Evoked Potentials; N-Methylaspartate; Neurons; Quinoxalines; Rats