Kynurenic acid, a tryptophan metabolite able to antagonize the actions of the excitatory amino acids, has been identified and measured for the first time in the brain of mice, rats, guinea pigs, and humans by using an HPLC method. Its content was 5.8 +/- 0.9 in mouse brain, 17.8 +/- 2.0 in rat brain, 16.2 +/- 1.5 in guinea pig brain, 26.8 +/- 2.9 in rabbit brain, and 150 +/- 30 in human cortex (pmol/g wet wt. mean +/- SE). The regional distribution of this molecule was uneven. In rats, guinea pigs, and rabbits, the brainstem was the area richest in this compound. Tryptophan administration (100-300 mg/kg, i.p.) to rats resulted in a significant increase of the brain content of kynurenic acid. Similarly, 1 h after probenecid administration (200 mg/kg, i.p.), the brain content of kynurenate increased by fourfold, thus suggesting that its turnover rate is relatively fast.

Excessive stimulation of excitatory amino acid (EAA) receptors and abnormal production of oxygen-derived free radicals have repeatedly been implicated in the series of events linking brain hypoxia or ischemia to neuronal death. We report here that in rat hippocampal slices the KCl-stimulated output of labeled D-3H aspartate or of endogenous aspartate and glutamate significantly increased under in vitro simulated hypoxic, hypoglycemic, or ischemic conditions. In particular, when the slices were incubated for 10 min at 32 degrees C under "ischemic" conditions (namely, lack of oxygen and glucose), endogenous aspartate and glutamate in the supernatant increased by 10 and 20 times, respectively. Since radical scavengers (D-mannitol), drugs reducing free radical formation (indomethacin, corticosteroid), or enzymes able to metabolize them (catalase and superoxide dismutase) significantly reduced this output, it was supposed that free radicals caused EAA release. A direct demonstration of this concept was obtained by showing a significant release of EAA after incubation of hippocampal slices with enzymes and substrates known to cause the formation of free radicals, such as xanthine plus xanthine oxidase or arachidonic acid plus prostaglandin synthase. Neither ischemia nor the enzymatic reactions leading to free radical production increased the activity of the cytoplasmic enzyme lactate dehydrogenase in the incubation medium, thus ruling out a nonspecific cellular lysis. It appears therefore that during ischemic states, brain production of reactive molecules (free radicals) causes an increased output of EAA. This may trigger a series of events which could help to explain the delayed loss of neurons after a transient ischemic period.

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1 We measured the e ects of agonists and antagonists of metabotropic glutamate (mGlu) receptors (types 1 and 5) on NMDA-induced depolarization of mouse cortical wedges in order to characterize the mGlu receptor type responsible for modulating NMDA responses. We also characterized a number of mGlu receptor agents by measuring [ 3 H]-inositol phosphate (IP) formation in cortical slices and in BHK cells expressing either mGlu 1 or mGlu 5 receptors. 2 (S)-3,5-dihydroxyphenylglycine (DHPG), an agonist of both mGlu 1 and mGlu 5 receptors, at concentrations ranging from 1 ± 10 mM, enhanced up to 105+15% the NMDA-induced depolarization. Larger concentrations (100 ± 300 mM) of the compound were inactive in this test. When evaluated on [ 3 H]-IP synthesis in cortical slices or in cells expressing either mGlu 1 or mGlu 5 receptors, DHPG responses (1 ± 300 mM) increased in a concentration-dependent manner. 3 (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) and (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG), had partial agonist activity on mGlu 5 receptors, with maximal e ects reaching approximately 50% that of the full agonists. These compounds, however, enhanced NMDA-evoked currents with maximal e ects not di erent from those induced by DHPG. Thus the enhancement of [ 3 H]-IP synthesis and the potentiation of NMDA currents were not directly related. 4 2-methyl-6-(phenylethynyl)-pyridine (MPEP, 1 ± 10 mM), a selective mGlu 5 receptor antagonist, reduced DHPG e ects on NMDA currents. 7-(hydroxyimino)cyclopropan[b]-chromen-1a-carboxylic acid ethylester (CPCCOEt, 30 mM), a preferential mGlu 1 receptor antagonist, did not reduce NMDA currents. 5 These results show that mGlu 5 receptor agonists enhance while mGlu 5 receptor antagonists reduce NMDA currents. Thus the use of mGlu 5 receptor agents may be suggested in a number of pathologies related to altered NMDA receptor function.