Neuroscience Letters, 146 (1992) 115-118

115

© 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 05.00 NSL 09041

The effect of peripheral loading with kynurenine and probenecid on extracellular striatal kynurenic acid concentrations J o a n n e M . Miller, U s h a M a c G a r v e y a n d M . F l i n t Beal Department of Neurology, Massachusetts General Hospital, Boston, MA 02114 (USA)

(Received 11 March 1992; Revised version received 10 June 1992; Accepted 28 July 1992) Key words: Kynurenicacid; Kynurenine; Microdialysis; Probenecid; Striatum

Kynurenic acid (KYA) is the only known endogenous excitatory amino acid antagonist in mammalian brain. In the present study we examined the effects of precursor loading with kynurenine (KYN) and blockade of organic acid transport with probenecid, either alone or in combination, on extracellular striatal KYA concentrations in unanesthetized rats. Baseline KYA concentrations were 1.61_+0.29pmol/ml. Following administration of KYN 150 mg/kg with increasing doses of probenecid a maximal increase in KYA to 946_+210 pmol/ml was seen with probenecid 200 mg/kg. Probenecid 200 mg/kg alone increased KYA levels to 16.0_+5.2pmol/ml. The combination of probenecid 200 mg/kg with KYN 450 mg/kg produced a maximal increase of KYA to 2085_+391 pmol/ml, a 1300-fold increase indicating marked potentiation. These results show that pharmacologic manipulation can markedly increase extracellular fluid concentrations of KYA into a range which may be useful in attempts to block NMDA receptor-mediated neurotoxicity.

Kynurenic acid (KYA), a tryptophan metabolite, is the only endogenous N M D A receptor antagonist currently identified in m a m m a l i a n brain. It has been shown to block synaptic transmission in the hippocampus [13], to attenuate quinolinic acid-induced neurodegeneration and seizures [3], decrease infarction size and neurological deficits after middle cerebral artery occlusion [6] and have neuroprotective effects after hypoxic ischemia in neonatal rats [1]. KYA has demonstrated only a limited ability to pass into the CNS from the periphery [4], however, kynurenine (KYN), its immediate precursor, has been shown to rapidly pass through the b l o o d - b r a i n barrier at a significant rate [5] via the large neutral amino acid carrier system [4]. The conversion of K Y N to KYA in rat brain is catalyzed primarily by the enzyme kynurenine-pyruvate aminotransferase [11] and increased brain levels of KYA after peripheral K Y N administration have been observed [19]. Recent studies have shown increased extracellular levels o f KYA in the striatum after peripheral administration of small doses of either K Y N or to a lesser extent, tryptophan [16]. Probenecid, a known inhibitor of organic acid transport has also been shown to Correspondence. M.F. Beal, Department of Neurology, Neurology Research 4, Massachusetts General Hospital, Fruit St., Boston, MA 02114, USA.

increase tissue levels of KYA in rat brain [10, 19]. In the present study large i.p. doses of the combination of K Y N and probenecid were used to pharmacologically maximize KYA concentrations measured in striatal dialysates of freely moving rats. Parts o f these data were previously published in abstract form [9]. Male Sprague-Dawley rats (250-350 g) were maintained on a 12 h light/dark cycle and allowed food and water ad libitum. On the day of surgery, rats were anesthetized with pentobarbital (65 mg/kg) and placed in a K o p f stereotaxic frame. Stainless steel guide shafts (21gauge tubing) were implanted in the striatum (AP + 2.0, M L + 2.4 from bregma and - 3 . 0 from the dura) and secured with dental cement. Animals were allowed to recover for approximately 24 h before probe insertion. Probes of concentric design (4 m m exposed membrane; cutoff 6000 MW) were perfused with modified Ringer's solution containing (in m M ) K + 2.7, N a ÷ 145, Ca 2+ 1.2, Mg 2+ 1.0, phosphate 2, p H 7.40 at a flow rate of 2.0 /.tl/min using a CMA/100 microinjection p u m p and samples were collected at 20-min intervals. In vitro probe recoveries measured at 37°C were 16.5+2.3% (mean + S.E.M.). Results were not corrected for recovery. Probe placement was verified by visual inspection of brain sections from each animal. Kynurenine sulfate and probenecid (Sigma Co., St. Louis, MO) were dissolved in saline with sodium hydrox-

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Fig. 1. Kynurenic acid concentrations in striatal dialysates after i.p. injection of kynurenine 150 mg/kg in combination with various doses of probenecid (PROB). Values represent mean+S.E.M. (n=4-5 at each dose). Comparisons to dose 0: dose 100, P

The effect of peripheral loading with kynurenine and probenecid on extracellular striatal kynurenic acid concentrations.

Kynurenic acid (KYA) is the only known endogenous excitatory amino acid antagonist in mammalian brain. In the present study we examined the effects of...
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