Kynurenic acid leads, dopamine follows: A new case of volume transmission in the brain?

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Abstract
Intrastriatal infusion of nanomolar concentrations of kynurenic acid (KYNA), an astrocyte-derived neuroinhibitory tryptophan metabolite, reduces basal extracellular dopamine (DA) levels in the rat striatum. This effect is initiated by the inhibition of α7 nicotinic acetylcholine receptors (α7nAChRs) on glutamatergic afferents. The present study was designed to further investigate this functional link between KYNA and DA using striatal microdialysis in awake animals. In rats, increases in KYNA, caused by intrastriatal infusions of KYNA itself (100 nM) or of KYNA’s bioprecursor L-kynurenine (2 µM), were associated with substantial reductions in DA. Co-infusion of KYNA with the α7nAChR agonist galantamine (5 µM), but not with the NMDA receptor agonist D-serine (100 nM), prevented this effect. Moreover, KYNA also reduced DA levels in the NMDA-lesioned striatum. Conversely, extracellular DA levels were enhanced when KYNA formation was compromised, either by astrocyte poisoning with fluorocitrate or by perfusion with aminooxyacetic acid (AOAA; 5 mM), a non-specific inhibitor of KYNA synthesis. Notably, this effect of AOAA was prevented by co-perfusion with 100 nM KYNA. In the striatum of 21 day-old mice with a targeted deletion of kynurenine aminotransferase II, extracellular KYNA levels were reduced by 67 ± 6%, while extracellular DA levels were simultaneously increased by 170 ± 14%. Taken together, a picture emerges where fluctuations in the astrocytic production of KYNA, possibly through volume transmission, inversely regulate dopaminergic tone. This newly uncovered mechanism may profoundly influence DA function under physiological and pathological conditions.