Astrocytic localization of kynurenine aminotransferase II in the rat brain visualized by immunocytochemistry

Kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation, is a neuroinhibitory agent present in the mammalian brain. Endogenous KYNA preferentially affects the alpha7 nicotinic acetylcholine (alpha7nACh) receptor and, possibly, the glycine co-agonist (glycineB) site of the NMDA receptor. Functionally relevant fluctuations in brain KYNA occur under both physiological and pathological conditions, affecting cholinergic and glutamatergic neurotransmission. Kynurenine aminotransferase II (KAT II), the major biosynthetic enzyme of KYNA in the rat brain, catalyzes the irreversible formation of KYNA from its immediate bioprecursor, kynurenine. We now purified rat kidney KAT II to homogeneity, generated a polyclonal rabbit anti-rat KAT II antibody, and purified the antibody using routine biochemical methods. The antibody selectively recognized KAT II by Western blot analysis and in immunotitration experiments. Used for immunocytochemistry, the antibody revealed discrete, specific staining of KAT II-positive astrocyte-like cells throughout the adult rat brain. The presence of KAT II in astrocytes was confirmed by double fluorescence immunostaining with an antibody against the astrocyte-specific marker glial fibrillary acidic protein (GFAP). No specific labeling was detected in neurons or microglia. However, KAT II-positive astrocytes were intimately associated with select neuron populations, supporting a neuromodulatory role of KYNA. Intense staining was frequently seen around brain capillaries, with astrocytic end feet contacting the capillary wall. This may explain the rapid access of blood-derived kynurenine to KAT II-containing astrocytes. The new anti-KAT II antibody should be useful in the further elucidation of the presumed role of KYNA in brain physiology and pathology.