Kynurenate production by cultured human astrocytes.
In the rodent brain, astrocytes are known to be the primary source of kynurenate (KYNA), an endogenous antagonist of both the glycine(B) and the alpha7 nicotinic acetylcholine receptor. In the present study, primary human astrocytes were used to examine the characteristics and regulation of de novo KYNA synthesis in vitro. To this end, cells were exposed to KYNA's bioprecursor L-kynurenine, and newly formed KYNA was recovered from the extracellular milieu. The production of KYNA was stereospecific and rose with increasing L-kynurenine concentrations, reaching a plateau in the high microM range. In an analogous experiment, astrocytes also readily produced and liberated the potent, specific glycine(B) receptor antagonist 7-chlorokynurenate from L-4-chlorokynurenine. KYNA synthesis was dose-dependently reduced by L-leucine or L-phenylalanine, two amino acids that compete with L-kynurenine for cellular uptake, and by aminooxyacetate, a non-specific aminotransferase inhibitor. In contrast, KYNA formation was stimulated by 5 mM pyruvate or oxaloacetate, which act as co-substrates of the transamination reaction. Aglycemic or depolarizing (50 mM KCl or 100 microM veratridine) conditions had no effect on KYNA synthesis. Subsequent studies using tissue homogenate showed that both known cerebral kynurenine aminotransferases (KAT I and KAT II) are present in astrocytes, but that KAT II appears to be singularly responsible for KYNA formation under physiological conditions. Taken together with previous results, these data suggest that very similar mechanisms control KYNA synthesis in the rodent and in the human brain. These regulatory events are likely to influence the neuromodulatory effects of astrocyte-derived KYNA in the normal and diseased human brain.
Kiss
C
C
Maryland Psychiatric Research Center, University of Maryland School of Medicine, Baltimore, Md, USA.
Ceresoli-Borroni
G
G
Guidetti
P
P
Zielke
C L
CL
Zielke
H R
HR
Schwarcz
R
R
D000625:Aminooxyacetic Acid / Q000008:administration & dosage / Q000378:metabolism; D001253:Astrocytes / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D001921:Brain / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D018691:Excitatory Amino Acid Antagonists / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D005998:Glycine / Q000037:antagonists & inhibitors; D006801:Humans; D007150:Immunohistochemistry; D007736:Kynurenic Acid / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000378:metabolism*; D007737:Kynurenine / Q000008:administration & dosage / Q000378:metabolism; D007930:Leucine / Q000008:administration & dosage / Q000378:metabolism; D018696:Neuroprotective Agents / Q000378:metabolism*; D009538:Nicotine / Q000037:antagonists & inhibitors; D062907:Oxaloacetic Acid / Q000008:administration & dosage / Q000378:metabolism; D010649:Phenylalanine / Q000008:administration & dosage / Q000378:metabolism; D019289:Pyruvic Acid / Q000008:administration & dosage / Q000378:metabolism; D011978:Receptors, Nicotinic / Q000378:metabolism; D000637:Transaminases / Q000378:metabolism
D018691:Excitatory Amino Acid Antagonists; D018696:Neuroprotective Agents; D011978:Receptors, Nicotinic; D000625:Aminooxyacetic Acid; D062907:Oxaloacetic Acid; D007737:Kynurenine; D010649:Phenylalanine; D009538:Nicotine; D019289:Pyruvic Acid; D000637:Transaminases; C021481:kynurenine-oxoglutarate transaminase; D007930:Leucine; D007736:Kynurenic Acid; C057013:7-chlorokynurenic acid; D005998:Glycine