Biochemical Regulation of Brain Kynurenic Acid Synthesis and Inhibition in Rats is Sensitive to the Time of Day

Neurochemical imbalances, including elevations of the tryptophan metabolite kynurenic acid (KYNA), an endogenous antagonist of glutamatergic and cholinergic receptors, are linked to cognitive and sleep disturbances in psychiatric and neurocognitive disorders. Therapeutic strategies to reduce brain KYNA by inhibiting kynurenine aminotransferase II (KAT II) are under investigation. However, few studies consider time as a biological variable, despite recent evidence that the time of day can affect brain metabolism and drug effectiveness. Therefore, we explore the hypothesis that KYNA formation and synthesis inhibition change throughout the day. Using rats of both sexes, we measured basal KYNA levels and the effects of kynurenine (100 mg/kg, i.p.), to stimulate de novo KYNA, and/or PF-04859989 (KAT II inhibitor, 30 mg/kg, s.c.), at the beginning of light or dark phases. Microdialysis was used to assess extracellular KYNA in the dorsal hippocampus, and ex vivo assays evaluated KAT enzyme activity in separate animals. Additionally, we examined KYNA levels and the effect of PF-04859989 during acute sleep deprivation in male rats. Regardless of phase, PF-04859989 reduced basal KYNA levels in male but not female rats, yet it reduced kynurenine-stimulated KYNA synthesis in both sexes, demonstrating a context-specific action in female rats. Importantly, we observed a novel effect of phase in males, as kynurenine-induced KYNA synthesis and its inhibition by PF-04859989 were greater during the dark phase than during the light phase. Ex vivo , male KAT II activity was higher, and PF-04859989 was more effective, in the dark than in the light phase, suggesting that properties of the KAT II enzyme itself fluctuate with time of day. Finally, sleep deprivation increased extracellular KYNA levels in the light phase, and PF-04859989 fully ameliorated this increase. Overall, our findings highlight the need to consider time-dependent factors when developing therapies impacting KYNA synthesis. Lay Summary Changes in brain neurochemistry, including elevations in the tryptophan metabolite kynurenic acid (KYNA), are common in brain disorders that present with sleep disturbances and cognitive deficits as symptoms. KYNA interferes with neurotransmission critical for cognition and sleep, so therapeutic strategies to reduce brain KYNA are being pursued. As recent literature has highlighted the impact of time on brain metabolism and drug efficacy, we, for the first time, explored the hypothesis that KYNA formation and synthesis inhibition change throughout the day. We measured extracellular KYNA levels in the brains of male and female rats and stimulated KYNA synthesis with physiological challenges (exogenous kynurenine, the KYNA bioprecursor, or acute sleep deprivation) and/or inhibited KYNA synthesis pharmacologically (PF-04859989, an inhibitor of the KYNA-synthesizing enzyme kynurenine aminotransferase II (KAT II)). KYNA formation, KAT II enzyme activity, and the effectiveness of PF-04859989 varied throughout the day, highlighting time as a key factor modulating KYNA brain metabolism. PF-04859989 reduced KYNA levels under most conditions. Our findings suggest that the timing of KYNA-targeted treatments should be carefully considered in therapeutic strategies to improve cognition and sleep in brain disorders.