Caffeine-induced anxiety and increase of kynurenine concentration in plasma of healthy subjects: a pilot study.

The concentration of kynurenine, a neuroactive tryptophan metabolite, in blood plasma after pharmacologically induced anxiety was studied. Anxiety was provoked in 15 healthy volunteers by an anxiogenic dose of caffeine. Kynurenine concentration was markedly increased at the peak of anxiety and returned to normal after anxiety had abated. Possible causes responsible for this effect are discussed. There is a correlation between kynurenine concentration in blood plasma and indices of state and trait anxiety (Spielberger-Khanin scale) and anxiety (Hamilton scale) at baseline. The correlation disappears at the peak of anxiety. It is suggested that kynurenine is involved in caffeine-induced anxiety in humans. The absence of correlation at the peak of caffeine-induced anxiety is discussed.