Inflammatory activation of the kynurenine pathway : studies with lipopolysaccharides

Abstract

Neuroinflammation is increasingly recognised as playing an important role in several major psychiatric disorders. The mechanisms by which neuroinflammation influences neurotransmitter systems have for long been unknown. However, the kynurenine pathway, a source of several neuroactive metabolites, is thought to serve as a link between immune signalling and neuronal activity in the brain. Kynurenic acid (KYNA), an end-metabolite of the kynurenine pathway, is an endogenous N-methyl-D-aspartate-receptor antagonist that strongly regulates brain dopamine activity. Elevated brain levels of KYNA are suggested to be involved in the pathophysiology of psychotic disorders and cognition.

The overall aim of this thesis is to investigate the interplay between inflammation and the kynurenine pathway. For this purpose, LPS is administered in doses that induce an inflammatory response in both animals and humans. Our findings show that dual, rather than a single, administration of LPS produces a robust induction of the kynurenine pathway, including increases in brain KYNA levels as well as increased turnover of brain serotonin and dopamine in rodents. Alterations in tryptophan metabolism via the kynurenine pathway in response to dual LPS administration is further shown to induce behavioural impairments, such as cognitive deficits and enhanced amphetamine-induced locomotor activity. In human primary dermal fibroblasts, IFN-γ and IL- 1β or their combination, were used to trigger the kynurenine pathway. Forty-eight hours poststimulation, IL-1β did not elevate extracellular kynurenine and KYNA levels, however IFN-γ induced an 11.5-fold increase in kynurenine and an 8-fold increase in KYNA and the combination of IL-1β with IFN-γ resulted in a synergistic increase in both kynurenine and KYNA. Kynurenine aminotransferase (KAT II) is the main enzyme involved in the synthesis of KYNA. Nevertheless, pharmacological inhibition of KAT II only to some extent reduced the cytokine-induced release of KYNA. In rodents, dual LPS administration increased brain KYNA levels despite pharmacological inhibition of KAT II or genetic ablation of the enzyme. To translate the experimental results obtained in the present thesis to humans, we investigated the effects of systemic LPS administration in healthy human subjects on the kynurenine pathway. Administration of LPS activated both the neurotoxic and the neuroprotective branch of the kynurenine pathway for at least 48 h post LPS-injection.

Overall, the results of the present thesis suggest that the dual LPS model can be used as an animal model, showing both face and construct validity regarding increased central and decreased peripheral KYNA levels as well as regarding aspects of behaviour reflecting psychosis and cognitive deficits. In combination with a cellular assay, this model would be suitable for translational studies of novel immunomodulatory agents, aiming at diminishing KYNA synthesis in psychotic disorders. This thesis, by the means of diverse experimental approaches ranging from the biochemical level, animal behaviour experiments and cellular systems to an experimental human endotoxemia model, confirms the hypothesis of the immune regulation of the kynurenine pathway in psychiatric disorders.