Cytokines, neurophysiology, neuropsychology, and psychiatric symptoms

Abstract

Recent research has overcome the old paradigms of the brain as an immunologically privileged organ, and of the exclusive role of neurotransmitters and neuropeptides as signal transducers in the central nervous system. Growing evidence suggests that the signal proteins of the immune system - the cytokines - are also involved in modulation of behavior and induction of psychiatric symptoms. This article gives an overview on the nature of cytokines and the proposed mechanisms of immune-to-brain interaction. The role of cytokines in psychiatric symptoms, syndromes, and disorders like sickness behavior, major depression, and schizophrenia are discussed together with recent immunogenetic findings.

Keywords: cytokine; immunogenetics; major depression; psychoneuroimmunology; schizophrenia; sickness behavior.

Figure 1.. The balance between Th1 (cell-mediated) and Th2 (humoral) response of the adaptive (specific) immune system. IL, interleukin; IFN-γ, interferon gamma; Th1 , T helper 1; Th2, T helper 2.

Figure 2.. The essential amino acid tryptophan is converted either into the neurotransmitter serotonin, or into the neuroactive metabolite kynurenine, which is further degraded to quinolinic acid. The rate-limiting enzyme in the kynurenine pathway, indoleamine2,3-dioxygenase (IDO), and kynurenine hydroxylase (KYNHydrox) are activated by the cytokine interferon gamma (IFN-γ).