How Immune-inflammatory Processes Link CNS and Psychiatric Disorders: Classification and Treatment Implications

In this article the emerging biological overlaps of CNS disorders and psychiatric conditions are reviewed. Recent work has highlighted how immune-inflammatory processes and their interactions with oxidative and nitrosative stress, couple to drive changes in neuroregulatory tryptophan catabolites, with consequences for serotonin availability, including as a precursor for the melatonergic pathways. Subsequent alterations in the regulation of local melatonin synthesis are likely to have direct impacts on the reactivity of immune cells, both centrally and systemically. These inflammatory processes also lead to the activation of wider immune processes. Such wider processes can include the production of immunoglobulin (Ig)A and IgM antibody responses, including to tryptophan catabolites, emphasizing the importance of immune responses, and their interactions with inflammatory processes, in the etiology and course of an array of medical conditions, including CNS disorders and psychiatric conditions. Such work poses questions as to the validity and utility of current, non-biologically based classification systems for psychiatric and CNS disorders. In this article, the biological underpinnings of CNS disorders and psychiatric conditions are reviewed in the context of how recent data, in reconceptualizing key processes in these classically-conceived brain-associated disorders, provides scope for novel, and hopefully more clinically useful, treatments. These processes are looked at in detail in Alzheimer's disease and major depressive disorder. One important treatment target is the gut. Alterations in the gut, including gut permeability and the composition of the microbiome, have now become an important target for treatment across an array of medical conditions, emphasizing the importance of targeting regulators of the immune system in developing novel treatments that are based on a more comprehensive and 'wholistic' understanding of currently poorly managed medical conditions, particularly psychiatric and CNS disorders.