The role of serotonin in schizophrenia. New findings

Paliperidone Reversion of Maternal Immune Activation-Induced Changes on Brain Serotonin and Kynurenine Pathways

Emerging evidence indicates that early-life exposure to environmental factors may increase the risk for schizophrenia via inflammatory mechanisms. Inflammation can alter the metabolism of tryptophan through the oxidative kynurenine pathway to compounds with neurotoxic and neuroprotective activity and compromise serotonin (5-HT) synthesis. Here we investigate the role of serotonergic and kynurenine pathways in the maternal immune activation (MIA) animal model of schizophrenia. The potential reversion exerted by long-term antipsychotic treatment was also evaluated. MIA was induced by prenatal administration of polyinosinic:polycytidylic acid (poly (I:C)) in mice. Expression of different proteins and the content of different metabolites involved in the function of serotonergic and kynurenine pathways was assessed by RT-PCR, immunoblot and ELISA analyses in frontal cortex of the offspring after puberty. MIA decreased tissue 5-HT content and promoted changes in the expression of serotonin transporter, 5-HT_2A and 5-HT_2C receptors. Expression of indoleamine 2,3-dioxygenase 2 (IDO2) and kynurenine 3-monooxygenase (KMO) was increased by poly (I:C) whereas kynurenine aminotransferase II and its metabolite kynurenic acid were not altered. Long-term paliperidone was able to counteract MIA-induced changes in 5-HT and KMO, and to increase tryptophan availability and tryptophan hydroxylase-2 expression in poly (I:C) mice but not in controls. MIA-induced increase of the cytotoxic risk ratio of kynurenine metabolites (quinolinic/kynurenic acid) was also reversed by paliperidone. MIA induces specific long-term brain effects on serotonergic activity. Such effects seem to be related with alternative activation of the kynurenine metabolic pathway towards a cytotoxic status. Atypical antipsychotic paliperodine partially remediates abnormalities observed after MIA.

The role of serotonin in schizophrenia

The hypothesis that the LSD psychosis and by inference schizophrenic psychoses are related to dysfunctions in central serotonergic systems, formulated by Woolley and Shaw in the early 1950s was the first testable theory of modern biological psychiatry. Initially, it did not get the scientific attention it deserved. First, because LSD fell into disrepute and was to all intents and purposes banned from human experimentation. Secondly, the antipsychotics were discovered in the same period, and it became clear that these compounds block dopaminergic transmission and hence for many years thereafter the dopaminergic system occupied center stage in biological schizophrenia research. Presently, interest in the relation between serotonin and schizophrenia has been revived, due to the development of serotonin-blocking agents that appear to exert therapeutic effects in schizophrenia. In this paper the evidence for and against a link between serotonergic defects and schizophrenia psychopathology is critically discussed. The conclusion to be reached is threefold. (1) Interruption of certain serotonergic circuits represents an antipsychotic principle. (2) Tentative evidence suggests the involvement of serotonergic dysfunctions in the pathogenesis of schizophrenic psychoses. (3) It is not yet known whether serotonergic lesions contribute directly to the occurrence of schizophrenic psychopathology or via alterations in the dopaminergic system.