Dorsal raphe neuroinflammation promotes dramatic behavioral stress dysregulation

Impulsivity, risk-taking behavior, and elevated stress responsivity are prominent symptoms of mania, a behavioral state common to schizophrenia and bipolar disorder. Though inflammatory processes activated within the brain are involved in the pathophysiology of both disorders, the specific mechanisms by which neuroinflammation drives manic behavior are not well understood. Serotonin cell bodies originating within the dorsal raphe (DR) play a major role in the regulation of behavioral features characteristic of mania. Therefore, we hypothesized that the link between neuroinflammation and manic behavior may be mediated by actions on serotonergic neurocircuitry. To examine this, we induced local neuroinflammation in the DR by viral delivery of Cre recombinase into interleukin (IL)-1β(XAT) transgenic male and female mice, resulting in overexpressing of the proinflammatory cytokine, IL-1β. For assertion of brain-region specificity of these outcomes, the prefrontal cortex (PFC), as a downstream target of DR serotonergic projections, was also infused. Inflammation within the DR, but not the PFC, resulted in a profound display of manic-like behavior, characterized by increased stress-induced locomotion and responsivity, and reduced risk-aversion/fearfulness. Microarray analysis of the DR revealed a dramatic increase in immune-related genes, and dysregulation of genes important in GABAergic, glutamatergic, and serotonergic neurotransmission. Behavioral and physiological changes were driven by a loss of serotonergic neurons and reduced output as measured by high-performance liquid chromatography, demonstrating inflammation-induced serotonergic hypofunction. Behavioral changes were rescued by acute selective serotonin reuptake inhibitor treatment, supporting the hypothesis that serotonin dysregulation stemming from neuroinflammation in the DR underlies manic-like behaviors.