Closed Head Traumatic Brain Injury Drives Protracted Inflammation and Remodeling of Serotonergic Architecture in the Dorsal Raphe Nucleus

Abstract

Traumatic Brain Injury (TBI) presents a monumental public health burden that affects over 3 million Americans annually, with mild TBI (mTBI)/concussion being the most common form of closed head injury. Chronic neuropsychiatric sequalae are common following TBI, with many individuals spontaneously developing persistent depression, anxiety, social withdrawal and/or PTSD, disorders that have been previously linked to alterations in serotonin (5‐HT) signaling. Currently, the effects of various forms of TBI on the function of the 5‐HT signaling system within the CNS are poorly understood and selective serotonin reuptake inhibitors (SSRIs) fail to mitigate the clinical neuropsychiatric sequalae elicited by TBI. The lack of any FDA‐approved therapies for the treatment of acute or chronic symptoms of TBI necessitates a greater understanding of how TBI alters the environmental milieu of the CNS, including normal, physiologic 5‐HT signaling. We hypothesize that TBI drives the formation of aberrant behavioral states linked to neuropsychiatric disorders through alterations of normal, physiologic 5‐HT signaling stemming from 5‐HT neurons located within the dorsal raphe nucleus (DRN). To test this hypothesis, adult, wild type C57Bl/6J mice were subjected to a single, blast induced mTBI or sham treatment, followed by behavioral and/or molecular analyses 0‐10 days post‐injury (dpi). mTBI subjects exhibited significantly increased righting reflex times (RRT) compared to their sham counterparts immediately post‐injury. Ten dpi, mTBI subjects exhibited significant reductions in sociability and social dominance as compared to their sham counterparts in the Crawley three chamber sociability assay and tube test, respectively. High performance liquid chromatography (HPLC) revealed a time‐dependent increase in total levels of 5‐HT and its metabolite, 5HIAA within the DRN 10 dpi. No effect of mTBI on 5‐HT or 5HIAA levels were found in prefrontal cortex (PFC) or somatosensory cortex (SSC), areas of high 5‐HT innervation. Ten dpi, DRN punches were subjected to RNA‐sequencing to delineate differential gene‐expression profiles elicited by mTBI. Genes indicative of cellular remodeling, cell‐to‐cell adhesion, monoaminergic processing, and proliferation were identified. Crucially, genes involved in ongoing inflammation including those relevant to peripheral immune recruitment and blood brain barrier permeation were found to be differentially expressed. Quantitative real‐time PCR was subsequently used to confirm differential expression of genes of interest, notably Vegfc, Slc18a1, and Icam1. These studies provide critical insight into ongoing molecular processes following closed head injury that may impact normal, physiologic 5‐HT signaling within the DRN. Further, these studies may provide critical insight into regulation of 5‐HT neuron function within the DRN and yield novel drug candidates for ameliorating the neuropsychiatric complications elicited by TBI.