The effect of an acute systemic inflammatory insult on the chronic effects of a single mild traumatic brain injury

A small but significant proportion of mild traumatic brain injury (mTBI) sufferers will report persistent symptoms, including depression, anxiety and cognitive deficits, in the months, or even years, following the initial event. This is known as post-concussion syndrome and its pathogenesis is not yet known. This study sought to investigate the role of a peripheral inflammatory insult in the development of ongoing behavioral symptoms following a mTBI. To investigate, male Sprague-Dawley rats were administered a single mTBI using the diffuse impact-acceleration model to generate ∼100 G of force. Sham animals underwent surgery only. At 5 days following surgery, rats were given either the TLR4 agonist, lipopolysaccharide (LPS, 0.1 mg/kg), or saline via an intraperitoneal injection. mTBI animals showed an exaggerated response to LPS, with an increase in the expression of pro-inflammatory cytokines within the hippocampus at 24 h post-dose, an effect not seen in sham animals. This was associated with the development of persistent behavioral deficits in the mTBI:LPS animals at 3 months post-injury. These behavioral deficits consisted of increased time spent immobile on the forced swim-test, indicative of depressive like behavior, impaired cognitive performance on the Barnes Maze and decreased anxiety on the Elevated Plus Maze. In contrast, animals administered mTBI alone had no deficits. This study provides evidence that a peripheral inflammatory stimulus can facilitate ongoing symptoms following a mTBI. As such this provides a basis for further exploration of exogenous factors which promote immune system activation as potential targets for intervention to allow the resolution of symptoms following a mTBI.