Acute Neuroimmune Modulation Attenuates the Development of Anxiety-Like Freezing Behavior in an Animal Model of Traumatic Brain Injury

Abstract

Chronic anxiety is a common and debilitating result of traumatic brain injury (TBI) in humans. While little is known about the neural mechanisms of this disorder, inflammation resulting from activation of the brain's immune response to insult has been implicated in both human post-traumatic anxiety and in recently developed animal models. In this study, we used a lateral fluid percussion injury (LFPI) model of TBI in the rat and examined freezing behavior as a measure of post-traumatic anxiety. We found that LFPI produced anxiety-like freezing behavior accompanied by increased reactive gliosis (reflecting neuroimmune inflammatory responses) in key brain structures associated with anxiety: the amygdala, insula, and hippocampus. Acute peri-injury administration of ibudilast (MN166), a glial cell activation inhibitor, suppressed both reactive gliosis and freezing behavior, and continued neuroprotective effects were apparent several months post-injury. These results support the conclusion that inflammation produced by neuroimmune responses to TBI play a role in post-traumatic anxiety, and that acute suppression of injury-induced glial cell activation may have promise for the prevention of post-traumatic anxiety in humans.

Reversal of Established Traumatic Brain Injury-Induced, Anxiety-Like Behavior in Rats after Delayed, Post-Injury Neuroimmune Suppression

“…92 Evidence for the specific involvement of these brain areas in human post-traumatic anxiety is complemented by animal models, including findings of increased PTSD-related traits and increased Stathmin 1 (a protein known to increase fear responses) expression in the amygdala after blast injury, 101 increased fear conditioning and up-regulation of excitatory N-methyl-D-aspartate receptors (crucial receptors for normal fear learning and memory) in the amygdala after concussive injury, 33 and our current results showing increased anxiety-like behavior and reactive gliosis in insula and amygdala at long-term time points after LFPI. However, though these results support findings from our previous study 34 indicating increased gliosis in these areas, the insula and amygdala are not the only regions involved in anxiety and do not exclude the possibility that other regions may be contributing to the results reported here. Future studies will be aimed at characterizing the model and inclusion of other brain regions involved in human anxiety, including cingulate cortex, hippocampus, medial prefrontal cortex, locus coeruleus, and the bed nucleus of the stria terminalis.…”

“…To our knowledge, this is the first study to examine delayed immunosuppression at long-term injury time points, because other immunosuppressive treatments targeting anxiety-like behaviors have been administered before or within hours of injury. 34,37,38,[66][67][68][69] These results indicate that the persistence of post-traumatic anxiety may reflect chronic neuroinflammatory neuropathy, a possibility supported by the observation of activated microglia and astrocytes, key cells mediating inflammatory processes, many years after injury in long-term survivors of TBI. [9][10][11][12] Chronic post-traumatic neuroinflammation suggests the presence of a self-perpetuating positive feedback loop, possibly involving reactivation and further promotion of inflammatory mediators after injury in an inflammation-damage-inflammation cycle.…”

“…However, we previously found that LFPI rats show increases in freezing responses even in the absence of fear conditioning. 34 These unconditioned freezing responses may reflect pathological anxiety, which involves exaggerated fear, hypervigilance, and readiness to respond to danger or negative events, 61 because freezing behavior is part of an anticipatory response to stress or danger. Shock was chosen as the stressor because it resulted in anxiety-like freezing behavior, which is a simple reproducible response elicited as a defense reaction in both conditioned and unconditioned fearful situations.…”

“…Several studies have reported increased anxiety-like behavior in rodent TBI models, [29][30][31][32] including increased conditioned 33 and unconditioned 34 fear responses to both learned and novel stimuli. TBI in rodents also increases levels of activated glial cells and proinflammatory cytokines, [34][35][36][37][38] and administration of these cytokines increases anxiety-like behaviors.…”

“…TBI in rodents also increases levels of activated glial cells and proinflammatory cytokines, [34][35][36][37][38] and administration of these cytokines increases anxiety-like behaviors. [29][30][31][32] The aim of the present study was therefore to determine whether neuroinflammation is associated with the long-term maintenance of post-traumatic anxiety in an animal model.…”