The dynorphin/kappa opioid receptor mediates adverse immunological and behavioral outcomes induced by repetitive blast trauma

BACKGROUND

Adverse pathophysiological and behavioral outcomes related to mild traumatic brain injury (mTBI), posttraumatic stress disorder (PTSD), and chronic pain are common following blast exposure and contribute to decreased quality of life, but underlying mechanisms and prophylactic/treatment options remain limited. The dynorphin/kappa opioid receptor (KOR) system helps regulate behavioral and inflammatory responses to stress and injury; however, it has yet to be investigated as a potential mechanism in either humans or animals exposed to blast. We hypothesized that blast-induced KOR activation mediates adverse outcomes related to inflammation and affective behavioral response.

METHODS

C57Bl/6 adult male mice were singly or repeatedly exposed to either sham (anesthesia only) or blast delivered by a pneumatic shock tube. The selective KOR antagonist norBNI or vehicle (saline) was administered 72 h prior to repetitive blast or sham exposure. Serum and brain were collected 10 min or 4 h post-exposure for dynorphin A-like immunoreactivity and cytokine measurements, respectively. At 1-month post-exposure, mice were tested in a series of behavioral assays related to adverse outcomes reported by humans with blast trauma.

RESULTS

Repetitive but not single blast exposure resulted in increased brain dynorphin A-like immunoreactivity. norBNI pretreatment blocked or significantly reduced blast-induced increase in serum and brain cytokines, including IL-6, at 4 h post exposure and aversive/anxiety-like behavioral dysfunction at 1-month post-exposure.

CONCLUSIONS

Our findings demonstrate a previously unreported role for the dynorphin/KOR system as a mediator of biochemical and behavioral dysfunction following repetitive blast exposure and highlight this system as a potential prophylactic/therapeutic treatment target.

Repetitive Blast Promotes Chronic Aversion to Neutral Cues Encountered in the Peri-Blast Environment

Repetitive mild traumatic brain injury (mTBI) has been called the "signature injury" of military service members in the Iraq and Afghanistan wars and is highly comorbid with post-traumatic stress disorder (PTSD). Correct attribution of adverse blast-induced mTBI and/or PTSD remains challenging. Pre-clinical research using animal models can provide important insight into the mechanisms by which blast produces injury and dysfunction-but only to the degree by which such models reflect the human experience. Avoidance of trauma reminders is a hallmark of PTSD. Here, we sought to understand whether a mouse model of blast reproduces this phenomenon, in addition to blast-induced physical injuries. Drawing on well-established work from the chronic stress and Pavlovian conditioning literature, we hypothesized that even while one is anesthetized during blast exposure, environmental cues encountered in the peri-blast environment could be conditioned to evoke aversion/dysphoria and re-experiencing of traumatic stress. Using a pneumatic shock tube that recapitulates battlefield-relevant open-field blast forces, we provide direct evidence that stress is inherent to repetitive blast exposure, resulting in chronic aversive/dysphoric-like responses to previous blast-paired cues. The results in this report demonstrate that, although both single and repetitive blast exposures produce acute stress responses (weight loss, corticosterone increase), only repetitive blast exposure also results in co-occurring aversive/dysphoric-like stress responses. These results extend appreciation of the highly complex nature of repetitive blast exposure; and lend further support for the potential translational relevance of animal modeling approaches currently used by multiple laboratories aimed at elucidating the mechanisms (both molecular and behavioral) of repetitive blast exposure.