Propranolol Elicits Long Term Systemic Effects After Repetitive Mild Traumatic Brain Injury

Abstract

There are almost 2 million new traumatic brain injuries (TBIs) every year in the US. Of these, 80% of these can be classified as mild TBI, also known as concussions, that can lead to pronounced long term symptoms months and years after injury. The presence of post traumatic headaches (PTH) is the most common chronic side effect with prevalence of 47-95% of mTBI patients within a week of injury. Though the mechanisms after TBI leading to these headaches and other post mTBI side effects are poorly understood, recent studies have suggested the role of the immune system after injury plays a causal role in this process. Peripheral immune cells can travel to the brain after mTBI as a result of blood brain barrier dysfunction, sympathetic nervous signaling, and the release of inflammatory mediators. Recent studies have shown sympathetic activation after injury can result in IL-10 dependent systemic immunosuppressive state after mTBI. In this study we sought to limit sympathetic dependent immune alterations after injury by injecting the beta blocker propranolol directly after injury and investigating the immune changes in the blood, brain, spleen, and bone marrow of mTBI animals. Together, these data show mTBI causes immune genetic and pathway level changes at least one month after injury and that propranolol alters genes important for metabolism, cytokine signaling, epigenetic modification, innate, and adaptive immunity. We also find that propranolol reduces the presence of Ly6C+ and increases the presence of Ly6C- monocytes in the blood one month after injury; however, it leads to increased Ly6C+ monocyte presence in the spleen of mTBI mice. In conclusion, propranolol administration directly after mTBI leads to immune changes that may lead to long-term improvement in post TBI symptomology.