Scientist department of Anatomy, Physiology and Genetics USUHS frederick, Maryland
Abstract Text: Post-traumatic headache (PTH) is the most common and debilitating symptom following traumatic brain injury (TBI), and in a substantial number of patients, PTH can become chronic and significantly impact the patients’ quality of life. There is a prevalence of PTH after mild TBI as opposed to moderate and severe TBI. The weight-drop model is commonly used for the PTH study but with limitation of being unable to produce speed acceleration induced brain injury, and the use of other invasive TBI animal models could complicate the interpretation of PTH due to the open surgery itself induced dura damage. Although there is an increased understanding on the pathogenic mechanisms of PTH, effective treatments for PTH are still unavailable. In this study we employed a novel, and non-invasive close-head impact model of engineered rotational acceleration (CHIMERA) to investigate PTH in mice and to explore the potential therapeutic effects of enhancement of the endocannabinoid signaling on mTBI induced headache. We found that mice impacted at 0.65J once a day for 4 days had a significant increase in the righting reflex latency and greatly enhanced periorbital tactile hypersensitivity compared to the sham control. MJN110, a potent and selective 2-AG hydrolytic enzyme monoacylglycerol lipase (MAGL) dose dependently attenuated periorbital allodynia demonstrated by an increased tactile threshold in the MJN110 treatment group compared to that in the TBI/vehicle group. Moreover, administration of the calcitonin gene related peptide (CGRP) at 0.01 mg/kg significantly exaggerated periorbital allodynia in TBI animals at 33- and 45-days post-injury, but it had no effect in the sham and MJN110 treated animals. These results suggested that MJN110 treatment not only ameliorated the development of PTH, but also prevented TBI animals from being triggered by other pain stimulus. Immunohistochemistry analysis revealed an activation of microglia and astrocytes and the increased production of CGRP in the trigeminal nucleus caudalis and trigeminal ganglion at 7 days post-TBI. Treatment with MJN110 alleviated the accumulation of inflammatory cell infiltrates demonstrated by greatly reduced Iba1 and GFAP positive staining and the CGRP production in both trigeminal nucleus caudalis and trigeminal ganglion. The therapeutic effects of MJN110 were partially reduced by co-administration of the CB1 and CB2 cannabinoid receptor antagonists AM 281 and AM 630, suggesting that activation of the endocannabinoid signaling is able to ameliorate PTH. We also found that the pain inhibitory effects of MJN110 was reversed by co-administration of DO34, an inhibitor of the 2-AG synthetic enzymes diacylglycerol lipases, further indicating that augmentation of 2-AG signaling contributes to the protective effects of MJN110 on PTH. Consistently, the increased levels of 2-AG by MJN110 treatment in several regions of the pain transmission pathway, such as trigeminal ganglion, trigeminal nucleus caudalis and periaqueductal gray was significantly reduced by co-administration of DO34 measured by mass spectrometry. In conclusion, our findings suggested that inhibition of the 2-AG hydrolytic enzyme MAGL by MJN110 attenuated TBI induced periorbital allodynia, suppressed the activation of inflammatory glial cells and reduced the neuropeptide CGRP expression in the trigeminal pathway. Our study demonstrated that modulation of the endocannabinoid signaling is likely an effective strategy for the treatment of PTH although the exact mechanisms are still under investigation.
