**5. Conclusions**

In conclusion, the results of this preliminary study sugges<sup>t</sup> that both 2-AG and cortisol concentrations are associated with the risk for development of chronic pain following injury and add the endocannabinoid system to the list of stress-responsive systems that are associated with long-term consequences of an injury. We have previously published that 2-AG concentrations at the time of injury are also positively associated with risk for the development of major depression 6–9 months later [32]. Together, these studies indicate that 2-AG concentrations, which are roughly six times higher following trauma than under normal conditions, could serve as a general biomarker for risk for negative psychological states following traumatic injury. Future studies will probe the more interesting possibility that strong engagemen<sup>t</sup> of endocannabinoid signaling in the periphery and brain occurs in the aftermath of significant physiological stress and while this response may be beneficial at the time of injury, it could result in long-term negative effects in certain individuals.

**Author Contributions:** All four authors; C.M.T., C.J.H., A.S. and T.A.d.-C. contributed to all aspects of conceptualization, methodology, validation, and formal analysis, writing—original draft preparation, review and editing, and T.A.d.-C. and C.J.H. completed funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** Funding for this study was provided by a gran<sup>t</sup> from the National Institute on Mental Health (5R21MH102838 TA deRoon-Cassini and CJ Hillard, MPI).

**Institutional Review Board:** The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of Medical College of Wisconsin IRB# PRO00022827 approved on 3 May 2019 for studies involving humans.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** Cecilia Hillard is a member of the Scientific Advisory Boards of Phytecs, Inc. and Formulate Biosciences; and has equity in Formulate Biosciences.
