Substrate-selective COX-2 inhibition by IMMA attenuates posttraumatic headache via endocannabinoid modulation and neuroinflammatory suppression

Background: Posttraumatic headache (PTH) is a common and debilitating consequence of traumatic brain injury (TBI), characterized by neuroinflammation and pain hypersensitivity. Current treatments are limited, and novel therapeutics are needed. Indomethacin morpholinamide (IMMA), a substrate-selective cyclooxygenase-2 (COX-2) inhibitor, enhances endocannabinoid signaling without disrupting prostaglandin homeostasis and may offer a mechanistically distinct approach to managing PTH. Methods: Male C57BL/6J mice were subjected to repetitive mild TBI (rmTBI) using the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) and treated with IMMA (10 mg/kg, i.p.) daily for 7 days post-injury. Mechanical allodynia was assessed using von Frey stimulation of the periorbital region. Neuroinflammation was evaluated through immunohistochemistry in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC). Endocannabinoid and prostaglandin levels were quantified by mass spectrometry and enzyme immunoassay, respectively. Results: IMMA significantly reduced rmTBI-induced periorbital allodynia, microglial and astrocyte activation, and CGRP expression in the TG and TNC. It also preserved meningeal mast cell integrity and elevated cortical anandamide (AEA) levels without altering prostaglandin E₂ (PGE₂) production, supporting a mechanism that enhances cannabinoid signaling while sparing COX-2-mediated prostaglandin synthesis. Conclusion: IMMA effectively attenuates neuroinflammation and pain hypersensitivity in the acute phase of PTH through a distinct mechanism that preserves endocannabinoid tone without suppressing physiological prostaglandins. While these results highlight its promise as a novel therapeutic strategy, further studies are warranted to determine its efficacy during the chronic phase of PTH and across anatomically targeted regions.