A new rat model for thrombus-induced ischemic pain (TIIP); development of bilateral mechanical allodynia

Patients with peripheral arterial disease (PAD) commonly suffer from ischemic pain associated with severe thrombosis. However, the pathophysiology of peripheral ischemic pain is not fully understood due to the lack of an adequate animal model. In this study, we developed a new rodent model of thrombus-induced ischemic pain (TIIP) to investigate the neuronal mechanisms underlying ischemic pain. Ischemia was induced by application of 20% FeCl₂ onto the surface of the femoral artery for 20 min. Induction of peripheral ischemia was confirmed by measurement of the concentration of Evans blue and by increases in the ischemia-specific markers, hypoxia-inducible factor-1 alpha and vascular endothelial growth factor in the ipsilateral plantar muscles. Ischemic pain, as indicated by the presence of mechanical allodynia, developed bilaterally and peaked at days 3-9 post-FeCl₂ application and gradually decreased through day 31. Systemic heparin pretreatment dose dependently suppressed ischemic pain, suggesting that thrombosis-induced ischemia might be a key factor in TIIP. Intraplantar injection of BMS-182874, an ET_A (endothelin-A) receptor antagonist, at day 3 selectively blocked ipsilateral pain, indicating that ET_A receptor activity mediated TIIP. Spinal GFAP expression was significantly increased by FeCl₂ and intrathecal injection of carbenoxolone (an astrocyte gap junction decoupler) at day 3 significantly reduced TIIP, suggesting that spinal astrocyte activation plays an important role. However, the anti-inflammatory agent, ibuprofen, did not affect TIIP. In conclusion, we have developed a novel animal model of TIIP that should be useful in investigating the pathophysiological mechanisms that underlie human peripheral ischemic pain.