Objective: To investigate the role of proteinase-activated receptor 4 (PAR-4) in mediating joint inflammation and pain in mice.
Methods: Knee joint blood flow, edema, and pain sensitivity (as induced by thermal and mechanical stimuli) were assessed in C57BL/6 mice following intraarticular injection of either the selective PAR-4 agonist AYPGKF-NH(2) or the inactive control peptide YAPGKF-NH(2). The mechanism of action of AYPGKF-NH(2) was examined by pretreatment of each mouse with either the PAR-4 antagonist pepducin P4pal-10 or the bradykinin antagonist HOE 140. Finally, the role of PAR-4 in mediating joint inflammation was tested by pretreating mice with acutely inflamed knees with pepducin P4pal-10.
Results: PAR-4 activation caused a long-lasting increase in joint blood flow and edema formation, which was not seen following injection of the control peptide. The PAR-4-activating peptide was also found to be pronociceptive in the joint, where it enhanced sensitivity to a noxious thermal stimulus and caused mechanical allodynia and hyperalgesia. The proinflammatory and pronociceptive effects of AYPGKF-NH(2) could be inhibited by pepducin P4pal-10 and HOE 140. Finally, pepducin P4pal-10 ameliorated the clinical and physiologic signs of acute joint inflammation.
Conclusion: This study demonstrates that local activation of PAR-4 leads to proinflammatory changes in the knee joint that are dependent on the kallikrein-kinin system. We also show for the first time that PARs are involved in the modulation of joint pain, with PAR-4 being pronociceptive in this tissue. Thus, blockade of articular PAR-4 may be a useful means of controlling joint inflammation and pain.