Protease-activated receptor 2 activation is sufficient to induce the transition to a chronic pain state

Protease-activated receptor type 2 (PAR₂) is known to play an important role in inflammatory, visceral, and cancer-evoked pain based on studies using PAR₂ knockout (PAR₂ ^-/-) mice. We have tested the hypothesis that specific activation of PAR₂ is sufficient to induce a chronic pain state through extracellular signal-regulated kinase (ERK) signaling to protein synthesis machinery. We have further tested whether the maintenance of this chronic pain state involves a brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (trkB)/atypical protein kinase C (aPKC) signaling axis. We observed that intraplantar injection of the novel highly specific PAR₂ agonist, 2-aminothiazol-4-yl-LIGRL-NH₂ (2-at), evokes a long-lasting acute mechanical hypersensitivity (median effective dose ∼12 pmoles), facial grimacing, and causes robust hyperalgesic priming as revealed by a subsequent mechanical hypersensitivity and facial grimacing to prostaglandin E₂ (PGE₂) injection. The promechanical hypersensitivity effect of 2-at is completely absent in PAR₂ ^-/- mice as is hyperalgesic priming. Intraplantar injection of the upstream ERK inhibitor, U0126, and the eukaryotic initiation factor (eIF) 4F complex inhibitor, 4EGI-1, prevented the development of acute mechanical hypersensitivity and hyperalgesic priming after 2-at injection. Systemic injection of the trkB antagonist ANA-12 similarly inhibited PAR₂ -mediated mechanical hypersensitivity, grimacing, and hyperalgesic priming. Inhibition of aPKC (intrathecal delivery of ZIP) or trkB (systemic administration of ANA-12) after the resolution of 2-at-induced mechanical hypersensitivity reversed the maintenance of hyperalgesic priming. Hence, PAR₂ activation is sufficient to induce neuronal plasticity leading to a chronic pain state, the maintenance of which is dependent on a BDNF/trkB/aPKC signaling axis.