Here, we investigated the mechanism of the antihyperalgesic effect of capsaicin cream in the nerve injury-induced neuropathic pain model in mice. In naive mice, application of capsaicin cream onto footpad caused no significant changes in the thermal latency in contrast to the severe thermal hyperalgesia induced by a capsaicin ointment. On the other hand, application of the cream 3 h before test concentration dependently reversed both thermal and mechanical hyperalgesia observed after partial sciatic nerve injury in mice. In algogenic-induced nociceptive flexion (ANF) test, application of 0.1% capsaicin cream in naive mice blocked intraplantar (i.pl.) nociceptin-and ATP-induced flexion responses, whereas prostaglandin I 2 (PGI 2 ) agonist-induced responses were unaffected. After nerve injury PGI 2 agonist-induced flexion responses were hypersensitized, and capsaicin cream concentration dependently blocked these hyperalgesic responses. Intraplantar injection of capsaicin solution in ANF test also produced potent flexion responses in naive mice that were lost after neonatal capsaicintreatment. Partial sciatic nerve injury in neonatal capsaicintreated mice caused reappearance of i.pl. capsaicin-induced flexion responses, suggesting novel expression of capsaicin receptors due to injury. The PGI 2 agonist-induced responses were also hypersensitized in such injured mice. Capsaicin cream completely reversed both i.pl. capsaicin-or i.pl. PGI 2 agonist-induced hyperalgesia in neonatal capsaicin-treated injured mice. Finally, novel expression of VR1 receptors on neonatal capsaicin-insensitive neurons after nerve injury was confirmed by immunohistochemistry. The newly expressed VR1 receptors after nerve injury were mainly confined to A-fibers. Together, our results suggest that novel expression of capsaicin receptors in neuropathic condition contributes to the analgesic effects of the capsaicin cream.

injection; i.t., intrathecal injection; IB4, isolectin B4; MS-ODN, mismatch scramble oligodeoxynucleotide; PKA, cAMP-dependent protein kinase; PTH, parathyroid hormone; TIP39, tuberoinfundibular peptide of 39; TRPV1, transient receptor potential vanilloid 1. AbstractWe have previously demonstrated that parathyroid hormone 2 (PTH2) receptors are expressed in dorsal root ganglion (DRG) neurons and that its endogenous agonist tuberoinfundibular peptide of 39 residues (TIP39) causes nociceptive paw flexor responses after intraplantar administration. Here we found that the PTH2 receptor is selectively localized on myelinated A-, but not unmyelinated C-fibers using immunohistochemical labeling, based on PTH2 receptor expression on antibody N52-positive medium/large-sized DRG neurons, but not on TRPV1, substance P, P2X 3 receptor or isolectin B4-binding protein-positive small-sized DRG neurons. Pharmacological studies showed that TIP39-induced nociceptive responses were mediated by activation of G s and cAMP-dependent protein kinase. We also found that nociceptive responses induced by TIP39-or the cAMP analog 8-bromo-cAMP were significantly greater following partial sciatic nerve injury induced neuropathic pain, without changes in PTH2 receptor expression. Together these data suggest that activation of PTH2 receptors stimulates nociceptive A-fiber through G scAMP-dependent protein kinase signaling, and this pathway has elevated sensitization following nerve injury.