Microglial cathepsin B contributes to the initiation of peripheral inflammation-induced chronic pain.

Interleukin (IL)-1ß and IL-18 play critical roles in the induction of chronic pain hypersensitivity. Their inactive forms are activated by caspase-1. However, little is known about the mechanism underlying the activation of pro-caspase-1. There is increasing evidence that cathepsin B (CatB), a typical lysosomal cysteine protease, is involved in the pro-caspase-1 activation and the subsequent maturation of IL-1ß and IL-18. In this context, CatB is considered to be an important molecular target to control chronic pain. However, no information is currently available about the role of CatB in chronic pain hypersensitivity. We herein show that CatB deficiency or the intrathecal administration of CA-074Me, a specific CatB inhibitor, significantly inhibited the induction of complete Freund's adjuvant-induced tactile allodynia in mice without affecting peripheral inflammation. In contrast, CatB deficiency did not affect the nerve injury-induced tactile allodynia. Furthermore, CatB deficiency or CA-074Me treatment significantly inhibited the maturation and secretion of IL-1ß and IL-18 by cultured microglia following treatment with the neuroactive glycoprotein chromogranin A (CGA), but not with ATP. Moreover, the IL-1ß expression in spinal microglia and the induction of tactile allodynia following the intrathecal administration of CGA depended on CatB, whereas those induced by the intrathecal administration of ATP or lysophosphatidic acid were CatB independent. These results strongly suggest that CatB is an essential enzyme for the induction of chronic inflammatory pain through its activation of pro-caspase-1, which subsequently induces the maturation and secretion of IL-1ß and IL-18 by spinal microglia. Therefore, CatB-specific inhibitors may represent a useful new strategy for treating inflammation-associated pain.

Calpain-mediated down-regulation of myelin-associated glycoprotein in lysophosphatidic acid-induced neuropathic pain.

Lysophosphatidic acid receptor (LPA(1)) signaling initiates neuropathic pain through demyelination of the dorsal root (DR). Although LPA is found to cause down-regulation of myelin proteins underlying demyelination, the detailed mechanism remains to be determined. In the present study, we found that a single intrathecal injection of LPA evoked a dose- and time-dependent down-regulation of myelin-associated glycoprotein (MAG) in the DR through LPA(1) receptor. A similar event was also observed in ex vivo DR cultures. Interestingly, LPA-induced down-regulation of MAG was significantly inhibited by calpain inhibitors (calpain inhibitor X, E-64 and E-64d) and LPA markedly induced calpain activation in the DR. The pre-treatment with calpain inhibitors attenuated LPA-induced neuropathic pain behaviors such as hyperalgesia and allodynia. Moreover, we found that sciatic nerve injury activates calpain activity in the DR in a LPA(1) receptor-dependent manner. The E-64d treatments significantly blocked nerve injury-induced MAG down-regulation and neuropathic pain. However, there was no significant calpain activation in the DR by complete Freund's adjuvant treatment, and E-64d failed to show anti-hyperalgesic effects in this inflammation model. The present study provides strong evidence that LPA-induced calpain activation plays a crucial role in the manifestation of neuropathic pain through MAG down-regulation in the DR.

Itch-scratch responses induced by lysophosphatidic acid in mice.

The present investigation was conducted in order to determine whether lysophosphatidic acid (LPA) induces itch-scratch responses (ISRs) in mice. Intradermal administration of LPA induces ISRs; furthermore, the time course for LPA-induced ISRs was similar to that for histamine-induced responses. Comparative study of the pruritogenic activity revealed that histamine possessed a potent effect characterized by a dose-response relationship; however, prostaglandin D2 failed to induce this response. Pretreatment with ketotifen, a histamine H1 receptor antagonist, and capsaicin inhibited LPA-induced ISRs. Additionally, LPA-induced ISRs were abolished by Y-27632, an inhibitor of Rho-associated protein kinase (ROCK). These findings suggest that LPA-induced ISRs are attributable to histamine- and substance-P-mediated pathways. Moreover, the Rho/ROCK-mediated pathway may be involved.