Activation and expression of µ-calpain in dorsal root contributes to RTX-induced mechanical allodynia.

Background Calpain is a calcium-dependent cysteine protease, and inhibition of calpain by pre-treatment with MDL28170 attenuated the rat mechanical allodynia in a variety of pain models. Postherpetic neuralgia (Shingles) is a neuropathic pain conditioned with the presence of profound mechanical allodynia. Systemic injection of resiniferatoxin can reproduce the clinical symptoms of postherpetic neuralgia. In this study, we determined to study whether activation of calpain contributes to cleave the myelin basic protein of dorsal root and is involved in resiniferatoxin-induced mechanical allodynia of postherpetic neuralgia animal model. Results Resiniferatoxin up-regulated the expression and activation of µ-calpain in dorsal root. The expression of µ-calpain was located in Schwann cell of dorsal root, and resiniferatoxin increased the expression of µ-calpain in Schwann cell in L4-L6 dorsal root at six weeks after injection. Resiniferatoxin also induced myelin basic protein degradation in L4-L6 dorsal root at six weeks after injection. Moreover, intraperitoneal injection of calpain inhibitor MDL28170 prevented the degradation of myelin basic protein and then reduced the sprouting of myelinated afferent fibers into spinal lamina II, thus relieving resiniferatoxin-induced mechanical allodynia. Conclusions Up-regulation and activation of µ-calpain located in Schwann cell may be the mechanism underlying resiniferatoxin-mediated proteolysis of myelin basic protein in dorsal root. Calpain inhibitor MDL28170 prevents resiniferatoxin-induced sprouting of myelinated afferent fibers and mechanical allodynia through inhibition of degradation of the myelin basic protein in dorsal root. Our results indicate that inhibition of pathological µ-calpain activation may present an interesting novel drug target in the treatment of postherpetic neuralgia.

Netrin-1 Contributes to Myelinated Afferent Fiber Sprouting and Neuropathic Pain.

Netrin-1 is a neuronal guidance molecule implicated in the development of spinal cord neurons and cortical neurons. In the adult spinal cord, UNC5H (repulsive receptor of netrin-1), but not deleted in colorectal cancer (DCC) (attractive receptor of netrin-1), constitutes a major mode of netrin-1 signal transduction, which may be involved in axon repulsion and inhibits neurite outgrowth. Abnormal sprouting of myelinated afferent fibers in the spinal dorsal horn can cause mechanical allodynia associated with postherpetic neuralgia (PHN, Shingles) and other neuropathic pains. However, whether netrin-1 participates in sprouting of myelinated afferent fibers and mechanical allodynia remains unknown. In an ultropotent TRPV1 agonist resiniferatoxin (RTX)-induced PHN-like model, RTX treatment for 6 weeks increased netrin-1 expression in dorsal horn neurons, including NK-1-positive projection neurons. In human neuroblastoma SH-SY5Y cells, we found that TRPV1 antagonist capsazepine antagonized RTX-induced upregulation of netrin-1. After RTX treatment, UNC5H2 expression was gradually decreased, whereas DCC expression was significantly increased. Silencing netrin-1 in the spinal dorsal horn significantly attenuated RTX-induced mechanical allodynia and sprouting of myelinated fibers into the spinal lamina II. Our results suggest that RTX treatment upregulates netrin-1 expression through activation of TRPV1 receptors and change UNC5H2-rich spinal dorsal horn into a growth-permissive environment by increasing DCC expression, thus enhancing the sprouting of myelinated afferent nerves. Netrin-1 may be targeted for reducing primary afferent sprouting and mechanical allodynia in PHN and other neuropathic pain conditions.