Early CALP2 expression and microglial activation are potential inducers of spinal IL‐6 up‐regulation and bilateral pain following motor nerve injury

Abstract

Previous work from our laboratory showed that motor nerve injury by lumbar 5 ventral root transection (L5-VRT) led to interleukin-6 (IL-6) over-expression in bilateral spinal cord, and that intrathecal administration of IL-6 neutralizing antibody delayed the induction of mechanical allodynia in bilateral hind paws. However, early events and upstream mechanisms underlying spinal IL-6 expression following L5-VRT require elucidation. The model of L5-VRT was used to induce neuropathic pain, which was assessed with von Frey hairs and the plantar tester in adult male Sprague-Dawley rats. Calpain-2 (CALP2, a calcium-dependent protease) knockdown or over-expression and microglia depletion were conducted intrathecally. Western blots and immunohistochemistry were performed to explore the possible mechanisms. Here, we provide the first evidence that both IL-6 and CALP2 levels are increased in lumbar spinal cord within 30 min following L5-VRT. IL-6 and CALP2 co-localized in both spinal dorsal horn (SDH) and spinal ventral horn. Post-operative (PO) increase in CALP2 in ipsilateral SDH was evident at 10 min PO, preceding increased IL-6 at 20 min PO. Knockdown of spinal CALP2 by intrathecal CALP2-shRNA administration prevented VRT-induced IL-6 overproduction in ipsilateral spinal cord and alleviated bilateral mechanical allodynia. Spinal microglia activation also played a role in early IL-6 up-regulation. Macrophage/microglia markers ED1/Iba1 were increased at 30 min PO, while glial fibrillary acidic protein (astrocyte) and CNPase (oligodendrocyte) markers were not. Increased Iba1 was detected as early as 20 min PO and peaked at 3 days. Morphology changed from a small soma with fine processes in resting cells to an activated ameboid shape. Depletion of microglia using Mac-1-saporin partially prevented IL-6 up-regulation and attenuated VRT-induced bilateral mechanical allodynia. Taken together, our findings provide evidence that increased spinal cord CALP2 and microglia cell activation may have early causative roles in IL-6 over-expression following motor nerve injury. Agents that inhibit CALP2 and/or microglia activation may therefore prove valuable for treating neuropathic pain.

Suppression of Superficial Microglial Activation by Spinal Cord Stimulation Attenuates Neuropathic Pain Following Sciatic Nerve Injury in Rats

“…Many studies have indicated that persistent mechanical hypersensitivity was induced in the hind paw following sciatic nerve injuries along with microglial cell accumulation in the spinal dorsal horn (SDH), and the suppression of microglial cell activation in the spinal cord significantly alleviated mechanical hypersensitivity [11][12][13]. In SDH, the increased Iba1 immunoreactivity that indicates the hyperactive states of microglia was reported to be detected as early as 20 min, peaking on day 3, and remaining at a significant level on day 7 following peripheral nerve injury [14].…”

Neuroinflammation in the anterior cingulate cortex: the potential supraspinal mechanism underlying the mirror-image pain following motor fiber injury

“…Chronic neuropathic pain (NP) results from multiple etiological factors that initiate diverse mechanisms and express both within, and across different disease states [1]. It is now clear that peripheral nerve in ammation or lesion can affect contralateral healthy structures, and thus results in mirror-image pain [2][3][4][5][6][7][8][9][10], a clinical pain-associated phenomenon [11][12][13][14]. Although the contralateral/mirror pain is commonly reported, it is hard to achieve satisfying therapeutic effect.…”

“…Ectopic discharges from uninjured but not injured afferents are important for the development of neuropathic pain [41]. Selective injury to the motor bers, leaving the sensory bers intact, by L5 ventral root transection (L5-VRT) can reliably induce mirror-image pain [2,9,10,21,[42][43][44]. In the present study, we systematically investigate and compare the spatiotemporal expression variations of chemokine CX3CL1, proin ammatory cytokines (TNF-α/interleukin-6) as well as voltage gated Nav1.6 sodium channel in bilateral ACC region of SNI and L5-VRT models, and discuss the possible mechanism underlying CX3CL1's involvement in ACC's descending facilitation of spinal cord.…”

Convolutional Neural Network Optimization Algorithm-Based Magnetic Resonance Imaging in Analysis of Chronic Pain Caused by the Myofascial Trigger Point

“…While the nodules contract, they can also stimulate the cells to release interleukin-6, capsaicin, bradykinin, and other pain-causing substances, which intensifies the pain of skeletal muscles. ese substances will also lower the activation threshold of nerves, leading to activation and sensitization of peripheral nerve endings and motor nerves, activating the secondary nerves, and leading to central sensitization [9][10][11]. When MTrP are properly treated clinically, local pain and involved pain can be more effectively reduced, perhaps due to the reduction of nociceptive stimulus signals introduced into the spinal dorsal horn neurons, which controls the spread of pain and central sensitization.…”