Low-dose methotrexate reduces peripheral nerve injury-evoked spinal microglial activation and neuropathic pain behavior in rats

Abstract

Peripheral nerve injuries that provoke neuropathic pain are associated with microglial activation in the spinal cord. We have investigated the characteristics of spinal microglial activation in three distinct models of peripheral neuropathic pain: spared nerve injury (SNI), chronic constriction injury, and spinal nerve ligation. In all models, dense clusters of cells immunoreactive for the microglial marker CD11b formed in the ipsilateral dorsal horn 7 days after injury. Microglial expression of ionized calcium binding adapter molecule 1 (Iba1) increased by up to 40% and phosphorylation of p38 mitogen-activated protein kinase, a marker of microglial activity, by 45%. Expression of the lysosomal ED1-antigen indicated phagocytic activity of the cells. Unlike the peripheral nerve lesions, rhizotomy produced only a weak microglial reaction within the spinal gray matter but a strong activation of microglia and phagocytes in the dorsal funiculus at lumbar and thoracic spinal cord levels. This suggests that although degeneration of central terminals is sufficient to elicit microglial activation, it does not account for the inflammatory response in the dorsal horn after peripheral nerve injury. Early intrathecal treatment with low-dose methotrexate, beginning at the time of injury, decreased microglial activation, reduced p38 phosphorylation, and attenuated pain-like behavior after SNI. In contrast, systemic or intrathecal delivery of the glucocorticoid dexamethasone did not inhibit the activation of microglia or reduce pain-like behavior. We confirm that microglial activation is crucial for the development of pain after nerve injury, and demonstrate that suppression of this cellular immune response is a promising approach for preventing neuropathic pain.

Nerve Injury-related Autoimmunity Activation Leads to Chronic Inflammation and Chronic Neuropathic Pain

“…Methotrexate, a known immune suppressor administered intrathecally with a low dose from the start of injury, reduced neuropathic pain behavior in rats. 20,21 Although immune suppressors are commonly used in chronic pain caused by arthritis, their efficacy and safety in neuropathic pain patients needs further investigation.…”

“…18,19 Lowdose methotrexate, a known immune suppressor when given intrathecally, reduced peripheral nerve injury-evoked neuropathic pain behavior in rats. 20,21 Moreover, congenitally PAIN MEDICINE athymic nude rats, which lack mature T cells, developed significantly less mechanical allodynia and thermal hyperalgesia after nerve injury when compared to their heterozygous littermates. however, passive transfer of proinflammatory cytokine-producing type 1 T cells into nude rats enhanced the recipients' pain hypersensitivity to a level similar to that of the heterozygous donor rats.…”

SNO-ing at the Nociceptive Synapse?

“…Apoptosis-related signaling kinase 1 (ASK1), a mitogen-activated protein kinase kinase, can activate NF-B by phosphorylation of IB (Zhao and Lee, 1999) and also stimulates the MAP signaling cascade, which involves the sequential phosphorylation of MAP kinase kinase, c-Jun N-terminal kinase, and activator protein 1 and culminates in c-Jun/c-Fos mediated gene transcription. Activation of MAP kinase pathways in nociceptive neurons in the DRGs and spinal cord mediates fast adaptation to noxious stimulation (Ma and Quirion, 2002;Obata et al, 2004) and is essential for the activation of microglial cells after nerve injury (Jin et al, 2003;Scholz et al, 2008). Nitric oxide controls the MAP kinase pathway by S-nitrosylation-mediated inhibition of ASK1 and of c-Jun N-terminal kinase 1 (Fig.…”

Inhibition of Mitogen-Activated Protein Kinases Phosphorylation Plays an Important Role in the Anti-nociceptive Effect of Pregabalin in Zymosan-Induced Inflammatory Pain Model

“…3) In neuropathic pain model, it is well documented that microglial activation (using markers CD11b and ED1) accompanying p38 phosphorylation in the spinal cord plays an important role in the development of pain. 2,4,13) Current study also revealed that zymosan dramatically increased spinal p38 phosphorylation as well as microg- lia proliferation (CD11b) and phagocytic marker (ED1) elevation in the spinal dorsal horn. It was notable that zymosan induced phosphorylation p38 exclusively observed in microglia but not in spinal neurons in present double labelling study.…”

“…3) In line with this view, microglia inhibitors successfully alleviate neuropathic pain in experimental models. 3,4) Similar with neuropathic condition, peripheral inflammation by intraplanar zymosan injection as an experimental rheumatoid arthritis model also results in the development of hyperalgesia and allodynia accompany with spinal microglia activation and cytokine expression. 5,6) Accumulative data have been revealed that activation of mitogen activated protein kinases (MAPKs, i.e., extracellular signal-regulated protein kinase (ERK) 1/2, p38 MAPK, and ERK5) in both spinal neurons and microglia may have a pivotal role in development of pain symptom following nerve injury and peripheral inflammation.…”