ABSTRACT
Spinal mGluR5 is a key mediator of neuroplasticity underlying persistent pain. Although brain mGluR5 is localized on cell surface and intracellular membranes, neither the presence nor physiological role of spinal intracellular mGluR5 is established. Here we show that in spinal dorsal horn neurons >80% of mGluR5 is intracellular, of which â¼60% is located on nuclear membranes, where activation leads to sustained Ca(2+) responses. Nerve injury inducing nociceptive hypersensitivity also increases the expression of nuclear mGluR5 and receptor-mediated phosphorylated-ERK1/2, Arc/Arg3.1 and c-fos. Spinal blockade of intracellular mGluR5 reduces neuropathic pain behaviours and signalling molecules, whereas blockade of cell-surface mGluR5 has little effect. Decreasing intracellular glutamate via blocking EAAT-3, mimics the effects of intracellular mGluR5 antagonism. These findings show a direct link between an intracellular GPCR and behavioural expression in vivo. Blockade of intracellular mGluR5 represents a new strategy for the development of effective therapies for persistent pain.
Subject(s)
Behavior, Animal , Calcium/metabolism , Glutamic Acid/metabolism , Hyperalgesia/metabolism , Neuralgia/metabolism , Posterior Horn Cells/metabolism , Receptor, Metabotropic Glutamate 5/metabolism , Sciatic Neuropathy/metabolism , Analgesics, Opioid/pharmacology , Animals , Blotting, Western , Cells, Cultured , Cytoskeletal Proteins/metabolism , Excitatory Amino Acid Transporter 3/antagonists & inhibitors , Glutamic Acid/pharmacology , Hyperalgesia/pathology , Immunohistochemistry , Injections, Spinal , Male , Microdialysis , Microscopy, Confocal , Microscopy, Electron , Mitogen-Activated Protein Kinase 1/metabolism , Mitogen-Activated Protein Kinase 3/metabolism , Morphine/pharmacology , Nerve Tissue Proteins/metabolism , Posterior Horn Cells/pathology , Proto-Oncogene Proteins c-fos/metabolism , Proto-Oncogene Proteins c-jun/metabolism , Rats , Rats, Long-Evans , Sciatic Nerve/injuries , Sciatic Neuropathy/pathologyABSTRACT
BACKGROUND: Chronic pain occurs when normally protective acute pain becomes pathologically persistent. We examined here whether an isoform of protein kinase C (PKC), PKMζ, that underlies long-term memory storage in various brain regions, also sustains nociceptive plasticity in spinal cord dorsal horn (SCDH) mediating persistent pain. RESULTS: Cutaneous injury or spinal stimulation produced persistent increases of PKMζ, but not other atypical PKCs in SCDH. Inhibiting spinal PKMζ, but not full-length PKCs, reversed plasticity-dependent persistent painful responses to hind paw formalin and secondary mechanical hypersensitivity and SCDH neuron sensitization after hind paw capsaicin, without affecting peripheral sensitization-dependent primary heat hypersensitivity after hind paw capsaicin. Inhibiting spinal PKMζ, but not full-length PKCs, also reversed mechanical hypersensitivity in the rat hind paw induced by spinal stimulation with intrathecal dihydroxyphenylglycine. Spinal PKMζ inhibition also alleviated allodynia 3 weeks after ischemic injury in rats with chronic post-ischemia pain (CPIP), at a point when allodynia depends on spinal changes. In contrast, spinal PKMζ inhibition did not affect allodynia in rats with chronic contriction injury (CCI) of the sciatic nerve, or CPIP rats early after ischemic injury, when allodynia depends on ongoing peripheral inputs. CONCLUSIONS: These results suggest spinal PKMζ is essential for the maintenance of persistent pain by sustaining spinal nociceptive plasticity.
