Neuropeptides
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Although chronic inflammatory pain is known to be associated with hypersensitivity to mu opioid receptor agonists, no evidence for changes in the expression and/or characteristics of central mu opioid receptors has yet been reported in relevant models of this type of pain. In the present study, both immunohistochemical and autoradiographic approaches were used to address this question in polyarthritic rats, on the 4th week after intradermal injection of complete Freund's adjuvant, when inflammatory pain was at its maximum. ⋯ These data indicate that chronic inflammatory pain caused by polyarthritis was associated with an increased expression of mu-opioid receptors in dorsal root ganglion sensory neurones that did not result in an increased spinal density of these receptors, in spite of their well established axonal transport in the central portion of primary afferent fibres to the dorsal horn. In contrast, axonal transport of mu-opioid receptors in the peripheral portion of these fibres probably accounts for the increased receptor density in inflamed tissues already reported in the literature.