Increased in vivo release of calcitonin gene-related peptide-like material from the spinal cord in arthritic rats

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Abstract
Possible alterations in spinal systems containing calcitonin gene-related peptide (CGRP) due to polyarthritis were assessed in rats 3–4 weeks after an intradermal injection of Freund's adjuvant in the low back. The tissue levels of CGRP-like material (CGRPLM) were ˜ 50% higher in the dorsal zone of the spinal cord and dorsal root ganglia at both the cervical and lumbar (but not thoracic) segments in polyarthritic rats than in age-paired control animals. In addition the rate of the spinal release of CGRPLM determined through an intrathecal perfusion procedure in halothane-anaesthetized animals was ˜ 15-fold higher in polyarthritic rats than in controls. The blockade of μ-opioid receptors by intrathecal perfusion with 10 μ M naloxone produced a larger increase in the spontaneous CGRPLM outflow in polyarthritic rats than in age-paired controls. Furthermore, the stimulation of μ-opioid receptors by intrathecal perfusion with 10 μ M DAGO significantly inhibited the spinal outflow of CGRPLM only in polyarthritic rats. These data indicate that CGRP-containing primary afferent fibres are markedly activated in chronic suffering polyarthritic rats. This activation occurs in spite of an increased tonic inhibitory control by endogenous opioids acting at μ receptors.