Abstract
Previous studies from our laboratory have demonstrated that both chronic inflammatory pain, induced by intraplantar injection of complete Freund's adjuvant (CFA), and prolonged (48 h) stimulation of mu-opioid receptors (μOR) by systemic administration of a variety of selective agonists, resulted in enhanced plasma membrane targeting of delta-opioid receptors (δOR) in neurons of the dorsal spinal cord. To determine whether δOR trafficking induced by chronic inflammation was dependent on the activation of μOR, we investigated by immunogold cytochemistry the effects of intraplantar CFA injection on the plasma membrane density of δOR in μOR knockout (KO) mice. In untreated wild-type (WT) mice, only a small proportion of δOR was associated with neuronal plasma membranes in the dorsal horn of the spinal cord. The CFA-induced inflammation produced a significantly higher ratio of plasma membrane to intracellular receptors, as well as a 75% increase in the membrane density of immunoreactive δOR, in dendrites of the ipsilateral dorsal horn as compared to untreated mice. This increase in the membrane density of δOR was likely due to a recruitment of receptors from intracellular stores since no difference in the overall δOR immunolabeling density was evident between CFA-treated and untreated mice. Most importantly, the CFA-induced changes in δOR plasma membrane insertion seen in WT animals were not present in the spinal cord of μOR KO mice. These results demonstrate that the integrity of μOR is necessary for CFA-induced changes in δOR trafficking to occur and suggest that these changes could be elicited by stimulation of μOR by endogenous opioids released in response to chronic inflammatory pain.
Highlights
Administration of opioid analgesics directly into the central nervous system (CNS) produces a profound increase in nociceptive thresholds in various animal models of acute and chronic pain
The percentage of gold particles associated with plasma membranes was significantly increased in dendritic profiles within the ipsilateral dorsal horn of complete Freund’s adjuvant (CFA)-treated mice as compared to untreated animals (P, 0:01; Fig. 2B; compare A and B in Fig. 1 and see Table 1)
These distributional changes translated into a 75% increase in the density of gold particles per unit length of plasma membrane in dendrites of the ipsilateral dorsal horn in CFA-treated as compared to untreated WT mice (Fig. 2C)
Summary
Administration of opioid analgesics directly into the central nervous system (CNS) produces a profound increase in nociceptive thresholds in various animal models of acute and chronic pain. Plastic changes have been documented to occur in the spinal cord in response to persistent inflammatory pain (Julius and Basbaum, 2001). Some of these changes affect the expression and subcellular distribution of dOR in neurons of the dorsal horn of the spinal cord and dorsal root ganglia (Besse et al, 1992; Cahill et al, 2003; Ji et al, 1995). It was suggested that these changes in the cellular compartmentalization of dOR might contribute to the enhanced opioid anti-nociception observed in this pain state (Cahill et al, 2003)
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