Functional regulation of opioid receptor signaling

Abstract

Studies have shown that long-term opioid agonist (such as morphine) treatment produces antinociceptive tolerance and increased pain sensitivity (hyperalgesia and/or allodynia), limiting the clinical efficacy of morphine. Prolonged opiate administration also upregulates spinal pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) levels and enhances evoked CGRP release in the dorsal horn of rats. It was suggested that augmented spinal pain neurotransmission may contribute to paradoxical pain sensitization and antinociceptive tolerance. The cellular signal transduction pathways involved in sustained opioid mediated augmentation of spinal pain neurotransmitter are not fully clarified.Sustained morphine treatment was shown to augment the concentrations of inflammatory mediators, such as PGE2 in the spinal cord. Studies have shown that PGE2 stimulates cAMP formation and CGRP release by activation of Gs protein-coupled prostaglandin receptor types in primary sensory neurons. Interestingly, it was found earlier that sustained opioid agonist treatment leads to a Raf-1-dependent sensitization of adenylyl cyclase(s) (AC superactivation), augmenting forskolin-stimulated cAMP formation upon opioid withdrawal (cAMP overshoot). It is well demonstrated that cAMP activates cAMP-dependent protein kinase (PKA), which plays an important role in the modulation of presynaptic neurotransmitter release. Therefore, in this study, we investigate the physiological role of Raf-1 mediated AC superactivation and subsequent PKA activation in A. sustained morphine-mediated augmentation of basal or evoked pain neurotransmitter release in vitro, in cultured primary sensory neurons, and B. in vivo, in sustained morphine mediated paradoxical pain sensitization and antinociceptive tolerance in rats.Our data demonstrates that A. sustained morphine treatment augments both basal and capsaicin-evoked CGRP release from isolated primary sensory neurons in a PKA- and Raf-1- dependent manner. B. sustained morphine treatment- augments of PGE2-evoked CGRP release from these cells. C. selective knockdown of spinal PKA or Raf-1 protein levels by intrathecal PKA- or Raf-1-specific siRNA pretreatment completely attenuates sustained morphine-mediated thermal hyperalgesia, tactile allodynia and greatly reduces antinociceptive tolerance in rats.In conclusion, we suggest that Raf-1-mediated AC superactivation may have a crucial trigger role in sustained morphine-mediated compensatory adaptations in the nervous system. Thus, we expect that pharmacological attenuation of Raf-1-mediated AC superactivation may improve the clinical treatment of chronic and neuropathic pain.