Sustained morphine treatment augments basal CGRP release from cultured primary sensory neurons in a Raf-1 dependent manner

Eur J Pharmacol. 2008 Apr 28;584(2-3):272-7. doi: 10.1016/j.ejphar.2008.02.013. Epub 2008 Feb 14.


Recent studies suggest that sustained morphine-mediated paradoxical pain may play an important role in the development of analgesic tolerance. The intracellular signal transduction pathways involved in sustained opioid mediated augmentation of spinal pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release are not fully clarified. Cyclic AMP (cAMP)-dependent protein kinase (PKA) plays an important role in the modulation of presynaptic neurotransmitter release. Moreover, we have shown 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). Therefore, in the present study we examined the role of Raf-1 in sustained morphine-mediated regulation of cAMP formation and basal CGRP release in vitro, in cultured neonatal rat dorsal root ganglion (DRG) neurons. We found that sustained morphine treatment significantly augments intracellular cAMP production as well as basal CGRP release from cultured neonatal rat DRG neurons. The selective PKA inhibitor, H-89, attenuates the sustained morphine-mediated augmentation of basal CGRP release, indicating that the cAMP/PKA pathway plays an important role in regulation of CGRP release from sensory neurons. Since our present data also demonstrated that selective Raf-1 inhibitor, GW 5074, attenuated both the cAMP overshoot and the augmentation of CGRP release mediated by sustained morphine in neonatal rat DRG neurons, we suggest that Raf-1-mediated sensitization of the intracellular cAMP formation may play an important role in sustained morphine-mediated augmentation of spinal pain neurotransmitter release.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Animals, Newborn
  • Calcitonin Gene-Related Peptide / metabolism*
  • Cells, Cultured
  • Cyclic AMP / metabolism
  • Cyclic AMP-Dependent Protein Kinases / antagonists & inhibitors
  • Cyclic AMP-Dependent Protein Kinases / metabolism
  • Dose-Response Relationship, Drug
  • Drug Tolerance
  • Ganglia, Spinal / drug effects*
  • Ganglia, Spinal / enzymology
  • Ganglia, Spinal / metabolism
  • Indoles / pharmacology
  • Isoquinolines / pharmacology
  • Morphine / pharmacology*
  • Narcotics / pharmacology*
  • Neurons, Afferent / drug effects*
  • Neurons, Afferent / enzymology
  • Neurons, Afferent / metabolism
  • Phenols / pharmacology
  • Protein Kinase Inhibitors / pharmacology
  • Proto-Oncogene Proteins c-raf / antagonists & inhibitors
  • Proto-Oncogene Proteins c-raf / metabolism*
  • Rats
  • Rats, Sprague-Dawley
  • Signal Transduction / drug effects
  • Sulfonamides / pharmacology
  • Up-Regulation


  • 5-iodo-3-((3,5-dibromo-4-hydroxyphenyl)methylene)-2-indolinone
  • Indoles
  • Isoquinolines
  • Narcotics
  • Phenols
  • Protein Kinase Inhibitors
  • Sulfonamides
  • Morphine
  • Cyclic AMP
  • Proto-Oncogene Proteins c-raf
  • Cyclic AMP-Dependent Protein Kinases
  • Calcitonin Gene-Related Peptide
  • N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide