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The analgesic effects of supraspinal mu and delta opioid receptor agonists are potentiated during persistent

R W Hurley1, D L Hammond

  • 1Department of Anesthesia and Critical Care and The Committee on Neurobiology, University of Chicago, Chicago, Illinois 60637, USA.

The Journal of Neuroscience : the Official Journal of the Society for Neuroscience
|January 29, 2000
PubMed

Insights

Peripheral inflammation enhances opioid effects in the brainstem (RVM) for pain relief. This study shows increased potency of opioid agonists in the RVM for both local and distant pain after injury.

Area of Science:

  • Neuroscience
  • Pharmacology
  • Pain Research

Background:

  • Peripheral inflammatory injuries can lead to chronic pain states.
  • The rostral ventromedial medulla (RVM) is a key brainstem region involved in pain modulation.
  • Opioid receptors in the RVM play a crucial role in regulating nociception.

Purpose of the Study:

  • To investigate the antihyperalgesic and antinociceptive effects of opioid receptor agonists in the RVM following inflammatory injury.
  • To determine how the timing and chronicity of inflammation affect opioid efficacy in the RVM.
  • To explore the impact of peripheral inflammation on pain modulation in both inflamed and uninflamed hindpaws.

Main Methods:

  • Induction of inflammatory injury in rat hindpaws using complete Freund's adjuvant (CFA).
  • Microinjection of mu and delta opioid receptor agonists into the RVM at various time points post-injury (4 hr, 4 d, 2 weeks).
  • Assessment of nociceptive sensitivity using the radiant-heat paw withdrawal test on both ipsilateral and contralateral hindpaws.

Main Results:

  • The antihyperalgesic potency of DAMGO (mu-opioid agonist) was enhanced at 4 days and 2 weeks after CFA injection.
  • The antinociceptive potency of DAMGO in the contralateral hindpaw progressively increased over time post-injury.
  • Both mu and delta opioid receptor agonists showed increased efficacy in the RVM 2 weeks after inflammatory injury, with greatest potentiation observed at this time point.

Conclusions:

  • Peripheral inflammatory injury significantly alters RVM neuronal activity, enhancing the effects of opioid agonists.
  • These RVM-mediated changes contribute to the alleviation of hyperalgesia at the injury site.
  • Enhanced opioid efficacy in the RVM following inflammation has implications for opioid-induced pain relief at sites remote from the initial injury.

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