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The opioid placebo analgesia is mediated exclusively through μ-opioid receptor in rat

Rui-Rui Zhang1, Wen-Cai Zhang, Jin-Yan Wang

  • 1Key Laboratory of Mental Health, Institute of Psychology, Chinese Academy of Sciences, Beijing, PR China.

The International Journal of Neuropsychopharmacology
|July 26, 2012
PubMed

Insights

Placebo analgesia in rats involves the rostral anterior cingulate cortex (rACC). This pain relief is mediated exclusively by the mu-opioid receptor, not delta or kappa opioid receptors.

Area of Science:

  • Neuroscience
  • Pharmacology
  • Pain Research

Background:

  • Placebo analgesia is a well-documented phenomenon.
  • Existing research implicates mu-opioid receptor signaling in human placebo analgesia.
  • The involvement of other opioid receptors in placebo analgesia remains largely unexplored.

Purpose of the Study:

  • To investigate the brain region responsible for placebo analgesia.
  • To determine which types of opioid receptors mediate placebo analgesia.
  • To establish and utilize a rat model for studying placebo analgesia.

Main Methods:

  • Established a placebo analgesia model in male Sprague-Dawley rats using morphine training.
  • Administered naloxone to block placebo analgesia, including via microinjection into the rostral anterior cingulate cortex (rACC).
  • Tested the effects of selective opioid receptor antagonists (CTOP for μ, NTI for δ, nor-BNI for κ) microinjected into the rACC.

Main Results:

  • Placebo analgesia in rats was blocked by naloxone, particularly when microinjected into the rACC.
  • Microinjection of CTOP (a μ-opioid receptor antagonist) into the rACC reduced the pain threshold in placebo analgesia rats.
  • NTI (δ-opioid receptor antagonist) and nor-BNI (κ-opioid receptor antagonist) did not affect placebo analgesia when administered into the rACC.

Conclusions:

  • The rostral anterior cingulate cortex (rACC) is a critical brain region for placebo analgesia.
  • Opioid-mediated placebo analgesia in rats is exclusively dependent on the μ-opioid receptor.
  • These findings provide insights into the neurobiological mechanisms of placebo effects.

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