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Updated: Jul 13, 2026

Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
07:23

Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities

Published on: July 29, 2014

Analgesia induced by morphine microinjected into the nucleus raphe magnus: effects on tonic pain

Christian Dualé1, Fernando Sierralta, Radhouane Dallel

  • 1INSERM, E216, Clermont-Ferrand, F-63000, France. cduale@chu-clermontferrand.fr

Current Drug Delivery
|July 14, 2007
PubMed

Insights

Morphine in the Nucleus Raphe Magnus provides analgesia for both transient and tonic pain in rats. However, its effect on tonic pain is not complete, suggesting other sites of action for morphine-induced analgesia.

Area of Science:

  • Neuroscience
  • Pharmacology
  • Pain Research

Background:

  • The Nucleus Raphe Magnus is a potential site for morphine's analgesic effects, demonstrated in transient pain models.
  • Investigating morphine's action in the Nucleus Raphe Magnus is crucial for understanding pain management.

Purpose of the Study:

  • To determine if morphine administered to the Nucleus Raphe Magnus exhibits analgesic properties in a tonic pain model.
  • To compare the analgesic efficacy of morphine in the Nucleus Raphe Magnus for both phasic and tonic pain stimuli.

Main Methods:

  • Microinjection of morphine (5 microg) or saline into the Nucleus Raphe Magnus of rats.
  • Assessment of analgesic effects using tail-flick test (phasic pain) and formalin injection in the orofacial region (tonic pain).

Main Results:

  • Morphine in the Nucleus Raphe Magnus significantly reduced the tail-flick response, indicating strong analgesia in phasic pain.
  • Analgesia was observed for both early and late phases of the formalin test (tonic pain), though not completely abolishing the response.
  • Statistical significance was noted for tail-flick (p <0.0001), early formalin (p = 0.007), and late formalin responses (p = 0.02).

Conclusions:

  • The Nucleus Raphe Magnus plays a role in morphine-induced analgesia for both transient and tonic pain.
  • The incomplete blunting of the tonic pain response suggests that the Nucleus Raphe Magnus is not the sole site of action for morphine's clinical analgesic effects.

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