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Effects of morphine microinjections into the trigeminal sensory complex on the formalin test in the rat
C Duale1, P Luccarini, R Cadet
1Laboratoire de Physiologie Oro-Faciale, Faculté de Chirurgie Dentaire,Université d'Auvergne, Clermont-Ferrand, France.
Abstract:
The aim of this study was to find out whether morphine locally applied into the different subnuclei of the spinal trigeminal nucleus could impair the behavioral response evoked by a tonic nociceptive stimulus. Microinjections of morphine were performed unilaterally in rats through a chronically implanted tube. The duration of the formalin-induced biphasic rubbing activity was used as a measure of nociception. Morphine, 0, 2, 7, or 14 microg, in 0.2 microl of saline were microinjected 10 min before a subcutaneous injection of formalin (1.5%) in the ipsilateral upper lip. Morphine microinjections into subnucleus oralis (Sp5O) and subnucleus caudalis (Sp5C) induced a significant decrease in rubbing duration of either the early or late phase of the response or both. The early response to formalin was depressed only after morphine injection into Sp5O. The late response was depressed by microinjections into both Sp5O and Sp5C. No significant effect was observed in subnucleus interpolaris (Sp5I). These results are further evidence for the role played by Sp5C in orofacial nociception. They also give support to the involvement of the Sp5O in perioral nociceptive mechanisms of short duration.
Insights
Morphine applied to specific brain areas, the subnucleus oralis and caudalis of the spinal trigeminal nucleus, reduced pain responses in rats. These findings highlight key areas involved in processing orofacial pain.
Area of Science:
- Neuroscience
- Pain Research
- Pharmacology
Background:
- The spinal trigeminal nucleus plays a crucial role in processing orofacial sensory information, including pain.
- Understanding the specific roles of its subnuclei in nociception is essential for developing targeted pain therapies.
Purpose of the Study:
- To investigate the effect of localized morphine application within different subnuclei of the spinal trigeminal nucleus on behavioral responses to nociceptive stimuli.
- To determine the involvement of subnucleus oralis (Sp5O), subnucleus interpolaris (Sp5I), and subnucleus caudalis (Sp5C) in orofacial nociception.
Main Methods:
- Unilateral microinjections of varying doses of morphine (0, 2, 7, 14 microg) into rat spinal trigeminal nucleus subnuclei.
- Assessment of nociception using the duration of formalin-induced biphasic rubbing activity in the ipsilateral upper lip.
- Behavioral responses were measured 10 minutes after morphine microinjection and prior to formalin administration.
Main Results:
- Morphine microinjections into Sp5O and Sp5C significantly reduced the duration of the formalin-induced rubbing response (early, late, or both phases).
- The early phase of the nociceptive response was specifically attenuated by morphine in Sp5O.
- The late phase was inhibited by morphine in both Sp5O and Sp5C, while Sp5I showed no significant effect.
Conclusions:
- The subnucleus caudalis (Sp5C) is confirmed to play a significant role in orofacial nociception.
- The subnucleus oralis (Sp5O) is implicated in the mechanisms of short-duration perioral pain processing.
- These findings provide valuable insights into the differential roles of spinal trigeminal nucleus subnuclei in pain modulation.