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Morphine application to peripheral tissues modulates nociceptive jaw reflex
1Department of Oral Function and Physiology, School of Dentistry, Faculty of Health Sciences, University of Copenhagen, Denmark.
Abstract:
This study assessed the effect of peripherally applied opioids on the electromyographic activity reflexly evoked in digastric and masseter muscles by injection of the small-fiber excitant and inflammatory irritant mustard oil (MO) into the temporomandibular joint. In 39 anaesthetized rats, local pretreatment of joint tissues with morphine (15 nmol) significantly depressed the jaw muscle responses compared with saline, and the depression was antagonized by simultaneous local injection of the opiate antagonist naloxone (2.7 nmol); systemic morphine pretreatment (15 nmol, i.v.) did not influence the muscle responses. The naloxone-reversible depression of the MO-evoked muscle responses by local, but not systemic morphine, supports the presence of peripheral opioid receptors that may have a role in modulating nociceptive responses.
Insights
Local application of morphine, an opioid, reduced jaw muscle activity evoked by mustard oil in rats. This effect was blocked by naloxone, suggesting peripheral opioid receptors modulate pain responses.
Area of Science:
- Neuroscience
- Pharmacology
- Pain Research
Background:
- Nociceptive stimuli can trigger reflex muscle activity.
- The role of peripheral opioid receptors in modulating these responses is not fully understood.
Purpose of the Study:
- To investigate the effect of peripherally applied opioids on reflex jaw muscle activity evoked by inflammatory irritant mustard oil (MO).
- To determine if local opioid action in the temporomandibular joint influences nociceptive reflex responses.
Main Methods:
- Electromyographic activity of digastric and masseter muscles was recorded in anesthetized rats.
- Mustard oil was injected into the temporomandibular joint to evoke responses.
- Local or systemic pretreatment with morphine and naloxone was administered.
Main Results:
- Local pretreatment with morphine significantly depressed MO-evoked jaw muscle responses.
- This depression was reversed by the opiate antagonist naloxone.
- Systemic morphine administration did not affect the muscle responses.
Conclusions:
- Local application of opioids to the temporomandibular joint can modulate nociceptive reflex activity.
- Findings support the existence of peripheral opioid receptors involved in pain modulation.