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Multiple effects of morphine on facial scratching in monkeys
D A Thomas1, G M Williams, K Iwata
1Neurobiology and Anesthesiology Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892.
Anesthesia and Analgesia
|November 1, 1993
Summary
Morphine injected into the medullary dorsal horn (MDH) causes facial scratching, but systemic morphine reduces it. This suggests morphine has dual effects on itch depending on its location in the brain.
Area of Science:
- Neuroscience
- Pharmacology
- Pruritus Research
Background:
- The medullary dorsal horn (MDH) is a brain region homologous to the spinal dorsal horn.
- Opioid receptor agonists administered to the MDH can induce facial scratching, a behavior associated with pruritus (itch).
Purpose of the Study:
- To investigate the dual role of morphine in modulating pruritus.
- To determine if morphine exhibits both itch-inducing and itch-relieving properties based on its administration site.
Main Methods:
- Morphine was micro-injected into the MDH of monkeys following a baseline observation period.
- Facial scratching behavior was recorded and quantified by independent raters.
- The effects of systemic naloxone and systemic morphine on MDH-induced scratching were assessed.
Main Results:
- Microinjection of morphine into the MDH significantly increased facial scratching behavior.
- Systemic administration of naloxone reversed the scratching induced by MDH morphine.
- Systemic administration of morphine substantially reduced facial scratching behavior.
Conclusions:
- Morphine demonstrates both pruritic (itch-inducing) and antipruritic (itch-relieving) effects.
- The observed effects depend on the site of morphine administration, with central MDH acting as a site for itch induction and systemic administration for itch relief.