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Updated: Aug 18, 2026

Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Naloxone-induced facilitation of C-fiber reflexes is reduced by chronic morphine
1Neuroscience Branch, National Institute on Drug Abuse, Baltimore, MD 21224.
Abstract:
The effects of chronic morphine on endogenous opioid function were examined in feline spinal cord. Naloxone-induced facilitation (disinhibition) of the C-fiber reflex was used as a model to reflect endogenous opioid function. Facilitation was reduced 24 h after withdrawal of chronic morphine (14 days). Acute morphine failed to depress C-fiber reflexes in the abstinent cats, indicating tolerance. Thus, these results demonstrated that, after chronic morphine, spinal endogenous opioid function was reduced due to tolerance.
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