Primary sensory neurones and naloxone‐precipitated morphine withdrawal

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Abstract
The C‐fibre‐evoked depressor reflex following i.a. injection of capsaicin and the wiping movements following chemical irritation of the cornea by capsaicin were both found to be augmented in the naloxone‐precipitated morphine withdrawal phase. The in vitro capsaicin‐evoked release of substance P from central terminals of C‐fibre afferents in the spinal cord was decreased in morphine‐treated rats. Following naloxone, the release in the morphine‐treated group was as large as in the control group. The C‐fibre mediated plasma extravasation in the rat paw after naloxone was found to be the same in morphine‐treated and control rats. It is concluded that C‐fibre‐evoked reflex reactions are augmented during morphine withdrawal, but that the mechanisms responsible are either located postsynaptically to the primary sensory neurone or further centrally.