Primary sensory neurones and naloxone-precipitated morphine withdrawal

1. 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. 2. 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. 3. The C-fibre mediated plasma extravasation in the rat paw after naloxone was found to be the same in morphine-treated and control rats. 4. 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.