Peripherally administrated morphine attenuates capsaicin-induced mechanical hypersensitivity in humans.

We examined the hypothesis that peripheral morphine can modulate pain and hyperalgesia/allodynia in the human capsaicin model. Subcutaneous injections of 1 mL morphine (1 mg/mL) in one arm and of 1 mL 0.9% saline in the other arm were made prior to bilateral intradermal injections of 50 microL (6 mg/mL) capsaicin. All injections were made on the volar aspect of the arm. Before and after the capsaicin injections, spontaneous pain and pain evoked by repetitive von Frey filament stimulation was rated on a numerical rating scale; furthermore, pressure pain thresholds were determined. The area in which von Frey filament stimulation evoked pain and the area of visible flare were mapped after the capsaicin injection. Capsaicin injection resulted in spontaneous pain on the saline-injected side not significantly different from that on the morphine-injected side. However, capsaicin injections gave rise to significantly less pain evoked by mechanical stimuli, as well as to a significantly smaller area of mechanical hypersensitivity, on the morphine-injected side compared with the saline-injected side. These results suggest that morphine can modulate sensitization mechanisms involved in the development of capsaicin-induced mechanical hypersensitivity.