The effect of peripheral opioid block and body cooling on sensitivity to heat in capsaicin-treated skin

We sought to determine whether stimulation of opioid receptors during body cooling would alter sensitivity to heat in the heat-sensitized, inflamed skin of 14 healthy volunteers. To investigate the contribution of opioid receptors to nociception, the opioid antagonist naloxone was introduced into the skin by iontophoresis after the topical application of capsaicin. For comparison, the same iontophoretic dose of saline was also administered. Shortly after the iontophoreses, sensitivity to heat was greater at the naloxone and saline sites than at iontophoresis-control sites in the capsaicin-treated skin, indicating that nonspecific aspects of the iontophoreses enhanced thermal hyperalgesia. The hyperalgesic effect of saline persisted during body cooling, whereas the naloxone site was less sensitive to heat (heat pain threshold 43.6°± 1.0°C) than either the saline site (40.8°± 0.9°C) or iontophoresis-control sites (41.7°± 1.0°C) (P < 0.05). We conclude that activation of opioid receptors contributed to thermal hyperalgesia in inflamed skin during body cooling. Implications: This study shows that opiate receptor block paradoxically inhibits sensitivity to heat-pain in inflamed skin during body cooling. The findings suggest that endogenous opioids release substances from nerves or other cells during inflammation, which heighten pain. Thus, opioids may fine- tune pain and the inflammatory response while healing takes place