OPIOIDS SUPPRESS SPONTANEOUS ACTIVITY OF POLYMODAL NOCICEPTORS IN RATPAW SKIN INDUCED BY ULTRAVIOLET-IRRADIATION

Changes in chemical sensitivity of periperal nociceptors following inj ury or inflammation have been studied in in vitro preparation of the s aphenous nerve-hind paw skin from adult rats. Heat hyperalgesia in the hind paw was induced by a prior ultraviolet irradiation and the skin from these animals was investigated five days later. Polymodal nocicep tors were quiescent in normal skin but were spontaneously active in th e majority of fibres after ultraviolet exposure. Capsaicin-induced act ivation of fine fibres was enhanced after ultraviolet pretreatment. Di rect administration of morphine, DAGOL (mu-receptor agonist) and U-695 93 (kappa-receptor agonist), but not DPDPE (delta-receptor agonist) to the receptive field produced a concentration related and naloxone-rev ersible suppression of spontaneous firing in polymodal nociceptors of ultraviolet-treated skin. Morphine did not reduce the activity of fibr es in normal skin when these were driven by KCl depolarization. These data show that polymodal nociceptors change their activity and sensiti vity to exogenous chemicals following the induction of peripheral hype ralgesia by ultraviolet irradiation. Specifically, evidence is provide d for the expression of opioid sensitivity and inhibition of polymodal nociceptor activity through mu- and kappa-opioid receptors. These obs ervations may account for peripheral antinociceptive actions of opioid s during specific states of peripheral hyperalgesia.