The analgesic effect induced by capsaicin is enhanced in inflammatory states

Agonists of the vanilloid receptor type 1 (VR1), such as capsaicin, induce an analgesic effect following an initial excitatory response. It has been demonstrated that the vanilloid system plays an important role in inflammatory hyperalgesia. In accordance, we show that the VR1 antagonist capsazepine (30 microg; i.pl.) prevented the thermal hyperalgesia induced by carrageenan or complete Freund's adjuvant (CFA) in mice. Furthermore, we studied whether this inflammation-induced activation of the vanilloid system could enhance the analgesic properties of capsaicin. A single administration of capsaicin (10 microg; i.pl.) induced in control mice an analgesic effect that lasted for 2 days. In contrast, in carrageenan-treated animals, the analgesic effect of this dose of capsaicin lasted for 6 days and in CFA-treated mice for 30 days. This prolongation of capsaicin-induced analgesia during inflammation was mediated through VR1 since it was completely blocked by coadministration of capsazepine (10 microg). Licking behavior induced by capsaicin in carrageenan- and CFA-treated mice was greater than in control animals. However, although capsaicin induced a more prolonged analgesia in CFA-treated mice, the licking behavior was greater in the carrageenan-treated group, suggesting that the prolongation of analgesia is independent of the initial nociceptive input. Overall, these results show that the analgesic effects of capsaicin are importantly enhanced during inflammation, supporting the fact that the stimulation of VR1 could perhaps constitute a suitable strategy to avoid inflammatory hyperalgesia.