Fig. 4. Algesic indexes of the long-lasting effects of the first and second carrageenan injections on the nociceptive threshold. Modulation by ketamine or fentanyl. The algesic indexes were calculated (see Material and Methods) using the variations of the nociceptive threshold of the carrageenan-injected paw measured after the first (open square) or the second (filled square) carrageenan injections as shown in figure 3. The plus symbol corresponds to pharmacologic treatments (carrageenan, fentanyl, ketamine) and the minus symbol to saline injections. Newman-Keuls test, P < 0.05: first versus second long-lasting hyperalgesia induced by carrageenan injections. *Paired Student t test was used for comparing the algesic indexes of the first and second carrageenan injections in the same group. #Newman-Keuls test, P < 0.05 compared with the reference algesic index: long-lasting hyperalgesia induced by the first carrageenan injection in saline–saline-treated rats.