Fig. 1. Long-lasting effects of an intraplantar carrageenan injection on the nociceptive threshold. Modulation by fentanyl. Nociceptive thresholds of both hind paws (A  and B : carrageenan injected paw;C  and D : non–carrageenan-injected paw) were measured by the paw-pressure vocalization test and expressed as the mean ± SD. The rats were injected in the left hind paw with carrageenan (0.2 ml, 1%) on day 0. Furthermore, four injections of fentanyl (4 × 60 [A  and C ] or 4 × 100 μg/kg [B  and D ], administered subcutaneously; n = 8 each group) or saline (n = 8) were performed. The first of four fentanyl or saline injections was performed 5 min before the carrageenan injection. The following injections were administered every 15 min. The nociceptive thresholds of both hind paws were measured 2 and 4 h after the carrageenan injection on day 0, and subsequently once daily. Open circles = saline-treated group; filled circles = fentanyl-treated group; bold line indicates the time of saline or fentanyl administrations; thin arrow indicates the time and the paw injected by carrageenan.