Fig. 1.

Male Sprague–Dawley rats were implanted with two placebo or morphine pellets. (A) Morphine induced a robust antinociception within 6 h of pellet implant. (B) Rats were tested 2 days post-pellet implantation. Paw-withdrawal latencies of morphine-pelleted animals were longer than placebo-pelleted rats, indicating hypoalgesia, P≤0.05. Intrathecal administration of the NK-1 receptor antagonist, L-732,138 (25 μg/5 μl) or the vehicle control (DMSO) did not alter the responses of placebo or morphine-pelleted rats to the thermal stimulus at 15 min post-injection. (C) Rats were tested 6 days post-pellet implantation. Paw-withdrawal latencies of morphine-pelleted animals were lower than the placebo-pelleted rats, indicating that thermal hyperalgesia emerges by 6 days post-morphine pellet. Paw-withdrawal latencies were measured 15 min after intrathecal administration of the NK-1 receptor antagonist, L-732,138 (25 μg/5 μl). Intrathecal administration of L-732,138 reversed thermal hyperalgesia within 15 min post-administration in morphine-pelleted rats, but did not alter the response of placebo-pelleted animals. Intrathecal administration of the vehicle did not alter responses of the morphine or placebo-pelleted animals. Graphs represent mean±SEM. Each treatment group consisted of 12 rats. * indicates significant difference from pre-pellet baselines.