Effects of inhibition of PKA or ERK on AM‐induced alteration in tail flick latency, mechanical threshold and naive morphine tolerance. Saline, AM (8 μg), AM plus H‐89 (5 μg), AM plus PD (PD98059, 1 μg), H‐89 (5 μg) or PD (PD98059, 1 μg) was administered i.t. once daily for 9 days. Tail flick latency (A) or mechanical threshold (B) was measured on days 1 and 9 or from days 2 to 10. *
P < 0.05 compared with that on day 1 in the AM group. #
P < 0.05 compared with that on day 0. The antinociceptive effect of cumulative doses of morphine was generated on day 10 (C); n = 6–7 for each group.