Figure 5. On day 5 acute injection of different doses of morphine (0.3–3 mg/kg) in animals that received saline over 4 days produced a significant dose-dependent antinociceptive response when compared with responses obtained in naive group.

The antinociceptive response to morphine was not altered in animals that were treated over 4 days with l-NAME (10 mg/kg/d), MnTBAP^3– (10 mg/kg/d), or FeTM-4-PyP⁵⁺ (30 mg/kg/d), indicating lack of acute interaction between morphine and l-NAME, MnTBAP^3–, or FeTM-4-PyP⁵⁺. Results are expressed as mean ± SEM for 10 animals. *P < 0.001 for the vehicle group when compared to values obtained in the absence of morphine.