The antinociceptive response to morphine was not altered in animals that were treated over 4 days with
l-NAME (10 mg/kg/d), MnTBAP3– (10 mg/kg/d), or FeTM-4-PyP5+ (30 mg/kg/d), indicating lack of acute interaction between morphine and
l-NAME, MnTBAP3–, 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.