Figure 3. Dose-dependent reduction of the antinociceptive effect of intravenously administered morphine (2.5 mg/kg) by intrathecal injection of the nonspecific NO synthase (NOS) antagonist N^{G-monomethyl-L-arginine}(NMMA; filled circles), the neuronal NOS specific antagonist 1-(2-trifluoromethylphenyl) imidazole (TRIM; open circles), and the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide (PTIO; filled squares). Antinociception is expressed as percent maximum possible effect (%MPE). Data are presented as mean +/- SD. *P < 0.05 versus morphine alone.