Figure 3. Dose-effect curves for the antinociceptive effects of morphine, clonidine, and a 1:3 fixed ratio of morphine to clonidine injected intrathecally to L5/L6 ligated rats are shown. Morphine, clonidine, and the morphine-clonidine mixture are represented by filled circles, filled squares, and filled triangles, respectively. Error bars represent +/- SEM. Dose is expressed as the total (i.e., morphine plus clonidine) administered. n = 5–10 rats per group.