Fig. 2.

The antinociceptive effect of acute subcutaneous morphine administration in naïve rats and in rats pretreated with either morphine or vehicle pellets. The ability of pretreatment protocol using morphine pellets to induce tolerance is indicated by the virtually complete disappearance of antinociception after acute morphine administration. In this and subsequent figures, antinociception is plotted as percentage attenuation from baseline of the JOR EMG amplitude on the y-axis (i.e., greater antinociception is represented as higher positive numbers). Baseline JOR recordings were obtained before interventions. Time 0 on thex-axis represents the time at which the last (or only) treatment was given for each group. Data are plotted as mean ± SEM. The number of rats in each group is shown inparentheses. Group numbers, precedinggroup names, refer to the Tukey post hoc analyses in Table 1.