Fig. 1.

Prevention of acute opioid tolerance (A) and dependence (B) by haloperidol. Separate groups of six mice received haloperidol (0.2, 0.6, and 1.0 mg/kg i.p.) or equal volume of saline 30 min before administration of morphine (100 mg/kg s.c.). Saline mice only received intraperitoneal injections of saline. Four hours later, all groups received a test dose of morphine (10 mg/kg s.c.). The antinociception was determined by the tail-flick assay 30 min later. A, development of morphine tolerance was prevented by haloperidol in a dose-dependent manner. B, development of morphine dependence, as revealed by naloxone (10 mg/kg i.p.)-precipitated withdraw jumping, was also prevented by haloperidol in a dose-dependent manner. Data are expressed in mean ± S.E.M. **, p < 0.01; ***, p < 0.001 compared with the saline group; #, p < 0.05; ###, p < 0.001 compared with the morphine (MS) group.