Fig. 4.  Intrathecal injection of ondansetron (OND) reverses morphine tolerance in mice. Morphine analgesic responses were assessed after the intrathecal administration of OND (1 μg) or saline either before or after chronic morphine treatment. MPE% = The percentage maximal possible effect. Data are presented as mean ± SEM; **P < 0.01; ***P < 0.001) (Chronic Morphine vs.  Chronic Morphine + OND). Six mice were used in acute morphine + Saline or chronic morphine + Saline group, eight mice were in the acute morphine + OND or chronic morphine + OND group.