Antinociceptive Effect of Dihydroetorphine and Its Tolerance/Dependence Liability in Mice.

The profile of actions of dihydroetorphine (DHE) concerning antinociception, tolerance and dependence was compared with those of morphine in mice. DHE at 1, 5, 10 or 20μg/kg produced an antinociceptive effect in a dose dependent manner and 10μg/kg was nearly equipotent to that of 10mg/kg of morphine. The antinociceptive effect of both drugs was completely suppressed by 1mg/kg of naloxone, while neither 10mg/kg of naltrindole nor 1mg/kg of nor-binaltorphimine had any suppressive effect. Mice tolerant to morphine antinociception were tolerant to DHE and vice versa. The naloxone-sensitive, locomotor accelerating activity was progressively enhanced by daily administration of DHE and morphine and a cross reverse tolerance developed between these compounds, suggesting that common mechanisms, especially mediating opioid receptors, underlay the activity enhancement. The development of physical dependence as evidenced by naloxone precipitated withdrawal signs, however, was not observed with daily treatment with DHE, 10, 20 and 100μg/kg for 6d. Thus, we demonstrated that DHE produces the antinociceptive effect mediated through μ opioid receptors without causing development of a physical dependence, suggesting that it is safe to use in the clinical therapy of patients suffering severe pain such as that accompanying cancer