Time- and dose-related influence of dexamethasone on morphine-induced hypermotility in mice

The effect of interaction between dexamethasone (DEX) and morphine on locomotor activity in mice was investigated. DEX alone (1.0 and 10 mg/kg i.p.) did not affect the locomotor activity of mice when injected immediately before the beginning of the session. DEX (1.0 mg) injected two hours before the session did not modify the activity of mice whereas the higher dose (10 mg) increased it. Morphine alone (30 mg/kp i.p.) induced a consistent increase in the locomotor activity. DEX (1.0 mg) injected at the same time as morphine reduced the morphine hypermotility, whereas that injected two hours before morphine had no influence on morphine hypermotility. DEX (10 mg) consistently potentiated the morphine hypermotility both when administered together and two hours before morphine. We conclude that DEX exerts an important time- and dose-related influence on morphine-induced hyperactivity in mice.