DOSE-DEPENDENT IMPAIRING EFFECTS OF MORPHINE ON AVOIDANCE ACQUISITIONAND PERFORMANCE IN MALE-MICE

The effects of morphine (6.3, 12.6, and 25.2 mg/kg) on active avoidanc e behavior of BALB/C mice are explored in three acquisition sessions a nd in two subsequent performance sessions. Morphine-treated animals sh owed an increase in avoidance acquisition with respect to control grou p without differences in performance. However, a dramatical, concomita nt rise in the locomotor activity of the animals (increase in the numb er of crossings during the intertrial intervals) prompted us to transf orm the data employing a formula with which a measure of actual learni ng was obtained. Applying this formula, we have observed that morphine administration impairs, dose-dependently, acquisition and performance of avoidance. Thus, the impairing effects of morphine on avoidance co uld be masked by their stimulant effects on locomotor activity. (C) 19 98 Academic Press.