Simultaneous assessment of the opiate-induced modifications in the vertical and horizontal components of locomotor activity in mice.

The effects of some opiates on the horizontal and vertical components of locomotor activity in mice were measured in actometers fitted with photoelectric cells counting horizontal displacements and the striding over a low partition wall. Morphine and fentanyl, used at low doses which did not modify the horizontal locomotor activity, decreased the striding over of the partition wall. Such a dissociation was not observed with either kappa or sigma agonists which affected in the same way the two components of the locomotor activity: both components were inhibited by kappa agonists and slightly stimulated by the sigma agonist SKF 10047. Although the opiate-induced inhibition of the striding over was induced by low doses of morphine, relatively high doses of naloxone were necessary for its antagonism. This apparently difficult antagonism could result in fact from the sedation which appeared when morphine was associated with naloxone.