MU and delta opioid receptors control differently the horizontal and vertical components of locomotor activity in mice

The present study investigated the role of mu and delta opioid receptors in the control of the horizontal and vertical components of locomotion. Mice received intracerebroventricularly (i.c.v.) enkephalin analogs specific for either the mu or delta opioid receptors. The administration of the specific mu agonist [D-Ala2-NMePhe4-Gly5(ol)] enkephalin (DAGO) induced a dose-dependent increase in horizontal activity and a decrease in vertical activity. The specific delta agonist [D-Pen2,D-Pen5] enkephalin (DPDPE) increased both components of motor activity. The opiate antagonist naltrexone reversed the effects of DAGO, but did not influence the effects of DPDPE on motor activity. The pretreatment with the delta opiate antagonist ICI 154, 129 completely reversed the effects of DPDPE on locomotion but antagonized only partially the effects of DAGO on locomotion. These results indicate that the two components of locomotor activity--horizontal and vertical activity--are modulated differently by the stimulation of mu or delta opioid receptors.