Biphasic effects of a potent enkephalin analogue (D-Met2,Pro5)-enkephalinamide and morphine on locomotor activity in mice.

The effects of morphine and a potent enkephalin analogue on spontaneous locomotion have been compared in mice. In doses of 3-10 mg/kg SC both compounds induced a brief reduction of motor activity followed by a period of behavioral hyperactivity. Similar receptorial mechanisms are suggested in mediation of their motor effects.