Effects of enkephalin analogs, morphine and naloxone on the behavioral responses of phencyclidine in mice

The effects of enkephalin analogs, morphine and naloxone were investigated on phencyclidine (PCP)-induced stereotyped behaviors and motor incoordination in mice. Enkephalin analogs (D-ala²-methionine-enkephalinamide; DAMEA 25 μg/mouse and D-ala²-leucine-enkephalin; DALE 12,5 μg/mouse, i.c.v.) significantly increased the degree and duration of pivotting at 10 mg/kg PCP. Morphine (2.5 and 5.0 mg/kg) also potenciated PCP-induced pivotting. On the contrary, naloxone (5.0 mg/kg) partially antagonized PCP-induced pivotting. PCP-induced motor incoordination was enhanced by DAMEA (25 μg/mouse), Dale (12.5 μg/mouse) and morphine (2.5 μg/kg), but not naloxone (5.0 mg/kg). These resuls suggest that an involvement of central opioid receptor mechanisms in the mediation of PCP-induced behaviours in mice.