Interactions of naloxone and haloperidol with phencyclidine: effects on milk intake.

Phencyclidine (PCP), naloxone and haloperidol were administered alone and in combination to rats trained to drink sweetened-condensed milk during a 20 min daily session. PCP (1.0-16.0 mg/kg) produced a dose-dependent decrease in milk intake. All doses of naloxone (0.1-16.0 mg/kg) produced approximately a 30% decrease in milk intake. Haloperidol (0.125 mg/kg) had virtually no effect on milk intake. When a dose of naloxone which reduced milk intake by approximately 30% (8.0 mg/kg) was administered as a pretreatment to the PCP, the PCP curve was shifted to the left (lowered) to that degree. When haloperidol (0.125 mg/kg) was administered as a pretreatment to the PCP, the PCP dose-response curve was shifted 1.5 fold to the right. These interactions are similar to those observed in other behavioral paradigms and are discussed in reference to PCP's actions as an indirect dopaminergic agonist.