Rats were trained to respond on a variable interval 60 sec (VI-60) schedule of water reinforcement. Phencyclidine (PCD) in doses of 0.25, 0.5, and 1.0 mg/kg i.p. increased response rates, respectively, to 118%, 140%, and 143% of the control rates. In contrast, doses of 2.0 and 4.0 mg/kg decreased response rates to 73% and 6% of the control rates. In the tolerance study, one group of rats received daily treatments of 4.0 mg/kg PCD immediately prior to the operant session, while a second group received the same daily dose of PCD after the operant session. When both groups of animals were treated prior to the operant session on a test day, they both displayed complete tolerance to the rate depressant effect of PCD. These results indicate that the observed tolerance to PCD was not dependent on behavioral compensatory mechanisms.
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