d-Amphetamine and phencyclidine alone and in combination: Effects on fixed-ratio and interresponse-time-greater-than-t responding of rats

The effects of three doses of d-amphetamine (0.5, 1.0, and 2.0 mg/kg) and phencylidine (0.5, 1.0, and 2.0 mg/kg), alone and in combination, were assessed in rats performing under fixed-ratio 30 and interresponse-time-greater-than-15-sec food reinforcement schedules. When given alone, phencyclidine and d-amphetamine produced similar increases in responding under the interresponse-time-greater-than-t schedule, and decreases in responding under the fixed-ration 30 schedule. Each drug decreased the number of reinforcers (food pellets) earned relative to control values under both schedules. The effects of the two drugs in combination were nearly always less than additive. That is, the effects of a given dose of phencyclidine and d-amphetamine together were less than an arithmetic summation of the effects of the drugs given alone.