Modulation of the stimulus effects of morphine by d-amphetamine

The stimulus effects of morphine and d-amphetamine coadministration were studied in rats. Place conditioning, drug discrimination, and taste conditioning were employed to assess the rewarding, discriminative, and aversive stimulus properties of both drugs. d-Amphetamine increased the rewarding and morphine-like discriminative stimulus effects of 1.25 mg/kg morphine. d-Amphetamine did not, however, change the aversive effects of 1.25 mg/kg morphine, or any effect of higher (5-20 mg/kg) morphine doses. Because the rewarding/discriminative properties and the aversive properties of a drug are considered the main attributes that regulate (facilitate and weaken, respectively) drug-seeking behavior, the present data are in keeping with clinical reports indicating that amphetamines are sometimes used by opiate abusers in an attempt to increase the effect obtained from poor-quality heroin.