Vol. 290, Issue 3, 1092-1100, September 1999

Discriminative Stimulus Effects of Morphine in Squirrel Monkeys: Stimulants, Opioids, and Stimulant-Opioid Combinations¹

Donna M. Platt, Doreen M. Grech², James K. Rowlett and Roger D. Spealman

Harvard Medical School, New England Regional Primate Research Center, Southborough, Massachusetts

Morphine and other µ opioids mimic and/or modulate the discriminative stimulus (DS) effects of cocaine, possibly reflecting mutual stimulation of mesolimbic dopamine activity. Less is known about the capacity of cocaine and related stimulants to modulate the DS effects of morphine. The present study investigated the effects of cocaine, amphetamine, and reference drugs, administered alone and with morphine, in squirrel monkeys trained to discriminate morphine from vehicle. Additional studies determined the ability of opioid and dopamine receptor antagonists to attenuate the DS effects of morphine and the morphine-like effects of other drugs. The DS effects of morphine were mimicked by the µ-opioid agonist fentanyl but not the -opioid agonists SNC 80 and BW 373U86 or the -opioid agonist U50,488H, and were antagonized by the opioid antagonist naltrexone but not the dopamine antagonist flupenthixol. In three of five monkeys, the DS effects of morphine also were mimicked by cocaine, amphetamine, and the dopamine transport inhibitor GBR 12909 but not the norepinephrine transport inhibitor talsupram or the serotonin transport inhibitor fluoxetine, and were antagonized by flupenthixol but not naltrexone. In this subgroup, pretreatment with cocaine or amphetamine enhanced the DS effects of morphine, whereas in the other two monkeys pretreatment with either stimulant attenuated the DS effects of morphine. The results demonstrated individual differences in morphine-like DS effects of stimulants that are mirrored by individual differences in their interactions with morphine. Furthermore, different mechanisms appear to mediate the DS effects of morphine and the morphine-like DS effects of cocaine and amphetamine.