Drug and alcohol dependence
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Drug Alcohol Depend · May 2001
The effects of chronic morphine on behavior reinforced by several opioids or by cocaine in rhesus monkeys.
The reinforcing effects of intravenously delivered cocaine, alfentanil, morphine, heroin, nalbuphine, or buprenorphine were evaluated in four rhesus monkeys before, during, and after daily administration of 3.2 mg/kg morphine. Morphine was given 21 h prior to measures of the reinforcing effects of each of the drugs. ⋯ Larger decreases in both the potency and effectiveness of low-efficacy mu agonists nalbuphine and buprenorphine developed during this time. These data suggest that the amount of tolerance that develops to the reinforcing effects of opioids depends on the efficacy of the drugs used to maintain responding.