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, mor phine, heroin, nalbuphine, or buprenorphine were evaluated in four rhesus m onkeys 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 e ach of the drugs. No changes in the potency of cocaine or the high efficacy mu agonist alfentanil were detectable during the period of chronic morphin e administration. Small (1/2-1) log unit decreases in the reinforcing poten cy of intermediate efficacy mu agonists morphine and heroin occurred during chronic morphine administration. Larger decreases in both the potency and effectiveness of low-efficacy mu agonists nalbuphine and buprenorphine deve loped during this time. These data suggest that the amount of tolerance tha t develops to the reinforcing effects of opioids depends on the efficacy of the drugs used to maintain responding. (C) 2001 Elsevier Science Ireland L td. All rights reserved.