Comparison of opioid agonists in maintaining responding and in suppressing morphine withdrawal in rhesus monkeys

Sixteen opioid agonists were studied for their capacity both to maintain responding previously reinforced by codeine and to suppress the withdrawal syndrome induced by morphine deprivation in rhesus monkeys. All compounds, which included examples from each of the major chemical families of opioids, maintained responding at rates above those maintained by saline. There were differences among the compounds in the maximal response rates maintained, and large differences in their potencies in maintaining responding. In morphine-dependent monkeys, the abstinence signs that developed 14 h after the last morphine dose were suppressed completely by all of the compounds except codeine. There was a strong positive correlation (r = 0.92) between the potency of a compound in maintaining drug-reinforced responding and the potency of the compound in suppressing the morphine withdrawal syndrome.