Effect of morphine on conditioned place preference in rhesus monkeys.

In rodents, a conditioned place preference (CPP) can be induced by morphine. In the current study, we designed a biased place conditioning paradigm to test the rewarding effects of morphine in freely moving rhesus monkeys. Five monkeys were first placed in three serial rooms with the doors open between them for three days. After this habituation period, during which baseline preference for each of the two end rooms was measured, CPP conditioning occurred when the monkeys were injected intramuscularly with morphine at an increasing dose (1.5, 3, 4.5 mg/kg) before they entered the non-preferred room and on alternate days, with saline before they entered the preferred room. Morphine and saline treatment lasted for six days, respectively. CPP was tested 24 hours after the end of CPP training. The result showed that in all five monkeys, CPP was induced by the morphine treatment. The preference lasted for at least 15.3 ± 1.7 months.