Morphine-induced place conditioning is not confounded by drug-induced alterations in locomotor activity

The influence of locomotor activity and environmental familiarity upon the reinforcing effects of morphine was examined in an unbiased place preference conditioning procedure. Groups of rats were trained to associate one distinctive environment with morphine and another with saline. One group was made tolerant to the locomotor activity effects of morphine by the SC administration of morphine (5.0 mg/kg/12 hr) for four days prior to conditioning. The other group received injections of saline. Administration of morphine, at doses which decreased locomotor activity, resulted in marked preferences for the drug-associated place in saline-treated rats. In contrast, chronic morphine treatment resulted in tolerance to the sedative effects of morphine and an abolition of the morphine-induced place preference. These results indicate that in the place conditioning procedure, measures of reinforcement are not confounded by drug-induced increases in activity.