Drug-seeking behavior in the dog: lack of effect of prior passive dependence on morphine.

Twelve dogs were made dependent on morphine (20 mg/kg per 24 hours) by intravenous passive administration of the drug. Six were gradually withdrawn and six were withdrawn abruptly. Subsequent tests for self-administration of morphine began within 8 to 18 weeks after morphine was last administered passively. Morphine was available for self-administration at several unit dose levels for 8 weeks. In comparison with control dogs administered with saline, there was no evidence that prior dependence on morphine influences subsequent self-administration of morphine, or that morphine is a primary reinforcer for dogs. Possible implications of these and related observations are discussed.