Characteristics of chronic self-administration of morphine by dogs.

Each of five dogs that had been trained to chronically self-administer IV morphine was tested with changes in the morphine dose from the baseline dose (1 mg/kg/infusion) to 0.125, 0.5, or 2 mg/kg/infusion, and with passive administration of the usual daily morphine intake while either continuing the morphine self-administration at the baseline dose or changing the self-administered solution to saline. Each treatment lasted 5 weeks. Results indicated that there is a significant negative regression of response on dose, and chronic self-administration of morphine is not entirely accounted for by a need to avoid abstinence or to obtain a direct drug effect. A third element, which may be an acquired need to obtain a response-contingent drug effect, is necessary to account for the chronic self-administration of morphine by the dog.