Psychostimulant self-administration by beagle dogs in a progressive-ratio paradigm

The relative reinforcing efficacy of cocaine, amphetamine, mazindol, and fenfluramine was quantified using a progressive-ratio paradigm. Catheterized beagle dogs (N = 6) were given access to response-contingent drug infusions during three 1-h trials each day. According to a predetermined schedule, the response requirement to obtain one infusion trial was increased daily until the dogs failed to complete the necessary fixed-ratio (FR); i.e., until they reached a "break-point" in their ratio behavior. Fenfluramine (in doses ranging from 0.0625-4.0 mg/kg/infusion) did not maintain self-administration behavior at or above the minimum requirement (FR 30). In contrast, all doses of cocaine (0.2-1.6 mg/kg/infusion), d-amphetamine (0.035-0.28 mg/kg/infusion), and mazindol (0.0225-0.18 mg/kg/infusion) sustained responding at large FR values. The highest FR values were maintained by cocaine, followed by d-amphetamine, then mazindol. Generally the dogs completed higher FRs for higher doses of a drug.