Dosage choices of rats for morphine, for heroin, and between morphine and heroin.

Each of four doses of intravenous morphine and four of intravenous heroin was tested for reinforcing efficacy, and comparisons were made among the four morphine doses, among the four heroin doses, and between morphine and heroin. Rats with venous catheters performed one daily forced run for 75 s to one of the two compartments of a spatial choice apparatus, with compartments and doses or drugs alternating over days. In each trial, the accessible compartment depended on which dose (including zero dose) or drug a rat was to receive. After 18-28 forced trials and 0 or 4 reinforced choice trials, efficacies of the different doses and drugs were measured by subsequent compartment choices. Increasing dose effects on choice were found for morphine (0.32, 1.0, 3.2, and 10.0 mg/kg intravenously) and for heroin (0.032, 0.10, 0.32, and 1.0 mg/kg intravenously). A 32:1 morphine:heroin dose ratio produced nearly equal choice. Dose effects were more evident in within-subjects experiments (each rat received two doses or two drugs) than in between-subjects experiments (each rat received one dose of one drug.