ACQUISITION OF IV AMPHETAMINE AND COCAINE SELF-ADMINISTRATION IN RATSAS A FUNCTION OF DOSE

The effect of dose on the acquisition of TV amphetamine and cocaine se lf-administration was examined. Three unit doses of amphetamine (0.03, 0.06 and 0.12 mg/kg) and three unit doses of cocaine (0.05, 0.2 and 0 .8 mg/kg) were tested in separate groups of ten (amphetamine) or 13 (c ocaine) rats. Autoshaping methods were used to train rats to press a l ever that resulted in drug infusion under a fixed-ratio (FR) 1 schedul e. A dally 6-h autoshaping component non-contingently delivered 60 inf usions according to a 60-s random time schedule with ten infusions del ivered during the first half of each h. Each day autoshaping sessions were followed by a 6-h self-administration session. The criterion for acquisition was a 5-day period during which a daily mean of 100, 50 or 25 infusions for the three amphetamine doses and 400, 100 or 25 infus ions were earned during the 6-h self-administration period for the thr ee cocaine doses, respectively. As dose increased, more rats per group acquired drug self-administration and the mean number of days to meet the acquisition criterion decreased. The percentage of rats acquiring amphetamine self-administration increased with dose and ranged from 8 0 to 100%. Only one rat at the lowest cocaine dose met the acquisition criterion, but 100 percent of the rats at the two higher doses acquir ed. During the last 2 days of acquisition, mean infusions decreased an d mean drug intake (mg/kg) increased as dose increased. On the last da y of acquisition, the time course of infusions during the 6-h self-adm inistration component was characterized by a steady rate of infusions per hour, and number of infusions was inversely related to dose. These findings indicate that the initial available dose of a drug is an imp ortant determinant of the rate and probability that successful acquisi tion will occur.