The synergistic effects of combining cocaine and heroin ("speedball") using a progressive-ratio schedule of drug reinforcement

The relative reinforcing value of cocaine/heroin combination ("speedball") was compared in the rat using a progressive-ratio (PR) reinforcement schedule. The initial training for all rats was a combined dose of 18 mu g/kg/inj of heroin (H) plus 300 mu g/kg/inj of cocaine (C). Break points for the training dose and individual component doses were determined for half and double the training dose. Of the three doses of each treatment, only C yielded the expected monotonic increase in break point as a function of dose. Also, break points for C (300 and 600 mu g/kg/inj) was greater than for the combination of C and H (18 H/300 C and 36 H/600 C mu g/kg/inj), suggesting a greater reward value for C alone. The doses for these three drug treatments that produced saline level break points were then determined. At these lower doses, significant break points were obtained with the H/C combination at which the respective doses of H or C had break points identical to those of saline. These lower dose results indicate that the combination is clearly synergistic and that the discrepancy with doses at the opposite end of the dose response curve suggest that the PR schedule is vulnerable to drug-induced motor effects. (C) 1998 Elsevier Science Inc.