POLYDRUG SELF-ADMINISTRATION IN RATS - COCAINE-HEROIN IS MORE REWARDING THAN COCAINE-ALONE

THE i.v. self-administration by rats of a polydrug combination of coca ine and heroin was explored. The rewarding efficacies of a range of co caine doses (0.25-8 mg/kg/ injection) alone or in combination with her oin (12.5 or 25 mu g/kg/injection) were compared using a progressive r atio (PR) schedule of reinforcement. Breaking points (BP) for one grou p of rats were determined at each dose of cocaine alone and for two ot her groups at each of the same cocaine doses plus one of the heroin do ses, respectively. The cocaine-heroin combination was associated with higher BPs than cocaine alone at all doses of cocaine. These data demo nstrate that cocaine-heroin combinations (speedballs) are more rewardi ng than the identical doses of cocaine alone. Some possible mechanisms by which heroin increases cocaine reward are discussed. NeuroReport 9 : 2463-2466 (C) 1998 Rapid Science Ltd.