Intravenous self-administration of methamphetamine-heroin (speedball) combinations under a progressive-ratio schedule of reinforcement in rats

The i.v. self-administration by rats of a polydrug combination of methamphe tamine and heroin was explored. The rewarding efficacy of a range of metham phetamine doses (0.0625-2.0 mg/kg/injection) alone or in combination with h eroin (12.5 mu g/kg/injection) was compared using a progressive-ratio (PR) schedule of reinforcement. Breaking points (BP) for one group of rats were determined at each dose of methamphetamine-alone and for another group at e ach of the same methamphetamine doses plus the heroin dose. The methampheta mine-heroin combination was associated with higher BPs than methamphetamine alone at all doses of methamphetamine. These data demonstrate that methamp hetamine-heroin combinations (speedballs) produce a greater rewarding effec t than the identical doses of methamphetamine alone and suggest that enhanc ed reward may underlie the popularity of speedball abuse. (C) 2000 Lippinco tt Williams & Wilkins.