ELEVATIONS OF NUCLEUS-ACCUMBENS DOPAMINE AND DOPAC LEVELS DURING INTRAVENOUS HEROIN SELF-ADMINISTRATION

Extracellular dopamine and DOPAC (3,4-dihydroxyphenylacetic acid) leve ls in nucleus accumbens were sampled by microdialysis and quantified w ith highperformance liquid chromatography during intravenous heroin se lf-administration sessions in rats. Dopamine levels in 10 and 20 min s amples were elevated following the first injection of each session, re aching a plateau of elevation within the first two or three injections and falling back toward baseline only when drug access was terminated . Elevations were in the range of 150-300% when unit dosages of 0.05-0 .2 mg/kg were given. Increasing the work requirement from FR-1 to FR-1 0 did not appear to alter the degree of elevation of dopamine levels, and dopamine levels fell during extinction while lever-pressing rates increased 20-fold. While animals compensated for unit dose changes bet ween 0.05 and 0-2 mg/kg/injection, adjusting their response rate such that the same hourly drug intake and the Rme asymptotic dopamine level s were maintained across these conditions, at 0.4 mg/kg/injection hour ly drug intake and asymptotic dopamine levels were elevated beyond the levels sustained by the lower doses. These findings confirm that self -administered doses of intravenous heroin are sufficient to activate t he mesolimbic dopamine system and suggest that significant heroin ''cr aving'' can emerge when dopamine levels are still moderately elevated, long before the development of dopamine depletion associated with opi ate withdrawal. (C) 1995 Wiley-Liss, Inc.