Synergistic elevations in nucleus accumbens extracellular dopamine concentrations during self-administration of cocaine/heroin combinations (Speedball) in rats.

The abuse of cocaine/opiate combinations (speedball) represents a growing trend in illicit drug use. Delineation of neurobiological substrates mediating the reinforcing effects of the combination may increase our knowledge of reinforcement mechanisms and provide useful new information for the development of pharmacotherapies. Several studies suggest dopaminergic innervations of the nucleus accumbens (NAc) have a central role in the brain processes underlying drug reinforcement. The present study was undertaken to determine the relationship between the self-administration of cocaine/heroin combinations and NAc extracellular dopamine concentrations ([DA]e) using in vivo microdialysis and microbore high-pressure liquid chromatography. Rats were assigned randomly to one of three groups to self-administer i.v. cocaine (125, 250, and 500 micrograms/infusion; n = 5), heroin (4.5, 9, and 18 micrograms/infusion; n = 5), or cocaine/heroin combinations (125/4.5; 250/9, and 500/18 micrograms/infusion; n = 4) under a fixed ratio (FR) 10: 20-s time-out schedule of reinforcement/multicomponent dosing session. After stable rates of responding were engendered and maintained, microdialysis samples were collected in 10-min intervals during the self-administration session. Self-administration of cocaine/heroin combinations produced synergisitic elevations in NAc [DA]e (1000% baseline) compared with cocaine (400% baseline) and heroin (not significantly different from baseline levels). Neither the number of infusions nor the interinfusion intervals was significantly different between the groups across the self-administration session. Moreover, cocaine concentrations were not significantly different between the cocaine and cocaine/heroin groups. These results demonstrate that heroin interacts with cocaine to produce synergistic elevations in [DA]e, providing a neurochemical basis for understanding the abuse liability of cocaine/opiate combinations.