Asymmetric generalization and interaction profiles in rhesus monkeys discriminating intravenous cocaine or intravenous heroin from vehicle.

Many polydrug abusers combine cocaine with heroin in the form of a "speedball." This study investigated the discriminative stimulus (DS) effects of speedballs in rhesus monkeys trained to discriminate either intravenous cocaine or intravenous heroin from vehicle. Initial substitution tests revealed an asymmetry in the generalization profile of dopamine and opioid agonists such that mu agonists partially substituted for cocaine, but direct and indirect dopamine agonists did not substitute for heroin. Subsequent speedball tests in which drug mixtures were administered by coinjecting the component drugs while keeping the dose-ratio constant revealed an additional asymmetry. In cocaine-trained monkeys, coadministration of cocaine and heroin produced leftward shifts in the cocaine dose-response function. Heroin's cocaine-enhancing effects were mimicked by the mu agonists fentanyl and methadone and less consistently by the delta agonist (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80) and reversed by the mu antagonist naltrexone and the delta antagonist naltrindole. In heroin-trained monkeys, coadministration of cocaine and heroin attenuated the DS effects of heroin. Cocaine's heroin-attenuating effects were mimicked by the D1-like agonist 6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine (SKF 81297) and the D2-like agonist R-(-)-propylnorapomorphine and reversed by the D1-like antagonist (6aS-trans)-11-chloro-6,6a,7,8,9,13b-hexahydro-7-methyl-5H- benzo[d] aphtha[2,1-b]azepin-12-ol hydrobromide (SCH 39166) and the D2-like antagonist raclopride. Attenuation of the effects of heroin was accompanied by decreases in response rate. These results suggest that heroin enhances the DS effects of cocaine via mu, and to a lesser extent delta, receptor mechanisms; whereas cocaine-induced inhibition of the DS effects of heroin probably was due at least in part to masking of the heroin DS presumably via stimulation of both D1- and D2-like receptors.