Demand for heroin in rats: effects of non-drug alternative substitutes and complements

Given the central role of economic demand for drugs in substance use disorder, identifying factors that increase or decrease drug value is of high importance. The present study investigated how the concurrent availability of non-drug reinforcers previously shown to be heroin substitutes or complements affected demand for heroin in rats. In Exp. 1, three groups of rats pressed one lever for intravenous heroin infusions on a series of prices that increased over sessions. The groups differed with respect to what they received for pressing a second lever: timeout-from-avoidance (TOA) reinforcers, heroin at a constant low price, or no programmed consequences. The essential value of heroin was significantly reduced in the groups that had either TOA or low-price heroin available on the second lever. Additionally, as the price of heroin on the first lever increased, consumption of TOA reinforcers or low-price heroin infusions increased, confirming that these were substitutes for expensive heroin. In Exp. 2, two groups of rats pressed one lever for heroin infusions on a series of increasing prices. One group could press a second lever for saccharin reinforcers at a constant low price. Concurrent saccharin availability increased demand for heroin in female rats, but not in male rats. Exp. 3 compared demand for saccharin in groups that had or did not have concurrent access to low-price heroin. Concurrent heroin availability caused an increase in estimated saccharin consumption at no cost (Q₀), but did not affect elasticity of demand for saccharin. The outcome of Exp. 1 suggests that non-drug means of reducing pain or stress can weaken demand for opioids. Exp. 2 and 3 show that, in contrast, availability of some non-drug alternatives can increase demand for heroin in female rats and that heroin can increase consumption of non-drug alternatives at low price. Overall, these results show, consistent with the contextualized reinforcer pathology model, that opioid value depends on the broader behavioral economic context in which opioids and non-drug alternatives are available.