Exposure to Δ-9-Tetrahydrocannabinol (THC) Increases Subsequent Heroin Taking but not Heroin's Reinforcing Efficacy: A Self-Administration Study in Rats

One concern about the widespread use of cannabis is that exposure to its active ingredient, Δ-9-tetrahydrocannabinol (THC), might increase future reinforcing effects of other abused drugs such as heroin. In this study, we investigated the effects of pre-exposure to THC on subsequent intravenous self-administration of heroin by Sprague–Dawley rats. In one group of rats, we studied (1) acquisition of heroin self-administration behavior using a continuous-reinforcement (fixed-ratio (FR) 1) schedule, (2) heroin dose–response relationships using an FR1/variable-dose schedule, and (3) reinforcing efficacy of heroin using a progressive-ratio schedule. The number of rats pre-exposed to THC that subsequently learned to self-administer 50 μg/kg injections of heroin within 10 daily sessions did not differ from vehicle-pretreated controls. In contrast, rats pre-exposed to THC subsequently self-administered significantly more heroin injections per session and showed significantly shorter post-injection pauses over a range of heroin doses (12.5–100 μg/kg/injection) using the variable-dose schedule. Interestingly, the maximum effort rats would exert to receive an injection of the different doses of heroin under the progressive-ratio schedule was not altered by THC pre-exposure. In a second group of rats, we varied the ‘price’ of heroin (responses required/dose), by manipulating FR response requirements at different doses of heroin across sessions, to calculate demand and response output curves. Again, consumption was significantly higher in the THC-treated rats at the lowest prices of heroin (FR1/100 μg/kg and FR1/50 μg/kg) but there were no differences in the reinforcing efficacy of heroin between THC- and vehicle-pretreated rats. Altogether, these results demonstrate that pre-exposure to THC alters some pharmacological effects of heroin that determine frequency of heroin taking, but offer no support for the hypothesis that pre-exposure to THC alters heroin's efficacy as a reinforcer.