title: Exposure to delta-9-tetrahydrocannabinol (THC) increases subsequent heroin taking but not heroin's reinforcing efficacy: a self-administration study in rats.
abstract: One concern about the widespread use of cannabis is that exposure to its active ingredient, delta-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 microg/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 microg/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 microg/kg and FR1/50 microg/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.
authors.affiliation: Preclinical Pharmacology Section, Behavioral Neuroscience Branch, Department of Health and Human Services, Baltimore, MD 21224, USA.
authors.forename: M
authors.initials: M
authors.lastname: Solinas
authors.forename: L V
authors.initials: LV
authors.lastname: Panlilio
authors.forename: S R
authors.initials: SR
authors.lastname: Goldberg
mesh_terms: D018712:Analgesics, Non-Narcotic / Q000494:pharmacology*; D000704:Analysis of Variance; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D003216:Conditioning, Operant / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D013759:Dronabinol / Q000494:pharmacology*; D004334:Drug Administration Schedule; D004347:Drug Interactions; D003932:Heroin / Q000008:administration & dosage*; D006556:Heroin Dependence; D009294:Narcotics / Q000008:administration & dosage*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D012646:Self Administration / Q000379:methods
chemical_list: D018712:Analgesics, Non-Narcotic; D009294:Narcotics; D003932:Heroin; D013759:Dronabinol