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BEHAVIORAL PHARMACOLOGY

The Cannabinoid CB1 Antagonist N-Piperidinyl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl) -4-methylpyrazole-3-carboxamide (SR-141716A) Differentially Alters the Reinforcing Effects of Heroin under Continuous Reinforcement, Fixed Ratio, and Progressive Ratio Schedules of Drug Self-Administration in Rats

Preclinical Pharmacology Section, Behavioral Neuroscience Branch, National Institute on Drug Abuse, Intramural Research Program, National Institutes of Health, Department of Health and Human Services, Baltimore, Maryland (M.S., L.V.P., L.A.P., S.R.G.); and Department of Pharmacology, Medical School, University of Athens, Goudi Athens, Greece (K.A.)

Activation or blockade of cannabinoid CB1 receptors markedly alters many effects of opioids. In the present study, we investigated whether the cannabinoid antagonist (N-piperidinyl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide (SR-141716A) could alter the reinforcing effects of heroin in rats. A ⁹-tetrahydrocannabinol (THC) drug-discrimination procedure was first used to determine effective CB1 antagonist doses of SR-141716A and optimal pretreatment times for self-administration studies. Subsequently, Sprague-Dawley rats learned to self-administer heroin under three different schedules of intravenous drug injection: a continuous reinforcement schedule [fixed ratio (FR)1], a five-response, fixed ratio schedule (FR5), and a progressive ratio schedule. Then, SR-141716A (1 mg/kg i.p.) was administered 60 min before the start of the session for three consecutive daily sessions. SR-141716A markedly decreased heroin self-administration under the progressive ratio schedule at heroin doses ranging from 12.5 to 100 µg/kg/injection. In contrast, SR-141716A had no effect on heroin self-administration under the FR1 schedule at heroin doses of 50 or 100 µg/kg/injection, but produced small decreases in self-administration at lower doses (25 and 12.5 µg/kg/injection). Consistent with a behavioral economics evaluation, SR-141716A produced a small but significant decrease in self-administration of the higher 50 µg/kg/injection dose of heroin when the fixed ratio requirement was raised to five (FR5). Thus, blockade of CB1 receptors differentially decreased the reinforcing efficacy of heroin depending on the number of responses required for each injection (price). These findings indicate a facilitatory modulation of opioid reward by endogenous cannabinoid activity and provide support for the use of cannabinoid CB1 antagonists as medications for heroin addiction.

Address correspondence to: Dr. Steven R. Goldberg, Preclinical Pharmacology Section, Neuroscience Research Branch, National Institute on Drug Abuse, Division of Intramural Research, National Institutes of Health, 5500 Nathan Shock Dr., Baltimore, MD 21224. E-mail: sgoldber{at}intra.nida.nih.gov

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