¹Preclinical Pharmacology Section, Behavioral Neuroscience Branch, National Institute on Drug Abuse, Intramural Research Program, National Institutes of Health, Department of Health and Human Services, Baltimore, MD, USA

Correspondence: Dr M Solinas, Current address: Laboratoire de Biologie et Physiologie Cellulaires, CNRS-6187, University of Poitiers, 40 Avenue du Recteur Pineau, Poitiers 86022, France. Tel: +33 5 49 366225; Fax: +33 5 49 454014; E-mail: msolinas@intra.nida.nih.gov or marcello.solinas@ext.univ-poitiers.fr

Received 10 December 2004; Revised 4 February 2005; Accepted 4 February 2005; Published online 6 April 2005.

Abstract

Strong functional interactions exist between endogenous cannabinoid and opioid systems. Here, we investigated whether cannabinoid–opioid interactions modulate motivational effects of food reinforcement. In rats responding for food under a progressive-ratio schedule, the maximal effort (break point) expended to obtain 45 mg pellets depended on the level of food deprivation, with free-feeding reducing break points and food-deprivation increasing break points. Delta-9-tetrahydrocannabinol (THC; 0.3–5.6 mg/kg intrapeitoneally (i.p.)) and morphine (1–10 mg/kg i.p.) dose-dependently increased break points for food reinforcement, while the cannabinoid CB1 receptor antagonist rimonabant (SR-141716A; 0.3–3 mg/kg i.p.) and the preferential mu-opioid receptor antagonist naloxone (0.3–3 mg/kg i.p.) dose-dependently decreased break points. THC and morphine only increased break points when food was delivered during testing, suggesting that these treatments directly influenced reinforcing effects of food, rather than increasing behavior in a nonspecific manner. Effects of THC were blocked by rimonabant and effects of morphine were blocked by naloxone, demonstrating that THC's effects depended on cannabinoid CB1 receptor activation and morphine's effects depended on opioid-receptor activation. Furthermore, THC's effects were blocked by naloxone and morphine's effects were blocked by rimonabant, demonstrating that mu-opioid receptors were involved in the effects of THC and cannabinoid CB1 receptors were involved in the effects of morphine on food-reinforced behavior. Thus, activation of both endogenous cannabinoid and opioid systems appears to jointly facilitate motivational effects of food measured under progressive-ratio schedules of reinforcement and this facilitatory modulation appears to critically depend on interactions between these two systems. These findings support the proposed therapeutic utility of cannabinoid agonists and antagonists in eating disorders.