Mice devoid of CB1 cannabinoid receptors (CB1Ϫ/Ϫ mice) provide a unique opportunity to further investigate the role of CB1 receptors in exocannabinoid and endocannabinoid effects. CB1Ϫ/Ϫ mice (N ϭ 18) and their wild-type littermates (CB1ϩ/ϩ mice; N ϭ 12) were placed in standard mouse operant chambers and trained to lever press under a fixed ratio 10 schedule of reinforcement. When stable lever press responding under the fixed ratio 10 schedule had been established, cannabinoids and noncannabinoids were administered to both groups. CB1ϩ/ϩ mice acquired the lever press response more readily than CB1Ϫ/Ϫ mice. ⌬ 9 -Tetrahydrocannabinol (⌬ 9 -THC) decreased lever press responding in CB1ϩ/ϩ mice only, whereas methanandamide, a metabolically stable endocannabinoid analog, produced similar response rate decreases in both genotypic groups. Similar to ⌬ 9 -THC, another endocannabinoid analog, (R)- (20-cyano-16,16-dimethyl docosa-cis-5,8,11,14--THC, but not those of methanandamide. Because methanandamide binds poorly to CB2 receptors, these results suggest possible non-CB1, non-CB2 mechanisms of action for methanandamide-induced behavioral disruption of lever press responding. Ethanol and morphine elicited greater response decreases in CB1Ϫ/Ϫ mice than in CB1ϩ/ϩ mice, suggesting a possible role of CB1 receptors in the rate disruptive effects of these drugs. In contrast, diazepam did not produce between group differences, suggesting that CB1 receptors are not involved in diazepam-induced disruption of lever press responding.
⌬9 -Tetrahydrocannabinol (⌬ 9 -THC), the principle psychoactive constituent of marijuana and other natural, synthetic, and endocannabinoids, elicits a well characterized syndrome of pharmacological effects in laboratory animals (Dewey, 1986;Martin et al., 1991). These effects, including hypothermia, hypoactivity, antinociception, and catalepsy, are attributed mainly to the actions of cannabinoids at CB1 cannabinoid receptors in the brain (Martin et al., 1991). Recently, recombinant DNA techniques have made possible the development of CB1 receptor knockout mice (CB1Ϫ/Ϫ) Zimmer et al., 1999). These mice lack CB1 receptors and provide a unique opportunity to further investigate the role of these receptors in the pharmacological effects of cannabinoids. Research with CB1Ϫ/Ϫ mice demonstrates that the characteristic antinociceptive, hypoactive, and cataleptic effects of ⌬ 9 -THC are absent in these mice Di Marzo et al., 2000). Moreover, ⌬ 9 -THC stimulates [ 35 S]GTP␥S binding in brain membranes of CB1ϩ/ϩ mice, but not in those of CB1Ϫ/Ϫ mice (Di Marzo et al., 2000).Thus far, the measures examining the pharmacological effects of cannabinoids in CB1Ϫ/Ϫ mice, albeit important, represent unlearned responses. Previous research has demonstrated that cannabinoids, including ⌬ 9 -THC, WIN 55,212-2, and methanandamide, also disrupt performance in the Morris water maze, a working memory task (Varvel and Lichtman, 2002). The effects of cannabinoids on schedulecontrolled operant behavior have not been investiga...