Antagonism of ∆9-THC induced behavioral effects by rimonabant: Time course studies in rats

The objective was to examine the time course of the cannabinoid 1 receptor antagonist/inverse agonist rimonabant's ability to antagonize in vivo cannabinergic agonist effects. We used two behavioral procedures sensitive to the effects of ∆9-tetrahydrocannabinol (∆9-THC): rat drug discrimination (EXP-1) and suppression of fixed-ratio responding (FR) for food reinforcement (EXP-2). Two training doses of ∆9-THC (1.8 and 3 mg/kg) served as discriminative cues in two groups discriminating ∆9-THC from vehicle; injections were i.p. 20 min before session onset. Tests assessed the dose–response functions of ∆9-THC and the time course for rimonabant in its ability to block the discriminative stimulus effects of ∆9-THC. For antagonism testing, the training doses of ∆9-THC were used and the rimonabant dose was 1 mg/kg. Tests were 20, 60, 120, and 240 min post rimonabant administration; ∆9-THC was always administered 20 min prior to testing. For EXP-2, only one response lever was activated and every 10th (FR-10) press on that lever resulted in food delivery. Once the response rate stabilized, tests occurred with ∆9-THC, rimonabant and combinations of the drugs. The ED50 estimates for the dose–response functions were 0.38 (± 0.28–0.51) and 0.50 (± 0.40–0.63) mg/kg for the training doses of 1.8 and 3 mg/kg ∆9-THC, respectively. The time course studies suggested functional half-life estimates of 128.4 (± 95.7–172.2) and 98.4 (± 64.2–150.7) min by rimonabant for the two groups in EXP-1, respectively. Similarly, the functional half-life of rimonabant was 118.9 (± 66.1–213.9) min in EXP-2. Thus, antagonism of ∆9-THC by rimonabant is relatively short lasting.