Microstructure analysis of the effects of the cannabinoid agents HU-210 and rimonabant in rats licking for sucrose

Stimulation of cannabinoid CB₁ receptors generally increases ingestion. However, the non-selective cannabinoid receptor agonist HU-210 exerts the opposite effect. The first objective of this study was to investigate the role of CB₁ receptors in this "atypical" effect. Studies on the endocannabinoid system investigating the microstructure of licking for palatable solutions reported inconsistent results as for the role of CB₁ receptors in "liking" and "wanting". The second objective was to deal with these inconsistencies investigating the within-session time-course of licking-burst number. The microstructure of licking for a 10% sucrose solution in 30-min sessions was analysed in two experiments. In Experiment 1, the effect of the CB₁ receptor antagonist-inverse agonist rimonabant (0.5, 1 mg/kg) on HU-210 effect (100 μg/kg) was investigated. A dose range of HU-210 (25, 50, 100 μg/kg) was examined in Experiment 2. In Experiment 1, both HU-210 and rimonabant reduced licking due to reduced burst number. Moreover, HU-210 reduced the intra-burst lick rate, an index of motor competence. HU-210 effects were antagonised by rimonabant, and vice versa. Rimonabant decreased burst number late in the session at 0.5 mg/kg, and since the beginning of the session at 1 mg/kg. In Experiment 2, HU-210 failed to affect overall ingestion. These results suggest that HU-210 effect - when present - depends on CB₁ receptor stimulation, possibly leading to impairment of the motor competence necessary for licking. The reduction of burst number late in the session observed with rimonabant 0.5 mg/kg, which resembles the effect of reward devaluation, might suggest reduced "liking".

Keywords: Behavioural activation; HU-210; Licking microstructure; Reward; Reward-evaluation; Rimonabant.