Reduced sensitivity to reward in CB1 knockout mice.

RATIONALE: Previous studies have demonstrated that the activation and blockade of the cannabinoid type 1 receptor (CB1) leads to an enhancement and decrease of the consumption of food and other orally ingested reinforcers, respectively.
OBJECTIVE: To gain further knowledge about the role of CB1 in sucrose/saccharin reinforcing efficacy and intake, we tested CB1 knockout (CB1-KO) and littermate wild-type (WT) control mice in several self-administration experimental protocols.
METHODS: Operant (fixed or progressive ratio schedule) and non-operant conditioning procedures were used. In addition, a choice analysis based on the "matching law" as well as a microstructural analysis of the intra-session pattern of self-administration was performed.
RESULTS: CB1-KO mice consume less sucrose under operant conditions or when using a two-bottle free choice procedure. Moreover, as revealed by additional behavioural analysis, CB1-KO mice exhibit a decreased sensitivity to the rewarding properties of sucrose. In agreement with this finding, the differences between WT and CB1-KO mice faded away when the palatability of sucrose was devaluated by adding quinine, but not when a non-caloric sweetener, saccharin, was available.
CONCLUSIONS: These results demonstrate a modulatory role of CB1 in the determination of the rewarding properties of sucrose and probably, as suggested by previous studies, other reinforcers.