Cannabinoid receptor-1 signaling contributions to sign-tracking and conditioned reinforcement in rats

Abstract

Rationale: Endocannabinoids (eCBs) are critical gatekeepers of dopaminergic signaling, and disrupting cannabinoid receptor-1 (CB1) signaling alters DA dynamics to attenuate cue-motivated behaviors. Prior studies suggest that dopamine (DA) release plays a critical role in driving sign-tracking.

Objectives: Here, we determine whether systemic injections of rimonabant, a CB1 receptor inverse agonist, during Pavlovian lever autoshaping impair the expression of sign-tracking. We next examine whether rimonabant blocks the reinforcing properties of the Pavlovian lever cue in a conditioned reinforcement test.

Methods: In Exp. 1, we trained rats in Pavlovian lever autoshaping prior to systemic rimonabant injections (0, 1, 3 mg/kg) during early and late Pavlovian lever autoshaping sessions. In Exp. 2, we trained rats in Pavlovian lever autoshaping prior to systemic rimonabant injections (0, 1 mg/kg) during a conditioned reinforcement test.

Results: Rimonabant dose-dependently decreased lever contact and probability, and increased sign-tracker's latency to approach the lever cue early in Pavlovian training. With extended training, many previously goal-tracking and intermediate rats shifted to lever approach, which remained dose-dependently sensitive to rimonabant. Rimonabant attenuated cue-evoked food cup approach early, but not late, in conditioning, and did not affect pellet retrieval or consumption. The inserted lever cue served as a robust conditioned reinforcer after Pavlovian lever autoshaping, and 1 mg/kg rimonabant blocked conditioned reinforcement.

Conclusions: Together, our results suggest that CB1 signaling mediates two critical properties of incentive stimuli; their ability to attract (Exp. 1) and their ability to reinforce (Exp. 2) behavior.

Keywords: Appetitive; Approach; CB1 receptor; Conditioned reinforcement; Cue-motivated; Endocannabinoids; Incentive; Pavlovian; Sign-tracking.

Figure 1.. Lever and food cup approach are attenuated by rimonabant treatment early in PLA.

A. Exp. 1 timeline. We trained male and female rats in four daily sessions (1–4) of PLA to determine their ST, GT or INT group assignments. We next tested rats with systemic injections of the CB1 inverse agonist, rimonabant (0, 1, 3 mg/kg), during early reinforced PLA sessions (5–7). Rats were retrained in daily autoshaping sessions (8–14) and again tested with systemic injections of rimonabant in late reinforced PLA sessions (15–17). B-D. Data are mean and ± standard error of the mean (SEM) for PCA scores (B) lever contacts (C) and food cup contacts (D) for training sessions 1–4 (left) and three counterbalanced early test sessions for each rimonabant dose (right). E. Population distribution of PCA scores under vehicle and 3 mg/kg rimonabant. * significant main effect of Session or Drug, # significant Session x Tracking or Drug x Tracking interaction, % significant shift in population. Main effects of Tracking are not indicated on figure.

Figure 2.. Extended training shifts behavior towards sign-tracking, which continues to be sensitive to rimonabant treatment.

A-C. Data are mean and ± standard error of the mean (SEM) for PCA scores (A) lever contacts (B) and food cup contacts (C). Data are shown for retraining sessions 8–14 (left) and the three late test sessions for each rimonabant dose (right). A. Extended training shifts behavior towards sign-tracking. B. Lever contacts increase with extended training across all groups (left), and lever directed behavior across all rats continues to be sensitive to the effects of rimonabant (right). C. Food cup contacts are not affected by extended training and are not affected by rimonabant treatment. D. Population distribution of PCA scores are significantly left-shifted by 3mg/kg rimonabant compared to vehicle. * significant main effect of Session or Drug, # significant Session x Tracking or Drug x Tracking interaction, % significant shift in population. Main effects of Tracking are not indicated on figure.

Figure 3.. After extended training in PLA, rimonabant attenuates conditioned reinforcement for Pavlovian lever cue presentations.

A. Exp. 2 timeline. We trained rats in 22 daily PLA sessions. We tested rats with systemic injection of rimonabant (0 and 1 mg/kg) during a conditioned reinforcement test. We gave rats Rimonabant (0, 1, or 3 mg/kg) before a motor activity and satiety test. B. Rats with GT and INT PCA scores early in lever autoshaping shifted towards ST PCA scores with extended PLA training. C. During conditioned reinforcement, rats discriminated between active and inactive ports, suggesting the inserted lever cue served as a robust conditioned reinforcer. Rimonabant decreased the number of active, but not inactive pokes suggesting rimonabant attenuates the conditioned reinforcing properties of the Pavlovian lever cue. D. Time-course of nosepokes (binned in five-minute blocks) during conditioned reinforcement test. Rimonabant blunted the number of active nosepokes, while extinction curves were unaffected. * significant main effect of Session, Response or Drug, # significant Session x Tracking, Drug x Response interaction