Cannabinoid system controls complex learning differently than simple associations

Since Ivan Pavlov first demonstrated classical conditioning by pairing a neutral stimulus with a reinforcer, researchers have applied this approach to understand not only direct associations, but also more complex learning processes, such as sensory preconditioning and second-order conditioning. Despite their relative immaturity, very young mammals such as newborn rabbits exhibit robust classical and higher-order conditioning. However, the neuromodulatory systems involved in these different types of neonatal conditioning remain to be identified. Here, we compared the role played by the noradrenergic and the endocannabinoid systems in classical conditioning, sensory preconditioning and second-order conditioning in newborn rabbits. Intraperitoneal injections of Propranolol, an antagonist of beta-adrenergic receptors, blocked classical conditioning promoted by the mammary pheromone in newborn rabbits but had no effect on sensory preconditioning or second-order conditioning. Conversely, intraperitoneal injections of Rimonabant, an antagonist of the main cannabinoid receptor CB1, had no effect on classical conditioning but blocked sensory preconditioning and second-order conditioning, indicating a specific impact on unreinforced association. Moreover, an effect of Rimonabant on memory reconsolidation was also revealed. These findings demonstrate a double dissociation in the role of noradrenergic and endocannabinoid modulations in first- and higher-order conditioning in newborn rabbits. Whereas our results indicate the noradrenergic system specifically promotes reinforced association in pups, they also establish that the endocannabinoid system selectively mediates higher-order conditioning by regulating unreinforced association in newborns. This also highlights that the rabbit is an excellent model for further investigating the neurobiology of neonatal first- and higher-order memory.