Repeated exposure to cocaine increases Cdk5 levels, and this appears to exert negative feedback on motivated responding for the drug.82 In rats, intra-NAc injections of the Cdk5 inhibitor olomoucine increase the work which an animal is willing to expend to obtain an injection of cocaine in a progressive ratio paradigm.150 The Cdk5 inhibitor roscovitine, injected into the NAc increases conditioned reinforcement induced by cocaine, which can also be considered a measure of incentive motivation.150
