Locomotor conditioning by amphetamine requires cyclin-dependent kinase 5 signaling in the nucleus accumbens

Intermittent systemic exposure to psychostimulants such as amphetamine leads to several forms of long-lasting behavioral plasticity including non-associative sensitization and associative conditioning. In the nucleus accumbens (NAcc), the protein serine/threonine kinase cyclin-dependent kinase 5 (Cdk5) and its phosphorylation target, the guanine-nucleotide exchange factor kalirin-7 (Kal7), may contribute to the neuroadaptations underlying each of these forms of plasticity. Pharmacological inhibition of Cdk5 in the NAcc prevents the increases in dendritic spine density in this site and enhances the locomotor sensitization normally observed following repeated cocaine. Mice lacking the Kal7 gene display similar phenotypes suggesting that locomotor sensitization and increased NAcc spine density need not be positively correlated. As increases in spine density may relate to the formation of associative memories and both Cdk5 and Kal7 regulate the generation of spines following repeated drug exposure, we hypothesized that either inhibiting Cdk5 or preventing its phosphorylation of Kal7 in the NAcc may prevent the induction of drug conditioning. In the present experiments, blockade in rats of NAcc Cdk5 activity with roscovitine (40 nmol/0.5µl/side) prior to each of 4 injections of amphetamine (1.5 mg/kg; i.p.) prevented the accrual of contextual locomotor conditioning but spared the induction of locomotor sensitization as revealed on tests conducted one week later. Similarly, transient viral expression in the NAcc exclusively during amphetamine exposure of a threonine-alanine mutant form of Kal7 [mKal7(T1590A)] that is not phosphorylated by Cdk5 also prevented the accrual of contextual conditioning and spared the induction of sensitization. These results indicate that Cdk5 phosphorylation of Kal7 in the NAcc is necessary for the formation of context-drug associations potentially through the modulation of dendritic spine dynamics in this site