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

Abstract

Intermittent systemic exposure to psychostimulants such as amphetamine leads to several forms of long-lasting behavioral plasticity including nonassociative 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 threoninealanine 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 signaling via Cdk5 and Kal7 in the NAcc is necessary for the formation of context-drug associations, potentially through the modulation of dendritic spine dynamics in this site.

Inhibiting cyclin-dependent kinase 5 in the nucleus accumbens enhances the expression of amphetamine-induced locomotor conditioning

“…The dose of Ros tested was selected because it has been used by others to study the effects of Cdk5 inhibition in the NAcc (see [16] and references therein). We also recently showed that when infused into the NAcc during drug exposure, this dose of Ros prevents the induction of locomotor conditioning by amphetamine [12]. The use of the same dose in the present study assessing the effects of inhibiting NAcc Cdk5 on the expression of locomotor conditioning will thus allow the direct comparison of the results obtained to those obtained in our recent study [12] as well as those reported by others (see [16]).…”

“…Indeed, different neuronal mechanisms appear to underlie the induction and expression of excitatory conditioning [10–11] and these may be differentially regulated by Cdk5. Recently, we reported that inhibiting Cdk5 signaling in the NAcc exclusively during exposure to amphetamine prevented the accrual of contextual locomotor conditioning by amphetamine, indicating that Cdk5 actions in the NAcc are necessary for the induction of excitatory contextual associative conditioning [12]. In the experiment reported here, we assessed the effect of inhibiting Cdk5 in the NAcc exclusively on the expression of contextual locomotor conditioning with amphetamine.…”

“…Locomotion was recorded in the open fields for 2-h. S(+)-amphetamine sulfate (Sigma-Aldrich Inc., Saint Louis, MO) was dissolved in sterile saline. The dose [12] refers to the weight of the salt.…”

Drug-Paired Contextual Stimuli Increase Dendritic Spine Dynamics in Select Nucleus Accumbens Neurons

“…As expected, rats that showed conditioned behavioral responding (Paired, IP AMPH) also showed conditioned increases in c-Fos expression in the NAcc shell (Figure 3c; one-way ANOVA, c-Fos/NeuN, % Control; F 2, 10 = 5.18, po0.05; Scheffé post hoc, Paired (264.25 ± 30.88 cells/rat) vs Unpaired (180.60 ± 16.17 cells/rat) or Control (216.50 ± 25.91 cells/rat), po0.05). This effect was not expected nor was it observed in the NAcc core (data not shown; one-way ANOVA, c-Fos/NeuN, F 2, 23 = 0.57, NS) as contextual stimuli are processed in the NAcc shell and not the NAcc core (Cruz et al, 2014a;Singer et al, 2014aSinger et al, , 2014b. Importantly, Paired rats that previously received intra-VTA amphetamine displayed neither conditioned locomotion nor increases in c-Fos expression in the NAcc shell (Figure 3d; one-way ANOVA, c-Fos/NeuN, % Control; F 2, 14 = 0.18, NS; Paired (590.33 ± 58.72 cells/rat), Unpaired (676.00 ± 62.73 cells/rat), Control (575.50 ± 44.76 cells/rat)).…”

“…Importantly, manipulations, such as inhibiting cyclindependent kinase 5 (cdk5), which are known to prevent drug-induced spine proliferation in the NAcc (Norrholm et al, 2003), also prevent the development of conditioning but spare sensitization (Singer et al, 2014b). Although we did not observe spine alterations in ΔFosB+ neurons in the present study, it remains possible under some conditions for changes to occur in these neurons that enable the development of conditioned associations, as ΔFosB is a transcription factor for the cdk5 gene.…”