Cocaine reward and locomotion stimulation in mice with reduced dopamine transporter expression

Abstract

Background: The dopamine transporter (DAT) plays a critical role in regulating dopamine neurotransmission. Variations in DAT or changes in basal dopaminergic tone have been shown to alter behavior and drug responses. DAT is one of the three known high affinity targets for cocaine, a powerful psychostimulant that produces reward and stimulates locomotor activity in humans and animals. We have shown that cocaine no longer produces reward in knock-in mice with a cocaine insensitive mutant DAT (DAT-CI), suggesting that cocaine inhibition of DAT is critical for its rewarding effect. However, in DAT-CI mice, the mutant DAT has significantly reduced uptake activity resulting in elevated basal dopaminergic tone, which might cause adaptive changes that alter responses to cocaine. Therefore, the objective of this study is to determine how elevated dopaminergic tone affects how mice respond to cocaine.

Results: We examined the cocaine induced behavior of DAT knockdown mice that have DAT expression reduced by 90% when compared to the wild type mice. Despite a dramatic reduction of DAT expression and marked elevation in basal dopamine tone, cocaine produced reward, as measured by conditioned place preference, and stimulated locomotor activity in these mice.

Conclusion: A reduction in DAT expression and elevation of dopaminergic tone do not lead to adaptive changes that abolish the rewarding and stimulating effects of cocaine. Therefore, the lack of reward to cocaine observed in DAT-CI mice is unlikely to have resulted from the reduced DAT activity but instead is likely due to the inability of cocaine to block the mutated DAT and increase extracellular dopamine. This study supports the conclusion that the blockade of DAT is required for cocaine reward and locomotor stimulation.

Figure 1

Effect of cocaine on locomotor activity in DAT-KD mutant mice compared to WT mice. Mice were habituated to the locomotor test chamber for 60 minutes. Cocaine or saline were injected (ip) and mice were returned to the test chamber and monitored for another 60 min. A) and B): Time course of locomotor activity of WT mice and DAT-KD mice. Saline, 5 mg/kg, 10 mg/kg or 20 mg/kg cocaine was given at the time indicated by the arrows. Data shown are average distance traveled in 5 min. C) Total distance traveled in 60 minutes for wild type and DAT-KD mice during their habituation to the chambers before drug or saline injection. DAT-KD mice are significantly more active than wild type ice (***, p < 0.001, t-test). D) Total distance traveled in 30 min after the injection of saline or 5, 10, or 20 mg/kg cocaine with 6 – 8 mice in each group. Two-way ANOVA was performed. Cocaine significantly increased locomotor activities in both genotypes of mice (for drug effect, F_3,66 = 22.38, p < 0.001) and had greater effect on DAT-KD mice than on the WT mice (for genotype, F_1,66 = 19.57, p < 0.001). Error bars represent standard error of means. Post hoc Bonferroni tests versus saline: *, p < 0.05; **, p < 0.01; ***, p < 0.001. Comparing between the two genotypes, 5 mg/kg and 10 mg/kg cocaine had a greater effect on locomotor activity in the DAT-KD mice than in wild type mice (#, p < 0.01).

Figure 2

Cocaine induced CPP in DAT-KD mutant mice and WT mice. CPP is represented by the difference in the time mice spend in the drug-paired chamber between pre-conditioning day and post-conditioning day. Both 5 and 20 mg/kg cocaine produced significant CPP in both WT mice and DAT-KD mice (for drug effect, F_2,1 = 24.25, p < 0.0001). Eight mice were examined in each group. Error bars represent standard error of means. Post hoc Bonferroni tests versus saline: *, p < 0.05; **, p < 0.01; ***, p < 0.001.