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Dopamine signaling in the nucleus accumbens is critical in mediating the effects of cocaine. There are two splice variants of dopamine D2 receptors, D2L and D2S, which are believed to have different functional roles. Here, we show, that knocking down D2L selectively using viral-mediated short-hairpin RNA led to a slight but significant decrease in basal locomotor activity with no significant change in cocaine-induced stimulation of locomotion. The knockdown appears to produce a trend of reduced conditioned place preference to cocaine but the difference was not statistically significant. Our results demonstrated that the splice variants of D2 receptors can be selectively manipulated in vivoin specific brain regions allowing more specific studies of each D2 receptor isoform.

Cocaine addiction is a worldwide public health problem for which there are no established treatments. The dopamine transporter (DAT) is suspected as the primary target mediating cocaine's abuse-related effects based on numerous pharmacological studies. However, in a previous study, DAT knockout mice were reported to self-administer cocaine, generating much debate regarding the importance of the DAT in cocaine's abuse-related effects. Here, we show that mice expressing a "knockin" of a cocaine-insensitive but functional DAT did not self-administer cocaine intravenously despite normal food-maintained responding and normal intravenous self-administration of amphetamine and a direct dopamine agonist. Our results have three implications. First, they imply a crucial role for high-affinity DAT binding of cocaine in mediating its reinforcing effects, reconciling mouse genetic engineering approaches with data from classic pharmacological studies. Second, they demonstrate the usefulness of knockin strategies that modify specific amino acid sequences within a protein. Third, they show that it is possible to alter the DAT protein sequence in such a way as to selectively target its interaction with cocaine, while sparing other behaviors dependent on DAT function. Thus, molecular engineering technology could advance the development of highly specialized compounds such as a dopamine-sparing "cocaine antagonist."

We have previously reported that knockin mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) do not experience cocaine reward, as measured by conditioned place preference. This conclusion has come under scrutiny because some genetically modified mice show cocaine-induced conditioned place preference in a narrow dose range, that is, responding at doses around 10 mgkg, but not at 5 and 20 mgkg, the doses we tested in DAT-CI mice. These results raise the possibility that we have missed the optimal dose for cocaine response. Here we report that cocaine does not produce reward in DAT-CI mice at low, moderate, and high doses, including 10 mgkg. This study strengthens our conclusion that DAT inhibition is required for cocaine reward in mice with a functional dopaminergic system.

Methylphenidate (Ritalin) is one of the most commonly abused prescription drugs. It is a psychostimulant that inhibits the dopamine and norepinephrine transporters with high affinity. In mice, methylphenidate stimulates locomotor activity, is self-administered, and produces conditioned place preference, typical properties of an addictive drug. We have generated a knockin mouse line bearing a mutant dopamine transporter that is approximately 80-fold less sensitive to cocaine inhibition than wild type. It is interesting to note that this mutant is also almost 50-fold less sensitive to methylphenidate inhibition, suggesting similarities in the binding site for cocaine and methylphenidate. Because methylphenidate is not effective at inhibiting the mutant dopamine transporter, we hypothesized that it would not stimulate locomotor activity or produce reward in the knockin mice. In these knockin mice, doses up to 40 mg/kg methylphenidate either inhibit or fail to stimulate locomotor activity and do not produce conditioned place preference. Doses up to 40 mg/kg methylphenidate also fail to produce stereotypy in the knockin mice. Nisoxetine and desipramine, selective norepinephrine transporter inhibitors, also reduce locomotor activity in wild-type and knockin mice. These results indicate that enhanced dopaminergic neurotransmission is required for methylphenidate's stimulating and rewarding effects. In addition, we observed that drugs enhancing noradrenergic neurotransmission inhibit locomotor activity in mice, which is consistent with the notion that methylphenidate's ability to inhibit the norepinephrine transporter may contribute to its efficacy in treating attention deficit hyperactivity disorder.

The primary mechanism by which cocaine induces stereotypy has been difficult to discern because cocaine has three high-affinity targets the reuptake transporters for dopamine (DAT), norepinephrine, and serotonin. To dissect out the role of DAT in cocaine effects, we generated a knock-in mouse line with a cocaine-insensitive DAT (DAT-CI mice). DAT-CI mice provide a powerful tool that can directly test whether DAT inhibition is important for cocaine-induced stereotypy. We found that acute cocaine failed to produce stereotypy in DAT-CI mice. In fact, 40 mgkg cocaine suppressed stereotypy in DAT-CI mice but produced profound stereotypy in wild-type mice. These findings suggest that DAT inhibition is necessary for cocaine-induced stereotypy. Furthermore, mechanisms independent of DAT inhibition appear to inhibit stereotypy.

When expressed in epithelial cells, dopamine transporter (DAT) was detected predominantly in the apical plasma membrane, whereas norepinephrine transporter (NET) was found in the basolateral membrane, despite 67% overall amino acid sequence identity. To identify possible localization signals responsible for this difference, DAT–NET chimeras were expressed in MDCK cells and localized by immunocytochemistry and transport assays. The results suggested that localization of these transporters in MDCK cells depends on their highly divergent NH2-terminal regions. Deletion of the first 58 amino acids of DAT (preceding TM1) did not change its apical localization. However, the replacement of that region with corresponding sequence from NET resulted in localization of the chimeric protein to the basolateral membrane, suggesting that the NH2-terminus of NET, which contains two dileucine motifs, contains a basolateral localization signal. Mutation of these leucines to alanines in the context of a basolaterally localized NET/DAT chimera restored transporter localization to the apical membrane, indicating that the dileucine motifs are critical to the basolateral localization signal embodied within the NET NH2-terminal region. However, the same mutation in the context of wild-type NET did not disrupt basolateral localization, indicating the presence of additional signals in NET directing its basolateral localization within the plasma membrane.

To search for an efficient expression cloning method, we mixed plasmid pmDATsv, which contains the mouse dopamine transporter (mDAT) cDNA, with a large amount of another plasmid prGlyTsv to mimic the situation of a cDNA library and examined COS cell expression. Both plasmids have an SV40 replication origin and thus will be replicated to high copy numbers in COS cells. After transfecting COS-7 cells with pmDATsv/prGlyTsv mixture at 1/1000 ratio, we could not detect any cells expressing strong mDAT activity. In contrast, when prGlyTsv was replaced by prSERTsk (no SV40 origin) in the transfection mixture, we observed hundreds of cells expressing strong mDAT activity. The results suggested that in many cells low mDAT expression was not due to the lack of pmDATsv plasmid but due to the presence of large numbers of replicable prGlyTsv. Analysis with a mathematical model suggests that diluting cDNA libraries with other plasmids without the SV40 origin should improve the detection of COS cells expressing target cDNAs. We tested this conclusion with pmDATsv/prGlyTsv mixture. When the mixture at 1/1000 ratio was diluted with prSERTsk and used for transfection, we could now easily detect cells expressing strong mDAT activity. Copyright 2000 Academic Press.