Cocaine is an inhibitor of the dopamine, norepinephrine, and serotonin reuptake transporters. Because its administration would therefore elevate signaling of all these three neurotransmitters, many studies have been aimed at attributing individual effects of cocaine to specific transmitter systems. Using mice with a cocaine insensitive dopamine transporter (DAT-CI mice), we previously showed that cocaine-induced dopamine elevations were necessary for its rewarding and stimulating effects. In this study, we observe that DAT-CI mice exhibit cocaine-conditioned place aversion, and that its expression depends on their genetic background. Specifically, DAT-CI mice backcrossed to the C57Bl/6J strain background did not display a preference or an aversion to cocaine, whereas DAT-CI mice that were on a mixed 129S1/SvImJ × C57Bl/6J (129B6) background had a robust conditioned place aversion to cocaine. These results indicate that while inhibition of the dopamine transporter (DAT) is necessary for cocaine reward, other cocaine targets and neurotransmitter systems may mediate the aversive properties of cocaine. Furthermore, the aversive effect of cocaine can be observed in the absence of a DAT-mediated rewarding effect, and it is affected by genomic differences between these two mouse strains.
Aversion; Addiction; Cocaine; Dopamine; Reward
We previously generated a knock-in mouse line with a cocaine-insensitive dopamine transporter (DAT-CI mice). These mice lost several behavioral responses to cocaine, but retained their response to amphetamine. DAT-CI mice are hyperdopaminergic due to reduced DAT function, and may thus be a good model for studying attention deficit hyperactivity disorder (ADHD). These mice had been behaviorally characterized while they were on a mixed genetic background. – However as the colony was propagated over time, the mixed genetics were shifted toward a pure C57Bl/6J background – via a common breeding scheme known as “backcrossing.” Several phenotypes appeared to have changed during this time frame. In this study, we investigated whether backcrossing altered the hyperlocomotive phenotype and behavioral responses to amphetamine, a drug used to treat ADHD.
C57-congenic DAT-CI mice had high spontaneous locomotor activity that could be suppressed by low doses of amphetamine. Furthermore, their locomotion was not stimulated by very high doses of amphetamine (20 mg/kg). After the reversion to a mixed genetic background by breeding with the 129 strain, the C57:129 hybrid DAT-CI mice displayed reduced basal locomotor activity compared to the C57-congenic mutant mice, and regained locomotor stimulation by high-dose amphetamine. The calming effect of amphetamine at low doses was retained in both strains.
In summary, reduced DAT function in DAT-CI mice leads to a hyperdopaminergic state, and an ADHD-like phenotype in both strains. The data show that the genetic background of DAT-CI mice affects their locomotor phenotypes and their responses to amphetamine. Since the differences in genetic background between the strains of mice have a significant impact on the ADHD-like phenotype and the response to amphetamine, further study with these strains could identify the genetic underpinnings affecting the severity of ADHD-related symptoms and the treatment response.
ADHD; Amphetamine; Dopamine; Habituation; Knock-in
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 shRNA 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 vivo in specific brain regions allowing more specific studies of each D2 receptor isoform.
Dopamine; cocaine; addiction; D2L; D2 receptors; shRNA; AAV
RNA interference is a cellular mechanism regulating levels of mRNAs. It has been widely exploited to knockdown specific protein targets. The selected interfering RNA sequence greatly influences its ability to knockdown the target. Here we present a method for constructing multiple testing plasmids which express small hairpin RNAs (shRNA) targeting different regions of an mRNA. A simple fluorescence test in cultured cells allows convenient evaluation of mRNA knockdown by many different shRNAs on 96-well plates. We show that software predicted shRNAs have varying efficacies and only 2 of the 7 tested shRNAs significantly knocked down their targets.
shRNA; interfering RNA; knock down; gene silencing; in vitro screening
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 mg/kg 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.
Cocaine; dopamine transporter; knock-in mice; stereotypy
Cocaine binds and inhibits dopamine transporter (DAT), norepinephrine transporter (NET) and serotonin transporter. The residues forming cocaine binding sites are unknown. RTI-113, a cocaine analog, is 100x more potent at inhibiting DAT than inhibiting NET. Here we show that removing the hydroxyl group from residue Tyr151 in NET by replacing it with Phe, the corresponding residue in DAT, increased the sensitivity of NET to RTI-113, while the reverse mutation in DAT decreased the sensitivity of DAT to RTI-113. In contrast, RTI-31, another cocaine analog having the same structure as RTI-113 but with the phenyl group at the 2β position replaced by a methyl group, inhibits the transporter mutants equally well whether a hydroxyl group is present at the residue or not. The data suggest that this residue contributes to cocaine binding site and is close to the 2β position of cocaine analogs. These results are consistent with our previously proposed cocaine-DAT binding model where cocaine initially binds to a site that does not overlap with, but is close to, the dopamine-binding site. Computational modeling and molecular docking yielded a binding model that explains the observed changes in RTI-113 inhibition potencies.
The gene encoding the dopamine transporter (DAT) has been implicated in CNS disorders, but the responsible polymorphisms remain uncertain. To search for regulatory polymorphisms, we measured allelic DAT mRNA expression in substantia nigra of human autopsy brain tissues, using two marker SNPs (rs6347 in exon 9 and rs27072 in the 3′-UTR). Allelic mRNA expression imbalance (AEI), an indicator of cis-acting regulatory polymorphisms, was observed in all tissues heterozygous for either of the two marker SNPs. SNP scanning of the DAT locus with AEI ratios as the phenotype, followed by in vitro molecular genetics studies, demonstrated that rs27072 C>T affects mRNA expression and translation. Expression of the minor T allele was dynamically regulated in transfected cell cultures, possibly involving microRNA interactions. Both rs6347 and rs3836790 (intron8 5/6 VNTR) also seemed to affect DAT expression, but not the commonly tested 9/10 VNTR in the 3′UTR (rs28363170). All four polymorphisms (rs6347, intron8 5/6 VNTR, rs27072 and 3′UTR 9/10 VNTR) were genotyped in clinical cohorts, representing schizophrenia, bipolar disorder, depression, and controls. Only rs27072 was significantly associated with bipolar disorder (OR=2.1, p=0.03). This result was replicated in a second bipolar/control population (OR=1.65, p=0.01), supporting a critical role for DAT regulation in bipolar disorder.
dopamine transporter; bipolar disorder; allelic expression imbalance; SLC6A3; rs27072; Dopamine; Depression; Unipolar/Bipolar; Pharmacogenetics/Pharmacogenomics; Neurogenetics; Allelic expression imbalance; Dopamine transporter; SLC6A3
Molecular modeling and dynamics simulations have been performed to study how cocaine inhibits dopamine transporter (DAT) for the transport of dopamine. The computationally determined DAT-ligand binding mode is totally different from previously proposed overlap binding mode in which cocaine- and dopamine-binding sites are the same (Beuming, T. et al. Nature Neurosci. 2008, 11, 780–789). The new cocaine-binding site does not overlap with, but close to, the dopamine-binding site. Analysis of all results reveals that when cocaine binds to DAT, the initial binding site is likely the one modeled in this study, as this binding site can naturally accommodate cocaine. Then, cocaine may move to the dopamine-binding site after DAT makes some necessary conformational change and expands the binding site cavity. It has been demonstrated that cocaine may inhibit the transport of dopamine through both blocking the initial DAT-dopamine binding and reducing the kinetic turnover of the transporter following the DAT-dopamine binding. The relative contributions to the phenomenological inhibition of the transport of dopamine from blocking the initial binding and reducing the kinetic turnover can be different in different types of assays. The obtained general structural and mechanistic insights are consistent with available experimental data and could be valuable for guiding future studies towards understanding cocaine inhibiting other transporters.
The transporters of dopamine, norepinephrine and serotonin are molecular targets of cocaine, amphetamine, and therapeutic antidepressants. The residues involved in binding these drugs are unknown. We have performed several rounds of random and site-directed mutagenesis in the mouse norepinephrine transporter and screened for mutants with altered sensitivity to cocaine inhibition of substrate uptake. We have identified a triple mutation that retains close to wild-type transport function but displays a 37-fold decrease in cocaine sensitivity and 24-fold decrease in desipramine sensitivity. In contrast, the mutant’s sensitivities to amphetamine, methamphetamine, and methylphenidate are only slightly changed. Our data reveal critical residues contributing to the potent uptake inhibitions by these important drugs. Furthermore, this drug-resistant triple mutant can be used to generate a unique knock-in mouse line to study the role of norepinephrine transporter in the addictive effects of cocaine and the therapeutic effects of desipramine.
norepinephrine transporter; dopamine transporter; cocaine; antidepressants; random mutagenesis
Cocaine methiodide (CM), a charged cocaine analog, cannot pass the blood brain barrier. It has been assumed the effects of systemic CM represent cocaine actions in peripheral tissues. However, the IC50 values of CM have not been clearly determined for the major cocaine targets: dopamine, norepinephrine, and serotonin transporters, and sodium channels. Using cells transfected with individual transporters from mice and synaptosomes from mouse striatum tissues, we observed that the inhibition IC50 values for monoamine uptake by CM were 31-fold to 184-fold higher compared to cocaine at each of the transporters. In dorsal root ganglion neurons, cocaine inhibited sodium channels with an apparent IC50 of 75 µM, while CM showed no observable effect at concentrations up to 3 mM. These results indicate that an equal dose of CM will not produce an equivalent peripheral effect of cocaine.
With recent advances in understanding of the neuroscience of risk taking, attention is now turning to genetic factors that may contribute to individual heterogeneity in risk attitudes. In this paper we test for genetic associations with risk attitude measures derived from both the psychology and economics literature. To develop a long-term prospective study, we first evaluate both types of risk attitudes and find that the economic and psychological measures are poorly correlated, suggesting that different genetic factors may underlie human response to risk faced in different behavioral domains. We then examine polymorphisms in a spectrum of candidate genes that affect neurotransmitter systems influencing dopamine regulation or are thought to be associated with risk attitudes or impulsive disorders. Analysis of the genotyping data identified two single nucleotide polymorphisms (SNPs) in the gene encoding the alpha 4 nicotine receptor (CHRNA4, rs4603829 and rs4522666) that are significantly associated with harm avoidance, a risk attitude measurement drawn from the psychology literature. Novelty seeking, another risk attitude measure from the psychology literature, is associated with several COMT (catechol-O-methyl transferase) SNPs while economic risk attitude measures are associated with several VMAT2 (vesicular monoamine transporter) SNPs, but the significance of these associations did not withstand statistical adjustment for multiple testing and requires larger cohorts. These exploratory results provide a starting point for understanding the genetic basis of risk attitudes by considering the range of methods available for measuring risk attitudes and by searching beyond the traditional direct focus on dopamine and serotonin receptor and transporter genes.
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.
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.
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.
The plasma membrane neurotransmitter transporters terminate neurotransmissions by the reuptake of the released neurotransmitters. The transporters for the monoamines dopamine, norepinephrine, and serotonin (DAT, NET, and SERT) are targets for several popular psychostimulant drugs of abuse. The potencies of the psychostimulant on the monoamine transporters have been studied by several laboratories. However, there are significant discrepancies in the reported data with differences up to 60-fold. In addition, the drug potencies of the 3 monoamine transporters from mouse have not been compared in the same experiments or along side the human transporters. Further studies and systematic comparisons are needed.
In this study, we compared the potencies of five psychostimulant drugs to inhibit human and mouse DAT, SERT and NET in the same cellular background. The KI values of cocaine to inhibit the 3 transporters are within a narrow range of 0.2 to 0.7 μM. In comparison, methylphenidate inhibited DAT and NET at around 0.1 μM, while it inhibited SERT at around 100 μM. The order of amphetamine potencies was NET (KI = 0.07–0.1 μM), DAT (KI ≈ 0.6 μM), and SERT (KI between 20 to 40 μM). The results for methamphetamine were similar to those for amphetamine. In contrast, another amphetamine derivative, MDMA (3–4 methylenedioxymethamphetamine), exhibited higher potency at SERT than at DAT. The human and mouse transporters were similar in their sensitivities to each of the tested drugs (KI values are within 4-fold).
The current and previous studies support the following conclusions: 1) cocaine blocks all 3 monoamine transporters at similar concentrations; 2) methylphenidate inhibits DAT and NET well but a 1000-fold higher concentration of the drug is required to inhibit SERT; 3) Amphetamine and methamphetamine are most potent at NET, while being 5- to 9-fold less potent at DAT, and 200- to 500-fold less potent at SERT; 4) MDMA has moderately higher apparent affinity for SERT and NET than for DAT. The relative potencies of a drug to inhibit DAT, NET and SERT suggest which neurotransmitter systems are disrupted the most by each of these stimulants and thus the likely primary mechanism of drug action.
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.