Molecular mechanisms of cocaine reward: combined dopamine and serotonin transporter knockouts eliminate cocaine place preference

[Pharmacology of monoamine neurotransmitter transporters]

Following exocytotic release, the biogenic amine neurotransmitters, norepinephrine, dopamine, and serotonin are removed from the synaptic cleft by the respective transporter, NET, DAT, and SERT, located on the plasma membrane and then re-stored into synaptic vesicles by vesicular monoamine transporter, VMAT. The molecular cloning of these transporters revealed that NET, DAT, and SERT are members of a sodium-dependent neurotransmitter transporter gene family, while VMATs arise from proton-dependent transporter gene family. Structural features common to NET, DAT, and SERT reveal a putative 12 transmembrane-spanning domain structure with cytosolic N- and C-terminal regions. Recent evidence suggest the regulation of the functional expression of these transporters via phosphorylation, which include direct phosphorylation of transporter proteins and/or of associated proteins that may control transporter function/expression. In addition, the substrates and inhibitors for these transporters appear capable of regulating transporter cell surface expression, thereby suggesting both activity-dependent and pharmacological regulatory mechanisms for transporter expression. Analyses of the genes provide new insight into their relation to neuronal diseases since NET, DAT and SERT are the molecular targets for many antidepressants as well as drugs of abuse such as cocaine and amphetamine. The availability of cDNAs of these and vesicular transporters has permitted detailed pharmacological studies in heterologous expression systems, and thus would promise the development of novel drugs with diverse chemical structures.

Experimental genetic approaches to addiction

Drugs of abuse are able to elicit compulsive drug-seeking behaviors upon repeated administration, which ultimately leads to the phenomenon of addiction. Evidence indicates that the susceptibility to develop addiction is influenced by sources of reinforcement, variable neuroadaptive mechanisms, and neurochemical changes that together lead to altered homeostasis of the brain reward system. Addiction is hypothesized to be a cycle of progressive dysregulation of the brain reward system that results in the compulsive use and loss of control over drug taking and the initiation of behaviors associated with drug seeking. The view that addiction represents a pathological state of reward provides an approach to identifying the factors that contribute to vulnerability, addiction, and relapse in genetic animal models.

Pharmacotherapy of addictions

Addiction to drugs, such as heroin, cocaine and alcohol, exacts great human and financial costs on society, but the development of pharmacotherapies for addiction has been largely neglected by the pharmaceutical industry. With advances in our understanding of the underlying biology of addictions now opening the door for the development of novel pharmacotherapies, it could be time for a reassessment of involvement in this increasingly important therapeutic area. Here, we summarize the current approved and implemented pharmacotherapeutic approaches to the treatment of addiction, and then highlight the most promising areas for future drug development from the perspective of our laboratory and our National Institutes of Health (NIH) National Institute on Drug Abuse (NIDA) Research Center.

Probes for the dopamine transporter: new leads toward a cocaine-abuse therapeutic--A focus on analogues of benztropine and rimcazole

In an attempt to discover a cocaine-abuse pharmacotherapeutic, extensive investigation has been directed toward elucidating the molecular mechanisms underlying the reinforcing effects of this psychostimulant drug. The results of these studies have been consistent with the inhibition of dopamine uptake, at the dopamine transporter (DAT), which results in a rapid and excessive accumulation of extracellular dopamine in the synapse as being the mechanism primarily responsible for the locomotor stimulant actions of cocaine. Nevertheless, investigation of the serotonin (SERT) and norepinephrine (NET) transporters, as well as other receptor systems, with which cocaine either directly or indirectly interacts, has suggested that the DAT is not solely responsible for the reinforcing effects of cocaine. In an attempt to further elucidate the roles of these systems in the reinforcing effects of cocaine, selective molecular probes, in the form of drug molecules, have been designed, synthesized, and characterized. Many of these compounds bind potently and selectively to the DAT, block dopamine reuptake, and are behaviorally cocaine-like in animal models of psychostimulant abuse. However, there have been exceptions noted in several classes of dopamine uptake inhibitors that demonstrate behavioral profiles that are distinctive from cocaine. Structure-activity relationships between chemically diverse dopamine uptake inhibitors have suggested that different binding interactions, at the molecular level on the DAT, as well as divergent actions at the other monoamine transporters may be related to the differing pharmacological actions of these compounds, in vivo. These studies suggest that novel dopamine uptake inhibitors, which are structurally and pharmacologically distinct from cocaine, may be developed as potential cocaine-abuse therapeutics.

Studying the neurobiology of stimulant and alcohol abuse and dependence in genetically manipulated mice

The ability to manipulate the genetic makeup of organisms by specific targeting of selected genes has provided a novel means of investigating the neurobiological mechanisms underlying drug abuse and dependence. However, as with other techniques, there are a number of potential pitfalls in the use of genetically manipulated animals (usually mice) in behavioural experiments. This review discusses the techniques involved in creating genetically manipulated mice, and points to opportunities and insights into addictive processes provided by the new science, while illustrating some of the potential problems encountered in interpretation of data obtained from such animals. The use of the mouse as an experimental animal also raises some specific problems which limit the usefulness of the technique at present. Examples taken from research into alcohol and psychostimulant abuse and dependence are used to illustrate the usefulness of genetically manipulated animals in addiction research, the problems of interpretation which sometimes arise, and how techniques are being developed to overcome present limitations to this exciting area of research.

No association between polymorphisms in the serotonin transporter gene and susceptibility to cocaine dependence among African-American individuals

Genetic research of cocaine abuse has been relatively limited among the African-American population. Since the serotonin transporter (5HTT) may be involved in modulating effects of cocaine, we investigated whether allelic variants of the 5HTT gene may confer susceptibility to cocaine dependence among African-American individuals. One hundred and fifty-six cocaine-dependent subjects and 82 controls were studied. Polymerase chain reaction-based genotyping of a variable-number-tandem-repeat (VNTR) marker yielded three alleles designated 12, 10 and 9. Genotype and allele frequencies were compared using chi-square analyses. We found no differences between subjects and controls with respect to genotype distribution (cocaine: 12/12 = 50%, 10/12 = 35.3%, 10/10 = 13.5%, 9/12 = 1.3%; controls: 12/12 = 42.7%, 10/12 = 39.0%, 10/10 = 17.1%, 9/12 = 1.2%). Similarly, allele frequencies of the VNTR marker did not differ between the two groups (cocaine: 12 = 68.3%, 10 = 31.1%, 9 = 0.6%; controls: 12 = 62.8%, 10 = 36.6%, 9 = 0.6%). Our findings do not seem to support a relationship between VNTR polymorphisms and cocaine dependence among African-American patients. Further studies involving larger samples are required to confirm our results.

The selective serotonin(1A)-receptor antagonist WAY 100635 blocks behavioral stimulating effects of cocaine but not ventral striatal dopamine increase

An increase in the extracellular dopamine (DA) concentration is generally accepted as an important neurochemical mediator of the behavioral effects of cocaine. Cocaine induced increases in serotonergic (5-HT) activity also appears to be involved in these effects. Here we describe the effects of the 5-HT(1A)-receptor antagonist WAY 100635 on the behavioral and neurochemical effects of cocaine. In-vivo microdialysis was used in behaving rats to measure extracellular concentration of DA in the nucleus accumbens (Nac). Four groups of animals received one of the following drug combinations: WAY 100635 (0.4 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), WAY 100635 (0.4 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 20 min apart. The pretreatment with WAY 100635 significantly attenuated the locomotor stimulant effects of cocaine without altering the DA overflow in the Nac. WAY 100635 itself did not modify locomotion or the extracellular DA concentration in the Nac. These results indicate that (1) the 5-HT(1A)-receptor is an important component in the mediation of cocaine locomotor stimulant effects, and (2) an increase in the extracellular DA concentration in the Nac might be a necessary but is not a sufficient condition for the locomotor stimulant effects of cocaine.

A cocaine insensitive chimeric insect serotonin transporter reveals domains critical for cocaine interaction

Serotonin transporters are key target sites for clinical drugs and psychostimulants, such as fluoxetine and cocaine. Molecular cloning of a serotonin transporter from the central nervous system of the insect Manduca sexta enabled us to define domains that affect antagonist action, particularly cocaine. This insect serotonin transporter transiently expressed in CV-1 monkey kidney cells exhibits saturable, high affinity Na+ and Cl- dependent serotonin uptake, with estimated Km and Vmax values of 436 +/- 19 nm and 3.8 +/- 0.6 x 10-18 mol.cell.min-1, respectively. The Manduca high affinity Na+/Cl- dependent transporter shares 53% and 74% amino acid identity with the human and fruit fly serotonin transporters, respectively. However, in contrast to serotonin transporters from these two latter species, the Manduca transporter is inhibited poorly by fluoxetine (IC50 = 1.23 micro m) and cocaine (IC50 = 12.89 micro m). To delineate domains and residues that could play a role in cocaine interaction, the human serotonin transporter was mutated to incorporate unique amino acid substitutions, detected in the Manduca homologue. We identified a domain in extracellular loop 2 (amino acids 148-152), which, when inserted into the human transporter, results in decreased cocaine sensitivity of the latter (IC50 = 1.54 micro m). We also constructed a number of chimeras between the human and Manduca serotonin transporters (hSERT and MasSERT, respectively). The chimera, hSERT1-146/MasSERT106-587, which involved N-terminal swaps including transmembrane domains (TMDs) 1 and 2, was remarkably insensitive to cocaine (IC50 = 180 micro m) compared to the human (IC50 = 0.431 micro m) and Manduca serotonin transporters. The chimera MasSERT1-67/hSERT109-630, which involved only the TMD1 swap, showed greater sensitivity to cocaine (IC50 = 0.225 micro m) than the human transporter. Both chimeras showed twofold higher serotonin transport affinity compared to human and Manduca serotonin transporters. Our results show TMD1 and TMD2 affect the apparent substrate transport and antagonist sensitivity by possibly providing unique conformations to the transporter. The availability of these chimeras facilitates elucidation of specific amino acids involved in interactions with cocaine.

Monoamine transporter pharmacology and mutant mice

Monoamine transporters, such as the dopamine transporter, 5-HT transporter and noradrenaline transporter, in the plasma membrane provide effective control over the intensity of monoamine-mediated signaling by recapturing neurotransmitters released by presynaptic neurons. These proteins represent established targets for several psychotropic drugs, including psychostimulants and antidepressants; however, important issues regarding the selectivity and mechanisms of action of these drugs remain unresolved. Although monoamine transporter knockout mice have profound changes in neurotransmission, they provide useful in vivo models to analyze the effects of psychotropic drugs. In this review, we summarize recent insights into the pharmacology of psychotropic drugs using mice in which the genes encoding these transporters have been deleted.

Cocaine increases serotonergic activity in the hippocampus and nucleus accumbens in vivo: 5-HT1a-receptor antagonism blocks behavioral but potentiates serotonergic activation

The hippocampus is an important mediator of learning and reinforcement, but its role in cocaine effects has received little attention. Neuronal activity in the hippocampus and the nucleus accumbens (Nac) depend on serotonergic (5-HT) transmission. Here we describe for the first time a cocaine-induced increase in 5-HT concentration in the hippocampus and the Nac parallel to behavioral activation. In addition, pretreatment with the 5-HT(1A)-receptor antagonist WAY 100635 blocked the behavioral activation after cocaine while potentiating the 5-HT increase in the hippocampus and the Nac. In vivo microdialysis was used in behaving rats to measure extracellular concentration of 5-HT in the hippocampus and the Nac. Four groups of animals received one of the following drug combinations: WAY 100635 (0.4 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), WAY 100635 (0.4 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 30 min apart. It was found that 1.) cocaine, at a dose that activates behavior, increases 5-HT levels in the hippocampus and in the Nac, and 2.) 5-HT(1A)-receptor antagonism can cause a dissociation of the hippocampal and Nac 5-HT activity from behavioral activation after cocaine. These results are discussed within the framework of the hippocampal-accumbens projection and its contribution to behavioral activity. They suggest that the hippocampus may have a role in mediating the behavioral and neurochemical effects of cocaine.

Substance abuse vulnerability loci: converging genome scanning data

Classical genetic studies suggest strong complex genetic contributions to a predisposition to abuse multiple addictive substances. Until recently, there were no reproducible genome scanning data identifying chromosomal positions likely to contain allelic variants that predispose the carrier to illegal substance addiction. Nominal results of linkage-based genome scanning studies for ethanol and nicotine addictions failed to display much agreement. Our recent data from association-based genome scans for illegal addictions, and reanalyses of previous results now provide a substantial body of converging results. The 15 reproducible chromosomal loci identified here are good candidates to harbor allelic variants that alter human substance abuse vulnerabilities. We discuss several approaches to identifying the specific gene variants that underlie these convergent association and linkage observations, and the impact that these convergent observations should have on understanding important human addictive disorders.

SAR studies of piperidine-based analogues of cocaine. 4. Effect of N-modification and ester replacement

A series of novel N- and 3alpha-modified piperidine-based analogues of cocaine were synthesized and tested for their ability to inhibit reuptake of DA, 5-HT, and NE by the DA, 5-HT, and NE transporters. N-Demethylation of trans-(+)-3alpha-piperidine-based ligands leads to improved activity at the SERT and NET and modest changes at the DAT. Replacement of the N-methyl group in trans-(+)-ester 1a with phenylalkyl groups leads to a modest 2.3-fold improvement in activity at the SERT (K(i) < or = 3.27 microM), insignificant changes at the NET, and a 3.5-fold loss in activity at the DAT (K(i) > or = 810 nM); however, such replacement in cis-(-)-ester 4, the more potent isomer of 1a, leads, in general, to a significant decrease in activity at all monoamine transporters (K(i) > 1 microM). Other N-modified ligands, including the ligands with polar groups incorporated in the N-alkyl substituent (3e-g) and ligands lacking the basic nitrogen (3i and 6d), show decreased activity at all monoamine transporters, though ligands 3e-g are similar in potency at the NET to 1a. N-Norester 2a, a possible metabolite of the lead compound 1a, and alcohol 1c, a compound with a 3alpha-substituent that is more stable to metabolism than 1a, were selected for further behavioral tests in animals. Alcohol 1c and ester 2a are similar in potency at the DAT to cocaine, ester 1a, and oxadiazole 1b, and both fully substitute for cocaine and have potency similar to that of cocaine in drug discrimination tests. Like cocaine, 1c increased locomotor activity (LMA) monotonically with time, whereas 2a produces biphasic effects consisting of initial locomotor depression followed by delayed locomotor stimulation. An interesting difference between cocaine, ester 1a, alcohol 1c, and N-norester 2a is that 1c and 2a are significantly longer acting in LMA tests. Although this result was anticipated for alcohol 1c, it is rather surprising for 2a which has an ester function susceptible to hydrolysis, a pathway of in vivo deactivation of cocaine and its ester analogues. The present results may have important implications for our understanding of the pharmacological mechanisms underlying the behavioral actions of cocaine and of the structural features needed for the design of the new leads in the discovery of a cocaine abuse medication.

Discovery of substituted 3,4-diphenyl-thiazoles as a novel class of monoamine transporter inhibitors through 3-D pharmacophore search using a new pharmacophore model derived from mazindol

Substituted 3,4-diphenyl-1,3-thiazols were identified as a class of novel and potent monoamine transporter inhibitors through a 3-D pharmacophore search using a new pharmacophore model derived from mazindol. The most potent compound (13) has K(i) values of 24 and 23 nM in binding to dopamine transporter and inhibition of dopamine reuptake, respectively.

Predicting treatment-outcome in cocaine dependence from admission urine drug screen and peripheral serotonergic measures

We investigated whether urine drug screens (UDS) at admission and platelet paroxetine binding, a measure of serotonin transporter sites, were related to outcome measures for cocaine patients in treatment. Tritiated paroxetine binding sites on platelets were assayed and UDS were obtained for 105 African American cocaine-dependent outpatients. Outcome measures included number of negative urines, days in treatment, dropouts, and number of treatment sessions attended. A significant association was found between cocaine-positive UDS at admission and negative urines, treatment retention, dropouts, and treatment sessions; while Bmax values of paroxetine binding (density of serotonin transporter sites) were significantly associated with treatment retention and negative urines. Moreover, UDS and paroxetine binding combined to enhance prediction of retention and abstinence. Although both admission UDS and paroxetine binding seem to contribute individually in predicting outcome of cocaine patients, a combination of the two variables seems to have a stronger effect in terms of predicting treatment-outcome.

Serotonin uptake into dopamine neurons via dopamine transporters: a compensatory alternative

Monoamine neurons are believed to use neuronal-specific transporters to remove their own transmitters from the extracellular space and thus terminate transmission to postsynaptic neurons. We report here, for the first time, conclusive evidence that a cross clearance of serotonin into dopamine neurons exists. Such alternative uptake by different neurons is adopted under circumstances when their own transporter function is no longer adequate. When the serotonin transporter (5-HTT) is disrupted in 5-HTT knockout mice, serotonin (5-HT) is found in dopamine (DA) neurons of homozygous (-/-) but not of heterozygous (+/-) mutant mice or their normal littermates. DA neurons containing 5-HT are seen in the substantia nigra and ventral tegmental area (VTA), but not in other brain areas of 5-HTT -/- mice. Normal rats treated with a 5-HT uptake blocker paroxetine also showed similar result. To verify the role of the DA transporter in such ectopic uptake, 5-HTT -/- mice were treated with DA uptake blocker GBR-12935, ectopic 5-HT in DA neurons was disappeared. These data indicate that: (a) 5-HT can be taken into DA neurons in rats and mice when the 5-HTT is not functionally adequate to remove extracellular 5-HT; (b) the 5-HT uptake into DA neurons is not affected by the 5-HT uptake blocker paroxetine; and (c) the DA transporter is responsible for the 5-HT uptake into DA neurons. This study thus demonstrates that cross neuronal type uptake exists and serves as a compensatory backup when a specific transporter is dysfunctional. This study also demonstrates that DA neurons can store 5-HT for possible "false neurotransmitter" or other usage.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

Metalearning and neuromodulation

This paper presents a computational theory on the roles of the ascending neuromodulatory systems from the viewpoint that they mediate the global signals that regulate the distributed learning mechanisms in the brain. Based on the review of experimental data and theoretical models, it is proposed that dopamine signals the error in reward prediction, serotonin controls the time scale of reward prediction, noradrenaline controls the randomness in action selection, and acetylcholine controls the speed of memory update. The possible interactions between those neuromodulators and the environment are predicted on the basis of computational theory of metalearning.

Age-related distribution of c-fos expression in the striatum of CD-1 mice after acute methylphenidate administration

Ritalin (methylphenidate hydrochloride, MPH) is the drug of choice for the treatment of attention deficit hyperactivity disorder. Previous research has shown that MPH administration affects the adult brain in a manner different from the young brain. In the current study, we set out to determine the target brain regions of acutely administered MPH at different stages of development. On postnatal days 3, 7, 11, 24, and 45, mice were treated with a single injection (s.c.) of saline, 5 or 20 mg/kg of MPH, and sacrificed 1 h later. Localization of c-fos expression was determined by immunocytochemistry. Compared to saline treated controls, mice treated with the high dose of MPH (20 mg/kg) showed dense Fos-immunoreactivity (Fos-IR) in the striatum. In most cases the low dose of MPH (5 mg/kg) produced only weak c-fos expression that was nearly indistinguishable from saline-treated controls. At PND 3 and 7, Fos-IR was localized in patches in the striatum. This patchy distribution of c-fos positive cells began to decline by PND 11 and was absent in PND 45 mice, with Fos-IR showing a scattered distribution throughout the striatum. The results of this study indicate that MPH induces the expression of c-fos in the same brain regions as cocaine and amphetamine, and that this expression is distributed differentially according to the age of the mouse.

Cocaine, reward, movement and monoamine transporters

Recent evidence enriches our understanding of the molecular sites of action of cocaine reward and locomotor stimulation. Dopamine transporter blockade by cocaine appears a sufficient explanation for cocaine-induced locomotion. Variation in DAT appears to cause differences in locomotion without drug stimulation. However, previously-held views that DAT blockade was the sole site for cocaine reward have been replaced by a richer picture of multitransporter involvement with the rewarding and aversive actions of cocaine. These new insights, derived from studies of knockout mice with simultaneous deletions and/or blockade of multiple transporters, provide a novel model for the rewarding action of this heavily-abused substance and implicate multiple monoamine systems in cocaine's hedonic activities.

5-HT1A agonist/antagonist modification of cocaine stimulant effects: implications for cocaine mechanisms

The 5-HT1A receptor site has been demonstrated to be an important pharmacological target in the modulation of unconditioned behavioral effects induced by cocaine. In this study, separate groups of rats (n=7) received a series of the 5-HT1A agonist treatments, 8-OHDPAT (0.2,0.4 mg/kg) in combination with saline or cocaine (10 mg/kg). Using a crossover design, the treatments were subsequently switched to the 5-HT1A antagonist, WAY 100635 (0.4,0.8 mg/kg) and then, switched back again to 8-OHDPAT (0.2,0.4 mg/kg). When the 8-OHDPAT was given in combination with cocaine, locomotion was substantially enhanced but when the treatment was switched to WAY 100635, the cocaine induced locomotion was suppressed. Neither the 8-OHDPAT or WAY 100635 given with saline affected locomotion as compared to saline treated animals. These findings indicated a reciprocal facilitatory/inhibitory influence of 5-HT1A agonists/antagonists upon cocaine induced locomotion. The 8-OHDPAT treatments, however, did not enhance all cocaine behavioral responses. Initially, 8-OHDPAT suppressed cocaine induced rearing and central zone entry, but with repeated treatments, these response suppression effects subsided. As a consequence, the facilitative influence of 8-OHDPAT upon cocaine induced locomotion could not be attributed to response redistribution effects. While WAY 100635 markedly reduced cocaine induced locomotion and rearing to nearly saline response levels, the same WAY 100635 treatments did not modify locomotor stimulant effects induced by caffeine (10 mg/kg). In that caffeine stimulant effects are not directly linked to serotonergic mechanisms, the absence of an influence of WAY 100635 upon caffeine induced locomotor stimulation lent further support to the proposition that the 5-HT1A receptor site contributes to locomotor behavior in situations where the serotonergic system is pharmacologically activated by drugs such as cocaine. These findings point to a potential role for 5-HT1A antagonists in treatment of cocaine abuse.

Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.

DARPP-32 mediates serotonergic neurotransmission in the forebrain

Serotonin is implicated in the regulation of complex sensory, motor, affective, and cognitive functions. However, the biochemical mechanisms whereby this neurotransmitter exerts its actions remain largely unknown. DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of molecular weight 32,000) is a phosphoprotein that has primarily been characterized in relation to dopaminergic neurotransmission. Here we report that serotonin regulates DARPP-32 phosphorylation both in vitro and in vivo. Stimulation of 5-hydroxy-tryptamine (5-HT4 and 5-HT6 receptors causes an increased phosphorylation state at Thr34-DARPP-32, the protein kinase A site, and a decreased phosphorylation state at Thr75-DARPP-32, the cyclin-dependent kinase 5 site. Furthermore, stimulation of 5-HT2 receptors increases the phosphorylation state of Ser137-DARPP-32, the casein kinase-1 site. Behavioral and gene transcriptional effects induced by compounds that selectively release serotonin were greatly reduced in DARPP-32 knockout mice. Our data indicate that DARPP-32 is essential not only for dopaminergic but also for serotonergic neurotransmission.

Dopamine uptake through the norepinephrine transporter in brain regions with low levels of the dopamine transporter: evidence from knock-out mouse lines

Selective blockers of the norepinephrine transporter (NET) inhibit dopamine uptake in the prefrontal cortex. This suggests that dopamine in this region is normally cleared by the somewhat promiscuous NET. We have tested this hypothesis by comparing the effects of inhibitors selective for the three monoamine transporters with those of a nonspecific inhibitor, cocaine, on uptake of 3H-dopamine into synaptosomes from frontal cortex, caudate nucleus, and nucleus accumbens from wild-type, NET, and dopamine transporter (DAT) knock-out mice. Dopamine uptake was inhibited by cocaine and nisoxetine, but not by GBR12909, in frontal cortex synaptosomes from wild-type or DAT knock-out mice. At transporter-specific concentrations, nisoxetine and GBR12909 failed to block dopamine uptake into frontal cortex synaptosomes from NET knock-out mice. The efficacy of cocaine at the highest dose (1 mm) was normal in DAT knock-out mice but reduced by 70% in NET knock-out mice. Nisoxetine inhibited dopamine uptake by 20% in caudate and nucleus accumbens synaptosomes from wild-type and DAT knock-out mice but had no effect in those from NET knock-out mice. Cocaine failed to block dopamine uptake into caudate or nucleus accumbens synaptosomes from DAT knock-out mice. Cocaine and GBR12909 each inhibited dopamine uptake into caudate synaptosomes from NET knock-out mice, but cocaine effectiveness was reduced in the case of nucleus accumbens synaptosomes. Thus, whereas dopamine uptake in caudate and nucleus accumbens depends primarily on the DAT, dopamine uptake in frontal cortex depends primarily on the NET. These data underscore the fact that which transporter clears dopamine from a given region depends on both the affinities and the local densities of the transporters.

Dopamine uptake through the norepinephrine transporter in brain regions with low levels of the dopamine transporter: evidence from knock-out mouse lines

Selective blockers of the norepinephrine transporter (NET) inhibit dopamine uptake in the prefrontal cortex. This suggests that dopamine in this region is normally cleared by the somewhat promiscuous NET. We have tested this hypothesis by comparing the effects of inhibitors selective for the three monoamine transporters with those of a nonspecific inhibitor, cocaine, on uptake of 3H-dopamine into synaptosomes from frontal cortex, caudate nucleus, and nucleus accumbens from wild-type, NET, and dopamine transporter (DAT) knock-out mice. Dopamine uptake was inhibited by cocaine and nisoxetine, but not by GBR12909, in frontal cortex synaptosomes from wild-type or DAT knock-out mice. At transporter-specific concentrations, nisoxetine and GBR12909 failed to block dopamine uptake into frontal cortex synaptosomes from NET knock-out mice. The efficacy of cocaine at the highest dose (1 mm) was normal in DAT knock-out mice but reduced by 70% in NET knock-out mice. Nisoxetine inhibited dopamine uptake by 20% in caudate and nucleus accumbens synaptosomes from wild-type and DAT knock-out mice but had no effect in those from NET knock-out mice. Cocaine failed to block dopamine uptake into caudate or nucleus accumbens synaptosomes from DAT knock-out mice. Cocaine and GBR12909 each inhibited dopamine uptake into caudate synaptosomes from NET knock-out mice, but cocaine effectiveness was reduced in the case of nucleus accumbens synaptosomes. Thus, whereas dopamine uptake in caudate and nucleus accumbens depends primarily on the DAT, dopamine uptake in frontal cortex depends primarily on the NET. These data underscore the fact that which transporter clears dopamine from a given region depends on both the affinities and the local densities of the transporters.

[3-cis-3,5-Dimethyl-(1-piperazinyl)alkyl]-bis-(4'-fluorophenyl)amine analogues as novel probes for the dopamine transporter

In a continuing effort to identify novel probes with which to study the dopamine transporter (DAT), we discovered that the sigma receptor antagonist, rimcazole, binds with moderate affinity (K(i)=224nM) to the DAT. The results from previous SAR studies suggested that substitution of the carbazole ring system of rimcazole with bis-(4'-fluorophenyl)amine might improve binding affinity and selectivity for the DAT. Thus, a novel series of [3-cis-3,5-dimethyl-(1-piperazinyl)alkyl]bis-(4'-fluorophenyl)amines were synthesized. The most potent compound in this series (9b) displaced [3H]WIN 35,428 binding in rat caudate-putamen (K(i)=17.6nM) with comparable affinity to GBR 12909. Despite high-affinity binding at DAT, and structural similarity to GBR 12909, preliminary studies suggest 9b behaves more like rimcazole than GBR 12909 and does not demonstrate cocaine-like psychostimulant behavior in mice.

Genetic perspectives on the serotonin transporter

The serotonin transporter (5-HTT) is most well known as the site of action of the serotonin reuptake inhibitors, which were initially developed as antidepressants, but now are the most widely used agents in the treatment of many additional neuropsychiatric and related disorders. The discovery that the gene that expresses the 5-HTT possesses a functional promoter-region polymorphism, which is associated with temperament and personality traits such as anxiety and negative emotionality as well as some behaviors, led to many studies examining this polymorphism in individuals with different neuropsychiatric disorders. The subsequent development of mice with a targeted disruption of the 5-HTT in our laboratory has provided an experimental model to examine the many consequences of diminished (in +/-, heterozygote mice) or absent (in -/-, homozygote knockout mice) function of the 5-HTT. The 5-HTT-deficient mice were also crossed with other knockout mice, allowing the study of multiple neurobiologic dysfunctions. As multiple genes are probably involved in the expression of complex behaviors such as anxiety, as well as neuropsychiatric disorders, these more genetically complex mice may more closely model disorders with complex etiologies. Thus, the combination of these comparative human and mouse studies may extend the opportunities to examine genetic alterations from a novel "bottom-up" approach [gene knockout or partial gene knockout in a combinational gene x gene x (yet unknown) gene approach], which is complementary to the traditional "top-down" genetic approach based upon studies of individuals with diagnosed neuropsychiatric disorders and their family members.

Behavioral screening for cocaine sensitivity in mutagenized zebrafish

Understanding the molecular basis of addiction could be greatly aided by using forward genetic manipulation to lengthen the list of candidate genes involved in this complex process. Here, we report that zebrafish exhibit cocaine-induced conditioned place preference. In a pilot screen of 18 F(2) generation families of mutagenized fish, we found three with abnormally low responses to cocaine. This behavior was inherited by the F(3) generation in a manner that suggests the abnormalities were because of dominant mutations in single genes. Performance profiles in secondary behavioral screens measuring visual dark-adaptation and learning suggest that the defects were the result of mutations in distinct genes that affect dopaminergic signaling in the retina and brain.