Cocaine and amphetamine increase extracellular dopamine in the nucleus accumbens of mice lacking the dopamine transporter gene.

Behavioral and biochemical studies suggest that dopamine (DA) plays a role in the reinforcing and addictive properties of drugs of abuse. Recently, this hypothesis has been challenged on the basis of the observation that, in mice genetically lacking the plasma membrane dopamine transporter [DAT-knock out (DAT-KO)], cocaine maintained its reinforcing properties of being self-administered and inducing place preference, despite the failure to increase extracellular dopamine in the dorsal striatum. Here we report that, in DAT-KO mice, cocaine and amphetamine increase dialysate dopamine in the medial part of the nucleus accumbens. Moreover, reboxetine, a specific blocker of the noradrenaline transporter, increased DA in the nucleus accumbens of DAT-KO but not of wild-type mice; in contrast, GBR 12909, a specific blocker of the dopamine transporter, increased dialysate dopamine in the nucleus accumbens of wild-type but not of DAT-KO mice. These observations provide an explanation for the persistence of cocaine reinforcement in DAT-KO mice and support the hypothesis of a primary role of nucleus accumbens dopamine in drug reinforcement.

Altered neurotensin mrna expression in mice lacking the dopamine transporter.

Psychostimulants and antipsychotic drugs increase mRNA expression of the neuropeptide neurotensin (NT) in the striatum and nucleus accumbens. In the present study, we used mice lacking the dopamine transporter (DAT) to investigate the consequences of a chronic hyperdopaminergic state on NT gene expression. NT mRNA expression was examined under basal conditions and after administration of haloperidol or amphetamine using in situ hybridization with a digoxigenin-labeled NT cRNA probe. DAT-/- mice exhibited a striking increase in the number of NT mRNA-expressing perikarya in the substantia nigra and ventral tegmental area, as well as a less pronounced increase in the lateral septum compared with wild-type littermates. No changes were detected in other regions expressing NT mRNA. Acute administration of haloperidol (1 mg/kg) induced a significant increase in the number of NT mRNA-expressing neurons in the dorsomedial and dorsolateral striatum of wild-type mice but failed to stimulate NT gene expression in DAT mutants. In contrast, a higher dose of haloperidol (5 mg/kg) stimulated striatal NT mRNA expression both in DAT+/+ and DAT-/- mice. Amphetamine (10 mg/kg) increased the number of hybridized neurons in the nucleus accumbens shell and fundus striati of wild-type and DAT-/- mice, indicating that the drug acted through a target other than DAT, such as the serotonin or the norepinephrine transporters. The up-regulation of NT mRNA observed in DAT-/- mice may represent an adaptive mechanism in response to constitutive hyperdopaminergia. These results illustrate the profound alterations in the NT system induced by chronic stimulation of DA receptors and underscore the potential clinical relevance of NT/DA interactions in schizophrenia and drug abuse.

Hypolocomotor effects of acute and daily d-amphetamine in mice lacking the dopamine transporter.

RATIONALE: Mice lacking the dopamine transporter (DAT(-/-)) exhibit high extracellular dopamine levels and marked hyperactivity. This hyperlocomotion is paradoxically decreased by acute administration of amphetamine-like psychostimulants, an effect that has been previously related to the activation of serotonergic neurotransmission. OBJECTIVES: The goal of the present study was to investigate the effects of acute and daily administration of d-amphetamine on the locomotor activity of DAT(-/-) mice and examine the development of behavioral sensitization. In addition, we tested the implication of the serotonin system in the observed effects. METHODS: DAT(+/+), DAT(+/-), and DAT(-/-) mice were injected with acute amphetamine (0, 0.3, 1, 3, or 10 mg/kg, SC), repeated amphetamine (1 mg/kg for 8 days, SC), or with the serotonin reuptake inhibitor fluoxetine (0, 5, 10, or 20 mg/kg, SC) and their locomotor activity was evaluated. Moreover, the expression of the serotonin transporter and 5-HT(1A) receptors in the brain of DAT(-/-) mice was studied using autoradiography. RESULTS: Acute and repeated d-amphetamine injection (1 mg/kg) induced an hypolocomotor response in DAT(-/-) and DAT(+/-) mice, but only DAT(+/-) mice developed locomotor sensitization to the drug. Acute treatment with fluoxetine decreased locomotion in DAT(-/-) mice in a dose-dependent manner. The common hypolocomotor effect induced by d-amphetamine and fluoxetine in DAT(-/-) mice suggests an action on the serotonin transporter. However, autoradiography of the serotonin transporter and 5-HT(1A) receptors showed normal density and distribution in the brain, suggesting no compensatory effects due to the deletion of the DAT. CONCLUSIONS: These findings indicate that partial or total DAT gene deletion result in decreased locomotion in response to d-amphetamine and modify behavioral sensitization depending on the proportion of DAT removed, suggesting that inhibition of the DAT is necessary for the development of sensitization to psychostimulant drugs.

Behavioural disturbances associated with hyperdopaminergia in dopamine-transporter knockout mice.

Mice lacking the dopamine transporter (DAT-/-) are characterized by high extracellular dopamine levels and spontaneous hyperlocomotion. We performed a detailed analysis of the behavioural phenotype of DAT-/- mice in order to identify other behavioural impairments associated with the hyperdopaminergic tone of these mutant mice. In particular, we investigated locomotor activity, exploration, and social and maternal behaviours, which are known to be regulated by dopamine. DAT-/- mice were easily aroused by novelty and always responded with hyperlocomotion, which interfered with habituation to the testing environment, exploratory behaviour in an open field and the coping response to forced swimming stress. Social behaviours such as interaction with an unknown congener or aggressiveness were not modified in DAT-/- mice compared with DAT+/- and DAT+/+ mice, although the maternal behaviour of mutant females was severely disturbed. Haloperidol and clozapine reversed the hyperactivity in DAT-/- mice, with a rightward shift of the dose-response curve compared with control animals, suggesting a dopamine-mediated effect. These results emphasize the role of dopamine regulation in locomotion, exploration and maternal behaviours and suggest that mice with a genetic deletion of DAT may represent a useful model to elucidate the altered behavioural processes accompanying pathological conditions associated with hyperdopaminergic function.

Increased rewarding properties of morphine in dopamine-transporter knockout mice.

The activation of dopamine (DA) neurotransmission plays a crucial role in the behavioural responses to drugs of abuse. In particular, increased extracellular levels of DA within the mesolimbic pathway have been implicated in the rewarding and locomotor stimulatory properties of morphine. We investigated the behavioural responses to morphine in mice with a genetic disruption of the DA transporter (DAT), resulting in a constitutively high level of extrasynaptic DA. In the conditioned place preference test, DAT-/- mice exhibited a stronger rewarding response to morphine (5 mg/kg, s.c.) compared with control littermates. However, the same dose of morphine failed to increase locomotor activity in DAT-/- mice, whilst enhancing locomotion in DAT+/- and DAT+/+ animals. Morphine-induced analgesia was unaffected in mutant mice, but the behavioural expression of naloxone-induced withdrawal signs was blunted. In vivo voltammetry in the shell of the nucleus accumbens revealed that morphine was able to stimulate DA neurons in DAT-/- mice, resulting in the accumulation of higher extracellular DA levels compared with control animals. Morphine also induced a higher rate of c-fos transcription in the shell of the nucleus accumbens in mutant mice. We conclude that morphine-induced rewarding responses are firmly established in DAT mutant mice despite a DA transmission that is already tonically activated, and independently of any effect on locomotion. These particular behavioural responses to morphine may be associated with the action of the drug on DA release and c-fos expression in the shell of the nucleus accumbens of DAT-/- mice.

Serotonin transporter gene polymorphisms and hyperserotonemia in autistic disorder.

Previous studies have provided conflicting evidence regarding the association of the serotonin transporter (5-HTT) gene with autism. Two polymorphisms have been identified in the human 5-HTT gene, a VNTR in intron 2 and a functional deletion/insertion in the promoter region (5-HTTLPR) with short and long variants. Positive associations of the 5-HTTLPR polymorphism with autism have been reported by two family-based studies, but one found preferential transmission of the short allele and the other of the long allele. Two subsequent studies failed to find evidence of transmission disequilibrium at the 5-HTTLPR locus. These conflicting results could be due to heterogeneity of clinical samples with regard to serotonin (5-HT) blood levels, which have been found to be elevated in some autistic subjects. Thus, we examined the association of the 5-HTTLPR and VNTR polymorphisms of the 5-HTT gene with autism, and we investigated the relationship between 5-HTT variants and whole-blood 5-HT. The transmission/disequilibrium test (TDT) revealed no linkage disequilibrium at either loci in a sample of 96 families comprising 43 trios and 53 sib pairs. Furthermore, no significant relationship between 5-HT blood levels and 5-HTT gene polymorphisms was found. Our results suggest that the 5-HTT gene is unlikely to play a major role as a susceptibility factor in autism.