Dopamine control of striatal gene expression during development: relevance to knockout mice for the dopamine transporter

Dopamine Transporter Deficient Rodents: Perspectives and Limitations for Neuroscience

The key element of dopamine (DA) neurotransmission is undoubtedly DA transporter (DAT), a transmembrane protein responsible for the synaptic reuptake of the mediator. Changes in DAT's function can be a key mechanism of pathological conditions associated with hyperdopaminergia. The first strain of gene-modified rodents with a lack of DAT were created more than 25 years ago. Such animals are characterized by increased levels of striatal DA, resulting in locomotor hyperactivity, increased levels of motor stereotypes, cognitive deficits, and other behavioral abnormalities. The administration of dopaminergic and pharmacological agents affecting other neurotransmitter systems can mitigate those abnormalities. The main purpose of this review is to systematize and analyze (1) known data on the consequences of changes in DAT expression in experimental animals, (2) results of pharmacological studies in these animals, and (3) to estimate the validity of animals lacking DAT as models for discovering new treatments of DA-related disorders.

Different sensitivity to anesthesia according to ECoG data in dopamine transporter knockout and heterozygous rats

Dopamine in the brain is involved in many important functions, including the regulation of wakefulness. There is also some evidence suggesting that the dopamine function is crucial in anesthetic function. The state of anesthesia is characterized by a change in the level of consciousness and a change in brain electrical activity. Due to impaired mechanisms of dopamine transportation back to the synaptic terminal, dopamine transporter (DAT) knockout and heterozygous rats have increased levels of the extracellular dopamine. In our work, we registered ECoG disturbances in knockout and heterozygous rats, as well as disturbances in tone and activity in acute experiments under the anesthesia Zoletil (tiletamine and zolazepam) from the somatosensory cortex using a NeuroNexus flat multielectrode array to study gamma activity. We also used four low-resistance electrodes to control the slow rhythm. Both low-resistance and high-resistance electrodes showed differences in the ECoG spectrum of heterozygotes and total knockouts from the wild type and from each other. Heterozygous rats for the DAT gene (HET) showed increased rapid beta and gamma activity and decreased slow delta activity, while complete knockouts (KO), on the contrary, showed increased delta activity and decreased beta and gamma activity. Thus, the ECoG spectrum of HET is shifted to the right, while that of KO is shifted to the left. Full knockouts also showed decreased spatial synchronization in the 30-100 Hz gamma range compared to the wild type (WT). It is assumed that sedation of HET and KO is shifted towards opposite directions compared to WT under the same anesthesia conditions.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Intrinsic exercise capacity in rats influences dopamine neuroplasticity induced by physical training

The study evaluates whether the intrinsic capacity for physical exercise influences dopamine neuroplasticity induced by physical training. Male rats were submitted to three progressive tests until fatigue. Based on the maximal time of exercise (TE), rats were considered as low performance (LP), standard performance (SP) or high performance (HP) to exercise. Eight animals from each group (LP, SP, and HP) were randomly subdivided in sedentary (SED) or trained (TR). Physical training was performed for 6 wk. After that, concentrations of dopamine (DA), serotonin (5-HT), and their metabolites and mRNA levels of D1 receptor ( Drd1), D2 receptor ( Drd2), dopamine transporter ( Dat), tyrosine hydroxylase ( Th), glia cell line neurotrophic factor ( Gdnf), and brain-derived neurotrophic factor ( Bdnf) were determined in the caudate-putamen (CPu). TE was increased with training in all performance groups. However, the relative increase was markedly higher in LP rats, and this was associated with a training-induced increase in dopaminergic activity in the CPu, which was determined by the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio. An opposite monoamine response was found in HP-TR rats, in which physical training decreased the DOPAC/DA ratio in the CPu. Moreover, LP-SED rats displayed higher levels of Drd2 in the CPu compared with the other SED groups, and this higher expression was decreased by physical training. Physical training also decreased Dat and increased Gdnf in the CPu of LP rats. Physical training decreased Bdnf in the CPu only in HP rats. Thus, we provide evidence that the intrinsic capacity to exercise affects the neuroplasticity of the dopaminergic system in response to physical training. NEW & NOTEWORTHY The findings reported reveal that dopaminergic neuroplasticity in caudate-putamen induced by physical training is influenced by the intrinsic capacity to exercise in rats. To evaluate the dopaminergic neuroplasticity, we analyzed mRNA levels of D1 receptor, D2 receptor, dopamine transporter, tyrosine hydroxylase, glia cell line neurotrophic factor, and brain-derived neurotrophic factor as well as concentrations of dopamine, serotonin, and their metabolites. These results expand our knowledge about the interrelationship between genetic background, physical training, and dopaminergic neuroplasticity.

Psychostimulants affect dopamine transmission through both dopamine transporter-dependent and independent mechanisms

The precise mechanisms by which cocaine and amphetamine-like psychostimulants exert their reinforcing effects are not yet fully defined. It is widely believed, however, that these drugs produce their effects by enhancing dopamine neurotransmission in the brain, especially in limbic areas such as the nucleus accumbens, by inducing dopamine transporter-mediated reverse transport and/or blocking dopamine reuptake though the dopamine transporter. Here, we present the evidence that aside from dopamine transporter, non-dopamine transporter-mediated mechanisms also participate in psychostimulant-induced dopamine release and contribute to the behavioral effects of these drugs, such as locomotor activation and reward. Accordingly, psychostimulants could increase norepinephrine release in the prefrontal cortex, the latter then alters the firing pattern of dopamine neurons resulting in changes in action potential-dependent dopamine release. These alterations would further affect the temporal pattern of dopamine release in the nucleus accumbens, thereby modifying information processing in that area. Hence, a synaptic input to a nucleus accumbens neuron may be enhanced or inhibited by dopamine depending on its temporal relationship to dopamine release. Specific temporal patterns of dopamine release may also be required for certain forms of synaptic plasticity in the nucleus accumbens. Together, these effects induced by psychostimulants, mediated through a non-dopamine transporter-mediated mechanism involving norepinephrine and the prefrontal cortex, may also contribute importantly to the reinforcing properties of these drugs.

Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout

Knockout (KO) mice that lack the dopamine transporter (SL6A3; DAT) display increased locomotion that can be attenuated, under some circumstances, by administration of drugs that normally produce psychostimulant-like effects, such as amphetamine and methylphenidate. These results have led to suggestions that DAT KO mice may model features of attention deficit hyperactivity disorder (ADHD) and that these drugs may act upon serotonin (5-HT) systems to produce these unusual locomotor decreasing effects. Evidence from patterns of brain expression and initial pharmacologic studies led us to use genetic and pharmacologic approaches to examine the influence of altered 5-HT_1B receptor activity on hyperactivity in DAT KO mice. Heterozygous 5-HT_1B KO and pharmacologic 5-HT_1B antagonism both attenuated locomotor hyperactivity in DAT KO mice. Furthermore, DAT KO mice with reduced, but not eliminated, 5-HT_1B receptor expression regained cocaine-stimulated locomotion, which was absent in DAT KO mice with normal levels of 5-HT_1B receptor expression. Further experiments demonstrated that the degree of habituation to the testing apparatus determined whether cocaine had no effect on locomotion in DAT KO or reduced locomotion, helping to resolve differences among prior reports. These findings of complementation of the locomotor effects of DAT KO by reducing 5-HT_1B receptor activity underscore roles for interactions between specific 5-HT receptors and dopamine (DA) systems in basal and cocaine-stimulated locomotion and support evaluation of 5-HT_1B antagonists as potential, non-stimulant ADHD therapeutics.

Striatal dopamine receptor plasticity in neurotensin deficient mice

Schizophrenia is thought to be caused, at least in part, by dysfunction in striatal dopamine neurotransmission. Both clinical studies and animal research have implicated the dopamine neuromodulator neurotensin (NT) in the pathophysiology of schizophrenia. Utilizing male mice lacking the NT gene (NT(-/-)), these studies examined the consequences of NT deficiency on dopaminergic tone and function, investigating (1) dopamine concentrations and dopamine receptor and transporter expression and binding in dopaminergic terminal regions, and (2) the behavioral effects of selective dopamine receptor agonists on locomotion and sensorimotor gating in adult NT(-/-) mice compared to wildtype (NT(+/+)) mice. NT(-/-) mice did not differ from NT(+/+) mice in concentrations of dopamine or its metabolite DOPAC in any brain region examined. However, NT(-/-) mice showed significantly increased D1 receptor, D2 receptor, and dopamine transporter (DAT) mRNA in the caudate putamen compared to NT(+/+) controls. NT(-/-) mice also showed elevated D2 receptor binding densities in both the caudate putamen and nucleus accumbens shell compared to NT(+/+) mice. In addition, some of the behavioral effects of the D1-type receptor agonist SKF-82958 and the D2-type receptor agonist quinpirole on locomotion, startle amplitude, and prepulse inhibition were dose-dependently altered in NT(-/-) mice, showing altered D1-type and D2-type receptor sensitivity to stimulation by agonists in the absence of NT. The results indicate that NT deficiency alters striatal dopamine receptor expression, binding, and function. This suggests a critical role for the NT system in the maintenance of striatal DA system homeostasis and implicates NT deficiency in the etiology of dopamine-associated disorders such as schizophrenia.

Modanifil activates the histaminergic system through the orexinergic neurons

Modafinil is a drug used to treat hypersomnolence of narcolepsy. We previously reported that modafinil increases hypothalamic histamine release in rats but did not increase locomotor activity in histamine-depleted mice, suggesting that modafinil-induced locomotor activity involves the histaminergic system. Modafinil is also thought to express its effect through the orexinergic neurons, and orexin increases hypothalamic histamine release. These findings led us to investigate whether modafinil activates the histaminergic system via the orexinergic system. In the present study, we performed in vivo microdialysis and c-Fos immunohistochemistry to investigate whether the orexinergic system mediates the activation of the histaminergic system by modafinil using orexin neuron-deficient mice. Two hours after the injection, modafinil (150 mg/kg) caused a significant increase of histamine release compared to the basal release in wild type mice. However, modafinil had no effect on the histamine release in orexin neuron-deficient mice. By immunohistochemical study, we found that there was no neuronal activation in the tuberomammillary nucleus where the cell bodies of the histaminergic neurons exclusively exist in orexin neuron-deficient mice. These findings indicate that modafinil-induced increment of histamine release requires intact orexinergic neurons.

Past, present and future therapeutics for cerebellar ataxias

Cerebellar ataxias are a group of disabling neurological disorders. Patients exhibit a cerebellar syndrome and can also present with extra-cerebellar deficits, namely pigmentary retinopathy, extrapyramidal movement disorders, pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioural symptoms), and peripheral neuropathy. Recently, deficits in cognitive operations have been unraveled. Cerebellar ataxias are heterogeneous both at the phenotypic and genotypic point of view. Therapeutical trials performed during these last 4 decades have failed in most cases, in particular because drugs were not targeting a deleterious pathway, but were given to counteract putative defects in neurotransmission. The identification of the causative mutations of many hereditary ataxias, the development of relevant animal models and the recent identifications of the molecular mechanisms underlying ataxias are impacting on the development of new drugs. We provide an overview of the pharmacological treatments currently used in the clinical practice and we discuss the drugs under development.

Cocaine-induced sensitization is associated with altered dynamics of transcriptional responses of the dopamine transporter, tyrosine hydroxylase, and dopamine D2 receptors in C57Bl/6J mice

RATIONALE

Behavioural sensitization is a long lasting phenomenon that has been proposed to be involved in drug addiction. Although the expression of cocaine-induced sensitization has been associated with the activity of the mesencephalic dopaminergic neurons, little is known about the transcriptional adaptations of these neurons to a new challenge with cocaine long after cessation of repeated exposure to the drug.

OBJECTIVES

We studied the time course of the mRNA levels of three main regulatory elements of dopaminergic transmission after a challenge with cocaine (15 mg/kg) that followed 21 days of withdrawal from a cocaine pretreatment (20 mg/kg, ip, every 2 days for 21 days) in C57Bl/6J mice.

MATERIALS AND METHODS

Mice were placed 45 min in activity chambers and were killed 45 min, 2 h or 24 h after the challenge injection. Dopamine transporter (DAT), D2 auto-receptor (D2) and tyrosine hydroxylase (TH) mRNA levels were assessed by in situ hybridization in the ventral tegmental area and the substantia nigra compacta.

RESULTS

As compared to vehicle challenge, cocaine challenge in vehicle pretreated mice induced a rapid increase (+208%) in DAT mRNA (45 min) followed by a delayed decrease (-70%) (24 h), while TH and D2 mRNA were both increased (+45%) 24 h after the challenge. In cocaine pretreated mice, cocaine-induced short-term increase and long-term decrease in DAT mRNA levels were amplified (+328%) and reduced (-40%), respectively.

CONCLUSIONS

Repeated exposure to cocaine alters the transcriptional response of DA neurons to a new cocaine challenge long after cessation of repeated exposure to the drug. They point to the DAT mRNA as a major responsive element to a new presentation of cocaine.

Expression of Cre recombinase in dopaminoceptive neurons

Background

Dopamine-activated signaling regulates locomotor and emotional responses and alterations in dopamine-signaling are responsible of several psychomotor disorders. In order to identify specific functions of these pathways, the Cre/loxP system has been used. Here, we describe the generation and the characterization of a transgenic mouse line expressing the Cre recombinase in dopaminoceptive neurons. To this purpose, we used as expression vector a 140 kb yeast artificial chromosome (YAC) containing the dopamine D1 receptor gene (Drd1a).

Results

In the chosen line, D1Cre, the spatio-temporal pattern of Cre expression closely recapitulated that of the endogenous Drd1a gene, as assessed by immunohistological approaches in embryonic and adult stages. Efficiency of recombination was confirmed by crossing D1Cre with three different loxP lines (Creb1^loxP, CaMKIV^loxP and GR^loxP) and with the R26R reporter. In the three loxP lines studied, recombination was restricted to the area of Cre expression.

Conclusion

In view of the patterns of recombination restricted to the major dopaminoceptive regions as seen in the context of the CREB, CaMKIV and GR mutations, the D1Cre line will be a useful tool to dissect the contributions of specific genes to biological processes involving dopamine signaling.

Dopamine transporters in the cerebellum of mutant mice

In the central nervous system, dopamine is known to play a critical role in motor and cognitive functions. Although the cerebellum plays a role in the control of movement and posture and in cognitive functions, it has not been considered to be a dopaminergic region and the dopamine present was thought to represent a precursor of noradrenaline. However, recent evidence suggests that in the cerebellum there is a small dopaminergic element, whose properties are similar to the well characterized system of striatum. In order to better understand the functional role of this system and to delineate its specific interactions within the cerebellum, the distribution and properties of dopamine transporter (DAT) in the cerebellum of reeler and Purkinje cell degeneration (Nna1pcd) mutant mice, which are characterized by severe loss of different cell populations and abnormalities in synapse formation, have been studied. Kinetic studies revealed that [3H]dopamine is transported into cerebellar synaptosomes prepared from normal mice with affinities similar to that into striatal synaptosomes but with much lower maximal velocities. In reeler cerebellar synaptosomes the number of transport sites is significantly reduced. In Nna1pcd cerebellar synaptosomes the kinetic properties of transport of [3H]dopamine are similar to the normal. However, in vitro quantitative DAT autoradiography revealed a significantly increased binding in cerebellar nuclei, a decreased binding in molecular layer and an unaltered binding in the granule cell layer. These observations confirm a dopaminergic innervation of the cerebellum and contribute to our understanding of the intracerebellar distribution of the dopaminergic system.

Modification of dopaminergic markers expression in the striatum by neonatal exposure to glutamate during development

Monosodium l-glutamate (MSG) was administered subcutaneously to male neonatal rats, and the effect on developmental profile of tyrosine hydroxylase (TH), D1, D2 receptors, and dopamine (DA) transporter expression in the striatum was examined using Western blot. In addition, TH-immunopositive neurons at substantia nigra (SN) were also examined. MSG treatment (4mg/g of body weight, administered on postnatal days 1, 3, 5, and 7) resulted in a reduction of D1 and D2 receptor expression from 30 days of age and persisted to adulthood (120 days of age), while DA transporter expression was significantly reduced from 14 days of age to adulthood. TH immunopositive neurons at SN showed a significant reduction, as well as TH expression on postnatal days 10, 30, 60, and 120 at striatum was reduced. No changes of TH were observed at 14 days of age. Results indicate that an over-stimulation of the glutamatergic system by neonatal exposure to a high glutamate concentration induces a partial loss in TH-positive neurons in the SN and an important reduction in dopaminergic markers expression in the striatum, suggesting that early excitotoxicity could contribute to developmental alterations in the nigrostriatal pathway, which may be associated with various disorders of the basal ganglia.

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.

Opioid receptor gene expression in dopamine transporter knock-out mice in adult and during development

Dopamine transporter knock-out mice display locomotor hyperactivity due to increased extracellular striatal levels of dopamine. Hyperdopaminergic activity within this mesolimbic pathway is involved in the rewarding properties of morphine which are also increased in these mice. Due to the hyperdopaminergia, profound alterations in gene expression for dopamine receptors and neuropeptides are observed in the caudate putamen and nucleus accumbens. Here we investigated (1) the levels of mu-, delta- and kappa-opioid receptors mRNAs in normal mice from gestational day 13 (G13) to adult, and (2) the adaptive changes in the expression of these receptors in mice lacking the dopamine transporter. Our results show that, in wild-type mice, mu-opioid receptor mRNA expression appears early during development (G13) with a homogeneous distribution that evolves towards a patchy distribution in adult. Delta-opioid receptor mRNA appears only at G17 and kappa-opioid receptor mRNA is not observed before adulthood. The levels of delta-opioid receptor mRNA are not modified during development in knock-out mice compared to wild-type, but are increased in adult caudate putamen (+39%, P<0.05) and nucleus accumbens (+66%, P<0.05) at a time when these receptors are believed to be functional. The mu- and kappa-opioid receptors mRNA levels are not modified in the adult knock-out mice. In addition, we observed no differences in any opioid receptor mRNA level in dopamine transporter knock-out mice during prenatal ontogeny compared to wild-type. Our results constitute a detailed neuroanatomical description of opioid receptor mRNA expression from the time of their appearance during prenatal development until adulthood. Furthermore, we show here that chronic constitutive hyperdopaminergia only affects delta-opioid receptor mRNA levels in adult. Even if the propensity of knock-out mice to show increased rewarding properties to morphine seems to be mainly due to the substantial and further increase in hyperdopaminergic activity following drug treatment, the involvement of increased delta-opioid receptor mRNA levels in this behavior remains to be elucidated.

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.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Cellular and molecular aspects of striatal development

The striatum is a key component of the basal ganglia and there is considerable evidence that it has an important role in motor, cognitive and limbic functions. However, very little is known about how this forebrain structure develops. This review considers the role of cellular and molecular mechanisms involved in the development of the striatum, and the potential application of this knowledge to the understanding of the pathology and treatment of primary disease of this structure.

Dopamine regulates brain-derived neurotrophic factor (BDNF) expression in cultured embryonic mouse striatal cells

The differentiation of striatal GABAergic neurons coincides with the perinatal establishment of nigrostriatal dopaminergic synaptic connections. We have shown previously that dopamine stimulates the maturation of striatal GABAergic neurons. Since BDNF also regulates the development of GABAergic cells, we hypothesized that dopamine might affect striatal BDNF expression. The influence of dopamine on BDNF protein/mRNA and trkB mRNA levels was studied in neuronal and astroglia cultures of the mouse striatum. Stimulation with dopamine and a dopamine D1 receptor agonist increased BDNF mRNA and protein but not trkB mRNA in neuronal cultures. Our data indicate a potential role for dopamine in the developmental regulation of striatal BDNF expression and suggest that dopamine effects on GABAergic cells may be intertwined with BDNF action.