Sustained elevation of extracellular dopamine causes motor dysfunction and selective degeneration of striatal GABAergic neurons

Motor impairments, striatal degeneration, and altered dopamine-glutamate interplay in mice lacking PSD-95

Excessive activation of the N-methyl-d-aspartate (NMDA) receptor and the neurotransmitter dopamine (DA) mediate neurotoxicity and neurodegeneration under many neurological conditions, including Huntington's disease (HD), an autosomal dominant neurodegenerative disease characterized by the preferential loss of medium spiny projection neurons (MSNs) in the striatum. PSD-95 is a major scaffolding protein in the postsynaptic density (PSD) of dendritic spines, where a classical role for PSD-95 is to stabilize glutamate receptors at sites of synaptic transmission. Our recent studies indicate that PSD-95 also interacts with the D1 DA receptor localized in spines and negatively regulates spine D1 signaling. Moreover, PSD-95 forms ternary protein complexes with D1 and NMDA receptors, and plays a role in limiting the reciprocal potentiation between both receptors from being escalated. These studies suggest a neuroprotective role for PSD-95. Here we show that mice lacking PSD-95, resulting from genetic deletion of the GK domain of PSD-95 (PSD-95-ΔGK mice), sporadically develop progressive neurological impairments characterized by hypolocomotion, limb clasping, and loss of DARPP-32-positive MSNs. Electrophysiological experiments indicated that NMDA receptors in mutant MSNs were overactive, suggested by larger, NMDA receptor-mediated miniature excitatory postsynaptic currents (EPSCs) and higher ratios of NMDA- to AMPA-mediated corticostriatal synaptic transmission. In addition, NMDA receptor currents in mutant cortical neurons were more sensitive to potentiation by the D1 receptor agonist SKF81297. Finally, repeated administration of the psychostimulant cocaine at a dose regimen not producing overt toxicity-related phenotypes in normal mice reliably converted asymptomatic mutant mice to clasping symptomatic mice. These results support the hypothesis that deletion of PSD-95 in mutant mice produces concomitant overactivation of both D1 and NMDA receptors that makes neurons more susceptible to NMDA excitotoxicity, causing neuronal damage and neurological impairments. Understanding PSD-95-dependent neuroprotective mechanisms may help elucidate processes underlying neurodegeneration in HD and other neurological disorders.

Dopaminergic manipulations and its effects on neurogenesis and motor function in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder that is classically defined by a triad of movement and cognitive and psychiatric abnormalities with a well-established pathology that affects the dopaminergic systems of the brain. This has classically been described in terms of an early loss of dopamine D2 receptors (D2R), although interestingly the treatments most effectively used to treat patients with HD block these same receptors. We therefore sought to examine the dopaminergic system in HD not only in terms of striatal function but also at extrastriatal sites especially the hippocampus, given that transgenic (Tg) mice also exhibit deficits in hippocampal-dependent cognitive tests and a reduction in adult hippocampal neurogenesis. We showed that there was an early reduction of D2R in both the striatum and dentate gyrus (DG) of the hippocampus in the R6/1 transgenic HD mouse ahead of any overt motor signs and before striatal neuronal loss. Despite downregulation of D2Rs in these sites, further reduction of the dopaminergic input to these sites by either medial forebrain bundle lesions or receptor blockade using sulpiride was able to improve both deficits in motor performance and adult hippocampal neurogenesis. In contrast, a reduction in dopaminergic innervation of the neurogenic niches resulted in impaired neurogenesis in healthy WT mice. This study therefore provides evidence that D2R blockade improves hippocampal and striatal deficits in HD mice although the underlying mechanism for this is unclear, and suggests that agents working within this network may have greater effects than previously thought.

Quercetin improves behavioral deficiencies, restores astrocytes and microglia, and reduces serotonin metabolism in 3-nitropropionic acid-induced rat model of Huntington's Disease

AIM

Huntington's disease (HD) is an autosomal dominant disorder, for which clinically available drugs offer only symptomatic relief. These prescription drugs are not free of side effects, and the patients usually suffer from anxiety and depression. We investigated quercetin, a dietary flavonoid with free radical scavenging properties, for its beneficial potential if any, in 3-nitropropionic acid (3-NP)-induced HD in rats where both drugs were administered simultaneously.

METHODS

Performance of rats on beam balancing, elevated plus maze and gait traits were investigated following 3-NP and/or quercetin treatments for 4 days. Striatal biogenic amine levels and monoamine oxidase activity were assayed. Striatal sections were examined for Cd11B and glial fibrillary acidic protein immunoreactivity, and for evidences of neuronal lesion.

RESULTS

Quercetin significantly attenuated 3-NP-induced anxiety, motor coordination deficits, and gait despair. While the dopaminergic hyper-metabolism was unaffected, quercetin provided a significant reduction of 3-NP mediated increase in serotonin metabolism. Quercetin failed to affect 3-NP-induced striatal neuronal lesion, but decreased microglial proliferation, and increased astrocyte numbers in the lesion core.

CONCLUSION

These results taken together suggest that quercetin could be of potential use not only for correcting movement disturbances and anxiety in HD, but also for addressing inflammatory damages.

Dissociations between cognitive and motor effects of psychostimulants and atomoxetine in hyperactive DAT-KO mice

RATIONALE

Psychostimulants such as amphetamine and methylphenidate, which target the dopamine transporter (DAT), are the most frequently used drugs for the treatment of hyperactivity and cognitive deficits in humans with attention deficit hyperactivity disorder (ADHD). While psychostimulants can increase activity in healthy subjects, they exert a "paradoxical" calming effect in humans with ADHD as well as in hyperactive mice lacking the dopamine transporter (DAT-KO mice). However, the mechanism of action of these drugs and their impact on cognition in the absence of DAT remain poorly understood.

OBJECTIVES

This study was conducted to investigate the effects of psychostimulants and noradrenergic and serotonergic drugs on cognition in DAT-KO mice and normal (WT) littermates.

METHODS

We used a recently developed behavioral apparatus, the automated H-maze. The H-maze involves the consecutive learning of three different rules: delayed alternation, nonalternation, and reversal tasks.

RESULTS

Treatment of WT animals with the psychostimulants replicated the behavior observed in untreated DAT-KO mice while "paradoxically" restoring cognitive performances in DAT-KO mice. Further investigation of the potential involvement of other monoamine systems in the regulation of cognitive functions showed that the norepinephrine transporter blocker atomoxetine restored cognitive performances in DAT-KO mice without affecting hyperactivity. In contrast, the nonselective serotonin receptor agonist 5CT, which antagonizes hyperactivity in DAT-KO mice, had no effect on cognitive functions.

CONCLUSIONS

Taken together, these data allow dissociation of the locomotor and cognitive effects of ADHD drugs and suggest that the combination of DAT-KO mice with the automated H-maze can constitute a powerful experimental paradigm for the preclinical development of therapeutic approaches for ADHD.

A mitochondrial basis for Huntington's disease: therapeutic prospects

Huntington's disease (HD) is an autosomal dominant disease, with overt movement dysfunctions. Despite focused research on the basis of neurodegeneration in HD for last few decades, the mechanism for the site-specific lesion of neurons in the brain is not clear. All the explanations that partially clarify the phenomenon of neurodegeneration leads to one organelle, mitochondrion, which is severely affected in HD at the level of electron transport chain, Ca(2+) buffering efficiency and morphology. But, with the existing knowledge, it is not clear whether the cell death processes in HD initiate from mitochondria, though the Huntingtin (Htt) aggregates show close proximity to this organelle, or do some extracellular stimuli like TNFα or FasL trigger the process. Mainly because of the disparity in the different available experimental models, the results are quite confusing or at least inconsistent to a great extent. The fact remains that the mutant Htt protein was seen to be associated with mitochondria directly, and as the striatum is highly enriched with dopamine and glutamate, it may make the striatal mitochondria more vulnerable because of the presence of dopa-quinones, and due to an imbalance in Ca(2+). The current therapeutic strategies are based on symptomatic relief, and, therefore, mainly target neurotransmitter(s) and their receptors to modulate behavioral outputs, but none of them targets mitochondria or try to address the basic molecular events that cause neurons to die in discrete regions of the brain, which could probably be resulting from grave mitochondrial dysfunctions. Therefore, targeting mitochondria for their protection, while addressing symptomatic recovery, holds a great potential to tone down the progression of the disease, and to provide better relief to the patients and caretakers.

Impaired cliff avoidance reaction in dopamine transporter knockout mice

RATIONALE

Impulsivity is a key feature of disorders that include attention-deficit/hyperactivity disorder (ADHD). The cliff avoidance reaction (CAR) assesses maladaptive impulsive rodent behavior. Dopamine transporter knockout (DAT-KO) mice display features of ADHD and are candidates in which to test other impulsive phenotypes.

OBJECTIVES

Impulsivity of DAT-KO mice was assessed in the CAR paradigm. For comparison, attentional deficits were also assessed in prepulse inhibition (PPI) in which DAT-KO mice have been shown to exhibit impaired sensorimotor gating.

RESULTS

DAT-KO mice exhibited a profound CAR impairment compared to wild-type (WT) mice. As expected, DAT-KO mice showed PPI deficits compared to WT mice. Furthermore, the DAT-KO mice with the most impaired CAR exhibited the most severe PPI deficits. Treatment with methylphenidate or nisoxetine ameliorated CAR impairments in DAT-KO mice.

CONCLUSION

These results suggest that DAT-KO mice exhibit impulsive CAR behavior that correlates with their PPI deficits. Blockade of monoamine transporters, especially the norepinephrine transporter (NET) in the prefrontal cortex (PFC), may contribute to pharmacological improvement of impulsivity in these mice.

Parkinson's disease: gene therapies

With the recent development of effective gene delivery systems, gene therapy for the central nervous system is finding novel applications. Here, we review existing viral vectors and discuss gene therapy strategies that have been proposed for Parkinson's disease. To date, most of the clinical trials were based on viral vectors to deliver therapeutic transgenes to neurons within the basal ganglia. Initial trials used genes to relieve the major motor symptoms caused by nigrostriatal degeneration. Although these new genetic approaches still need to prove more effective than existing symptomatic treatments, there is a need for disease-modifying strategies. The investigation of the genetic factors implicated in Parkinson's disease is providing precious insights in disease pathology that, combined with innovative gene delivery systems, will hopefully offer novel opportunities for gene therapy interventions to slow down, or even halt disease progression.

Exclusive expression of VMAT2 in noradrenergic neurons increases viability of homozygous VMAT2 knockout mice

The vesicular monoamine transporter 2 (VMAT2) translocates monoamine neurotransmitters from the neuronal cytoplasm into synaptic vesicles. Since VMAT2-/- mice die within a few days of birth, it is difficult to analyze the detailed VMAT2 functions using these mice. In this study, we generated human VMAT2 transgenic mice that expressed VMAT2 in noradrenergic neurons with the aim to rescue the lethality of VMAT2 deletion. The expression of human VMAT2 in noradrenergic neurons extended the life of VMAT2-/- mice for up to three weeks, and these mice showed severe growth deficiency compared with VMAT2+/+ mice. These results may indicate that VMAT2 expressed in noradrenergic neurons has crucial roles in survival during the first several weeks after birth, and VMAT2 functions in other monoaminergic systems could be required for further extended survival. Although VMAT2 rescue in noradrenergic neurons did not eliminate the increased morbidity and lethality associated with VMAT2 deletion, the extension of the lifespan in VMAT2 transgenic mice will enable behavioral, pharmacological and pathophysiological studies of VMAT2 function.

A truncated peptide from p35, a Cdk5 activator, prevents Alzheimer's disease phenotypes in model mice

Alzheimer's disease (AD), one of the leading neurodegenerative disorders of older adults, which causes major socioeconomic burdens globally, lacks effective therapeutics without significant side effects. Besides the hallmark pathology of amyloid plaques and neurofibrillary tangles (NFTs), it has been reported that cyclin-dependent kinase 5 (Cdk5), a critical neuronal kinase, is hyperactivated in AD brains and is, in part, responsible for the above pathology. Here we show that a modified truncated 24-aa peptide (TFP5), derived from the Cdk5 activator p35, penetrates the blood-brain barrier after intraperitoneal injections, inhibits abnormal Cdk5 hyperactivity, and significantly rescues AD pathology (up to 70-80%) in 5XFAD AD model mice. The mutant mice, injected with TFP5 exhibit behavioral rescue, whereas no rescue was observed in mutant mice injected with either saline or scrambled peptide. However, TFP5 does not inhibit cell cycle Cdks or normal Cdk5/p35 activity, and thereby has no toxic side effects (even at 200 mg/kg), a common problem in most current therapeutics for AD. In addition, treated mice displayed decreased inflammation, amyloid plaques, NFTs, cell death, and an extended life by 2 mo. These results suggest TFP5 as a potential therapeutic, toxicity-free candidate for AD.

Development of hyperactivity and anxiety responses in dopamine transporter-deficient mice

Dopamine (DA) is a catecholamine neurotransmitter that regulates many aspects of motivated behavior in animals. Extracellular DA is highly regulated by the presynaptic high-affinity dopamine transporter (DAT), and drug- or genetically induced deficiencies in DAT function result in loss of DA reuptake. Mice in which DAT expression has been ablated have been previously proposed to be a relevant model of attention deficit hyperactivity disorder and have led to mechanistic insights regarding psychostimulant drug actions. However, very little previous work has emphasized the biobehavioral development of DAT-deficient mice. We therefore examined motoric, emotional and cognitive phenotypes in preadolescent (P22-26) DAT mutant mice. Consistent with previous reports in adult DAT(-/-) mice, we observed a hyperlocomotive phenotype in preadolescent mice across multiple assays. Somewhat surprisingly, spatial working memory in a Y-maze appeared intact, suggesting that cognitive phenotypes may emerge relatively late in development following hyperdopaminergia. Anxiety levels appeared to be reduced in DAT(-/-) mice, as defined by elevated plus maze and light-dark preference assays. No significant differences were observed between wild-type and heterozygous mice, suggesting a minimal impact of DAT haploinsufficiency on neurobehavioral status. Taken together, these data for the first time establish behavioral phenotypes of DAT mutant mice during development and suggest complex developmental stage-dependent effects of DA signaling on cognitive and emotional behaviors.

The cannabinoid agonist WIN55212-2 decreases L-DOPA-induced PKA activation and dyskinetic behavior in 6-OHDA-treated rats

Chronic Levodopa (L-DOPA), the gold standard therapy for Parkinson's disease (PD), causes disabling motor complications (dyskinesias) that are associated with changes in the activity of striatal protein kinase A (PKA) and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). In this study, we showed that systemic administration of the cannabinoid agonist WIN55212-2 ameliorated L-DOPA-induced abnormal involuntary movements (AIMs) in the 6-OHDA rat model of PD and reversed L-DOPA-induced PKA hyperactivity via a CB(1)-mediated mechanism. This effect was accompanied by increased phosphorylation of DARPP-32 at threonine 34, which was partially blocked by CB(1) antagonism. Striatal PKA activity was positively correlated with the severity of L-DOPA-induced axial and limb dyskinesias, suggesting a role for the cAMP/PKA signaling pathway in the expression of these motor disturbances. Our results indicate that activation of CB(1) receptors, as well as reduction of striatal PKA hyperactivity, might be an effective strategy for the treatment of L-DOPA-induced dyskinesias.

Loss of dopaminoreceptive neuron causes L-dopa resistant parkinsonism in tauopathy

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a family of inherited dementias caused by tauopathy. A mutation in exon 10 of the tau gene, N279K, causes a particular kindred of FTDP-17, which is predominant for parkinsonism. The disease initially presents as L-dopa resistant parkinsonism which then rapidly progresses. The final pathological features reveal disappearing dopamine (DA) neurons, but the causes remain poorly understood. We previously established a transgenic mouse with human N279K mutant tau as a model for FTDP-17, which showed cognitive dysfunctions caused by the mutant. Here we analyze L-dopa resistant parkinsonism by several behavioral tests, and focus on the distributions and accumulations of the mutant tau in the DA system by immunohistochemistry and Western blot. Interestingly, dopaminoreceptive (DAr) neurons in the striatum showed neurofibrils degeneration and apoptosis through caspase-3 activation by mutant tau accumulation. The DAr neuron loss in the caudoputamen, the target of the nigrostriatal system occurred before DA neuron loss in young symptomatic mice. Residual DA neurons in the mouse functioned in DA transportation, whereas dysregulation of intracellular DA compartmentalization implied an excess level of DA caused by DAr neuron loss. In the final stages, both DAr and DA neurons decreased equally, unlike Parkinson's disease. Therefore, DAr neurons were fundamentally vulnerable to the mutation indicating a critical role for the L-dopa resistant parkinsonism in tauopathy.

Subchronic polychlorinated biphenyl (Aroclor 1254) exposure produces oxidative damage and neuronal death of ventral midbrain dopaminergic systems

Recent epidemiologic studies have demonstrated a link between organochlorine and pesticide exposure to an enhanced risk for neurodegenerative disorders such as Parkinson's disease (PD). A common biological phenomenon underlying cell injury associated with both polychlorinated biphenyl (PCB) exposure and dopaminergic neurodegeneration during aging is oxidative stress (OS). In this study, we tested the hypothesis that oral PCB exposure, via food ingestion, impairs dopamine systems in the adult murine brain. We determined whether PCB exposure was associated with OS in dopaminergic neurons, a population of cells that selectively degenerate in PD. After 4 weeks of oral exposure to the PCB mixture Aroclor 1254, several congeners, mostly ortho substituted, accumulated throughout the brain. Significant increases in locomotor activity were observed within 2 weeks, which persisted after cessation of PCB exposure. Stereologic analyses revealed a significant loss of dopaminergic neurons within the substantia nigra and ventral tegmental area. However, striatal dopamine levels were elevated, suggesting that compensatory mechanisms exist to maintain dopamine homeostasis, which could contribute to the observed increases in locomotor activity following PCB exposure. Biochemical experiments revealed alterations in OS markers, including increases in SOD and HO-1 levels and the presence of oxidatively modified lipids and proteins. These findings were accompanied by elevated iron levels within the striatal and midbrain regions, perhaps due to the observed dysregulation of transferrin receptors and ferritin levels following PCB exposure. In this study, we suggest that both OS and the uncoupling of iron regulation contribute to dopamine neuron degeneration and hyperactivity following PCB exposure.

GSK-3 Mouse Models to Study Neuronal Apoptosis and Neurodegeneration

Increased GSK-3 activity is believed to contribute to the etiology of chronic disorders like Alzheimer’s disease (AD), schizophrenia, diabetes, and some types of cancer, thus supporting therapeutic potential of GSK-3 inhibitors. Numerous mouse models with modified GSK-3 have been generated in order to study the physiology of GSK-3, its implication in diverse pathologies and the potential effect of GSK-3 inhibitors. In this review we have focused on the relevance of these mouse models for the study of the role of GSK-3 in apoptosis. GSK-3 is involved in two apoptotic pathways, intrinsic and extrinsic pathways, and plays opposite roles depending on the apoptotic signaling process that is activated. It promotes cell death when acting through intrinsic pathway and plays an anti-apoptotic role if the extrinsic pathway is occurring. It is important to dissect this duality since, among the diseases in which GSK-3 is involved, excessive cell death is crucial in some illnesses like neurodegenerative diseases, while a deficient apoptosis is occurring in others such as cancer or autoimmune diseases. The clinical application of a classical GSK-3 inhibitor, lithium, is limited by its toxic consequences, including motor side effects. Recently, the mechanism leading to activation of apoptosis following chronic lithium administration has been described. Understanding this mechanism could help to minimize side effects and to improve application of GSK-3 inhibitors to the treatment of AD and to extend the application to other diseases.

Beyond cAMP: The Regulation of Akt and GSK3 by Dopamine Receptors

Brain dopamine receptors have been preferred targets for numerous pharmacological compounds developed for the treatment of various neuropsychiatric disorders. Recent discovery that D2 dopamine receptors, in addition to cAMP pathways, can engage also in Akt/GSK3 signaling cascade provided a new framework to understand intracellular signaling mechanisms involved in dopamine-related behaviors and pathologies. Here we review a recent progress in understanding the role of Akt, GSK3, and related signaling molecules in dopamine receptor signaling and functions. Particularly, we focus on the molecular mechanisms involved, interacting partners, role of these signaling events in the action of antipsychotics, psychostimulants, and antidepressants as well as involvement in pathophysiology of schizophrenia, bipolar disorder, and Parkinson’s disease. Further understanding of the role of Akt/GSK3 signaling in dopamine receptor functions could provide novel targets for pharmacological interventions in dopamine-related disorders.

Huntington's Disease and Striatal Signaling

Huntington’s Disease (HD) is the most frequent neurodegenerative disease caused by an expansion of polyglutamines (CAG). The main clinical manifestations of HD are chorea, cognitive impairment, and psychiatric disorders. The transmission of HD is autosomal dominant with a complete penetrance. HD has a single genetic cause, a well-defined neuropathology, and informative pre-manifest genetic testing of the disease is available. Striatal atrophy begins as early as 15 years before disease onset and continues throughout the period of manifest illness. Therefore, patients could theoretically benefit from therapy at early stages of the disease. One important characteristic of HD is the striatal vulnerability to neurodegeneration, despite similar expression of the protein in other brain areas. Aggregation of the mutated Huntingtin (HTT), impaired axonal transport, excitotoxicity, transcriptional dysregulation as well as mitochondrial dysfunction, and energy deficits, are all part of the cellular events that underlie neuronal dysfunction and striatal death. Among these non-exclusive mechanisms, an alteration of striatal signaling is thought to orchestrate the downstream events involved in the cascade of striatal dysfunction.

L-DOPA: a scapegoat for accelerated neurodegeneration in Parkinson's disease?

There is consensus that amelioration of the motor symptoms of Parkinson's disease is most effective with L-DOPA (levodopa). However, this necessary therapeutic step is biased by an enduring belief that L-DOPA is toxic to the remaining substantia nigra dopaminergic neurons by itself, or by specific metabolites such as dopamine. The concept of L-DOPA toxicity originated from pre-clinical studies conducted mainly in cell culture, demonstrating that L-DOPA or its derivatives damage dopaminergic neurons due to oxidative stress and other mechanisms. However, the in vitro data remain controversial as some studies showed neuroprotective, rather than toxic action of the drug. The relevance of this debate needs to be considered in the context of the studies conducted on animals and in clinical trials that do not provide convincing evidence for L-DOPA toxicity in vivo. This review presents the current views on the pathophysiology of Parkinson's disease, focusing on mitochondrial dysfunction and oxidative/proteolytic stress, the factors that can be affected by L-DOPA or its metabolites. We then critically discuss the evidence supporting the two opposing views on the effects of L-DOPA in vitro, as well as the animal and human data. We also address the problem of inadequate experimental models used in these studies. L-DOPA remains the symptomatic 'hero' of Parkinson's disease. Whether it contributes to degeneration of nigral dopaminergic neurons, or is a 'scapegoat' for explaining undesirable or unexpected effects of the treatment, remains a hotly debated topic.

Cyclin-dependent kinase 5 with phosphorylation of tyrosine 15 residue is enriched in striatal matrix compartment in adult mice

Accumulating evidence suggests that the striosome-matrix systems have a tight link with motor and behavioral brain functions and their disorders. Cyclin-dependent kinase 5 (Cdk5) is a versatile protein kinase that plays a role in synaptic functions and cell survival in adult brain, and its kinase activity is stimulated by phosphorylation at tyrosine 15 residue (pY15). In this study, we used an immunohistochemical method to show differential localization of Cdk5-pY15 in the striatal compartments of adult mice, with a heightened density of Cdk5-pY15 labeling in the matrix relative to the striosomes. Our findings indicate that Cdk5-pY15 can be a new marker for the striatal matrix compartment, and suggest a possible involvement of Cdk5-mediated signaling in compartment-specific neurotransmission and disease pathology in the striatum.

The physiology, signaling, and pharmacology of dopamine receptors

G protein-coupled dopamine receptors (D1, D2, D3, D4, and D5) mediate all of the physiological functions of the catecholaminergic neurotransmitter dopamine, ranging from voluntary movement and reward to hormonal regulation and hypertension. Pharmacological agents targeting dopaminergic neurotransmission have been clinically used in the management of several neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, bipolar disorder, Huntington's disease, attention deficit hyperactivity disorder (ADHD(1)), and Tourette's syndrome. Numerous advances have occurred in understanding the general structural, biochemical, and functional properties of dopamine receptors that have led to the development of multiple pharmacologically active compounds that directly target dopamine receptors, such as antiparkinson drugs and antipsychotics. Recent progress in understanding the complex biology of dopamine receptor-related signal transduction mechanisms has revealed that, in addition to their primary action on cAMP-mediated signaling, dopamine receptors can act through diverse signaling mechanisms that involve alternative G protein coupling or through G protein-independent mechanisms via interactions with ion channels or proteins that are characteristically implicated in receptor desensitization, such as β-arrestins. One of the future directions in managing dopamine-related pathologic conditions may involve a transition from the approaches that directly affect receptor function to a precise targeting of postreceptor intracellular signaling modalities either directly or through ligand-biased signaling pharmacology. In this comprehensive review, we discuss dopamine receptor classification, their basic structural and genetic organization, their distribution and functions in the brain and the periphery, and their regulation and signal transduction mechanisms. In addition, we discuss the abnormalities of dopamine receptor expression, function, and signaling that are documented in human disorders and the current pharmacology and emerging trends in the development of novel therapeutic agents that act at dopamine receptors and/or on related signaling events.

GSK-3β activity and hyperdopamine-dependent behaviors

Dopamine plays important roles in normal brain function and many neuropsychiatric disorders. Classically, dopamine receptors are positively coupled to G protein-mediated signaling to regulate cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) and Ca(2+) pathways. However, emerging evidence indicates that under hyperdopaminergic conditions, the protein kinase B (Akt)-glycogen synthase kinase 3β (GSK-3β) signaling cascade may mediate dopamine actions via D(2)-like receptors. This cAMP-independent signaling pathway involves the regulation of downstream synaptic targets, e.g., AMPA receptor, NMDA receptors, and thus synaptic plasticity. Here we provide an overview of how this novel signaling pathway relays dopamine receptor-mediated responses, particularly hyperdopamine-dependent behaviors. We discuss the relevance of the Akt/GSK-3β signaling cascade for the expression of dopamine-dependent behaviors and the drug actions associated with dopaminergic systems.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

Corticostriatal circuit dysfunction in Huntington's disease: intersection of glutamate, dopamine and calcium

Huntington's disease (HD) is a noncurable and progressive autosomal-dominant neurodegenerative disorder that results from a polyglutamine expansion in the amino-terminal region of the huntingtin protein. The generation of rodent HD models has revealed that cellular dysfunction, rather than cell death alone, occurs early in the disease progression, appearing even before overt symptom onset. Much evidence has now established that dysfunction of the corticostriatal circuit is key to HD symptomology. In this article, we summarize the most current findings that implicate glutamate, dopamine and calcium signaling in this system and discuss how they work in concert to disrupt corticostriatal function. In addition, we highlight therapeutic strategies related to altered corticostriatal signaling in HD.

Tetrabenazine is neuroprotective in Huntington's disease mice

Background

Huntington's disease (HD) is a neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in Huntingtin protein (Htt). PolyQ expansion in Httexp causes selective degeneration of striatal medium spiny neurons (MSN) in HD patients. A number of previous studies suggested that dopamine signaling plays an important role in HD pathogenesis. A specific inhibitor of vesicular monoamine transporter (VMAT2) tetrabenazine (TBZ) has been recently approved by Food and Drug Administration for treatment of HD patients in the USA. TBZ acts by reducing dopaminergic input to the striatum.

Results

In previous studies we demonstrated that long-term feeding with TBZ (combined with L-Dopa) alleviated the motor deficits and reduced the striatal neuronal loss in the yeast artificial chromosome transgenic mouse model of HD (YAC128 mice). To further investigate a potential beneficial effects of TBZ for HD treatment, we here repeated TBZ evaluation in YAC128 mice starting TBZ treatment at 2 months of age ("early" TBZ group) and at 6 months of age ("late" TBZ group). In agreement with our previous studies, we found that both "early" and "late" TBZ treatments alleviated motor deficits and reduced striatal cell loss in YAC128 mice. In addition, we have been able to recapitulate and quantify depression-like symptoms in TBZ-treated mice, reminiscent of common side effects observed in HD patients taking TBZ.

Conclusions

Our results further support therapeutic value of TBZ for treatment of HD but also highlight the need to develop more specific dopamine antagonists which are less prone to side-effects.

Age-related changes in nigrostriatal dopaminergic function in heterozygous mutant dopamine transporter knock-out mice

In this report we compared three different parameters of nigrostriatal dopaminergic (NSDA) function - locomotor activity, striatal dopamine (DA) levels and 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratios between heterozygous mutant dopamine transporter mice (+/- DAT) and their wild type controls (+/+ DAT) at three different age range periods: 4-10, 11-17 and 18-24 months of age. Locomotor activity of the +/- DAT mice failed to differ over the three age periods sampled. In +/+ DAT mice a significant decrease in locomotor activity was obtained at the 18-24-month old period compared with scores at the two earlier age periods. In addition, locomotor scores of +/+ DAT mice at 18-24 months of age were significantly decreased as compared with scores of the +/- DAT mice at this age. Striatal DA concentrations of +/- DAT mice also failed to differ over the three age periods sampled, while that of +/+ DAT mice showed significant decreases in striatal DA at 11-17 and 18-24 months of age as compared to their 4-10-month old cohorts. Striatal DOPAC/DA ratios were significantly increased in both +/+ and +/- DAT mice at the 11-17 and 18-24 month age periods as compared with their respective 4-10-month old groups. Striatal DOPAC/DA ratios of +/- DAT mice were significantly greater than that of the +/+ DAT mice at 18-24 months of age. These findings reveal the significance of interactions between a mutation of the dopamine transporter and aging upon NSDA function and the importance of isolating such variables when using knock-out models.

Occupational exposure to PCBs reduces striatal dopamine transporter densities only in women: a beta-CIT imaging study

We hypothesize that occupational exposure to PCBs is associated with a reduction in central dopamine (DA) similar to changes previously seen in PCB exposed adult non-human primates. To test that hypothesis, we used [(123)I]beta-CIT SPECT imaging to estimate basal ganglia DA transporter density in former capacitor workers. Women, but not men, showed an inverse relationship between lipid-adjusted total serum PCB concentrations and DA transporter densities in the absence of differences in serum PCB concentrations. These sex differences may reflect age-related reductions in the levels of gonadal hormones since these hormones have been shown experimentally to alter response to DA neurotoxicants. These findings may aid in better understanding the roles that sex and age play in modifying central DA function following exposure, not only to PCBs, but also to other DA neurotoxicants as well as further elucidating the role of gonadal hormones in influencing the initiation and/or progression of neurodegenerative disorders.

Striatal inhibition of PKA prevents levodopa-induced behavioural and molecular changes in the hemiparkinsonian rat

l-3,4-dihydroxyphenylalanine methyl ester hydrochloride (l-DOPA) is the gold standard for symptomatic treatment of Parkinson's disease (PD), but long-term therapy is associated with the emergence of abnormal involuntary movements (AIMS) known as l-DOPA-induced dyskinesias (LID). The molecular changes underlying LID are not completely understood. Using the 6-hydroxydopamine-lesioned rat model of PD, we showed that l-DOPA elicits profound alterations in the activity of three LID molecular markers, namely DeltaFosB, dopamine, cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) and extracellular signal-regulated kinases 1 and 2 (ERK1/2), as well as in phosphorylation levels of the cytoskeletal-associated protein tau. These modifications are triggered by protein kinase A (PKA) activation and intermittent stimulation of dopamine receptors as they are totally prevented by intrastriatal injections of Rp-cAMPS, a PKA inhibitor, or by continuous administration of l-DOPA via subcutaneous mini-pump. Importantly, Rp-cAMPS does not modulate the positive effect of l-DOPA on locomotor deficits and significantly attenuates the emergence of AIMS in 6-hydroxydopamine hydrobromide-lesioned rats. Even if decreased PKA signalling in the striatum may represent a clinical challenge, these data provide novel evidence that PKA activation, through modification of striatal signalling and alterations of cytoskeletal constituents, plays a key role in the manifestation of LID.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.

Dopamine D2 receptor stimulation potentiates PolyQ-Huntingtin-induced mouse striatal neuron dysfunctions via Rho/ROCK-II activation

BACKGROUND

Huntington's disease (HD) is a polyglutamine-expanded related neurodegenerative disease. Despite the ubiquitous expression of expanded, polyQ-Huntingtin (ExpHtt) in the brain, striatal neurons present a higher susceptibility to the mutation. A commonly admitted hypothesis is that Dopaminergic inputs participate to this vulnerability. We previously showed that D2 receptor stimulation increased aggregate formation and neuronal death induced by ExpHtt in primary striatal neurons in culture, and chronic D2 antagonist treatment protects striatal dysfunctions induced by ExpHtt in a lentiviral-induced model system in vivo. The present work was designed to elucidate the signalling pathways involved, downstream D2 receptor (D2R) stimulation, in striatal vulnerability to ExpHtt.

METHODOLOGY/PRINCIPAL FINDINGS

Using primary striatal neurons in culture, transfected with a tagged-GFP version of human exon 1 ExpHtt, and siRNAs against D2R or D1R, we confirm that DA potentiates neuronal dysfunctions via D2R but not D1R stimulation. We demonstrate that D2 agonist treatment induces neuritic retraction and growth cone collapse in Htt- and ExpHtt expressing neurons. We then tested a possible involvement of the Rho/ROCK signalling pathway, which plays a key role in the dynamic of the cytoskeleton, in these processes. The pharmacological inhibitors of ROCK (Y27632 and Hydroxyfasudil), as well as siRNAs against ROCK-II, reversed D2-related effects on neuritic retraction and growth cone collapse. We show a coupling between D2 receptor stimulation and Rho activation, as well as hyperphosphorylation of Cofilin, a downstream effector of ROCK-II pathway. Importantly, D2 agonist-mediated potentiation of aggregate formation and neuronal death induced by ExpHtt, was totally reversed by Y27632 and Hydroxyfasudil and ROCK-II siRNAs.

CONCLUSIONS/SIGNIFICANCE

Our data provide the first demonstration that D2R-induced vulnerability in HD is critically linked to the activation of the Rho/ROCK signalling pathway. The inclusion of Rho/ROCK inhibitors could be an interesting therapeutic option aimed at forestalling the onset of the disease.

ISTA13-catecholamine toxicity and metabolism in the ciliated protozoan, Tetrahymena pyriformis

A high throughput culture methodology of unicellular eukaryote Tetrahymena pyriformis, strain GL were used for the determination of catecholamines toxicity and their metabolism. Catecholamines exhibited acute toxicity to Tetrahymena cells where dopamine and L-DOPA showed higher toxic potential of EC(10) (0.39 and 0.63 mg/L, respectively) and EC(20) (1.1 and 1.0 mg/L, respectively). All the testing catecholamines were highly degradable in the PPY-medium due to the oxidizing environment during incubation. They were also naturally synthesized and released by Tetrahymena cells into the culture medium and increasingly accumulated with time where as noradrenalin demonstrated significant results. Cells were exposed with physiological concentration (0.12 mg/L) and one higher concentration (8.0 mg/L) of catecholamines, resulting noradrenalin depletion and in vivo generation of a metabolite in response to dopamine with higher concentration treatment. This dopamine metabolite was relatively nonpolar compared with the catecholamines and was eluted later from the reverse phase C-18 column.

Methamphetamine treatment causes delayed decrease in novelty-induced locomotor activity in mice

Methamphetamine (METH) is a psychostimulant that causes damage to dopamine (DA) axons and to non-monoaminergic neurons in the brain. The aim of the present study was to investigate short- and long-term effects of neurotoxic METH treatment on novelty-induced locomotor activity in mice. Male BALB/c mice, 12-14 weeks old, were injected with saline or METH (i.p., 7.5 mg/kg x 4 times, every 2 h). Behavior and neurotoxic effects were assessed at 10 days, 3 and 5 months following drug treatment. METH administration caused marked decreases in DA levels in the mouse striatum and cortex at 10 days post-drug. However, METH did not induce any changes in novelty-induced locomotor activity. At 3 and 5 months after treatment METH-exposed mice showed significant recovery of DA levels in the striatum and cortex. In contrast, these animals demonstrated significant decreases in locomotor activity at 5 months in comparison to aged-matched control mice. Further assessment of METH toxicity using TUNEL staining showed that the drug induced increased cell death in the striatum and cortex at 3 days after administration. Taken together, these data suggest that delayed deficits in novelty-induced locomotor activity observed in METH-exposed animals are not due to neurodegeneration of DA terminals but to combined effects of METH and age-dependent dysfunction of non-DA intrinsic striatal and/or corticostriatal neurons.

Dopamine induces supernumerary centrosomes and subsequent cell death through Cdk2 up-regulation in dopaminergic neuronal cells

Aggregation of proteins in the centrosome is implicated in the pathophysiology of Parkinson's disease. However, the relevance of the centrosome in neurodegeneration is still obscure. Centrosome duplication is initiated by the cyclin E/cyclin-dependent kinase 2 (Cdk2) complex. The present study determined changes in cyclin E or Cdk2 expression and in the centrosomal structure in dopaminergic neuronal CATH.a cells exposed to 50, 100 and 150 micromolar dopamine (DA) for 24 h. DA induced significant increase in Cdk2 protein and cyclin E protein, but not cyclin e mRNA. In DA-treated cells, the intense cyclin E- and Cdk2-immunofluorescence signals were co-localized around large and supernumerary centrosomes, and these two parameters of centrosome amplification were significantly increased compared with the control. Simultaneous co-treatment with DA and a Cdk2 inhibitor blocked centrosome amplification and enhanced cell viability. Our results demonstrated that DA could lead to cyclin E accumulation and Cdk2 up-regulation triggering supernumerary centrosomes and apoptotic cell death.

PSD-95 uncouples dopamine-glutamate interaction in the D1/PSD-95/NMDA receptor complex

Classical dopaminergic signaling paradigms and emerging studies on direct physical interactions between the D(1) dopamine (DA) receptor and the NMDA glutamate receptor predict a reciprocally facilitating, positive feedback loop. This loop, if not controlled, may cause concomitant overactivation of both D(1) and NMDA receptors, triggering neurotoxicity. Endogenous protective mechanisms must exist. Here, we report that PSD-95, a prototypical structural and signaling scaffold in the postsynaptic density, inhibits D(1)-NMDA receptor subunit 1 (NR1) NMDA receptor association and uncouples NMDA receptor-dependent enhancement of D(1) signaling. This uncoupling is achieved, at least in part, via a disinhibition mechanism by which PSD-95 abolishes NMDA receptor-dependent inhibition of D(1) internalization. Knockdown of PSD-95 immobilizes D(1) receptors on the cell surface and escalates NMDA receptor-dependent D(1) cAMP signaling in neurons. Thus, in addition to its role in receptor stabilization and synaptic plasticity, PSD-95 acts as a brake on the D(1)-NMDA receptor complex and dampens the interaction between them.

Dopamine promotes striatal neuronal apoptotic death via ERK signaling cascades

Although the mechanisms underlying striatal neurodegeneration are poorly understood, we have shown that striatal pathogenesis may be initiated by high synaptic levels of extracellular dopamine (DA). Here we investigated in rat striatal primary neurons the mobilization of the mitogen-activated protein kinase (MAPK) signaling pathways after treatment with DA. Instead of observing an elevation of the archetypical pro-cytotoxic MAPKs, p-JNK and p-p38 MAPK, we found that DA, acting through D1 DA receptors, induced a sustained stimulation of the phosphorylated form of extracellular signal-regulated kinase (p-ERK) via a cAMP/protein kinase A (PKA)/Rap1/B-Raf / MAPK/ERK kinase (MEK) pathway. Blockade of D2 DA receptors, beta-adrenergic receptors or N-methyl-D-aspartate receptors with receptor-specific antagonists had no significant effect on this process. Activation of D1 DA receptors and PKA by DA caused phosphorylation and inactivation of the striatal-enriched tyrosine phosphatase, an important phosphatase for the dephosphorylation and subsequent inactivation of p-ERK in the striatum. Interestingly, p-ERK was primarily retained in the cytoplasm, with only low amounts translocated to the nucleus. The scaffold protein beta-arrestin2 interacted with both p-ERK and D1 DA receptor, triggering the cytosolic retention of p-ERK and inducing striatal neuronal apoptotic death. These data provide unique insight into a novel role of p-ERK in striatal neurodegeneration.

Growth inhibition and biodegradation of catecholamines in the ciliated protozoan Tetrahymena pyriformis

A 96-well plate culture methodology for the unicellular eukaryote Tetrahymena pyriformis, strain GL was used for the determination of toxicity and metabolism of catecholamines. Catecholamines exhibited moderate acute toxicity to Tetrahymena cells where dopamine and L-DOPA showed higher toxic potential at EC(10) (0.39 ppm and 0.63 ppm, respectively) and EC(20) (1.1 ppm and 1.0 ppm respectively) after 48 h exposure. All tested catecholamines were highly degradable in the PPY-medium due to the oxidizing environment during incubation. Also the catecholamines were naturally synthesized and released by Tetrahymena cells into the culture medium and increasingly accumulated with time where noradrenalin exhibited the highest degree of accumulation. However, the exogenous exposure of catecholamines to the cells caused the depletion of natural noradrenalin synthesis even with the addition of very low physiological concentration (0.12 ppm). Dopamine caused the higher effect on inhibiting noradrenalin synthesis. Treatment with a higher concentration (8.0 ppm) of dopamine in 96-well plates caused strong excitation of the cells and ascertained a new metabolite in vivo while the other representative catecholamines were not responsible for the production of this metabolite. This dopamine metabolite is relatively non-polar as compared to noradrenalin, adrenaline and dopamine and eluting later through the reverse phase C-18 column.

Deletion of GAD67 in dopamine receptor-1 expressing cells causes specific motor deficits

The medium spiny neurons (MSNs), which comprise the direct and indirect output pathways from the striatum, use gamma-aminobutyric acid (GABA) as their major fact-acting neurotransmitter. We generated mice carrying a conditional allele of the Gad1 gene, which encodes GAD67, one of the two enzymes responsible for GABA biosynthesis, and bred them to mice expressing Cre recombinase at the dopamine D1 receptor locus (Drd1a) to selectively reduce GABA synthesis in the direct output pathway from the striatum. We show that these mice are deficient in some types of motor skills, but normal for others, suggesting a differential role for GABA release from D1 receptor-containing neurons.

Dopamine determines the vulnerability of striatal neurons to the N-terminal fragment of mutant huntingtin through the regulation of mitochondrial complex II

In neurodegenerative disorders associated with primary or secondary mitochondrial defects such as Huntington's disease (HD), cells of the striatum are particularly vulnerable to cell death, although the mechanisms by which this cell death is induced are unclear. Dopamine, found in high concentrations in the striatum, may play a role in striatal cell death. We show that in primary striatal cultures, dopamine increases the toxicity of an N-terminal fragment of mutated huntingtin (Htt-171-82Q). Mitochondrial complex II protein (mCII) levels are reduced in HD striatum, indicating that this protein may be important for dopamine-mediated striatal cell death. We found that dopamine enhances the toxicity of the selective mCII inhibitor, 3-nitropropionic acid. We also demonstrated that dopamine doses that are insufficient to produce cell loss regulate mCII expression at the mRNA, protein and catalytic activity level. We also show that dopamine-induced down-regulation of mCII levels can be blocked by several dopamine D2 receptor antagonists. Sustained overexpression of mCII subunits using lentiviral vectors abrogated the effects of dopamine, both by high dopamine concentrations alone and neuronal death induced by low dopamine concentrations together with Htt-171-82Q. This novel pathway links dopamine signaling and regulation of mCII activity and could play a key role in oxidative energy metabolism and explain the vulnerability of the striatum in neurodegenerative diseases.

Unregulated cytosolic dopamine causes neurodegeneration associated with oxidative stress in mice

The role of dopamine as a vulnerability factor and a toxic agent in Parkinson's disease (PD) is still controversial, yet the presumed dopamine toxicity is partly responsible for the "DOPA-sparing" clinical practice that avoids using L-3,4-dihydroxyphenylalanine (L-DOPA), a dopamine precursor, in early PD. There is a lack of studies on animal models that directly isolate dopamine as one determining factor in causing neurodegeneration. To address this, we have generated a novel transgenic mouse model in which striatal neurons are engineered to take up extracellular dopamine without acquiring regulatory mechanisms found in dopamine neurons. These mice developed motor dysfunctions and progressive neurodegeneration in the striatum within weeks. The neurodegeneration was accompanied by oxidative stress, evidenced by substantial oxidative protein modifications and decrease in glutathione. Ultrastructural morphologies of degenerative cells suggest necrotic neurodegeneration. Moreover, L-DOPA accelerated neurodegeneration and worsened motor dysfunction. In contrast, reducing dopamine input to striatum by lesioning the medial forebrain bundle attenuated motor dysfunction. These data suggest that pathology in genetically modified striatal neurons depends on their dopamine supply. These neurons were also supersensitive to neurotoxin. A very low dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (5 mg/kg) caused profound neurodegeneration of striatal neurons, but not midbrain dopamine neurons. Our results provide the first in vivo evidence that chronic exposure to unregulated cytosolic dopamine alone is sufficient to cause neurodegeneration. The present study has significant clinical implications, because dopamine replacement therapy is the mainstay of PD treatment. In addition, our model provides an efficient in vivo approach to test therapeutic agents for PD.

Dopamine transporter mutant mice in experimental neuropharmacology

An opportunity to perform targeted genetic manipulations in mice has provided another dimension for modern pharmacological research. Genetically modified mice have become important tools to investigate functions of previously unexplored proteins, define mechanism of action of new and known pharmacological drugs, and validate novel targets for treatment of human disorders. One of the best examples of such use of genetic models in experimental pharmacology represents investigations involving mice deficient in the gene encoding the dopamine transporter (DAT). The dopamine transporter tightly regulates the extracellular dynamics of dopamine by recapturing released neurotransmitter into the presynaptic terminals, and genetic deletion of this protein results in profound alterations in both the presynaptic homeostasis and the extracellular dynamics of dopamine. By using this model of severe dopaminergic dysregulation, significant progress has been made in defining the major target of psychotropic drugs, understanding the mechanisms of their action, unraveling novel signaling events relevant for dopaminergic transmission, and mapping neuronal pathways involved in dopamine-related behaviors. Furthermore, DAT mutant mice provided an opportunity to model in vivo conditions of extreme dopaminergic dysfunction that could be relevant for human disorders such as ADHD, schizophrenia, and Parkinson's disease and, thus, could serve as test systems for developing novel treatments for these and related disorders.

Haloperidol protects striatal neurons from dysfunction induced by mutated huntingtin in vivo

Huntington's disease (HD) results from an abnormal polyglutamine extension in the N-terminal region of the huntingtin protein. This mutation causes preferential degeneration of striatal projection neurons. We previously demonstrated, in vitro, that dopaminergic D2 receptor stimulation acted synergistically with mutated huntingtin (expHtt) to increase aggregate formation and striatal death. In the present work, we extend these observations to an in vivo system based on lentiviral-mediated expression of expHtt in the rat striatum. The early and chronic treatment with the D2 antagonist haloperidol decanoate protects striatal neurons from expHtt-induced dysfunction, as analyzed by DARPP-32 and NeuN stainings. Haloperidol treatment also reduces aggregates formation, an effect that is maintained over time. These findings indicate that D2 receptors activation contributes to the deleterious effects of expHtt on striatal function and may represent an interesting early target to alter the subsequent course of neuropathology in HD.

Dopaminergic signaling and striatal neurodegeneration in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by polyglutamine (polyQ) expansion in Huntingtin protein (Htt). PolyQ expansion in Htt(exp) causes selective degeneration of striatal medium spiny neurons (MSNs) in HD patients. Striatal MSN neurons receive glutamatergic input from the cortex and dopaminergic input from the substantia nigra. In previous studies with the yeast artificial chromosome (YAC128) transgenic HD mouse model, we established a connection between glutamate receptor activation, disturbed calcium (Ca2+) signaling, and apoptosis of HD MSNs (Tang et al., 2005). Here, we used the same YAC128 mouse model to investigate the role of dopaminergic signaling in HD. We discovered that glutamate and dopamine signaling pathways act synergistically to induce elevated Ca2+ signals and to cause apoptosis of YAC128 MSNs in vitro. We demonstrated that potentiating effects of dopamine are mediated by D1-class dopamine receptors (DARs) and not by D2-class DARs. Consistent with in vitro findings, in whole-animal experiments we found that persistent elevation of striatal dopamine levels exacerbated the behavioral motor deficits and MSN neurodegeneration in YAC128 mice. We further discovered that the clinically relevant dopamine pathway inhibitor tetrabenazine alleviated the motor deficits and reduced striatal cell loss in YAC128 mice. Our results suggest that dopamine signaling pathway plays an important role in HD pathogenesis and that antagonists of dopamine pathway such as tetrabenazine or dopamine receptor blockers may have a therapeutic potential for treatment of HD beyond well established "symptomatic" benefit.

Altered corticostriatal neurotransmission and modulation in dopamine transporter knock-down mice

Dopamine (DA) modulates glutamate neurotransmission in the striatum. Abnormal DA modulation has been implicated in neurological and psychiatric disorders. The development of DA transporter knock-down (DAT-KD) mice has permitted modeling of these disorders and has shed new light on DA modulation. DAT-KD mice exhibit increased extracellular DA, hyperactivity, and alterations in habituation. We used whole cell patch-clamp recordings from visually identified striatal neurons in slices to examine the effects of DAT-KD on corticostriatal transmission. Electrophysiological recordings from medium-sized spiny neurons in the dorsal striatum revealed alterations in both amplitude and frequency, of spontaneous glutamate receptor-mediated synaptic currents in cells from DAT-KD mice. Furthermore, kinetic analyses revealed that these currents had shorter half-amplitude durations and faster decay times. In contrast, GABA-receptor-mediated synaptic currents were not altered. Striatal neurons from DAT-KD mice also responded differently to amphetamine, cocaine, and DA D2-receptor agonists or antagonists compared with wildtype (WT) littermate controls. In WTs amphetamine and cocaine reduced the frequency of spontaneous glutamate currents and these effects appeared to be mediated by activation of D2 receptors. In contrast, in DAT-KD mice either no changes or only small increases in frequency occurred. D2-receptor agonists or antagonists also had opposing effects in WT and DAT-KD mice. Together, these results indicate that chronically increased extracellular DA produces long-lasting changes in corticostriatal communication that may be mediated by changes in D2-receptor function. These findings have implications for understanding mechanisms underlying attention deficit hyperactivity disorder and Tourette's syndrome and may provide insights into novel therapeutic approaches.

Regulation of Akt signaling by D2 and D3 dopamine receptors in vivo

The serine/threonine kinase Akt is a downstream target of dopamine receptor signaling that is inhibited/dephosphorylated in response to direct and indirect dopamine receptor agonists. Although pharmacological studies uncovered the involvement of D2-class dopamine receptors in Akt regulation, they did not identify the role of individual receptor subtypes in this process. Here we used knock-out mice lacking the D1, D2, D2 long, or D3 dopamine receptors as well as a D4 receptor-selective antagonist to address the function of each of these receptors in the regulation of Akt in vivo. Under basal conditions, D2, D2 long, and D3 knock-out mice display enhanced striatal Akt activation, whereas D1 knock-out mice and mice treated with the D4 receptor antagonist L745870 (3-[[4-(4-chlorophenyl)piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride) have phospho-Akt levels comparable with those of normal control animals. Furthermore, both amphetamine and apomorphine lose their ability to inhibit Akt in D2 knock-out mice but retain their normal effect on this signaling molecule in D1 knock-out animals. Finally, D3 knock-out mice show a reduced sensitivity of Akt-mediated signaling to dopaminergic drugs but retain the action of these drugs on Akt at high dose regimens. These results indicate that D2 receptors are essential for the inhibition of Akt by dopamine and that D3 receptors also participate in this signaling potentially by enhancing D2 receptor response. Identification of the functions of individual dopamine receptor subtypes in Akt regulation may help the development of new pharmaceutical approaches for mental disorders related to abnormal dopamine transmission such as bipolar disorder and schizophrenia.

The importance of A9 dopaminergic neurons in mediating the functional benefits of fetal ventral mesencephalon transplants and levodopa-induced dyskinesias

Intrastriatal transplantation of fetal ventral mesencephalon (VM) tissue provides the potential to alleviate motor symptoms of Parkinson's disease (PD) and levodopa-induced dyskinesia (LID). However, the degree of recovery varies among individuals with an incidence of "off-phase", graft-induced dyskinesia (GID) in some patients. We hypothesised that this variability is due to the heterogeneous nature of dopaminergic neurons in the transplant. We therefore investigated this in the unilateral 6-hydroxydopamine-lesioned rat model of PD. These animals were primed to develop LID and then transplanted with fetal VM into the caudal aspects of the striatum. No GID was observed but in a significant number of animals the transplants ameliorated LID. There was a correlation between the degree of behavioural and LID recovery with the number of A9 dopaminergic neurons in the transplant, based on their expression of a G-protein-coupled inward rectifying current potassium channel (Girk2). Furthermore, we showed that LID development is related to an abnormal expression profile of cyclin-dependent kinase 5 (Cdk5) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) in the striatum and that intrastriatal VM transplants normalised both Cdk5 expression and DARPP-32 phosphorylation in animals exhibiting functional improvement. These results suggest that an A9 dopaminergic neuron-enriched transplant may be the key to an effective PD cell replacement therapy through normalisation of the altered striatal expression of Cdk5/DARPP-32.