Dopamine and glutamate in Huntington's disease: A balancing act

Striatal pre-enkephalin overexpression improves Huntington's disease symptoms in the R6/2 mouse model of Huntington's disease

The reduction of pre-enkephalin (pENK) mRNA expression might be an early sign of striatal neuronal dysfunction in Huntington’s disease (HD), due to mutated huntingtin protein. Indeed, striatopallidal (pENK-containing) neurodegeneration occurs at earlier stage of the disease, compare to the loss of striatonigral neurons. However, no data are available about the functional role of striatal pENK in HD. According to the neuroprotective properties of opioids that have been recognized recently, the objective of this study was to investigate whether striatal overexpression of pENK at early stage of HD can improve motor dysfunction, and/or reduce striatal neuronal loss in the R6/2 transgenic mouse model of HD. To achieve this goal recombinant adeno-associated-virus (rAAV2)-containing green fluorescence protein (GFP)-pENK was injected bilaterally in the striatum of R6/2 mice at 5 weeks old to overexpress opioid peptide pENK. Striatal injection of rAAV2-GFP was used as a control. Different behavioral tests were carried out before and/or after striatal injections of rAAV2. The animals were euthanized at 10 weeks old. Our results demonstrate that striatal overexpression of pENK had beneficial effects on behavioral symptoms of HD in R6/2 by: delaying the onset of decline in muscular force; reduction of clasping; improvement of fast motor activity, short-term memory and recognition; as well as normalization of anxiety-like behavior. The improvement of behavioral dysfunction in R6/2 mice having received rAAV2-GFP-pENK associated with upregulation of striatal pENK mRNA; the increased level of enkephalin peptide in the striatum, globus pallidus and substantia nigra; as well as the slight increase in the number of striatal neurons compared with other groups of R6/2. Accordingly, we suggest that at early stage of HD upregulation of striatal enkephalin might play a key role at attenuating illness symptoms.

Roles of cocaine- and amphetamine-regulated transcript in the central nervous system

1. Cocaine- and amphetamine-regulated transcript (CART), first isolated from the ovine hypothalamus, is a potential neurotransmitter widely distributed throughout the central and peripheral nervous systems, as well as in endocrine cells in the pituitary and adrenal glands, pancreatic islets and stomach. 2. Numerous studies have established the role of CART in food intake, maintenance of bodyweight, stress control, reward and pain transmission. Recently, it was demonstrated that CART, as a neurotrophic peptide, had a cerebroprotective against focal ischaemic stroke and inhibited the neurotoxicity of β-amyloid protein, which focused attention on the role of CART in the central nervous system (CNS) and neurological diseases. 3. In fact, little is known about the way in which CART peptide interacts with its receptors, initiates downstream cascades and finally exerts its neuroprotective effect under normal or pathological conditions. The literature indicates that there are many factors, such as regulation of the immunological system and protection against energy failure, that may be involved in the cerebroprotection afforded by CART. 4. The present review provides a brief summary of the current literature on CART synthesis and active fragments, its distribution in the CNS and, in particular, the role of CART peptide (and its receptors and signalling) in neurological diseases.

Therapeutic Strategies for Huntington's Disease

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease, caused by expansion of the CAG repeat in the huntingtin (HTT) gene and characterized pathologically by the loss of pyramidal neurons in several cortical areas, of striatal medium spiny neurons, and of hypothalamic neurons. Clinically, a distinguishing feature of the disease is uncontrolled involuntary movements (chorea, dyskensias) accompanied by progressive cognitive, motor, and psychiatric impairment. This review focuses on the current state of therapeutic development for the treatment of HD, including the preclinical and clinical development of small molecules and molecular therapies.

Biological sources of inflexibility in brain and behavior with aging and neurodegenerative diseases

Almost unequivocally, aging and neurodegeneration lead to deficits in neural information processing. These declines are marked by increased neural noise that is associated with increased variability or inconsistency in behavioral patterns. While it is often viewed that these problems arise from dysregulation of dopamine (DA), a monoamine modulator, glutamate (GLU), an excitatory amino acid that interacts with DA, also plays a role in determining the level of neural noise. We review literature demonstrating that neural noise is highest at both high and low levels of DA and GLU, allowing their interaction to form a many-to-one solution map for neural noise modulation. With aging and neurodegeneration, the range over which DA and GLU can be modulated is decreased leading to inflexibility in brain activity and behavior. As the capacity to modulate neural noise is restricted, the ability to shift noise from one brain region to another is reduced, leading to greater uniformity in signal-to-noise ratios across the entire brain. A negative consequence at the level of behavior is inflexibility that reduces the ability to: (1) switch from one behavior to another; and (2) stabilize a behavioral pattern against external perturbations. In this paper, we develop a theoretical framework where inflexibility across brain and behavior, rather than inconsistency and variability is the more important problem in aging and neurodegeneration. This theoretical framework of inflexibility in aging and neurodegeneration leads to the hypotheses that: (1) dysfunction in either or both of the DA and GLU systems restricts the ability to modulate neural noise; and (2) levels of neural noise and variability in brain activation will be dedifferentiated and more evenly distributed across the brain; and (3) changes in neural noise and behavioral variability in response to different task demands and changes in the environment will be reduced.

Circadian and sleep disorder in Huntington's disease

Huntington's disease is a progressive neurological disorder that starts insidiously with motor, cognitive or psychiatric disturbance, and progresses through a distressing range of symptoms to end with a devastating loss of function, both motor and executive. There is a growing awareness that, in addition to cognitive and psychiatric symptoms, there are other important non-motor symptoms in HD, including sleep and circadian abnormalities. It is not clear if sleep-wake changes are caused directly by HD gene-related pathology, or if they are simply a consequence of having a neurodegenerative disease. From a patient point of view, the answer is irrelevant, since sleep and circadian disturbances are deleterious to good daily living, even in neurologically normal people. The assumption should be that, at the very least, sleep and/or circadian disturbance in HD patients will contribute to their symptoms. At worst, they may contribute to the progressive decline in HD. Here I review the state of our understanding of sleep and circadian abnormalities in HD. I also outline a set of simple rules that can be followed to improve the chances of a good night's sleep, since preventing any 'preventable' symptoms is the a logical first step in treating disease. The long-term impact of sleep disruption in HD is unknown. There have been no large-scale systematic studies of in sleep in HD. Furthermore, there has never been a study of the efficacy of pharmaceuticals that are typically used to treat sleep deficits in HD patients. Thus treatment of sleep disturbance in HD is necessarily empirical. A better understanding of the relationship between sleep/circadian abnormalities and HD pathology is needed, if treatment of this aspect of HD is to be optimized.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Brief mitochondrial inhibition causes lasting changes in motor behavior and corticostriatal synaptic physiology in the Fischer 344 rat

The striatum is particularly vulnerable to mitochondrial dysfunction and this problem is linked to pathology created by environmental neurotoxins, stimulants like amphetamine, and metabolic disease and ischemia. We studied the course of recovery following a single systemic injection of the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP) and found 3-NP caused lasting changes in motor behavior that were associated with altered activity-dependent plasticity at corticostriatal synapses in Fischer 344 rats. The changes in synapse behavior varied with the time after exposure to the 3-NP injection. The earliest time point studied, 24h after 3-NP, revealed 3-NP-induced an exaggeration of D1 Dopamine (DA) receptor dependent long-term potentiation (LTP) that reversed to normal by 48 h post-3-NP exposure. Thereafter, the likelihood and degree of inducing D2 DA receptor dependent long-term depression (LTD) gradually increased, relative to saline controls, peaking at 1 month after the 3-NP exposure. NMDA receptor binding did not change over the same post 3-NP time points. These data indicate even brief exposure to 3-NP can have lasting behavioral effects mediated by changes in the way DA and glutamate synapses interact.

Evidence-based guideline: pharmacologic treatment of chorea in Huntington disease: report of the guideline development subcommittee of the American Academy of Neurology

OBJECTIVE

To develop an evidence-based guideline assessing pharmacologic options for treating Huntington disease (HD) chorea.

METHODS

We evaluated available evidence from a structured literature review performed through February 2011.

RESULTS AND RECOMMENDATIONS

If HD chorea requires treatment, clinicians should prescribe tetrabenazine (up to 100 mg/day), amantadine (300-400 mg/day), or riluzole (200 mg/day) (Level B) for varying degrees of expected benefit. Occurrence of adverse events should be discussed and monitored, particularly depression/suicidality and parkinsonism with tetrabenazine and elevated liver enzymes with riluzole. Clinicians may also prescribe nabilone for modest decreases (1- to <2-point changes on the Unified Huntington's Disease Rating Scale [UHDRS] chorea score) in HD chorea (Level C), but information is insufficient to recommend long-term use, particularly given abuse potential concerns (Level U). Clinicians should not prescribe riluzole 100 mg/day for moderate (2- to < 3-point UHDRS chorea change) short-term benefits (Level B) or for any long-term (3-year) HD antichoreic goals (Level B). Clinicians may choose not to prescribe ethyl-EPA (Level B), minocycline (Level B), or creatine (Level C) for very important improvements (>3-point UHDRS chorea change) in HD chorea. Clinicians may choose not to prescribe coenzyme Q10 (Level B) for moderate improvements in HD chorea. Data are insufficient to make recommendations regarding the use of neuroleptics or donepezil for HD chorea treatment (Level U).

Corticostriatal dysfunction and glutamate transporter 1 (GLT1) in Huntington's disease: interactions between neurons and astrocytes

Huntington's Disease (HD) is a fatally inherited neurodegenerative disorder caused by an expanded glutamine repeat in the N-terminal region of the huntingtin (HTT) protein. The result is a progressively worsening triad of cognitive, emotional, and motor alterations that typically begin in adulthood and end in death 10-20 years later. Autopsy of HD patients indicates massive cell loss in the striatum and its main source of input, the cerebral cortex. Further studies of HD patients and transgenic animal models of HD indicate that corticostriatal neuronal processing is altered long before neuronal death takes place. In fact, altered neuronal function appears to be the primary driver of the HD behavioral phenotype, and dysregulation of glutamate, the excitatory amino acid released by corticostriatal afferents, is believed to play a critical role. Although mutant HTT interferes with the operation of multiple proteins related to glutamate transmission, consistent evidence links the expression of mutant HTT with reduced activity of glutamate transporter 1 (rodent GLT1 or human EAAT2), the astrocytic protein responsible for the bulk of glutamate uptake. Here, we review corticostriatal dysfunction in HD and focus on GLT1 and its expression in astrocytes as a possible therapeutic target.

Self-induced accumulation of glutamate in striatal astrocytes and basal ganglia excitotoxicity

Excitotoxicity induced by high levels of extracellular glutamate (GLU) has been proposed as a cause of cell degeneration in basal ganglia disorders. This phenomenon is normally prevented by the astrocytic GLU-uptake and the GLU-catabolization to less dangerous molecules. However, high-GLU can induce reactive gliosis which could change the neuroprotective role of astrocytes. The striatal astrocyte response to high GLU was studied here in an in vivo rat preparation. The transient striatal perfusion of GLU (1 h) by reverse microdialysis induced complex reactive gliosis which persisted for weeks and which was different for radial-like glia, protoplasmic astrocytes and fibrous astrocytes. This gliosis was accompanied by a persistent cytosolic accumulation of GLU (immunofluorescence quantified by confocal microscope), which persisted for weeks (self-induced glutamate accumulation), and which was associated to a selective decrease of glutamine synthetase activity. This massive and persistent self-induced glutamate accumulation in striatal astrocytes could be an additional factor for the GLU-induced excitotoxicity, which has been implicated in the progression of different basal ganglia disorders.

Pathophysiology of Huntington's disease: time-dependent alterations in synaptic and receptor function

Huntington's disease (HD) is a progressive, fatal neurological condition caused by an expansion of CAG (glutamine) repeats in the coding region of the Huntington gene. To date, there is no cure but great strides have been made to understand pathophysiological mechanisms. In particular, genetic animal models of HD have been instrumental in elucidating the progression of behavioral and physiological alterations, which had not been possible using classic neurotoxin models. Our groups have pioneered the use of transgenic HD mice to examine the excitotoxicity hypothesis of striatal neuronal dysfunction and degeneration, as well as alterations in excitation and inhibition in striatum and cerebral cortex. In this review, we focus on synaptic and receptor alterations of striatal medium-sized spiny (MSNs) and cortical pyramidal neurons in genetic HD mouse models. We demonstrate a complex series of alterations that are region-specific and time-dependent. In particular, many changes are bidirectional depending on the degree of disease progression, that is, early vs. late, and also on the region examined. Early synaptic dysfunction is manifested by dysregulated glutamate release in striatum followed by progressive disconnection between cortex and striatum. The differential effects of altered glutamate release on MSNs originating the direct and indirect pathways is also elucidated, with the unexpected finding that cells of the direct striatal pathway are involved early in the course of the disease. In addition, we review evidence for early N-methyl-D-aspartate receptor (NMDAR) dysfunction leading to enhanced sensitivity of extrasynaptic receptors and a critical role of GluN2B subunits. Some of the alterations in late HD could be compensatory mechanisms designed to cope with early synaptic and receptor dysfunctions. The main findings indicate that HD treatments need to be designed according to the stage of disease progression and should consider regional differences.

Striatal-enriched protein tyrosine phosphatase expression and activity in Huntington's disease: a STEP in the resistance to excitotoxicity

Striatal-enriched protein tyrosine phosphatase (STEP) is highly expressed in striatal projection neurons, the neuronal population most affected in Huntington's disease. Here, we examined STEP expression and phosphorylation, which regulates its activity, in N-terminal exon-1 and full-length mutant huntingtin mouse models. R6/1 mice displayed reduced STEP protein levels in the striatum and cortex, whereas its phosphorylation was increased in the striatum, cortex, and hippocampus. The early increase in striatal STEP phosphorylation levels correlated with a deregulation of the protein kinase A pathway, and decreased calcineurin activity at later stages further contributes to an enhancement of STEP phosphorylation and inactivation. Accordingly, we detected an accumulation of phosphorylated ERK2 and p38, two targets of STEP, in R6/1 mice striatum at advanced stages of the disease. Activation of STEP participates in excitotoxic-induced cell death. Because Huntington's disease mouse models develop resistance to excitotoxicity, we analyzed whether decreased STEP activity was involved in this process. After intrastriatal quinolinic acid (QUIN) injection, we detected higher phosphorylated STEP levels in R6/1 than in wild-type mice, suggesting that STEP inactivation could mediate neuroprotection in R6/1 striatum. In agreement, intrastriatal injection of TAT-STEP increased QUIN-induced cell death. R6/2, Tet/HD94, and Hdh(Q7/Q111) mice striatum also displayed decreased STEP protein and increased phosphorylation levels. In Tet/HD94 mice striatum, mutant huntingtin transgene shutdown reestablished STEP expression. In conclusion, the STEP pathway is severely downregulated in the presence of mutant huntingtin and may participate in compensatory mechanisms activated by striatal neurons that lead to resistance to excitotoxicity.

Dissecting the contribution of individual receptor subunits to the enhancement of N-methyl-d-aspartate currents by dopamine D1 receptor activation in striatum

Dopamine, via activation of D1 receptors, enhances N-methyl-d-aspartate (NMDA) receptor-mediated responses in striatal medium-sized spiny neurons. However, the role of specific NMDA receptor subunits in this enhancement remains unknown. Here we used genetic and pharmacological tools to dissect the contribution of NR1 and NR2A/B subunits to NMDA responses and their modulation by dopamine receptors. We demonstrate that D1 enhancement of NMDA responses does not occur or is significantly reduced in mice with genetic knock-down of NR1 subunits, indicating a critical role of these subunits. Interestingly, spontaneous and evoked α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid (AMPA) receptor-mediated responses were significantly enhanced in NR1 knock-down animals, probably as a compensatory mechanism for the marked reduction in NMDA receptor function. The NMDA receptor subunits NR2A and NR2B played differential roles in D1 modulation. Whereas genetic deletion or pharmacological blockade of NR2A subunits enhanced D1 potentiation of NMDA responses, blockade of NR2B subunits reduced this potentiation, suggesting that these regulatory subunits of the NMDA receptor counterbalance their respective functions. In addition, using D1 and D2 receptor EGFP-expressing mice, we demonstrate that NR2A subunits contribute more to NMDA responses in D1-MSSNs, whereas NR2B subunits contribute more to NMDA responses in D2 cells. The differential contribution of discrete receptor subunits to NMDA responses and dopamine modulation in the striatum has important implications for synaptic plasticity and selective neuronal vulnerability in disease states.

Neural tissue engineering using embryonic and induced pluripotent stem cells

With the recent start of the first clinical trial evaluating a human embryonic stem cell-derived therapy for the treatment of acute spinal cord injury, it is important to review the current literature examining the use of embryonic stem cells for neural tissue engineering applications with a focus on diseases and disorders that affect the central nervous system. Embryonic stem cells exhibit pluripotency and thus can differentiate into any cell type found in the body, including those found in the nervous system. A range of studies have investigated how to direct the differentiation of embryonic cells into specific neural phenotypes using a variety of cues to achieve the goal of replacing diseased or damaged neural tissue. Additionally, the recent development of induced pluripotent stem cells provides an intriguing alternative to the use of human embryonic stem cell lines for these applications. This review will discuss relevant studies that have used embryonic stem cells to replicate the tissue found in the central nervous system as well as evaluate the potential of induced pluripotent stem cells for the aforementioned applications.

Mechanisms mediating brain and cognitive reserve: experience-dependent neuroprotection and functional compensation in animal models of neurodegenerative diseases

'Brain and cognitive reserve' (BCR) refers here to the accumulated neuroprotective reserve and capacity for functional compensation induced by the chronic enhancement of mental and physical activity. BCR is thought to protect against, and compensate for, a range of different neurodegenerative diseases, as well as other neurological and psychiatric disorders. In this review we will discuss BCR, and its potential mechanisms, in neurodegenerative disorders, with a focus on Huntington's disease (HD) and Alzheimer's disease (AD). Epidemiological studies of AD, and other forms of dementia, provided early evidence for BCR. The first evidence for the beneficial effects of enhanced mental and physical activity, and associated mechanistic insights, in an animal model of neurodegenerative disease was provided by experiments using HD transgenic mice. More recently, experiments on animal models of HD, AD and various other brain disorders have suggested potential molecular and cellular mechanisms underpinning BCR. We propose that sophisticated insight into the processes underlying BCR, and identification of key molecules mediating these beneficial effects, will pave the way for therapeutic advances targeting these currently incurable neurodegenerative diseases.

The importance of integrating basic and clinical research toward the development of new therapies for Huntington disease

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder that results from expansion of the polyglutamine repeat in the huntingtin (HTT) gene. There are currently no effective treatments for this devastating disease. Given its monogenic nature, disease modification therapies for HD should be theoretically feasible. Currently, pharmacological therapies aimed at disease modification by altering levels of HTT protein are in late-stage preclinical development. Here, we review current efforts to develop new treatments for HD based on our current understanding of HTT function and the main pathological mechanisms. We emphasize the need to enhance translational efforts and highlight the importance of aligning the clinical and basic research communities to validate existing hypotheses in clinical studies. Human and animal therapeutic trials are presented with an emphasis on cellular and molecular mechanisms relevant to disease progression.

Role of inositol 1,4,5-trisphosphate receptors in pathogenesis of Huntington's disease and spinocerebellar ataxias

Huntington's disease (HD) and spinocerebellar ataxias (SCAs) are autosomal-dominant neurodegenerative disorders. HD is caused by polyglutamine (polyQ) expansion in the amino-terminal region of a protein huntingtin (Htt) and primarily affects medium spiny striatal neurons (MSN). Many SCAs are caused by polyQ-expansion in ataxin proteins and primarily affect cerebellar Purkinje cells. The reasons for neuronal dysfunction and death in HD and SCAs remain poorly understood and no cure is available for the patients. Our laboratory discovered that mutant huntingtin, ataxin-2 and ataxin-3 proteins specifically bind to the carboxy-terminal region of the type 1 inositol 1,4,5-trisphosphate receptor (IP(3)R1), an intracellular Ca(2+) release channel. Moreover, we found that association of mutant huntingtin or ataxins with IP(3)R1 causes sensitization of IP(3)R1 to activation by IP(3) in planar lipid bilayers and in neuronal cells. These results suggested that deranged neuronal Ca(2+) signaling might play an important role in pathogenesis of HD, SCA2 and SCA3. In support of this idea, we demonstrated a connection between abnormal Ca(2+) signaling and neuronal cell death in experiments with HD, SCA2 and SCA3 transgenic mouse models. Additional data in the literature indicate that abnormal neuronal Ca(2+) signaling may also play an important role in pathogenesis of SCAl, SCA5, SCA6, SCA14 and SCA15/16. Based on these results I propose that IP(3)R and other Ca(2+) signaling proteins should be considered as potential therapeutic targets for treatment of HD and SCAs.

Functional increase of brain histaminergic signaling in Huntington's disease

To evaluate whether central histaminergic signaling in Huntington's disease (HD) patients is affected, we assessed mRNA levels of histidine decarboxylase (HDC), volume of and neuron number in the hypothalamic tuberomamillary nucleus (TMN) (HD n = 8, controls n = 8). In addition, we assessed histamine N-methyltransferase (HMT) and histamine receptor (H(1) R, H(2) R and H(3) R) mRNA levels in the inferior frontal gyrus (IFG) (n = 9 and 9) and caudate nucleus (CN) (n = 6 and 6) by real-time polymerase chain reaction. In HD patients, TMN volume and neuronal number was unaltered (P = 0.72, P = 0.25). The levels of HDC mRNA (P = 0.046), IFG HMT (P < 0.001), H(1) R (P < 0.001) and H(3) R mRNA levels (P = 0.011) were increased, while CN H(2) R and H(3) R mRNA levels were decreased (P = 0.041, P = 0.009). In HD patients, we observed a positive correlation between IFG H(3) R mRNA levels and CAG repeat length (P = 0.024) and negative correlations between age at onset of disease and IFG HMT (P = 0.015) and H(1) R (P = 0.021) mRNA levels. These findings indicate a functional increase in brain histaminergic signaling in HD, and provide a rationale for the use of histamine receptor antagonists.

The BMP signaling pathway at the Drosophila neuromuscular junction and its links to neurodegenerative diseases

The Drosophila neuromuscular junction (NMJ) has recently provided new insights into the roles of various proteins in neurodegenerative diseases including Amyotrophic Lateral Sclerosis (ALS), Spinal Muscular Atrophy (SMA), Multiple Sclerosis (MS) Hereditary Spastic Paraplegia (HSP), and Huntington's Disease (HD). Several developmental signaling pathways including WNT, MAPK and BMP/TGF-β signaling play important roles in the formation and growth of the Drosophila NMJ. Studies of the fly homologues of genes that cause neurodegenerative disease at the NMJ have resulted in a better understanding of the roles of these proteins in vivo. These studies may shed light on the pathological mechanisms of these diseases, with implications for reduced BMP/TGF-β signaling in ALS, SMA and HD and increased signaling in HSP and MS.

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.