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TOR1A-associated dystonia, otherwise known as DYT1 dystonia, is an inherited dystonia caused by a three base-pair deletion in the TOR1A gene (TOR1AΔE). Although the mechanisms underlying the dystonic movements are largely unknown, abnormalities in striatal dopamine and acetylcholine neurotransmission are consistently implicated whereby dopamine release is reduced while cholinergic tone is increased. Because striatal cholinergic neurotransmission mediates dopamine release, it is not known if the dopamine release deficit is mediated indirectly by abnormal acetylcholine neurotransmission or if Tor1a(ΔE) acts directly within dopaminergic neurons to attenuate release. To dissect the microcircuit that governs the deficit in dopamine release, we conditionally expressed Tor1a(ΔE) in either dopamine neurons or cholinergic interneurons in mice and assessed striatal dopamine release using ex vivo fast scan cyclic voltammetry or dopamine efflux using in vivo microdialysis. Conditional expression of Tor1a(ΔE) in cholinergic neurons did not affect striatal dopamine release. In contrast, conditional expression of Tor1a(ΔE) in dopamine neurons reduced dopamine release to 50% of normal, which is comparable to the deficit in Tor1a +/ΔE knockin mice that express the mutation ubiquitously. Despite the deficit in dopamine release, we found that the Tor1a(ΔE) mutation does not cause obvious nerve terminal dysfunction as other presynaptic mechanisms, including electrical excitability, vesicle recycling/refilling, Ca 2+ signaling, D2 dopamine autoreceptor function and GABA B receptor function, are intact. Although the mechanistic link between Tor1a(ΔE) and dopamine release is unclear, these results clearly demonstrate that the defect in dopamine release is caused by the action of the Tor1a(ΔE) mutation within dopamine neurons., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Trihexyphenidyl, a nonselective muscarinic receptor antagonist, is the small molecule drug of choice for the treatment of DYT1 dystonia, but it is poorly tolerated due to significant side effects. A better understanding of the mechanism of action of trihexyphenidyl is needed for the development of improved treatments. Because DTY1 dystonia is associated with both abnormal cholinergic neurotransmission and abnormal dopamine regulation, we tested the hypothesis that trihexyphenidyl normalizes striatal dopamine release in a mouse model of DYT1 dystonia using ex vivo fast scan cyclic voltammetry and in vivo microdialysis. Trihexyphenidyl increased striatal dopamine release and efflux as assessed by ex vivo voltammetry and in vivo microdialysis respectively. In contrast, ʟ-DOPA, which is not usually effective for the treatment of DYT1 dystonia, did not increase dopamine release in either Dyt1 or control mice. Trihexyphenidyl was less effective at enhancing dopamine release in Dyt1 mice relative to controls ex vivo (mean increase WT: 65% vs Dyt1: 35%). Trihexyphenidyl required nicotinic receptors but not glutamate receptors to increase dopamine release. Dyt1 mice were more sensitive to the dopamine release decreasing effects of nicotinic acetylcholine receptor antagonism (IC 50 : WT = 29.46 nM, Dyt1 = 12.26 nM) and less sensitive to acetylcholinesterase inhibitors suggesting that nicotinic acetylcholine receptor neurotransmission is altered in Dyt1 mice, that nicotinic receptors indirectly mediate the differential effects of trihexyphenidyl in Dyt1 mice, and that nicotinic receptors may be suitable therapeutic targets for DYT1 dystonia., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Objective: Lesch-Nyhan disease (LND) is caused by congenital deficiency of the purine recycling enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). Affected patients have a peculiar neurobehavioral syndrome linked with reductions of dopamine in the basal ganglia. The purpose of the current studies was to determine the anatomical basis for the reduced dopamine in human brain specimens collected at autopsy., Methods: Histopathological studies were conducted using autopsy tissue from 5 LND cases and 6 controls. Specific findings were replicated in brain tissue from an HGprt-deficient knockout mouse using immunoblots, and in a cell model of HGprt deficiency by flow-activated cell sorting (FACS)., Results: Extensive histological studies of the LND brains revealed no signs suggestive of a degenerative process or other consistent abnormalities in any brain region. However, neurons of the substantia nigra from the LND cases showed reduced melanization and reduced immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis. In the HGprt-deficient mouse model, immunohistochemical stains for TH revealed no obvious loss of midbrain dopamine neurons, but quantitative immunoblots revealed reduced TH expression in the striatum. Finally, 10 independent HGprt-deficient mouse MN9D neuroblastoma lines showed no signs of impaired viability, but FACS revealed significantly reduced TH immunoreactivity compared to the control parent line., Interpretation: These results reveal an unusual phenomenon in which the neurochemical phenotype of dopaminergic neurons is not linked with a degenerative process. They suggest an important relationship between purine recycling pathways and the neurochemical integrity of the dopaminergic phenotype., (© 2014 American Neurological Association.)

The dystonias are a group of disorders characterized by involuntary twisting movements and abnormal posturing. The most common of the inherited dystonias is DYT1 dystonia, which is due to deletion of a single GAG codon (ΔE) in the TOR1A gene that encodes torsinA. Since some forms of dystonia have been linked with dysfunction of brain dopamine pathways, the integrity of these pathways was explored in a knock-in mouse model of DYT1 dystonia. In DYT1(ΔE) knock-in mice, neurochemical measures revealed only small changes in the content of dopamine or its metabolites in tissue homogenates from caudoputamen or midbrain, but microdialysis studies revealed robust decreases in baseline and amphetamine-stimulated extracellular dopamine in the caudoputamen. Quantitative stereological methods revealed no evidence for striatal or midbrain atrophy, but substantia nigra neurons immunopositive for tyrosine hydroxylase were slightly reduced in numbers and enlarged in size. Behavioral studies revealed subtle abnormalities in gross motor activity and motor coordination without overt dystonia. Neuropharmacological challenges of dopamine systems revealed normal behavioral responses to amphetamine and a minor increase in sensitivity to haloperidol. These results demonstrate that this DYT1(ΔE) knock-in mouse model of dystonia harbors neurochemical and structural changes of the dopamine pathways, as well as motor abnormalities., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Restless Legs Syndrome (RLS), first chronicled by Willis in 1672 and described in more detail by Ekbom in 1945, is a prevalent sensorimotor neurological disorder (5%-10% in the population) with a circadian predilection for the evening and night. Characteristic clinical features also include a compelling urge to move during periods of rest, relief with movement, involuntary movements in sleep (viz., periodic leg movements of sleep), and fragmented sleep. Although the pathophysiology of RLS is unknown, dopaminergic neurotransmission and deficits in iron availability modulate expressivity. Genome-wide association studies have identified a polymorphism in an intronic region of the BTBD9 gene on chromosome 6 that confers substantial risk for RLS. Here, we report that loss of the Drosophila homolog CG1826 (dBTBD9) appreciably disrupts sleep with concomitant increases in waking and motor activity. We further show that BTBD9 regulates brain dopamine levels in flies and controls iron homeostasis through the iron regulatory protein-2 in human cell lines. To our knowledge, this represents the first reverse genetic analysis of a "novel" or heretofore poorly understood gene implicated in an exceedingly common and complex sleep disorder and the development of an RLS animal model that closely recapitulates all disease phenotypes., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency results in Lesch-Nyhan disease (LND), where affected individuals exhibit a characteristic neurobehavioral disorder that has been linked with dysfunction of dopaminergic pathways of the basal ganglia. Since the functions of HPRT, a housekeeping enzyme responsible for recycling purines, have no direct relationships with the dopaminergic pathways, the mechanisms whereby HPRT deficiency affect them remain unknown. The current studies demonstrate that HPRT deficiency influences early developmental processes controlling the dopaminergic phenotype, using several different cell models for HPRT deficiency. Microarray methods and quantitative PCR were applied to 10 different HPRT-deficient (HPRT(-)) sublines derived from the MN9D cell line. Despite the variation inherent in such mutant sublines, several consistent abnormalities were evident. Most notable were increases in the mRNAs for engrailed 1 and 2, transcription factors known to play a key role in the specification and survival of dopamine neurons. The increases in mRNAs were accompanied by increases in engrailed proteins, and restoration of HPRT reverted engrailed expression towards normal levels, demonstrating a functional relationship between HPRT and engrailed. The functional relevance of the abnormal developmental molecular signature of the HPRT(-) MN9D cells was evident in impoverished neurite outgrowth when the cells were forced to differentiate chemically. To verify that these abnormalities were not idiosyncratic to the MN9D line, HPRT(-) sublines from the SK-N-BE(2) M17 human neuroblastoma line were evaluated and an increased expression of engrailed mRNAs was also seen. Over-expression of engrailed occurred even in primary fibroblasts from patients with LND in a manner that suggested a correlation with disease severity. These results provide novel evidence that HPRT deficiency may affect dopaminergic neurons by influencing early developmental mechanisms.

A unique sensitivity to specific biochemical processes is responsible for selective vulnerability of midbrain dopamine neurons in several diseases. Prior studies have shown these neurons are susceptible to energy failure and mitochondrial dysfunction, oxidative stress, and impaired disposal of misfolded proteins. These neurons also are especially vulnerable to the loss of purine recycling. In the brains of humans or mice with inherited defects of the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT), the most prominent defect is loss of basal ganglia dopamine. To investigate the nature of the relationship between HPRT deficiency and dopamine, the mouse MN9D dopaminergic neuronal cell line was used to prepare 10 sublines lacking HPRT. The mutant sublines grew more slowly than the parent line, but without morphological signs of impaired viability. As a group, the mutant sublines had significantly lower dopamine than the parent line. The loss of dopamine in the mutants did not reflect impaired energy status, as judged by ATP levels or vulnerability to inhibitors of energy production. Indeed, the mutant lines as a group appeared energetically more robust than the parent line. The loss of dopamine also was not accompanied by enhanced susceptibility to oxidative stress or proteasome inhibitors. Instead, the loss of dopamine reflected only one aspect of a broad change in the molecular phenotype of the cells affecting mRNAs encoding tyrosine hydroxylase, the dopamine transporter, the vesicular monoamine transporter, monoamine oxidase B, catechol-O-methyltransferase, and GTP-cyclohydrolase. These changes were selective for the dopamine phenotype, since multiple control mRNAs were normal. These studies suggest purine recycling is an intrinsic metabolic process of particular importance to the molecular phenotype of dopaminergic neurons independent of previously established mechanisms involving energy failure, oxidative stress, or proteasome dysfunction.

Several rare inherited disorders have provided valuable experiments of nature highlighting specific biological processes of particular importance to the survival or function of midbrain dopamine neurons. In both humans and mice, deficiency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) is associated with profound loss of striatal dopamine, with relative preservation of other neurotransmitters. In the current studies of knockout mice, no morphological signs of abnormal development or degeneration were found in an exhaustive battery that included stereological and morphometric measures of midbrain dopamine neurons, electron microscopic studies of striatal axons and terminals, and stains for degeneration or gliosis. A novel culture model involving HPRT-deficient dopaminergic neurons also exhibited significant loss of dopamine without a morphological correlate. These results suggest that dopamine loss in HPRT deficiency has a biochemical rather than anatomical basis and imply that purine recycling to be a biochemical process of particular importance to the function of dopaminergic neurons.

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) is an enzyme that catalyses the conversion of hypoxanthine and guanine into their respective nucleotides. Inherited deficiency of the enzyme is associated with a loss of striatal dopamine in both mouse and man. Although HPRT is not directly involved in the metabolism of dopamine, it contributes to the supply of GTP, which is used in the first and rate-limiting step in the synthesis of tetrahydrobiopterin (BH4). Since BH4 is required as a cofactor for tyrosine hydroxylase in the synthesis of dopamine, any limitation in the supply of GTP could interfere with the synthesis of dopamine. The current studies were designed to address the hypothesis that the reduced striatal dopamine in mice with HPRT deficiency results from reduced availability of BH4. The mutant mice had small reductions in striatal BH4, with normal BH4 levels in other brain regions. Liver BH4 was normal in HPRT-deficient mutant mice, and a phenylalanine challenge test failed to reveal any evidence for impaired hepatic phenylalanine hydroxylase, another BH4-dependent enzyme. Although striatal BH4 content is not normal, supplementation with BH4 or L-dopa failed to correct the striatal dopamine deficiency of the mutant mice, suggesting that BH4 limitation is not responsible for the dopamine loss.

The L-type calcium channel activator +/-Bay K 8644 induces repetitive self-biting and self-injurious behavior in young mice. Since dopaminergic systems have been implicated in prior studies of these behaviors in both humans and animals, the present experiments were designed to test whether drugs influencing the dopaminergic systems could modify the behavioral responses to +/-Bay K 8644. The ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior was increased by amphetamine and GBR 12909, drugs that augment synaptic dopaminergic concentrations by blocking the reuptake and/or stimulating the release of dopamine. Conversely, self-biting and self-injurious behavior were decreased by tetrabenazine or reserpine, two drugs that deplete vesicular stores of dopamine. These results suggest that dopaminergic systems may play a role in the ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior., (Copyright 2003 S. Karger AG, Basel)

Lesch-Nyhan disease, a neurogenetic disorder caused by congenital deficiency of the purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase, is associated with a prominent loss of striatal dopamine. The current studies address the hypothesis that oxidant stress causes damage or dysfunction of nigrostriatal dopamine neurons in a knockout mouse model of the disease, by assessing several markers of oxidative damage and free radical scavenging systems. Some of these measures provided evidence for an increase in oxidative stress in the mutant mice (aconitase activity, oxidized glutathione, and lipid peroxides), but others did not (superoxide dismutase, protein thiol content, carbonyl protein content, total glutathione, glutathione peroxidase, catalase, and thiobarbituric reducing substances). Immunolocalization of heme-oxygenase 1 provided no evidence for oxidative stress restricted to specific elements of the striatum or midbrain in the mutants. Striatal dopamine systems of the mutant mice were more vulnerable to a challenge with the neurotoxin 6-hydroxydopamine, but they were not protected by cross-breeding the mutants with transgenic mice over-expressing superoxide dismutase. Overall, these data provide evidence for increased oxidative stress, but the failure to protect the knockout mice by over-expressing SOD1 argues that oxidative stress is not the sole process responsible for the loss of striatal dopamine.

Lesch-Nyhan disease is a neurogenetic disorder caused by deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Affected individuals exhibit a characteristic pattern of neurological and behavioral features attributable in part to dysfunction of basal ganglia dopamine systems. In the current studies, striatal dopamine loss was investigated in five different HPRT-deficient strains of mice carrying one of two different HPRT gene mutations. Caudoputamen dopamine concentrations were significantly reduced in all five of the strains, with deficits ranging from 50.7 to 61.1%. Mesolimbic dopamine was significantly reduced in only three of the five strains, with a range of 31.6-38.6%. The reduction of caudoputamen dopamine was age dependent, emerging between 4 and 12 weeks of age. Tyrosine hydroxylase and aromatic amino acid decarboxylase, two enzymes responsible for the synthesis of dopamine, were reduced by 22.4-37.3 and 22.2-43.1%, respectively. These results demonstrate that HPRT deficiency is strongly associated with a loss of basal ganglia dopamine. The magnitude of dopamine loss measurable is dependent on the genetic background of the mouse strain used, the basal ganglia subregion examined, and the age of the animals at assessment.

We have examined several aspects of neurotransmitter function in the brains of mice carrying a deletion mutation in the gene encoding the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). During the first 6 weeks of postnatal development, dopamine levels in whole-brain extracts from the mutant mice (HPRT-) failed to increase at rates comparable to normal animals, resulting in 40% lower dopamine levels throughout adulthood. Regional analysis in adult animals showed the caudoputamen to be the most severely affected region, with dopamine deficits of 48-64%. Dopamine levels in other regions were normal or less severely affected. The decrease in dopamine was accompanied by a decrease in tyrosine hydroxylase (TH) activity, the rate-limiting step in dopamine synthesis. Kinetic analysis of TH extracted from the caudoputamen of normal and HPRT- mice demonstrated a 45% decrease in Vmax with an increased affinity for the tetrahydropterin cofactor in the mutants. Labeling of midbrain dopamine neurons using TH immunohistochemistry revealed no obvious deficits in the number of midbrain dopamine neurons, but quantitative autoradiographic studies revealed significant reductions in the binding of 3H-N-[1-(2-benzo(beta)thiophenyl)cyclohexyl]piperidine (3H-BTCP) to dopamine uptake sites in the forebrain of the mutants. In contrast to these abnormalities of the dopamine systems in the mutant mice, other neurotransmitter systems appeared relatively unaffected. Norepinephrine, 5-HT, tryptophan hydroxylase, and glutamic acid decarboxylase were present at normal levels in the brains of the mutants. ChAT activity was slightly lower than normal in the caudoputamen of the mutant animals, but was normal in all other brain regions examined. These results indicate that HPRT deficiency is associated with a relatively specific deficit in basal ganglia dopamine systems that emerges during the first 2 months of postnatal development.

The Lesch-Nyhan syndrome is a neurogenetic disorder caused by congenital deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). The disorder is characterized by prominent neurobehavioral abnormalities which appear to result in part from dysfunction of striatal dopamine systems. HPRT-deficient (HPRT-) mutant strains of mice have been produced as animal models for this syndrome, but these animals exhibit none of the neurobehavioral abnormalities seen in Lesch-Nyhan patients. The present studies describe the behavioral responses of three strains of mice carrying one of two mutations in the HPRT gene to agents which interact with brain dopamine systems. HPRT- mice are more sensitive than age- and sex-matched littermates to the motor-activating properties of dopamine-releasing agents (amphetamine, amfonelic acid and methylphenidate), but not dopamine uptake inhibitors (GBR 12909 and nomifensine). The enhanced sensitivity of the HPRT- mice to the dopamine-releasing agents is not caused by dopamine receptor supersensitivity, because the HPRT- mice do not show enhanced motor responses to the direct D1/D2 dopamine receptor agonist apomorphine or to the selective D1 dopamine receptor agonist SKF 38393. The function of regulatory dopamine autoreceptors, as assessed by suppression of spontaneous motor activity by low doses of R(-)-propylnorapomorphine, also appears normal in the HPRT- mice. Biochemical analysis shows that the HPRT- mice have significantly lower levels of dopamine (-45%), but normal levels of tyrosine, 3,4-dihydroxyphenylacetic acid, homovanillic acid and 3-methoxytyramine in the caudoputamen. In contrast to the deficit in caudoputamen dopamine, no deficits were noted in the accumbens of the HPRT- mice. These results indicate the existence of an inherent abnormality in the dopamine systems in the brains of HPRT- mice, despite their apparently normal spontaneous behavior.

Abstract: Background: Recent neuroimaging studies implicate nigrostriatal degeneration as a critical factor in producing late‐onset parkinsonism in patients with l‐dopa‐responsive dystonia‐causing mutations. However, postmortem anatomical studies do not reveal neurodegeneration in l‐dopa‐responsive dystonia patients. These contrasting findings make it unclear how parkinsonism develops in l‐dopa‐responsive dystonia mutation carriers. Methods: We prospectively assessed motor dysfunction, responses to dopaminergic challenge, and dopamine neuron degeneration with aging in a validated knockin mouse model bearing a l‐dopa‐responsive dystonia‐causing mutation found in humans. Results: As l‐dopa‐responsive dystonia mice aged, dystonic movements waned while locomotor activity decreased and initiation of movements slowed. Despite the age‐related reduction in movement, there was no evidence for degeneration of midbrain dopamine neurons. Presynaptically mediated dopaminergic responses did not change with age in l‐dopa‐responsive dystonia mice, but responses to D1 dopamine receptor agonists decreased with age. Conclusions: We have demonstrated for the first time the co‐occurrence of dystonia and Parkinson's‐like features (mainly consisting of hypokinesia) in a genetic mouse model. In this model we show that these features evolve without dopaminergic neurodegeneration, suggesting that postsynaptic plasticity, rather than presynaptic degeneration, may contribute to the development of parkinsonism in patients with l‐dopa‐responsive dystonia. © 2017 International Parkinson and Movement Disorder Society [ABSTRACT FROM AUTHOR]

Abstract: Lesch-Nyhan disease is a neurogenetic disorder caused bydeficiency of the purine salvage enzyme hypoxanthine-guaninephosphoribosyltransferase (HPRT). Affected individuals exhibit acharacteristic pattern of neurological and behavioral features attributable inpart to dysfunction of basal ganglia dopamine systems. In the current studies,striatal dopamine loss was investigated in five different HPRT-deficientstrains of mice carrying one of two different HPRT gene mutations.Caudoputamen dopamine concentrations were significantly reduced in all five ofthe strains, with deficits ranging from 50.7 to 61.1%. Mesolimbic dopamine wassignificantly reduced in only three of the five strains, with a range of31.6-38.6%. The reduction of caudoputamen dopamine was age dependent, emergingbetween 4 and 12 weeks of age. Tyrosine hydroxylase and aromatic amino aciddecarboxylase, two enzymes responsible for the synthesis of dopamine, werereduced by 22.4-37.3 and 22.2-43.1%, respectively. These results demonstratethat HPRT deficiency is strongly associated with a loss of basal gangliadopamine. The magnitude of dopamine loss measurable is dependent on thegenetic background of the mouse strain used, the basal ganglia sub-regionexamined, and the age of the animals at assessment. [ABSTRACT FROM AUTHOR]