Regional distribution of monoamine vesicular uptake sites in the mesencephalon of control subjects and patients with Parkinson's disease: a postmortem study using tritiated tetrabenazine

Neurodegenerative Diseases: New Hopes and Perspectives

Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, Huntington's disease, and Friedrich ataxia are all incurable neurodegenerative diseases defined by the continuous progressive loss of distinct neuronal subtypes. Despite their rising prevalence among the world's ageing population, fewer advances have been made in the concurrent massive efforts to develop newer drugs. Recently, there has been a shift in research focus towards the discovery of new therapeutic agents for neurodegenerative diseases. In this review, we have summarized the recently developed therapies and their status in the management of neurodegenerative diseases.

Genetic elimination of dopamine vesicular stocks in the nigrostriatal pathway replicates Parkinson's disease motor symptoms without neuronal degeneration in adult mice

The type 2 vesicular monoamine transporter (VMAT2), by regulating the storage of monoamines transmitters into synaptic vesicles, has a protective role against their cytoplasmic toxicity. Increasing evidence suggests that impairment of VMAT2 neuroprotection contributes to the pathogenesis of Parkinson’s disease (PD). Several transgenic VMAT2 mice models have been developed, however these models lack specificity regarding the monoaminergic system targeting. To circumvent this limitation, we created VMAT2-KO mice specific to the dopamine (DA) nigrostriatal pathway to analyze VMAT2’s involvement in DA depletion-induced motor features associated to PD and examine the relevance of DA toxicity in the pathogenesis of neurodegeneration. Adult VMAT2 floxed mice were injected in the substancia nigra (SN) with an adeno-associated virus (AAV) expressing the Cre-recombinase allowing VMAT2 removal in DA neurons of the nigrostriatal pathway solely. VMAT2 deletion in the SN induced both DA depletion exclusively in the dorsal striatum and motor dysfunction. At 16 weeks post-injection, motor symptoms were accompanied with a decreased in food and water consumption and weight loss. However, despite an accelerating death, degeneration of nigrostriatal neurons was not observed in this model during this time frame. This study highlights a non-cytotoxic role of DA in our genetic model of VMAT2 deletion exclusively in nigrostriatal neurons.

New symptomatic therapies for Huntington disease

Huntington disease (HD), an inherited neurodegenerative disease, results from a CAG repeat expansion creating mutant huntingtin protein and widespread neuronal damage. Motor symptoms such as chorea are often preceded by cognitive and behavioral changes. Tetrabenazine and deutetrabebenazine are the two drugs approved by the Federal Food and Drug Administrationfor HD symptoms, is an effective therapy for chorea. However, there is still a large need for other symptomatic therapies impacting functional issues, including impaired gait, behavioral, and cognitive symptoms. A number of pharmacologic agents are under investigation. Additionally, other mechanisms are being targeted in motor symptom drug development, including phosphodiesterase 10 enzyme inhibition, dopamine modulation, and inhibition of deacetylation. There is perhaps the greatest unmet need in treating nonmotor effects, such as cognition and change in disease course. PBT2, a metal chaperone, and latrepirdine, a mitochondrial stabilizer, are under investigation specifically for the possibility of cognitive benefit. Unfortunately, there is a lack of HD-specific evidence on effective treatments for behavioral and psychiatric symptoms. Further investigation of nonmedication interventions such as physical therapy is necessary. As our understanding of molecular and cellular mechanisms underlying HD broadens, a new set of mechanistic targets will become the focus of HD symptomatic therapies.

Molecular changes in the postmortem parkinsonian brain

Parkinson disease (PD) is the second most common neurodegenerative disease after Alzheimer disease. Although PD has a relatively narrow clinical phenotype, it has become clear that its etiological basis is broad. Post-mortem brain analysis, despite its limitations, has provided invaluable insights into relevant pathogenic pathways including mitochondrial dysfunction, oxidative stress and protein homeostasis dysregulation. Identification of the genetic causes of PD followed the discovery of these abnormalities, and reinforced the importance of the biochemical defects identified post-mortem. Recent genetic studies have highlighted the mitochondrial and lysosomal areas of cell function as particularly significant in mediating the neurodegeneration of PD. Thus the careful analysis of post-mortem PD brain biochemistry remains a crucial component of research, and one that offers considerable opportunity to pursue etiological factors either by 'reverse biochemistry' i.e. from defective pathway to mutant gene, or by the complex interplay between pathways e.g. mitochondrial turnover by lysosomes. In this review we have documented the spectrum of biochemical defects identified in PD post-mortem brain and explored their relevance to metabolic pathways involved in neurodegeneration. We have highlighted the complex interactions between these pathways and the gene mutations causing or increasing risk for PD. These pathways are becoming a focus for the development of disease modifying therapies for PD. Parkinson's is accompanied by multiple changes in the brain that are responsible for the progression of the disease. We describe here the molecular alterations occurring in postmortem brains and classify them as: Neurotransmitters and neurotrophic factors; Lewy bodies and Parkinson's-linked genes; Transition metals, calcium and calcium-binding proteins; Inflammation; Mitochondrial abnormalities and oxidative stress; Abnormal protein removal and degradation; Apoptosis and transduction pathways. This article is part of a special issue on Parkinson disease.

Medicinal plant Combretum leprosum mart ameliorates motor, biochemical and molecular alterations in a Parkinson's disease model induced by MPTP

ETHNOPHARMACOLOGICAL RELEVANCE

Combretum leprosum is a popular medicinal plant distributed in north and northeastern regions of Brazil. Many different parts of this plant are used in traditional medicine to treat several inflammatory diseases. Parkinson's disease (PD) is a disorder associated with inflammatory toxic factors and the treatments available provide merely a delay of the neurodegeneration.

AIM OF THE STUDY

We investigated the potential neuroprotective properties of the C. leprosum ethanolic extract (C.l.EE) in a murine model of PD using the toxin 1-methyl-4 phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP).

MATERIALS AND METHODS

The mice were split into four groups: V/S (vehicle/saline), E/S (extract/saline), V/M (vehicle/MPTP) and E/M (extract/ MPTP). Mice received MPTP (30mg/kg, i.p.) or vehicle (10ml/kg, i.p.) once a day for 5 consecutive days and vehicle (10ml/kg) or C.l.EE (100mg/kg) orally by intra-gastric gavage (i.g.) during a 14-d period, starting 3 days before the first MPTP injection. All groups were assessed for behavioural impairments (amphetamine-induced locomotor activity and muscle strength), dopamine content in striatum using high performance liquid chromatography (HPLC), tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene expressions using qPCR.

RESULTS

Animals were injected with d-amphetamine (2mg/kg) and the activity was recorded. Amphetamine-induced hyperlocomotion was observed in all groups; however animals treated with MPTP showed exacerbated hyperlocomotion (approximately 3 fold increase compared to control groups). By contrast, mice treated with MPTP that received C.l.EE exhibited attenuation of the hyperlocomotion and did not differ from control groups. Muscle strength test pointed that C.l.EE strongly avoided muscular deficits caused by MPTP (approximately 2 fold increase compared to V/M group). Dopamine and its metabolites were measured in the striatum. The V/M group presented a dopamine reduction of 80%. On the other hand, the E/M group exhibited an increase in dopamine and its metabolites levels (approximately 3 fold increase compared to V/M group). Tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene expressions were significantly reduced in the V/M group (60%). Conversely, C.l.EE treatment was able to increase the mRNA levels of those genes in the E/M group (approximately 2 fold for TH and DAT).

CONCLUSIONS

These data show, for the first time, that C. leprosum ethanolic extract prevented motor and molecular changes induced by MPTP, and partially reverted dopamine deficit. Thus, our results demonstrate that C.l.EE has potential for the treatment and prevention of PD.

Treatment of Huntington's disease

Huntington's disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to family members at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate, mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic interventions have yielded positive results and current treatments have focused on the motor aspects of HD. Tetrabenazine is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer neuroleptic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse effect profile than older neuroleptic agents for treating chorea and psychosis. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

Tetrabenazine for the treatment of chorea and other hyperkinetic movement disorders

Tetrabenazine (TBZ; Xenazine) is a potent, selective, reversible depletor of monoamines from nerve terminals. TBZ inhibits the vesicular monoamine transporter type 2 which, in humans, is expressed nearly exclusively in the brain. TBZ is rapidly metabolized in the liver by carbonyl reductase to stereoisomers of hydrotetrabenazine, some of which are potent inhibitors of vesicular monoamine transporter type 2. Initially developed in the 1950s for schizophrenia, since the 1970s several publications have reported on the efficacy of TBZ in the treatment of various hyperkinetic movement disorders. Although quite effective in controlling the involuntary movements, there were considerable inter-individual differences in the optimal dose, defined as the dose judged by the investigator to provide the greatest efficacy with minimal or tolerable adverse events. This variability is in part owing to differences in severity and mechanism of the target symptoms and to variable activity of the enzyme carbonyl reductase that metabolizes TBZ to its active metabolites. Dose-limiting adverse events, consisting mainly of sedation, parkinsonism, akathisia and depression, are usually rapidly reversible upon dosage reduction. In addition to its established antichorea efficacy in Huntington's disease, the drug has been reported to also be effective in a variety of other hyperkinetic movement disorders, including tardive dyskinesia and tics associated with Tourette's syndrome.

An evidence-based approach in the treatment of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease with diverse symptoms for which there is no curative or disease-modifying treatment available. Currently, tetrabenazine is the only drug approved for HD by a regulatory agency, and only for the treatment of chorea. In the current review, we present updated results from recent clinical trials and ongoing clinical research efforts to find effective and safe treatments for HD motor, and neuropsychiatric and cognitive symptoms. We used a systematic review approach that included data from well-designed randomised controlled trials. The authors conclude that there is weak evidence to support most of the treatment decisions in HD and thus clinicians may be guided only by expert opinion-based therapeutic recommendations. Ongoing research is considerable and is expected to have an impact in the management of HD in upcoming years.

Tetrabenazine: for chorea associated with Huntington's disease

Oral tetrabenazine is currently the only drug approved by the US FDA for the treatment of chorea associated with Huntington's disease (HD). Although the precise antichorea mechanism of action is unknown, it most likely involves reversible depletion of monoamines, particularly dopamine, from presynaptic terminals via inhibition of human vesicular monoamine transporter type 2. In a 12-week, double-blind, placebo-controlled trial conducted in the US in patients with HD, oral tetrabenazine (≤100 mg/day; n = 54) was significantly (p = 0.0001) more efficacious than placebo (n = 30) at improving adjusted mean Unified HD Rating Scale (UHDRS) total maximum chorea scores (reduced from baseline by 5 vs 1.5) [primary endpoint]. After 12 weeks, improvements in UHDRS total maximum chorea scores of >3 were achieved by significantly (p < 0.0001) more patients in the tetrabenazine group than in the placebo group. The antichorea efficacy of tetrabenazine was maintained in an 80-week extension study (n = 75), with the adjusted mean UHDRS total maximum chorea score significantly (p < 0.001) reduced from baseline (score of 14.9) by 4.6 points (primary outcome). In the 12-week trial and 80-week extension study, treatment-emergent adverse events in the tetrabenazine group mainly occurred during the dosage-titration phase, a period during which the dosage was individually optimized. Most of these events were mild to moderate and were manageable with dosage adjustments or discontinuation of study drug.

Impact of ω-3 fatty acids in Parkinson's disease

Current epidemiological, preclinical and clinical data suggest that omega-3 polyunsaturated fatty acids (n-3 PUFAs) may constitute therapeutic strategy for several disorders of the central nervous system, including Parkinson's disease (PD). PD is a neurodegenerative disorder primarily characterized by motor symptoms but which also includes several other pathological features such as autonomic system failures, mood disorders, and cognitive deficits. Current pharmacological options for the disease are limited to symptom management and their long-term use leads to important side effects. In this review, we discuss the evidence for the effects of n-3 PUFAs in PD both from an epidemiological perspective as well as in light of data gathered on various pathological features of the disease. Effects of n-3 PUFAs on the dopaminergic system, α-synucleinopathy, their possible mechanisms of action as well as their therapeutic potential for PD patients are also reviewed. n-3 PUFAs are inexpensive, readily transferable to the clinical setting and their use could represent a neuroprotective strategy or a disease-modifying option to delay the appearance of symptoms. It could also be beneficial as a symptomatologic treatment or serve as an add-on therapy to current pharmacological approaches. Review of the current literature as well as the undertaking of future clinical trials will shed light on these possibilities.

Management of Huntington's disease: role of tetrabenazine

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder characterized by progressive involuntary movements, neuropsychiatric disturbances, and cognitive impairment. The use of tetrabenazine (TBZ), a specific inhibitor of vesicular monoamine transporter, is approved for chorea in HD patients. We aimed to review the medical literature concerning the efficacy and tolerability of TBZ in the treatment of HD patients and to report our personal experience about TBZ use in a cohort of HD patients. We searched PubMed (1960 to July 2010) using the following keywords: “tetrabenazine” + “huntington’s disease + chorea”. We included randomized controlled trials, open-label trials, and retrospective studies. We excluded case reports and studies conducted on fewer than 20 patients. In addition, we retrospectively evaluated 2 years’ follow-up of TBZ treatment on motor and cognitive performances and functional abilities in 28 HD patients, compared with 10 patients treated by other neuroleptics (clotiapine). Only four papers fulfilled the requested criteria. In the first study, which included 84 randomized outpatients, TBZ showed a significant improvement of chorea compared with placebo. In the open-label study extension, TBZ confirmed its efficacy on chorea, with a frequent occurrence of withdrawals due to side effects. In a retrospective study of long-term efficacy, 63 patients under TBZ therapy for an average period of 34 months showed a stable effect on chorea, despite a slight reduction of effect over time. In a telephone survey conducted on a total of 118 patients affected by different movement disorders, TBZ showed the most favorable effect for the 28 included HD patients. Our HD patients showed a slight deterioration of motor performances over time that was nonsignificant compared with TBZ or clotiapine treatments. Despite the fact that the global effect of TBZ seems positive in HD, more attention on evaluating symptomatic treatments for cognitive and psychiatric deterioration as well as motor deterioration would alleviate this devastating disorder until a neuroprotective treatment becomes available.

Tetrabenazine: the first approved drug for the treatment of chorea in US patients with Huntington disease

Huntington disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to those at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic or neuroprotective interventions yielded positive results. Tetrabenazine (TBZ) is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer antipsychotic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse-effect profile than older antipsychotic agents for treating chorea and psychosis. This review will address the epidemiology and diagnosis of HD as background for understanding potential pharmacological treatment options. Because TBZ is the only US Food and Drug Administration-approved medication in the United States for HD, the focus of this review will be on its pharmacology, efficacy, safety, and practical uses. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

Tetrabenazine

BACKGROUND

Tetrabenazine (TBZ) depletes presynaptic dopamine in the CNS. It has been found to be beneficial in hyperkinetic movement disorders without carrying the extrapyramidal side effects that are characteristic of neuroleptics.

OBJECTIVE

To summarize current knowledge on the use of TBZ and draw conclusions about its efficacy and safety.

METHODS

PubMed literature searches using the term 'tetrabenazine' were carried out for the period prior to May 2009. Additional relevant studies referenced by these publications were included.

CONCLUSIONS

Both short- and long-term studies have consistently yielded favorable results for the use of TBZ in the treatment of hyperkinetic movement in terms of efficacy and safety. TBZ is most effective in reducing chorea (including Huntington's disease associated chorea), tic associated with Tourette's syndrome and tardive dyskinesias. Furthermore, TBZ might also have potential for use in other hyperkinetic disorders (e.g., myoclonus and dystonia), for which future clinical trials are needed.

Tetrabenazine as anti-chorea therapy in Huntington disease: an open-label continuation study. Huntington Study Group/TETRA-HD Investigators

Background

Tetrabenazine (TBZ) selectively depletes central monoamines by reversibly binding to the type-2 vesicular monoamine transporter. A previous double blind study in Huntington disease (HD) demonstrated that TBZ effectively suppressed chorea, with a favorable short-term safety profile (Neurology 2006;66:366-372). The objective of this study was to assess the long-term safety and effectiveness of TBZ for chorea in HD.

Methods

Subjects who completed the 13-week, double blind protocol were invited to participate in this open label extension study for up to 80 weeks. Subjects were titrated to the best individual dose or a maximum of 200 mg/day. Chorea was assessed using the Total Maximal Chorea (TMC) score from the Unified Huntington Disease Rating Scale.

Results

Of the 75 participants, 45 subjects completed 80 weeks. Three participants terminated due to adverse events (AEs) including depression, delusions with associated previous suicidal behavior, and vocal tics. One subject died due to breast cancer. The other 26 subjects chose not to continue on with each ensuing extension for various reasons. When mild and unrelated AEs were excluded, the most commonly reported AEs (number of subjects) were sedation/somnolence (18), depressed mood (17), anxiety (13), insomnia (10), and akathisia (9). Parkinsonism and dysphagia scores were significantly increased at week 80 compared to baseline. At week 80, chorea had significantly improved from baseline with a mean reduction in the TMC score of 4.6 (SD 5.5) units. The mean dosage at week 80 was 63.4 mg (range 12.5-175 mg).

Conclusions

TBZ effectively suppresses HD-related chorea for up to 80 weeks. Patients treated chronically with TBZ should be monitored for parkinsonism, dysphagia and other side effects including sleep disturbance, depression, anxiety, and akathisia.

Trial Registration

Clinicaltrials.gov registration number (initial study): NCT00219804

The long-term effect of tetrabenazine in the management of Huntington disease

OBJECTIVES

To enhance the knowledge on the long-term efficacy and safety of tetrabenazine (TBZ) in managing chorea.

METHODS

We analyzed 68 Huntington disease patients (mean disease duration, 55.8 +/- 34.7 months) who had been treated with TBZ for a mean period of 34.4 +/- 25.2 months (median, 34 months; mode, 48 months; range, 3-104 months). We measured the variation from pretreatment of the motor score of Unified Huntington's Disease Rating Scale at the first follow-up visit and at the latest.

RESULTS

Mean Unified Huntington's Disease Rating Scale-chorea underscore at the time of the pretreatment visit was 10.4 +/- 4.1 (range, 0-28). At the first follow-up, 9.7 +/- 7.8 months after the prescription of TBZ (mean dose, 35.3 +/- 14.7 mg), mean score of chorea was 8.2 +/- 4.1 (-21% compared with baseline), whereas at the latest follow-up visit (mean dose, 57.5 +/- 14.7 mg), it was 9.5 +/- 5.0 (9%). During the follow-up, the clinical benefit persisted, but the magnitude was reduced despite a progressive increase of the doses (up to 60%). Motor improvement was not influenced by sex, or doses or duration of therapy; age at onset was the only predictor of a good outcome. Five patients (7%) did not gain any improvement, and TBZ was discontinued. There were 2 withdrawals because of side effects; 34 patients reported at least 1 side effect.

CONCLUSIONS

Tetrabenazine was well tolerated and produced long-term improvement of motor symptoms in Huntington disease patients, although a slight reduction of benefit occurred during the course of treatment.

Long-term effect of moderate and profound hypothermia on morphology, neurological, cognitive and behavioural functions in a rat model of perinatal asphyxia

BACKGROUND

Perinatal asphyxia is a frequent cause of neurological handicap with no known therapy. However, hypothermic therapy has recently attracted attention owing to its neuroprotective property in brain of immature organisms.

OBJECTIVES

Hypothermia appears to be promising in reversing the immediate effect of perinatal asphyxia, but data on long-term neuroprotection is still lacking. We therefore intended to test the long-term effect of moderate and profound hypothermia on brain morphology and functions using a well established rat model of perinatal asphyxia.

METHODS

Rat pups delivered by caesarean section were placed into a water bath, still in patent membranes, at 37 degrees C and variable hypothermic conditions to induce asphyxia and thereafter given to surrogate mothers. Examinations were performed at the age of three months, consisting of a battery of motor, behavioural, cognition and reflex tests including rota-rod, Morris water maze, multiple T-maze, elevated plus maze and open field studies. Morphological alterations were evaluated by Nissl staining of brain areas known to be hypoxia sensitive. Neurotransmission system markers, including tyrosine hydroxylase, vesicular monoamine transporter, vesicular acetylcholine transporter and excitatory amino acid carrier1 were analyzed by immunohistochemistry.

RESULTS

Survival increased with hypothermia. The Nissl stain revealed neuronal loss in hippocampus and hypothalamus of normothermic asphyxiated group (20/37) compared to controls (0/37), but no neuroprotective patterns emerged from hypothermia. An overall inconsistent protection of the neural systems was noted by variable periods of hypothermia. Motor function was significantly impaired in 20/37 as compared to 0/37. In the Morris water maze and multiple T-maze, results were comparable between the groups. In the elevated plus maze, time spent in the closed arm was reduced and in the open field, vertical behaviour was altered in the 20/37 group with horizontal motor behaviour being unaffected. Hypothermia reversed all abnormalities seen in 20/37, with short-term moderate and profound hypothermia being superior to long-term hypothermia.

CONCLUSION

Hypothermia not only significantly increased survival, but also resulted in unimpaired motor as well as improved cognitive functions. Those findings are in contrast to altered brain morphology. As neuronal loss was present in various brain regions, we conclude that deficits may be compensated in the maturing animal. Intrahypoxic hypothermia was able to protect the rat from the devastating effect of perinatal asphyxia not in morphological, but in functional terms.

Psychostimulants and Vesicle Trafficking: A Novel Mechanism and Therapeutic Implications

The monoamine vesicular transporter 2 (VMAT-2) has been associated with dopamine (DA) sequestration and protection against neurodegeneration caused by the intracellular oxidation of this monoamine. The data presented herein suggest that methylphenidate treatment enhances the amount of VMAT-2 protein and possibly its activity in the presynaptic cytosol, where it is able to increase the sequestration of DA and likely protect against its instability. In contrast, methamphetamine (METH) has an opposite effect on cytosolic VMAT-2 resulting in degradation of DA terminals. The fact that posttreatment of methylphenidate after a neurotoxic regimen of METH protects against resulting loss of DA parameters suggests that treatment with methylphenidate, or other DA transporter blockers, may be protective against degenerative disorders of DA pathways, such as Parkinson's disease.

Inverse relationship between the contents of neuromelanin pigment and the vesicular monoamine transporter-2: human midbrain dopamine neurons

The dopaminergic neurons in the ventral substantia nigra (SN) are significantly more vulnerable to degeneration in Parkinson's disease (PD) than the dopaminergic neurons in the ventral tegmental area (VTA). The ventral SN neurons also contain significantly more neuromelanin pigment than the dopaminergic neurons in the VTA. In vitro data indicate that neuromelanin pigment is formed from the excess cytosolic catecholamine that is not accumulated into synaptic vesicles by the vesicular monoamine transporter-2 (VMAT2). By using quantitative immunohistochemical methods in human postmortem brain, we sought to examine the relative contents of VMAT2 within neurons that contain different amounts of neuromelanin pigment. The immunostaining intensity (ISI) was measured for VMAT2 and also for the rate-limiting enzyme for the synthesis of dopamine, tyrosine hydroxylase (TH). ISI measures were taken from the ventral SN region where neurons are most vulnerable to degeneration in PD, nigrosome-1 (N1); from the ventral SN region where cells are moderately vulnerable to degeneration in PD, the matrix (M); and from VTA neurons near the exit of the third nerve (subregion III). The data indicate that 1) subregion III neurons have significantly higher levels of VMAT2 ISI compared with N1 neurons (more than twofold) and M neurons (45%); 2) there is an inverse relationship between VMAT2 ISI and neuromelanin pigment in the N1 and III neurons; 3) there is an inverse relationship between VMAT2 ISI and the vulnerability to degeneration in PD in the N1, M, and III subregions; and 4) neurons with high VMAT2 ISI also have high TH ISI. These data support the hypothesis that midbrain dopaminergic neurons that synthesize greater amounts of dopamine have more vesicular storage capacity for action potential-induced release of transmitter and that the ventral SN neurons accumulate the most neuromelanin pigment, in part because they have the least VMAT2 protein.

Impact of psychostimulants on vesicular monoamine transporter function

The vesicular monoamine transporter-2 (VMAT-2) facilitates the sequestration of catecholamines and serotonin into synaptic vesicles, and is therefore an essential regulator of monoaminergic neuronal function. VMAT-2 proteins may also play a role in neuroprotection, since these transporters have the capacity to sequester neurotoxins within vesicles. Recent studies have demonstrated that psychostimulants, particularly dopamine "releasers" and "reuptake inhibitors", differentially alter VMAT-2 function. As described in this review, these studies not only provide insight into the pharmacological actions of stimulants, but also mechanisms underlying neurodegenerative disorders, including Parkinson's disease.

Inhibition of calpains prevents neuronal and behavioral deficits in an MPTP mouse model of Parkinson's disease

The molecular mechanisms mediating degeneration of midbrain dopamine neurons in Parkinson's disease (PD) are poorly understood. Here, we provide evidence to support a role for the involvement of the calcium-dependent proteases, calpains, in the loss of dopamine neurons in a mouse model of PD. We show that administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) evokes an increase in calpain-mediated proteolysis in nigral dopamine neurons in vivo. Inhibition of calpain proteolysis using either a calpain inhibitor (MDL-28170) or adenovirus-mediated overexpression of the endogenous calpain inhibitor protein, calpastatin, significantly attenuated MPTP-induced loss of nigral dopamine neurons. Commensurate with this neuroprotection, MPTP-induced locomotor deficits were abolished, and markers of striatal postsynaptic activity were normalized in calpain inhibitor-treated mice. However, behavioral improvements in MPTP-treated, calpain inhibited mice did not correlate with restored levels of striatal dopamine. These results suggest that protection against nigral neuron degeneration in PD may be sufficient to facilitate normalized locomotor activity without necessitating striatal reinnervation. Immunohistochemical analyses of postmortem midbrain tissues from human PD cases also displayed evidence of increased calpain-related proteolytic activity that was not evident in age-matched control subjects. Taken together, our findings provide a potentially novel correlation between calpain proteolytic activity in an MPTP model of PD and the etiology of neuronal loss in PD in humans.

In vivo imaging of the vesicular acetylcholine transporter and the vesicular monoamine transporter

Validation of the vesicular acetylcholine transporter (VAChT) and the neuronal vesicular monoamine transporter (VMAT2) as important molecular targets in the cholinergic and dopamine neurons, respectively, has sparked interest in the development of radiotracers for studying these markers in vitro and in vivo. Currently, a number of selective high-affinity radiotracers are available for studying these targets in vivo with positron emission tomography (PET) or single photon emission computed tomography (SPECT). PET studies of VMAT2 in neuropathology reveal changes in the density of this marker that can be verified independently. Similarly, in vivo studies with VAChT ligands suggest that the latter are potentially useful in detecting cholinergic lesions in vivo; however, additional development is required to fully realize the potential of these radioligands.

Immunochemical analysis of vesicular monoamine transporter (VMAT2) protein in Parkinson's disease

The vesicular monoamine transporter (VMAT2) has been suggested to be an excellent marker of presynaptic dopaminergic nerve terminals in the striatum of Parkinson's disease patients based on its high level of expression and insensitivity to drugs used to treat the disease. Previous in vivo imaging and postmortem binding studies have detected a loss in striatal VMAT2 binding in Parkinson's diseased (PD) brain; however, these techniques have poor spatial resolution and may suffer from nonspecific binding of some ligands. In this study, we use novel polyclonal antibodies to distinct regions of human VMAT2 to quantify and localize the protein. Western blot analysis demonstrated marked reductions in VMAT2 immunoreactivity in putamen, caudate, and nucleus accumbens of PD brain compared to control cases. Immunohistochemistry revealed VMAT2 immunoreactive fibers and puncta that were dense throughout the striatum of control brains, but which were drastically reduced in putamen of PD brains. In PD brains the caudate showed a significant degree of sparing along the border of the lateral ventricle and the nucleus accumbens was relatively preserved. The distribution of VMAT2 in striatum and its loss in PD paralleled that of the dopamine transporter (DAT), a phenotypic marker of dopamine neurons. Thus, immunochemical analysis of VMAT2 protein provides novel and sensitive means for localizing and quantifying VMAT2 protein and nigrostriatal dopamine terminals in PD. Furthermore, the relative expression of VMAT2 compared to that of DAT may predict the differential vulnerability of dopamine neurons in PD.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: in vitro binding studies and positron emission tomography studies in nonhuman primates

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)-4-fluorobenzyltrozamicol ([18F](+)-FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)- and (-)-stereoisomers of FBT for the VAcChT as well as sigma (sigma 1 and sigma 2) receptors. (+)-FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for sigma 1 (21.6 nM) and sigma 2 (35.9 nM) receptors, whereas (-)-FBT had similar affinities for the VAcChT and sigma 1 receptors (approximately 20 nM) and a lower affinity for sigma 2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)-FBT. [18F](+)-FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)-FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood-brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)-FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT.