Structural mechanisms for VMAT2 inhibition by tetrabenazine

The vesicular monoamine transporter 2 (VMAT2) is a proton-dependent antiporter responsible for loading monoamine neurotransmitters into synaptic vesicles. Dysregulation of VMAT2 can lead to several neuropsychiatric disorders including Parkinson's disease and schizophrenia. Furthermore, drugs such as amphetamine and MDMA are known to act on VMAT2, exemplifying its role in the mechanisms of actions for drugs of abuse. Despite VMAT2's importance, there remains a critical lack of mechanistic understanding, largely driven by a lack of structural information. Here, we report a 3.1 Å resolution cryo-electron microscopy (cryo-EM) structure of VMAT2 complexed with tetrabenazine (TBZ), a non-competitive inhibitor used in the treatment of Huntington's chorea. We find TBZ interacts with residues in a central binding site, locking VMAT2 in an occluded conformation and providing a mechanistic basis for non-competitive inhibition. We further identify residues critical for cytosolic and lumenal gating, including a cluster of hydrophobic residues which are involved in a lumenal gating strategy. Our structure also highlights three distinct polar networks that may determine VMAT2 conformational dynamics and play a role in proton transduction. The structure elucidates mechanisms of VMAT2 inhibition and transport, providing insights into VMAT2 architecture, function, and the design of small-molecule therapeutics.

Transport and inhibition mechanism for VMAT2-mediated synaptic vesicle loading of monoamines

Monoamine neurotransmitters such as serotonin and dopamine are loaded by vesicular monoamine transporter 2 (VMAT2) into synaptic vesicles for storage and subsequent release in neurons. Impaired VMAT2 function underlies various neuropsychiatric diseases. VMAT2 inhibitors reserpine and tetrabenazine are used to treat hypertension, movement disorders associated with Huntington's Disease and Tardive Dyskinesia. Despite its physiological and pharmacological significance, the structural basis underlying VMAT2 substrate recognition and its inhibition by various inhibitors remains unknown. Here we present cryo-EM structures of human apo VMAT2 in addition to states bound to serotonin, tetrabenazine, and reserpine. These structures collectively capture three states, namely the lumen-facing, occluded, and cytosol-facing conformations. Notably, tetrabenazine induces a substantial rearrangement of TM2 and TM7, extending beyond the typical rocker-switch movement. These functionally dynamic snapshots, complemented by biochemical analysis, unveil the essential components responsible for ligand recognition, elucidate the proton-driven exchange cycle, and provide a framework to design improved pharmaceutics targeting VMAT2.

Noninvasive Quantitative PET Imaging in Humans of the Pancreatic Beta-Cell Mass Biomarkers VMAT2 and Dopamine D2/D3 Receptors In Vivo

Noninvasive quantitative imaging of beta-cells can provide information on changes in cellular transporters, receptors, and signaling proteins that may affect function and/or loss of mass, both of which contribute to the loss of insulin secretion and glucose regulation of patients with type 1 or type 2 diabetes (T1D/T2D). We have developed and optimized the use of two positron emission tomography (PET) radioligands, [¹⁸F]FP-(+)-DTBZ and [¹¹C](+)-PHNO, targeting beta-cell VMAT2 and dopamine (D2/D3) receptors, respectively. Here we describe our optimized methodology for the clinical use of these two tracers for quantitative PET imaging of beta-cell biomarkers in vivo. We also briefly discuss our previous results and their implications and value towards extending the use of PET radioligand beyond the original goal of quantitative imaging of beta-cell mass to the potential to provide insight into the biology of beta-cell loss of mass and/or function and to evaluate the efficacy of therapeutics to prevent or restore functional beta-cell mass.

Tetrabenazine Mitigates Aberrant Release and Clearance of Dopamine in the Nigrostriatal System, and Alleviates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease

BACKGROUND

L-DOPA-induced dyskinesia (LID), occurring with aberrant processing of exogenous L-DOPA in the dopamine-denervated striatum, is a main complication of levodopa treatment in Parkinson's disease.

OBJECTIVE

To characterize the effects of the vesicular antagonist tetrabenazine (TBZ) on L-DOPA-induced behavior, neurochemical signals, and underlying protein expressions in an animal model of Parkinson's disease.

METHODS

20-week-old MitoPark mice were co-treated or separately administered TBZ and L-DOPA for 14 days. Abnormal involuntary movements (AIMs) and locomotor activity were analyzed. To explore dopamine (DA) transmission, fast scan cyclic voltammetry was used to assess presynaptic DA dynamics in striatal slices following treatments. PET imaging with 4-[18F]-PE2I, ADAM and immunoblotting assays were used to detect receptor protein changes in the DA-denervated striatum. Finally, nigrostriatal tissues were collected for HPLC measures of DA, serotonin and their metabolites.

RESULTS

A single injection of TBZ given in the interval between the two L-DOPA/Carbidopa treatments significantly attenuated L-DOPA-induced AIMs expression and locomotor hyperactivity. TBZ was shown to reduce tonic and phasic release of DA following L-DOPA treatment in DA-denervated striatal tissue. In the DA-depleted striatum, TBZ decreased the expression of L-DOPA-enhanced D1 receptors and the serotonin reuptake transporter. Neurochemical analysis indicated that TBZ attenuated L-DOPA-induced surges of DA levels by promoting DA turnover in the nigrostriatal system.

CONCLUSIONS

Our findings demonstrate that TBZ diminishes abnormal striatal DA transmission, which involves the ability of TBZ to modulate the presymptomatic dynamics of DA, and then mitigate aberrant release of exogenous L-DOPA from nerve terminals. The results support the potential of repositioning TBZ to counteract LID development.

Typical features of Parkinson disease and diagnostic challenges with microdeletion 22q11.2

OBJECTIVE

To delineate the natural history, diagnosis, and treatment response of Parkinson disease (PD) in individuals with 22q11.2 deletion syndrome (22q11.2DS), and to determine if these patients differ from those with idiopathic PD.

METHODS

In this international observational study, we characterized the clinical and neuroimaging features of 45 individuals with 22q11.2DS and PD (mean follow-up 7.5 ± 4.1 years).

RESULTS

22q11.2DS PD had a typical male excess (32 male, 71.1%), presentation and progression of hallmark motor symptoms, reduced striatal dopamine transporter binding with molecular imaging, and initial positive response to levodopa (93.3%). Mean age at motor symptom onset was relatively young (39.5 ± 8.5 years); 71.4% of cases had early-onset PD (<45 years). Despite having a similar age at onset, the diagnosis of PD was delayed in patients with a history of antipsychotic treatment compared with antipsychotic-naive patients (median 5 vs 1 year, p = 0.001). Preexisting psychotic disorders (24.5%) and mood or anxiety disorders (31.1%) were common, as were early dystonia (19.4%) and a history of seizures (33.3%).

CONCLUSIONS

Major clinical characteristics and response to standard treatments appear comparable in 22q11.2DS-associated PD to those in idiopathic PD, although the average age at onset is earlier. Importantly, treatment of preexisting psychotic illness may delay diagnosis of PD in 22q11.DS patients. An index of suspicion and vigilance for complex comorbidity may assist in identifying patients to prioritize for genetic testing.

CSF Biomarkers and Its Associations with 18F-AV133 Cerebral VMAT2 Binding in Parkinson's Disease-A Preliminary Report

OBJECTIVE

Cerebrospinal fluid (CSF) biomarkers, such as α-synuclein (α-syn), amyloid beta peptide 1-42 (Aβ1-42), phosphorylated tau (181P) (p-tau), and total tau (t-tau), have long been associated with the development of Parkinson disease (PD) and other neurodegenerative diseases. In this investigation, we reported the assessment of CSF biomarkers and their correlations with vesicular monoamine transporter 2 (VMAT2) bindings measured with 18F-9-fluoropropyl-(+)-dihydrotetrabenazine (18F-AV133) that is being developed as a biomarker for PD. We test the hypothesis that monoaminergic degeneration was correlated with CSF biomarker levels in untreated PD patients.

METHODS

The available online data from the Parkinson's Progression Markers Initiative study (PPMI) project were collected and analyzed, which include demographic information, clinical evaluations, CSF biomarkers (α-syn, Aβ1-42, p-tau, and t-tau), 18F-AV133 brain PET, and T1 weighted MRIs. Region of interest (ROI) and voxel-wise Pearson correlation between standardized uptake value ratio (SUVR) and CSF biomarkers were calculated.

RESULTS

Our major findings are: 1) Compared with controls, CSF α-syn and tau levels decreased significantly in PD; 2) α-syn was closely correlated with Aβ1-42 and tau in PD, especially in early-onset patients; and 3) hypothesis-driven ROI analysis found a significant negative correlation between CSF Aβ1-42 levels and VMAT2 densities in post cingulate, left caudate, left anterior putamen, and left ventral striatum in PDs. CSF t-tau and p-tau levels were significantly negatively related to VMAT2 SUVRs in substantia nigra and left ventral striatum, respectively. Voxel-wise analysis showed that left caudate, parahippocampal gyrus, insula and temporal lobe were negatively correlated with Aβ1-42. In addition, superior frontal gyrus and transverse temporal gyrus were negatively correlated with CSF p-tau levels.

CONCLUSION

These results suggest that monoaminergic degeneration in PD is correlated with CSF biomarkers associated with cognitive impairment in neurodegenerative diseases including Alzheimer's disease. The association between loss of dopamine synaptic function and pathologic protein accumulations in PD indicates an important role of CSF biomarkers in PD development.

Evaluation of pancreatic VMAT2 binding with active and inactive enantiomers of 18F-FP-DTBZ in baboons

INTRODUCTION

¹⁸F-Fluoropropyl-(+)-dihydrotetrabenazine (¹⁸F-FP-(+)-DTBZ) is a vesicular monoamine transporter type 2 (VMAT2) radiotracer for positron emission tomography (PET) imaging to quantify human β-cell mass. Renal cortex and spleen have been suggested as reference regions, however, little is known about ¹⁸F-FP-(+)-DTBZ binding in these regions including the fraction of radiometabolite. We compared the kinetics of ¹⁸F-FP-(+)-DTBZ and its inactive enantiomer ¹⁸F-FP-(-)-DTBZ in baboons, estimated the non-displaceable binding (V_ND) of the tracers, and used ex vivo studies to measure radiometabolite fractions.

METHODS

PET scans were conducted for up to 4h with (+) and (-) enantiomers. Displacement experiments using unlabeled (+) and (-) enantiomers of FP-DTBZ and fluvoxamine (to evaluate sigma-1 receptor binding) were performed. SUV curves were used to calculate displacement values in the pancreas, renal cortex, and spleen. Distribution volumes (V_T) were computed, and three approaches for calculation of V_ND were compared: (1) ¹⁸F-FP-(+)-DTBZ reference V_T, (2) ¹⁸F-FP-(-)-DTBZ pancreatic V_T, and (3) a scaled ¹⁸F-FP-(+)-DTBZ reference V_T values. Ex vivo study was conducted to measure radiometabolite fraction in homogenized tissue samples from baboons at 90min post-injection.

RESULTS

Spleen uptake was lowest for both tracers. Highest uptake was in the pancreas with ¹⁸F-FP-(+)-DTBZ and renal cortex with ¹⁸F-FP-(-)-DTBZ. Substantial displacement effect was observed only with unlabeled FP-(+)-DTBZ in the ¹⁸F-FP-(+)-DTBZ studies. Radiometabolite fraction was higher in the renal cortex than the spleen. Approaches (1) and (3) with spleen to estimate V_ND provided lowest inter-subject variability of BP_ND.

CONCLUSIONS

V_T differences among organs and between enantiomers indicated that scaling of reference region values is needed for quantification of VMAT2 binding in the pancreas with ¹⁸F-FP-(+)-DTBZ. Since the kidney PET signal has greater partial volume averaging and more radiometabolites, the spleen was considered a more practical candidate for use as a scaled-reference region in the quantification of ¹⁸F-FP-(+)-DTBZ in the pancreas.

Effects of anesthetics on vesicular monoamine transporter type 2 binding to ¹⁸F-FP-(+)-DTBZ: a biodistribution study in rat brain

OBJECTIVES

The in vivo binding analysis of vesicular monoamine transporter type 2 (VMAT2) to radioligand has provided a means of investigating related disorders. Anesthesia is often inevitable when the investigations are performed in animals. In the present study, we tested effects of four commonly-used anesthetics: isoflurane, pentobarbital, chloral hydrate and ketamine, on in vivo VMAT2 binding to (18)F-FP-(+)-DTBZ, a specific VMAT2 radioligand, in rat brain.

METHODS

The transient equilibrium time window for in vivo binding of (18)F-FP-(+)-DTBZ after a bolus injection was firstly determined. The brain biodistribution studies under anesthetized and awake rats were then performed at the equilibrium time. Standard uptake values (SUVs) of the interest brain regions: the striatum (ST), hippocampus (HP), cortex (CX) and cerebellum (CB) were obtained; and ratios of tissue to cerebellum were calculated.

RESULTS

Isoflurane and pentobarbital did not alter distribution of (18)F-FP-(+)-DTBZ in the brain relative to the awake group; neither SUVs nor ratios of ST/CB and HP/CB were altered significantly. Chloral hydrate significantly increased SUVs of all the brain regions, but did not significantly alter ratios of ST/CB and HP/CB. Ketamine significantly increased SUVs of the striatum, hippocampus and cortex, and insignificantly increased the SUV of the cerebellum; consequently, ketamine significantly increased ratios of ST/CB and HP/CB.

CONCLUSIONS

It is concluded that in vivo VMAT2 binding to (18)F-FP-(+)-DTBZ are not altered by isoflurane and pentobarbital, but altered by chloral hydrate and ketamine. Isoflurane and pentobarbital may be promising anesthetic compounds for investigating in vivo VMAT2 binding. Further studies are warranted to investigate the interactions of anesthetics with VMAT2 binding potential with in vivo PET studies.

Species-specific vesicular monoamine transporter 2 (VMAT2) expression in mammalian pancreatic beta cells: implications for optimising radioligand-based human beta cell mass (BCM) imaging in animal models

AIMS/HYPOTHESIS

Imaging of beta cell mass (BCM) is a major challenge in diabetes research. The vesicular monoamine transporter 2 (VMAT2) is abundantly expressed in human beta cells. Radiolabelled analogues of tetrabenazine (TBZ; a low-molecular-weight, cell-permeant VMAT2-selective ligand) have been employed for pancreatic islet imaging in humans. Since reports on TBZ-based VMAT2 imaging in rodent pancreas have been fraught with confusion, we compared VMAT2 gene expression patterns in the mouse, rat, pig and human pancreas, to identify appropriate animal models with which to further validate and optimise TBZ imaging in humans.

METHODS

We used a panel of highly sensitive VMAT2 antibodies developed against equivalently antigenic regions of the transporter from each species in combination with immunostaining for insulin and species-specific in situ hybridisation probes. Individual pancreatic islets were obtained by laser-capture microdissection and subjected to analysis of mRNA expression of VMAT2.

RESULTS

The VMAT2 protein was not expressed in beta cells in the adult pancreas of common mouse or rat laboratory strains, in contrast to its expression in beta cells (but not other pancreatic endocrine cell types) in the pancreas of pigs and humans. VMAT2- and tyrosine hydroxylase co-positive (catecholaminergic) innervation was less abundant in humans than in rodents. VMAT2-positive mast cells were identified in the pancreas of all species.

CONCLUSIONS/INTERPRETATION

Primates and pigs are suitable models for TBZ imaging of beta cells. Rodents, because of a complete lack of VMAT2 expression in the endocrine pancreas, are a 'null' model for assessing interference with BCM measurements by VMAT2-positive mast cells and sympathetic innervation in the pancreas.

Sigma receptor binding of tetrabenazine series tracers targeting VMAT2 in rat pancreas

The vesicular monoamine transporter type II (VMAT2) is highly expressed in pancreatic β-cells and thus has been proposed to be a potential target for measuring β-cell mass (BCM) by molecular imaging. Several tracers based on the TBZ backbone, including 9-fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]AV-133), have shown some promising results as potential biomarkers for BCM despite a relatively high background signal in the pancreas. In the present study, we explore the background binding characteristics of [(18)F]AV-133 in rat pancreas.

METHODS

Pancreatic exocrine cells and islet cells were isolated and purified from Sprague-Dawley rats. Membrane homogenates, prepared from both pancreatic exocrine and islet cells as well as from brain striatum regions, were used for in vitro binding studies of [(18)F]AV-133 under a selective masking condition. 1,3-Di-o-tolylguanidine (DTG), displaying high and roughly equal affinity for both sigma-1 and sigma-2 receptors, was chosen at 5 μM concentration for the masking/blocking studies.

RESULTS

[(18)F]AV-133 binding to rat striatum homogenates was not significantly altered by the presence of DTG. In contrast, [(18)F]AV-133 showed significant competition with DTG for binding sites in rat pancreatic exocrine homogenates as well as in rat islet cell homogenates. Importantly, in the presence of DTG, [(18)F]AV-133 showed a single high-affinity binding site on islet cell homogenates with a K(d) value of 3.8 nM which is consistent with the affinity reported previously for VMAT2 sites in rat pancreas.

CONCLUSIONS

[(18)F]AV-133, in addition to a high-affinity VMAT2 binding site, binds with low affinity (but high capacity) to sigma components that are present in the rat pancreas. Identification of the cause of background binding of [(18)F]AV-133 to rat pancreatic tissue may lead to improved methods for quantification.

Computational neural network analysis of the affinity of lobeline and tetrabenazine analogs for the vesicular monoamine transporter-2

Back-propagation artificial neural networks (ANNs) were trained on a dataset of 104 VMAT2 ligands with experimentally measured log(1/K(i)) values. A set of related descriptors, including topological, geometrical, GETAWAY, aromaticity, and WHIM descriptors, was selected to build nonlinear quantitative structure-activity relationships. A partial least squares (PLS) regression model was also developed for comparison. The nonlinearity of the relationship between molecular descriptors and VMAT2 ligand activity was demonstrated. The obtained neural network model outperformed the PLS model in both the fitting and predictive ability. ANN analysis indicated that the computed activities were in excellent agreement with the experimentally observed values (r(2)=0.91, rmsd=0.225; predictive q(2)=0.82, loormsd=0.316). The generated models were further tested by use of an external prediction set of 15 molecules. The nonlinear ANN model has r(2)=0.93 and root-mean-square errors of 0.282 compared with the experimentally measured activity of the test set. The stability test of the model with regard to data division was found to be positive, indicating that the generated model is predictive. The modeling study also reflected the important role of atomic distribution in the molecules, size, and steric structure of the molecules when they interact with the target, VMAT2. The developed models are expected to be useful in the rational design of new chemical entities as ligands of VMAT2 and for directing synthesis of new molecules in the future.

Fluoroalkyl derivatives of dihydrotetrabenazine as positron emission tomography imaging agents targeting vesicular monoamine transporters

Imaging of vesicular monoamine transporter 2 (VMAT2) in the brain with [(11)C]-dihydrotetrabenazine (DTBZ) in conjunction with positron emission tomography (PET) has demonstrated its usefulness in the diagnosis and monitoring of neurodegenerative diseases such as Parkinson's disease and Huntington's disease. We report on the development of (18)F analogs of DTBZ with a longer half-life (t=110 vs. 20 min for C-11) to increase the availability of VMAT2 imaging agents for routine clinical studies with PET. Racemic 9-fluoroethyl (FE) and 9-fluoropropyl (FP)-9-desmethyl-DTBZ and the corresponding hydroxyl derivatives were successfully prepared. No-carrier-added racemic (18)F-DTBZ derivatives were synthesized by an [(18)F]fluoride displacement of the corresponding mesylates with good yields (30-40%) and high specific activity (SA=1500-2000 Ci/mmol). Racemic (+/-)-FE-DTBZ (6a) and (+/-)-FP-DTBZ (6b) displayed excellent binding affinities (K(i)=0.76 and 0.56 nM, respectively) for VMAT2 binding sites in rat striatal homogenates, whereas the known compounds (+/-)-DTBZ and (+/-)-tetrabenazine (TBZ) showed K(i) values of 1.7+/-0.2 and 1.3+/-0.1 nM, respectively. Consistently, racemic [(18)F]6a and [(18)F]6b exhibited K(d) values of 0.52 and 0.48 nM, respectively (based on an SA of 2000 Ci/mmol), for VMAT2 binding sites using mouse striatal homogenates. Both agents showed comparable binding densities with those obtained with [(3)H](+/-)-TBZ. Results of in vitro autoradiography with [(18)F]6b showed a distinct binding in the caudate putamen region consistent with the localization of VMAT2 in the mouse brain, which was blocked by nonradioactive TBZ efficiently. Biodistribution studies on mice after an intravenous injection of the tracer exhibited excellent brain uptakes (4.66% and 7.08% ID/g at 2 min for racemic [(18)F]6a and [(18)F]6b, respectively). It was determined that [(18)F]6b displayed a faster brain washout than [(18)F]6a did. As a result, [(18)F]6b yielded a better target-to-background ratio (striatum/cerebellum=3.0 and 1.7 at 30 min after an intravenous injection for [(18)F]6b and [(18)F]6a, respectively). The blocking study with the nonradioactive (+/-)-DTBZ clearly confirmed the in vivo competition and specificity of [(18)F]6b binding for VMAT2 sites. In conclusion, these findings strongly suggest that the novel racemic [(18)F]6b is potentially useful as a molecular imaging agent for VMAT2 binding sites in the brain. Further studies are warranted to assess the utility of these (18)F-labeled DTBZ derivatives as PET tracers for the diagnosis of various neurodegenerative diseases.

Interaction of amphetamines and related compounds at the vesicular monoamine transporter

Amphetamine-type agents interact with the vesicular monoamine transporter type 2 (VMAT(2)), promoting the release of intravesicular neurotransmitter and an increase in cytoplasmic neurotransmitter. Some compounds, such as reserpine, "release" neurotransmitter by inhibiting the ability of VMAT(2) to accumulate neurotransmitter in the vesicle, whereas other types of compounds can release neurotransmitter via a carrier-mediated exchange mechanism. The purpose of this study was to determine, for 42 mostly amphetamine-related compounds, their mode of interaction with the VMAT(2). We used a crude vesicular fraction prepared from rat caudate to assay VMAT(2) activity. Test compounds were assessed in several assays, including 1) inhibition of [(3)H]dihydrotetrabenazine binding, 2) inhibition of vesicular [(3)H]dopamine uptake, and 3) release of preloaded [(3)H]dopamine and [(3)H]tyramine. Several important findings derive from this comprehensive study. First, our work indicates that most agents are VMAT(2) substrates. Second, our data strongly suggest that amphetamine-type agents deplete vesicular neurotransmitter via a carrier-mediated exchange mechanism rather than via a weak base effect, although this conclusion needs to be confirmed via direct measurement of vesicular pH. Third, our data fail to reveal differential VMAT(2) interactions among agents that do and do not produce long-term 5-hydroxytryptamine depletion. Fourth, the data reported revealed the presence of two pools of [(3)H]amine within the vesicle, one pool that is free and one pool that is tightly associated with the ATP/protein complex that helps store amine. Finally, the VMAT(2) assays we have developed should prove useful for guiding the synthesis and evaluation of novel VMAT(2) agents as possible treatment agents for addictive disorders.

Longitudinal noninvasive PET-based beta cell mass estimates in a spontaneous diabetes rat model

Diabetes results from an absolute or relative reduction in pancreatic beta cell mass (BCM) leading to insufficient insulin secretion and hyperglycemia. Measurement of insulin secretory capacity is currently used as a surrogate measure of BCM. However, serum insulin concentrations provide an imprecise index of BCM, and no reliable noninvasive measure of BCM is currently available. Type 2 vesicular monoamine transporters (VMAT2) are expressed in human islet beta cells, as well as in tissues of the CNS. [11C]Dihydrotetrabenazine ([11C]DTBZ) binds specifically to VMAT2 and is a radioligand currently used in clinical imaging of the brain. Here we report the use of [11C]DTBZ to estimate BCM in a rodent model of spontaneous type 1 diabetes (the BB-DP rat). In longitudinal PET studies of the BB-DP rat, we found a significant decline in pancreatic uptake of [11C]DTBZ that anticipated the loss of glycemic control. Based on comparison of standardized uptake values (SUVs) of [11C]DTBZ and blood glucose concentrations, loss of more than 65% of the original SUV correlated significantly with the development of persistent hyperglycemia. These studies suggest that PET-based quantitation of VMAT2 receptors provides a noninvasive measurement of BCM that could be used to study the pathogenesis of diabetes and to monitor therapeutic interventions.

PET in LRRK2 mutations: comparison to sporadic Parkinson's disease and evidence for presymptomatic compensation

Parkinson's disease may arise from multiple aetiologies, including genetic mutations that are for the most part uncommon. We describe here the positron emission tomography (PET) findings in clinically affected and asymptomatic, high-risk members of two autosomal dominantly inherited Parkinson's disease kindreds with recently described mutations at the PARK8 locus, in a novel gene encoding a leucine-rich repeat kinase (LRRK2). Affected family members have L-dopa-responsive parkinsonism with loss of dopaminergic nigral neurons and pleomorphic subcellular pathology. Fifteen family members underwent PET using: 18F-6-fluoro-L-dopa (18F-dopa) to assess dopamine (DA) synthesis and storage, 11C-(+/-)-dihydrotetrabenazine (11C-DTBZ) for the vesicular monoamine transporter, and 11C-d-threo-methylphenidate (11C-MP) for the membrane dopamine transporter (DAT). Measurements were compared with normal (n = 33) and sporadic Parkinson's disease (sPD) (n = 67) control groups. Four clinically affected members had findings similar to sPD, with impaired presynaptic DA function affecting the putamen more than the caudate. In two affected members, D2 dopamine receptor binding was intact. Two asymptomatic mutation carriers had abnormal DAT binding with another two developing such abnormalities over 4 years of follow-up. In these individuals, 18F-dopa uptake remained normal, although two of them also displayed abnormal 11C-DTBZ binding. Our study demonstrates that the in vivo neurochemical phenotype of LRRK2 mutations is indistinguishable from that of sPD, despite the pathological heterogeneity of the condition. Furthermore, we suggest that compensatory changes including downregulation of the DAT and upregulation of decarboxylase activity may delay the onset of parkinsonian symptoms.

Decreased platelet vesicular monoamine transporter density in habitual smokers

The brain vesicular monoamine transporter (VMAT2) is part of the re-uptake mechanism which regulates monoaminergic neurotransmission. We demonstrated previously a high degree of similarity between the pharmacodynamic characteristics of platelet and brain VMAT2. Nicotine induced increase of dopamine and serotonin neurotransmission in limbic structures may alter the expression of VMAT2 in brains of smokers. In this study we measured the VMAT2 pharmacodynamic characteristics using high-affinity [3H]dihydrotetrabenazine (TBZOH) binding to platelets of smokers (n=15) compared to sex and age matched healthy nonsmokers controls (n=14). A significant decrease (17%, P=0.02) in VMAT2 density (Bmax) was observed in platelets of smokers compared to nonsmokers. There was no significant difference in the affinity of [3H]TBZOH to its platelet binding site and the VMAT2 density did not correlate with the heaviness of smoking. The decreased density of the VMAT2 in the platelets of smokers may reflect nicotine induced desensitization of VMAT2, a phenomenon that may be relevant to the addictive properties of nicotine.

Decreased platelet vesicular monoamine transporter density in children and adolescents with attention deficit/hyperactivity disorder

The aim of the present study was to assess vesicular monoamine transporter (VMAT2) density in attention deficit/hyperactivity disorder (ADHD), a disorder involving monoaminergic dysregulation. It was hypothesized that the hypoactivity of monoaminergic neurotransmission related to ADHD could be associated with an under-expression of VMAT2. We assessed high affinity [3H]dihydrotetrabenazine [TBZOH] binding to platelet VMAT2 in untreated male ADHD children and adolescents (n=11) as compared to age-matched controls (n=14), as well as the correlation between VMAT2 density and the severity of ADHD symptoms as measured by the clinician-administered DSM-IV ADHD Scale (DAS) and the parent-administered Abbreviated Conners' Rating Scale (ACPRS). The [3H]TBZOH binding capacity (Bmax) was significantly lower (17%) in the ADHD group as compared to the controls. There was no difference between the two groups in the affinity (Kd value) of [3H]TBZOH to its binding site. An inverse correlation was found between the ADHD symptom scales and the Bmax values. It remains unclear whether the under-expression of platelet VMAT2 in ADHD children is reflective of a parallel change in the brain, and whether it is primary or an epiphenomenon of ADHD.

Lack of Regional Selectivity During the Progression of Parkinson Disease

BACKGROUND

Dopamine terminal loss in the putamen of patients with Parkinson disease (PD) shows a regional heterogeneity, reflecting selective vulnerability of degenerating neurons to mechanisms of cell death.

HYPOTHESIS

If the same pathogenic mechanisms are responsible for the onset and progression of PD, the regional selectivity of dopamine cell loss will be the same throughout the course of the disorder.

OBJECTIVE

To investigate the regional selectivity of dopamine terminal loss during the progression of PD.

PARTICIPANTS

We studied 67 patients with PD and 20 healthy subjects using positron emission tomography with [(11)C](+/-)dihydrotetrabenazine (DTBZ).

RESULTS

Regional values of DTBZ binding potential (calculated as maximum specific binding [B(max)] divided by the equilibrium dissociation constant K(d)) against disease duration in the putamen of PD patients were best described by a multivariate exponential model with distinct parallel asymptotic values that were significantly (P<.001) different across 4 regions of the putamen. The extent of loss of DTBZ binding potential with disease progression during the clinical stage of PD (early vs late PD) was similar between the anterior (-33%, using early PD as the baseline) and posterior (-29%) putamen. In contrast, the extent of loss of DTBZ binding potential in early PD, which reflects the cumulated loss of DTBZ binding potential from the onset of the disorder (in healthy subjects vs those with early PD), was significantly (P<.001) lower in the posterior (-58%, using healthy subjects as the baseline) than the anterior (-42%) putamen.

CONCLUSION

To the extent that DTBZ positron emission tomography provides an accurate estimate of loss of dopamine neurons, our findings suggest that the mechanisms responsible for the progression of PD may not be the same as those responsible for its onset.

Identification of tissue-restricted transcripts in human islets

The purpose of our study was to identify transcripts specific for tissue-restricted, membrane-associated proteins in human islets that, in turn, might serve as markers of healthy or diseased islet cell masses. Using oligonucleotide chips, we obtained gene expression profiles of human islets for comparison with the profiles of exocrine pancreas, liver, and kidney tissue. As periislet presence of type 1 interferon is associated with the development of type 1 diabetes, the expression profile of human islets treated ex vivo with interferon-alpha2beta (IFNalpha2beta) was also determined. A set of genes encoding transmembrane- or membrane-associated proteins with novel islet-restricted expression was resolved by determining the intersection of the islet set with the complement of datasets obtained from other tissues. Under the influence of IFNalpha2beta, the expression levels of transcripts for several of the identified gene products were up- or down-regulated. One of the islet-restricted gene products identified in this study, vesicular monoamine transporter type 2, was shown to bind [3H]dihydrotetrabenazine, a ligand with derivatives suitable for positron emission tomography imaging. We report here the first comparison of gene expression profiles of human islets with other tissues and the identification of a target molecule with possible use in determining islet cell masses.

Assessment of neuroimaging techniques as biomarkers of the progression of Parkinson's disease

A major goal of research in Parkinson's disease (PD) has been the development of treatments to slow the progressive degeneration of the nigrostriatal dopaminergic system and to reduce the functional decline of patients. Because of the uncertainty in the ability of the clinical evaluation to reflect the status of the nigrostriatal dopaminergic system once dopaminergic therapy has commenced, investigators in PD have sought to develop alternative measures of disease. One approach, which has been extensively explored, is neuroimaging with radiotracers that interact with processes central to dopaminergic neurotransmission in the nigrostriatal dopaminergic axons-conversion of levodopa to dopamine through aromatic amino acid decarboxylase (AADC), [(18)F]fluorodopa PET, storage of dopamine in synaptic vesicles via the vesicular monoamine transporter 2 (VMAT2), (+)-[(11)C]dihydrotetrabenazine PET, and reuptake of dopamine into axons via the dopamine transporter (DAT), [(123)I]beta-CIT SPECT, and a number of other PET and SPECT ligands. During the 54(th) Annual Meeting of the American Academy of Neurology, a group of investigators active in the fields of biomakers, neuroimaging, and neuroprotection met to review the three techniques mentioned above. Prior to the meeting, the participants developed consensus on a set of 10 criteria for a neuroimaging technique to be considered adequate as a biomarker for progression of PD and levels at which the available data for each technique indicate that the criterion was met. The criteria and each of the three imaging techniques mentioned above were reviewed, and the results of that meeting are presented.

Raclopride studies of dopamine release: dependence on presynaptic integrity

BACKGROUND

Raclopride and dopamine (DA) compete for in vivo binding to the D(2) receptors. Thus, measurements of raclopride binding provide a method to evaluate endogenous release in a variety of conditions. Amphetamine elicits DA release, provoking a rapid increase in synaptic DA and leading to a reduction in raclopride binding, which outlasts the temporary increase in extracellular DA concentrations by several hours. The mechanism responsible for the decrease in raclopride binding is still unclear.

METHODS

We used a multiple ligand concentration receptor assay method in normal monkeys and in monkeys with varying degrees of lesion of the DA nigrostriatal terminals to measure the density and affinity of D(2) receptors after methamphetamine challenge.

RESULTS

The reduced raclopride binding can be accounted for by a decreased affinity of the ligand to the receptors. There is a direct, nonlinear relationship between the presynaptic storage capacity and the change in raclopride binding after methamphetamine.

CONCLUSIONS

This observation may bear important implications for the understanding of diseases such as schizophrenia in which the marked increase in amphetamine-induced displacement of raclopride compared with normal control subjects suggests increased release of DA from presynaptic stores and potential abnormalities in presynaptic DA function.

Decreased limbic vesicular monoamine transporter 2 in a genetic rat model of depression

Psychopharmacological and neurochemical research suggests that alterations in monoamine transporters may be involved in the etiology of depression. We studied the expression of the brain-type vesicular monoamine transporter (VMAT2) in the Flinders sensitive line (FSL) rats, which represent a genetic animal model for clinical depression in humans. VMAT2 expression at the protein level was assessed by autoradiography using [(3)H]dihydrotetrabenazine ([(3)H]TBZOH) binding, in the prefrontal cortex, the striatum and its subregions, nucleus accumbens (NAC), ventral tegmental area (VTA) and the substantia nigra pars compacta (SNC). The VMAT2 mRNA level was analyzed by in situ hybridization, in the VTA, SNC and the dorsal raphe (DR) nucleus. Reduced levels of [(3)H]TBZOH binding were detected in the striatum and its subregions, NAC shell but not in the NAC core. A marked reduction of 21% was found in the VTA while only a slight reduction (13%) was observed in the SNC. The reduced levels of VMAT2 protein binding capacity were not accompanied by a parallel alteration in VMAT2 mRNA levels in the VTA, the SNC and the DR. Since the VMAT2 is responsible for the intracellular storage and regulated release of monoamines, the reduced [(3)H]TBZOH binding levels in limbic brain regions of FSL rats may imply a reduced density of vesicular monoamine transporters, which can result in reduced monoamine transmission. Such reduction in the limbic neurotransmission, especially in NAC shell and VTA regions, may be involved in the depressive features of anhedonia and lack of motivation reported in the FSL rats.

Lack of effect of testosterone and dihydrotestosterone compared to 17beta-oestradiol in 1-methyl-4-phenyl-1,2,3,6, tetrahydropyridine-mice

Previous work from our laboratory has demonstrated prevention of 1-methyl-4-phenyl-1,2,3,6, tetrahydropyridine (MPTP)-induced striatal dopamine depletion in C57Bl/6 mice by 17beta-oestradiol, progesterone and raloxifene. The activity of androgenic compounds in MPTP mice has received less attention and was the object of the present investigation. The effects of 17beta-oestradiol (2 microg/day), testosterone (100 microg/day) and dihydrotestosterone (DHT) (2 microg/day or 100 microg/day) were studied during 5 days before and after an acute treatment of four MPTP (10 mg/kg) injections in male C57Bl/6 mice. Striatal concentrations of dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid were measured by high-performance liquid chromatography. MPTP mice treated with saline showed large decreases in dopamine and its metabolites compared to control mice. 17beta-oestradiol partially spared this decrease whereas testosterone and DHT did not. Striatal specific binding to the dopamine transporter (DAT) and to the vesicular monoamine transporter (VMAT2) were measured using [125I] RTI-121 and [3H] dihydrotetrabenazine autoradiography, respectively. As with striatal dopamine concentrations, MPTP treatment caused a decrease in DAT and VMAT2 specific binding. 17beta-oestradiol partially spared this decrease, whereas androgens did not. In the substantia nigra, DAT mRNA was measured by in situ hybridization. MPTP treatment induced a significant, but smaller decrease in substantia nigra DAT mRNA than striatal DAT protein. In addition, 17beta-oestradiol completely prevented the MPTP-induced decrease of DAT mRNA, whereas androgens did not. The present results show that androgens are unable to protect against MPTP-induced dopaminergic toxicity.

Increased platelet vesicular monoamine transporter density in adult schizophrenia patients

Vesicular monoamine transporter (VMAT2) plays a major role in the synaptic accumulation and release of monoamines. In the present study, we assessed high affinity [(3)H]dihydrotetrabenazine binding to platelet VMAT2 in a group of treated (n = 9) and untreated (n = 4) patients with schizophrenia and age- and sex-matched controls. Significantly elevated platelet VMAT2 density (B(max)) (53%, P<0.0001) was observed in the mixed population of schizophrenia patients. The affinity of the ligand (K(d)) to platelet VMAT2 was similar in both groups. The increased platelet VMAT2 density may indicate a schizophrenia-related hyperactivity of the monoaminergic system or an adaptive response to chronic drug treatment.

Chronic clozapine, but not haloperidol, treatment affects rat brain vesicular monoamine transporter 2

We compared the effect of chronic clozapine and haloperidol treatment on the expression of rat brain vesicular monoamine transporter (VMAT(2)) as well as on the membranal presynaptic transporters for serotonin, dopamine and noradrenaline. Rats were treated for 21 days with clozapine (25 mg/kg), haloperidol (0.5 mg/kg) or saline. VMAT(2) expression was assessed on the protein level by high affinity [3H]dihydrotetrabenazine binding using autoradiography, and on the mRNA level by in situ hybridization. The densities of the monoamine transporters were evaluated by autoradiography using specific ligands. Clozapine administration led to an increase in [3H]TBZOH binding in the nucleus accumbens, prefrontal cortex and striatum, whereas haloperidol had no effect on VMAT(2) binding capacity. The clozapine-induced increase in VMAT(2) was accompanied by a parallel increase in the membrane serotonin transporter in the prefrontal cortex and the striatum. Haloperidol induced an increase in the serotonin transporter in the striatum and the core of the nucleus accumbens. The special effect of clozapine on VMAT(2) expression may be relevant to its unique therapeutic advantages.

In vivo measures of dopaminergic radioligands in the rat brain: equilibrium infusion studies

The application of an equilibrium infusion method for measuring specific in vivo radioligand binding in the conscious rat brain was evaluated for two ligands of the dopaminergic system, (+)-alpha-[(11)C]dihydrotetrabenazine (DTBZ) and d-threo-[(11)C]methylphenidate (MePhen). Both radioligands can be successfully utilized to reach equilibrium distributions in rat brain within 1 h; combinations of tritiated and carbon-11-labeled radiotracers can furthermore be used to obtain simultaneous measures of the neuronal membrane dopamine transporter (using [(3)H]MePhen) and vesicular monoamine transporter (using [(11)C]DTBZ) in the same animal. These studies provided quantitative measures of distribution volume ratios, which represent specific radioligand binding. Stereospecificity of in vivo binding was demonstrated using equilibrium infusions of the low-affinity isomers of each ligand, (-)-alpha-[(11)C]dihydrotetrabenazine (DTBZ) and l-threo-[(11)C]methylphenidate, both of which produced uniform brain distributions and no specific binding. Specific binding of (+)-alpha-[(11)C]dihydrotetrabenazine was blocked by co-infusion of tetrabenazine, but was unaffected by administration of methylphenidate, haloperidol, or apomorphine. Specific binding of d-threo-[(11)C]methylphenidate, conversely, was blocked with unlabeled methylphenidate but not affected by tetrabenazine or the dopamine receptor ligands. Equilibrium measures of in vivo radioligand binding, as utilized in this study, offer a quantitative means to evaluate acute and chronic drug effects on in vivo radioligand binding in the rat brain.

Phencyclidine increases vesicular dopamine uptake

Phencyclidine (PCP) rapidly (within 1 h) increased vesicular dopamine uptake and binding of the vesicular monoamine transporter-2 (VMAT-2) ligand, dihydrotetrabenazine. Uptake returned to basal values 3 h in the striatum after a high-dose administration of this drug (15 mg/kg i.p.). In contrast, a similar pretreatment with another non-competitive NMDA receptor antagonist, dizocilpine;([5R,10S]-[+]-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine; MK-801; 1 mg/kg, i.p.), was without effect on vesicular dopamine uptake. Pretreatment with the dopamine D2 receptor antagonist, eticlopride, blocked the increase in vesicular dopamine uptake caused by PCP administration. These data demonstrate a heretofore unreported mechanism that may contribute to the ability of PCP to influence dopamine neuronal function and exert its pharmacological effects.

Decreased densities of dopamine and serotonin transporters and of vesicular monoamine transporter 2 in severely kainic acid lesioned subregions of the striatum

Fifteen days after a striatal kainic acid (KA) injection, we have examined presynaptic modifications of dopamine and serotonin terminals in the striatum through (i) autoradiographic labeling of dopamine, serotonin and vesicular monoamine transporters respectively with 3H-mazindol, 3H-citalopram and 3H-dihydrotetrabenazine, and (ii) determination of the contents in dopamine, serotonin and their metabolites. Acetylcholinesterase histochemical labeling enabled the definition of severely and moderately KA-lesioned subregions within the striatum. A significant decrease of the three transporters labeling density was observed only in the severely lesioned subregions. The strong decrease in serotonin transporter labeling revealed here has not been described until now. Besides, the striatal contents of homovanillic acid (dopamine metabolite) and 5-hydroxyindolacetic acid (serotonin metabolite) were significantly increased in the lesioned striatum. The whole data evidence an incomplete sparing of dopamine and serotonin terminals in the striatum 15 days after a KA injection, especially in the areas where the degeneration of postsynaptic neurons was the most extensive.

Characterization of high-affinity [3H]TBZOH binding to the human platelet vesicular monoamine transporter

The present study indicates that human platelets can be used as an accessible peripheral model not only for the plasma membrane serotonin transporter, but also for the vesicular monoamine transporter. The vesicular monoamine transporter (VMAT2) is responsible for the accumulation of monoamines in the synaptic vesicles. VMAT2 differs from the plasma membrane transporters in its capability to recognize serotonin, histamine, norepinephrine and dopamine with almost the same affinity. Dihydrotetrabenazine (TBZOH) is a very potent inhibitor of VMAT2 that binds with high affinity to this transporter. [3H]TBZOH has been used as a ligand to label VMAT2 in human, bovine and rodent brain. In this study we characterized the pharmacodynamic and pharmacokinetic parameters of [3H]TBZOH binding in human platelets as compared to rat brain. The density (Bmax) and affinity (Kd) of [3H]TBZOH specific binding was assessed by Scatchard analysis. Association and dissociation rate constants (k(on), K(off)) were assessed by kinetic binding studies. In this study high-affinity and saturable binding sites for [3H]TBZOH were demonstrated in human platelets. Both the affinity of [3H]TBZOH to its binding site in platelets (Kd = 3.2+/-0.5 nM) and the kinetic rate constants (K(on) = 2.8 x 10(7) M(-1) min(-1); K(off) = 0.099 min(-1)) were similar to that in rat brain (Kd(striatum) = 1.5 nM; Kd(cerebral cortex) = 1.35 nM; K(on) = 2 x 10(7) M(-1) min(-1); K(off) = 0.069 min(-1)). Only the VMAT2 blockers tetrabenazine and reserpine inhibited [3H]TBZOH specific binding.

Cocaine-induced increases in vesicular dopamine uptake: role of dopamine receptors

The vesicular monoamine transporter-2 is the sole transporter responsible for sequestration of monoamines, including dopamine (DA), into synaptic vesicles. Previous studies demonstrate that agents that inhibit DA transporter function, such as cocaine, increase vesicular [(3)H]DA uptake and binding of the ligand [(3)H]dihydrotetrabenazine ([(3)H]DHTBZ), as assessed in vesicles prepared from treated rats. The present studies examine the role of DA receptors in these cocaine-induced effects. Results demonstrate that administration of the D(2) DA receptor antagonist, eticlopride, but not the D(1) DA receptor antagonist, SCH23390, inhibited these cocaine-induced increases. Similar to the effects of cocaine, treatment with the D(2) agonist, quinpirole, increased both vesicular [(3)H]DA uptake and [(3)H]DHTBZ binding. In contrast, administration of the D(1) agonist, SKF81297, was without effect on vesicular [(3)H]DA uptake or [(3)H]DHTBZ binding. Finally, coadministration of quinpirole and cocaine did not further increase vesicular [(3)H]DA uptake or [(3)H]DHTBZ binding when compared with treatment with either agent alone. These data suggest that cocaine-induced increases in vesicular DA uptake and DHTBZ binding are mediated by a D(2) receptor-mediated pathway. Furthermore, results indicate that D(2) receptor activation, per se, is sufficient to increase vesicular DA uptake.

Absolute configuration of (+)-alpha-dihydrotetrabenazine, an active metabolite of tetrabenazine

Chiral column liquid chromatography and enantiospecific enzymatic hydrolysis were utilized to separate the enantiomers of alpha- and beta-dihydrotetrabenazine and alpha-9-O-desmethyldihydrotetrabenazine, three benzo[a]quinolizines derived from the amine-depleting drug tetrabenazine. An X-ray crystal structure analysis of (-)-alpha-9-O-desmethyldihydrotetrabenazine gave an absolute structure of that compound as the 2S, 3S, 11bS isomer. Therefore, (-)-alpha-dihydrotetrabenazine also has the 2S, 3S, 11bS absolute configuration. (+)-alpha-Dihydrotetrabenazine, the single biologically active isomer from the metabolic reduction of tetrabenazine, thus has the absolute configuration of 2R, 3R, 11bR. For further in vitro and in vivo studies of the vesicular monoamine transporter, it is now possible to use the single enantiomer of radiolabeled alpha-dihydrotetrabenazine.

Methamphetamine rapidly decreases vesicular dopamine uptake

Vesicular sequestration is important in the regulation of cytoplasmic concentrations of monoamines such as dopamine. Moreover, recent evidence suggests that increases in cytoplasmic dopamine levels, perhaps attributable to changes in vesicular monoamine transporter function, contribute to methamphetamine-induced dopaminergic deficits. Hence, we examined whether striatal vesicular uptake is altered following methamphetamine treatment. Multiple administrations of methamphetamine rapidly (within 1 h) decreased vesicular dopamine uptake and dihydrotetrabenazine binding, an effect that (a) persisted at least 24 h, (b) was associated with dopamine and not serotonin neurons, and (c) was unrelated to residual drug introduced by the original methamphetamine treatment. These data suggest that methamphetamine rapidly decreases vesicular monoamine transporter function in dopaminergic neurons, a phenomenon that may be associated with the long-term damage caused by this stimulant.

Reserpine or chronic paroxetine treatments do not modify the vesicular monoamine transporter 2 expression in serotonin-containing regions of the rat brain

To date, very little information is available about the regulation of vesicular monoamine transporter in central serotonergic regions. The expression of the vesicular monoamine transporter 2 (VMAT2) has been studied in the serotonergic system of the rat brain after an 18 day treatment with the serotonin selective-reuptake inhibitor paroxetine (10 mg/kg, i.p., once daily). This treatment, while increasing serotonergic transmission, did not modify either VMAT2 mRNA expression or (3)H-dihydrotetrabenazine binding site density in any of the studied regions. These results suggest that VMAT2 regulation in the central serotonergic system is not involved in the mechanism of action of antidepressants. In addition, a single administration of reserpine (5 mg/kg, s.c.), while blocking the vesicular monoamine uptake function, had no effect on VMAT2 immunoreactivity in the dorsal raphe nucleus 2 or 30 days after injection. It is concluded that neither a reduction (reserpine) nor an enhancement (paroxetine) of the serotonin transmission induces VMAT2 regulation in serotonergic system in the rat brain.

Analysis of four dopaminergic tracers kinetics using two different tissue input function methods

The integrity of the dopaminergic system can be studied using positron emission tomography. The presynaptic tracers [11C]-methylphenidate and [11C]dihydrotetrabenazine (DTBZ) are used to investigate the dopamine transporter availability, the dopamine vesicular transporter integrity; the postsynaptic tracers [11C]-raclopride and [11C]-Schering 23990 (SCH) are used to probe the D2 and D1 receptors. These are reversible tracers, where the binding potential (BP) = Bmax/Kd often is used to quantify the amount of their specific binding to the sites of interest. The simplified tissue input methods to calculate BP are attractive, since they do not require a blood input function. The suitability and performance of two such methods were evaluated: the Logan graphical tissue method, and the Lammertsma reference tissue method (RTM). The BP estimates obtained with the two methods were nearly identical in most cases, with similar reliability and reproducibility indicating that all four tracers satisfy the assumptions required by each method. The correlations among the fitted parameters obtained from the RTM were estimated and were found not to introduce noticeable bias in the RTM BP and R1 estimates. R1 showed low intersubject and intrasubject variability. The k2 estimate showed good reliability for SCH with cerebellar input function and DTBZ with occipital input function.

The effect of polychlorinated biphenyls on the uptake of dopamine and other neurotransmitters into rat brain synaptic vesicles

Studies have shown that polychlorinated biphenyls may affect cognitive functions both in human and also in experimental animals. One of the neurochemical parameters that is changed after exposure to these compounds is a reduction in the dopamine level in the brain, although the mechanism behind this reduction is not known. We have therefore investigated whether this reduction could be caused by an effect on vesicular uptake. ortho-Chlorinated biphenyls are found to be competitive inhibitors of dopamine transport into synaptic vesicles from rat brain with K(i) concentrations as low as 4 microM. In contrast, several nonortho-chlorinated biphenyls did not inhibit vesicular uptake. The inhibition was specific for dopamine, in that the uptake of glutamate and GABA was inhibited at higher PCB concentrations under identical conditions. The vesicular Mg-ATPase proton pump was also inhibited at higher concentrations of PCBs than the dopamine transport. Uptake of methylamine gave no indication of any disruption of the vesicular proton gradient. The inhibition of dopamine vesicular uptake by PCBs was competitive. Several of the ortho-PCBs also inhibited the binding of tetrabenazine, which is known to bind to a site close to the dopamine binding site, at the vesicular transporter. The results show that inhibition of vesicular uptake may contribute to the decrease of dopamine reported in nervous tissue after exposure to PCBs under different conditions.

Decreased striatal monoaminergic terminals in multiple system atrophy detected with positron emission tomography

We examined the density of striatal presynaptic monoaminergic terminals, using a ligand for the type 2 vesicular monoamine transporter, (+)-[11C]dihydrotetrabenazine, with positron emission tomography in 7 normal control subjects, 8 multiple system atrophy (MSA) patients with predominantly parkinsonian features (MSA-P), 8 MSA patients with principally cerebellar dysfunction (MSA-C), and 6 sporadic olivopontocerebellar atrophy (sOPCA) patients. The findings were correlated with the results of neurological evaluations and magnetic resonance imaging studies. Specific binding was significantly reduced in the putamen of all patient groups in the order MSA-P < MSA-C < sOPCA, compared with controls. Mean blood-to-brain ligand transport (K1) was significantly decreased in the putamen of all patient groups and in the cerebellar hemispheres of MSA-C and sOPCA but not MSA-P groups, compared with controls. Significant negative correlations were found between striatal binding and the intensity of parkinsonian features and between cerebellar K1 and the intensity of cerebellar dysfunction. The results suggest fundamental differences between MSA-P and MSA-C groups reflecting differential severity of degeneration of nigrostriatal and cerebellar systems in these two forms of MSA. The findings also show that some sOPCA patients have subclinical nigrostriatal dysfunction and are at risk of developing MSA with disease progression.

[125I]EGF binding in basal ganglia of patients with Parkinson's disease and progressive supranuclear palsy and in MPTP-treated monkeys

Since EGF is known to protect and stimulate the activity of dopaminergic neurons, an autoradiographic study of [125I]EGF binding sites was performed in the striatum and pallidal complex in parkinsonian syndromes. The analysis was performed on postmortem brain tissues of three control subjects, three patients with Parkinson's disease, and three patients with progressive supranuclear palsy, another parkinsonian syndrome in which dopaminergic neurons also degenerate. Since all six patients had been treated with L-Dopa, we also analyzed the effects of this drug in an animal model of Parkinson's disease. Quantitative analysis of [125I]EGF binding was performed on the brains of three control monkeys, nine monkeys rendered parkinsonian by MPTP intoxication, three of which were treated with L-Dopa. An increased density of [125I]EGF binding was observed at anterior levels in the dorsal striatum, but not in the pallidum, of patients with Parkinson's disease and progressive supranuclear palsy. [125I]EGF binding was unchanged in parkinsonian monkeys whether or not they had been treated with L-Dopa. The data suggest an increased expression of EGFRs in the striatum in chronic parkinsonian syndromes but not in acute models of the disease.

Transient expression of the vesicular monoamine transporter during development in the rat thalamus and cortex

The postnatal developmental pattern of the central vesicular monoamine transporter-2 (VMAT2) was analyzed in the rat brain by means of quantitative autoradiography with a specific and high affinity ligand [3H]dihydrotetrabenazine ([3H]TBZOH). We show a dense expression of VMAT2 in the cortex (especially area 17) and thalamus (particularly the dorsal lateral geniculate nucleus) at postnatal days 1 and 8. This pattern of VMAT2 distribution was transient since it was no longer observed at day 20 or in the adult rat brain where VMAT2 density was weak and uniform in these regions. These data suggest that monoamine vesicular storage participates in the early postnatal maturation of thalamus and cortex.

Regulation of rat brain vesicular monoamine transporter by chronic treatment with ovarian hormones

Ovarian steroids play an important role in neuroregulation and in the pathophysiology of various neuropsychiatric disorders. Most of the studies focused on the impact of gonadal steroids on post-synaptic receptors and plasma membrane transporters. In the present study, we evaluated the effect of chronic treatment with ovarian steroids on the expression of rat brain vesicular monoamine transporter (VMAT2). Ovariectomized rats were treated for 21 days with estradiol, progesterone or both. VMAT2 gene expression was assessed on the protein level by high affinity [3H]dihydrotetrabenazine ([3H]TBZOH) binding using autoradiography and on the mRNA level by in situ hybridization. Progesterone administration led to a decrease in [3H]TBZOH binding in the middle striatum and in the nucleus accumbens and to a parallel decrease in VMAT2 mRNA level in the substantia nigra pars compacta and dorsal raphè nuclei. Chronic estradiol treatment reduced VMAT2 mRNA level in the dorsal raphè and [3H]TBZOH binding in middle part of the striatum and nucleus accumbens but did not affect VMAT2 mRNA level in the substantia nigra pars compacta. Simultaneous administration of both ovarian steroids did not modulate VMAT2 mRNA in the substantia nigra pars compacta as well as [3H]TBZOH binding in the striatum or the nucleus accumbens but reduced VMAT2 mRNA level in the dorsal raphè. It appears that ovarian steroids may play a crucial role in the regulation of VMAT2 gene expression in the dopamine and serotonin systems. This modulatory activity may be relevant to synaptic and neuronal plasticity as well as to the molecular and cellular pathophysiology of gender-specific neuropsychiatric disorders.

High-efficiency expression and characterization of the synaptic-vesicle monoamine transporter from baculovirus-infected insect cells

The full-length cDNA for the rat synaptic-vesicle monoamine transporter (VMAT2) containing a C-terminal polyhistidine epitope has been engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) insect cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed at levels of 7.8x10(6) transporters per cell, as assessed by [3H]dihydrotetrabenazine binding. A 1l culture of infected cells produced approx. 15 nmol (900 microg) of transporter. rVMAT2 expressed in the Sf9 cells bound [3H]dihydrotetrabenazine with a KD of 31.2 nM and a Bmax of 19.9 pmol/mg. Two polypeptides of 55 and 63 kDa were identified using the photolabel, 7-azido-8-[125I]iodoketanserin ([125I]AZIK). Photoaffinity labelling of rVMAT2 by 1 nM [125I]AZIK was protectable by 10 microM tetrabenazine and 10 microM 7-aminoketanserin. Digitonin-solubilized VMAT2 was purified to greater than 95% homogeneity using immobilized Ni2+-affinity chromatography, followed by lectin (Concanavalin A) chromatography. The purified transporter migrates as a single broad band with a molecular mass of approx. 63kDa, as analyzed by SDS/PAGE. The purified transporter retained the ability to bind ligands ([125I]AZIK and [3H]dihydrotetrabenazine). The purified VMAT2 bound [3H]dihydrotetrabenazine with a KD of 86.2 nM. As is the case with the monoamine transporter from bovine chromaffin granule membranes, purified VMAT2 is covalently modified by dicyclohexylcarbodi-imide (DCCD) and is specifically labelled by [14C]DCCD. This labelling is inhibited by tetrabenazine and ketanserin. These data indicate that VMAT2 can be overexpressed using the baculovirus expression system and purified.

Peptide mapping of the [125I]Iodoazidoketanserin and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine binding sites for the synaptic vesicle monoamine transporter

The full-length cDNA for the rat recombinant synaptic vesicle monoamine transporter (rVMAT2) containing a COOH-terminal polyhistidine epitope was engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed to high levels and purified to >95% homogeneity using immobilized Ni2+-affinity chromatography followed by lectin (concanavalin A) chromatography. Purified transporter was photolabeled using [125I]-7-azido-8-iodoketanserin ([125I]AZIK) and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine ([125I]TBZ-AIPP). Both [125I]AZIK and [125I]TBZ-AIPP photoaffinity labeling of purified rVMAT2 were protectable by 10 microM tetrabenazine (TBZ), 10 microM 7-aminoketanserin, and 1 mM concentrations of the transporter substrates dopamine, norepinephrine, and serotonin. Radiolabeled peptides were generated using enzymatic and chemical methods, purified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal microsequenced. Radiosequencing of [125I]AZIK-labeled rVMAT2 indicated derivatization of Lys-20 in the NH2 terminus, just prior to putative transmembrane domain 1 (TMD1). [125I]TBZ-AIPP derivatized a segment of rVMAT2 between Gly-408 and Cys-431 in TMD10 and 11. These data implicate juxtaposition of TMD1 and 10/11.

Reduced striatal vesicular monoamine transporters after neurotoxic but not after behaviorally-sensitizing doses of methamphetamine

Prior studies indicate long-term reductions of striatal dopaminergic markers after sustained, high dose methamphetamine exposures in vivo, suggesting a neurotoxic effect. We have reported lack of regulation of vesicular monoamine transporter type-2 expression, as opposed to other markers of striatal dopaminergic terminals, under conditions that alter dopaminergic transmission without synaptic terminal losses. In the present study, we evaluated the vesicular monoamine transporter and the neuronal membrane dopamine transporter in rat striata after in vivo exposure to neurotoxic or to intermittent, low dose (behaviorally-sensitizing, non-neurotoxic) methamphetamine administrations. Vesicular monoamine transporter binding was measured by autoradiography of (+)-[3H]dihydrotetrabenazine, the active isomer of (+/-)-[3H]dihydrotetrabenazine. (+)-Dihydrotetrabenazine bound to a homogeneous population of striatal sites in controls with a Kd of 1.5 nM and a Bmax of 3.8 fmol/microg protein. Neurotoxic methamphetamine treatment reduced both striatal vesicular monoamine transporter (-26%) and dopamine transporter (-39%) bindings. There were no changes after the non-neurotoxic treatment regimen. The vesicular monoamine transporter may thus be a valuable marker in the further clinical study of psychostimulant drug neurotoxicity.

In vivo binding of (+)-alpha-[3H]dihydrotetrabenazine to the vesicular monoamine transporter of rat brain: bolus vs. equilibrium studies

The regional rat brain distribution of (+)-alpha-[3H]dihydrotetrabenazine was determined following (a) infusion to equilibrium between brain and blood or (b) bolus injection. Infusions provide for direct measurement of total distribution volumes. The free plus nonspecific distribution volume for the brain was determined using infusion of very low specific activity (+)-alpha-[3H]dihydrotetrabenazine; specific distribution volumes, which represent specific radioligand binding, were then calculated as total minus the free + nonspecific distribution volume. Both total and specific distribution volumes correlated very well (r2 > 0.99) with in vitro distributions of the vesicular monoamine transporter binding site. Bolus injection, and measurement of radioactivity at a single time point, also provided regional estimates of radioligand binding which correlated well (r2 > 0.98) with in vitro values. The bolus method shows a small positive bias (+ 10-15%) in regions of high binding site concentrations. Both infusion and bolus injection methods give acceptable in vivo measures of (+)-alpha-[3H]dihydrotetrabenazine binding to the vesicular monoamine transporter of rat brain.

Striatal dopamine nerve terminal markers but not nigral cellularity are reduced in spinocerebellar ataxia type 1

We previously reported markedly reduced (-76%) dopamine (DA) levels in the putamen of seven patients with spinocerebellar ataxia type 1 (SCA1) who had no evidence of nigral cell loss or parkinsonism. To determine whether the DA reduction was accompanied by loss of DA nerve terminals, we measured levels of the DA transporter ([3H]WIN, 35,428 binding; DA transporter protein) and the vesicular monoamine transporter ([3H]DTBZ binding) in the putamen of these patients. As compared with the controls (n = 14), mean putamen concentrations of [3H]WIN 35,428 binding (-45%), dopamine transporter protein (-61%), and [3H]DTBZ binding (-48%) were significantly reduced in this SCA1 subgroup. We conclude that the degeneration in nigrostriatal DA neurons begins at the nerve ending in SCA1.

Effects of nerve growth factor on dihydrotetrabenazine binding to PC12 cells

Tetrabenazine and dihydrotetrabenzaine (TBZOH) are potent inhibitors of substrate transport by the predominant forms of the vesicular monoamine transporter (VMAT) present in bovine brain synaptic vesicles and bovine adrenal medullary chromaffin vesicles. Radiolabeled TBZOH binds to these preparations with apparent dissociation constants in the low nanomolar range. However, tetrabenazine is a much less potent inhibitor of transport by rVMAT1, a form of the transporter cloned from a rat pheochromocytoma (PC12) cDNA library and expressed in CHO cells. Reported attempts to observe binding of [3H]TBZOH to rVMAT1 have not been successful. We examined binding of [3H]TBZOH to a crude membrane fraction from PC12 cells. Computerized nonlinear least squares curve fitting revealed two classes of binding sites (Kd1 = 1.5 nM, R1 = 0.2 pmol/mg protein, Kd2 = 340 nM, R2 = 15.2 pmol/mg protein). While the identity of the higher affinity sites is not certain, their high affinity for TBZOH suggests that they may be associated with rVMAT2. The lower affinity sites are likely to be associated with rVMAT1 on the basis of their affinity for TBZOH and sensitivity to inhibition of TBZOH binding by transporter substrates and inhibitors. NGF-treated PC12 cells also exhibited two classes of sites (Kd1 = 1.9 nM, R1 = 0.18 pmol/mg protein; Kd2 = 370 nM, R2 = 23.7 pmol/mg protein). While there were no significant differences between control and NGF-treated cells in binding capacity of the higher affinity sites, nor in apparent dissociation constants for either class of sites, there was a highly significant increase in number of lower affinity binding sites in the NGF-treated cells (p = 0.001). These results provide direct evidence that the differential sensitivity of rat brain and adrenal catecholamine stores to depletion by tetrabenazine and its derivatives is due to a much lower affinity of rVMAT1 for these compounds, and that NGF treatment may increase levels of rVMAT1 expression in PC12 cells.

Time-dependent recovery of in vivo binding sites after drug dosing: a method for radiotracer evaluation

The recovery of in vivo binding sites for (+/-)-alpha-[11C] methoxytetrabenazine, a radioligand for the monoamine vesicular transporter (VMAT2), was determined in mouse brain at various times following a pharmacological dose of tetrabenazine. Concentrations of in vivo radioligand binding sites progressively increased and had reached control values by 8.5 h, and this recovery was consistent with the pharmacokinetics of the competing drug tetrabenazine and its active metabolite, dihydrotetrabenazine. This study demonstrates a simple experimental protocol of using a single dose of a reversible competing drug and time-dependent measurements of in vivo binding of a radioligand. This protocol is suitable for testing the sensitivity of an in vivo radiotracer for measurement of varying concentrations of in vivo binding sites.