Reproducibility studies with 11C-DTBZ, a monoamine vesicular transporter inhibitor in healthy human subjects.
The reproducibility of (+/-)-alpha-[11C] dihydrotetrabenazine (DTBZ) measures in PET was studied in 10 healthy human subjects, aged 22-76 y. METHODS: The scan-to-scan variation of several measures used in PET data analysis was determined, including the radioactivity ratio (target-to-reference), plasma-input Logan total distribution volume (DV), plasma-input Logan Bmax/Kd and tissue-input Logan Bmax/Kd values. RESULTS: The radioactivity ratios, plasma-input Bmax/Kd and tissue-input Bmax/Kd all have higher reliability than plasma-input total DV values. In addition, measures using the occipital cortex as the reference region have higher reliability than the same measures using the cerebellum as the reference region. CONCLUSION: Our results show that DTBZ is a reliable PET tracer that provides reproducible in vivo measurement of striatal vesicular monoamine transporter density. In the selection of reference regions for DTBZ PET data analysis, caution must be exercised in circumstances when DTBZ binding in the occipital cortex or the cerebellum may be altered. (+info)
Methamphetamine (METH) is a powerful psychostimulant that is increasingly abused worldwide. Although it is commonly accepted that the dopaminergic system and oxidation of dopamine (DA) play pivotal roles in the neurotoxicity produced by this phenylethylamine, the primary source of DA responsible for this effect has remained elusive. In this study, we used mice heterozygous for vesicular monoamine transporter 2 (VMAT2 +/- mice) to determine whether impaired vesicular function alters the effects of METH. METH-induced dopaminergic neurotoxicity was increased in striatum of VMAT2 +/- mice compared with wild-type mice as revealed by a more consistent DA and metabolite depletion and a greater decrease in dopamine transporter expression. Interestingly, increased METH neurotoxicity in VMAT2 +/- mice was accompanied by less pronounced increase in extracellular DA and indices of free radical formation compared with wild-type mice. These results indicate that disruption of vesicular monoamine transport potentiates METH-induced neurotoxicity in vivo and point, albeit indirectly, to a greater contribution of intraneuronal DA redistribution rather than extraneuronal overflow on mediating this effect. (+info)
Effects of rubidium on behavioral responses to methamphetamine and tetrabenazine.
Different groups of mice were injected subcutaneously every other day with rubidium chloride at three doses (0.41(50), 1.23(150) and 3.69(450) meq/kg (mg/kg)) or with saline as a control for a period of 2-3 weeks. Rubidium administered acutely did not affect spontaneous locomotor activities, while it tended to increase the activities when administered repeatedly though the increase was not statistically significant. The methamphetamine-induced hyperlocomotor activities were potentiated in the rubidium groups as compared with those in the saline group, this effect of ribidium being increased with prolongation of repeated administrations. Monotonic decreases in ambulation after tetrabenazine were not significantly affected in the rubidium-treated animals though the decreases were sometimes preceded by slight increases and recovery from the decrement tended to be more rapid. After tetrabenazine in the rubidium-treated groups, incidences of catalepsy were increased and jumping behavior and Straub tail responses occurred in a few cases. The results suggest that rubidium potentiates the excitatory action of methamphetamine on spontaneous locomotor activities, as contrasted with inhibitory influence of lithium. (+info)
Assessment of extrastriatal vesicular monoamine transporter binding site density using stereoisomers of [11C]dihydrotetrabenazine.
Previous studies have demonstrated the utility of [11C]dihydrotetrabenazine ([11C]DTBZ) as a ligand for in vivo imaging of the vesicular monoamine transporter system. The (+)-isomer has a high affinity (approximately 1 nmol/L) for the vesicular monoamine transporter (VMAT2) binding site, whereas the (-)-isomer has an extremely low affinity (approximately 2 micromol/L). Efforts to model dynamic (+)-[11C]DTBZ data demonstrate the difficulty in separating the specific binding component from the free plus nonspecific component of the total positron emission tomography (PET) measure. The authors' previous PET work, as well as in vitro studies, indicate that there is little specific VMAT2 binding in neocortical regions. However, precise determination of in vivo binding levels have not been made, leaving important questions unanswered. At one extreme, is there sufficient specific binding in cortex or other extrastriate regions to be estimated reliably with PET? At the other extreme, is there sufficiently little binding in cortex so that it can be used as a reference region representing nonsaturable tracer uptake? The authors address these questions using paired studies with both active (+) and inactive (-) stereoisomers of [11C]DTBZ. Six normal control subjects were scanned twice, 2 hours apart, after injections of 16 mCi of (+)- and (-)-[11C]DTBZ (order counter-balanced). Three-dimensional PET acquisition consisted of 15 frames over 60 minutes for each scan. Arterial samples were acquired throughout, plasma counted, and corrected for radiolabeled metabolites. Analysis of specific binding was assessed by comparison of total distribution volume measures from the (+)- and (-)-[11C]DTBZ scans. The authors' findings indicate that only approximately 5% of the cortical signal in (+)-[11C]DTBZ scans results from binding to VMAT2 sites. The strongest extrastriatal signal comes from the midbrain regions where approximately 30% of the PET measure results from specific binding. The authors conclude that (1) the density of VMAT2 binding sites in cortical regions is not high enough to be quantified reliably with DTBZ PET, and (2) binding does appear to be low enough so that cortex can be used as a free plus nonspecific reference region for striatum. (+info)
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. (+info)
In vivo measurement of the vesicular monoamine transporter in schizophrenia.
Given evidence for excessive striatal dopamine activity in schizophrenia, we sought to test the hypothesis that dopaminergic innervation in the striatum is abnormally elevated, and a secondary hypothesis that age-related loss is accelerated. Twelve schizophrenic subjects on stable doses of medications, along with 12 age and sex-matched healthy control subjects, underwent positron emission tomography (PET) studies with [11C]dihydrotetrabenazine (DTBZ), which binds to the vesicular monoamine transporter, type 2 (VMAT2). DTBZ binding reflects principally dopaminergic projections in the striatum and appears in animal models, over treatment periods as long as two weeks, not to be regulated by antipsychotic drugs. Using an equilibrium analysis, we obtained measurements of the binding potential (BP) of [11C]DTBZ, as well as a transport (K(1)) measure, corresponding to regional cerebral blood flow. BP in the striatum showed no difference between the patient and control groups, and no differential effect of age. We did not find evidence supporting the hypothesis that excessive dopamine activity in schizophrenia could be explained by increased density of striatal dopamine terminals. (+info)
Vesicular monoamine transporters heterologously expressed in the yeast Saccharomyces cerevisiae display high-affinity tetrabenazine binding.
A mammalian vesicular neurotransmitter transporter has been expressed in the yeast Saccharomyces cerevisiae. The gene encoding the rat vesicular monoamine transporter (rVMAT(1)) was cloned in several expression plasmids. The transporter was expressed at detectable levels only when short sequences using codons favored by S. cerevisiae were fused preceding the start of translation of rVMAT(1). The scarce expression of the wild-type protein was, most likely, due to the fact that part of the N-terminus of the protein is encoded by codons not preferred in S. cerevisiae. Furthermore, low growth temperatures increased rVMAT(1) expression and altered its processing. Whereas at 30 degrees C the protein is not glycosylated, at lower temperatures ( approximately 16 degrees C) half of the expressed transporters undergo core glycosylation. In addition, under these conditions the levels of protein expression significantly increase. Using a functional chimeric protein composed by VMAT and the green fluorescent protein (GFP), it is shown that the punctate pattern of intracellular distribution remains invariable at the different temperatures. Using a similar fusion sequence, the bovine VMAT isoform 2 (bVMAT(2)) was also expressed in yeast. The yeast-expressed bVMAT(2) binds [(3)H]dihydrotetrabenazine ([(3)H]TBZOH) with the same characteristics found in the native protein from bovine chromaffin granules. Dodecyl maltoside-solubilized bVMAT(2) retains the conformation required for [(3)H]TBZOH binding. We exploited the robust binding to follow the transporter during purification assays on a Ni(2+)-chelating column. In this report we describe for the first time the heterologous expression of a neurotransmitter transporter in the yeast S. cerevisiae. (+info)
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. (+info)