Changes in vesicular monoamine transporter (VMAT2) and synaptophysin in rat Substantia nigra and prefrontal cortex induced by psychotropic drugs

Effects of chronic exposure to haloperidol, olanzapine or lithium on SV2A and NLGN synaptic puncta in the rat frontal cortex

Positron emission tomography studies using the synaptic vesicle glycoprotein 2A (SV2A) radioligand [11C]-UCB-J provide in vivo evidence for synaptic dysfunction and/or loss in the cingulate and frontal cortex of patients with schizophrenia. In exploring potential confounding effects of antipsychotic medication, we previously demonstrated that chronic (28-day) exposure to clinically relevant doses of haloperidol does not affect [3H]-UCB-J radioligand binding in the cingulate and frontal cortex of male rats. Furthermore, neither chronic haloperidol nor olanzapine exposure had any effect on SV2A protein levels in these brain regions. These data do not exclude the possibility, however, that more subtle changes in SV2A may occur at pre-synaptic terminals, or the post-synaptic density, following chronic antipsychotic drug exposure. Moreover, relatively little is known about the potential effects of psychotropic drugs other than antipsychotics on SV2A. To address these questions directly, we herein used immunostaining and confocal microscopy to explore the effect of chronic (28-day) exposure to clinically relevant doses of haloperidol, olanzapine or the mood stabilizer lithium on presynaptic SV2A, postsynaptic Neuroligin (NLGN) puncta and their overlap as a measure of total synaptic density in the rat prefrontal and anterior cingulate cortex. We found that, under the conditions tested here, exposure to antipsychotics had no effect on SV2A, NLGN, or overall synaptic puncta count. In contrast, chronic lithium exposure significantly increased NLGN puncta density relative to vehicle, with no effect on either SV2A or total synaptic puncta. Future studies are required to understand the functional consequences of these changes.

Pharmacologic Characterization of Valbenazine (NBI-98854) and Its Metabolites

The vesicular monoamine transporter 2 (VMAT2) is an integral presynaptic protein that regulates the packaging and subsequent release of dopamine and other monoamines from neuronal vesicles into the synapse. Valbenazine (NBI-98854), a novel compound that selectively inhibits VMAT2, is approved for the treatment of tardive dyskinesia. Valbenazine is converted to two significant circulating metabolites in vivo, namely, (+)-α-dihydrotetrabenazine (R,R,R-HTBZ) and a mono-oxy metabolite, NBI-136110. Radioligand-binding studies were conducted to assess and compare valbenazine, tetrabenazine, and their respective metabolites in their abilities to selectively and potently inhibit [3H]-HTBZ binding to VMAT2 in rat striatal, rat forebrain, and human platelet homogenates. A broad panel screen was conducted to evaluate possible off-target interactions of valbenazine, R,R,R-HTBZ, and NBI-136110 at >80 receptor, transporter, and ion channel sites. Radioligand binding showed R,R,R-HTBZ to be a potent VMAT2 inhibitor in homogenates of rat striatum (Ki = 1.0-2.8 nM), rat forebrain (Ki = 4.2 nM), and human platelets (Ki = 2.6-3.3 nM). Valbenazine (Ki = 110-190 nM) and NBI-136110 (Ki = 160-220 nM) also exhibited inhibitory effects on VMAT2, but with lower potency than R,R,R-HTBZ. Neither valbenazine, R,R,R-HTBZ, nor NBI-136110 had significant off-target interactions at serotonin (5-HT1A, 5-HT2A, 5-HT2B) or dopamine (D1 or D2) receptor sites. In vivo studies measuring ptosis and prolactin secretion in the rat confirmed the specific and dose-dependent interactions of tetrabenazine and R,R,R-HTBZ with VMAT2. Evaluations of potency and selectivity of tetrabenazine and its pharmacologically active metabolites were also performed. Overall, the pharmacologic characteristics of valbenazine appear consistent with the favorable efficacy and tolerability findings of recent clinical studies [KINECT 2 (NCT01733121), KINECT 3 (NCT02274558)].

Vesicular integrity in Parkinson's disease

The defining motor characteristics of Parkinson's disease (PD) are mediated by the neurotransmitter dopamine (DA). Dopamine molecules spend most of their lifespan stored in intracellular vesicles awaiting release and very little time in the extracellular space or the cytosol. Without proper packaging of transmitter and trafficking of vesicles to the active zone, dopamine neurotransmission cannot occur. In the cytosol, dopamine is readily oxidized; excessive cytosolic dopamine oxidation may be pathogenic to nigral neurons in PD. Thus, factors that disrupt vesicular function may impair signaling and increase the vulnerability of dopamine neurons. This review outlines the many mechanisms by which disruption of vesicular function may contribute to the pathogenesis of PD. From direct inhibition of dopamine transport into vesicles by pharmacological or toxicological agents to alterations in vesicle trafficking by PD-related gene products, variations in the proper compartmentalization of dopamine can wreak havoc on a functional dopamine pathway. Findings from patient populations, imaging studies, transgenic models, and mechanistic studies will be presented to document the relationship between impaired vesicular function and vulnerability of the nigrostriatal dopamine system. Given the deleterious effects of impaired vesicular function, strategies aimed at enhancing vesicular function may be beneficial in the treatment of PD.

Altered neuronal markers following treatment with mood stabilizer and antipsychotic drugs indicate an increased likelihood of neurotransmitter release

Objective

Given the ability of mood stabilizers and antipsychotics to promote cell proliferation, we wanted to determine the effects of these drugs on neuronal markers previously reported to be altered in subjects with psychiatric disorders.

Methods

Male Sprauge-Dawley rats were treated with vehicle (ethanol), lithium (25.5 mg per day), haloperidol (0.1 mg/kg), olanzapine (1.0 mg/kg) or a combination of lithium and either of the antipsychotic drugs for 28 days. Levels of cortical synaptic (synaptosomal associated protein-25, synaptophysin, vesicle associated protein and syntaxin) and structural (neural cell adhesion molecule and alpha-synuclein) proteins were determined in each treatment group using Western blots.

Results

Compared to the vehicle treated group; animals treated with haloperidol had greater levels of synaptosomal associated protein-25 (p<0.01) and neural cell adhesion molecule (p<0.05), those treated with olanzapine had greater levels of synaptophysin (p<0.01) and syntaxin (p<0.01). Treatment with lithium alone did not affect the levels of any of the proteins. Combining lithium and haloperidol resulted in greater levels of synaptophysin (p<0.01), synaptosomal associated protein-25 (p<0.01) and neural cell adhesion molecule (p<0.01). The combination of lithium and olanzapine produced greater levels of synaptophysin (p<0.01) and alpha-synuclein (p<0.05).

Conclusion

Lithium alone had no effect on the neuronal markers. However, haloperidol and olanzapine affected different presynaptic markers. Combining lithium with olanzapine additionally increased alpha-synuclein. These drug effects need to be taken into account by future studies examining presynaptic and neuronal markers in tissue from subjects with psychiatric disorders.

Platelet vesicular monoamine transporter 2 density in the disruptive behavior disorders

In a former study, we reported decreased platelet vesicular monoamine transporter 2 (VMAT2) density (Bmax) in patients with ADHD. The current study aimed at measuring platelet VMAT2 in the disruptive behavior disorders (DBDs) to assess whether this finding is specific to ADHD or generalizable to the broader DBD concept. The study included 13 patients with DBDs aged 10-12 years and 16 healthy volunteers aged 8-17 years. All participants underwent a thorough clinical evaluation using Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version for diagnosis, the Nisonger Child Behavior Rating Form, the Clinical Global Impressions Scale-Severity version, and the DSM-IV ADHD Scale (DAS). The study group's DAS scores did not differ from those of the control group. There was no significant difference between the patients with DBDs and the control group either in VMAT2 density (Bmax) or affinity (Kd) as measured by high-affinity [(3)H]TBZOH binding. We conclude that the formerly reported decreased platelet VMAT2 Bmax in patients with ADHD may be specific to ADHD and not present in DBDs. Larger-scale replication is needed.

Altered affinity of the platelet vesicular monoamine transporter 2 to dihydrotetrabenazine in children with major depression

Platelet vesicular monoamine transporter (VMAT2) binding characteristics were assessed, using high affinity dihydrotetrabenazine ([(3)H]TBZOH) binding, in 14 children with major depression (MDD) and 16 matched controls. All participants underwent a thorough diagnostic evaluation and the levels of depression and anxiety were measured. K (d) values were significantly lower in children with MDD versus controls (2.93 ± 0.84 vs. 3.63 ± 0.56 nM, respectively, t = 2.4, df = 18.4, p = 0.025). B (max) values did not differ significantly. This preliminary finding indicates a possible structural change in platelet VMAT2 in children with MDD.

Deglycosylation and subcellular redistribution of VMAT2 in the mesostriatal system during normal aging

The vesicular monoamine transporter type 2 (VMAT2) is a transmembrane glycoprotein responsible for the vesicular monoamine uptake in the brain. This function declines in the dopaminergic mesostriatal system during normal aging, but the mechanisms responsible for this deficit are unknown. We investigated possible age-related changes in the expression and subcellular distribution of VMAT2 in the rat mesostriatal system. VMAT2 is constitutively expressed as glycosylated (75 kDa), partially glycosylated (55 kDa) and native (45 kDa) forms, they are all present in both synaptosomal compartments (synaptosomal membrane and synaptic vesicle-enriched fractions) of the striatal terminals in young rats. In aged rats, no changes were found in midbrain VMAT2mRNA and VMAT2 total protein levels in whole striatal extracts. However, its subcellular distribution and glycosylation pattern were severely modified. The three VMAT2 forms virtually disappeared from the synaptic vesicle-enriched fraction, while the 55 kDa form was accumulated in the soluble compartment. These changes may be responsible for the loss of VMAT2 activity during aging and may contribute to the high susceptibility of aged midbrain dopaminergic cells to degeneration.

PACAP38 increases vesicular monoamine transporter 2 (VMAT2) expression and attenuates methamphetamine toxicity

Pituitary adenylyl cyclase activating polypeptide, 38 amino acids (PACAP38) is a brain-gut peptide with diverse physiological functions and is neuroprotective in several models of neurological disease. In this study, we show that systemic administration of PACAP38, which is transported across the blood-brain barrier, greatly reduces the neurotoxicity of methamphetamine (METH). Mice treated with PACAP38 exhibited an attenuation of striatal dopamine loss after METH exposure as well as greatly reduced markers of oxidative stress. PACAP38 treatment also prevented striatal neuroinflammation after METH administration as measured by overexpression of glial fibrillary acidic protein (GFAP), an indicator of astrogliosis, and glucose transporter 5 (GLUT5), a marker of microgliosis. In PACAP38 treated mice, the observed protective effects were not due to an altered thermal response to METH. Since the mice were not challenged with METH until 28 days after PACAP38 treatment, this suggests the neuroprotective effects are mediated by regulation of gene expression. At the time of METH administration, PACAP38 treated animals exhibited a preferential increase in the expression and function of the vesicular monoamine transporter (VMAT2). Genetic reduction of VMAT2 has been shown to increase the neurotoxicity of METH, thus we propose that the increased expression of VMAT2 may underlie the protective actions of PACAP38 against METH. The ability of PACAP38 to increase VMAT2 expression suggests that PACAP38 signaling pathways may constitute a novel therapeutic approach to treat and prevent disorders of dopamine storage.

Striatal dopaminergic terminals in type 1 and type 2 alcoholics measured with [3H]dihydrotetrabenazine and human whole hemisphere autoradiography

A number of studies have pointed to the importance of dopamine system in the context of alcoholism. Previous studies have shown lower dopamine transporter levels on late-onset Cloninger type 1 alcoholics. However, whether this lower level is due to a lower level of dopamine transporter protein or a lower level of dopaminergic nerve terminals remains unclear. The aim of this study was to compare putative alterations of dopaminergic terminals in caudate, putamen and nucleus accumbens of type 1 and type 2 alcoholics and healthy controls by using [(3)H]dihydrotetrabenazine as a radioligand in postmortem human whole hemisphere autoradiography. We compared the present results with the findings of our earlier studies on the dopamine transporter in these same subjects, demonstrating that alcoholics do not differ significantly from controls in striatal [(3)H]dihydrotetrabenazine binding. Although type 1 alcoholics have been reported to have up to 36% lower striatal dopamine transporter levels than controls, the results suggest that the density of their dopaminergic nerve terminals is not altered.

Vmat2 heterozygous mutant mice display a depressive-like phenotype

The vesicular monoamine transporter 2 (VMAT2) is localized primarily within the CNS and is responsible for transporting monoamines from the cytoplasm into secretory vesicles. Because reserpine (a VMAT inhibitor) can precipitate depressive-like symptoms in humans, we investigated whether Vmat2 heterozygous (HET) mice present with depressive-like behaviors. The mutants showed locomotor and rearing retardation in the open field and appeared anhedonic to 1 and 1.5% sucrose solutions. Immobility times for Vmat2 heterozygotes were prolonged in forced swim and imipramine normalized this behavior. HET animals also showed enhanced immobility in tail suspension and this response was alleviated by fluoxetine, reboxetine, and bupropion. Stimulated GTPgammaS binding indicated that alpha2-adrenergic receptors in HET hippocampus were more sensitive to UK 14,304 (5-bromo-N-(4,5-dihydro-1-H-imidazol-2-yl)-6-quinoxalinamine) stimulation than in wild type (WT) mice. In learned helplessness, mice were exposed to a shuttle box for 4 d or were given inescapable foot-shocks for the same time period. On day 5, all animals were tested in shock escape. Failure rates and the latency to escape were similar for WT and HET mice that were only pre-exposed to the test apparatus. In foot-shock groups, learned helplessness was more robust in heterozygotes than in WT controls. Basal secretion of serum corticosterone was not distinguished by genotype; however, corticosterone levels in mutants were more responsive to stress. Anxiety-like responses of WT and HET animals in the open field, light-dark exploration, zero maze, and novelty-suppressed feeding tests were indistinguishable. Collectively, these findings suggest that Vmat2 heterozygotes display a depressive-like phenotype that is devoid of anxiety-like behavior.

Pharmacogenomic evaluation of the antidepressant citalopram in the mouse tail suspension test

The identification of genetic variants regulating antidepressant response in human patients would allow for more individualized, rational, and successful drug treatments. We have previously identified the BALB/cJ inbred mouse strain as highly responsive to the selective serotonin reuptake inhibitor (SSRI) citalopram in the tail suspension test (TST), a widely used and well-established screening paradigm for detecting compounds with antidepressant activity. In contrast, A/J mice did not show a significant response to citalopram in this test despite exposure to equivalent plasma levels of the drug. To identify genetic determinants of this differential response, 506 F2 mice from an intercross between BALB/cJ and A/J mice were phenotyped. Composite interval mapping of 92 mice from the phenotypic extremes revealed three loci on chromosomes 7, 12, and 19 affecting citalopram response in the TST. The quantitative trait locus (QTL) at the telomeric end of chromosome 19 showed the greatest level of significance. Three candidate genes residing in this locus include those for vesicular monoamine transporter 2 (VMAT2, slc18a2), alpha 2A adrenergic receptor (adra2a), and beta 1 adrenergic receptor (adrb1). The protein coding regions of these three genes in BALB/cJ and A/J mice were sequenced and two polymorphisms were found in VMAT2 (Leu117Pro and Ser505Pro), while the transcribed regions of adra2a and adrb1 were of identical sequence between strains. Follow-up studies are needed to determine if the VMAT2 polymorphisms are functional and if they could explain the chromosome 19 QTL. The present quantitative trait study suggests possible candidate genes for human pharmacogenetic studies of therapeutic responses to SSRIs such as citalopram.

Positron emission tomography of monoaminergic vesicular binding in aging and Parkinson disease

The type-2 vesicular monoamine transporter (VMAT2) might serve as an objective biomarker of Parkinson disease (PD) severity. Thirty-one subjects with early-stage PD and 75 normal subjects underwent continuous intravenous infusion of (+)-[(11)C]dihydrotetrabenazine (DTBZ) and positron emission tomography (PET) imaging to estimate the striatal VMAT2 binding site density with equilibrium tracer modeling. Parkinson disease patients were evaluated clinically in the practically defined 'off' state with the Unified Parkinson Disease Rating Scale (UPDRS), the Hoehn and Yahr Scale (HY), and the Schwab and England Activities of Daily Living Scale (SE). In normal subjects there was age-related decline in striatal DTBZ binding, approximating 0.5% per year. In PD subjects, specific DTBZ binding was reduced in the caudate nucleus (CD; -44%), anterior putamen (-68%), and posterior putamen (PP; -77%). The PP-to-CD ratio of binding was reduced significantly in PD subjects. Dihydrotetrabenazine binding was also reduced by approximately 50% in the PD substantia nigra. Striatal binding reductions correlated significantly with PD duration and SE scores, but not with HY stage or with UPDRS motor subscale (UPDRS(III)) scores. Striatal and midbrain DTBZ binding was asymmetric in PD subjects, with greatest reductions contralateral to the most clinically affected limbs. There was significant correlation between asymmetry of DTBZ binding and clinical asymmetry measured with the UPDRS(III). In HY stage 1 and 1.5 subjects (n=16), PP DTBZ binding contralateral to the clinically unaffected body side was reduced by 73%, indicating substantial preclinical nigrostriatal pathology in PD. We conclude that (+)-[(11)C]DTBZ-PET imaging displays many properties necessary of a PD biomarker.

Evidence for increased expression of the vesicular glutamate transporter, VGLUT1, by a course of antidepressant treatment

The therapeutic effect of a course of antidepressant treatment is believed to involve a cascade of neuroadaptive changes in gene expression leading to increased neural plasticity. Because glutamate is linked to mechanisms of neural plasticity, this transmitter may play a role in these changes. This study investigated the effect of antidepressant treatment on expression of the vesicular glutamate transporters, VGLUT1-3 in brain regions of the rat. Repeated treatment with fluoxetine, paroxetine or desipramine increased VGLUT1 mRNA abundance in frontal, orbital, cingulate and parietal cortices, and regions of the hippocampus. Immunoautoradiography analysis showed that repeated antidepressant drug treatment increased VGLUT1 protein expression. Repeated electroconvulsive shock (ECS) also increased VGLUT1 mRNA abundance in regions of the cortex and hippocampus compared to sham controls. The antidepressant drugs and ECS did not alter VGLUT1 mRNA abundance after acute administration, and no change was detected after repeated treatment with the antipsychotic agents, haloperidol and chlorpromazine. In contrast to VGLUT1, the different antidepressant treatments did not commonly increase the expression of VGLUT2 or VGLUT3 mRNA. These data suggest that a course of antidepressant drug or ECS treatment increases expression of VGLUT1, a key gene involved in the regulation of glutamate secretion.

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.

Normal biogenesis and cycling of empty synaptic vesicles in dopamine neurons of vesicular monoamine transporter 2 knockout mice

The neuronal isoform of vesicular monoamine transporter, VMAT2, is responsible for packaging dopamine and other monoamines into synaptic vesicles and thereby plays an essential role in dopamine neurotransmission. Dopamine neurons in mice lacking VMAT2 are unable to store or release dopamine from their synaptic vesicles. To determine how VMAT2-mediated filling influences synaptic vesicle morphology and function, we examined dopamine terminals from VMAT2 knockout mice. In contrast to the abnormalities reported in glutamatergic terminals of mice lacking VGLUT1, the corresponding vesicular transporter for glutamate, we found that the ultrastructure of dopamine terminals and synaptic vesicles in VMAT2 knockout mice were indistinguishable from wild type. Using the activity-dependent dyes FM1-43 and FM2-10, we also found that synaptic vesicles in dopamine neurons lacking VMAT2 undergo endocytosis and exocytosis with kinetics identical to those seen in wild-type neurons. Together, these results demonstrate that dopamine synaptic vesicle biogenesis and cycling are independent of vesicle filling with transmitter. By demonstrating that such empty synaptic vesicles can cycle at the nerve terminal, our study suggests that physiological changes in VMAT2 levels or trafficking at the synapse may regulate dopamine release by altering the ratio of fillable-to-empty synaptic vesicles, as both continue to cycle in response to neural activity.

Treatment effects on nigrostriatal projection integrity in partial 6-OHDA lesions: comparison of L-DOPA and pramipexole

There is controversy over potential effects of dopaminergic replacement therapies on the partially lesioned nigrostriatal dopaminergic projection. We evaluated indirect (levodopa, L-DOPA) versus direct (pramipexole, PRA) dopaminergic treatment effects on nigrostriatal lesion severity as measured with vesicular monoamine transporter type-2 (VMAT2) binding. Prior studies have shown that striatal VMAT2 density provides an objective estimate of dopaminergic neuronal integrity, without confounding effects of compensatory regulation. Partial unilateral median forebrain bundle lesions were made by injection of 6-hydroxydopamine in adult male Sprague-Dawley rats. Lesion severity was estimated using rotational behavior after injections of apomorphine and amphetamine. Rats were ranked and matched in pairs by rotation and assigned to receive either PRA (1 mg/kg/day) or L-DOPA/benserazide (100/25 mg/kg/day) ip via osmotic pump. After 4 weeks of drug treatment, in vitro autoradiography was performed with [(3)H]methoxytetrabenazine to measure striatal VMAT2 binding density. Lesion-to-intact VMAT2 density correlated with rotation in both treatment groups. There was no treatment effect on VMAT2 expression in the partially lesioned striatum and thus no differential effect of indirect versus direct dopamimetic treatment on nigrostriatal integrity.

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.

Elevated platelet vesicular monoamine transporter density in untreated patients diagnosed with major depression

The intraneuronal uptake of monoamines into brain synaptic vesicles is mediated by the vesicular monoamine transporter (VMAT2). This transporter plays a major role in monoamine storage and quantal release. Recently we demonstrated a high degree of similarity between the pharmacodynamic characteristics of platelet and brain VMAT2. In the present study we measured the VMAT2 density, using [3H]dihydrotetrabenazine ([3H]TBZOH) as a ligand, in platelets of untreated patients diagnosed with major depressive disorder (MDD) (n=10; three with recurrent depression and seven with first episode depression) compared to sex- and age-matched healthy control subjects (n=23). A significant elevation in the VMAT2 density (B(max)) was observed in the platelets of untreated MDD patients (+24%) compared to healthy control subjects. No significant change was found in the affinity constant (K(d)). The increased platelet VMAT2 density may reflect depression-related enhancement of the capacity to accumulate monoamines in the vesicles in the presence of lower monoamine turnover.