The decrease of erythrocyte catechol-O-methyltransferase activity in depressed patients and its diagnostic significance

Analytical methodologies for sensing catechol-O-methyltransferase activity and their applications

Mammalian catechol-O-methyltransferases (COMT) are an important class of conjugative enzymes, which play a key role in the metabolism and inactivation of catechol neurotransmitters, catechol estrogens and a wide range of endobiotics and xenobiotics that bear the catechol group. Currently, COMT inhibitors are used in combination with levodopa for the treatment of Parkinson's disease in clinical practice. The crucial role of COMT in human health has raised great interest in the development of more practical assays for highly selective and sensitive detection of COMT activity in real samples, as well as for rapid screening and characterization of COMT inhibitors as drug candidates. This review summarizes recent advances in analytical methodologies for sensing COMT activity and their applications. Several lists of biochemical assays for measuring COMT activity, including the probe substrates, along with their analytical conditions and kinetic parameters, are presented. Finally, the challenges and future perspectives in the field, such as visualization of COMT activity in vivo and in situ, are highlighted. Collectively, this review article overviews the practical assays for measuring COMT activities in complex biological samples, which will strongly facilitate the investigations on the relevance of COMT to human diseases and promote the discovery of COMT inhibitors via high-throughput screening.

Structural and functional neuroimaging studies in generalized anxiety disorder: a systematic review

OBJECTIVES

Brain imaging studies carried out in patients suffering from generalized anxiety disorder (GAD) have contributed to better characterize the pathophysiological mechanisms underlying this disorder. The present study reviews the available functional and structural brain imaging evidence on GAD, and suggests further strategies for investigations in this field.

METHODS

A systematic literature review was performed in PubMed, PsycINFO, and Google Scholar, aiming to identify original research evaluating GAD patients with the use of structural and functional magnetic resonance imaging as well as diffusion tensor imaging.

RESULTS

The available studies have shown impairments in ventrolateral and dorsolateral prefrontal cortex, anterior cingulate, posterior parietal regions, and amygdala in both pediatric and adult GAD patients, mostly in the right hemisphere. However, the literature is often tentative, given that most studies have employed small samples and included patients with comorbidities or in current use of various medications. Finally, different methodological aspects, such as the type of imaging equipment used, also complicate the generalizability of the findings.

CONCLUSIONS

Longitudinal neuroimaging studies with larger samples of both juvenile and adult GAD patients, as well as at risk individuals and unaffected relatives, should be carried out in order to shed light on the specific biological signature of GAD.

Catechol-O-methyltransferase Val158Met genotype and the clinical responses to duloxetine treatment or plasma levels of 3-methoxy-4-hydroxyphenylglycol and homovanillic acid in Japanese patients with major depressive disorder

PURPOSE

This study investigated the relationships among the plasma levels of catecholamine metabolites, the clinical response to duloxetine treatment, and Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene.

SUBJECTS AND METHODS

Sixty-four patients and 30 healthy control subjects were recruited. Major depressive episodes were diagnosed using the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision criteria. The severity of depression was evaluated using the 17-item Hamilton Rating Scale for Depression (HAMD17). Patients whose HAMD17 scores were 15 or greater were enrolled in the study. Blood sampling and clinical evaluation were performed at week 0 and week 8. The levels of plasma catecholamine metabolites were measured using high-performance liquid chromatography with electrochemical detection. Genotyping was performed using direct sequencing.

RESULTS

Thirty of 45 patients (67%) responded to duloxetine treatment during the 8 weeks of treatment. The baseline plasma levels of 3-methoxy-4-hydroxyphenylglycol (MHPG), but not homovanillic acid (HVA), were lower in patients with major depressive disorder (MDD) who had the Val/Val genotype than in patients who were Met-carriers. Patients with MDD and the Val/Val genotype, but not Met carriers, had increased plasma levels of MHPG after 8 weeks of duloxetine treatment. The baseline plasma MHPG levels in healthy control subjects with the Val/Val genotype were significantly higher than those in patients with MDD. Among the subjects in the MDD group with the Val/Val genotype, the plasma MHPG levels increased to the same degree as in the healthy control subjects with the Val/Val genotype after 8 weeks of duloxetine treatment.

CONCLUSION

The relationship among the COMT Val158Met polymorphism, plasma levels of catecholamine metabolites, and responses to duloxetine is complex. Nevertheless, our results suggest that patients with MDD and the Val/Val genotype are more sensitive to the influence of noradrenergic neurons by duloxetine treatment.

Association of TMEM132D, COMT, and GABRA6 genotypes with cingulate, frontal cortex and hippocampal emotional processing in panic and major depressive disorder

OBJECTIVE

The aim of the study was to evaluate the association of transmembrane protein 132D (TMEM132D), catechol-O-methyltransferase (COMT), and gamma-aminobutyric acid (GABA) receptor alpha 6 subunit (GABRA6) genotypes with cingulate, frontal cortex and hippocampal emotional processing in panic disorder (PD) and major depressive disorder (MDD).

METHOD

The single nucleotide polymorphisms (SNPs) in TMEM132D, COMT, and GABRA6 were examined in patients with MDD, PD, and healthy controls. Functional magnetic resonance imaging (fMRI) was performed in patients with MDD, PD, and healthy controls.

RESULTS

rs4680 in COMT and rs3219151 in GABRA6 showed positive associations with PD and MDD. A dynamic fearful face was shown to the participants during fMRI scanning. In PD patients, responses in the bilateral anterior cingulate were stronger in carriers of the AA genotype of SNP rs11060369 in TMEM132D compared with carriers of the AC + CC genotype, and stronger in CT + TT genotype carriers of SNP rs3219151 in GABRA6 compared with carriers of the CC genotype. The response in the medial orbital frontal cortex was stronger in carriers of the CT + TT genotypes of SNP rs3219151 in PD. In MDD patients, the response in the right parahippocampus of carriers of the GG genotype of rs4680 in COMT was stronger than that of carriers of the AA + AG genotype.

CONCLUSION

These results suggest that TMEM132D, GABRA6, and COMT variants may increase vulnerability to panic.

COMT Val158Met, but not BDNF Val66Met, is associated with white matter abnormalities of the temporal lobe in patients with first-episode, treatment-naïve major depressive disorder: a diffusion tensor imaging study

We investigated the association between the Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene, the Val66Met polymorphism of the brain-derived neurotrophic factor (BDNF) gene, and white matter changes in patients with major depressive disorder (MDD) and healthy subjects using diffusion tensor imaging (DTI). We studied 30 patients with MDD (17 males and 13 females, with mean age ± standard deviation [SD] =44±12 years) and 30 sex- and age-matched healthy controls (17 males and 13 females, aged 44±13 years). Using DTI analysis with a tract-based spatial statistics (TBSS) approach, we investigated the differences in fractional anisotropy, radial diffusivity, and axial diffusivity distribution among the three groups (patients with the COMT gene Val158Met, those with the BDNF gene Val66Met, and the healthy subjects). In a voxel-wise-based group comparison, we found significant decreases in fractional anisotropy and axial diffusivity within the temporal lobe white matter in the Met-carriers with MDD compared with the controls (P<0.05). No correlations in fractional anisotropy, axial diffusivity, or radial diffusivity were observed between the MDD patients and the controls, either among those with the BDNF Val/Val genotype or among the BDNF Met-carriers. These results suggest an association between the COMT gene Val158Met and the white matter abnormalities found in the temporal lobe of patients with MDD.

Association analysis of the COMT/MTHFR genes and geriatric depression: an MRI study of the putamen

OBJECTIVE

Catechol-O-Methyltransferase (COMT) and Methylenetetrahydrofolate reductase (MTHFR) had been reported to relate to depression but with inconsistent results. The basal ganglia are also important in the pathophysiology of affective disorder via connections with limbic system and prefrontal cortex. The authors examined the relationship between an interaction of COMT/MTHFR polymorphisms and volumes of putamen in depressed and nondepressed elders.

METHODS

Participants included 170 depressed and 83 nondepressed subjects aged 60 years or older. Subjects completed cross-sectional assessments, including clinical evaluation, brain magnetic resonance imaging (MRI) scan, and COMT Val158Met and MTHFR C677T genotyping. Putamen volumes were measured using 1.5-Tesla whole-body MRI system. Statistical models examined the relationship between COMT/MTHFR genotype, proportional volumes of putamen and depression while controlling for age and sex.

RESULTS

After controlling for covariates, among depressed subjects with MTHFR C/C, both the right and left putamen had smaller volumes as the number of COMT 158Val increased. The left putamen volumes of depressed subjects with COMT Met/Met were smaller as the number of MTHFR 677T increased compared to nondepressed subjects.

CONCLUSIONS

Our findings do not support a major role for COMT or MTHFR alone. However, an epigenetic interaction of COMT Val158Met and MTHFR C677T polymorphisms may contribute to putamen volumes differences between depressed and nondepressed subjects. Further studies with a larger sample size are necessary to support a genetically based role for basal ganglia structures in the etiopathogenesis of depression.

The promises and pitfalls of reboxetine

The antidepressant compound, morpholine, 2-[(2-ethoxyphenoxy)phenylmethyl]-,methanesulfonate, or reboxetine, is a selective noradrenergic reuptake inhibitor that acts by binding to the norepinephrine (NE) transporter and blocking reuptake of extracellular NE back into terminals. This compound has low affinity for other transporters and receptors. The development of reboxetine as a potential antidepressant stems from the prior demonstration that blockade of the NE transporter imparts antidepressant activity. Desipramine, lofepramine, and nortryptiline are examples of tricyclic antidepressant (TCA) compounds from the first generation of antidepressants that exert their effects by blockade of NE reuptake. Maprotiline, a non-tricyclic compound, is also a NE selective reuptake inhibitor. Unfortunately, these antidepressants are also associated with interactions with muscarinic, histaminergic, and adrenergic receptors, which are known to contribute to a variety of untoward side effects. Despite the positive pharmacological profile of reboxetine, i.e., selectivity and specificity, with relatively fewer side effects, its use as an antidepressant is currently limited to Europe. Reboxetine is marketed as Edronax in the UK, Norebox in Italy, and as Irenor in Spain. It is registered in Germany, Sweden, Denmark, Ireland, Austria and Finland. Based on studies conducted primarily outside the US, the FDA granted a preliminary letter of approval in 1999. However, more recent clinical studies conducted in the US and Canada, prompted by the FDA, resulted in a letter of non-approval. To date, it is unclear why the further development of reboxetine as an antidepressant in the US has been halted. Despite this setback, reboxetine has been a valuable pharmacological tool to assess the role of the noradrenergic system in preclinical studies of depressive disorder.

Rat liver and kidney catechol-O-methyltransferase activity measured by high-performance liquid chromatography with fluorescence detection

We have previously reported a highly sensitive method for the measurement of catechol-O-methyltransferase (COMT) activities in rat erythrocytes with norepinephrine (NE), an endogenous native substrate, using high-performance liquid chromatography (HPLC)-fluorescence or peroxyoxalate chemiluminescence reaction detection. Applying this method to COMT activities in rat liver and kidney, known to have the highest activities of all organs, the optimum reaction conditions were investigated. Under the optimum conditions, soluble (S)-COMT and membrane-bound (MB)-COMT activities in rat liver, with NE as a substrate, were 2.17 +/- 0.33 and 0.16 +/- 0.02 nmol/min/mg protein (n = 5), respectively. In rat kidney, S-COMT and MB-COMT activities were 1.81 +/- 0.20 and 0.079 +/- 0.009 nmol/min/mg protein (n = 5), respectively. Since liver and kidney play important roles in inactivating catecholamines, using the proposed method would yield critical information to delineate the role of metabolism of catecholamines in rat tissues.

Pharmacogenetics of lithium prophylaxis in mood disorders: analysis of COMT, MAO-A, and Gbeta3 variants

We studied the possible association between the prophylactic efficacy of lithium in mood disorders and the following gene variants: catechol-O-methyltransferase (COMT) G158A, monoamine oxydase A (MAO-A) 30-bp repeat, G-protein beta 3-subunit (Gbeta3) C825T. A total of 201 subjects affected by bipolar (n = 160) and major depressive (n = 41) disorder were followed prospectively for an average of 59.8 months and were typed for their gene variants using PCR techniques. COMT, MAO-A, and Gbeta3 variants were not associated with lithium outcome, even when possible stratification effects such as sex, polarity, age at onset, duration of lithium treatment, and previous episodes were included in the model. The pathways influenced by those variants are not therefore involved with long-term lithium outcome in our sample.

Catechol-O-methyltransferase gene polymorphism in schizophrenia: evidence for association between symptomatology and prognosis

Catechol-O-methyltransferase (COMT) gene has long been implicated to play a role in the pathogenesis of schizophrenia. The aim of this study is to assess the relationship of schizophrenia and its subgroups with COMT gene polymorphism. We have attempted to evaluate a possible correlation between the severity and prognosis of the illness (the psychopathology of symptoms) and COMT gene polymorphisms. The study comprised 129 unrelated subjects who strictly met DSM-IV criteria for schizophrenia, and 65 healthy unrelated controls. All subjects were of Turkish origin. A clinical evaluation of all patients was accomplished by applying the Brief Psychiatric Rating Scale (BPRS) test. The analysis of COMT polymorphism was performed using the polymerase chain reaction technique. Regarding COMT gene polymorphisms, no statistically significant difference was found between schizophrenic patients and control subjects. However, within the schizophrenic group, the average of BPRS points of patients with the L/L genotype was significantly higher than those of the L/H and H/H genotypes (F = 6.25, degrees of freedom = 2, P = 0.003). Although no statistically significant difference was found between the duration of illness and COMT variations, a higher frequency of hospitalization was found in patients with the L/L genotype compared with other groups (t = 3.048, P = 0.003). In conclusion, the findings indicate that COMT gene polymorphisms were not statistically significant between patient and control groups. However, the patients with the L/L genotype may have much more severe clinical signs in Turkish schizophrenics. COMT variations, however, do not help to evaluate the susceptibility of the patients, but can help in the estimation of severity of clinical manifestations. Further studies are required to better understand the association of symptomatology of schizophrenia and other psychiatric disorders with COMT gene polymorphism.

Investigation of a correlation between monoamine oxidase B and catechol-O-methyltransferase activity in human blood cells

Monoamine oxidase B (MAO-B) and catechol-O-methyltransferase (COMT) are pivotal enzymes in the catabolism of several neurotransmitters. MAO-B and COMT activity can be reliably measured in human platelets and erythrocytes, respectively. This study investigated whether a correlation exists between the activity of the two enzymes in a panel of 47 elderly subjects (age range 55-80 years). No correlation was apparent between the two activities (r(2)<0.01), which suggests that, genetically, they are determined independently. COMT activity in a panel of 163 subjects showed a bimodal distribution with a nadir at approximately 38 pmol/h/mg Hb.

Determination of differential activities of soluble and membrane-bound catechol-O-methyltransferase in tissues and erythrocytes

Catechol-O-methyltransferase (COMT) exists as two isoenzymes, a membrane-bound form (MB-COMT) and a soluble form (S-COMT), with different roles in the metabolism of catecholamines and other catechol compounds. This report documents an HPLC assay for separate estimation of S-COMT and MB-COMT activity and examines activities of the two isoenzymes among different rat tissues and in human and rat erythrocytes. Activities of MB-COMT and S-COMT varied widely among tissues. There were higher activities of S-COMT than MB-COMT in all tissues except the adrenal medulla where MB-COMT was the predominant isoenzyme, consistent with the importance of this tissue and MB-COMT for the O-methylation of catecholamines. MB-COMT and S-COMT in rat and human erythrocytes showed divergent levels and patterns of activity. The assay represents a rapid and accurate method for quantifying MB-COMT and S-COMT in various tissues and examining the relative roles of COMT isoenzymes in the metabolism of catechol compounds in health and disease.

No evidence for an association of affective disorders with high- or low-activity allele of catechol-o-methyltransferase gene

Catechol-o-methyltransferase (COMT) is an enzyme that inactivates biologically active or toxic catechols. Previous studies have yielded inconsistent results on the relationship between erythrocyte COMT activity and affective disorders. Recently an amino acid change (Val-108-Met) of the COMT protein was shown to determine high- and low-activity alleles of the enzyme. Using polymerase chain reaction and the restriction enzyme NLaIII, we genotyped 107 patients with bipolar disorder, 62 with unipolar depression, and 121 controls. Neither bipolar nor unipolar patients differ significantly in the genotypic or allelic frequency from the control group. Even when the bipolar and unipolar patients were pooled into a single group, the distributions of both the genotypes and the alleles for the patient group were similar to those for the controls. We conclude that genetic variation that determines high and low activities of COMT does not have a major effect on the vulnerability to affective disorders in our sample.

Analysis of a functional catechol-O-methyltransferase gene polymorphism in schizophrenia: evidence for association with aggressive and antisocial behavior

We have recently characterized a functional polymorphism in the catechol-O-methyltransferase (COMT) gene that is responsible for substantial variability in COMT enzymatic activity found in humans. A common low-activity variant of the enzyme contains a methionine residue at amino acid 158 of membrane-bound COMT whereas the common high activity variant has a valine at this site. Considering the role of COMT in dopamine metabolism and the involvement of dopaminergic pathways in the pathogenesis of schizophrenia and violence, we screened 37 patients with schizophrenia to determine whether or not a behavioral association with the COMT polymorphism exists. Patients were assessed for dangerousness on the basis of a history of violent and threatening behavior, crime, cocaine and alcohol abuse, and other antisocial behaviors. We found that schizophrenic patients who were homozygous for the low activity allele were judged by their psychiatrists to be at higher risk for aggressive and dangerous behavior than those who were homozygous for the high activity allele (Kruskal-Wallis statistic = 10.43; P = 0.003).

Association of codon 108/158 catechol-O-methyltransferase gene polymorphism with the psychiatric manifestations of velo-cardio-facial syndrome

Velo-cardio-facial-syndrome (VCFS) is a common congenital disorder associated with typical facial appearance, cleft palate, cardiac defects, and learning disabilities. The majority of patients have an interstitial deletion on chromosome 22q11. In addition to physical abnormalities, a variety of psychiatric illnesses have been reported in patients with VCFS, including schizophrenia, bipolar disorder, and attention deficit hyperactivity disorder. The psychiatric manifestations of VCFS could be due to haploin-sufficiency of a gene(s) within 22q11. One candidate that has been mapped to this region is catechol-O-methyltransferase (COMT). We recently identified a polymorphism in the COMT gene that leads to a valine-->methionine substitution at amino acid 158 of the membrane-bound form of the enzyme. Homozygosity for COMT158met leads to a 3-4-fold reduction in enzymatic activity, compared with homozygotes for COMT158val. We now report that in a population of patients with VCFS, there is an apparent association between the low-activity allele, COMT158met, and the development of bipolar spectrum disorder, and in particular, a rapid-cycling form.

Cultured human keratinocytes as a model for studying the dopamine metabolism in schizophrenia

The dopamine hypothesis is the major etiological hypothesis of schizophrenia which proposes that enhanced central nervous system dopaminergic activity is the causative factor for this disease. The hypothesis remains unproven despite decades of research. The major difficulty in studying the disease is due to the unavailability of a suitable animal model. Studies with human blood, cerebrospinal fluid or post-mortem brains lead only to inconclusive results, due to the effects of medication and other environmental factors. No extra-neuronal cells, with the exception of adrenal medulla, have been reported to contain a dopamine metabolic pathway. Literature evidence and our own study suggest that human keratinocytes express the enzymes to synthesize and degrade dopamine. We have compared the properties of tyrosine hydroxylase, the rate-limiting enzyme, from mouse striatum and from human skin keratinocytes cultured in vitro. Moreover we could also detect dopamine beta hydroxylase and catechol-o-methyl transferase in keratinocytes. We propose that human keratinocytes cultured in vitro can be used to study the relevance of dopamine metabolism to schizophrenia under controlled conditions avoiding the effects of medication and other environmental factors.

Expression of functional membrane-bound and soluble catechol-O-methyltransferase in Escherichia coli and a mammalian cell line

Human catechol-O-methyltransferase (hCOMT) cDNA was used to express the recombinant hCOMT enzyme in sufficient quantities in prokaryotic as well as in eukaryotic cells to allow kinetic studies. When human membrane-bound catechol-O-methyltransferase (MB-COMT; amino acids 1-271) and the soluble catechol-O-methyltransferase COMT (S-COMT; delta membrane anchor hCOMT; amino acids 27-271), with the latter lacking the first 26 hydrophobic amino acids, were expressed in Escherichia coli, a relatively high-level synthesis of catalytically active enzymes was obtained. Insertion of the human MB-COMT-coding sequence into an eukaryotic expression vector under transcriptional control of the cytomegalovirus (CMV) promoter and enhancer yielded large quantities of hCOMT in human kidney 293 cells. Subcellular fractionation of 293 cells transfected with pBC12/CMV-hCOMT showed hCOMT to be located predominantly in the membrane fraction. The catechol-O-methyltransferase (COMT) activity was measured in cytosolic and membrane fractions at 37 degrees C, giving values of 33 and 114 units/mg of protein, respectively (1 unit produces 1 nmol of guaiacol/h). Km values were 10 microM for MB-COMT and 108 microM for S-COMT, indicating that recombinant MB-COMT exhibits a higher affinity for catechol as the substrate than the soluble form. RNA blot analysis of human hepatome cells (Hep G2), kidney, liver, and fetal brain revealed only one species of hCOMT mRNA of approximately 1.4 kb. Its level in these various tissues was similar to those of COMT protein in each tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological studies of dysthymia

Dysthymic disorder (DD) is a chronic subsyndromal depressive condition that has generated increasing interest since its formal introduction into the psychiatric nomenclature in 1980. Although DD was included among the affective disorders in DSM-III, this classification was controversial. Some clinical and family studies support an association between DD and major depression disorder (MDD), but there has been little additional research firmly establishing the diagnostic validity of DD or clarifying its relation to MDD and to personality disorders. In this article, the literature on the biology of DD is reviewed. Studies of rapid eye movement (REM) latency, electrodermal activity, and the thyroid axis show similarities between DD and MDD, but the findings are mixed. Other investigations, including the Dexamethasone Suppression Test (DST), catecholamines, and several other electroencephalogram (EEG) sleep variables, show more consistent differences between DD and MDD. These findings suggest that DD manifests primarily trait characteristics of depression, thus differentiating it from the state characteristics of MDD. The methodological problems and implications of these studies, and suggestions for future research, are discussed.

Human catechol-O-methyltransferase: cloning and expression of the membrane-associated form

A cDNA clone for human catechol-O-methyltransferase (hCOMT; S-adenosyl-L-methionine:catechol O-methyltransferase; EC 2.1.1.6) was isolated from a human hepatoma cell line (Hep G2) cDNA library by hybridization screening with a porcine cDNA probe. The cDNA clone was sequenced and found to have an insert of 1226 nucleotides. The deduced primary structure of hCOMT is composed of 271 amino acid residues with the predicted molecular mass of 30 kDa. At its N terminus it has a hydrophobic segment of 21 amino acid residues that may be responsible for insertion of hCOMT into the endoplasmic reticulum membrane. The primary structure of hCOMT exhibits high homology to the porcine partial cDNA sequence (93%). The deduced amino acid sequence contains two tryptic peptide sequences (T-22, T-33) found in porcine liver catechol-O-methyltransferase (COMT). The coding region of hCOMT cDNA was placed under the control of the cytomegalovirus promoter to transfect human kidney 293 cells. The endogenous COMT activity, which was approximately 9.98 units per mg of protein in the untransfected cells, increased to 206 units per mg of protein upon transfection with a plasmid containing the COMT cDNA. The COMT activity of recombinant protein was inhibited competitively (IC50 = 700 nM) by the selective COMT inhibitor Ro 40-7592. An anti-COMT monoclonal antibody recognized, on immunoblots, a major polypeptide with apparent molecular mass of 29 kDa, in reasonable agreement with the predicted molecular mass. The recombinant hCOMT was shown by immunoblot analysis to be mainly associated with the membrane fraction. RNA blot analysis revealed one COMT mRNA transcript of 1.4 kilobases in Hep G2 poly(A)+ RNA.

The heterogeneity of 3-methoxy-4-hydroxyphenylglycol levels among depressed patients

A previous report by our group and results published by other research groups have indicated a dichotomy in MHPG levels in depressed patients. This study attempted to characterize phenomena associated with this dichotomy in plasma MHPG levels. First, we have noted, at least in female patients, that homogeneity of MHPG levels, as tested by fitting with a normal curve, can be ruled out. In contrast, in the total population, a dichotomy was less evident in this study; the 2 subgroups, high and low levels of MHPG, partially overlap, and this results in a gaussian distribution. However, based on the hypothesis of a bimodal distribution, as shown by others and not excluded here, it was possible to find some factors associated with heterogeneity. Thus, polarity of depression, sex and age of the patients, age of disease onset and depression score affect MHPG levels. On the other hand, other elements suspected to modify MHPG values, such as the different lengths of the washout period (minimum 10 days), or some medication (chloralhydrate or levomepromazine) required during the washout period, did not show any effect. The aim of this study was to better characterize this peripheral index for its possible use in clinical application.