Platelet serotonin and plasma tryptophan in depressed patients: effect of drug treatment and clinical outcome

The tryptophan catabolite or kynurenine pathway in major depressive and bipolar disorder: A systematic review and meta-analysis

Background

There is now evidence that affective disorders including major depressive disorder (MDD) and bipolar disorder (BD) are mediated by immune-inflammatory and nitro-oxidative pathways. Activation of these pathways may be associated with activation of the tryptophan catabolite (TRYCAT) pathway by inducing indoleamine 2,3-dioxygenase (IDO, the rate-limiting enzyme) leading to depletion of tryptophan (TRP) and increases in tryptophan catabolites (TRYCATs).

Aims

To systematically review and meta-analyze central and peripheral (free and total) TRP levels, its competing amino-acids (CAAs) and TRYCATs in MDD and BD.

Methods

This review searched PubMed, Google Scholar and SciFinder and included 121 full-text articles and 15470 individuals, including 8024 MDD/BD patients and 7446 healthy controls.

Results

TRP levels (either free and total) and the TRP/CAAs ratio were significantly decreased (p < 0.0001) in MDD/BD as compared with controls with a moderate effect size (standardized mean difference for TRP: SMD = -0.513, 95% confidence interval, CI: -0.611; -0.414; and TRP/CAAs: SMD = -0.558, CI: -0.758; -0.358). Kynurenine (KYN) levels were significantly decreased in patients as compared with controls with a small effect size (p < 0.0001, SMD = -0.213, 95%CI: -0.295; -0.131). These differences were significant in plasma (p < 0.0001, SMD = -0.304, 95%CI: -0.415, -0.194) but not in serum (p = 0.054) or the central nervous system (CNS, p = 0.771). The KYN/TRP ratio, frequently used as an index of IDO activity, and neurotoxicity indices based on downstream TRYCATs were unaltered or even lowered in MDD/BD.

Conclusions

Our findings suggest that MDD and BD are accompanied by TRP depletion without IDO and TRYCAT pathway activation. Lowered TRP availability is probably the consequence of lowered serum albumin during the inflammatory response in affective disorders.

Is there a relationship between morphological and functional platelet changes and depressive disorder?

BACKGROUND

Blood platelets, due to shared biochemical and functional properties with presynaptic serotonergic neurons, constituted, over the years, an attractive peripheral biomarker of neuronal activity. Therefore, the literature strongly focused on the investigation of eventual structural and functional platelet abnormalities in neuropsychiatric disorders, particularly in depressive disorder. Given their impact in biological psychiatry, the goal of the present paper was to review and critically analyze studies exploring platelet activity, functionality, and morpho-structure in subjects with depressive disorder.

METHODS

According to the PRISMA guidelines, we performed a systematic review through the PubMed database up to March 2020 with the search terms: (1) platelets in depression [Title/Abstract]"; (2) "(platelets[Title]) AND depressive disorder[Title/Abstract]"; (3) "(Platelet[Title]) AND major depressive disorder[Title]"; (4) (platelets[Title]) AND depressed[Title]"; (5) (platelets[Title]) AND depressive episode[Title]"; (6) (platelets[Title]) AND major depression[Title]"; (7) platelet activation in depression[All fields]"; and (8) platelet reactivity in depression[All fields]."

RESULTS

After a detailed screening analysis and the application of specific selection criteria, we included in our review a total of 106 for qualitative synthesis. The studies were classified into various subparagraphs according to platelet characteristics analyzed: serotonergic system (5-HT2A receptors, SERT activity, and 5-HT content), adrenergic system, MAO activity, biomarkers of activation, responsivity, morphological changes, and other molecular pathways.

CONCLUSIONS

Despite the large amount of the literature examined, nonunivocal and, occasionally, conflicting results emerged. However, the findings on structural and metabolic alterations, modifications in the expression of specific proteins, changes in the aggregability, or in the responsivity to different pro-activating stimuli, may be suggestive of potential platelet dysfunctions in depressed subjects, which would result in a kind of hyperreactive state. This condition could potentially lead to an increased cardiovascular risk. In line with this hypothesis, we speculated that antidepressant treatments would seem to reduce this hyperreactivity while representing a potential tool for reducing cardiovascular risk in depressed patients and, maybe, in other neuropsychiatric conditions. However, the problem of the specificity of platelet biomarkers is still at issue and would deserve to be deepened in future studies.

Serotonin Receptor and Transporter Endocytosis Is an Important Factor in the Cellular Basis of Depression and Anxiety

Depression and anxiety are common, debilitating psychiatric conditions affecting millions of people throughout the world. Current treatments revolve around selective serotonin reuptake inhibitors (SSRIs), yet these drugs are only moderately effective at relieving depression. Moreover, up to 30% of sufferers are SSRI non-responders. Endocytosis, the process by which plasma membrane and extracellular constituents are internalized into the cell, plays a central role in the regulation of serotonin (5-hydroxytryptophan, 5-HT) signaling, SSRI function and depression and anxiety pathogenesis. Despite their therapeutic potential, surprisingly little is known about the endocytosis of the serotonin receptors (5-HT receptors) or the serotonin transporter (SERT). A subset of 5-HT receptors are endocytosed by clathrin-mediated endocytosis following serotonin binding, while for the majority of 5-HT receptors the endocytic regulation is not known. SERT internalizes serotonin from the extracellular space into the cell to limit the availability of serotonin for receptor binding and signaling. Endocytosis of SERT reduces serotonin uptake, facilitating serotonin signaling. SSRIs predominantly inhibit SERT, preventing serotonin uptake to enhance 5-HT receptor signaling, while hallucinogenic compounds directly activate specific 5-HT receptors, altering their interaction with endocytic adaptor proteins to induce alternate signaling outcomes. Further, multiple polymorphisms and transcriptional/proteomic alterations have been linked to depression, anxiety, and SSRI non-response. In this review, we detail the endocytic regulation of 5-HT receptors and SERT and outline how SSRIs and hallucinogenic compounds modulate serotonin signaling through endocytosis. Finally, we will examine the deregulated proteomes in depression and anxiety and link these with 5-HT receptor and SERT endocytosis. Ultimately, in attempting to integrate the current studies on the cellular biology of depression and anxiety, we propose that endocytosis is an important factor in the cellular basis of depression and anxiety. We will highlight how a thorough understanding 5-HT receptor and SERT endocytosis is integral to understanding the biological basis of depression and anxiety, and to facilitate the development of a next generation of specific, efficacious antidepressant treatments.

Alcohol-related phenotypes and platelet serotonin concentration

Alcohol dependence is a chronic relapsing mental disorder with heterogeneous and complex underlying biology. It is frequently associated with nicotine dependence, severity of alcohol dependence symptoms, and diverse alcohol-related phenotypes, including the presence of delirium tremens and withdrawal symptoms, early or late onset of alcohol abuse, aggression, suicidal behavior, and anxiety. While searching for peripheral biomarkers of altered serotonergic (5-HT) function in alcohol dependence and alcohol-related behaviors, we determined a peripheral biomarker, i.e., platelet 5-HT concentration in a large group of Caucasian subjects with alcohol dependence subdivided according to the presence of specific alcohol-related phenotypes and smoking status. Individuals with alcohol dependence (n = 661) of both sexes were evaluated using Structural Clinical Interview based on DSM-IV criteria, while platelet 5-HT concentration was determined using the spectrophotofluorimetric method. Smoking is significantly associated, while sex and age are not, with platelet 5-HT concentration. Severe alcohol dependence and lack of withdrawal symptoms were associated with significantly decreased platelet 5-HT concentration in alcohol-dependent non-smokers. In smokers, significantly lower platelet 5-HT concentration was found in patients with the late onset of alcohol abuse. These results suggested that platelet 5-HT concentration might be used as a peripheral marker of different alcohol-related phenotypes, after controlling for the effects of smoking and sex.

The kynurenine pathway in major depressive disorder, bipolar disorder, and schizophrenia: a meta-analysis of 101 studies

The importance of tryptophan as a precursor for neuroactive compounds has long been acknowledged. The metabolism of tryptophan along the kynurenine pathway and its involvement in mental disorders is an emerging area in psychiatry. We performed a meta-analysis to examine the differences in kynurenine metabolites in major depressive disorder (MDD), bipolar disorder (BD), and schizophrenia (SZ). Electronic databases were searched for studies that assessed metabolites involved in the kynurenine pathway (tryptophan, kynurenine, kynurenic acid, quinolinic acid, 3-hydroxykynurenine, and their associate ratios) in people with MDD, SZ, or BD, compared to controls. We computed the difference in metabolite concentrations between people with MDD, BD, or SZ, and controls, presented as Hedges' g with 95% confidence intervals. A total of 101 studies with 10,912 participants were included. Tryptophan and kynurenine are decreased across MDD, BD, and SZ; kynurenic acid and the kynurenic acid to quinolinic acid ratio are decreased in mood disorders (i.e., MDD and BD), whereas kynurenic acid is not altered in SZ; kynurenic acid to 3-hydroxykynurenine ratio is decreased in MDD but not SZ. Kynurenic acid to kynurenine ratio is decreased in MDD and SZ, and the kynurenine to tryptophan ratio is increased in MDD and SZ. Our results suggest that there is a shift in the tryptophan metabolism from serotonin to the kynurenine pathway, across these psychiatric disorders. In addition, a differential pattern exists between mood disorders and SZ, with a preferential metabolism of kynurenine to the potentially neurotoxic quinolinic acid instead of the neuroprotective kynurenic acid in mood disorders but not in SZ.

Centrally Acting Angiotensin-Converting Enzyme Inhibitor Suppresses Type I Interferon Responses and Decreases Inflammation in the Periphery and the CNS in Lupus-Prone Mice

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multi-organ damage. Neuropsychiatric lupus (NPSLE) is one of the most common manifestations of human SLE, often causing depression. Interferon-α (IFNα) is a central mediator in disease pathogenesis. Administration of IFNα to patients with chronic viral infections or cancers causes depressive symptoms. Angiotensin-converting enzyme (ACE) is part of the kallikrein-kinin/renin-angiotensin (KKS/RAS) system that regulates many physiological processes, including inflammation, and brain functions. It is known that ACE degrades bradykinin (BK) into inactive peptides. We have previously shown in an in vitro model of mouse bone-marrow-derived dendritic cells (BMDC) and human peripheral blood mononuclear cells that captopril (a centrally acting ACE inhibitor-ACEi) suppressed Type I IFN responsive gene (IRG) expression. In this report, we used the MRL/lpr lupus-prone mouse model, an established model to study NPSLE, to determine the in vivo effects of captopril on Type I IFN and associated immune responses in the periphery and brain and effects on behavior. Administering captopril to MRL/lpr mice decreased expression of IRGs in brain, spleen and kidney, decreased circulating and tissue IFNα levels, decreased microglial activation (IBA-1 expression) and reduced depressive-like behavior. Serotonin levels that are decreased in depression were increased by captopril treatment. Captopril also reduced autoantibody levels in plasma and immune complex deposition in kidney and brain. Thus, ACEi's may have potential for therapeutic use for systemic and NPSLE.

Peripheral tryptophan, serotonin, kynurenine, and their metabolites in major depression: A case-control study

AIM

Tryptophan is the sole precursor of both peripherally and centrally produced serotonin and kynurenine. In depressed patients, tryptophan, serotonin, kynurenine, and their metabolite levels remain unclear. Therefore, peripheral tryptophan and metabolites of serotonin and kynurenine were investigated extensively in 173 patients suffering from a current major depressive episode (MDE) and compared to 214 healthy controls (HC).

METHODS

Fasting plasma levels of 11 peripheral metabolites were quantified: tryptophan, serotonin pathway (serotonin, its precursor 5-hydroxytryptophan and its metabolite 5-hydroxyindoleacetic acid), and kynurenine pathway (kynurenine and six of its metabolites: anthranilic acid, kynurenic acid, nicotinamide, picolinic acid, xanthurenic acid, and 3-hydroxyanthranilic acid).

RESULTS

Sixty (34.7%) patients were antidepressant-drug free. Tryptophan levels did not differ between MDE patients and HC. Serotonin and its precursor (5-hydroxytryptophan) levels were lower in MDE patients than in HC, whereas, its metabolite (5-hydroxyindoleacetic acid) levels were within the standard range. Kynurenine and four of its metabolites (kynurenic acid, nicotinamide, picolinic acid, and xanthurenic acid) were lower in MDE patients.

CONCLUSION

Whilst the results of this study demonstrate an association between the metabolites studied and depression, conclusions about causality cannot be made. This study uses the largest ever sample of MDE patients, with an extensive assessment of peripheral tryptophan metabolism in plasma. These findings provide new insights into the peripheral signature of MDE. The reasons for these changes should be further investigated. These results might suggest new antidepressant therapeutic strategies.

Changes in plasma 5-HT levels and equine leukocyte SERT expression in response to treadmill exercise

Serotonin (5-HT) is a neurohormone transported from plasma into platelets and leukocytes by a specific transporter (SERT). While it is known that the brain 5-HT system is modulated by physical exercise, the peripheral serotoninergic response to exercise is not yet fully elucidated. In particular, this study aimed to evaluate changes in plasma 5-HT levels and equine leukocyte SERT expression in response to treadmill exercise in untrained horses. Analyses were carried out pre- and post-treadmill exercise. 5-HT plasma levels were analysed by HPLC. Leukocytes and platelets were isolated to perform Real Time PCR for the evaluation of SERT mRNA levels. Western blot was conducted for the detection of SERT protein levels. The presence of SERT in leukocytes was analysed by flow cytometry. The functionality of SERT on leukocytes was investigated by using paroxetine as inhibitor of 5-HT reuptake. Results showed a significant decrease in SERT levels after exercise in both leukocytes and platelets and a significant increase in plasma 5-HT levels. Flow cytometry revealed that SERT is functional in one specific horse leukocyte subpopulation, still not identified, and paroxetine was able to block 5-HT reuptake into leukocytes. The exercise may have induced an increased mobilization of free-tryptophan and a release of 5-HT from the stores in the blood. High concentrations of plasma 5-HT could have caused a reduction in SERT expression affecting cellular 5-HT storage/uptake. The increase of cortisol levels after treadmill exercise was not significant. Exercise modulates the peripheral serotonin metabolism. More research is needed to assess its physiological implications.

Serum serotonin levels and bone in rheumatoid arthritis patients

In rheumatoid arthritis (RA), a disease characterized by bone loss, increased levels of serotonin have been reported. Recent studies have demonstrated a role for circulating serotonin as a regulator of osteoblastogenesis, inhibiting bone formation. Thus, we measured serum serotonin levels (SSL) in a Portuguese sample of 205 RA patients and related these to anthropometric variables, disease parameters, serum bone biomarkers, and bone mineral density (BMD) assessed by dual-energy X-ray absorptiometry at several sites (total proximal femur, lumbar spine, left hand, and left second proximal phalange). SSL were inversely associated with body mass index (BMI) in RA women (r = - 0.218; p = 0.005), independent of exposure to biologics and/or bisphosphonates. Among biologic naïves, there was an inverse association between SSL and osteoprotegerin in RA women (r = - 0.260; p = 0.022). Serum β-CTX and dickkopf-1 were strongly associated with SSL in RA men not treated with bisphosphonates (r = 0.590; p < 0.001/r = 0.387; p = 0.031, respectively). There was also an inverse association between SSL and sclerostin in RA men (r = - 0.374; p < 0.05), stronger among biologic naïve or bisphosphonates-unexposed RA men. In crude models, SSL presented as a significant negative predictor of total proximal femur BMD in RA women as well as in postmenopausal RA women. After adjustment for BMI, disease duration, and years of menopause, SSL remained a significant negative predictor of total proximal femur BMD only in postmenopausal RA women. Our data reinforce a role, despite weak, for circulating serotonin in regulating bone mass in RA patients, with some differences in terms of gender and anatomical sites.

Platelets as a surrogate disease model of neurodevelopmental disorders: Insights from Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability and the leading monogenic cause of autism spectrum disorders (ASD). Despite a large number of therapeutics developed in past years, there is currently no targeted treatment approved for FXS. In fact, translation of the positive and very promising preclinical findings from animal models to human subjects has so far fallen short owing in part to the low predictive validity of the Fmr1 ko mouse, an overly simplistic model of the complex human disease. This issue stresses the critical need to identify new surrogate human peripheral cell models of FXS, which may in fact allow for the identification of novel and more efficient therapies. Of all described models, blood platelets appear to be one of the most promising and appropriate disease models of FXS, in part owing to their close biochemical similarities with neurons. Noteworthy, they also recapitulate some of FXS neuron's core molecular dysregulations, such as hyperactivity of the MAPK/ERK and PI3K/Akt/mTOR pathways, elevated enzymatic activity of MMP9 and decreased production of cAMP. Platelets might therefore help furthering our understanding of FXS pathophysiology and might also lead to the identification of disease-specific biomarkers, as was shown in several psychiatric disorders such as schizophrenia and Alzheimer's disease. Moreover, there is additional evidence suggesting that platelet signaling may assist with prediction of cognitive phenotype and could represent a potent readout of drug efficacy in clinical trials. Globally, given the neurobiological overlap between different forms of intellectual disability, platelets may be a valuable window to access the molecular underpinnings of ASD and other neurodevelopmental disorders (NDD) sharing similar synaptic plasticity defects with FXS. Platelets are indeed an attractive model for unraveling pathophysiological mechanisms involved in NDD as well as to search for diagnostic and therapeutic biomarkers.

Serotonin receptors in depression: from A to B

The role of serotonin in major depressive disorder (MDD) is the focus of accumulating clinical and preclinical research. The results of these studies reflect the complexity of serotonin signaling through many receptors, in a large number of brain regions, and throughout the lifespan. The role of the serotonin transporter in MDD has been highlighted in gene by environment association studies as well as its role as a critical player in the mechanism of the most effective antidepressant treatments – selective serotonin reuptake inhibitors. While the majority of the 15 known receptors for serotonin have been implicated in depression or depressive-like behavior, the serotonin 1A (5-HT _1A) and 1B (5-HT _1B) receptors are among the most studied. Human brain imaging and genetic studies point to the involvement of 5-HT _1A and 5-HT _1B receptors in MDD and the response to antidepressant treatment. In rodents, the availability of tissue-specific and inducible knockout mouse lines has made possible the identification of the involvement of 5-HT _1A and 5-HT _1B receptors throughout development and in a cell-type specific manner. This, and other preclinical pharmacology work, shows that autoreceptor and heteroreceptor populations of these receptors have divergent roles in modulating depression-related behavior as well as responses to antidepressants and also have different functions during early postnatal development compared to during adulthood.

Serum serotonin concentration associated with bone mineral density in Chinese postmenopausal women

Recent studies have shown that circulating serotonin plays a potential role in bone metabolism. However, conflicting results have been reported for the relationship between serum serotonin concentrations and bone mineral density (BMD). We investigated whether the serum serotonin concentrations related to BMD in Chinese postmenopausal women. Serum serotonin and bone turnover concentrations of 117 premenopausal women and 262 asymptomatic postmenopausal women were analyzed by enzyme-linked immunosorbent assay. BMD at the lumbar spine and femoral neck was measured by dual energy X-ray absorptiometry. The relationship between serotonin and BMD was investigated. The postmenopausal women had lower mean serum serotonin concentrations compared to the premenopausal women. Serotonin concentrations were negatively associated with age, weight, BMI, fat mass, and β-CTX concentrations in postmenopausal women. No significant correlations were found between serotonin and these parameters in premenopausal women. In postmenopausal women, age- and BMI-adjusted serotonin concentrations were positively correlated with BMD of the lumbar spine and femoral neck. Multiple regression analyses showed serum serotonin and β-CTX were the predictors for lumbar spine BMD. Only serum serotonin was the determinant for femoral neck BMD. In conclusion, lower serum serotonin concentrations are linked to low lumbar spine and femoral neck BMD in postmenopausal women.

Plasma amino acid profiling in major depressive disorder treated with selective serotonin reuptake inhibitors

AIMS

Amino acids are important body metabolites and seem to be helpful for understanding pathogenesis and predicting therapeutic response in major depressive disorder (MDD). We performed amino acid profiling to discover potential biomarkers in major depressive patients treated with selective serotonin reuptake inhibitors (SSRIs).

METHODS

Amino acid profiling using aTRAQ™ kits for Amino Acid Analysis in Physiological Fluids on a liquid chromatography-tandem mass spectrometry (LC-MS/MS) system was performed on 158 specimens at baseline and at 6 weeks after the initiation of SSRI treatment for 68 patients with MDD and from 22 healthy controls.

RESULTS

Baseline alpha-aminobutyric acid (ABA) discriminated the patients according to the therapeutic response. Plasma glutamic acid concentration and glutamine/glutamic acid ratio were different between before and after SSRI treatment only in the response group. Comparing patients with MDD with healthy controls, alterations of ten amino acids, including alanine, beta-alanine, beta-aminoisobutyric acid, cystathionine, ethanolamine, glutamic acid, homocystine, methionine, O-phospho-L-serine, and sarcosine, were observed in MDD.

CONCLUSION

Metabolism of amino acids, including ABA and glutamic acid, has the potential to contribute to understandings of pathogenesis and predictions of therapeutic response in MDD.

The associations of serum serotonin with bone traits are age- and gender-specific

Context

Serotonin plays a potential role in bone metabolism, but the nature and extent of this relationship is unclear and human studies directly addressing the skeletal effect of circulating serotonin are rare.

Objective

The study aimed to investigate the associations between serum serotonin and bone traits at multiple skeletal sites in women and men.

Subjects and Methods

Subjects were part of the CALEX-family study and comprised 235 young women, 121 premenopausal women, 124 postmenopausal women, and 168 men. Body composition was assessed using DXA, as was areal bone mineral density (aBMD) of spine, femur and whole body. In addition, pQCT was used to determine bone properties at tibial midshaft and distal radius. Fasting serum serotonin concentration was assessed using a competitive enzyme-linked immunosorbent assay.

Results

Serum serotonin declined with advancing age both in females and males (all p<0.01). Serotonin was negatively correlated with weight, BMI, lean and fat mass in women (r = −0.22 to −0.39, all p<0.001), but positively with height and lean mass in men (all p<0.01). In the premenopausal women, serotonin was negatively correlated with lumbar spine aBMD (r = −0.23, p<0.05) but the statistical significance disappeared after adjustment for weight. Conversely, in postmenopausal women, serotonin was positively correlated with whole body and femur aBMD, as well as with distal radius bone mineral content and volumetric BMD (r = 0.20 to 0.30, all p<0.05), and these associations remained significant after adjustment for weight. In men, no significant associations were found between serotonin and bone traits.

Conclusion

Serum serotonin is positively associated with bone traits in postmenopausal women, but not in premenopausal women or men. This partially supports the idea of circulating serotonin playing a role in the regulation of bone metabolism, but also indicates the importance of gender and age specific factors.

Efficacy of exercise in reducing depressive symptoms across 5-HTTLPR genotypes

INTRODUCTION

Exercise is effective in the alleviation of depressive symptoms and may have physiological effects similar to those of selective serotonin reuptake inhibitors (SSRI). Recent research has identified the difference in treatment effects across genetic polymorphisms of the serotonin transporter polymorphic region (5-HTTLPR), in which the l allele has been associated with a better response to SSRI compared with the s allele. The purpose of the current research was to examine the antidepressant effects of exercise across 5-HTTLPR genotypes.

METHODS

Participants, ages 18–23 yr, were randomly assigned to a 5-wk exercise intervention or a no-treatment control group. Participants completed the Beck Depression Inventory before and after the intervention and provided a saliva sample for DNA analysis.

RESULTS

Exercise resulted in a significant reduction in depressive symptoms compared with the control group. In addition, individuals with at least one l allele demonstrated greater reductions in depressive symptoms compared with ss individuals.

CONCLUSIONS

The effects of exercise on depressive symptoms appear to be moderated by 5-HTTLPR genotype, suggesting that the mechanisms responsible for the alleviation of depressive symptoms are similar for exercise and SSRI treatment. Furthermore, these findings suggest that 5-HTTLPR genotype should be a factor in determining the proper line of treatment for depression.

Evaluation of the effect of selective serotonin-reuptake inhibitors on lymphocyte subsets in patients with a major depressive disorder

To date, only the effect of a short-term antidepressant treatment (<12 weeks) on neuroendocrinoimmune alterations in patients with a major depressive disorder has been evaluated. Our objective was to determine the effect of a 52-week long treatment with selective serotonin-reuptake inhibitors on lymphocyte subsets. The participants were thirty-one patients and twenty-two healthy volunteers. The final number of patients (10) resulted from selection and course, as detailed in the enrollment scheme. Methods used to psychiatrically analyze the participants included the Mini-International Neuropsychiatric Interview, Hamilton Depression Scale and Beck Depression Inventory. The peripheral lymphocyte subsets were measured in peripheral blood using flow cytometry. Before treatment, increased counts of natural killer (NK) cells in patients were statistically significant when compared with those of healthy volunteers (312+/-29 versus 158+/-30; cells/mL), but no differences in the populations of T and B cells were found. The patients showed remission of depressive episodes after 20 weeks of treatment along with an increase in NK cell and B cell populations, which remained increased until the end of the study. At the 52nd week of treatment, patients showed an increase in the counts of NK cells (396+/-101 cells/mL) and B cells (268+/-64 cells/mL) compared to healthy volunteers (NK, 159+/-30 cells/mL; B cells, 179+/-37 cells/mL). We conclude that long-term treatment with selective serotonin-reuptake inhibitors not only causes remission of depressive symptoms, but also affects lymphocyte subset populations. The physiopathological consequence of these changes remains to be determined.

Low tryptophan diet increases stress-sensitivity, but does not affect habituation in rats

Cerebral dysfunction of 5-HT (serotonin) has been associated with stress response and with affective disorders. Stress alone is insufficient to induce depression, since only a minor proportion of subjects that have experienced stressful life events develop depressive episodes. We investigated whether long-term brain 5-HT depletion induced in rats by a diet with low content of its precursor tryptophan affects stress-responsiveness in rats. Stress-sensitivity was measured through various physiological parameters and by measuring the rats' response to acoustic stimuli. One group of rats was subjected to daily acoustic stimulus sessions for 5 days. Other groups received both immobilization stress and acoustic stimulus sessions daily for either 9 days (chronic experiment) or 1 day (acute experiment). A low tryptophan diet led to decreases in plasma tryptophan levels, low ratio of tryptophan/large neutral amino acid, whole blood 5-HT, and neuronal 5-HT content in the Dorsal and Median Raphe Nuclei, as well as altered c-fos expression in the brain. Without concomitant immobilization, the diet alone did not affect reactivity and habituation to acoustic stimuli, although plasma corticosterone levels, but not the adrenal weights, were increased on day 5. Low tryptophan and chronic immobilization stress together with the acoustic testing procedure increased adrenal weight, plasma corticosterone levels and reactivity to the acoustic stimuli, but not the rate of habituation to acoustic stimuli. These results show that cerebral dysfunction of serotonin achieved through a low tryptophan diet, increases the sensitivity of rats to external and stressful stimuli, but does not impair the capacity to adapt to these stimuli. Accordingly, brain-serotonin modulates reactivity to stress, but not stress coping.

Peripheral serotonergic markers in acutely suicidal patients. 1. Comparison of serotonergic platelet measures between suicidal individuals, nonsuicidal patients with major depression and healthy subjects

BACKGROUND

A robust association between "suicidality" and deficits of the serotoninergic neurotransmission has been claimed in the past. However, many studies having investigated the relationship between suicidality and peripheral indicators of serotoninergic neurotransmission suffer from considering only one or a very small number of potentially useful serotoninergic parameters, whereas a synoptic multidimensional approach appears to be more appropriate. Furthermore, the psychiatric context within which suicidal behaviour occurs should be considered when interpreting biochemical findings of patients with suicidal ideation and suicide attempts.

METHODS

In the present study 5 peripheral serotonergic markers, (platelet 5HT concentration, 5HT uptake activity, 5HT(2A) receptor binding characteristics, MAO-B activity and tryptophan concentration in plasma) were assessed simultaneously. Of the 60 acutely suicidal inpatients (ICD-10: F43.xx, n = 52; F31/32/33, n = 8), 45 were suicide attempters. Data of 28 nonsuicidal patients with major depression (F31, n = 4; F32, n = 14; F33, n = 10) and 123 healthy volunteers represented the control groups.

RESULTS

Mean platelet 5HT concentration was significantly lower in suicidal inpatients when compared to nonsuicidal depressed patients, but did not differ from the figure in healthy subjects. Nonsuicidal depressed patients showed significantly higher mean platelet-5HT concentration than healthy controls. Mean V(max) of 5HT uptake in washed platelets, but not in platelet-rich plasma, was significantly higher in suicidal patients than in healthy controls, not, however, when compared to nonsuicidal depressed patients. Mean K(D) for the platelet 5HT(2A) receptor and MAO-B activity were significantly lower in suicidal patients as compared to nonsuicidal depressed patients and healthy controls. The observed differences in peripheral serotonergic markers between groups are partially due to a significant gender effect. A lower MAO-B activity was observed only in suicidal females, while the higher V(max) of 5HT uptake in washed platelets of suicidal patients was due to suicidal males.

CONCLUSIONS

In view of conflicting observations made by other authors and the present findings on suicidal patients with adjustment disorder it remains doubtful whether and if so to which extent platelet studies can provide valid information on serotonergic mechanisms related to suicidal behaviour.

Impact of the CYP2D6 ultra-rapid metabolizer genotype on doxepin pharmacokinetics and serotonin in platelets

INTRODUCTION

CYP2D6 gene duplication causing ultrafast metabolism is one reason for failure in responding to CYP2D6-metabolized antidepressants. We studied the effect of the CYP2D6 duplication genotype on doxepin pharmacokinetics and platelet serotonin uptake and concentrations.

METHODS

Pharmacokinetics of trans (E)- and cis (Z)-doxepin and N-desmethyldoxepin were analyzed after a single dose of 75 mg doxepin in 11 ultrafast metabolizers (UM), 11 extensive metabolizers (EM) and 3 poor metabolizers (PM), identified by genotyping for CYP2D6 alleles *2, *3, *4, *5, *6, *9, *10, *35, *41 and specific analyses to characterize gene duplication. Platelet serotonin concentrations were measured by HPLC.

RESULTS

A trend for lower AUC of the active principle (sum of doxepin and N-desmethyldoxepin) in UMs versus EMs was detected (575 versus 1,000 nmol h/l, P=0.07), mainly due to the differences in desmethyldoxepin concentrations (P=0.003). Stereoselective analysis showed a significant effect of the UM genotype on (E)-doxepin pharmacokinetic parameters whereas those of (Z)-doxepin did not differ between the CYP2D6 genotype groups. The 75-mg doxepin dose had no effect on platelet serotonin concentration and uptake, but serotonin concentrations in platelets were significantly higher in UM in comparison to the EM and PM groups. At baseline, these concentrations were 462, 399, and 292 ng/10 platelets in UM, EM and PM (P<0.0001 for trend).

CONCLUSIONS

At the same dose, internal exposure to doxepin differed by more than ten-fold between the CYP2D6 genotype groups. CYP2D6 may have an effect on platelet serotonin explained by salvage pathways of 5-methoxytryptamine to serotonin mediated by CYP2D6; however, this finding requires further confirmatory experiments.

Peripheral and central neurochemical effects of the selective serotonin reuptake inhibitors (SSRIs) in humans and nonhuman primates: assessing bioeffect and mechanisms of action

It is clear that selective serotonin reuptake inhibitors (SSRIs) act powerfully to inhibit serotonin (5-hydroxytryptamine, 5-HT) uptake centrally and peripherally. However, there are a number of critical unanswered questions concerning the effects of the drugs in adults and children. The influence of age and duration of treatment on the extent of uptake inhibition and on the enhancement of central serotonergic functioning are unclear. In addition, the relationship of these factors and effects to the therapeutic and adverse effects of the SSRIs remain to be clarified. The general clinical utility of platelet 5-HT measurement is reviewed and studies assessing central and peripheral uptake blockade in infants and children and non-human primates are discussed. Recent investigations of central neurochemical effects of the SSRIs in primates assessed through measurement of 5-HT and related compounds in cisternal cerebrospinal fluid (CSF) of the rhesus monkey are presented. In summary, the studies described have found that: human fetal exposure to SSRIs has substantial effects on 5-HT transport in utero; exposure to SSRIs through breastmilk of mothers treated for postpartum depression usually has negligible effects on 5-HT uptake; prescribed SSRIs appear to exert similar effects on 5-HT transporter blockade in children and adults; and rapid and sustained increases are seen in monkey cisternal CSF levels of 5-HT upon initiation of SSRI administration. The implications of the observations in terms of behavioral effects, clinical practice, and underlying mechanisms of action of the SSRIs are discussed.

Platelet serotonin in newborns and infants: ontogeny, heritability, and effect of in utero exposure to selective serotonin reuptake inhibitors

Ontogeny of platelet serotonin (5-hydroxytryptamine, 5-HT) during the first year of life was examined in newborns and infants. The effects of in utero exposure to selective serotonin reuptake inhibitors (SSRI, including fluoxetine, sertraline, and citalopram) were examined by comparing cord blood 5-HT levels in exposed and unexposed newborns. Heritability was assessed by correlation of the platelet 5-HT values observed for mother-infant pairs. No age effect was observed in 1-49 wk-old infants (r = 0.13, p = 0.49) and mean platelet 5-HT levels in infants (241 +/- 102 ng/mL, n = 33; 615 +/- 320 ng/10(9) platelets, n = 32) were similar to those reported for older children and adults. However, significantly lower blood 5-HT levels were observed in newborns (81.3 +/- 32.5 ng/mL, n = 16, p < 0.0001; 297 +/- 101 ng/10(9) platelets, n = 11, p = 0.0007) compared with the 1-49 wk-old infants. The mean cord blood 5-HT concentrations in newborns exposed in utero to SSRI (n = 8) were substantially lower than that seen in unexposed (n = 16) newborns (20.6 +/- 14.4 versus. 81.3 +/- 32.5 ng/mL, p = 0.0001; 90.7 +/- 55.4 versus. 297 +/- 101 ng/10(9) platelets, p = 0.0005). Platelet serotonin levels (ng/10(9) platelets) in mother-child pairs (n = 32) were significantly correlated (r = 0.415, p = 0.018). The results indicate that, although platelet 5-HT is low at birth, values quickly increase and stabilize at near-adult levels by 1 mo of age. Gestational exposure to SSRI appears to substantially reduce platelet 5-HT uptake in the fetus, strongly suggesting that such exposure has important physiologic effects. The observed mother-infant correlation agrees with a previous report of high heritability in a large adult population.

Analgesic effect of amitriptyline in chronic tension-type headache is not directly related to serotonin reuptake inhibition

The tricyclic antidepressant amitriptyline is the only documented and most widely used prophylactic drug for chronic tension-type headache (CTTH). However, it is not fully clarified whether the serotonin (5-HT) reuptake inhibition plays a major role for the analgesic effect of amitriptyline. To explore the importance of 5-HT reuptake inhibition for mechanism of action of the analgesic effect of amitriptyline we investigated platelet 5-HT levels during preventive treatment of CTTH with amitriptyline, the selective serotonin reuptake inhibitor citalopram, and placebo. Thirty-four patients with CTTH were given preventive treatment with amitriptyline 75 mg/day, the selective 5-HT reuptake inhibitor citalopram 20 mg/day, and placebo in a 32-week, double-blind, placebo-controlled, three-way crossover trial. Blood samples were collected in the last week of each treatment period. Platelet 5-HT was used as a measure of 5-HT reuptake inhibition and determined by high performance liquid chromatography. Area under the headache curve was 308 (157-715) (median with quartiles in parentheses) with amitriptyline and significantly lower than 377 (158-1121) with citalopram (P = 0.04) and 441 (178-1408) with placebo (P = 0.002). There was no difference between citalopram and placebo (P = 0.23). Platelet 5-HT was 0.4 (0.3-0.7) x 10(-18)mol/platelet with citalopram, which was significantly lower than 1.7 (1.2-2.4) x 10(-18)mol/platelet with amitriptyline (P < 0.001), and 3.5 (2.8-4.3) x 10(-18)mol/platelet with placebo (P < 0.001). The lower platelet 5-HT during treatment with citalopram than amitriptyline indicates that 5-HT reuptake was most effectively inhibited by citalopram. In contrast, amitriptyline was most effective in reduction of headache. This suggests that the analgesic effect of amitriptyline in CTTH is not solely due to 5-HT reuptake inhibition and that other mechanisms such as norepinephrine reuptake inhibition, NMDA receptor antagonism, blockade of muscarinic receptors and ion channels should be addressed in the future research.

Maternal fluoxetine treatment in the postpartum period: effects on platelet serotonin and plasma drug levels in breastfeeding mother-infant pairs

OBJECTIVE

Postpartum major depression, a frequently (10%) occurring complication of childbirth, adversely affects the mother's functioning, the mother-infant relationship, and the child's subsequent development and propensity for later psychopathology. Although selective serotonin reuptake inhibitors (SSRIs) are effective in treating postpartum depression, concerns have been raised regarding their use in lactating women. Although plasma drug levels of infants who are exposed to SSRIs through breast milk are low compared with those typically seen in patients, infant levels in some reports do seem to be at or near the drugs' reported affinities (K(D)s) and IC(50)s for inhibition at the serotonin (5-hydroxytryptamine [5-HT]) transporter. The impact of central serotonin 5-HT modulation by SSRIs during critical periods of brain development is unknown. These concerns have led our group to examine whether exposure through breast milk has a discernible effect on platelet 5-HT uptake. Taking advantage of the similarities between platelet and neuronal serotonin transporters, we previously used measurements of platelet 5-HT before and during maternal sertraline treatment to determine the degree of 5-HT transporter blockade in breastfed infants. We found that infants who were exposed to sertraline through their mothers' breast milk experienced little to no change in platelet 5-HT levels, suggestive of minimal effects on peripheral and central 5-HT transporter blockade. Compared with sertraline and most other SSRIs, fluoxetine and its active metabolite, norfluoxetine, have substantially longer plasma half-lives, and both compounds have been found in measurable quantities in plasma of nursing infants. Thus, to extend our previous work in this area, we measured platelet 5-HT levels and plasma drug levels in breastfeeding mother-infant pairs before and during maternal treatment with fluoxetine.

METHODS

Maternal and infant transporter blockade was assessed by measurement of platelet 5-HT in 11 breastfeeding mother-infant pairs before and after 4 to 12 weeks of maternal fluoxetine (20-40 mg/d) treatment for postpartum depression. The study was approved by the Human Investigation Committee of Yale University School of Medicine, and each mother (mean age: 34.5 years; standard deviation [SD]: 5.3) gave written informed consent. Whole-blood 5-HT levels and plasma fluoxetine and norfluoxetine levels were determined by high-performance liquid chromatography.

RESULTS

Five mothers were taking 20 mg of fluoxetine daily, 4 were taking 30 mg daily, and 2 were taking 40 mg daily. Mean infant age at the start of the study was 16.8 (SD: 8.8) weeks. All infants except 1 were <6 months of age and 4 were <3 months of age when their mothers began treatment. Six infants were breastfed exclusively; the remaining were breastfed between 3 and 8 times daily and were given supplemental feedings. Mean maternal postexposure 5-HT levels of 22.9 ng/mL (SD: 12.5) were markedly lower than mean preexposure (baseline) levels of 156.6 ng/mL (SD: 71.4; paired t = 6.9, df = 10). In contrast, the mean infant pre- and postexposure 5-HT concentrations of 217.1 (SD: 66.5) and 229.9 (SD: 83.5) ng/mL, respectively, were similar (paired t = -0.24, df = 10). However, the 1 infant with measurable plasma fluoxetine had a substantial decline in 5-HT to 40% of baseline. In samples obtained from the same infant 4 months later, plasma drug levels were undetectable (<1 ng/mL) and the platelet serotonin levels were no longer reduced (12% increase from baseline).

CONCLUSIONS

The marked declines (to 9%-28% of baseline) in platelet 5-HT concentrations seen in mothers who were treated with the SSRI fluoxetine were similar to those observed in our study of sertraline in breastfeeding and other previous studies. In contrast, all but 1 infant experienced little or no decline in whole-blood (platelet) 5-HT concentrations after exposure to fluoxetine through breast milk. The substantial drop in platelet 5-HT seen in 1 infant and the coupling of this drop with measurable plasma fluoxetine leves drop with measurable plasma fluoxetine level raises some concern. Possible reasons for the infant's measurable plasma fluoxetine level include his mother's high plasma drug level and his being breastfed exclusively. However, the observations may be coincidental, and the infant experienced no discernible adverse effects. These data suggest that most infants may continue to breastfeed without experiencing meaningful changes in platelet 5-HT transport while their mothers are treated with 20 to 40 mg of fluoxetine daily. Given the limited data regarding occurrence and extent of SSRI exposure and the uncertainties concerning the possible effects of exposure, it is premature to propose treatment guidelines. Our own advice to women who are thinking of combining breastfeeding and SSRI treatment will weigh a range of factors, including severity of postpartum depression, any demonstrated preferential response to a specific SSRI, and the mother's commitment to breastfeeding. Additional research is needed to establish more definitively the frequency of physiologically meaningful infant SSRI exposure during breastfeeding and to determine the behavioral consequences of such exposure.

Pretreatment platelet 5-HT concentration predicts the short-term response to paroxetine in major depression. Grupo de Trastornos Afectivos

BACKGROUND

A previous retrospective study revealed that a high pretreatment platelet serotonin (5-HT) concentration was associated with a low response to serotonergic antidepressants in drug-free major depressives. We have examined such a relationship in depressive patients treated with paroxetine.

METHODS

Seventy-four drug-free major depressives (DSM-IV) were admitted to the study. Clinical ratings were performed and blood was drawn prior to the initiation of treatment and after 4 weeks of paroxetine (20 mg/day). The concentrations of 5-HT, 5-hydroxyindoleacetic acid, and tryptophan were determined in plasma and blood.

RESULTS

Paroxetine treatment reduced platelet 5-HT to 17% of baseline after 4 weeks of treatment. Responder patients had a pretreatment platelet 5-HT concentration 22% lower than nonresponders (p < .035). Admission HAMD scores, plasma paroxetine concentration, or platelet 5-HT concentration at endpoint did not differ between responders and nonresponders. Yet, the response rate was 11% in patients with high pretreatment platelet 5-HT (> 900 ng/10(9) platelets) and 50% in those below that value (p < .004).

CONCLUSIONS

These findings support that depressed patients with a high pretreatment platelet 5-HT concentration have a poor therapeutic outcome after treatment with a standard paroxetine dose. These differences may be related to the existence of molecular differences in the 5-HT transporter.

Platelet 5-HT and plasma MHPG levels in patients with bipolar I and bipolar II depressions and normal controls

BACKGROUND

It has been suggested that platelet serotonin (5-HT) content may reflect aspects of the presynaptic reuptake of 5-HT, while plasma 3-methoxy-4-hydroxy-phenylglycol (MHPG) levels may provide an index of central noradrenergic function.

METHODS

In order to determine if there is a biological distinction in 5-HT or noradrenergic function within bipolar I and bipolar II depressions, we measured levels of platelet 5-HT and plasma MHPG in 12 patients with bipolar I depression, 12 patients with bipolar II depression, and 20 normal healthy controls. All subjects were medication free for at least 4 weeks prior to the study.

RESULTS

There was a trend towards higher platelet 5-HT in bipolar I or II depressions when compared to normal controls, whereas there was no difference in platelet 5-HT levels between bipolar I and II depressed patients. When bipolar I and II patients were pooled, there was a significant increase in platelet 5-HT levels in bipolar depressives compared to normal controls, and there was a trend towards a weak positive correlation between platelet 5-HT and 21-item HAMD scores in the patient group. In contrast, there was no difference in plasma MHPG levels between the three groups.

LIMITATIONS

This study was limited to a small sample size, single point sampling and did not match seasons.

CONCLUSIONS

Our findings did not provide supportive evidence for a distinctive 5-HT or noradrenergic function within bipolar I and bipolar II depressions. However, the finding of increased platelet 5-HT levels in bipolar depressed patients compared to normal controls is consistent with the results of previous studies, and may suggest an increase in presynaptic 5-HT reuptake, presumably resulting from diminished synaptic 5-HT availability in this condition.

Mood correlates with blood serotonin, but not with glucose measures in patients with recurrent suicidal behavior

Depressive mood and impulsive-aggressive behavior are relevant to suicidal behavior and have been associated with decreased central serotonergic function. Impulsive behavior and aggression have also been related to abnormal glucose metabolism. To investigate these relationships further, blood serotonin (5-HT), blood glucose measures (glycated hemoglobin (HbA1C) and fructosamine) and self-reported depressive symptoms and anger were repeatedly measured, during a 1-year follow-up, in 106 patients who had recently attempted suicide for at least a second time. A major DSM-III-R Axis I diagnosis or use of antidepressants were reasons for exclusion. Intra-individual variations in blood 5-HT were inversely correlated with variations in depressive mood, self-depreciation, and anger, and positively with variations in fructosamine. Intra-individual variations of HbA1c or fructosamine were not correlated with mood. Mean blood 5-HT levels per patient were positively correlated with mean scores of depression and self-depreciation. The contrast between the intra-individual and the inter-individual correlations may reflect a difference between short- vs. long-term effects. The results do support an association between serotonergic function and mood, but not between glucose metabolism and mood, in recurrent suicide attempters.

Biological parameters in major depression: effects of paroxetine, viloxazine, moclobemide, and electroconvulsive therapy. Relation to early clinical outcome

BACKGROUND

Clinical and pharmacologic studies report a relative or absolute serotonergic deficiency in major depression; however, the variability of clinical characteristics of illness has led to controversial results. In the present work, we looked for a possible relationship between i) biochemical values that indirectly reflect aminergic neurons activity and clinical characteristics and ii) their evolution and the early clinical outcome under antidepressive therapies (ATs).

METHODS

Platelet serotonin content, platelet monoamine oxydase activity, and urinary biopterins were measured in 27 depressed patients before and during four different ATs (paroxetine, viloxazine, moclobemide, or electroconvulsive therapy). Depressive symptomatology and its evolution under ATs were quantified using three clinical rating scales.

RESULTS

A severe symptomatology, high serotonin (5-HT) platelet content, and high or low urinary B could represent risk factors leading to a smaller or delayed response to an AT. Furthermore, the early improvement under ATs was negatively correlated to pretreatment 5-HT platelet content.

CONCLUSIONS

Determination of 5-HT level could be useful in the choice of an AT.

Relationship between blood serotonergic variables, melancholic traits, and response to antidepressant treatments

The relationship between peripheral serotonergic variables, melancholic traits, and clinical improvement after antidepressant treatment was examined in 83 drug-free major depressive patients. Plasma serotonin (5-HT) concentrations was lower in untreated melancholic patients (1.00 +/- 0.11 vs. 1.84 +/- 0.28 ng/mL, p < 0.008; N = 40 and 43, respectively). A tendency was observed for plasma 5-hydroxyindoleacetic acid (p < 0.06), whereas platelet 5-HT and plasma tryptophan did not differ between groups. After blood sampling and clinical ratings, treatment began with fixed doses of 5-HT uptake inhibitors (clomipramine or fluvoxamine), monoamine oxidase inhibitors, or tianeptine, a 5-HT uptake enhancer. There was no significant difference in response rates between patients with and without melancholic traits. The relationship between the clinical response at 6 weeks (>50% reduction of baseline Hamilton score) and the pretreatment values of biochemical variables was examined. Responders had a lower pretreatment platelet 5-HT (530 +/- 36 vs. 664 +/- 50 ng/10(9) platelets, p < 0.03; N = 44 and 39, respectively). Patients with a platelet 5-HT concentration above 800 ng/10(9) platelets had a lower response rate than those below this value (p < 0.003). This difference was maximal in the subgroup of patients treated with 5-HT uptake inhibitors (N = 49). In this subgroup, the response rates of patients with 5-HT concentrations below and above the cutoff point were, respectively, 70% and 17% (p < 0.001). A pretreatment platelet 5-HT value above 800 ng/10(9) platelets had a predictive value for a negative response of 92%. These results suggest the presence of biochemical differences in the peripheral serotonergic system between melancholic and nonmelancholic patients. The inverse relationship between the pretreatment platelet 5-HT content and clinical response may be useful in the investigation of the relationship between the 5-HT system and antidepressant response.

Serotonin transporter-blocking properties of nefazodone assessed by measurement of platelet serotonin

This study aims to determine whether the newly introduced serotonin type 2 (5-HT2) antagonist nefazodone has serotonin (5-HT) transporter-blocking properties at clinical doses in depressed patients. Change in platelet 5-HT was used as an index of the cumulative peripheral 5-HT uptake blockade produced by nefazodone and selective serotonin reuptake inhibitors (SSRIs). Platelet 5-HT was measured before and during treatment with nefazodone (N = 6) or an SSRI (N = 10) in patients with major depression. Corresponding Hamilton Rating Scale for Depression (HAM-D) scores were also obtained over the course of the study. The results of the study demonstrated that the decrease from mean baseline platelet 5-HT after SSRI treatment was significantly greater than the change after nefazodone treatment (-88% vs. -3%; Mann-Whitney U-test, p = 0.0002). Pretreatment platelet 5-HT level, posttreatment platelet 5-HT level, nor the percent decrease in platelet 5-HT correlated with the percent change in HAM-D scores in either treatment group. In conclusion, therapeutic doses of nefazodone do not cause sustained 5-HT uptake inhibition at the platelet 5-HT transporter.

Evidence for tryptophan hydroxylase and hydroxy-indol-O-methyl-transferase mRNAs in human blood platelets

Human blood platelets were tested for the presence of mRNAs coding for tryptophan hydroxylase (TPOH) and hydroxy-indol-o-methyl-transferase (HIOMT). Total RNA was extracted from platelets (12.9 +/- 3.3 mg RNA/100 ml blood, mean +/- SEM of 6 preparations) and cDNA synthesized by reverse transcription using random hexamers, oligo-dT or TPOH- or HIOMT-specific primers, designed to amplify a 254 bp fragment for TPOH and a 301 bp fragment for HIOMT. Positive controls were performed using RNA extracted from human normal or tumoral pineal glands. The PCR products were analyzed by gel electrophoresis, transferred to a nylon membrane and hybridized with a 32P-labeled internal probe. When random hexamers, oligo-dT or specific primers were used for reverse transcription, amplification products of the predicted sizes were detectable following electrophoresis in the case of pineal glands and following transfer and hybridization in the case of platelets. These results show TPOH and HIOMT mRNAs to be present in human blood and support the hypothesis that serotonin and melatonin may be synthesized in blood and, more particularly, in platelets.