Receptor Occupancy of the κ-Opioid Antagonist LY2456302 Measured with Positron Emission Tomography and the Novel Radiotracer 11C-LY2795050.

The κ-opioid receptor (KOR) is thought to play an important therapeutic role in a wide range of neuropsychiatric and substance abuse disorders, including alcohol dependence. LY2456302 is a recently developed KOR antagonist with high affinity and selectivity and showed efficacy in the suppression of ethanol consumption in rats. This study investigated brain penetration and KOR target engagement after single oral doses (0.5-25 mg) of LY2456302 in 13 healthy human subjects. Three positron emission tomography scans with the KOR antagonist radiotracer (11)C-LY2795050 were conducted at baseline, 2.5 hours postdose, and 24 hours postdose. LY2456302 was well tolerated in all subjects without serious adverse events. Distribution volume was estimated using the multilinear analysis 1 method for each scan. Receptor occupancy (RO) was derived from a graphical occupancy plot and related to LY2456302 plasma concentration to determine maximum occupancy (rmax) and IC50. LY2456302 dose dependently blocked the binding of (11)C-LY2795050 and nearly saturated the receptors at 10 mg, 2.5 hours postdose. Thus, a dose of 10 mg of LY2456302 appears well suited for further clinical testing. Based on the pharmacokinetic (PK)-RO model, the rmax and IC50 of LY2456302 were estimated as 93% and 0.58 ng/ml to 0.65 ng/ml, respectively. Assuming that rmax is 100%, IC50 was estimated as 0.83 ng/ml.

Test-retest reliability of the novel 5-HT1B receptor PET radioligand [11C]P943.

PURPOSE: [(11)C]P943 is a novel, highly selective 5-HT1B PET radioligand. The aim of this study was to determine the test-retest reliability of [(11)C]P943 using two different modeling methods and to perform a power analysis with each quantification technique. METHODS: Seven healthy volunteers underwent two PET scans on the same day. Regions of interest (ROIs) were the amygdala, hippocampus, pallidum, putamen, insula, frontal, anterior cingulate, parietal, temporal and occipital cortices, and cerebellum. Two multilinear radioligand quantification techniques were used to estimate binding potential: MA1, using arterial input function data, and the second version of the multilinear reference tissue model analysis (MRTM2), using the cerebellum as the reference region. Between-scan percent variability and intraclass correlation coefficients (ICC) were used to assess test-retest reliability. We also performed power analyses to determine the method that would allow the least number of subjects using within-subject or between-subject study designs. A voxel-wise ICC analysis for MRTM2 BPND was performed for the whole brain and all the ROIs studied. RESULTS: Mean percent variability between two scans across regions ranged between 0.4 % and 12.4 % for MA1 BPND, 0.5 % and 11.5 % for MA1 BPP, 16.7 % and 28.3 % for MA1 BPF, and between 0.2 % and 5.4 % for MRTM2 BPND. The power analyses showed a greater number of subjects were required using MA1 BPF compared with other outcome measures for both within-subject and between-subject study designs. ICC values were the highest using MRTM2 BPND and the lowest with MA1 BPF in ten ROIs. Small regions and regions with low binding had lower ICC values than large regions and regions with high binding. CONCLUSION: Reliable measures of 5-HT1B receptor binding can be obtained using the novel PET radioligand [(11)C]P943. Quantification of 5-HT1B receptor binding with MRTM2 BPND and with MA1 BPP provided the least variability and optimal power for within-subject and between-subject designs.

Decreased norepinephrine transporter availability in obesity: Positron Emission Tomography imaging with (S,S)-[(11)C]O-methylreboxetine.

OBJECTIVES: Noradrenergic dysfunction is implicated in obesity. The norepinephrine transporter (NET) regulates the synaptic availability of norepinephrine. However, NET availability has not been previously characterized in vivo in obese people using Positron Emission Tomography (PET) imaging. Here we report findings evaluating NET availability in individuals with obesity and matched lean (i.e., normal weight) comparison subjects. METHODS: Seventeen obese but otherwise healthy individuals with a mean±SD body mass index (BMI) of 34.7±2.6 and 17 lean individuals with a mean±SD BMI of 23.1±1.4 were studied using a high-resolution research tomograph (HRRT) and (S,S)-[(11)C]O-methylreboxetine ([(11)C]-MRB), a radioligand selective for the NET. The regional brain NET binding potential (BPND) was estimated by the multilinear reference tissue model 2 (MRTM2) with the occipital cortex as a reference region. BPND for regions of interest were obtained with the Automated Anatomic Labeling (AAL) template registered to individual's structural MR scans. RESULTS: Obese individuals had lower NET BPND values in the thalamus (p<0.038, 27% reduction) including within the pulvinar (p<0.083, 30% reduction), but not in the hypothalamus, locus coeruleus or the raphe nuclei, compared to lean individuals. When age was included as a covariate, the difference in NET BPND values remained significant in the thalamus (p<0.025) and pulvinar (p<0.042). CONCLUSIONS: These results indicate that NET availability is decreased in the thalamus, including the pulvinar, in obese individuals. These findings further support data indicating noradrenergic dysfunction in obesity and suggest impaired NE clearance in obesity.

Anxiety symptoms, cerebral amyloid burden and memory decline in healthy older adults without dementia: 3-year prospective cohort study.

Although beta-amyloid, anxiety and depression have linked cross-sectionally to reduced memory function in healthy older adults without dementia, prospective data evaluating these associations are lacking. Using data an observational cohort study of 178 healthy older adults without dementia followed for 3 years, we found that anxiety symptoms significantly moderated the relationship between beta-amyloid level and decline in verbal (Cohen's d = 0.65) and episodic (Cohen's d = 0.38) memory. Anxiety symptoms were additionally linked to greater decline in executive function, irrespective of beta-amyloid and other risk factors. These findings suggest that interventions to mitigate anxiety symptoms may help delay memory decline in otherwise healthy older adults with elevated beta-amyloid.

Attention bias variability and symptoms of posttraumatic stress disorder.

Cognitive theories implicate information-processing biases in the etiology of anxiety disorders. Results of attention-bias studies in posttraumatic stress disorder (PTSD) have been inconsistent, suggesting biases towards and away from threat. Within-subject variability of attention biases in posttraumatic patients may be a useful marker for attentional control impairment and the development of posttrauma symptoms. This study reports 2 experiments investigating threat-related attention biases, mood and anxiety symptoms, and attention-bias variability following trauma. Experiment 1 included 3 groups in a cross-sectional design: (a) PTSD, (b) trauma-exposed without PTSD, and (c) healthy controls with no trauma or Axis I diagnoses. Greater attention-bias variability was found in the PTSD group compared to the other 2 groups (η(p)2=.23); attention-bias variability was significantly and positively correlated (r = .37) with PTSD symptoms. Experiment 2 evaluated combat-exposed and nonexposed soldiers before and during deployment. Attention-bias variability did not differentiate groups before deployment, but did differentiate groups during deployment (ηp2=.16); increased variability was observed in groups with acute posttraumatic stress symptoms and acute depression symptoms only. Attention-bias variability could be a useful marker for attentional impairment related to threat cues associated with mood and anxiety symptoms after trauma exposure.

Age-modulated association between prefrontal NAA and the BDNF gene.

Brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology of psychiatric and neurological disorders and in the mechanisms of antidepressant pharmacotherapy. Psychiatric and neurological conditions have also been associated with reduced brain levels of N-acetyl-aspartate (NAA), which has been used as a putative marker of neural integrity. However, few studies have explored the relationship between BDNF polymorphisms and NAA levels directly. Here, we present data from a single-voxel proton magnetic resonance spectroscopy study of 64 individuals and explore the relationship between BDNF polymorphisms and prefrontal NAA level. Our results indicate an association between a single nucleotide polymorphism (SNP) within BDNF, known as rs1519480, and reduced NAA level (p = 0.023). NAA levels were further predicted by age and Asian ancestry. There was a significant rs1519480 × age interaction on NAA level (p = 0.031). Specifically, the effect of rs1519480 on NAA level became significant at age ⩾34.17 yr. NAA level decreased with advancing age for genotype TT (p = 0.001) but not for genotype CT (p = 0.82) or CC (p = 0.34). Additional in silico analysis of 142 post-mortem brain samples revealed an association between the same SNP and reduced BDNF mRNA expression in the prefrontal cortex. The rs1519480 SNP influences BDNF mRNA expression and has an impact on prefrontal NAA level over time. This genetic mechanism may contribute to inter-individual variation in cognitive performance seen during normal ageing, as well as contributing to the risk for developing psychiatric and neurological conditions.

Normative database of the serotonergic system in healthy subjects using multi-tracer PET.

The highly diverse serotonergic system with at least 16 different receptor subtypes is implicated in the pathophysiology of most neuropsychiatric disorders including affective and anxiety disorders, obsessive compulsive disorder, post-traumatic stress disorder, eating disorders, sleep disturbance, attention deficit/hyperactivity disorder, drug addiction, suicidal behavior, schizophrenia, Alzheimer, etc. Alterations of the interplay between various pre- and postsynaptic receptor subtypes might be involved in the pathogenesis of these disorders. However, there is a lack of comprehensive in vivo values using standardized procedures. In the current PET study we quantified 3 receptor subtypes, including the major inhibitory (5-HT(1A) and 5-HT(1B)) and excitatory (5-HT(2A)) receptors, and the transporter (5-HTT) in the brain of healthy human subjects to provide a database of standard values. PET scans were performed on 95 healthy subjects (age=28.0 ± 6.9 years; 59% males) using the selective radioligands [carbonyl-(11)C]WAY-100635, [(11)C]P943, [(18)F]altanserin and [(11)C]DASB, respectively. A standard template in MNI stereotactic space served for region of interest delineation. This template follows two anatomical parcellation schemes: 1) Brodmann areas including 41 regions and 2) AAL (automated anatomical labeling) including 52 regions. Standard values (mean, SD, and range) for each receptor and region are presented. Mean cortical and subcortical binding potential (BP) values were in good agreement with previously published human in vivo and post-mortem data. By means of linear equations, PET binding potentials were translated to post-mortem binding (provided in pmol/g), yielding 5.89 pmol/g (5-HT(1A)), 23.5 pmol/g (5-HT(1B)), 31.44 pmol/g (5-HT(2A)), and 11.33 pmol/g (5-HTT) being equivalent to the BP of 1, respectively. Furthermore, we computed individual voxel-wise maps with BP values and generated average tracer-specific whole-brain binding maps. This knowledge might improve our interpretation of the alterations taking place in the serotonergic system during neuropsychiatric disorders.

Positron emission tomography shows elevated cannabinoid CB1 receptor binding in men with alcohol dependence.

BACKGROUND: Several lines of evidence link cannabinoid (CB) type 1 (CB (1) ) receptor-mediated endogenous CB (eCB) signaling to the etiology of alcohol dependence (AD). However, to date, only peripheral measures of eCB function have been collected in living humans with AD and no human in vivo data on the potentially critical role of the brain CB (1) receptor in AD have been published. This is an important gap in the literature, because recent therapeutic developments suggest that these receptors could be targeted for the treatment for AD. METHODS: Medication-free participants were scanned during early abstinence 4 weeks after their last drink. Using positron emission tomography (PET) with a high-resolution research tomograph and the CB (1) receptor selective radiotracer [(11) C]OMAR, we determined [(11) C]OMAR volume of distribution ( V (T) ) values, a measure of CB (1) receptor density, in a priori selected brain regions in men with AD (n = 8, age 37.4 ± 7.9 years; 5 smokers) and healthy control (HC) men (n = 8, age 32.5 ± 6.9 years; all nonsmokers). PET images reconstructed using the MOLAR algorithm with hardware motion correction were rigidly aligned to the subject-specific magnetic resonance (MR) image, which in turn was warped to an MR template. Time-activity curves (TACs) were extracted from the dynamic PET data using a priori selected regions of interest delineated in the MR template space. RESULTS: In AD relative to HC, [(11) C]OMAR V (T) values were elevated by approximately 20% (p = 0.023) in a circuit, including the amygdala, hippocampus, putamen, insula, anterior and posterior cingulate cortices, and orbitofrontal cortex. Age, body mass index, or smoking status did not influence the outcome. CONCLUSIONS: These findings agree with preclinical evidence and provide the first, albeit still preliminary in vivo evidence suggesting a role for brain CB (1) receptors in AD. The current study design does not answer the important question of whether elevated CB (1) receptors are a preexisting vulnerability factor for AD or whether elevations develop as a consequence of AD.

Prefrontal cortical abnormalities in currently depressed versus currently remitted patients with major depressive disorder.

Previous neuromorphometric investigations of major depressive disorder (MDD) have reported abnormalities in gray matter in several regions, although the results have been inconsistent across studies. Some discrepancies in the results across studies may reflect design limitations such as small sample sizes, whereas others may reflect biological variability that potentially manifests as differences in clinical course. For example, it remains unclear whether the abnormalities found in persistently depressed MDD subjects extend to or persist in patients who experience prolonged remission. The aim of the present study was to investigate gray matter (GM) differences in unmedicated, currently-depressed participants (dMDD) and unmedicated, currently-remitted (rMDD) participants with MDD compared to healthy controls (HC). The GM density and volume were compared across groups using voxel-based morphometry, a quantitative neuroanatomical technique, and high-resolution MRI images from 107 HC, 58 dMDD and 27 rMDD subjects. Relative to the HC group the dMDD group had reduced GM in the dorsal anterolateral (DALPFC), the dorsomedial (DMPFC) and the ventrolateral prefrontal cortex (VLPFC). Relative to the rMDD group the dMDD group showed reduced GM in the DALPFC, the VLPFC, the anterior cingulate cortex (ACC), the precuneus and the inferior parietal lobule. No regions were identified in which the rMDD group showed significantly lower GM compared to the HC group after p-values were corrected for the number of comparisons performed. In unmedicated patients in the depressed phase of MDD, we found evidence of morphometric abnormalities in DALPFC and in medial prefrontal cortical regions belonging to the visceromotor network. These findings, along with the absence of GM abnormalities in the remitted sample imply a possible link between greater GM tissue and better clinical outcome. Consistent with other neuroimaging and post-mortem neuropathological studies of MDD, we also found evidence of decreased white matter in patients with dMDD and rMDD.

Cognitive dysfunction in depression: neurocircuitry and new therapeutic strategies.

Major depressive disorder (MDD) is a disabling medical condition associated with significant morbidity, mortality and public health costs. However, neurocircuitry abnormalities underlying depression remain incompletely understood and consequently current treatment options are unfortunately limited in efficacy. Recent research has begun to focus specifically on cognitive aspects of depression and potential neurobiological correlates. Two fundamental types of cognitive dysfunction observed in MDD are cognitive biases, which include distorted information processing or attentional allocation toward negative stimuli, and cognitive deficits, which include impairments in attention, short-term memory and executive functioning. In this article, we present a selective review of current research findings in these domains and examine neuroimaging research that is beginning to characterize the neurocircuitry underlying these biases and deficits. We propose that deficient cognitive functioning, attention biases and the sustained negative affect characteristic of MDD can be understood as arising in part from dysfunctional prefrontal-subcortical circuitry and related disturbances in the cognitive control of emotion. Finally, we highlight potential new pharmacological and non-pharmacological therapeutic strategies for MDD based on an evolving mechanistic understanding of the disorder.

Differential conditioned fear response predicts duloxetine treatment outcome in male veterans with PTSD: a pilot study.

This pilot study tested whether posttraumatic stress disorder (PTSD) patients with impaired conditioned fear acquisition were refractory to open-label duloxetine treatment. Patients with a differential conditioned fear response at pre-treatment subsequently demonstrated significant reductions in PTSD symptoms. These data provide initial evidence of a putative biomarker of selective treatment response in PTSD.

PET imaging of the effects of age and cocaine on the norepinephrine transporter in the human brain using (S,S)-[(11)C]O-methylreboxetine and HRRT.

OBJECTIVES: The role of the norepinephrine transporter (NET) in cocaine dependence has never been demonstrated via in vivo imaging due to the lack of suitable NET radioligands. Here we report our preliminary studies evaluting the NET in individuals with cocaine dependence (COC) in comparison to healthy controls (HC) using (S,S)-[(11)C]methylreboxetine ([(11)C]MRB), the most promising C-11 labeled positron-emission tomography (PET) radioligand for NET developed to date. METHODS: Twenty two human volunteers (10 COC and 12 HC) underwent dynamic (11)C-MRB-PET acquisition using a High Resolution Research Tomograph (HRRT). Binding potential (BP(ND)) parametric images were computed using the simplified reference tissue model (SRTM2) with occipital cortex as reference region. BP(ND) values were compared between the two groups. RESULTS: Locus coeruleus (LC), hypothalamus, and pulvinar showed a significant inverse correlation with age among HC (age range = 25-54 years; P = 0.04, 0.009, 0.03 respectively). The BP(ND) was significantly increased in thalamus (27%; P < 0.02) and dorsomedial thalamic nuclei (30%; P < 0.03) in COC as compared to HC. Upon age normalization, the upregulation of NET in COC also reached significance in LC (63%, P < 0.01) and pulvinar (55%, P < 0.02) regions. CONCLUSION: Our results suggest that (a) brain NET concentration declines with age in HC, and (b) there is a significant upregulation of NET in thalamus and dorsomedial thalamic nucleus in COC as compared to HC. Our results also suggest that the use of [(11)C]MRB and HRRT provides an effective strategy for studying alterations of the NET system in humans.

Retraction Note: Cannabinoid type 1 receptor availability in the amygdala mediates threat processing in trauma survivors.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Serotonin transporter genotype and depressive phenotype determination by discriminant analysis of glucose metabolism under acute tryptophan depletion.

Acute tryptophan depletion (ATD) putatively results in a transient reduction in central serotonin transmission, and induces depressed mood in some un-medicated subjects with remitted major depressive disorder (MDD). The 5-HT transporter promoter region length polymorphism (5-HTTLPR) has been shown to influence behavioral and metabolic responses to ATD, as well as the risk for developing MDD within the context of stress. The current study investigates the relationships between 5-HTTLPR genotype, neurophysiologic response to ATD, and diagnostic phenotype (healthy control subjects versus MDD subjects differentiated by their depressive response to ATD) using (18)FDG-PET. Un-medicated subjects with remitted MDD and healthy controls were genotyped for the long (l) and short (s) alleles of the 5-HTTLPR polymorphism and categorized into one of three genotypes. On two separate occasions, subjects received either a placebo or an amino acid mixture designed to deplete plasma tryptophan, followed by (18)FDG-PET scanning. Depressive symptoms were rated to determine the diagnostic phenotype. Descriptive and predictive discriminant analyses were performed using brain regional metabolic data to classify according to phenotype and genotype. Overall, 79% of the cases were classified correctly by genotype, and 85% were classified correctly by phenotype. In a leave-one-out cross-validation, 72% of the subjects were classified correctly as carrying an s-allele, and 79% of the subjects were classified correctly by primary diagnosis. The robust nature of the classification results indicates that much of the variance in metabolic response to ATD is accounted for by genotypic and phenotypic category.

Plasma NPY concentrations during tryptophan and sham depletion in medication-free patients with remitted depression.

BACKGROUND: Neuropeptide Y (NPY) and serotonergic systems have been implicated in the pathophysiology of depression but have not yet been linked together. METHODS: In a randomized, double-blind crossover study, 28 medication-free patients with remitted depression and 26 healthy control subjects underwent tryptophan depletion (TD) and sham depletion. Plasma NPY concentrations were determined at baseline and at +5, +7, and +24 h during TD and sham depletion, respectively. Hamilton Depression Rating Scale (HDRS, 24-item) scores were assessed at baseline and at +7 and +24 h after TD and sham depletion, respectively. RESULTS: There was no difference between healthy subjects and patients with remitted depression in baseline plasma NPY concentrations and in plasma NPY concentrations during TD and sham depletion, respectively. Plasma NPY concentrations did not differ between TD and sham depletion. At no time point there was an association between HDRS scores and plasma NPY concentrations in patients with remitted depression. LIMITATIONS: Plasma NPY concentrations in rMDD patients were not obtained during the symptomatic phase of the illness. Only peripheral measurements of NPY were used. CONCLUSIONS: Decreased plasma NPY concentrations, as described previously during a spontaneous episode of major depression, appear as state but not as trait marker in depression. No evidence was found for an involvement of plasma NPY in relapse during TD. There appears no direct functional link between serotonergic neurotransmission and plasma NPY concentrations.

Altered cerebral benzodiazepine receptor binding in post-traumatic stress disorder.

Agonists of the γ-aminobutyric acid (GABA) type A benzodiazepine (BZD) receptor exert anxiolytic effects in anxiety disorders, raising the possibility that altered GABA-ergic function may play a role in the pathophysiology of anxiety disorders, such as post-traumatic stress disorder (PTSD). However, few neuroimaging studies have assessed the function or binding potential of the central GABAA BZD receptor system in PTSD. Therefore, our aim was to compare the BZD receptor binding potential between PTSD patients and healthy controls. Twelve medication-free participants with a current diagnosis of PTSD and 15 matched healthy controls underwent positron emission tomography (PET) imaging using [11C] flumazenil. Structural magnetic resonance imaging (MRI) scans were obtained and co-registered to the PET images to permit co-location of neuroanatomical structures in the lower resolution PET image data. Compared to healthy controls, PTSD patients exhibited increased BZD binding in the caudal anterior cingulate cortex and precuneus (p's < 0.05). Severity of PTSD symptoms positively correlated with BZD binding in the left mid- and anterior insular cortices. This study extends previous findings by suggesting that central BZD receptor system involvement in PTSD includes portions of the default mode and salience networks, along with insular regions that support interoception and autonomic arousal.

Interactive effects of sex and 5-HTTLPR on mood and impulsivity during tryptophan depletion in healthy people.

BACKGROUND: Serotonin (5-HT) plays a central role in mood regulation and impulsivity. We studied whether healthy men and women react differently on mood and impulsivity measures during acute tryptophan depletion (ATD). We also studied the relative contribution of a functional length triallelic polymorphism in the promoter region of the serotonin transporter, designated 5-HTTLPR, to the behavioral responses to ATD. METHODS: Thirty-nine men and 44 women participated in a randomized, double-blind, parallel group ATD study. Behavioral measures of impulsivity and mood were obtained. RESULTS: During ATD, women reported mood reduction and showed a cautious response style, which is commonly associated with depression. Men showed an impulsive response style and did not report mood reduction. The 5-HTTLPR influenced the mood response to ATD in women. CONCLUSIONS: Healthy men became more impulsive, whereas healthy women showed mood reduction in response to ATD. This suggests that 5-HT could be one mechanism contributing to the sex differences in the prevalence of mood and impulsivity disorders. The influence of 5-HTTLPR on mood responses in women further substantiates the relevance of this variant in the pathophysiology of at least a subgroup of patients with major depressive disorder.

Sustained low-grade pro-inflammatory state in unmedicated, remitted women with major depressive disorder as evidenced by elevated serum levels of the acute phase proteins C-reactive protein and serum amyloid A.

BACKGROUND: Major depressive disorder (MDD) shows increased coronary artery disease (CAD) risk of unknown mechanism(s). MDD is more common in women than men; CAD diagnosis can be difficult in women. Elevations of the inflammatory markers C-reactive protein (CRP) and serum amyloid A (SAA) predict increased CAD risk in populations; few data on these markers exist in MDD, particularly in remitted patients. METHODS: We measured fasting am serum CRP (high sensitivity, CRP(hs)) and SAA in 18 unmedicated, remitted women with MDD (mean age 41 +/- (SD)12, body mass index (BMI) 25.2 +/- 4.1 kg/m(2)) and 18 BMI-matched healthy control subjects (age 36 +/- 10, BMI 25.3 +/- 3.8 kg/m(2)) on 2 separate occasions, > or = 6 days apart. RESULTS: Repeat SAA and CRP(hs) measurements strongly correlated across study days (SAA: r = .83, p < .001; CRP(hs): r = .94, p < .001). Both SAA (5.30 +/- 3.39 vs. 2.84 +/- 1.87 mg/L, p < .005) and CRP(hs) (3.23 +/- 3.17 vs. 1.12 +/- 1.45 mg/L; p < .01) were significantly elevated in MDD women versus controls. CONCLUSIONS: Elevated SAA and CRP(hs) in remitted, unmedicated women with MDD indicate a pro-inflammatory state unrelated to current depressive symptoms or pharmacotherapy. These findings suggest that inflammatory mechanisms may in part underlie findings of increased CAD risk in MDD.

Differential effects of 5-HTTLPR genotypes on the behavioral and neural responses to tryptophan depletion in patients with major depression and controls.

CONTEXT: Tryptophan depletion (TD) is a model used to study the contribution of reduced serotonin transmission to the pathogenesis of major depressive disorder (MDD). Recent studies have not sufficiently addressed the relative contribution of a functional-length triallelic polymorphism in the promoter of the serotonin transporter, 5-HTTLPR, to the behavioral and neural responses to TD in individuals with remitted MDD (rMDD) and controls. OBJECTIVE: To determine the role of 5-HTTLPR on the behavioral and neural responses to TD in medication-free patients with rMDD and individually matched controls. DESIGN: Participants were stratified according to diagnosis and 5-HTTLPR genotypes and underwent TD on one test day and sham depletion on the other test day in a prospective, double-blind, randomized order. SETTING: Outpatient clinic. PARTICIPANTS: Twenty-seven medication-free patients with rMDD (18 women and 9 men) and 26 controls (17 women and 9 men). INTERVENTIONS: Tryptophan depletion was induced by administration of capsules containing an amino acid mixture without tryptophan. Sham depletion used identical capsules containing lactose. Fludeoxyglucose F 18 positron emission tomography was performed 6 hours after TD. Magnetic resonance images were obtained for each participant. MAIN OUTCOME MEASURES: Quantitative positron emission tomography of regional cerebral metabolic rates for glucose and measures of depression using the Hamilton Depression Rating Scale. RESULTS: Behavioral responses to TD are affected by 5-HTTLPR in patients with rMDD and controls. A direct effect of 5-HTTLPR on the regulation of regional cerebral metabolic rates for glucose was identified in patients with rMDD for the amygdala, hippocampus, and subgenual anterior cingulate cortex. CONCLUSIONS: Variations in 5-HTTLPR modulate the sensitivity of patients with rMDD and controls to the behavioral effects of TD. In patients with rMDD, variations in triallelic 5-HTTLPR have a direct effect on regulation of regional cerebral metabolic rates for glucose in a corticolimbic circuit that has been implicated in rMDD.