Replicating predictive serum correlates of greater translocator protein distribution volume in brain.

Greater activation of glia, a key component of neuroinflammation, is an important process to target in neuropsychiatric illnesses. However, the magnitude of gliosis varies across cases so low-cost predictors are needed to stratify subjects for clinical trials. Here, several such blood serum measures were assessed in relation to TSPO VT, an index of translocator protein density, measured with positron emission tomography. Blood serum concentration of several products known to be synthesized by activated microglia (and to some extent astroglia) [prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2α), and tumor necrosis factor alpha (TNFα)], controlled by an index of peripheral inflammation [C-reactive protein (CRP)] and TSPO VT were measured in 3 cohorts: prefrontal cortex TSPO VT of 20 subjects with major depressive episodes (MDEs) from major depressive disorder (MDD); and 56 subjects with treatment resistant MDEs from MDD; and dorsal caudate TSPO VT of 20 subjects with obsessive-compulsive disorder. Ln(PGE2/CRP) and ln(TNFα/CRP) consistently correlated with TSPO VT (R2 = 0.36 to 0.11, p = 0.0030 to p = 0.0076). Assessment of threshold serum values to predict highly elevated TSPO VT, demonstrated that a positive predictive value (PPV) of 80% was possible while retaining 40% of participant samples and that receiver operating curves (ROC) ranged from 75 to 81%. Post-hoc selection of ln(CRP) was more predictive (R2 = 0.23 to 0.39, p = 0.0058 to p = 0.00013; ROC > 80%). Systematic assessment of selected peripheral inflammatory markers is promising for developing low cost predictors of TSPO VT. Marker thresholds with high PPV will improve subject stratification for clinical trials of glial targeting therapeutics.

Monoamine Oxidase B Total Distribution Volume in the Prefrontal Cortex of Major Depressive Disorder: An [11C]SL25.1188 Positron Emission Tomography Study.

Importance: Monoamine oxidase B (MAO-B) is an important, high-density enzyme in the brain that generates oxidative stress by hydrogen peroxide production, alters mitochondrial function, and metabolizes nonserotonergic monoamines. Recent advances in positron emission tomography radioligand development for MAO-B in humans enable highly quantitative measurement of MAO-B distribution volume (MAO-B VT), an index of MAO-B density. To date, this is the first investigation of MAO-B in the brain of major depressive disorder that evaluates regions beyond the raphe and amygdala. Objective: To investigate whether MAO-B VT is elevated in the prefrontal cortex in major depressive episodes (MDEs) of major depressive disorder. Design, Setting, and Participants: This case-control study was performed at a tertiary care psychiatric hospital from April 1, 2014, to August 30, 2018. Twenty patients with MDEs without current psychiatric comorbidities and 20 age-matched controls underwent carbon 11-labeled [11C]SL25.1188 positron emission tomography scanning to measure MAO-B VT. All participants were drug and medication free, nonsmoking, and otherwise healthy. Main Outcomes and Measures: The MAO-B VT in the prefrontal cortex (PFC). The second main outcome was to evaluate the association between MAO-B VT in the PFC and duration of major depressive disorder illness. Results: Twenty patients with MDEs (mean [SD] age, 34.2 [13.2] years; 11 women) and 20 healthy controls (mean [SD] age, 33.7 [13.1] years; 10 women) were recruited. Patients with MDEs had significantly greater MAO-B VT in the PFC (mean, 26%; analysis of variance, F1,38 = 19.6, P < .001). In individuals with MDEs, duration of illness covaried positively with MAO-B VT in the PFC (analysis of covariance, F1,18 = 15.2, P = .001), as well as most other cortex regions and the thalamus. Conclusions and Relevance: Fifty percent (10 of 20) of patients with MDEs had MAO-B VT values in the PFC exceeding those of healthy controls. Greater MAO-B VT is an index of MAO-B overexpression, which may contribute to pathologies of mitochondrial dysfunction, elevated synthesis of neurotoxic products, and increased metabolism of nonserotonergic monoamines. Hence, this study identifies a common pathological marker associated with downstream consequences poorly targeted by the common selective serotonin reuptake inhibitor treatments. It is also recommended that the highly selective MAO-B inhibitor medications that are compatible for use with other antidepressants and have low risk for hypertensive crisis should be developed or repurposed as adjunctive treatment for MDEs.

Association of translocator protein total distribution volume with duration of untreated major depressive disorder: a cross-sectional study.

BACKGROUND: People with major depressive disorder frequently exhibit increasing persistence of major depressive episodes. However, evidence for neuroprogression (ie, increasing brain pathology with longer duration of illness) is scarce. Microglial activation, which is an important component of neuroinflammation, is implicated in neuroprogression. We examined the relationship of translocator protein (TSPO) total distribution volume (VT), a marker of microglial activation, with duration of untreated major depressive disorder, and with total illness duration and antidepressant exposure. METHODS: In this cross-sectional study, we recruited participants aged 18-75 years from the Toronto area and the Centre for Addiction and Mental Health (Toronto, ON, Canada). Participants either had major depressive episodes secondary to major depressive disorder or were healthy, as confirmed with a structured clinical interview and consultation with a study psychiatrist. To be enrolled, participants with major depressive episodes had to score a minimum of 17 on the 17-item Hamilton Depression Rating Scale, and had to be medication free or taking a stable dose of medication for at least 4 weeks before PET scanning. Eligible participants were non-smokers; had no history of or concurrent alcohol or substance dependence, neurological illness, autoimmune disorder, or severe medical problems; and were free from acute medical illnesses for the previous 2 weeks before PET scanning. Participants were excluded if they had used brain stimulation treatments within the 6 months before scanning, had used anti-inflammatory drugs lasting at least 1 week within the past month, were taking hormone replacement therapy, had psychotic symptoms, had bipolar disorder (type I or II) or borderline antisocial personality disorder, or were pregnant or breastfeeding. We scanned three primary grey-matter regions of interest (prefrontal cortex, anterior cingulate cortex, and insula) and 12 additional regions and subregions using 18F-FEPPA PET to measure TSPO VT. We investigated the duration of untreated major depressive disorder, and the combination of total duration of disease and duration of antidepressant treatment, as predictor variables of TSPO VT, assessing their significance. FINDINGS: Between Sept 1, 2009, and July 6, 2017, we screened 134 participants for eligibility, of whom 81 were included in the study (current major depressive episode n=51, healthy n=30). We excluded one participant with a major depressive episode from the analysis because of unreliable information about previous medication use. Duration of untreated major depressive disorder was a strong predictor of TSPO VT (p<0·0001), as were total illness duration (p=0·0021) and duration of antidepressant exposure (p=0·037). The combination of these predictors accounted for about 50% of variance in TSPO VT in the prefrontal cortex, anterior cingulate cortex, and insula. In participants who had untreated major depressive disorder for 10 years or longer, TSPO VT was 29-33% greater in the prefrontal cortex, anterior cingulate cortex, and insula than in participants who were untreated for 9 years or less. TSPO VT was also 31-39% greater in the three primary grey-matter regions of participants with long duration of untreated major depressive disorder compared with healthy participants (p=0·00047). INTERPRETATION: Microglial activation, as shown by TSPO VT, is greater in patients with chronologically advanced major depressive disorder with long periods of no antidepressant treatment than in patients with major depressive disorder with short periods of no antidepressant treatment, which is strongly suggestive of a different illness phase. Consistent with this, the yearly increase in microglial activation is no longer evident when antidepressant treatment is given. FUNDING: Canadian Institutes of Health Research and Neuroscience Catalyst Fund.

Inflammation in the Neurocircuitry of Obsessive-Compulsive Disorder.

Importance: For a small percentage of obsessive-compulsive disorder (OCD) cases exhibiting additional neuropsychiatric symptoms, it was proposed that neuroinflammation occurs in the basal ganglia as an autoimmune response to infections. However, it is possible that elevated neuroinflammation, inducible by a diverse range of mechanisms, is important throughout the cortico-striato-thalamo-cortical circuit of OCD. Identifying brain inflammation is possible with the recent advance in positron emission tomography (PET) radioligands that bind to the translocator protein (TSPO). Translocator protein density increases when microglia are activated during neuroinflammation and the TSPO distribution volume (VT) is an index of TSPO density. Objective: To determine whether TSPO VT is elevated in the dorsal caudate, orbitofrontal cortex, thalamus, ventral striatum, dorsal putamen, and anterior cingulate cortex in OCD. Design, Setting, and Participants: This case-control study was conducted at a tertiary care psychiatric hospital from May 1, 2010, to November 30, 2016. Participants with OCD (n = 20) and age-matched healthy control individuals (n = 20) underwent a fluorine F 18-labeled N-(2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide PET scan. It is a high-quality second-generation TSPO-binding PET radiotracer. All participants were drug and medication free, nonsmoking, and otherwise healthy. Main Outcomes and Measures: The TSPO VT was measured in the dorsal caudate, orbitofrontal cortex, thalamus, ventral striatum, dorsal putamen, and anterior cingulate cortex. Compulsions were assessed with the Yale-Brown Obsessive Compulsive Scale. Results: In the OCD and healthy groups, the mean (SD) ages were 27.4 (7.1) years and 27.6 (6.6) years, respectively, and 11 (55%) and 8 (40%) were women, respectively. In OCD, TSPO VT was significantly elevated in these brain regions (mean, 32%; range, 31%-36% except anterior cingulate cortex, 24%; analysis of variance, effect of diagnosis: P < .001 to P = .004). Slightly lower elevations in TSPO VT (22%-29%) were present in other gray matter regions. The Yale-Brown Obsessive Compulsive Scale measure of distress associated with preventing compulsive behaviors significantly correlated with TSPO VT in the orbitofrontal cortex (uncorrected Pearson correlation r = 0.62; P = .005). Conclusions and Relevance: To our knowledge, this is the first study demonstrating inflammation within the neurocircuitry of OCD. The regional distribution of elevated TSPO VT argues that the autoimmune/neuroinflammatory theories of OCD should extend beyond the basal ganglia to include the cortico-striato-thalamo-cortical circuit. Immunomodulatory therapies should be investigated in adult OCD, rather than solely childhood OCD, particularly in cases with prominent distress when preventing compulsions.

Monoamine Oxidase-A Occupancy by Moclobemide and Phenelzine: Implications for the Development of Monoamine Oxidase Inhibitors.

BACKGROUND: Monoamine oxidase inhibitors (MAOIs) are being developed for major depressive disorder, Alzheimer's, and Parkinson's Disease. Newer MAOIs have minimal sensitivity to tyramine, but a key limitation for optimizing their development is that standards for in vivo monoamine oxidase-A (MAO-A) occupancy in humans are not well established. The objectives were to determine the dose-occupancy relationship of moclobemide and the occupancy of phenelzine at typical clinical dosing. METHODS: Major depressive episode (MDE) subjects underwent [(11)C]harmine positron emission tomography scanning prior to and following 6 weeks of treatment with moclobemide or phenelzine. RESULTS: Mean brain MAO-A occupancies were 74.23±8.32% for moclobemide at 300-600 mg daily (n = 11), 83.75±5.52% for moclobemide at 900-1200 mg daily (n = 9), and 86.82±6.89% for phenelzine at 45-60 mg daily (n = 4). The regional dose-occupancy relationship of moclobemide fit a hyperbolic function [F(x) = a(x/[b + x]); F(1,18) = 5.57 to 13.32, p = 0.002 to 0.03, mean 'a': 88.62±2.38%, mean 'b': 69.88±4.36 mg]. Multivariate analyses of variance showed significantly greater occupancy of phenelzine (45-60mg) and higher-dose moclobemide (900-1200 mg) compared to lower-dose moclobemide [300-600 mg; F(7,16) = 3.94, p = 0.01]. CONCLUSIONS: These findings suggest that for first-line MDE treatment, daily moclobemide doses of 300-600mg correspond to a MAO-A occupancy of 74%, whereas for treatment-resistant MDE, either phenelzine or higher doses of moclobemide correspond to a MAO-A occupancy of at least 84%. Therefore, novel MAO inhibitor development should aim for similar thresholds. The findings provide a rationale in treatment algorithm design to raise moclobemide doses to inhibit more MAO-A sites, but suggest switching from high-dose moclobemide to phenelzine is best justified by binding to additional targets.

Role of translocator protein density, a marker of neuroinflammation, in the brain during major depressive episodes.

IMPORTANCE: The neuroinflammatory hypothesis of major depressive disorder is supported by several main findings. First, in humans and animals, activation of the immune system causes sickness behaviors that present during a major depressive episode (MDE), such as low mood, anhedonia, anorexia, and weight loss. Second, peripheral markers of inflammation are frequently reported in major depressive disorder. Third, neuroinflammatory illnesses are associated with high rates of MDEs. However, a fundamental limitation of the neuroinflammatory hypothesis is a paucity of evidence of brain inflammation during MDE. Translocator protein density measured by distribution volume (TSPO VT) is increased in activated microglia, an important aspect of neuroinflammation. OBJECTIVE: To determine whether TSPO VT is elevated in the prefrontal cortex, anterior cingulate cortex (ACC), and insula in patients with MDE secondary to major depressive disorder. DESIGN, SETTING, AND PARTICIPANTS: Case-control study in a tertiary care psychiatric hospital from May 1, 2010, through February 1, 2014. Twenty patients with MDE secondary to major depressive disorder and 20 healthy control participants underwent positron emission tomography with fluorine F 18-labeled N-(2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide ([18F]FEPPA). Patients with MDE were medication free for at least 6 weeks. All participants were otherwise healthy and nonsmokers. MAIN OUTCOMES AND MEASURES: Values of TSPO VT in the prefrontal cortex, ACC, and insula. RESULTS: In MDE, TSPO VT was significantly elevated in all brain regions examined (multivariate analysis of variance, F15,23 = 4.5 [P = .001]). The magnitude of TSPO VT elevation was 26% in the prefrontal cortex (mean [SD] TSPO VT, 12.5 [3.6] in patients with MDE and 10.0 [2.4] in controls), 32% in the ACC (mean [SD] TSPO VT, 12.3 [3.5] in patients with MDE and 9.3 [2.2] in controls), and 33% in the insula (mean [SD] TSPO VT, 12.9 [3.7] in patients with MDE and 9.7 [2.3] in controls). In MDE, greater TSPO VT in the ACC correlated with greater depression severity (r = 0.63 [P = .005]). CONCLUSIONS AND RELEVANCE: This finding provides the most compelling evidence to date of brain inflammation, and more specifically microglial activation, in MDE. This finding is important for improving treatment because it implies that therapeutics that reduce microglial activation should be promising for MDE. The correlation between higher ACC TSPO VT and the severity of MDE is consistent with the concept that neuroinflammation in specific regions may contribute to sickness behaviors that overlap with the symptoms of MDE.

Kinetic modeling of the monoamine oxidase B radioligand [¹¹C]SL25.1188 in human brain with high-resolution positron emission tomography.

This article describes the kinetic modeling of [(11)C]SL25.1188 ([(S)-5-methoxymethyl-3-[6-(4,4,4-trifluorobutoxy)-benzo[d]isoxazol-3-yl]-oxazolidin-2-[(11)C]one]) binding to monoamine oxidase B (MAO-B) in the human brain using high-resolution positron emission tomography (PET). Seven healthy subjects underwent two separate 90- minute PET scans after an intravenous injection of [(11)C]SL25.1188. Complementary arterial blood sampling was acquired. Radioactivity was quickly eliminated from plasma with 80% of parent compound remaining at 90 minutes. Metabolites were more polar than the parent compound. Time-activity curves showed high brain uptake, early peak and washout rate consistent with known regional MAO-B concentration. A two-tissue compartment model (2-TCM) provided better fits to the data than a 1-TCM. Measurement of total distribution volume (VT) showed very good identifiability (based on coefficient of variation (COV)) for all regions of interest (ROIs) (COV(VT)<8%), low between-subject variability (∼20%), and quick temporal convergence (within 5% of final value at 45 minutes). Logan graphical method produces very good estimation of VT. Regional VT highly correlated with previous postmortem report of MAO-B level (r(2)= ≥ 0.9). Specific binding would account from 70% to 90% of VT. Hence, VT measurement of [(11)C]SL25.1(1)88 PET is an excellent estimation of MAO-B concentration.

Evaluation of safety and tolerability, pharmacokinetics, and pharmacodynamics of BMS-820836 in healthy subjects: a placebo-controlled, ascending single-dose study.

RATIONALE: BMS-820836, a novel triple monoamine reuptake inhibitor, is an experimental monotherapy for sufferers of major depressive disorder who have had an inadequate response to an existing antidepressant treatment. OBJECTIVES: This study was conducted to evaluate the safety and tolerability, pharmacokinetics (PK), and serotonin transporter (SERT) and dopamine transporter (DAT) occupancy for single doses of BMS-820836 in healthy subjects. METHODS: Healthy subjects were assigned to seven BMS-820836 dose panels (0.025, 0.1, 0.5, 1, 2, 3, and 5 mg; n = 8 each), in which subjects were randomly allocated 3:1 to a single BMS-820836 dose or matched placebo. Serial blood samples were collected on Days 1, 2, 3, 4, 7, and 14 to characterize the PK of BMS-820836. Following evaluation of the maximum tolerated dose, SERT occupancy was determined by applying [(11)C]DASB positron emission tomography (PET) after single-dose BMS-820836 (0.5 or 3 mg; n = 3 each) and DAT occupancy by applying [(11)C]PE2I PET after single-dose BMS-820836 (3 mg; n = 6). RESULTS: Single oral doses of BMS-820836 (0.025-3 mg) were generally safe and well tolerated. BMS-820836 had a median T max of 5.0-7.2 h and a mean apparent terminal T 1/2 of 34-57 h. Mean striatal SERT occupancies were 19 ± 9 % and 82 ± 8 % after single doses of 0.5 and 3 mg BMS-820836, respectively. The mean striatal DAT occupancy was 19 ± 9 % after a single 3 mg BMS-820836 dose. CONCLUSIONS: Single doses of BMS-820836 have meaningful SERT and DAT occupancy and demonstrate an acceptable safety and tolerability profile in healthy control subjects.

Elevated monoamine oxidase a binding during major depressive episodes is associated with greater severity and reversed neurovegetative symptoms.

Inadequate treatment response occurs in approximately 40% of major depressive episodes (MDEs), and one approach to solve this is careful matching of treatment to the specific pathologies of MDE. One such biological abnormality is elevated monoamine oxidase A (MAO-A) levels, which occurs in the prefrontal and anterior cingulate cortex (PFC and ACC) during MDE; however, the subtypes for which this abnormality is most prominent are unknown. We hypothesized that MAO-A levels in the PFC and ACC are most elevated in MDE with greater severity and reversed neurovegetative symptoms (hypersomnia and either hyperphagia or weight gain). MAO-A VT (an index of MAO-A density) was measured using [(11)C]harmine positron emission tomography (PET) in 42 subjects with MDEs secondary to major depressive disorder and 37 healthy controls. The effect of severity and reversed neurovegetative symptoms on MAO-A VT in the PFC and ACC was analyzed using a multivariate analysis of variance (MANOVA). Greater severity and reversed neurovegetative symptoms were associated with elevated MAO-A VT in the PFC and ACC (MANOVA, severity: F(2,38)=5.44, p=0.008; reversed neurovegetative symptoms: F(2,38)=5.13, p=0.01). Increased MAO-A level, when greater severity and reversed neurovegetative symptoms are present, may explain the association of these clinical features with a preferential response to MAO inhibitors, which is especially well-evidenced for reversed neurovegetative symptoms in MDE. As MAO-A creates oxidative stress, facilitates apoptosis, and metabolizes monoamines, therapeutics opposing these processes are predicted to best treat MDE with greater severity and reversed neurovegetative symptoms.

Convergent effects of acute stress and glucocorticoid exposure upon MAO-A in humans.

Monoamine oxidase-A (MAO-A), a key brain enzyme which metabolizes monoamines, is implicated in the pathophysiology of stress-related illnesses, including major depressive disorder, addiction, and violent behavior. Chronic stressors and glucocorticoid-administration typically associate with elevated MAO-A levels/activity. However, the relationship of shorter stress or glucocorticoid exposures and MAO-A levels/activity is not well established. Our objectives are to assess effects of acute stress upon MAO-A V(T,) an index of MAO-A density, in human brain and acute glucocorticoid exposure upon MAO-A levels in human neuronal and glial cell lines. Twelve healthy, non-smoking participants aged 18-50 underwent [(11)C]harmine positron emission tomography to measure brain MAO-A V(T) on two different days: One under acute psychosocial stress (via Trier Social Stress and Montreal Imaging Stress Tasks) and one under a non-stress condition. MAO-A density (by Western blot) and activity (by [(14)C]-5-HT metabolism and liquid scintillation spectroscopy) were measured in human neuronal and glial cell lines after 4 h exposure to dexamethasone. We observed a significant reduction in whole-brain MAO-A binding as reflected by reductions in 10 of 11 brain regions. Acute dexamethasone exposure in neuronal and glial cells significantly decreased MAO-A activity and protein levels. We observed a highly consistent relationship between acute stressors and glucocorticoid administration and decreased MAO-A binding, activity and protein levels. Since MAO-A metabolizes monoamines, this phenomenon may explain why acute stressors benefit healthy animals even though chronic stress is associated with illness.

Monoamine oxidase A binding in the prefrontal and anterior cingulate cortices during acute withdrawal from heavy cigarette smoking.

CONTEXT: Greater prefrontal cortex and anterior cingulate cortex monoamine oxidase A (MAO-A) binding is associated with depressed mood. Substances in cigarette smoke, such as harman, inhibit MAO-A, and cigarette withdrawal is associated with depressed mood. Dysphoria during cigarette withdrawal predicts relapse. It is unknown whether MAO-A binding increases during early cigarette withdrawal. OBJECTIVES: To measure prefrontal and anterior cingulate cortex MAO-A binding during acute cigarette withdrawal and to assess the relationship with smoking severity, plasma levels of harman, and severity of depression. DESIGN: Study via positron emission tomography of healthy control and cigarette-smoking individuals. PATIENTS: Twenty-four healthy nonsmoking and 24 otherwise healthy cigarette-smoking individuals underwent positron emission tomography with harmine labeled with carbon 11. Healthy nonsmoking individuals underwent scanning once. Cigarette-smoking individuals underwent scanning after acute withdrawal and after active cigarette smoking. Cigarette smoking was heavy (≥25 cigarettes per day) or moderate (15-24 cigarettes per day). SETTING: Tertiary care psychiatric hospital. MAIN OUTCOME MEASURE: An index of MAO-A density, MAO-A V(T), was measured in the prefrontal and anterior cingulate cortices. RESULTS: In heavy-smoking individuals, prefrontal and anterior cingulate cortex MAO-A V(T) was greater during withdrawal (23.7% and 33.3%, respectively; repeated-measures multivariate analysis of variance, F(1,22) = 25.58, P < .001). During withdrawal from heavy smoking, prefrontal and anterior cingulate cortex MAO-A V(T) was greater than in healthy controls (25.0% and 25.6%, respectively; multivariate analysis of variance, F(2,33) = 6.72, P = .004). The difference in MAO-A V(T) in the prefrontal cortex and anterior cingulate cortex between withdrawal and active, heavy smoking covaried with change in plasma harman levels in the prefrontal cortex and anterior cingulate cortex (multivariate analysis of covariance, F(1,10) = 9.97, P = .01). The change in MAO-A V(T) between withdrawal and active, heavy smoking also covaried with severity of depression (multivariate analysis of covariance, F(1,10) = 11.91, P = .006). CONCLUSIONS: The increase in prefrontal and anterior cingulate cortex MAO-A binding and associated reduction in plasma harman level represent a novel, additional explanation for depressed mood during withdrawal from heavy cigarette smoking. This finding resolves a longstanding paradox regarding the association of cigarette smoking with depression and suicide and argues for additional clinical trials on the effects of MAO-A inhibitors on quitting heavy cigarette smoking.

Brain monoamine oxidase A binding in major depressive disorder: relationship to selective serotonin reuptake inhibitor treatment, recovery, and recurrence.

CONTEXT: Highly significant elevations in regional brain monoamine oxidase A (MAO-A) binding were recently reported during major depressive episodes (MDEs) of major depressive disorder (MDD). The relationship between MAO-A levels and selective serotonin reuptake inhibitor (SSRI) treatment, recovery, and recurrence in MDD is unknown. OBJECTIVES: To determine whether brain MAO-A binding changes after SSRI treatment, whether brain MAO-A binding normalizes in subjects with MDD in recovery, and whether there is a relationship between prefrontal and anterior cingulate cortex MAO-A binding in recovery and subsequent recurrence of MDE. DESIGN: Case-control study. SETTING: Tertiary care psychiatric hospital. PARTICIPANTS: Twenty-eight healthy subjects, 16 subjects with an MDE secondary to MDD, and 18 subjects with MDD in recovery underwent carbon 11-labeled harmine positron emission tomography scans. Subjects with MDE were scanned before and after 6 weeks of SSRI treatment. All were otherwise healthy, nonsmoking, and medication free. Subjects with MDD in recovery were followed up for 6 months after MAO-A binding measurement. MAIN OUTCOME MEASURE: Monoamine oxidase A V(T), an index of MAO-A density, was measured in the prefrontal cortex, anterior cingulate cortex, posterior cingulate cortex, dorsal putamen, ventral striatum, thalamus, anterior temporal cortex, midbrain, and hippocampus. RESULTS: Monoamine oxidase A V(T) was significantly elevated in each brain region both during MDE and after SSRI treatment as compared with healthy controls. During recovery, MAO-A V(T) was significantly elevated in each brain region; however, those who went on to recurrence had significantly higher MAO-A V(T) in the prefrontal and anterior cingulate cortex than those who did not. CONCLUSIONS: Elevated MAO-A binding after SSRI treatment indicates persistence of a monoamine-lowering process not present in health. This provides a strong conceptual rationale for continuing SSRI treatment during early remission. Greater MAO-A binding in the prefrontal and anterior cingulate cortex in subjects with MDD in recovery and its association with subsequent recurrence argue that deficient monoamine neuromodulation may persist into recovery and contribute to recurrence.