Acute occupancy of brain serotonin transporter by sertraline as measured by [11C]DASB and positron emission tomography

Update Lessons from PET Imaging Part II: A Systematic Critical Review on Therapeutic Plasma Concentrations of Antidepressants

BACKGROUND

Compared with antipsychotics, the relationship between antidepressant blood (plasma or serum) concentrations and target engagement is less well-established.

METHODS

We have discussed the literature on the relationship between plasma concentrations of antidepressant drugs and their target occupancy. Antidepressants reviewed in this work are citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, sertraline, venlafaxine, duloxetine, milnacipran, tricyclic antidepressants (amitriptyline, nortriptyline, and clomipramine), bupropion, tranylcypromine, moclobemide, and vortioxetine. Four electronic databases were systematically searched.

RESULTS

We included 32 articles published 1996-2022. A strong relationship between serotonin transporter (SERT) occupancy and drug concentration is well established for selective serotonin reuptake inhibitors. Lower limits of recommended therapeutic reference ranges largely corroborate with the findings from positron emission tomography studies (80% SERT occupancy). Only a few novel studies have investigated alternative targets, that is, norepinephrine transporters (NETs), dopamine transporters (DATs), or monoamine oxidase A (MAO-A). For certain classes of drugs, positron emission tomography study data are inconclusive. Low DAT occupancy after bupropion treatment speculates its discussed mechanism of action. For MAO inhibitors, a correlation between drug concentration and MAO-A occupancy could not be established.

CONCLUSIONS

Neuroimaging studies are critical in TDM-guided therapy for certain antidepressants, whereas for bupropion and MAO inhibitors, the available evidence offers no further insight. Evidence for selective serotonin reuptake inhibitors is strong and justifies a titration toward suggested ranges. For SNRIs, duloxetine, and venlafaxine, NETs are sufficiently occupied, well above the SERT efficacy threshold. For these drugs, a titration toward higher concentrations (within the recommended range) should be considered in case of no response at lower concentrations.

Mechanisms of SSRI Therapy and Discontinuation

SSRIs are one of the most widely used drug therapies in primary care and psychiatry, and central to the management of the most common mental health problems in today's society. Despite this, SSRIs suffer from a slow onset of therapeutic effect and relatively poor efficacy as well as adverse effects, with recent concerns being focused on a disabling SSRI discontinuation syndrome. The mechanism underpinning their therapeutic effect has long shifted away from thinking that SSRIs act simply by increasing 5-HT in the synapse. Rather, a current popular view is that increased 5-HT is just the beginning of a series of complex downstream signalling events, which trigger changes in neural plasticity at the functional and structural level. These changes in plasticity are then thought to interact with neuropsychological processes to enhance re-learning of emotional experiences that ultimately brings about changes in mood. This compelling view of SSRI action is underpinning attempts to understand fast-acting antidepressants, such as ketamine and psychedelic drugs, and aid the development of future therapies. An important gap in the theory is evidence that changes in plasticity are causally linked to relevant behavioural effects. Also, predictions that the SSRI-induced neural plasticity might have applicability in other areas of medicine have not yet been borne out. In contrast to the sophisticated view of the antidepressant action of SSRIs, the mechanism underpinning SSRI discontinuation is little explored. Nevertheless, evidence of rebound increases in 5-HT neuron excitability immediately on cessation of SSRI treatment provide a starting point for future investigation. Indeed, this evidence allows formulation of a mechanistic explanation of SSRI discontinuation which draws on parallels with the withdrawal states of other psychotropic drugs.

30 years of [11C]methyl triflate

Some scientific discoveries are well known only to a core group of researchers working on technical subjects. Nevertheless, they open new research directions, allow existing knowledge to be viewed in entirely new and useful ways, or provide a way to make something that was hard or impossible to make before. Carbon-11 methyl triflate ([¹¹C]MeOTf) is one such advance, facilitating the synthesis of many carbon-11 radio tracers and broadening the range of applications of carbon-11 radiochemistry. The year 2022 marked the 30th anniversary of the original paper in Applied Radiation and Isotopes introducing a simple synthesis of [¹¹C]MeOTf from carbon-11 methyl iodide ([¹¹C]MeI) and it also marked the end of the fruitful career and life of the researcher who developed it, Douglas Jewett. It seems fitting to say a few words on how it came to be and how it has helped advance carbon-11 radiochemistry.

Multivariate analysis of PET pharmacokinetic parameters improves inferential efficiency

PURPOSE

In positron emission tomography quantification, multiple pharmacokinetic parameters are typically estimated from each time activity curve. Conventionally all but the parameter of interest are discarded before performing subsequent statistical analysis. However, we assert that these discarded parameters also contain relevant information which can be exploited to improve the precision and power of statistical analyses on the parameter of interest. Properly taking this into account can thereby draw more informative conclusions without collecting more data.

METHODS

By applying a hierarchical multifactor multivariate Bayesian approach, all estimated parameters from all regions can be analysed at once. We refer to this method as Parameters undergoing Multivariate Bayesian Analysis (PuMBA). We simulated patient-control studies with different radioligands, varying sample sizes and measurement error to explore its performance, comparing the precision, statistical power, false positive rate and bias of estimated group differences relative to univariate analysis methods.

RESULTS

We show that PuMBA improves the statistical power for all examined applications relative to univariate methods without increasing the false positive rate. PuMBA improves the precision of effect size estimation, and reduces the variation of these estimates between simulated samples. Furthermore, we show that PuMBA yields performance improvements even in the presence of substantial measurement error. Remarkably, owing to its ability to leverage information shared between pharmacokinetic parameters, PuMBA even shows greater power than conventional univariate analysis of the true binding values from which the parameters were simulated. Across all applications, PuMBA exhibited a small degree of bias in the estimated outcomes; however, this was small relative to the variation in estimated outcomes between simulated datasets.

CONCLUSION

PuMBA improves the precision and power of statistical analysis of PET data without requiring the collection of additional measurements. This makes it possible to study new research questions in both new and previously collected data. PuMBA therefore holds great promise for the field of PET imaging.

Neurotransmitter transporter occupancy following administration of centanafadine sustained-release tablets: A phase 1 study in healthy male adults

BACKGROUND

Centanafadine is an inhibitor of reuptake transporters for norepinephrine (NET), dopamine (DAT) and serotonin (SERT).

AIMS

This phase 1, adaptive-design positron emission tomography study investigated the occupancy time course of NET, DAT, and SERT and the relationship to centanafadine plasma concentrations.

METHODS

Healthy adult males received centanafadine sustained-release 400 mg/day for 4 days (N = 6) or 800 mg in a single day (N = 4). Assessments included safety monitoring; time course of occupancy of NET, DAT, and SERT; and centanafadine plasma concentrations.

RESULTS

Transporter occupancy was numerically higher for NET versus DAT or SERT. For NET, estimated (mean ± standard error [SE]) maximal observable target occupancy (TO_max) and concentration at half maximal occupancy (IC₅₀) were 64 ± 7% and 132 ± 65 ng/mL, respectively, for all regions and 82 ± 13% and 135 ± 97 ng/mL after excluding the thalamus, which showed high nonspecific binding. For DAT and SERT, TO_max could not be established and was assumed to be 100%; estimated IC₅₀ (mean ± SE) values were 1580 ± 186 ng/mL and 1,760 ± 309 ng/mL, respectively. For centanafadine, the estimated in vivo affinity ratio was 11.9 ± 6.0 (mean ± SE) for NET/DAT, 13.3 ± 7.0 for NET/SERT, and 1.1 ± 0.2 for DAT/SERT. DAT and SERT occupancies at a plasma concentration of 1400 ng/mL were estimated to be 47 and 44%, respectively.

CONCLUSIONS

High occupancy at NET and moderate occupancy at DAT and SERT was observed at peak concentrations achieved following 400 mg total daily doses of centanafadine.

Molecular imaging of the association between serotonin degeneration and beta-amyloid deposition in mild cognitive impairment

BACKGROUND

Degeneration of the serotonin system has been observed in Alzheimer's disease (AD) and in mild cognitive impairment (MCI). In transgenic amyloid mouse models, serotonin degeneration is detected prior to widespread cortical beta-amyloid (Aβ) deposition, also suggesting that serotonin degeneration may be observed in preclinical AD.

METHODS

The differences in the distribution of serotonin degeneration (reflected by the loss of the serotonin transporter, 5-HTT) relative to Aβ deposition was measured with positron emission tomography in a group of individuals with MCI and a group of healthy older adults. A multi-modal partial least squares (mmPLS) algorithm was applied to identify the spatial covariance pattern between 5-HTT availability and Aβ deposition.

RESULTS

Forty-five individuals with MCI and 35 healthy older adults were studied, 22 and 27 of whom were included in the analyses who were "amyloid positive" and "amyloid negative", respectively. A pattern of lower cortical, subcortical and limbic 5-HTT availability and higher cortical Aβ deposition distinguished the MCI from the healthy older control participants. Greater expression of this pattern was correlated with greater deficits in memory and executive function in the MCI group, not in the control group.

CONCLUSION

A spatial covariance pattern of lower 5-HTT availability and Aβ deposition was observed to a greater extent in an MCI group relative to a control group and was associated with cognitive impairment in the MCI group. The results support the application of mmPLS to understand the neurochemical changes associated with Aβ deposition in the course of preclinical AD.

Serotonin Transporter Imaging in Multiple System Atrophy and Parkinson's Disease

BACKGROUND

Both Parkinson's disease (PD) and multiple system atrophy (MSA) exhibit degeneration of brainstem serotoninergic nuclei, affecting multiple subcortical and cortical serotoninergic projections. In MSA, medullary serotoninergic neuron pathology is well documented, but serotonin system changes throughout the rest of the brain are less well characterized.

OBJECTIVES

To use serotonin transporter [¹¹ C]3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile positron emission tomography (PET) to compare serotoninergic innervation in patients with MSA and PD.

METHODS

We performed serotonin transporter PET imaging in 18 patients with MSA, 23 patients with PD, and 16 healthy controls to explore differences in brainstem, subcortical, and cortical regions of interest.

RESULTS

Patients with MSA showed lower serotonin transporter distribution volume ratios compared with patients with PD in the medulla, raphe pontis, ventral striatum, limbic cortex, and thalamic regions, but no differences in the dorsal striatal, ventral anterior cingulate, or total cortical regions. Controls showed greater cortical serotonin transporter binding compared with PD or MSA groups but lower serotonin transporter binding in the striatum and other relevant basal ganglia regions. There were no regional differences in binding between patients with MSA-parkinsonian subtype (n = 8) and patients with MSA-cerebellar subtype (n = 10). Serotonin transporter distribution volume ratios in multiple different regions of interest showed an inverse correlation with the severity of Movement Disorders Society Unified Parkinson's Disease Rating Scale motor score in patients with MSA but not patients with PD.

CONCLUSIONS

Brainstem and some forebrain subcortical region serotoninergic deficits are more severe in MSA compared with PD and show an MSA-specific correlation with the severity of motor impairments. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Serotonin transporter binding in major depressive disorder: impact of serotonin system anatomy

Serotonin transporter (5-HTT) binding deficits are reported in major depressive disorder (MDD). However, most studies have not considered serotonin system anatomy when parcellating brain regions of interest (ROIs). We now investigate 5-HTT binding in MDD in two novel ways: (1) use of a 5-HTT tract-based analysis examining binding along serotonergic axons; and (2) using the Copenhagen University Hospital Neurobiology Research Unit (NRU) 5-HT Atlas, based on brain-wide binding patterns of multiple serotonin receptor types. [11C]DASB 5-HTT PET scans were obtained in 60 unmedicated participants with MDD in a current depressive episode and 31 healthy volunteers (HVs). Binding potential (BPP) was quantified with empirical Bayesian estimation in graphical analysis (EBEGA). Within the [11C]DASB tract, the MDD group showed significantly lower BPP compared with HVs (p = 0.02). This BPP diagnosis difference also significantly varied by tract location (p = 0.02), with the strongest MDD binding deficit most proximal to brainstem raphe nuclei. NRU 5-HT Atlas ROIs showed a BPP diagnosis difference that varied by region (p < 0.001). BPP was lower in MDD in 3/10 regions (p-values < 0.05). Neither [11C]DASB tract or NRU 5-HT Atlas BPP correlated with depression severity, suicidal ideation, suicide attempt history, or antidepressant medication exposure. Future studies are needed to determine the causes of this deficit in 5-HTT binding being more pronounced in proximal axon segments and in only a subset of ROIs for the pathogenesis of MDD. Such regional specificity may have implications for targeting antidepressant treatment, and may extend to other serotonin-related disorders.

Tools for optimising pharmacotherapy in psychiatry (therapeutic drug monitoring, molecular brain imaging and pharmacogenetic tests): focus on antidepressants

Objectives: More than 40 drugs are available to treat affective disorders. Individual selection of the optimal drug and dose is required to attain the highest possible efficacy and acceptable tolerability for every patient.Methods: This review, which includes more than 500 articles selected by 30 experts, combines relevant knowledge on studies investigating the pharmacokinetics, pharmacodynamics and pharmacogenetics of 33 antidepressant drugs and of 4 drugs approved for augmentation in cases of insufficient response to antidepressant monotherapy. Such studies typically measure drug concentrations in blood (i.e. therapeutic drug monitoring) and genotype relevant genetic polymorphisms of enzymes, transporters or receptors involved in drug metabolism or mechanism of action. Imaging studies, primarily positron emission tomography that relates drug concentrations in blood and radioligand binding, are considered to quantify target structure occupancy by the antidepressant drugs in vivo. Results: Evidence is given that in vivo imaging, therapeutic drug monitoring and genotyping and/or phenotyping of drug metabolising enzymes should be an integral part in the development of any new antidepressant drug.Conclusions: To guide antidepressant drug therapy in everyday practice, there are multiple indications such as uncertain adherence, polypharmacy, nonresponse and/or adverse reactions under therapeutically recommended doses, where therapeutic drug monitoring and cytochrome P450 genotyping and/or phenotyping should be applied as valid tools of precision medicine.

Molecular imaging of the serotonin transporter availability and occupancy by antidepressant treatment in late-life depression

Patients with late-life depression (LLD) have a more variable response to pharmacotherapy relative to patients with mid-life depression. Degeneration of the serotonergic system and lower occupancy of the initial target for antidepressant medications, the serotonin transporter (5-HTT), may contribute to variability in treatment response. The focus of this study was to test the hypotheses that lower cortical and limbic serotonin transporter (5-HTT) availability in LLD patients relative to controls and less 5-HTT occupancy by antidepressant medications would be associated with less improvement in mood and cognition with treatment in LLD patients. Twenty LLD patients meeting DSM-IV criteria for a current major depressive episode and 20 non-depressed controls underwent clinical and neuropsychological assessments, magnetic resonance imaging to measure gray matter volumes and high-resolution positron emission tomography (PET) scanning to measure 5-HTT before and after 10-12 weeks of treatment with Citalopram or Sertraline (patients only). Prior to treatment, 5-HTT was lower in LLD patients relative to controls in mainly temporal cortical and limbic (amygdala and hippocampus) regions. Gray matter volumes were not significantly different between groups. 5-HTT occupancy was detected throughout cortical, striatal, thalamic and limbic regions. The magnitude of regional 5-HTT occupancy by antidepressants was 70% or greater across cortical and sub-cortical regions, consistent with the magnitude of 5-HTT occupancy observed in mid-life depressed patients. Greater regional 5-HTT occupancy correlated with greater improvement in depressive symptoms and visual-spatial memory performance. These data support the hypothesis that serotonin degeneration and variability in 5-HTT occupancy may contribute to heterogeneity in treatment response in LLD patients.

Quantification of Positron Emission Tomography Data Using Simultaneous Estimation of the Input Function: Validation with Venous Blood and Replication of Clinical Studies

PURPOSE

To determine if one venous blood sample can substitute full arterial sampling in quantitative modeling for multiple positron emission tomography (PET) radiotracers using simultaneous estimation of the input function (SIME).

PROCEDURES

Participants underwent PET imaging with [¹¹C]ABP688, [¹¹C]CUMI-101, and [¹¹C]DASB. Full arterial sampling and additional venous blood draws were performed for quantification with the arterial input function (AIF) and SIME using one arterial or venous (vSIME) sample.

RESULTS

Venous and arterial metabolite-corrected plasma activities were within 6 % of each other at varying time points. vSIME- and AIF-derived outcome measures were in good agreement, with optimal sampling times of 12 min ([¹¹C]ABP688), 90 min ([¹¹C]CUMI-101), and 100 min ([¹¹C]DASB). Simulation-based power analyses revealed that SIME required fewer subjects than the AIF method to achieve statistical power, with significant reductions for [¹¹C]CUMI-101 and [¹¹C]DASB with vSIME. Replication of previous findings and test-retest analyses bolstered the simulation analyses.

CONCLUSIONS

We demonstrate the feasibility of AIF recovery using SIME with one venous sample for [¹¹C]ABP688, [¹¹C]CUMI-101, and [¹¹C]DASB. This method simplifies PET acquisition while allowing for fully quantitative modeling, although some variability and bias are present with respect to AIF-based quantification, which may depend on the accuracy of the single venous blood measurement.

Prediction of lithium treatment response in bipolar depression using 5-HTT and 5-HT1A PET

BACKGROUND

Lithium, one of the few effective treatments for bipolar depression (BPD), has been hypothesized to work by enhancing serotonergic transmission. Despite preclinical evidence, it is unknown whether lithium acts via the serotonergic system. Here we examined the potential of serotonin transporter (5-HTT) or serotonin 1A receptor (5-HT_1A) pre-treatment binding to predict lithium treatment response and remission. We hypothesized that lower pre-treatment 5-HTT and higher pre-treatment 5-HT_1A binding would predict better clinical response. Additional analyses investigated group differences between BPD and healthy controls and the relationship between change in binding pre- to post-treatment and clinical response. Twenty-seven medication-free patients with BPD currently in a depressive episode received positron emission tomography (PET) scans using 5-HTT tracer [¹¹C]DASB, a subset also received a PET scan using 5-HT_1A tracer [¹¹C]-CUMI-101 before and after 8 weeks of lithium monotherapy. Metabolite-corrected arterial input functions were used to estimate binding potential, proportional to receptor availability. Fourteen patients with BPD with both [¹¹C]DASB and [¹¹C]-CUMI-101 pre-treatment scans and 8 weeks of post-treatment clinical scores were included in the prediction analysis examining the potential of either pre-treatment 5-HTT or 5-HT1A or the combination of both to predict post-treatment clinical scores.

RESULTS

We found lower pre-treatment 5-HTT binding (p = 0.003) and lower 5-HT_1A binding (p = 0.035) were both significantly associated with improved clinical response. Pre-treatment 5-HTT predicted remission with 71% accuracy (77% specificity, 60% sensitivity), while 5-HT_1A binding was able to predict remission with 85% accuracy (87% sensitivity, 80% specificity). The combined prediction analysis using both 5-HTT and 5-HT_1A was able to predict remission with 84.6% accuracy (87.5% specificity, 60% sensitivity). Additional analyses BPD and controls pre- or post-treatment, and the change in binding were not significant and unrelated to treatment response (p > 0.05).

CONCLUSIONS

Our findings suggest that while lithium may not act directly via 5-HTT or 5-HT_1A to ameliorate depressive symptoms, pre-treatment binding may be a potential biomarker for successful treatment of BPD with lithium.

CLINICAL TRIAL REGISTRATION

PET and MRI Brain Imaging of Bipolar Disorder Identifier: NCT01880957; URL: https://clinicaltrials.gov/ct2/show/NCT01880957.

Attenuation Correction Approaches for Serotonin Transporter Quantification With PET/MRI

Background

Several MR-based attenuation correction (AC) approaches were developed to conquer the challenging AC in hybrid PET/MR imaging. These AC methods are commonly evaluated on standardized uptake values or tissue concentration. However, in neurotransmitter system studies absolute quantification is more favorable due to its accuracy. Therefore, our aim was to investigate the accuracy of segmentation- and atlas-based MR AC approaches on serotonin transporter (SERT) distribution volumes and occupancy after a drug challenge.

Methods

18 healthy subjects (7 male) underwent two [¹¹C]DASB PET/MRI measurements in a double-blinded, placebo controlled, cross-over design. After 70 min the selective serotonin reuptake inhibitor (SSRI) citalopram or a placebo was infused. The parameters total and specific volume of distribution (V_T, V_S = BP_P) and occupancy were quantified. All subjects underwent a low-dose CT scan as reference AC method. Besides the standard AC approaches DIXON and UTE, a T1-weighted structural image was recorded to estimate a pseudo-CT based on an MR/CT database (pseudoCT). Another evaluated AC approach superimposed a bone model on AC DIXON. Lastly, an approach optimizing the segmentation of UTE images was analyzed (RESOLUTE). PET emission data were reconstructed with all 6 AC methods. The accuracy of the AC approaches was evaluated on a region of interest-basis for the parameters V_T, BP_P, and occupancy with respect to the results of AC CT.

Results

Variations for V_T and BP_P were found with all AC methods with bias ranging from -15 to 17%. The smallest relative errors for all regions were found with AC pseudoCT (<|5%|). Although the bias between BP_P SSRI and BP_P placebo varied markedly with AC DIXON (<|12%|) and AC UTE (<|9%|), a high correlation to AC CT was obtained (r ²∼1). The relative difference of the occupancy for all tested AC methods was small for SERT high binding regions (<|4%|).

Conclusion

The high correlation might offer a rescaling from the biased parameters V_T and BP_P to the true values. Overall, the pseudoCT approach yielded smallest errors and the best agreement with AC CT. For SERT occupancy, all AC methods showed little bias in high binding regions, indicating that errors may cancel out in longitudinal assessments.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Brain serotonin transporter binding, plasma arachidonic acid and depression severity: A positron emission tomography study of major depression

BACKGROUND

Serotonin transporter (5-HTT) binding and polyunsaturated fatty acids (PUFAs) are implicated in major depressive disorder (MDD). Links between the two systems in animal models have not been investigated in humans.

METHODS

Using positron emission tomography (PET) and [¹¹C]DASB, we studied relationships between 5-HTT binding potential and plasma levels of PUFAs docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) in medication-free MDD patients (n = 21). PUFAs were quantified using transesterification and gas chromatography. Binding potential BP_P, and alternative outcome measures BP_F and BP_ND, were determined for [¹¹C]DASB in six a priori brain regions of interest (ROIs) using likelihood estimation in graphical analysis (LEGA) to calculate radioligand total distribution volume (V_T), and a validated hybrid deconvolution approach (HYDECA) that estimates radioligand non-displaceable distribution volume (V_ND) without a reference region. Linear mixed models used PUFA levels as predictors and binding potential measures as outcomes across the specified ROIs; age and sex as fixed effects; and subject as random effect to account for across-region binding correlations. As nonlinear relationships were observed, a quadratic term was added to final models.

RESULTS

AA predicted both 5-HTT BP_P and depression severity nonlinearly, described by an inverted U-shaped curve. 5-HTT binding potential mediated the relationship between AA and depression severity.

LIMITATIONS

Given the small sample and multiple comparisons, results require replication.

CONCLUSIONS

Our findings suggest that AA status may impact depression pathophysiology through effects on serotonin transport. Future studies should examine whether these relationships explain therapeutic effects of PUFAs in the treatment of MDD.

Cerebral serotonin transporter measurements with [11C]DASB: A review on acquisition and preprocessing across 21 PET centres

Positron Emission Tomography (PET) imaging has become a prominent tool to capture the spatiotemporal distribution of neurotransmitters and receptors in the brain. The outcome of a PET study can, however, potentially be obscured by suboptimal and/or inconsistent choices made in complex processing pipelines required to reach a quantitative estimate of radioligand binding. Variations in subject selection, experimental design, data acquisition, preprocessing, and statistical analysis may lead to different outcomes and neurobiological interpretations. We here review the approaches used in 105 original research articles published by 21 different PET centres, using the tracer [¹¹C]DASB for quantification of cerebral serotonin transporter binding, as an exemplary case. We highlight and quantify the impact of the remarkable variety of ways in which researchers are currently conducting their studies, while implicitly expecting generalizable results across research groups. Our review provides evidence that the foundation for a given choice of a preprocessing pipeline seems to be an overlooked aspect in modern PET neuroscience. Furthermore, we believe that a thorough testing of pipeline performance is necessary to produce reproducible research outcomes, avoiding biased results and allowing for better understanding of human brain function.

Parcellation of the Human Cerebral Cortex Based on Molecular Targets in the Serotonin System Quantified by Positron Emission Tomography In vivo

Parcellation of distinct areas in the cerebral cortex has a long history in neuroscience and is of great value for the study of brain function, specialization, and alterations in neuropsychiatric disorders. Analysis of cytoarchitectonical features has revealed their close association with molecular profiles based on protein density. This provides a rationale for the use of in vivo molecular imaging data for parcellation of the cortex with the advantage of whole-brain coverage. In the current work, parcellation was based on expression of key players of the serotonin neurotransmitter system. Positron emission tomography was carried out for the quantification of serotonin 1A (5-HT1A, n = 30) and 5-HT2A receptors (n = 22), the serotonin-degrading enzyme monoamine oxidase A (MAO-A, n = 32) and the serotonin transporter (5-HTT, n = 24) in healthy participants. Cortical protein distribution maps were obtained using surface-based quantification. Based on k-means clustering, silhouette criterion and bootstrapping, five distinct clusters were identified as the optimal solution. The defined clusters proved of high explanatory value for the effects of psychotropic drugs acting on the serotonin system, such as antidepressants and psychedelics. Therefore, the proposed method constitutes a sensible approach towards integration of multimodal imaging data for research and development in neuropharmacology and psychiatry.

Serotonin receptors and suicide, major depression, alcohol use disorder and reported early life adversity

Serotonin neurotransmitter deficits are reported in suicide, major depressive disorder (MDD) and alcohol use disorder (AUD). To compare pathophysiology in these disorders, we mapped brain serotonin transporter (SERT), 5-HT1A, and 5-HT2A receptor binding throughout prefrontal cortex and in anterior cingulate cortex postmortem. Cases and controls died suddenly minimizing agonal effects and had a postmortem interval ≤24 h to avoid compromised brain integrity. Neuropathology and toxicology confirmed absence of neuropathology and psychotropic medications. For most subjects (167 of 232), a DSM-IV Axis I diagnosis was made by psychological autopsy. Autoradiography was performed in right hemisphere coronal sections at a pre-genual level. Linear model analyses included sex and age with group and Brodmann area as interaction terms. SERT binding was lower in suicides (p = 0.004) independent of sex (females < males, p < 0.0001), however, the lower SERT binding was dependent on MDD diagnosis (p = 0.014). Higher SERT binding was associated with diagnosis of alcoholism (p = 0.012). 5-HT1A binding was greater in suicides (p < 0.001), independent of MDD (p = 0.168). Alcoholism was associated with higher 5-HT1A binding (p < 0.001) but only in suicides (p < 0.001). 5-HT2A binding was greater in suicides (p < 0.001) only when including MDD (p = 0.117) and alcoholism (p = 0.148) in the model. Reported childhood adversity was associated with higher SERT and 5-HT1A binding (p = 0.004) in nonsuicides and higher 5-HT2A binding (p < 0.001). Low SERT and more 5-HT1A and 5-HT2A binding in the neocortex in depressed suicides is dependent on Axis I diagnosis and reported childhood adversity. Findings in alcoholism differed from those in depression and suicide indicating a distinct serotonin system pathophysiology.

Functional Data Analysis of Dynamic PET Data

One application of positron emission tomography (PET), a nuclear imaging technique, in neuroscience involves in vivo estimation of the density of various proteins (often, neuroreceptors) in the brain. PET scanning begins with the injection of a radiolabeled tracer that binds preferentially to the target protein; tracer molecules are then continuously delivered to the brain via the bloodstream. By detecting the radioactive decay of the tracer over time, dynamic PET data are constructed to reflect the concentration of the target protein in the brain at each time. The fundamental problem in the analysis of dynamic PET data involves estimating the impulse response function (IRF), which is necessary for describing the binding behavior of the injected radiotracer. Virtually all existing methods have three common aspects: summarizing the entire IRF with a single scalar measure; modeling each subject separately; and the imposition of parametric restrictions on the IRF. In contrast, we propose a functional data analytic approach that regards each subject's IRF as the basic analysis unit, models multiple subjects simultaneously, and estimates the IRF nonparametrically. We pose our model as a linear mixed effect model in which population level fixed effects and subject-specific random effects are expanded using a B-spline basis. Shrinkage and roughness penalties are incorporated in the model to enforce identifiability and smoothness of the estimated curves, respectively, while monotonicity and non-negativity constraints impose biological information on estimates. We illustrate this approach by applying it to clinical PET data with subjects belonging to three diagnosic groups. We explore differences among groups by means of pointwise confidence intervals of the estimated mean curves based on bootstrap samples.

Likelihood estimation of drug occupancy for brain PET studies

Neuroimaging with PET is unique in its capability to measure in vivo the occupancy of a drug. The occupancy is typically obtained by conducting PET measurements before and after administration of the drug. For radioligands for which no reference region exists, however, the only established procedure to estimate the occupancy from these data is via linear regression analysis, forming the basis for the so-called Lassen plot. There are several reasons why simple linear regression analysis is not ideal for analyzing these data, including regression attenuation and correlated errors. Here, we propose the use of Likelihood Estimation of Occupancy (LEO) in such a situation. Similar to the Lassen plot, LEO uses the total distribution volume estimates at baseline and at block condition as input, but estimates the non-displaceable distribution volume (V_ND) and fractional occupancy (Δ) via direct maximum likelihood estimation (MLE). This study outlines the rationale for using MLE to estimate Δ and V_ND from PET data, and evaluates its performance in relation to the Lassen Plot via two separate simulation experiments. Finally, LEO and Lassen plot are applied to a PET dataset acquired with [¹¹C]WAY-100635. LEO can exploit the covariance structure of the data to improve the accuracy and precision of the estimates of Δ and V_ND. Theoretically, the covariance matrix can be extracted from a test-retest dataset for the radioligand at hand. Several procedures to estimate the covariance matrix were considered as part of the simulation experiments, and the effect of the test-retest sample size was also assessed. The results are conclusive in that MLE can be used to estimate Δ and V_ND from PET data, avoiding the limitations associated with linear regression. The performance of LEO was, naturally, dependent on the procedure used to estimate the covariance matrix, and the test-retest sample size. Given a test-retest sample size of at least 5, but preferably 10 individuals, LEO provides higher accuracy and precision than Lassen plot in the estimation of Δ and V_ND. We conclude that LEO is valuable in drug occupancy studies.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Regional Differences in Serotonin Transporter Occupancy by Escitalopram: An [11C]DASB PK-PD Study

BACKGROUND AND OBJECTIVE

Escitalopram is one of the most commonly prescribed selective serotonin reuptake inhibitors (SSRIs). It is thought to act by blocking the serotonin transporter (SERT). However, its dose-SERT occupancy relationship is not well known, so it is not clear what level of SERT blockade is achieved by currently approved doses.

METHODS

To determine the dose-occupancy relationship, we measured serial SERT occupancy using [¹¹C]DASB [3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile] positron emission tomography (PET) and plasma drug concentrations after the administration of escitalopram in 12 healthy volunteers. We then built a pharmacokinetic-pharmacodynamic model to characterize the dose-occupancy relationship in the putamen and the dorsal raphe nucleus.

RESULTS

Escitalopram at approved doses occupied less SERT than expected and the SERT occupancy showed regional effects [occupancy was higher in the dorsal raphe nucleus than in the putamen (p < 0.001)]. The drug concentration when 50 % of receptors are occupied (EC₅₀) value and Hill coefficient were significantly different between the putamen (EC₅₀ 4.30, Hill coefficient 0.459) and the dorsal raphe nucleus (EC₅₀ 2.89, Hill coefficient 0.817).

CONCLUSIONS

Higher doses of escitalopram than 20 mg are needed to achieve 80 % or greater SERT occupancy. Higher occupancy by escitalopram in the dorsal raphe nucleus relative to the striatum may explain the delayed onset of action of SSRIs by modulating autoreceptor function. The prevention of the 5-HT_1A autoreceptor-mediated negative feedback could be a strategy for accelerating the clinical antidepressant effects.

Assessment of Ketamine Binding of the Serotonin Transporter in Humans with Positron Emission Tomography

BACKGROUND

Comprehensive description of ketamine's molecular binding profile becomes increasingly pressing as use in real-life patient cohorts widens. Animal studies attribute a significant role in the substance's antidepressant effects to the serotonergic system. The serotonin transporter is a highly relevant target in this context, because it is central to depressive pathophysiology and treatment. This is, to our knowledge, the first study investigating ketamine's serotonin transporter binding in vivo in humans.

METHODS

Twelve healthy subjects were assessed twice using [11C]DASB positron emission tomography. A total of 0.50 mg/kg bodyweight ketamine was administered once i.v. prior to the second positron emission tomography scan. Ketamine plasma levels were determined during positron emission tomography. Serotonin transporter nondisplaceable binding potential was computed using a reference region model, and occupancy was calculated for 4 serotonin transporter-rich regions (caudate, putamen, thalamus, midbrain) and a whole-brain region of interest.

RESULTS

After administration of the routine antidepressant dose, ketamine showed <10% occupancy of the serotonin transporter, which is within the test-retest variability of [11C]DASB. A positive correlation between ketamine plasma levels and occupancy was shown.

CONCLUSIONS

Measurable occupancy of the serotonin transporter was not detectable after administration of an antidepressant dose of ketamine. This might suggest that ketamine binding of the serotonin transporter is unlikely to be a primary antidepressant mechanism at routine antidepressant doses, as substances that facilitate antidepressant effects via serotonin transporter binding (e.g., selective serotonin reuptake inhibitors) show 70% to 80% occupancy. Administration of high-dose ketamine is widening. Based on the positive relationship we find between ketamine plasma levels and occupancy, there is a need for investigation of ketamine's serotonin transporter binding at higher doses.

A positron emission tomography study of the serotonergic system in relation to anxiety in depression

Symptoms of anxiety are highly comorbid with major depressive disorder (MDD) and are known to alter the course of the disease. To help elucidate the biological underpinnings of these prevalent disorders, we previously examined the relationship between components of anxiety (somatic, psychic and motoric) and serotonin 1A receptor (5-HT_1A) binding in MDD and found that higher psychic and lower somatic anxiety was associated with greater 5-HT_1A binding. In this work, we sought to examine the correlation between these anxiety symptom dimensions and 5-HTT binding. Positron emission tomography with [¹¹C]-3-amino-4-(3-dimethylamino-methylphenylsulfanyl)-benzonitrile ([¹¹C]DASB) and a metabolite-corrected arterial input function were used to estimate regional 5-HTT binding in 55 subjects with MDD and anxiety symptoms. Somatic anxiety was negatively correlated with 5-HTT binding in the thalamus (β=-.33, p=.025), amygdala (β=-.31, p=.007) and midbrain (β=-.72, p<.001). Psychic anxiety was positively correlated with 5-HTT binding in midbrain only (β=.46, p=.0025). To relate to our previous study, correlation between 5-HT_1A and 5-HTT binding was examined, and none was found. We also examined how much of the variance in anxiety symptom dimensions could be explained by both 5-HTT and 5-HT_1A binding. The developed model was able to explain 68% (p<.001), 38% (p=.012) and 32% (p=.038) of the total variance in somatic, psychic, and motoric anxiety, respectively. Results indicate the tight coupling between the serotonergic system and anxiety components, which may be confounded when using aggregate anxiety measures. Uncovering serotonin's role in anxiety and depression in this way may give way to a new generation of therapeutics and treatment strategies.

Decreased Pretreatment Amygdalae Serotonin Transporter Binding in Unipolar Depression Remitters: A Prospective PET Study

Major depressive disorder (MDD) is a debilitating condition that affects over 14 million Americans. Remission occurs only in a minority of individuals after first-line antidepressant treatment (∼35%); predictors of treatment outcome are therefore needed. Using PET imaging with a radiotracer specific for the serotonin transporter (5-HTT), ¹¹C-McN5652, we found that patients with MDD who did not achieve remission after 12 mo of naturalistic treatment had lower pretreatment midbrain and amygdala binding than healthy volunteers. Here, using a superior 5-HTT tracer, ¹¹C-DASB, we repeated this study with a prospective design with 8 wk of standardized treatment with escitalopram. As this same cohort also underwent ¹¹C-WAY100635 scans (serotonin-1A receptor [5-HT_1A]), we examined whether using both pretreatment 5-HTT and 5-HT_1A binding could improve prediction of posttreatment remission status. Methods: Thirty-one healthy controls (Hamilton Depression Rating Scale-24 item [HDRS-24] = 1.7) and 26 medication-free patients with MDD (HDRS-24 = 24.8) underwent PET scanning using ¹¹C-DASB. MDD subjects then received 8 wk of standardized pharmacotherapy with escitalopram. The relationship between pretreatment binding and posttreatment clinical status was examined. Arterial blood samples were collected to calculate the metabolite-corrected arterial input function. The outcome measure was V_T/f_P (V_T is volume of distribution in region of interest, f_P is free fraction in plasma). Remission was defined as a posttreatment depression score of less than 10 as well as 50% or more reduction in the score from baseline, resulting in 14 nonremitters (HDRS-24 = 17.6) and 12 remitters (HDRS-24 = 5.3). Results: A linear mixed-effects model comparing group differences in the a priori regions of interest (amygdala and midbrain) revealed a significant difference in amygdala binding between controls and remitters (P = 0.03, unadjusted), where remitters had an 11% lower amygdala binding than controls. Differences in amygdala binding between remitters and nonremitters approached significance (P = 0.06). No additional differences were found between any groups (all P > 0.05). Additionally, we found no relationship between pretreatment amygdala binding and posttreatment depression score, and were unable to predict posttreatment depression severity using both pretreatment 5-HTT (in the amygdala) and 5-HT_1A binding (in the raphe). Conclusion: These results suggest 5-HTT amygdala binding should be examined further, in conjunction with other measures, as a potential biomarker for remission after standardized escitalopram treatment.

Cerebellar heterogeneity and its impact on PET data quantification of 5-HT receptor radioligands

In the quantification of positron emission tomography (PET) radiotracer binding, a commonly used method is reference tissue modeling (RTM). RTM necessitates a proper reference and a ubiquitous choice for G-protein coupled receptors is the cerebellum. We investigated regional differences in uptake within the grey matter of the cerebellar hemispheres (CH), the cerebellar white matter (CW), and the cerebellar vermis (CV) for five PET radioligands targeting the serotonin system. Furthermore, we evaluated the impact of choosing different reference regions when quantifying neocortical binding. The PET and MR images are part of the Cimbi database: 5-HT1AR ([11C]CUMI-101, n = 8), 5-HT1BR ([11C]AZ10419369, n = 36), 5-HT2AR ([11C]Cimbi-36, n = 29), 5-HT4R ([11C]SB207145, n = 59), and 5-HTT ([11C]DASB, n = 100). We employed SUIT and FreeSurfer to delineate CV, CW, and CH and quantified mean standardized uptake values (SUV) and nondisplaceable neocortical binding potential (BPND). Statistical difference was assessed with paired nonparametric two-sided Wilcoxon signed-rank tests and multiple comparison corrected via false discovery rate. We demonstrate significant radioligand specific regional differences in cerebellar uptake. These differences persist when using different cerebellar regions for RTM, but the influence on the neocortical BPND is small. Nevertheless, our data highlight the importance of validating each radioligand carefully for defining the optimal reference region.

Association between serotonin denervation and resting-state functional connectivity in mild cognitive impairment

Resting-state functional connectivity alterations have been demonstrated in Alzheimer's disease (AD) and mild cognitive impairment (MCI) before the observation of AD neuropathology, but mechanisms driving these changes are not well understood. Serotonin neurodegeneration has been observed in MCI and AD and is associated with cognitive deficits and neuropsychiatric symptoms, but the role of the serotonin system in relation to brain network dysfunction has not been a major focus of investigation. The current study investigated the relationship between serotonin transporter availability (SERT; measured using positron emission tomography) and brain network functional connectivity (measured using resting-state functional MRI) in 20 participants with MCI and 21 healthy controls. Two SERT regions of interest were selected for the analysis: the Dorsal Raphe Nuclei (DRN) and the precuneus which represent the cell bodies of origin and a cortical target of projections of the serotonin system, respectively. Both regions show decreased SERT in MCI compared to controls and are the site of early AD pathology. Average resting-state functional connectivity did not differ between MCI and controls. Decreased SERT in DRN was associated with lower hippocampal resting-state connectivity in MCI participants compared to controls. Decreased SERT in the right precuneus was also associated with lower resting-state connectivity of the retrosplenial cortex to the dorsal lateral prefrontal cortex and higher resting-state connectivity of the retrosplenial cortex to the posterior cingulate and in patients with MCI but not in controls. These results suggest that a serotonergic mechanism may underlie changes in brain functional connectivity in MCI. Hum Brain Mapp 38:3391-3401, 2017. © 2017 Wiley Periodicals, Inc.

Possibility of Predicting Serotonin Transporter Occupancy From the In Vitro Inhibition Constant for Serotonin Transporter, the Clinically Relevant Plasma Concentration of Unbound Drugs, and Their Profiles for Substrates of Transporters

Accurate prediction of target occupancy facilitates central nervous system drug development. In this review, we discuss the predictability of serotonin transporter (SERT) occupancy in human brain estimated from in vitro K_i values for human SERT and plasma concentrations of unbound drug (C_u,plasma), as well as the impact of drug transporters in the blood-brain barrier. First, the geometric means of in vitro K_i values were compared with the means of in vivo K_i values (K_i,u,plasma) which were calculated as C_u,plasma values at 50% occupancy of SERT obtained from previous clinical positron emission tomography/single photon emission computed tomography imaging studies for 6 selective serotonin transporter reuptake inhibitors and 3 serotonin norepinephrine reuptake inhibitors. The in vitro K_i values for 7 drugs were comparable to their in vivo K_i,u,plasma values within 3-fold difference. SERT occupancy was overestimated for 5 drugs (P-glycoprotein substrates) and underestimated for 2 drugs (presumably uptake transporter substrates, although no evidence exists as yet). In conclusion, prediction of human SERT occupancy from in vitro K_i values and C_u,plasma was successful for drugs that are not transporter substrates and will become possible in future even for transporter substrates, once the transporter activities will be accurately estimated from in vitro experiments.

Molecular imaging of serotonin degeneration in mild cognitive impairment

Neuropathological and neuroimaging studies have consistently demonstrated degeneration of monoamine systems, especially the serotonin system, in normal aging and Alzheimer's disease. The evidence for degeneration of the serotonin system in mild cognitive impairment is limited. Thus, the goal of the present study was to measure the serotonin transporter in vivo in mild cognitive impairment and healthy controls. The serotonin transporter is a selective marker of serotonin terminals and of the integrity of serotonin projections to cortical, subcortical and limbic regions and is found in high concentrations in the serotonergic cell bodies of origin of these projections (raphe nuclei). Twenty-eight participants with mild cognitive impairment (age 66.6±6.9, 16 males) and 28 healthy, cognitively normal, demographically matched controls (age 66.2±7.1, 15 males) underwent magnetic resonance imaging for measurement of grey matter volumes and high-resolution positron emission tomography with well-established radiotracers for the serotonin transporter and regional cerebral blood flow. Beta-amyloid imaging was performed to evaluate, in combination with the neuropsychological testing, the likelihood of subsequent cognitive decline in the participants with mild cognitive impairment. The following hypotheses were tested: 1) the serotonin transporter would be lower in mild cognitive impairment compared to controls in cortical and limbic regions, 2) in mild cognitive impairment relative to controls, the serotonin transporter would be lower to a greater extent and observed in a more widespread pattern than lower grey matter volumes or lower regional cerebral blood flow and 3) lower cortical and limbic serotonin transporters would be correlated with greater deficits in auditory-verbal and visual-spatial memory in mild cognitive impairment, not in controls. Reduced serotonin transporter availability was observed in mild cognitive impairment compared to controls in cortical and limbic areas typically affected by Alzheimer's disease pathology, as well as in sensory and motor areas, striatum and thalamus that are relatively spared in Alzheimer's disease. The reduction of the serotonin transporter in mild cognitive impairment was greater than grey matter atrophy or reductions in regional cerebral blood flow compared to controls. Lower cortical serotonin transporters were associated with worse performance on tests of auditory-verbal and visual-spatial memory in mild cognitive impairment, not in controls. The serotonin system may represent an important target for prevention and treatment of MCI, particularly the post-synaptic receptors (5-HT4 and 5-HT6), which may not be as severely affected as presynaptic aspects of the serotonin system, as indicated by the observation of lower serotonin transporters in MCI relative to healthy controls.

A hybrid deconvolution approach for estimation of in vivo non-displaceable binding for brain PET targets without a reference region

BACKGROUND AND AIM

Estimation of a PET tracer's non-displaceable distribution volume (VND) is required for quantification of specific binding to its target of interest. VND is generally assumed to be comparable brain-wide and is determined either from a reference region devoid of the target, often not available for many tracers and targets, or by imaging each subject before and after blocking the target with another molecule that has high affinity for the target, which is cumbersome and involves additional radiation exposure. Here we propose, and validate for the tracers [11C]DASB and [11C]CUMI-101, a new data-driven hybrid deconvolution approach (HYDECA) that determines VND at the individual level without requiring either a reference region or a blocking study.

METHODS

HYDECA requires the tracer metabolite-corrected concentration curve in blood plasma and uses a singular value decomposition to estimate the impulse response function across several brain regions from measured time activity curves. HYDECA decomposes each region's impulse response function into the sum of a parametric non-displaceable component, which is a function of VND, assumed common across regions, and a nonparametric specific component. These two components differentially contribute to each impulse response function. Different regions show different contributions of the two components, and HYDECA examines data across regions to find a suitable common VND. HYDECA implementation requires determination of two tuning parameters, and we propose two strategies for objectively selecting these parameters for a given tracer: using data from blocking studies, and realistic simulations of the tracer. Using available test-retest data, we compare HYDECA estimates of VND and binding potentials to those obtained based on VND estimated using a purported reference region.

RESULTS

For [11C]DASB and [11C]CUMI-101, we find that regardless of the strategy used to optimize the tuning parameters, HYDECA provides considerably less biased estimates of VND than those obtained, as is commonly done, using a non-ideal reference region. HYDECA test-retest reproducibility is comparable to that obtained using a VND determined from a non-ideal reference region, when considering the binding potentials BPP and BPND.

CONCLUSIONS

HYDECA can provide subject-specific estimates of VND without requiring a blocking study for tracers and targets for which a valid reference region does not exist.

Intrinsic functional connectivity in late-life depression: trajectories over the course of pharmacotherapy in remitters and non-remitters

Previous studies in late-life depression (LLD) have found that patients have altered intrinsic functional connectivity in the dorsal default mode network (DMN) and executive control network (ECN). We aimed to detect connectivity differences across a treatment trial among LLD patients as a function of remission status. LLD patients (N=37) were enrolled into a 12-week trial of venlafaxine and underwent five functional magnetic resonance imaging resting state scans during treatment. Patients had no history of drug abuse, psychosis, dementia/neurodegenerative diseases or medical conditions with known effects on mood. We investigated whether there were differences in three networks: DMN, ECN and anterior salience network connectivity, as well as a whole brain centrality measure (eigenvector centrality). We found that remitters showed increases in ECN connectivity in the right precentral gyrus and decreases in DMN connectivity in the right inferior frontal gyrus and supramarginal gyrus. The ECN and DMN had regions (middle temporal gyrus and bilateral middle/inferior temporal/fusiform gyrus, respectively) that showed reversed effects (decreased ECN and increased DMN, respectively). Early changes in functional connectivity can occur after initial medication exposure. This study offers new data, indicating that functional connectivity changes differ depending on treatment response and can occur shortly after exposure to antidepressant medication.

Modeling Strategies for Quantification of In Vivo 18F-AV-1451 Binding in Patients with Tau Pathology

Aggregation of hyperphosphorylated tau is a major hallmark of many neurodegenerative diseases, including Alzheimer disease (AD). In vivo imaging with PET may offer important insights into pathophysiologic mechanisms, diagnosis, and disease progression. We describe different strategies for quantification of ¹⁸F-AV-1451 (T807) tau binding, including models with blood sampling and noninvasive alternatives. Methods: Fifteen subjects (4 controls, 6 AD, 3 progressive supranuclear palsy, 2 cortico basal syndrome) underwent 180-min PET with ¹⁸F-AV-1451 and arterial blood sampling. Modeling with arterial input functions included 1-, 2-, and 3-tissue-compartment models and the Logan plot. Using the cerebellum as reference region, we applied the simplified reference tissue model 2 and Logan reference plot. Finally, simplified outcome measures were calculated as ratio, with reference to cerebellar concentrations (SUV ratio [SUVR]) and SUVs. Results: Tissue compartment models were not able to describe the kinetics of ¹⁸F-AV-1451, with poor fits in 33%-53% of cortical regions and 80% in subcortical areas. In contrast, the Logan plot showed excellent fits and parameter variance (total volume of distribution SE < 5%). Compared with the 180-min arterial-based Logan model, strong agreement was obtained for the Logan reference plot also for a reduced scan time of 100 min (R² = 0.91) and SUVR 100-120 min (R² = 0.94), with 80-100 min already representing a reasonable compromise between duration and accuracy (R² = 0.93). Time-activity curves and kinetic parameters were equal for cortical regions and the cerebellum in control subjects but different in the putamen. Cerebellar total volumes of distribution were higher in controls than patients. For these methods, increased cortical binding was observed for AD patients and to some extent for cortico basal syndrome, but not progressive supranuclear palsy. Conclusion: The Logan plot provided the best estimate of tau binding using arterial input functions. Assuming that the cerebellum is a valid reference region, simplified methods seem to provide robust alternatives for quantification, such as the Logan reference plot with 100-min scan time. Furthermore, SUVRs between target and cerebellar activities obtained from an 80- to 100-min static scan offer promising potential for clinical routine application.

Relationship of recent stress to amygdala volume in depressed and healthy adults

BACKGROUND

The amygdala is an integral part of the extrahypothalamic stress-response system, and its volume related to childhood trauma has been studied, but less is known of associations with recent stressful life events. Amygdala volume differences also have been studied in depression, with conflicting results. We hypothesized that effects of stress may be a confound for amygdala volumetric differences in the context of depression.

METHODS

Right-handed participants (n=61) experiencing a major depressive episode during major depressive disorder (n=40) or bipolar depression (n=21) and healthy volunteers (n=60) underwent 1.5T magnetic resonance imaging (MRI). The amygdala perimeter was manually traced with an electronic mouse, based on anatomical landmarks on consecutive coronal slices, by raters blind to diagnosis. The effects of stress on amygdala volume were examined in linear regression models with self-reported physical/sexual abuse or highest category score on the St. Paul-Ramsey scale of stressful life events within the past 6 months as predictors, testing separately for age, sex, race, and depression status as covariates.

RESULTS

Diagnostic groups did not differ significantly with respect to mean age (depressed, 37.8±11.8yrs; healthy, 34.9±13.8yrs) or proportion of males (depressed, 39%, healthy, 50%). We found no association between physical and/or sexual abuse history and amygdala volume. Life stress within the last six months, however, was associated with smaller left amygdala volume. The association between stress and amygdala volume did not differ by diagnostic group.

LIMITATIONS

Most depressed patients were off medications for at least 2 weeks; however, this may not have been long enough to reverse effects of medications on amygdala structure.

CONCLUSIONS

That life stress of relatively short duration was associated with amygdala size in the entire sample, while temporally distant life stress was not, suggests that amygdala volume changes may occur rapidly and reversibly, and independent of depression status.

Time-course of serotonin transporter occupancy by single dose of three SSRIs in human brain: A positron emission tomography study with [(11)C]DASB

Sixteen healthy volunteers were enrolled and divided into four groups according to the single administration of 10mg or 20mg escitalopram, 50mg sertraline, or 20mg paroxetine. Four positron emission tomography scans with [(11)C]DASB were performed on each subject, the first prior to taking the drug, followed by the others at 4, 24, and 48h after. Serotonin transporter occupancies of the drugs at each time point were calculated. All drugs showed maximum occupancy at 4h after dosing and then decreasing occupancies with time. Escitalopram and sertraline showed high occupancies of 69.1-77.9% at 4h, remaining at 52.8-57.8% after 48h. On the other hand, paroxetine showed relatively low occupancy of 44.6%, then decreasing to 10.3% at 48h. Escitalopram (both 10mg and 20mg) and sertraline (50mg) showed high and sustained occupancy. Paroxetine (20mg) showed relatively low and rapidly decreasing occupancy, possibly due to the low plasma concentration by single dosing schedule. Applying the reported concentration of multiple dosing, 20mg paroxetine will induce over 80% occupancy. The present study suggested that these drugs and doses would be sufficient for the treatment of depression.

In Vivo Evaluation of Blood Based and Reference Tissue Based PET Quantifications of [11C]DASB in the Canine Brain

This first-in-dog study evaluates the use of the PET-radioligand [¹¹C]DASB to image the density and availability of the serotonin transporter (SERT) in the canine brain. Imaging the serotonergic system could improve diagnosis and therapy of multiple canine behavioural disorders. Furthermore, as many similarities are reported between several human neuropsychiatric conditions and naturally occurring canine behavioural disorders, making this tracer available for use in dogs also provide researchers an interesting non-primate animal model to investigate human disorders. Five adult beagles underwent a 90 minutes dynamic PET scan and arterial whole blood was sampled throughout the scan. For each ROI, the distribution volume (V_T), obtained via the one- and two- tissue compartment model (1-TC, 2-TC) and the Logan Plot, was calculated and the goodness-of-fit was evaluated by the Akaike Information Criterion (AIC). For the preferred compartmental model BP_ND values were estimated and compared with those derived by four reference tissue models: 4-parameter RTM, SRTM2, MRTM2 and the Logan reference tissue model. The 2-TC model indicated in 61% of the ROIs a better fit compared to the 1-TC model. The Logan plot produced almost identical V_T values and can be used as an alternative. Compared with the 2-TC model, all investigated reference tissue models showed high correlations but small underestimations of the BP_ND-parameter. The highest correlation was achieved with the Logan reference tissue model (Y = 0.9266 x + 0.0257; R² = 0.9722). Therefore, this model can be put forward as a non-invasive standard model for future PET-experiments with [¹¹C]DASB in dogs.

Relationship of the serotonin transporter gene promoter polymorphism (5-HTTLPR) genotype and serotonin transporter binding to neural processing of negative emotional stimuli

BACKGROUND

The lower-expressing (S') alleles of the serotonin transporter (5-HTT) gene promoter polymorphism (5-HTTLPR) are linked to mood and anxiety related psychopathology. However, the specific neural mechanism through which these alleles may influence emotional and cognitive processing remains unknown. We examined the relationship between both 5-HTTLPR genotype and in vivo 5-HTT binding quantified via PET with amygdala reactivity to emotionally negative stimuli. We hypothesized that 5-HTT binding in both raphe nuclei (RN) and amygdala would be inversely correlated with amygdala reactivity, and that number of S' alleles would correlate positively with amygdala reactivity.

METHODS

In medication-free patients with current major depressive disorder (MDD; N=21), we determined 5-HTTLPR genotype, employed functional magnetic resonance imaging (fMRI) to examine amygdala responses to negative emotional stimuli, and used positron emission tomography with [(11)C]DASB to examine 5-HTT binding.

RESULTS

[(11)C]DASB binding in RN and amygdala was inversely correlated with amygdala response to negative stimuli. 5-HTTLPR S' alleles were not associated with amygdala response to negative emotional stimuli.

LIMITATIONS

Primary limitations are small sample size and lack of control group.

CONCLUSIONS

Consistent with findings in healthy volunteers, 5-HTT binding is associated with amygdala reactivity to emotional stimuli in MDD. 5-HTT binding may be a stronger predictor of emotional processing in MDD as compared with 5-HTTLPR genotype.

Positron emission tomography quantification of serotonin transporter binding in medication-free bipolar disorder

OBJECTIVES

Bipolar disorder (BD) is associated with abnormalities in the serotonin transporter (5-HTT), but specific in vivo findings have been discrepant. Using positron emission tomography (PET) and [(11)C]DASB, we compared 5-HTT binding between unmedicated depressed BD subjects and healthy volunteers (HVs).

EXPERIMENTAL DESIGN

5-HTT binding in six brain regions was compared between 17 depressed, unmedicated BD subjects and 31 HVs, using the outcome measure of VT/fP (proportional to the total number of available transporters). Alternative outcome measures were examined as well. 47% of BD were BP I; and 65% reported a prior suicide attempt.

PRINCIPAL OBSERVATIONS

5-HTT binding (VT/fP ) did not differ between BD and HV groups considering six brain regions of interest simultaneously (P = 0.24). In contrast, alternative outcome measures (BPF*, BPP*, and BPND*) indicated lower binding in BD compared with HV across these six regions of interest (BPF*: P = 0.047; BPP*: P = 0.032; BPND*: P = 0.031). 5-HTT binding was unrelated to suicide attempt history, depression severity, bipolar subtype, or history of past substance use disorder.

CONCLUSIONS

Choice of outcome measure strongly affects comparisons of serotonin transporter binding using PET with [(11)C]DASB. We do not find evidence of abnormal 5-HTT binding in bipolar depression using our primary outcome measure, VT /fP . However, we did observe lower 5-HTT binding in BD with alternative outcome measures that are frequently used with [(11)C]DASB. Relative merits and assumptions of different outcome measures are discussed. Evaluation in larger samples and during different mood states, including remission, is warranted.

In Vivo Evaluation of 11C-DASB for Quantitative SERT Imaging in Rats and Mice

Serotonin, or 5-hydroxytryptamine (5-HT), plays a key role in the central nervous system and is involved in many essential neurologic processes such as mood, social behavior, and sleep. The serotonin transporter ligand (11)C-3-amino-4(2-dimethylaminomethyl-phenylsufanyl)-benzonitrile ((11)C-DASB) has been used to determine nondisplaceable binding potential (BPND), which is defined as the quotient of the available receptor density (Bavail) and the apparent equilibrium dissociation rate constant (1/appKD) under in vivo conditions. Because of the increasing number of animal models of human diseases, there is a pressing need to evaluate the applicability of (11)C-DASB to rats and mice. Here, we assessed the feasibility of using (11)C-DASB for quantification of serotonin transporter (SERT) density and affinity in vivo in rats and mice.

METHODS

Rats and mice underwent 4 PET scans with increasing doses of the unlabeled ligand to calculate Bavail and appKD using the multiple-ligand concentration transporter assay. An additional PET scan was performed to calculate test-retest reproducibility and reliability. BPND was calculated using the simplified reference tissue model, and the results for different reference regions were compared.

RESULTS

Displaceable binding of (11)C-DASB was found in all brain regions of both rats and mice, with the highest binding being in the thalamus and the lowest in the cerebellum. In rats, displaceable binding was largely reduced in the cerebellar cortex, which in mice was spatially indistinguishable from cerebellar white matter. Use of the cerebellum with fully saturated binding sites as the reference region did not lead to reliable results. Test-retest reproducibility in the thalamus was more than 90% in both mice and rats. In rats, Bavail, appKD, and ED50 were 3.9 ± 0.4 pmol/mL, 2.2 ± 0.4 nM, and 12.0 ± 2.6 nmol/kg, respectively, whereas analysis of the mouse measurements resulted in inaccurate fits due to the high injected tracer mass.

CONCLUSION

Our data showed that in rats, (11)C-DASB can be used to quantify SERT density with good reproducibility. BPND agreed with the distribution of SERT in the rat brain. It remains difficult to estimate quantitative parameters accurately from mouse measurements because of the high injected tracer mass and underestimation of the binding parameters due to high displaceable binding in the reference region.

High-Dose Testosterone Treatment Increases Serotonin Transporter Binding in Transgender People

BACKGROUND

Women are two times more likely to be diagnosed with depression than men. Sex hormones modulating serotonergic transmission are proposed to partly underlie these epidemiologic findings. Here, we used the cross-sex steroid hormone treatment of transsexuals seeking sex reassignment as a model to investigate acute and chronic effects of testosterone and estradiol on serotonin reuptake transporter (SERT) binding in female-to-male and male-to-female transsexuals.

METHODS

Thirty-three transsexuals underwent [(11)C]DASB positron emission tomography before start of treatment, a subset of which underwent a second scan 4 weeks and a third scan 4 months after treatment start. SERT nondisplaceable binding potential was quantified in 12 regions of interest. Treatment effects were analyzed using linear mixed models. Changes of hormone plasma levels were correlated with changes in regional SERT nondisplaceable binding potential.

RESULTS

One and 4 months of androgen treatment in female-to-male transsexuals increased SERT binding in amygdala, caudate, putamen, and median raphe nucleus. SERT binding increases correlated with treatment-induced increases in testosterone levels, suggesting that testosterone increases SERT expression on the cell surface. Conversely, 4 months of antiandrogen and estrogen treatment in male-to-female transsexuals led to decreases in SERT binding in insula, anterior, and mid-cingulate cortex. Increases in estradiol levels correlated negatively with decreases in regional SERT binding, indicating a protective effect of estradiol against SERT loss.

CONCLUSIONS

Given the central role of the SERT in the treatment of depression and anxiety disorders, these findings may lead to new treatment modalities and expand our understanding of the mechanism of action of antidepressant treatment properties.

Serotonin transporter availability may moderate the association between perceiving stress and depressive tendencies - A SPECT with 5-HTTLPR genotyping study

It was found that serotonin transporter (SERT) gene (5-HTTLPR) polymorphism may moderate the association between perceiving stress and depressive tendency. Although SERT availability in the central nervous system could be associated with 5-HTTLPR polymorphism, whether SERT availability moderates the association between stress and depressive tendency is unclear. This study aimed to investigate whether there is a SERT availability×environmental stress interaction effect, as well as a gene-by-environmental (G×E) interaction effect, using single-photon emission computed tomography (SPECT) with a serotonin transporter radiotracer, [(123)I]ADAM. 87 healthy volunteers were enrolled. The SERT availability was approximated using SPECT with [(123)I]ADAM. Stress and depressive tendencies were measured by the Recent Life Change Questionnaire (RLCQ) and the Taiwanese Depression Questionnaire (TDQ), respectively. A significant interaction of sex×RLCQ×thalamic SERT availability on the TDQ was found, and this effect was robust after controlling for the effect of the SS genotype. The interaction of RLCQ×thalamic SERT availability on the TDQ was significant among males. In particular, a significant association between RLCQ and TDQ (Spearman correlation, ρ=0.64, p<0.01) was found among male subjects with a lower level of thalamic SERT availability. SERT availability may play a role in depressive tendency when under perceived stress among healthy individuals, independent of G×E. This finding provides new evidence that confirms the role of the serotonergic system in the association between stress and depression. Males with lower levels of SERT availability may be more vulnerable to the effects of negative life events.

Disproportionate Reduction of Serotonin Transporter May Predict the Response and Adherence to Antidepressants in Patients with Major Depressive Disorder: A Positron Emission Tomography Study with 4-[18F]-ADAM

BACKGROUND

Many lines of evidence suggest the role of serotonin transporter (SERT)-mediated reuptake of serotonin in the pathophysiology and treatment of major depressive disorder (MDD). This study aimed to examine whether the pretreatment of SERT binding potential or SERT binding ratio between terminal projection regions relative to the midbrain raphe nuclei was associated with treatment outcomes to SERT-targeted antidepressants.

METHODS

We recruited 39 antidepressant-naïve patients with MDD and 39 heathy controls. Positron emission tomography with N,N-dimethyl-2-(2-amino-4-[(18)F]fluorophenylthio)benzylamine (4-[(18)F]-ADAM) was used to measure in vivo SERT availability prior to antidepressant treatment. The 21-item Hamilton Depression Rating Scale (HDRS) was use to assess the severity of depression from baseline to week 6. All the patients with MDD had HDRS scores of 18 or more.

RESULTS

Pretreatment SERT binding in the thalamus and striatum positively correlated with an early reduction in HDRS scores at week 3. Nonresponders and dropout patients showed a proportionate reduction in SERT binding in the terminal projection regions and midbrain compared to healthy controls. In contrast, a disproportionate reduction in SERT binding in the terminal projection regions relative to midbrain was observed in responders.

CONCLUSIONS

The results of this study suggested that a disproportionate reduction in SERT binding between terminal projection regions and midbrain may predict better treatment outcomes in patients with MDD.

Noninvasive blood-free full quantification of positron emission tomography radioligand binding

Full quantification of a positron emission tomography (PET) radioligand binding to its target is preferred because it requires the fewest assumptions, but generally involves measuring the concentration of free radioligand in the arterial plasma by collecting blood samples from the subject's radial artery during the scan, and performing metabolite analysis. This invasive, costly procedure deters subjects' participation, and requires specialized staff and equipment. Simultaneous estimation (SIME) can fully quantify binding using only PET data from multiple brain regions and one individual anchor value, which is based on a single arterial blood sample. Drawing this sample can still be challenging in clinical settings, particularly when using simultaneous PET/magnetic resonance scanners. Here we propose a methodology for full quantification of binding that does not require any blood samples. The methodology substitutes the SIME blood-based anchor with a value predicted using multiple linear regression of noninvasive, easy-to-collect variables related to the radioligand blood concentration, and individual metabolism, such as injected dose, body mass index, or body surface area. As a study case, we show here the methodology in comparison to analysis with full arterial-line blood sampling in a cohort of 23 available scans with [(11)C]CUMI-101, a partial agonist of the serotonin 5-HT1A receptors.

Exploring the impact of BDNF Val66Met genotype on serotonin transporter and serotonin-1A receptor binding

Background

The brain-derived neurotrophic factor (BDNF) Val66Met polymorphism (rs6265) may impact on the in-vivo binding of important serotonergic structures such as the serotonin transporter (5-HTT) and the serotonin-1A (5-HT_1A) receptor. Previous positron emission tomography (PET) studies on the association between Val66Met and 5-HTT and 5-HT_1A binding potential (BP_ND) have demonstrated equivocal results.

Methods

We conducted an imaging genetics study investigating the effect of Val66Met genotype on 5-HTT or 5-HT_1A BP_ND in 92 subjects. Forty-one subjects (25 healthy subjects and 16 depressive patients) underwent genotyping for Val66Met and PET imaging with the 5-HTT specific radioligand [¹¹C]DASB. Additionally, in 51 healthy subjects Val66Met genotypes and 5-HT_1A binding with the radioligand [carbonyl-¹¹C]WAY-100635 were ascertained. Voxel-wise and region of interest-based analyses of variance were used to examine the influence of Val66Met on 5-HTT and 5-HT_1A BP_ND.

Results

No significant differences of 5-HTT nor 5-HT_1A BP_ND between BDNF Val66Met genotype groups (val/val vs. met-carrier) were detected. There was no interaction between depression and Val66Met genotype status.

Conclusion

In line with previous data, our work confirms an absent effect of BDNF Val66Met on two major serotonergic structures. These results could suggest that altered protein expression associated with genetic variants, might be compensated invivo by several levels of unknown feedback mechanisms. In conclusion, Val66Met genotype status is not associated with changes of in-vivo binding of 5-HTT and 5-HT_1A receptors in human subjects.

Quantifying serotonin transporters by PET with [11C]-DASB before and after interferon-α treatment

BACKGROUND

Interferon-α (IFN-α) therapy is frequently associated with disabling depression, fatigue, and related neuropsychiatric effects. Although depression in major depressive disorder is associated with low serotonin transporter binding, animal models suggest that IFN-associated mood effects are linked to increased presynaptic serotonin transporter binding. This study tested the hypotheses that IFN administration to human subjects increases presynaptic serotonin binding activity, and that this effect correlates with incident depression symptoms.

METHODS

Positron emission tomography (PET) scans using [11C]-DASB were obtained for nine hepatitis C patients before and after IFN-α treatment for 8 weeks. Serotonin transporter binding was estimated using the likelihood estimation in graphical analysis (LEGA) model and measured as the volume of distribution (VT) divided by the free fraction of ligand (fP). Depression was measured with the Structured Clinical Interview for DSM-IV Diagnosis (SCID) and the Hamilton Rating Scale for Depression (HAM-D).

RESULTS

Compared to pre-IFN treatment values, changes in serotonin transporter binding and depression symptoms were not significant. There was no correlation between changes in serotonin transporter binding and depression symptoms.

LIMITATIONS

The study is limited by small sample size, minimal effect on observed mood symptoms within the sample, and brief duration of follow-up.

CONCLUSION

These findings do not support the hypothesis of an IFN-induced change in serotonin transporter function as the cause of incident depressive symptoms in patients treated with IFN-α. Additional study of these possible relationships should be of longer duration and include more subjects with more pronounced changes in mood.

Occupancy of serotonin transporter by tramadol: a positron emission tomography study with [11C]DASB

Tramadol is used for the treatment of pain, and it is generally believed to activate the μ-opioid receptor and inhibit serotonin (5-HT) and norepinephrine (NE) transporters. Recent findings from animal experiments suggest that 5-HT reuptake inhibition in brain is related to pain reduction. However, there has been no report of 5-HT transporter (5-HTT) occupancy by tramadol at clinical doses in humans. In the present study, we investigated 5-HTT occupancy by tramadol in five subjects receiving various doses of tramadol by using positron emission tomography (PET) scanning with the radioligand [11C]DASB. Our data showed that mean 5-HTT occupancies in the thalamus by single doses of tramadol were 34.7% at 50 mg and 50.2% at 100 mg. The estimated median effective dose (ED50) of tramadol was 98.1 mg, and the plasma concentration was 0.33 μg/ml 2 h after its administration; 5-HTT occupancy by tramadol was dose-dependent. We estimated 5-HTT occupancy at 78.7% upon taking an upper limit dose (400 mg) of tramadol. The results of the present study support the finding that 5-HTT inhibition is involved in the mechanism underlying the analgesic effect of tramadol in humans, and a clinical dose of tramadol sufficiently inhibits 5-HTT reuptake; this inhibition is similar to that shown by selective serotonin reuptake inhibitors (SSRIs).

Positron emission tomography molecular imaging in late-life depression

Molecular imaging represents a bridge between basic and clinical neuroscience observations and provides many opportunities for translation and identifying mechanisms that may inform prevention and intervention strategies in late-life depression (LLD). Substantial advances in instrumentation and radiotracer chemistry have resulted in improved sensitivity and spatial resolution and the ability to study in vivo an increasing number of neurotransmitters, neuromodulators, and, importantly, neuropathological processes. Molecular brain imaging studies in LLD will be reviewed, with a primary focus on positron emission tomography. Future directions for the field of molecular imaging in LLD will be discussed, including integrating molecular imaging with genetic, neuropsychiatric, and cognitive outcomes and multimodality neuroimaging.

Occupancy of serotonin transporters in the amygdala by paroxetine in association with attenuation of left amygdala activation by negative faces in major depressive disorder

Amygdala hyperactivation in major depressive disorder (MDD) might be attenuated by selective serotonin reuptake inhibitors (SSRIs), but the working mechanism remains unclear. We hypothesized that higher amygdala serotonin transporter (SERT) occupancy by paroxetine results in greater attenuation of amygdala activation by negative facial expressions in MDD patients. We treated fifteen MDD patients (22-55 years) with paroxetine 20-50mg/day. After 6 and 12 weeks, we quantified (1) clinical response (≥50% decrease in Hamilton Depression Rating Scale (HDRS), (2) SERT occupancy in both amygdala measured by repeated [123I]β-CIT single photon emission computed tomography (SPECT), and (3) amygdala activation when viewing fearful and angry (negative) faces with repeated functional MRI scans. Response rates were 4/15 and 9/15 at 6 and 12 weeks, respectively. Attenuation of left amygdala activation was associated with amygdala SERT occupancy (P=0.006) and response (P=0.015). This association may provide a rationale for decreased limbic activity seen during treatment of MDD. It might also explain the rapid decrease in negative attentional bias and amygdala activation caused by SSRIs.