Regional and subcellular localization in human brain of [3H]paroxetine binding, a marker of serotonin uptake sites

Association between body mass index (BMI) and [123I]Ioflupane (DaTSCAN) availabilities in patients with parkinsonism using single-photon emission computed tomography-computed tomography (SPECT-CT)

AIM

[123I]Ioflupane (DaTSCAN) has a high binding affinity to the dopamine (DA) transporter (DaT) and tenfold less affinity to serotonin (5-HT) transporter (SERT). Both neurotransmitters are considered to contribute to body weight regulation. This study assesses the association between body mass index (BMI) and DaTSCAN availability in brain.

METHOD

Scans from 74 consecutive patients who had undergone DaTSCAN single-photon emission computed tomography-computed tomography (SPECT-CT) were used to obtain semi- and absolute quantitative data in several volumes of interest (VOIs). Relative semi-quantitative specific binding ratios (SBRs) from Chang attenuated SPECT were obtained from GE DaTQUANT. Absolute normalised concentration (NC) was calculated from attenuation/scatter corrected SPECT-CT images, using an adapted version of the EARL Ltd (European Association of Nuclear Medicine (EANM) Research 4 Life) template. Scans were subdivided into either degenerative parkinsonism (abnormal = 49), borderline (n = 14) or scan without evidence of dopaminergic deficit (SWEDD = 11) using visual assessment and SBR values by two nuclear medicine consultants.

RESULTS

SBRs did not correlate with BMI. However, NC values correlated negatively in the entire cohort, with the strongest correlation in the frontal (r = - 0.649. p = 0.000), occipital (r = - 0.555, p = 0.000) regions and pons (r = - 0.555, p = 0.000). In the abnormal (n = 49) and SWEDD group (n = 11), NC of the frontal region was the most correlated with BMI (r = - 0.570, p = 0.000; r = - 0.813, p = 0.002, respectively). In the borderline group (n = 14), the left posterior putamen displayed the strongest correlation (r = - 0.765, p = 0.001).

CONCLUSION

Absolute NC values demonstrate a strong inverse correlation with BMI, strongest in the extrastriatal regions. Due to the predominately non-overlapping distribution of DaT and SERT, this study suggests greater involvement of SERT in obesity with possible interplay with DA transmission.

Translational PET Imaging of Spinal Cord Injury with the Serotonin Transporter Tracer [11C]AFM

PURPOSE

The descending raphespinal serotonin (5-HT) system contributes to neural activities required for locomotion. The presynaptic serotonin transporter (SERT) is a marker of 5-HT innervation. In this study, we explored the use of PET imaging with the SERT radioligand [¹¹C]AFM as a biomarker of 5-HT axon damage after spinal cord injury (SCI) in a rodent model and its translation to imaging SCI in humans.

PROCEDURES

PET imaging with [¹¹C]AFM was performed in healthy rats under baseline and citalopram blocking conditions and a mid-thoracic transection rat model of SCI. The lumbar-to-cervical activity (L/C) ratio was calculated for the healthy and SCI animals to assess SERT binding decrease after SCI. Finally, translation of [¹¹C]AFM PET was attempted to explore its potential to image SCI in humans.

RESULTS

Intense uptake in the brain and intact spinal cord was observed at 30-60 min post-injection of [¹¹C]AFM in healthy rats. About 65% of [¹¹C]AFM uptake in the spinal cord was blocked by citalopram. In the SCI rat model, the cervical uptake of [¹¹C]AFM was similar to that in healthy rats, but the lumbar uptake was dramatically reduced, resulting in about half the L/C ratio in SCI rats compared to healthy rats. In contrast, [¹¹C]AFM uptake in the human spinal cord showed no obvious decrease after treatment with citalopram. In the human subjects with SCI, decreases in [¹¹C]AFM uptake were also not obvious in the section of spinal cord caudal to the injury point.

CONCLUSION

[¹¹C]AFM PET imaging of SERT provides a useful preclinical method to non-invasively visualize the rodent spinal cord and detect SERT changes in SCI rodent models. However, there appears to be little detectable specific binding signal for [¹¹C]AFM in the human spinal cord. An SERT tracer with higher affinity and lower non-specific binding signal is needed to image the spinal cord in humans and to assess the axonal status in SCI patients.

Transcranial Magnetic Stimulation Indices of Cortical Excitability Enhance the Prediction of Response to Pharmacotherapy in Late-Life Depression

BACKGROUND

Older adults with late-life depression (LLD) often experience incomplete or lack of response to first-line pharmacotherapy. The treatment of LLD could be improved using objective biological measures to predict response. Transcranial magnetic stimulation (TMS) can be used to measure cortical excitability, inhibition, and plasticity, which have been implicated in LLD pathophysiology and associated with brain stimulation treatment outcomes in younger adults with depression. TMS measures have not yet been investigated as predictors of treatment outcomes in LLD or pharmacotherapy outcomes in adults of any age with depression.

METHODS

We assessed whether pretreatment single-pulse and paired-pulse TMS measures, combined with clinical and demographic measures, predict venlafaxine treatment response in 76 outpatients with LLD. We compared the predictive performance of machine learning models including or excluding TMS predictors.

RESULTS

Two single-pulse TMS measures predicted venlafaxine response: cortical excitability (neuronal membrane excitability) and the variability of cortical excitability (dynamic fluctuations in excitability levels). In cross-validation, models using a combination of these TMS predictors, clinical markers of treatment resistance, and age classified patients with 73% ± 11% balanced accuracy (average correct classification rate of responders and nonresponders; permutation testing, p < .005); these models significantly outperformed (corrected t test, p = .025) models using clinical and demographic predictors alone (60% ± 10% balanced accuracy).

CONCLUSIONS

These preliminary findings suggest that single-pulse TMS measures of cortical excitability may be useful predictors of response to pharmacotherapy in LLD. Future studies are needed to confirm these findings and determine whether combining TMS predictors with other biomarkers further improves the accuracy of predicting LLD treatment outcome.

Test-Retest Reliability of the SERT Imaging Agent 11C-HOMADAM in Healthy Humans

The aim of this study was to measure the test-retest reliability of ¹¹C-N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (¹¹C-HOMADAM) imaging of serotonin transporter (SERT) density in healthy control subjects. Methods: Two female and 2 male volunteers participated in the study, with each undergoing three 90-min ¹¹C-HOMADAM PET scans. Time-activity curves were derived from SERT-rich structures and fit to 2 models: a simplified reference tissue model and a multilinear graphical model. Binding potential, the ratio of specifically bound uptake to nondisplaceable uptake at equilibrium, was calculated from the model parameter estimates. Ninety-five percent confidence intervals and the intraclass correlation coefficient (ICC) were calculated and adjusted for repeated measures. Results: The ICC values ranged from -0.13 in the dorsal raphe to 0.88 in the caudate nucleus. The highest average ICC values were in the striatum, but other regions were sensitive to measurement outliers. Conclusion: Good-to-excellent test-retest reliability was observed for SERT binding in the striatum. The dorsal raphe ICC value was sensitive to a measurement outlier. ¹¹C-HOMADAM binding potential calculated from the simplified reference tissue model and the multilinear graphical model were robust and in good agreement.

Serotonin transporter binding is reduced in seasonal affective disorder following light therapy

OBJECTIVE

To investigate the effects of light therapy on serotonin transporter binding (5-HTT BP_ND ), an index of 5-HTT levels, in the anterior cingulate and prefrontal cortices (ACC and PFC) during winter in seasonal affective disorder (SAD). 5-HTT BP_ND fluctuates seasonally to a greater extent in SAD relative to health. We hypothesized that in SAD, 5-HTT BP_ND would be reduced in the ACC and PFC following light therapy.

METHODS

Eleven SAD participants underwent [¹¹ C] DASB positron emission tomography (PET) scans to measure 5-HTT BP_ND before and after 2 weeks of daily morning light therapy.

RESULTS

The primary finding was a main effect of treatment on 5-HTT BP_ND in the ACC and PFC (repeated-measures manova, F_(2,9) = 6.82, P = 0.016). This effect was significant in the ACC (F_(1,10) = 15.11 and P = 0.003, magnitude of decrease, 11.94%) and PFC (F_(1,10) = 8.33, P = 0.016, magnitude of decrease, 9.13%). 5-HTT BP_ND also decreased in other regions assayed following light therapy (repeated-measures manova, F_(4,7) = 8.54, P = 0.028) including the hippocampus, ventral striatum, dorsal putamen, thalamus and midbrain (F_(1,10) = 8.02-36.94, P < 0.0001-0.018; magnitude -8.83% to -16.74%).

CONCLUSIONS

These results demonstrate that light therapy reaches an important therapeutic target in the treatment of SAD and provide a basis for improvement of this treatment via application of [¹¹ C]DASB PET.

In vivo quantification of the [(11)C]DASB binding in the normal canine brain using positron emission tomography

Background

[¹¹C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([¹¹C]DASB) is currently the mostly used radiotracer for positron emission tomography (PET) quantitative studies of the serotonin transporter (SERT) in the human brain but has never been validated in dogs. The first objective was therefore to evaluate normal [¹¹C]DASB distribution in different brain regions of healthy dogs using PET. The second objective was to provide less invasive and more convenient alternative methods to the arterial sampling-based kinetic analysis.

Results

A dynamic acquisition of the brain was performed during 90 min. The PET images were coregistered with the magnetic resonance images taken prior to the study in order to manually drawn 20 regions of interest (ROIs). The highest radioactivity concentration of [¹¹C]DASB was observed in the hypothalamus, raphe nuclei and thalamus and lowest levels in the parietal cortex, occipital cortex and cerebellum.

The regional radioactivity in those 20 ROIs was quantified using the multilinear reference tissue model 2 (MRTM2) and a semi-quantitative method. The values showed least variability between 40 and 60 min and this time interval was set as the optimal time interval for [¹¹C]DASB quantification in the canine brain. The correlation (R²) between the MRTM2 and the semi-quantitative method using the data between 40 and 60 min was 99.3 % (two-tailed p-value < 0.01).

Conclusions

The reference tissue models and semi-quantitative method provide a more convenient alternative to invasive arterial sampling models in the evaluation of the SERT of the normal canine brain. The optimal time interval for static scanning is set at 40 to 60 min after tracer injection.

Light therapy and serotonin transporter binding in the anterior cingulate and prefrontal cortex

OBJECTIVE

To investigate the effects of light therapy on serotonin transporter binding (5-HTT BPND ), an index of 5-HTT levels, in the anterior cingulate and prefrontal cortices (ACC and PFC) of healthy individuals during the fall and winter. Twenty-five per cent of healthy individuals experience seasonal mood changes that affect functioning. 5-HTT BPND has been found to be higher across multiple brain regions in the fall and winter relative to spring and summer, and elevated 5-HTT BPND may lead to extracellular serotonin loss and low mood. We hypothesized that, during the fall and winter, light therapy would reduce 5-HTT BPND in the ACC and PFC, which sample brain regions involved in mood regulation.

METHOD

In a single-blind, placebo-controlled, counterbalanced, crossover design, [(11) C]DASB positron emission tomography was used measure 5-HTT BPND following light therapy and placebo conditions during fall and winter.

RESULTS

In winter, light therapy significantly decreased 5-HTT BPND by 12% in the ACC relative to placebo (F1,9 = 18.04, P = 0.002). In the fall, no significant change in 5-HTT BPND was found in any region across conditions.

CONCLUSION

These results identify, for the first time, a central biomarker associated with the intervention of light therapy in humans which may be applied to further develop this treatment for prevention of seasonal depression.

Altered serotonin and dopamine transporter availabilities in brain of depressed patients upon treatment with escitalopram: A [123 I]β-CIT SPECT study

Altered SERT and DAT availabilities during treatment with escitalopram were investigated with [(123)I]2β-carbomethoxy-3β-(4-iodophenyl)tropane (β-CIT) SPECT in a series of patients fulfilling the criteria for unipolar major depressive disorder (MDD). 27 patients (10m, 42±16y) with diagnosis of MDD were recruited for the study. All patients underwent neuropsychiatric testing for assessment of Hamilton Depression (HAM-D) and Beck Depression Inventory (BDI) scores. At baseline, [(123)I]β-CIT SPECT recordings were acquired 4h (SERT-weighted) and 20-24h p.i (DAT-weighted). Follow-up scans and neuropsychiatric testing were performed after six weeks of stable escitalopram medication. Voxel-wise parametric maps of specific/ non-specific ratios-1 (~BPND) were calculated. At baseline, DAT-weighted BPND was 5.06±0.81 in striatum and SERT-weighted BPND was 0.94±0.18 in thalamus. There were significant negative correlations with age for DAT in striatum (R=-0.60; p<0.01) and SERT in thalamus (R=-0.45; p<0.05). Under SSRI treatment there was an apparent 42% occupancy of SERT in thalamus (p<0.0001), whereas DAT availability increased significantly by 20% in striatum (p<0.001); higher apparent SERT occupancy in thalamus was associated with lesser DAT increase in striatum (R=-0.62; p<0.005). The low apparent SERT occupancy may be confounded by alterations in SERT expression during treatment. Thus, [(123)I]β-CIT SPECT revealed age-dependent declines in DAT and SERT availabilities in un-medicated MDD patients, comparable to that seen previously in healthy controls. At follow-up, the SSRI-evoked increase in DAT was less pronounced in the older patients, even though apparent SERT occupancy and clinical improvement were not age-dependent. Present findings may have implications for escitalopram dosage and side effect profile in younger MDD patients.

Kinetic modeling of (11)C-LY2795050, a novel antagonist radiotracer for PET imaging of the kappa opioid receptor in humans

(11)C-LY2795050 is a novel kappa opioid receptor (KOR) antagonist tracer for positron emission tomography (PET) imaging. The purpose of this first-in-human study was to determine the optimal kinetic model for analysis of (11)C-LY2795050 imaging data. Sixteen subjects underwent baseline scans and blocking scans after oral naltrexone. Compartmental modeling and multilinear analysis-1 (MA1) were applied using the arterial input functions. Two-tissue compartment model and MA1 were found to be the best models to provide reliable measures of binding parameters. The rank order of (11)C-LY2795050 distribution volume (VT) matched the known regional KOR densities in the human brain. Blocking scans with naltrexone indicated no ideal reference region for (11)C-LY2795050. Three methods for calculation of the nondisplaceable distribution volume (VND) were assessed: (1) individual VND estimated from naltrexone occupancy plots, (2) mean VND across subjects, and (3) a fixed fraction of cerebellum VT. Approach (3) produced the lowest intersubject variability in the calculation of binding potentials (BPND, BPF, and BPP). Therefore, binding potentials of (11)C-LY2795050 can be determined if the specific binding fraction in the cerebellum is presumed to be unchanged by diseases and experimental conditions. In conclusion, results from the present study show the suitability of (11)C-LY2795050 to image and quantify KOR in humans.

Evaluation of the agonist PET radioligand [¹¹C]GR103545 to image kappa opioid receptor in humans: kinetic model selection, test-retest reproducibility and receptor occupancy by the antagonist PF-04455242

INTRODUCTION

Kappa opioid receptors (KOR) are implicated in several brain disorders. In this report, a first-in-human positron emission tomography (PET) study was conducted with the potent and selective KOR agonist tracer, [(11)C]GR103545, to determine an appropriate kinetic model for analysis of PET imaging data and assess the test-retest reproducibility of model-derived binding parameters. The non-displaceable distribution volume (V(ND)) was estimated from a blocking study with naltrexone. In addition, KOR occupancy of PF-04455242, a selective KOR antagonist that is active in preclinical models of depression, was also investigated.

METHODS

For determination of a kinetic model and evaluation of test-retest reproducibility, 11 subjects were scanned twice with [(11)C]GR103545. Seven subjects were scanned before and 75 min after oral administration of naltrexone (150 mg). For the KOR occupancy study, six subjects were scanned at baseline and 1.5 h and 8 h after an oral dose of PF-04455242 (15 mg, n=1 and 30 mg, n=5). Metabolite-corrected arterial input functions were measured and all scans were 150 min in duration. Regional time-activity curves (TACs) were analyzed with 1- and 2-tissue compartment models (1TC and 2TC) and the multilinear analysis (MA1) method to derive regional volume of distribution (V(T)). Relative test-retest variability (TRV), absolute test-retest variability (aTRV) and intra-class coefficient (ICC) were calculated to assess test-retest reproducibility of regional VT. Occupancy plots were computed for blocking studies to estimate occupancy and V(ND). The half maximal inhibitory concentration (IC50) of PF-04455242 was determined from occupancies and drug concentrations in plasma. [(11)C]GR103545 in vivo K(D) was also estimated.

RESULTS

Regional TACs were well described by the 2TC model and MA1. However, 2TC VT was sometimes estimated with high standard error. Thus MA1 was the model of choice. Test-retest variability was ~15%, depending on the outcome measure. The blocking studies with naltrexone and PF-04455242 showed that V(T) was reduced in all regions; thus no suitable reference region is available for the radiotracer. V(ND) was estimated reliably from the occupancy plot of naltrexone blocking (V(ND)=3.4±0.9 mL/cm(3)). The IC50 of PF-04455242 was calculated as 55 ng/mL. [(11)C]GR103545 in vivo K(D) value was estimated as 0.069 nmol/L.

CONCLUSIONS

[(11)C]GR103545 PET can be used to image and quantify KOR in humans, although it has slow kinetics and variability of model-derived kinetic parameters is higher than desirable. This tracer should be suitable for use in receptor occupancy studies, particularly those that target high occupancy.

Extrastriatal binding of [¹²³I]FP-CIT in the thalamus and pons: gender and age dependencies assessed in a European multicentre database of healthy controls

PURPOSE

Apart from binding to the dopamine transporter (DAT), [(123)I]FP-CIT shows moderate affinity for the serotonin transporter (SERT), allowing imaging of both monoamine transporters in a single imaging session in different brain areas. The aim of this study was to systematically evaluate extrastriatal binding (predominantly due to SERT) and its age and gender dependencies in a large cohort of healthy controls.

METHODS

SPECT data from 103 healthy controls with well-defined criteria of normality acquired at 13 different imaging centres were analysed for extrastriatal binding using volumes of interest analysis for the thalamus and the pons. Data were examined for gender and age effects as well as for potential influence of striatal DAT radiotracer binding.

RESULTS

Thalamic binding was significantly higher than pons binding. Partial correlations showed an influence of putaminal DAT binding on measured binding in the thalamus but not on the pons. Data showed high interindividual variation in extrastriatal binding. Significant gender effects with 31 % higher binding in women than in men were observed in the thalamus, but not in the pons. An age dependency with a decline per decade (±standard error) of 8.2 ± 1.3 % for the thalamus and 6.8 ± 2.9 % for the pons was shown.

CONCLUSION

The potential to evaluate extrastriatal predominant SERT binding in addition to the striatal DAT in a single imaging session was shown using a large database of [(123)I]FP-CIT scans in healthy controls. For both the thalamus and the pons, an age-related decline in radiotracer binding was observed. Gender effects were demonstrated for binding in the thalamus only. As a potential clinical application, the data could be used as a reference to estimate SERT occupancy in addition to nigrostriatal integrity when using [(123)I]FP-CIT for DAT imaging in patients treated with selective serotonin reuptake inhibitors.

Tracer kinetic modeling of [(11)C]AFM, a new PET imaging agent for the serotonin transporter

[(11)C]AFM, or [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine, is a new positron emission tomography (PET) radioligand with high affinity and selectivity for the serotonin transporter (SERT). The purpose of this study was to determine the most appropriate kinetic model to quantify [(11)C]AFM binding in the healthy human brain. Positron emission tomography data and arterial input functions were acquired from 10 subjects. Compartmental modeling and the multilinear analysis-1(MA1) method were tested using the arterial input functions. The one-tissue model showed a lack of fit in low-binding regions, and the two-tissue model failed to estimate parameters reliably. Regional time-activity curves were well described by MA1. The rank order of [(11)C]AFM binding potential (BPND) matched well with the known regional SERT densities. For routine use of [(11)C]AFM, several noninvasive methods for quantification of regional binding were evaluated, including simplified reference tissue models (SRTM and SRTM2), and multilinear reference tissue models (MRTM and MRTM2). The best methods for region of interest (ROI) analysis were MA1, MRTM2, and SRTM2, with fixed population kinetic values ( or b') for the reference methods. The MA1 and MRTM2 methods were best for parametric imaging. These results showed that [(11)C]AFM is a suitable PET radioligand to image and quantify SERT in humans.

Decreased brainstem and putamen SERT binding potential in depressed suicide attempters using [11C]-zient PET imaging

BACKGROUND

Deficits in serotonergic neurotransmission have been implicated in the pathogenesis of depression and suicidality. The present study utilized a novel positron-emission tomography (PET) ligand to quantitate and compare brain regional serotonin transporter (SERT) binding potential in depressed patients with a past history of suicide attempts to that of healthy comparison subjects.

METHOD

We used [(11) C]-ZIENT PET to label SERT in the serotonergic cell body rich brainstem, and forebrain projection fields. Quantitative PET emission data from 21 adults (10 healthy controls and 11 drug-free patients with major depression) was used for group comparison. SERT binding potential (BPND ) in eight MRI-based brain regions of interest (ROI) were compared in high-resolution PET images.

RESULTS

SERT binding potential was significantly decreased in the midbrain/pons (P = .029) and putamen (P = .04) of depressed patients with a past suicide attempt relative to comparison subjects. Forebrain SERT binding was also reduced in the patient sample, though these region effects did not survive a multiple comparison correction.

CONCLUSION

These results suggest that decreased availability of the brainstem and basal ganglia SERT represents a biomarker of depression and thus confirm and extend the role of dysregulation of brain serotonergic neurotransmission in the pathophysiology of depression and suicide.

PET imaging of the brain serotonin transporters (SERT) with N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM) in humans: a preliminary study

PURPOSE

The aim of this study was to assess the feasibility of using 4-[(18)F]-ADAM as a brain SERT imaging agent in humans.

METHODS

Enrolled in the study were 19 healthy Taiwanese subjects (11 men, 8 women; age 33 ± 9 years). The PET data were semiquantitatively analyzed and expressed as specific uptake ratios (SUR) and distribution volume ratios (DVR) using the software package PMOD. The SUR and DVR of 4-[(18)F]-ADAM in the raphe nucleus (RN), midbrain (MB), thalamus (TH), striatum (STR) and prefrontal cortex (PFC) were determined using the cerebellum (CB) as the reference region.

RESULTS

4-[(18)F]-ADAM bound to known SERT-rich regions in human brain. The order of the regional brain uptake was MB (RN) > TH > STR > PFC > CB. The DVR (n = 4, t* = 60 min) in the RN, TH, STR and PFC were 3.00 ± 0.50, 2.25 ± 0.45, 2.05 ± 0.31 and 1.40 ± 0.13, respectively. The optimal time for imaging brain SERT with 4-[(18)F]-ADAM was 120-140 min after injection. At the optimal imaging time, the SURs (n = 15) in the MB, TH, STR, and PFC were 2.25 ± 0.20, 2.28 ± 0.20, 2.12 ± 0.18 and 1.47 ± 0.14, respectively. There were no significant differences in SERT availability between men and women (p < 0.05).

CONCLUSION

The results of this study showed that 4-[(18)F]-ADAM was safe for human studies and its distribution in human brain appeared to correlate well with the known distribution of SERT in the human brain. In addition, it had high specific binding and a reasonable optimal time for imaging brain SERT in humans. Thus, 4-[(18)F]-ADAM may be feasible for assessing the status of brain SERT in humans.

Sustained recreational use of ecstasy is associated with altered pre and postsynaptic markers of serotonin transmission in neocortical areas: a PET study with [¹¹C]DASB and [¹¹C]MDL 100907

3,4-Methylenedioxymethamphetamine (MDMA), the main psychoactive component of the recreational drug ecstasy, is a potent serotonin (5-HT) releaser. In animals, MDMA induces 5-HT depletion and toxicity in 5-HT neurons. The aim of this study was to investigate both presynaptic (5-HT transporter, SERT) and postsynaptic (5-HT(2A) receptor) markers of 5-HT transmission in recently abstinent chronic MDMA users compared with matched healthy controls. We hypothesized that MDMA use is associated with lower SERT density and concomitant upregulation of 5-HT(2A) receptors. Positron emission tomography studies using the SERT ligand [¹¹C]DASB and the 5-HT(2A) receptor ligand [¹¹C]MDL 100907 were evaluated in 13 current and recently detoxified MDMA users and 13 matched healthy controls. MDMA users reported a mean duration of ecstasy use of 8 years, regular exposure, and at least 2 weeks of abstinence before the scans. SERT and 5-HT(2A) receptor availability (binding potential, BP(ND)) were analyzed with a two-tissue compartment model with arterial input function. Current recreational MDMA use was significantly associated with lower SERT BP(ND) and higher 5-HT(2A) receptor BP(ND) in cortical, but not subcortical regions. Decreased SERT BP(ND) was regionally associated with upregulated 5-HT(2A) receptor BP(ND). In light of the animal literature, the most parsimonious interpretation is that repeated exposure to MDMA in humans, even in moderate amounts, leads to damage in 5-HT neuron terminals innervating the cortex. Alterations in mood, cognition, and impulse control associated with these changes might contribute to sustain MDMA use. The reversibility of these changes upon abstinence remains to be firmly established.

Differential modulation of emotion processing brain regions by noradrenergic and serotonergic antidepressants

RATIONALE

Most widely used antidepressant drugs affect the serotonergic and noradrenergic pathways. However, there are currently no neurobiological criteria for selecting between these targets and predicting the treatment response in individual depressed patients.

OBJECTIVES

The current study is aimed at differentiating brain regions known to be pathophysiologically and functionally involved in depression-related emotion processing with respect to their susceptibility to serotonergic and noradrenergic modulation.

METHODS

In a single-blind pseudo-randomized crossover study, 16 healthy subjects (out of 21 enrolled) were included in analysis after ingesting a single dose of citalopram (a selective serotonin-reuptake inhibitor, 40 mg), reboxetine (a selective noradrenaline-reuptake inhibitor, 8 mg), or placebo at three time points prior to functional magnetic resonance imaging (fMRI). During fMRI, subjects anticipated and subsequently viewed emotional pictures. Effects of serotonergic and noradrenergic modulation versus placebo on brain activity during the perception of negative pictures were analyzed with a repeated measures ANOVA in the whole brain and in specific regions of interest relevant to depression.

RESULTS

Noradrenergic modulation by reboxetine increased brain activity in the thalamus, right dorsolateral prefrontal cortex and occipital regions during the perception of negative emotional stimuli. Citalopram primarily affected the ventrolateral prefrontal cortical regions.

CONCLUSION

The brain regions involved in the processing of negative emotional stimuli were differentially modulated by selective noradrenergic and serotonergic drugs: thalamic activity was increased by reboxetine, whereas citalopram primarily affected ventrolateral prefrontal regions. Thus, dysfunction in these regions, which could be identified in depressed patients, may predict treatment responses to either noradrenergic or serotonergic antidepressants.

Dopamine and serotonin transporter availability in chronic heroin users: a [¹²³I]β-CIT SPECT imaging study

Dopamine (DA) and serotonin (5-HT) transporter availability in heroin users and healthy controls was measured using [¹²³I]β-CIT and SPECT imaging. Heroin users had statistically similar striatal DA and brainstem and diencephalon 5-HT transporter availability compared with controls. No associations between transporter availability and heroin use characteristics were found.

Genetic variation in HTR2A influences serotonin transporter binding potential as measured using PET and [11C]DASB

In a previous study we showed that genetic variation in HTR2A, which encodes the serotonin 2A receptor, influenced outcome of citalopram treatment in patients with major depressive disorder. Since chronic administration of citalopram, which selectively and potently inhibits the serotonin transporter (5-HTT), putatively enhances serotonergic transmission, it is conceivable that genetic variation within HTR2A also influences pretreatment 5-HTT function or serotonergic transmission. The present study used positron emission tomography (PET) and the selective 5-HTT ligand, [11C]DASB, to investigate whether the HTR2A marker alleles that predict treatment outcome also predict differences in 5-HTT binding. Brain levels of 5-HTT were assessed in vivo using PET measures of the non-displaceable component of the [11C]DASB binding potential (BPND). DNA from 43 patients and healthy volunteers, all unmedicated, was genotyped with 14 single nucleotide polymorphisms located within or around HTR2A. Allelic association with BPND was assessed in eight brain regions, with covariates to control for race and ethnicity. We detected allelic association between [11C]DASB BPND in thalamus and three markers in a region spanning the 3' untranslated region and second intron of HTR2A (rs7333412, p=0.000045; rs7997012, p=0.000086; rs977003, p=0.000069). The association signal at rs7333412 remained significant (p<0.05) after applying corrections for multiple testing via permutation. Genetic variation in HTR2A that was previously associated with citalopram treatment outcome was also associated with thalamic 5-HTT binding. While further work is needed to identify the actual functional genetic variants involved, these results suggest that a relationship exists between genetic variation in HTR2A and either 5-HTT expression or central serotonergic transmission that influences the therapeutic response to 5-HTT inhibition in major depression.

Serotonergic and noradrenergic modulation of emotion processing by single dose antidepressants

Serotonergic and noradrenergic pathways are the main targets of antidepressants. Their differential effects on emotion processing-related brain activation are, however, to be further characterized. We aimed at elucidating the neural sites of action of an acute differential serotonergic and noradrenergic influence on an emotion-processing task, which was earlier shown to be associated with depressiveness. In a single-blind pseudo-randomized crossover study, 21 healthy subjects (16 subjects finally included in the analysis) participated to ingest a single dose at three time points of either 40 mg citalopram, a selective serotonin-reuptake inhibitor, 8 mg reboxetine, a selective noradrenaline-reuptake inhibitor, or placebo 2-3 h before functional magnetic resonance imaging (fMRI). During fMRI, subjects performed a task comprising the anticipation and perception of pictures of either 'known' (positive, negative, neutral) or 'unknown' valence (randomly 50% positive or negative). In direct comparison with citalopram and with placebo, reboxetine increased brain activity in the medial thalamus. Citalopram modulated certain prefrontal and insular areas more prominently. Other frontal and parieto-occipital areas were modulated by both drugs. In conclusion, the functional network involved in emotional information processing could be modulated by the acute application of selective noradrenergic and serotonergic drugs revealing a noradrenergic effect in thalamic and frontal areas, and a prefrontal and insular focus of serotonergic modulation. These findings could have implications for future selection criteria concerning personalized antidepressant medication in depression.

Dopamine and serotonin transporter availability during acute alcohol withdrawal: effects of comorbid tobacco smoking

Tobacco smoking is highly comorbid with heavy alcohol drinking, yet the interaction of tobacco smoking and alcohol drinking on brain catecholaminergic synaptic markers is unexplored. Here we evaluate the effects of alcohol drinking alone from comorbid alcohol drinking and tobacco smoking on dopamine (DA) and serotonin (5-HT) transporter availability. A total of 14 heavy alcohol drinking smokers (n=6) and nonsmokers (n=8) and 14 age-matched control smokers (n=6) and nonsmokers (n=8) were imaged with [(123)]beta-CIT single photon emission computed tomography. Alcohol drinking smokers and nonsmokers consumed 134.3+/-100.3 and 196.5+/-139.9 drinks, respectively, over the previous month and were imaged during acute withdrawal, eg within 5 days of their last drink. Striatal DA transporter availability was significantly higher (16%, P=0.04) in alcohol drinkers compared to controls. 5-HT transporter availability was also significantly higher in alcohol drinkers vs controls in the brainstem (25%, P=0.001) and the diencephalon (8%, P=0.01). This elevation was restricted to alcohol drinking nonsmokers with higher DA transporter availability in the striatum (26%, P=0.006), and higher 5-HT transporter availability in the diencephalon (26%, P=0.04) and brainstem (42%, P<0.0002). There was a significant positive correlation between days since last drink and 5-HT transporter availability in the diencephalon (r=0.60, P=0.023) and brainstem (r=0.54, P=0.047), in the total group of alcohol drinkers and in the nonsmokers, but not the smokers. During the first week of abstinence, DA and 5-HT transporter availability is higher in alcohol drinking nonsmokers but not in alcohol drinking smokers. Smoking appears to suppress neuroadaptive changes in DA and 5-HT transporters during acute withdrawal from alcohol.

Serotonin transporter gene promoter polymorphisms modify the association between paroxetine serotonin transporter occupancy and clinical response in major depressive disorder

BACKGROUND

In major depressive disorder, selective serotonin reuptake inhibitors target the serotonin transporter (SERT). Their response rates (30-50%) are modified by SERT promotor polymorphisms (5-HTTLPR).

OBJECTIVES

To quantify the relationship between SERT occupancy and response, and whether 5-HTTLPR is a modifier.

METHODS

Drug-free depressed outpatients (n=49; both sexes; aged 25-55 years), received paroxetine (20 mg/day). We quantified SERT occupancy with iodine-123-labeled 2beta-carbomethoxy-3beta-(4-iodophenyl)-tropane single-photon emission computed tomography imaging at baseline and after 6 weeks; we genotyped 5-HTTLPR (S, L(G), L(A)).

PRIMARY OUTCOMES

percentage decrease in 17-item Hamilton Depression Rating Scale and response (> or =50% decrease of 17-item Hamilton Depression Rating Scale).

RESULTS

A significant positive relationship between SERT occupancy and clinical response existed only in the L(A)/L(A) genotype (P<0.002). Relative to paroxetine serum concentrations maximal midbrain SERT occupancy was numerically higher for L(A)/L(A) compared with other genotypes, but this difference was nonsignificant (P=0.188).

CONCLUSION

Higher SERT occupancy is only associated with more clinical improvement in the L(A)/L(A) genotype. We hypothesize that the L(A)/L(A) carriers have a more dynamic serotonergic system, which seems more responsive to selective serotonin reuptake inhibitors. (ISRCTN Trial Register ISRCTN44111488; http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=193).

Evidence why paroxetine dose escalation is not effective in major depressive disorder: a randomized controlled trial with assessment of serotonin transporter occupancy

Dose escalation is often used in depressed patients who fail to respond to standard doses of selective serotonin reuptake inhibitors, but clinical efficacy is equivocal. We aimed to reassess the efficacy of paroxetine dose escalation and quantify whether paroxetine dose escalation increases occupancy of the serotonin transporter (SERT) more than placebo dose escalation in a randomized controlled trial. We recruited 107 nonpsychotic, unipolar depressed outpatients (18-70 years; Hamilton Depression Rating Scale (HDRS(17)) >18) from primary care and psychiatric outpatient departments. After 6 weeks, open-label paroxetine 20 mg per day (T0), nonresponding patients (HDRS(17) decrease <50%; n=60) were randomized to double-blind paroxetine (30-50 mg per day as tolerable) or placebo dose escalation (paroxetine 20 mg per day+placebo). Patients were followed until 6 weeks after randomization (T1). Forty-nine patients, drug free at study entry, underwent single-photon emission-computed tomography (SPECT) scanning before treatment and were scanned repeatedly at T0 and T1. Paroxetine serum concentrations and SERT occupancy were determined at T0 and T1 (n=32). We terminated the dose-escalation trial after an interim analysis. Thirty nonresponding patients were randomized to paroxetine (46.7+/-5.5 mg per day), 27 to placebo dose escalation. Response rates were 10/30 (33.3%) and 10/27 (37.0%), respectively. Repeated measurement analyses showed no significant effect for treatment (p=0.88, exceeding a priori stopping rules for futility (p>0.5)). Overall dropout was higher for placebo (26.7%) than paroxetine (3.3%; p=0.03). Paroxetine dose escalation increased paroxetine serum concentrations (p<0.001). SPECT measurements (12 patients randomized to paroxetine (46.9+/-4.8 mg) and 14 to placebo dose escalation) showed no significant increase of midbrain SERT occupancy (2.5+/-26.4%, paroxetine; 3.1+/-25.8% placebo; p=0.687) nor in diencephalon (p=0.529). Paroxetine dose escalation in depressed patients has no clinical benefit over placebo dose escalation. This is explained by the absence of significant increases of SERT occupancy by paroxetine dose escalation, despite increased paroxetine serum concentrations (ISRCTN44111488).

Compartmental modeling of 11C-HOMADAM binding to the serotonin transporter in the healthy human brain

The novel PET radioligand (11)C-N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine ((11)C-HOMADAM) binds with high affinity and selectively to the serotonin transporter (SERT). The purpose of this study was to develop a reliable kinetic model to describe the uptake of (11)C-HOMADAM in the healthy human brain.

METHODS

Eight volunteers participated in the study; 5 of them were fitted with arterial catheters for blood sampling and all were scanned on a high-resolution research tomograph after the injection of (11)C-HOMADAM. Regional distribution volumes and binding potentials were calculated with 2- and 4-parameter arterial-input compartment models, a 3-parameter reference tissue compartment model, and the Logan graphic approach.

RESULTS

The 2-parameter arterial-input compartment model was statistically superior to the 4-parameter model and described all brain regions. Calculated binding potentials agreed well between the arterial-input model and the reference tissue model when the cerebellum was used as the reference tissue. The Logan graphic approach was not able to estimate the higher concentration of SERT in the dorsal raphe than in the midbrain.

CONCLUSION

(11)C-HOMADAM is a highly promising radioligand with high ratios of specific binding to nonspecific binding in known SERT-rich structures, such as the raphe nuclei. The 3-parameter reference tissue model approach permits a simplified quantitatively accurate method for estimating SERT binding potentials.

Serotonin transporter binding with [123I]beta-CIT SPECT in major depressive disorder versus controls: effect of season and gender

PURPOSE

The serotonin system is undoubtedly involved in the pathogenesis of major depressive disorder (MDD). More specifically the serotonin transporter (SERT) serves as a major target for antidepressant drugs. There are conflicting results about SERT availability in depressed patients versus healthy controls. We aimed to measure SERT availability and study the effects of age, gender and season of scanning in MDD patients in comparison to healthy controls.

METHODS

We included 49 depressed outpatients (mean+/-SD 42.3 +/- 8.3 years) with a Hamilton depression rating scale score above 18, who were drug-naive or drug-free for >or=4 weeks, and 49 healthy controls matched for age (+/-2 years) and sex. Subjects were scanned with single photon emission computed tomography (SPECT) using [(123)I]beta-CIT. SERT availability was expressed as specific to nonspecific binding ratios (BP(ND)) in the midbrain and diencephalon with cerebellar binding as a reference.

RESULTS

In crude comparisons between patients and controls, we found no significant differences in midbrain or diencephalon SERT availability. In subgroup analyses, depressed males had numerically lower midbrain SERT availability than controls, whereas among women SERT availability was not different (significant diagnosis x gender interaction; p = 0.048). In the diencephalon we found a comparable diagnosis x gender interaction (p = 0.002) and an additional smoking x gender (p = 0.036) interaction. In the midbrain the season of scanning showed a significant main effect (p = 0.018) with higher SERT availability in winter.

CONCLUSION

Differences in SERT availability in the midbrain and diencephalon in MDD patients compared with healthy subjects are affected by gender. The season of scanning is a covariate in the midbrain. The diagnosis x gender and gender x smoking interactions in SERT availability should be considered in future studies of the pathogenesis of MDD.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

Ecstasy-induced reduction of the availability of the brain serotonin transporter as revealed by [11C](+)McN5652-PET and the multi-linear reference tissue model: loss of transporters or artifact of tracer kinetic modelling?

In a previous positron emission tomography (PET) study with the serotonin transporter (SERT) ligand [(11)C](+)McN5652, we found protracted reduction of the availability of the brain SERT in users of the drug ecstasy. However, the multi-linear reference tissue method for the quantification of SERT availability used in this study is prone to effects of altered levels of statistical noise that could simulate reduction of SERT. The aim of the present study was to take into account this confound by re-evaluation of the data now using a modelling approach that is rather insensitive to alterations in the level of statistical noise. A total of 116 subjects (30 current, 29 former ecstasy users, 29 drug-naive, 28 polydrug controls) in whom [(11)C](+)McN5652-PET had been performed previously were re-evaluated. The equilibrium specific-to-non-specific partition coefficient V"( 3) was obtained voxel-wise by application of the simplified reference tissue method (SRTM), which provides quite unbiased results up to rather large noise levels. Voxel-based comparisons between the groups were performed using statistical parametric mapping. V"(3) was reduced in the striatum and in the thalamus in current ecstasy users. This was confirmed by volume-of-interest-based analysis. This result suggests that the ecstasy-induced reduction of SERT availability in SERT-rich brain regions reported previously indicates reduced SERT binding potential rather than being an artifact of tracer kinetic modelling. SRTM analysis did not confirm previous findings in neocortical brain areas.

Architectonic distribution of the serotonin transporter within the orbitofrontal cortex of the vervet monkey

To elucidate the organization of the serotoninergic innervation within the orbitofrontal cortex (OFC), serotonin transporter (SERT) density was quantified by autoradiography using [(3)H]cyanoimipramine binding. In six adult vervet monkeys, 15 architectonic areas were delineated according to cytoarchitectonic (Nissl), myeloarchitectonic (Gallyas) and chemoarchitectonic (acetylcholinesterase) criteria to assess SERT distribution at two levels of organization: cortical area and cortical type. For cortical type, the 15 areas were evenly divided into three different categories primarily based upon the degree of granularization of layer IV: agranular, dysgranular, and granular. Within agranular and dysgranular, but not granular cortical types, SERT density was area-specific and progressively decreased in a medial to lateral gradient. Across cortical types, SERT density decreased in a caudal to rostral gradient: agranular>dysgranular>granular. A similar caudal to rostral gradient was seen when serotonin content was measured (using high performance liquid chromatography) in areas representative of each cortical type. Collectively, these results suggest that the serotoninergic innervation is organized according to both cortical type and area, and is thus structured to differentially modulate information processing within the OFC.

Stereoselective neuroimaging in vivo

Stereoselectivity is a basic property of many neuronal processes due to the spatial features of molecules involved in neurotransmission. Today, neuroimaging procedures are available for studying stereoselectivity in the living brain. Mirror-image radiotracers are the molecular tools that are used, together with single photon emission tomography (SPECT) and positron emission tomography (PET), for studying stereoselective neuronal mechanisms. This review presents the findings obtained in those studies of cholinergic, noradrenergic, dopaminergic, serotonergic, glutamatergic, opioid, cannabinoid, and second messenger neurotransmission.

PET and SPECT imaging in psychiatric disorders

OBJECTIVES

To review recent findings from positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies that investigate the pathophysiology and treatment of schizophrenia, depression, and dementia.

METHODS

We carried out a review of the literature.

RESULTS

PET and SPECT studies have provided evidence of dopamine system dysregulation in patients with schizophrenia and variable loss of monoamines in patients with depression. Antipsychotic response has been demonstrated to be associated with blockade of dopamine D2 receptors, and antidepressant response has now been linked to blockade of serotonin transporter receptors. PET and SPECT have been extensively evaluated as diagnostic procedures for dementia. Substantial progress has been made in developing radioligands that bind to amyloid deposits in the brain, which should provide new opportunities for early diagnosis and treatment monitoring in Alzheimer's disease.

CONCLUSION

Advances in PET and SPECT imaging have provided new insights into the biology of major psychiatric disorders and their treatment. In the future, we can expect that these imaging techniques will become more central to the management of psychiatric disorders.

Effects of image reconstruction algorithm on neurotransmission PET studies in humans: comparison between filtered backprojection and ordered subsets expectation maximization

OBJECTIVES

Both reconstruction algorithms, filtered backprojection (FBP) and ordered subsets expectation maximization (OSEM), are widely used in clinical positron emission tomography (PET) studies. Image reconstruction for most neurotransmission PET scan data is performed by FBP, while image reconstruction for whole-body [18F]FDG scan data is usually performed by OSEM. Although several investigators have compared FBP and OSEM in terms of the quantification of regional radioactivity and physiological parameters calculated from PET data, only a few studies have compared the two reconstruction algorithms in PET studies that estimate neurotransmission, i.e., neuroreceptor and neurotransporter binding. In this study we compared mean regional radioactivity concentration in the late phase and binding potential (BP) between FBP and OSEM algorithms in neurotransmission PET studies for [11C]raclopride and [11C]DASB.

METHODS

Dynamic PET scans with [11C]raclopride in 3-dimensional mode were performed on seven healthy subjects. Dynamic PET scans with [11C]DASB in 2-dimensional mode were performed on another seven subjects. OSEM images were post-filtered so that its transverse spatial resolution became similar to that of FBP with the same Hanning filter (Kernel FWHM 6 mm). In both PET studies we calculated the BP of [11C]raclopride and [11C]DASB by a reference tissue model for each ROI (region of interest).

RESULTS

There was no significant difference in mean regional radioactivity concentration between FBP and OSEM for [11C]raclopride and [11C]DASB. Only +2.4 - +3.2%, but still a significant difference in BP of [11C]raclopride between FBP and OSEM was observed in the striatum. There was no significant difference in BP between FBP and OSEM in other than the striatum for [11C]raclopride and in all regions for [11C]DASB. In addition, there was no significant difference in root mean square error between FBP and OSEM when BP was calculated.

CONCLUSIONS

The BP values were similar between FBP and OSEM algorithms with [11C]raclopride and [11C]DASB. This study indicates that OSEM can be used for human neurotransmission PET studies for calculating BP although OSEM was not necessarily superior to FBP in the present study.

[11C]SMe-ADAM, an imaging agent for the brain serotonin transporter: synthesis, pharmacological characterization and microPET studies in rats

N,N-Dimethyl-2-(2-amino-4-methylthiophenylthio)benzylamine (SMe-ADAM, 1) is a highly potent and selective inhibitor of the serotonin transporter (SERT). This compound was labeled with carbon-11 by methylation of the S-desmethyl precursor 10 with [(11)C]methyl iodide to obtain the potential positron emission tomography (PET) radioligand [(11)C]SMe-ADAM. The radiochemical yield was 27 +/- 5%, and the specific radioactivity was 26-40 GBq/micromol at the end of synthesis. Ex vivo and in vivo biodistribution experiments in rats demonstrated a rapid accumulation of the radiotracer in brain regions known to be rich in SERT, such as the thalamus/hypothalamus region (3.59 +/- 0.41%ID/g at 5 min after injection). The specific uptake reached a thalamus to cerebellum ratio of 6.74 +/- 0.95 at 60 min postinjection. The [(11)C]SMe-ADAM uptake in the thalamus was significantly decreased by pretreatment with fluoxetine to 38 +/- 11% of the control value. Furthermore, no metabolites of [(11)C]SMe-ADAM could be detected in the SERT-rich regions of the rat brain. It is concluded that [(11)C]SMe-ADAM may be a suitable PET ligand for SERT imaging in the living brain.

Parkinson's disease and dopamine transporter neuroimaging: a critical review

Parkinson's disease (PD) is a common neurodegenerative disorder that is mainly caused by dopaminergic neuron loss in the substantia nigra. Several nuclear medicine radiotracers have been developed to evaluate PD diagnoses and disease evolution in vivo in PD patients. Positron emission tomography (PET) and single photon computerized emission tomography (SPECT) radiotracers for the dopamine transporter (DAT) provide good markers for the integrity of the presynaptic dopaminergic system affected in PD. Over the last decade, radiotracers suitable for imaging the DAT have been the subject of most efforts. In this review, we provide a critical discussion on the utility of DAT imaging for Parkinson's disease diagnosis (sensitivity and specificity).

Mood, cognition and serotonin transporter availability in current and former ecstasy (MDMA) users: the longitudinal perspective

Although 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a known serotonergic neurotoxin in different animal species, there is to date no conclusive evidence of its neurotoxicity in humans. MDMA use was associated with impairments of psychological well-being, verbal memory and altered serotonergic functioning in a number of cross-sectional studies. Due to inherent methodological limitations, such as the notorious polydrug use of ecstasy users and lack of control of possible pre-existing differences between ecstasy users and control participants, researchers have called for well-controlled, prospective longitudinal studies to shed more light on the issue of MDMA neurotoxicity to the human brain. This longitudinal study investigated whether mood, cognition and central serotonin transporters (SERT) would deteriorate with continued MDMA use and whether or not they would recover over increasing periods of MDMA abstinence. In a repeated-measures design, 11 current and ten ex-ecstasy users, and 11 polydrug (but not MDMA) and 15 drug-naive controls participated three times within approximately two years. Both ecstasy user groups reported a polydrug use pattern besides heavy ecstasy use. Subjective reports of ecstasy use or abstinence were verified by toxicological analyses. On each trial, the participants underwent a cognitive test battery and filled in the Symptom Check List. The availability of central SERT was assessed with positron emission tomography using the McN5652 ligand for all groups at t1, and only for the ecstasy user groups on follow-ups. The factor Group yielded significant results in the SCL-90 scales Global Severity Index, Anxiety, Obsessive/compulsive and Interpersonal sensitivity, with the ex-ecstasy users reporting the highest symptom scores. There were significant Group effects in all measures of verbal memory, with the lowest performance in the group of ex-ecstasy users. The repeated-measures analyses yielded no significant Group x Time interactions in any SCL-90 scales or measures of memory performance, with the exception of AVLT 1 immediate recall. Thus the ex-ecstasy users' psychopathological symptoms and memory performance failed to improve, and the current ecstasy users' failed to deteriorate, over time relative to the other groups. While there was a significant effect of Group in all brain regions examined (except the control region white matter), the current users' SERT availability seems to have recovered in the mesencephalon, as indicated by a significant Group x Time interaction. Reduced SERT availability might be a transient effect of heavy ecstasy use, since it partially recovered as the current users reduced their MDMA use. However, this measure may not necessarily be a valid indicator of the number or integrity of serotonergic neurons. Ex-ecstasy users' verbal memory showed no sign of improvement even after over 2.5 years of abstinence and thus may represent persistent functional consequences of MDMA neurotoxicity. However, alternative causes such as pre-existing group differences cannot be completely ruled out in spite of the longitudinal design.

[(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.

Measurement of serotonin transporter binding with PET and [11C]MADAM: A test–retest reproducibility study

[(11)C]MADAM, or [(11)C]N,N-dimethyl-2-(2-amino-4-methylphenyl thio)benzylamine, is a radioligand suitable for positron emission tomography (PET) studies of the serotonin transporter (5-HTT) in man. The purpose of this study was to examine the test-retest reproducibility using a design tailored for future applied studies. Nine healthy male subjects were examined with PET and [(11)C]MADAM under baseline conditions at two occasions 4-8 weeks apart. The subjects participated in a Phase 1 trial to which the present study was an addendum. Eight regions of interest were studied, including frontal cortex, hippocampal complex, and the raphe nuclei. All regions, but the raphe nuclei, were defined on MR-images to which the PET-images were coregistered using SPM2. Binding potentials were calculated using the simplified reference tissue model, with cerebellum as reference region. Test-retest data were calculated from the binding potentials, and included binding potential (BP) quotient, BP difference, and the intraclass correlation coefficient. The quotient was about one in all regions, and the mean difference varied between 0 and 11%. The intraclass correlation coefficient varied between 0.96 and 0.51 in the raphe nuclei and averaged bilateral regions. [(11)C]MADAM was shown to have good to excellent reliability in measurements of 5-HTT binding in brain regions of interest in research on psychiatric disorders.

Striatal dopamine transporter availability with [123I]beta-CIT SPECT is unrelated to gender or menstrual cycle

OBJECTIVES

The effect of gender and female menstrual cycle on human striatal dopamine transporters (DATs) was investigated with single-photon emission computed tomography (SPECT) using the ligand 2beta-carbomethoxy-3beta-(4-[(123)I]iodophenyl)tropane.

METHODS

Ten female subjects aged 18-40 years (25.3+/-7.3 years) were scanned twice during the early follicular and the mid-luteal phases to detect any hormone-mediated changes in DAT availability in the striatum or serotonin transporter (SERT) availability in brainstem-diencephalon. Plasma estradiol and progesterone levels were obtained at the time of SPECT and confirmed the expected increases from the follicular to the luteal phases. Finally, in a post hoc analysis of a previously published healthy-subject sample, striatal DAT availability was compared between 70 male and 52 female subjects who ranged in age from 18 to 88 years.

RESULTS

In the ten menstrual cycle subjects, DAT availability (V(3)'') in striatum and SERT availability in brainstem-diencephalon did not differ between follicular and luteal phases. Moreover, change in V(3)'' for striatum or brainstem-diencephalon was uncorrelated with change in plasma estradiol or progesterone from the follicular to the luteal phase. In the larger healthy-subject sample, there was no significant effect of gender or the interaction of age and gender on striatal V(3)''.

CONCLUSIONS

These findings suggest that in using DAT or SERT ligands in the study of neuropsychiatric disorders, matching of female subjects according to a menstrual cycle phase is unnecessary. Although the present investigation did not confirm previous reports of gender differences in striatal DAT availability, controlling for gender in such studies still seems advisable.

Reversibility of ecstasy-induced reduction in serotonin transporter availability in polydrug ecstasy users

PURPOSE

Animal data suggest that the synthetic drug ecstasy may damage brain serotonin neurons. Previously we reported protracted reductions in the availability of the serotonin transporter (SERT), an index of integrity of the axon terminals of brain serotonergic neurons, in SERT-rich brain regions in current human ecstasy users. Comparison of current ecstasy users and former ecstasy users yielded some evidence that this reduction might be reversible. However, participant selection effects could not be ruled out. Therefore, follow-up examinations were performed in these subjects to test the following a priori hypothesis in a prospective longitudinal design that eliminates participant selection effects to a large extent: availability of the SERT increases towards normal levels when ecstasy use is stopped, and remains unchanged or is further decreased if use is continued.

METHODS

Two follow-up positron emission tomography measurements using the SERT ligand [11C](+)McN5652 were completed by 15 current and nine former ecstasy users. All subjects used illicit drugs other than ecstasy, too. The time interval between repeated measurements was about 1 year. The time course of the availability of the SERT was analysed in the following SERT-rich regions: mesencephalon, putamen, caudate and thalamus.

RESULTS

Current ecstasy users showed a consistent increase in the availability of the SERT in the mesencephalon during the study (Friedman test: p = 0.010), which most likely was caused by a decrease in the intensity of ecstasy consumption (Spearman correlation coefficient -0.725, p = 0.002). Former ecstasy users showed a consistent increase in SERT availability in the thalamus (Friedman test: p = 0.006).

CONCLUSION

Ecstasy-induced protracted alterations in the availability of the SERT might be reversible.

The new PET imaging agent [11C]AFE is a selective serotonin transporter ligand with fast brain uptake kinetics

A new positron emission tomography (PET) radioligand for the serotonin transporter (SERT), [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-(2-fluoroethyl)phenylamine ([(11)C]AFE, 12), was synthesized and evaluated in vivo in rats and baboons. [(11)C]AFE (12) was prepared from its monomethylamino precursor 11 by reaction with high specific activity [(11)C]methyl triflate. Radiochemical yield was 32+/-17% based on [(11)C]methyl triflate (n=6) and specific activity was 1670+/-864 Ci/mmol at end of synthesis (EOS, n=6). Binding assays indicated that AFE displays high affinity for SERT (K(i)=1.80 nM for hSERT) and lower affinity for norepinephrine transporter (K(i)=946 nM for hNET) or dopamine transporter (K(i)>10,000 nM for hDAT). In addition, AFE displays negligible binding affinities for other serotonin and dopamine receptors, indicating an excellent binding selectivity in vitro. Biodistribution studies in rats indicated that [(11)C]AFE enters the brain readily and localizes in regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich brain regions is reduced significantly by pretreatment with either citalopram or the cold compound itself, but not by nisoxetine or GBR 12935, thus demonstrating that [(11)C]AFE binding in the rat brain is saturable, specific and selective for the SERT. Imaging experiments in baboons indicated that the uptake pattern of [(11)C]AFE is consistent with the known distribution of SERT in the baboon brain, with high levels of radioactivity detected in the midbrain and thalamus, moderate levels in the hippocampus and striatum and low levels in the cortical regions. The uptake kinetics of [(11)C]AFE in the baboon brain is rapid, with activity in the midbrain and thalamus peaking at 15-40 min postinjection. Pretreatment of the baboon with citalopram (4 mg/kg) 20 min before radioactivity injection reduced the binding of [(11)C]AFE in all SERT-containing brain regions to the level in the cerebellum. Kinetic analysis revealed that in all brain regions examined, [(11)C]AFE specific-to-nonspecific partition coefficients (V(3)'') are similar to those of [(11)C]McN5652 and [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluorophenylamine ([(11)C]AFA), but lower than those of [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluoromethylphenylamine ([(11)C]AFM) or [(11)C]DASB. In summary, [(11)C]AFE appears to be a PET radioligand with fast brain uptake kinetics and can be used for the visualization and quantification of SERT in vivo.

Serotonin transporter availability in patients with schizophrenia: a positron emission tomography imaging study with [11C]DASB

BACKGROUND

Postmortem studies have reported several alterations in serotonin transporter (SERT) binding parameters in patients with schizophrenia. The aim of this study was to compare SERT availability in vivo in patients with schizophrenia and matched control subjects.

METHODS

Ten medication-free patients with schizophrenia and 10 healthy subjects underwent positron emission tomography (PET) scans for 90 min after 11C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([11C]DASB) injection. Metabolite-corrected arterial input function was measured. Regional distribution volumes (mL/g) were derived with a two tissue compartment kinetic model. Outcome measures for SERT availability included binding potential (BP) and the specific-to-nonspecific equilibrium partition coefficient (V3''). Ten brain regions with high density of SERT and where SERT availability can be reliably quantified with [11C]DASB were included in the analysis.

RESULTS

No significant differences were observed in regional BP or V3'' between patients and control subjects. No significant relationships were observed between regional SERT availability and severity of positive, negative, and depressive symptoms.

CONCLUSIONS

This study failed to detect alterations of SERT availability in patients with schizophrenia; however, this study does not rule out the possibility that schizophrenia might be associated with alterations of SERT density in the cortical regions, where the [11C]DASB-specific binding signal is too low for reliable quantification of SERT.

A PET imaging agent with fast kinetics: synthesis and in vivo evaluation of the serotonin transporter ligand [11C]2-[2-dimethylaminomethylphenylthio)]-5-fluorophenylamine ([11C]AFA)

A new serotonin transporter (SERT) ligand, [11C]2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine (10, [11C]AFA), was synthesized and evaluated as a candidate PET radioligand in pharmacological and pharmacokinetic studies. As a PET radioligand, AFA (8) can be labeled with either C-11 or F-18. In vitro, AFA displayed high affinity for SERT (Ki 1.46 +/- 0.15 nM) and lower affinity for norepinephrine transporter (NET, Ki 141.7 +/- 47.4 nM) or dopamine transporter (DAT, Ki > 10,000 nM). [11C]AFA (10) was prepared from its monomethylamino precursor 9 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 43 +/- 20% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 2,129 +/- 1,369 Ci/mmol at end of synthesis (EOS, n = 10). Biodistribution studies in rats indicated that [11C]AFA accumulated in brain regions known to contain high concentrations of SERT. Binding in SERT-rich brain regions was reduced significantly by pretreatment with either the cold compound 8 or with the selective serotonin reuptake inhibitor (SSRI) citalopram, but not by the selective norepinephrine reuptake inhibitor nisoxetine, thus underlining its in vivo binding selectivity and specificity for SERT. Imaging experiments in baboons demonstrated that the uptake pattern of [11C]AFA in the baboon brain is consistent with the known distribution of SERT, with highest activity levels in the midbrain and thalamus, followed by striatum, hippocampus, and cortical regions. Activity levels in the baboon brain peaked at 15-40 min after radioligand injection, indicating a fast uptake kinetics for [11C]AFA. Pretreatment of the baboon with citalopram (4 mg/kg) significantly reduced the specific binding of [11C]AFA in all SERT-containing brain regions. Kinetic analysis revealed that the regional equilibrium specific to non-specific partition coefficients (V3") of [11C]AFA are similar to those of [11C]McN5652, but lower than those of [11C]AFM or [11C]DASB. In summary, [11C]AFA appears to be an appropriate PET radioligand with a fast brain uptake kinetics:

Potential of [11C]DASB for measuring endogenous serotonin with PET: binding studies

The serotonin transporter radioligand [11C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile, or [11C]DASB, was examined in order to assess its potential for measuring fluctuations in endogenous serotonin concentrations with positron emission tomography. Binding characteristics of [11C]DASB and the propensity for serotonin to displace the tracer were explored in rat brain homogenates. Experiments showed that serotonin displaced [11C]DASB in vitro. Ex vivo experiments performed after tranylcypromine injection (3 or 15 mg/kg) showed a dose-dependent trend in radioactivity uptake and suggested that serotonin may compete with [11C]DASB for transporter binding.

Regional distribution of serotonin transporter protein in postmortem human brain

INTRODUCTION

The primary approach in assessing the status of brain serotonin neurons in human conditions such as major depression and exposure to the illicit drug ecstasy has been the use of neuroimaging procedures involving radiotracers that bind to the serotonin transporter (SERT). However, there has been no consistency in the selection of a "SERT-free" reference region for the estimation of free and nonspecific binding, as occipital cortex, cerebellum and white matter have all been employed.

OBJECTIVE AND METHODS

To identify areas of human brain that might have very low SERT levels, we measured, by a semiquantitative Western blotting procedure, SERT protein immunoreactivity throughout the postmortem brain of seven normal adult subjects.

RESULTS

Serotonin transporter could be quantitated in all examined brain areas. However, the SERT concentration in cerebellar cortex and white matter were only at trace values, being approximately 20% of average cerebral cortex and 5% of average striatum values.

CONCLUSION

Although none of the examined brain areas are completely free of SERT, human cerebellar cortex has low SERT binding as compared to other examined brain regions, with the exception of white matter. Since the cerebellar cortical SERT binding is not zero, this region will not be a suitable reference region for SERT radioligands with very low free and nonspecific binding. For SERT radioligands with reasonably high free and nonspecific binding, the cerebellar cortex should be a useful reference region, provided other necessary radioligand assumptions are met.

Synthesis and characterization of EADAM: a selective radioligand for mapping the brain serotonin transporters by positron emission tomography

[11C]N,N-Dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine ([11C]EADAM) was synthesized in the development of a serotonin transporter (SERT) imaging ligand for positron emission tomography (PET). The methods of ligand synthesis, results of in vitro characterization, 11C labeling and in vivo micro-PET imaging studies of [11C]EADAM in cynomolgus monkey brain are described. 11C was introduced into N,N-dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (5) by alkylation of N-methyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (10) in 32% radiochemical yield (end of bombardment [EOB], decay-corrected from [11C]methyl iodide). Competition binding assays in cells stably expressing the transfected human dopamine transporter (DAT), SERT and norepinephrine transporter (NET) labeled with [3H]WIN 35428 or [(125)I]RTI-55, [3H]citalopram and [3H]nisoxetine, respectively, indicated the following order of SERT affinity: ADAM>EADAM>>fluvoxamine. The affinity of EADAM for DAT and NET was 500- and >1000-fold lower, respectively, than for SERT. Micro-PET brain imaging studies in a cynomolgus monkey demonstrated high [11C]EADAM uptake in the striatum, thalamus and brainstem. [11C]EADAM uptake in these brain regions peaked in less than 60 min following administration of [11C]EADAM. The tissue-to-cerebellum ratios of the striatum, thalamus and brainstem were 1.67, 1.71 and 1.63, respectively, at 120 min postinjection of [11C]EADAM. Analysis of monkey arterial plasma samples using high-pressure liquid chromatography determined there was no detectable formation of lipophilic radiolabeled metabolites capable of entering the brain. In a displacement experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was displaced 20-60 min after administration of citalopram. In a blocking experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was significantly reduced. These results support the candidacy of [11C]EADAM as a radioligand for visualizing brain SERT using PET.

Effects of reduced endogenous 5-HT on the in vivo binding of the serotonin transporter radioligand11C-DASB in healthy humans

Although abnormal serotonin (5-HT) function is implicated in a range of mental disorders, there is currently no method to directly assess 5-HT synaptic levels in the living human brain. The in vivo binding of some dopamine (DA) radioligands such as (11)C-raclopride is affected by fluctuations in endogenous DA, thus providing an indirect measure of DA presynaptic activity. Attempts to identify a serotonergic radiotracer with similar properties have proved unsuccessful. Here, we investigated in humans the effects of reduced synaptic 5-HT on the in vivo binding of the 5-HT transporter (SERT) radioligand (11)C-DASB, using Positron Emission Tomography (PET) and the rapid tryptophan depletion (RTD) technique. Eight (8) subjects (5M, 3F) were scanned with (11)C-DASB under control and reduced endogenous 5-HT conditions, in a within-subject, double-blind, counterbalanced, crossover design. Regional distribution volumes (V(T)) were calculated using kinetic modeling and metabolite-corrected arterial input function. (11)C-DASB specific binding was estimated as binding potential (BP) and specific to nonspecific equilibrium partition coefficient (V(")(3)), using the cerebellum as reference region. RTD caused small but significant mean reductions in (11)C-DASB V(T) (-6.1%) and BP (-4.5%) across brain regions, probably explained by a concomitant reduction in (11)C-DASB plasma free fraction (f(1)) of similar magnitude. No significant change in (11)C-DASB V(")(3) was observed between control and reduced 5-HT conditions. Nor was there a significant relationship between the magnitude of tryptophan depletion and change in BP and V(")(3) across individual subjects. These results suggest that (11)C-DASB in vivo binding is not affected by reductions in endogenous 5-HT.