Daniela Haeusler

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

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

Prediction of SSRI treatment response in major depression based on serotonin transporter interplay between median raphe nucleus and projection areas

Recent mathematical models suggest restored serotonergic burst-firing to underlie the antidepressant effect of selective serotonin reuptake inhibitors (SSRI), resulting from down-regulated serotonin transporters (SERT) in terminal regions. This mechanism possibly depends on the interregional balance between SERTs in the raphe nuclei and in terminal regions before treatment. To evaluate these hypotheses on a systems level in humans in vivo, we investigated SERT availability and occupancy longitudinally in patients with major depressive disorder using positron emission tomography (PET) and the radioligand [11C]DASB. Measurements were performed before and after a single oral dose, as well as after three weeks (mean 24.73±3.3 days) of continuous oral treatment with either escitalopram (10 mg/day) or citalopram (20 mg/day). Data were analyzed using voxel-wise linear regression and ANOVA to evaluate SERT binding, occupancy and binding ratios (SERT binding of the entire brain compared to SERT binding in the dorsal and median raphe nuclei) in relation to treatment outcome. Regression analysis revealed that treatment response was predicted by pre-treatment SERT binding ratios, i.e., SERT binding in key regions of depression including bilateral habenula, amygdala-hippocampus complex and subgenual cingulate cortex in relation to SERT binding in the median but not dorsal raphe nucleus (p<0.05 FDR-corrected). Similar results were observed in the direct comparison of responders and non-responders. Our data provide a first proof-of-concept for recent modeling studies and further underlie the importance of the habenula and subgenual cingulate cortex in the etiology of and recovery from major depression. These findings may indicate a promising molecular predictor of treatment response and stimulate new treatment approaches based on regional differences in SERT binding.

Regional differences in SERT occupancy after acute and prolonged SSRI intake investigated by brain PET

Blocking of the serotonin transporter (SERT) represents the initial mechanism of action of selective serotonin reuptake inhibitors (SSRIs) which can be visualized due to the technical proceedings of SERT occupancy studies. When compared to the striatum, higher SERT occupancy in the midbrain and lower values in the thalamus were reported. This indicates that occupancy might be differently distributed throughout the brain, which is supported by preclinical findings indicating a regionally varying SERT activity and antidepressant drug concentration. The present study therefore aimed to investigate regional SERT occupancies with positron emission tomography and the radioligand [(11)C]DASB in 19 depressed patients after acute and prolonged intake of oral doses of either 10mg/day escitalopram or 20mg/day citalopram. Compared to the mean occupancy across cortical and subcortical regions, we detected increased SERT occupancies in regions commonly associated with antidepressant response, such as the subgenual cingulate, amygdala and raphe nuclei. When acute and prolonged drug intake was compared, SERT occupancies increased in subcortical areas that are known to be rich in SERT. Moreover, SERT occupancy in subcortical brain areas after prolonged intake of antidepressants was predicted by plasma drug levels. Similarly, baseline SERT binding potential seems to impact SERT occupancy, as regions rich in SERT showed greater binding reduction as well as higher residual binding. These findings suggest a region-specific distribution of SERT blockage by SSRIs and relate the postulated link between treatment response and SERT occupancy to certain brain regions such as the subgenual cingulate cortex.

Simple and fully automated preparation of [carbonyl-11C]WAY-100635

So far, [carbonyl-11C]WAY-100635 is the PET-tracer of choice for 5HT1A-receptor-imaging. Since the preparation is still a challenge, we aimed at (1) the evaluation of various essential parameters for the successful preparation, (2) the simplification of the radiosynthesis and (3) the establishment of a safe and fully automated system. The preparation is based on a commercial synthesizer and all chemicals are used without further processing. We found a low failure rate (7.7%), high average yield (4.0 ± 1.0 GBq) and a specific radioactivity of 292 ± 168 GBq/μmol (both at the end of synthesis, EOS).

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 [11C]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.

Attenuated serotonin transporter association between dorsal raphe and ventral striatum in major depression.

Suffering from anhedonia, patients with major depressive disorder (MDD) exhibit alterations in several parts of the serotonergic neurotransmitter system, which are in turn involved in reward processing. However, previous investigations of the serotonin transporter (SERT) focused on regional differences with varying results depending on the clinical syndrome. Here, we aimed to describe the serotonergic system of MDD patients on a network level by evaluating SERT associations across brain regions. Twenty medication free patients with major depression and 20 healthy controls underwent positron emission tomography using the radioligand [11C]DASB. SERT binding potentials (BPND) were quantified voxel‐wise with the multilinear reference tissue model 2. In addition, SERT BPND was extracted from the dorsal raphe nucleus (DRN) as an indicator of midbrain serotonergic neurotransmission. Whole‐brain linear regression analysis was applied to evaluate the association of DRN SERT bindings to those in projection areas, which was followed by ANCOVA to assess differences in interregional relationships between patients and controls. Although both groups showed widespread positive correlations, group differences were restricted to decreased SERT associations between the DRN and the ventral striatum (right and left respectively: t = 5.85, P < 0.05 corrected and t = 5.07, P < 0.1 corrected) when comparing MDD patients (R2 = 0.11 and 0.24) to healthy subjects (R2 = 0.72 and 0.66, P < 0.01 and 0.05 corrected). Adjusting for age and sex did not change these findings. This study indicates a disturbed regulation between key regions involved in reward processing via the SERT. Our interregional approach highlights the importance of evaluating pathophysiological alterations on a network level to gain complementary information in addition to regional investigations. Hum Brain Mapp 35:3857–3866, 2014.

Microfluidic preparation of [18F]FE@SUPPY and [18F]FE@SUPPY:2 — comparison with conventional radiosyntheses

INTRODUCTION Recently, first applications of microfluidic principles for radiosyntheses of positron emission tomography compounds were presented, but direct comparisons with conventional methods were still missing. Therefore, our aims were (1) the set-up of a microfluidic procedure for the preparation of the recently developed adenosine A(3)-receptor tracers [(18)F]FE@SUPPY [5-(2-[(18)F]fluoroethyl)2,4-diethyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate] and [(18)F]FE@SUPPY:2 [5-ethyl-2,4-diethyl-3-((2-[(18)F]fluoroethyl)sulfanylcarbonyl)-6-phenylpyridine-5-carboxylate] and (2) the direct comparison of reaction conditions and radiochemical yields of the no-carrier-added nucleophilic substitution with [(18)F]fluoride between microfluidic and conventional methods. METHODS For the determination of optimal reaction conditions within an Advion NanoTek synthesizer, 5-50 μl of precursor and dried [(18)F]fluoride solution were simultaneously pushed through the temperature-controlled reactor (26 °C-180 °C) with defined reactant bolus flow rates (10-50 μl/min). Radiochemical incorporation yields (RCIYs) and overall radiochemical yields for large-scale preparations were compared with data from conventional batch-mode syntheses. RESULTS Optimal reaction parameters for the microfluidic set-up were determined as follows: 170 °C, 30-μl/min pump rate per reactant (reaction overall flow rate of 60 μl/min) and 5-mg/ml precursor concentration in the reaction mixture. Applying these optimized conditions, we observed a significant increase in RCIY from 88.2% to 94.1% (P < .0001, n ≥ 11) for [(18)F]FE@SUPPY and that from 42.5% to 95.5% (P<.0001, n ≥ 5) for [(18)F]FE@SUPPY:2 using microfluidic instead of conventional heating. Precursor consumption was decreased from 7.5 and 10 mg to 1 mg per large-scale synthesis for both title compounds, respectively. CONCLUSION The direct comparison of radiosyntheses data applying a conventional method and a microfluidic approach revealed a significant increase of RCIY using the microfluidic approach.

Simple and rapid preparation of [11C]DASB with high quality and reliability for routine applications.

[(11)C]DASB combines all major prerequisites for a successful SERT-ligand, providing excellent biological properties and in-vivo behaviour. Thus, we aimed to establish a fully automated procedure for the synthesis and purification of [(11)C]DASB with a high degree of reliability reducing the overall synthesis time while conserving high yields and purity. The optimized [(11)C]DASB synthesis was applied in more than 60 applications with a very low failure rate (3.2%). We obtained yields up to 8.9 GBq (average 5.3+/-1.6 GBq). Radiochemical yields based on [(11)C]CH(3)I, (corrected for decay) were 66.3+/-6.9% with a specific radioactivity (A(s)) of 86.8+/-24.3 GBq/micromol (both at the end of synthesis, EOS). Time consumption was kept to a minimum, resulting in 43 min from end of bombardment to release of the product after quality control. From our data, it is evident that the presented method can be implemented for routine preparations of [(11)C]DASB with high reliability.

Cerebral serotonin transporter asymmetry in females, males and male-to-female transsexuals measured by PET in vivo

The serotonergic system modulates brain functions that are considered to underlie affective states, emotion and cognition. Several lines of evidence point towards a strong lateralization of these mental processes, which indicates similar asymmetries in associated neurotransmitter systems. Here, our aim was to investigate a potential asymmetry of the serotonin transporter distribution using positron emission tomography and the radioligand [11C]DASB in vivo. As brain asymmetries may differ between sexes, we further aimed to compare serotonin transporter asymmetry between females, males and male-to-female (MtF) transsexuals whose brains are considered to be partly feminized. Voxel-wise analysis of serotonin transporter binding in all groups showed both strong left and rightward asymmetries in several cortical and subcortical structures including temporal and frontal cortices, anterior cingulate, hippocampus, caudate and thalamus. Further, male controls showed a rightward asymmetry in the midcingulate cortex, which was absent in females and MtF transsexuals. The present data support the notion of a lateralized serotonergic system, which is in line with previous findings of asymmetric serotonin-1A receptor distributions, extracellular serotonin concentrations, serotonin turnover and uptake. The absence of serotonin transporter asymmetry in the midcingulate in MtF transsexuals may be attributed to an absence of brain masculinization in this region.

Serotonin-1A receptor binding is positively associated with gray matter volume -- a multimodal neuroimaging study combining PET and structural MRI.

Animal models revealed that the serotonin-1A (5-HT(1A)) receptor modulates gray matter structure. However, there is a lack of evidence showing the relationship between 5-HT(1A) receptor concentration and gray matter in the human brain in vivo. Here, to demonstrate an association between the 5-HT(1A) receptor binding potential, an index for receptor concentration, and the local gray matter volume (GMV), an index for gray matter structure, we measured 35 healthy subjects with both positron emission tomography (PET) and structural magnetic resonance imaging (MRI). We found that regional heteroreceptor binding was positively associated with GMV in distinctive brain regions such as the hippocampi and the temporal cortices in both hemispheres (R(2) values ranged from 0.308 to 0.503, p<0.05 cluster-level FDR-corrected). Furthermore, autoreceptor binding in the midbrain raphe region was positively associated with GMV in forebrain projection sites (R(2)=0.656, p=0.001). We also observed a broad range between 5-HT(1A) receptor binding and GMV. Given the congruence of altered 5-HT(1A) receptor concentrations and GMV reduction in depression or Alzheimers disease as reported by numerous studies, these results might provide new insights towards understanding the mechanisms behind GMV alterations observed in these brain disorders.