Measurement of changes in endogenous serotonin level by positron emission tomography with [18F]altanserin.

OBJECTIVE: Positron emission tomography (PET) has been used to investigate changes in the concentration of endogenous neurotransmitters. Recently, this technique has been applied to the imaging of serotonin2A receptors using [18F]altanserin. In these measurements, a reduction in binding potential (BP) suggests an increase in endogenous serotonin levels caused by pharmacological or cognitive stimulations, and the sensitivity of BP reduction depends on the characteristics of [18F]altanserin. In this study, we evaluated an analytical method for estimating the changes in endogenous serotonin levels based on PET scans with [18F]altanserin at baseline and stimulated states and validated it using simulations and small animal PET studies. METHODS: First, in the simulations, the time-activity curves at baseline and the stimulated states were generated using an extended compartment model including the competition for the receptors between the administered [18F]altanserin and endogenous serotonin. In the stimulated state, the magnitude and onset of the endogenous serotonin elevation were altered to varying degrees. In these time-activity curves, BP was estimated using the simplified reference tissue model (SRTM), and the reduction in BP was evaluated by comparison with that of the baseline state. Next, the proposed method was applied to mouse PET studies. Endogenous serotonin levels were elevated by treatment with selective serotonin reuptake inhibitors (SSRIs), and PET studies were performed twice, once with and once without treatment. In both scans, BP was estimated using the SRTM with the cerebellum as a reference region, and the reduction in BP after SSRI treatment was evaluated. RESULTS: In the simulations, the BP estimate of the stimulated state was smaller than that of the baseline state, and their reduction was related to the amount of change in the serotonin concentration. BP reduction was also affected by the onset of serotonin elevation. In the mouse studies, the BP of the cerebral cortex decreased in the scans with SSRI treatment. CONCLUSIONS: The reduction in BP estimated using the SRTM from [18F]altanserin-PET studies at baseline and in stimulated states can detect changes in the binding conditions of serotonin2A receptors. This may be useful for investigating the elevation of endogenous serotonin levels caused by stimulations.

Common HTR2A variants and 5-HTTLPR are not associated with human in vivo serotonin 2A receptor levels.

The serotonin 2A receptor (5-HT2AR) is implicated in the pathophysiology and treatment of various psychiatric disorders. [18 F]altanserin and [11 C]Cimbi-36 positron emission tomography (PET) allow for high-resolution imaging of 5-HT2AR in the living human brain. Cerebral 5-HT2AR binding is strongly genetically determined, though the impact of specific variants is poorly understood. Candidate gene studies suggest that HTR2A single nucleotide polymorphisms including rs6311/rs6313, rs6314, and rs7997012 may influence risk for psychiatric disorders and mediate treatment response. Although known to impact in vitro expression of 5-HT2AR or other serotonin (5-HT) proteins, their effect on human in vivo brain 5-HT2AR binding has as of yet been insufficiently studied. We thus assessed the extent to which these variants and the commonly studied 5-HTTLPR predict neocortex in vivo 5-HT2AR binding in healthy adult humans. We used linear regression analyses and likelihood ratio tests in 197 subjects scanned with [18 F]altanserin or [11 C]Cimbi-36 PET. Although we observed genotype group differences in 5-HT2AR binding of up to ~10%, no genetic variants were statistically significantly predictive of 5-HT2AR binding in what is the largest human in vivo 5-HT2AR imaging genetics study to date. Thus, in vitro and post mortem results suggesting effects on 5-HT2AR expression did not carry over to the in vivo setting. To any extent these variants might affect clinical risk, our findings do not support that 5-HT2AR binding mediates such effects. Our observations indicate that these individual variants do not significantly contribute to genetic load on human in vivo 5-HT2AR binding.

Kinetic analysis of [18F] altanserin bolus injection in the canine brain using PET imaging.

BACKGROUND: Currently, [18F] altanserin is the most frequently used PET-radioligand for serotonin2A (5-HT2A) receptor imaging in the human brain but has never been validated in dogs. In vivo imaging of this receptor in the canine brain could improve diagnosis and therapy of several behavioural disorders in dogs. Furthermore, since dogs are considered as a valuable animal model for human psychiatric disorders, the ability to image this receptor in dogs could help to increase our understanding of the pathophysiology of these diseases. Therefore, five healthy laboratory beagles underwent a 90-min dynamic PET scan with arterial blood sampling after [18F] altanserin bolus injection. Compartmental modelling using metabolite corrected arterial input functions was compared with reference tissue modelling with the cerebellum as reference region. RESULTS: The distribution of [18F] altanserin in the canine brain corresponded well to the distribution of 5-HT2A receptors in human and rodent studies. The kinetics could be best described by a 2-Tissue compartment (2-TC) model. All reference tissue models were highly correlated with the 2-TC model, indicating compartmental modelling can be replaced by reference tissue models to avoid arterial blood sampling. CONCLUSIONS: This study demonstrates that [18F] altanserin PET is a reliable tool to visualize and quantify the 5-HT2A receptor in the canine brain.

Low frontal serotonin 2A receptor binding is a state marker for schizophrenia?

Here we imaged serotonin 2A receptor (5-HT2AR) binding in a very rare population of monozygotic twins discordant for schizophrenia to provide insight into trait and state components in brain 5-HT2AR patterns. In four twin pairs not medicated with drugs that target 5-HT2AR, frontal 5-HT2AR binding was consistently lower (33%) in schizophrenic- relative to their healthy co-twins. Our results strongly imply low frontal 5-HT2AR availability as a state feature of schizophrenia. If replicated, ideally in a larger sample also including dizygotic twin pairs and drug-naïve patients, this finding critically advance our understanding of the complex pathophysiology of schizophrenia.

Serotonin 2A receptor agonist binding in the human brain with [(11)C]Cimbi-36: Test-retest reproducibility and head-to-head comparison with the antagonist [(18)F]altanserin.

INTRODUCTION: [(11)C]Cimbi-36 is a recently developed serotonin 2A (5-HT2A) receptor agonist positron emission tomography (PET) radioligand that has been successfully applied for human neuroimaging. Here, we investigate the test-retest variability of cerebral [(11)C]Cimbi-36 PET and compare [(11)C]Cimbi-36 and the 5-HT2A receptor antagonist [(18)F]altanserin. METHODS: Sixteen healthy volunteers (mean age 23.9 ± 6.4years, 6 males) were scanned twice with a high resolution research tomography PET scanner. All subjects were scanned after a bolus of [(11)C]Cimbi-36; eight were scanned twice to determine test-retest variability in [(11)C]Cimbi-36 binding measures, and another eight were scanned after a bolus plus constant infusion with [(18)F]altanserin. Regional differences in the brain distribution of [(11)C]Cimbi-36 and [(18)F]altanserin were assessed with a correlation of regional binding measures and with voxel-based analysis. RESULTS: Test-retest variability of [(11)C]Cimbi-36 non-displaceable binding potential (BPND) was consistently <5% in high-binding regions and lower for reference tissue models as compared to a 2-tissue compartment model. We found a highly significant correlation between regional BPNDs measured with [(11)C]Cimbi-36 and [(18)F]altanserin (mean Pearson's r: 0.95 ± 0.04) suggesting similar cortical binding of the radioligands. Relatively higher binding with [(11)C]Cimbi-36 as compared to [(18)F]altanserin was found in the choroid plexus and hippocampus in the human brain. CONCLUSIONS: Excellent test-retest reproducibility highlights the potential of [(11)C]Cimbi-36 for PET imaging of 5-HT2A receptor agonist binding in vivo. Our data suggest that Cimbi-36 and altanserin both bind to 5-HT2A receptors, but in regions with high 5-HT2C receptor density, choroid plexus and hippocampus, the [(11)C]Cimbi-36 binding likely represents binding to both 5-HT2A and 5-HT2C receptors.

Central 5-HT neurotransmission modulates weight loss following gastric bypass surgery in obese individuals.

The cerebral serotonin (5-HT) system shows distinct differences in obesity compared with the lean state. Here, it was investigated whether serotonergic neurotransmission in obesity is a stable trait or changes in association with weight loss induced by Roux-in-Y gastric bypass (RYGB) surgery. In vivo cerebral 5-HT2A receptor and 5-HT transporter binding was determined by positron emission tomography in 21 obese [four men; body mass index (BMI), 40.1 ± 4.1 kg/m(2)] and 10 lean (three men; BMI, 24.6 ± 1.5 kg/m(2)) individuals. Fourteen obese individuals were re-examined after RYGB surgery. First, it was confirmed that obese individuals have higher cerebral 5-HT2A receptor binding than lean individuals. Importantly, we found that higher presurgical 5-HT2A receptor binding predicted greater weight loss after RYGB and that the change in 5-HT2A receptor and 5-HT transporter binding correlated with weight loss after RYGB. The changes in the 5-HT neurotransmission before and after RYGB are in accordance with a model wherein the cerebral extracellular 5-HT level modulates the regulation of body weight. Our findings support that the cerebral 5-HT system contributes both to establish the obese condition and to regulate the body weight in response to RYGB.

Neocortical serotonin2A receptor binding predicts quetiapine associated weight gain in antipsychotic-naive first-episode schizophrenia patients.

Antipsychotic-induced weight gain is of major clinical importance since it is associated with severe metabolic complications and increased mortality. The serotonin2A receptor system has been suggested to be implicated in weight gain and obesity. However, no previous in vivo imaging data have related serotonin2A receptor binding to weight gain before and after antipsychotic monotherapy. Fifteen antipsychotic-naive first-episode schizophrenia patients were included and investigated before and after six months of quetiapine treatment. We examined the relationship between serotonin2A receptor binding as measured with positron emission tomography (PET) and [18F]altanserin and change in body mass index (BMI). Quetiapine was chosen because it is characterized by a moderately high affinity for the serotonin2A receptor and a fast dissociation rate from the dopamine D2 receptor. At baseline the mean BMI was 24.2 kg/m2, range 18-36 kg/m2. After six months of quetiapine treatment (mean dose: 383 mg/day) the BMI had, on average, increased by 6.7%, corresponding to an average weight gain of 5.0 kg. We found a significant positive correlation both between neocortical serotonin2A receptor binding prior to treatment and subsequent increase in BMI (rho=0.59, p=0.022). At follow-up, the serotonin2A receptor occupancy was positively correlated with BMI increase (rho=0.54, p=0.038). To our knowledge, these are the first in vivo receptor imaging data in initially antipsychotic-naive first-episode schizophrenia patients to show that the cerebral serotonin2A receptor is associated with antipsychotic-induced weight gain.

Effects of serotonin-2A receptor binding and gender on personality traits and suicidal behavior in borderline personality disorder.

Impulsivity and aggressiveness are personality traits associated with a vulnerability to suicidal behavior. Behavioral expression of these traits differs by gender and has been related to central serotonergic function. We assessed the relationships between serotonin-2A receptor function, gender, and personality traits in borderline personality disorder (BPD), a disorder characterized by impulsive-aggression and recurrent suicidal behavior. Participants, who included 33 BPD patients and 27 healthy controls (HC), were assessed for Axis I and II disorders with the Structured Clinical Interview for DSM-IV and the International Personality Disorders Examination, and with the Diagnostic Interview for Borderline Patients-Revised for BPD. Depressed mood, impulsivity, aggression, and temperament were assessed with standardized measures. Positron emission tomography with [(18)F]altanserin as ligand and arterial blood sampling was used to determine the binding potentials (BPND) of serotonin-2A receptors in 11 regions of interest. Data were analyzed using Logan graphical analysis, controlling for age and non-specific binding. Among BPD subjects, aggression, Cluster B co-morbidity, antisocial PD, and childhood abuse were each related to altanserin binding. BPND values predicted impulsivity and aggression in BPD females (but not BPD males), and in HC males (but not HC females.) Altanserin binding was greater in BPD females than males in every contrast, but it did not discriminate suicide attempters from non-attempters. Region-specific differences in serotonin-2A receptor binding related to diagnosis and gender predicted clinical expression of aggression and impulsivity. Vulnerability to suicidal behavior in BPD may be related to serotonin-2A binding through expression of personality risk factors.

[¹8F]Altanserin and small animal PET: impact of multidrug efflux transporters on ligand brain uptake and subsequent quantification of 5-HT2A receptor densities in the rat brain.

INTRODUCTION: The selective 5-hydroxytryptamine type 2a receptor (5-HT(2A)R) radiotracer [(18)F]altanserin is a promising ligand for in vivo brain imaging in rodents. However, [(18)F]altanserin is a substrate of P-glycoprotein (P-gp) in rats. Its applicability might therefore be constrained by both a differential expression of P-gp under pathological conditions, e.g. epilepsy, and its relatively low cerebral uptake. The aim of the present study was therefore twofold: (i) to investigate whether inhibition of multidrug transporters (MDT) is suitable to enhance the cerebral uptake of [(18)F]altanserin in vivo and (ii) to test different pharmacokinetic, particularly reference tissue-based models for exact quantification of 5-HT(2A)R densities in the rat brain. METHODS: Eighteen Sprague-Dawley rats, either treated with the MDT inhibitor cyclosporine A (CsA, 50 mg/kg, n=8) or vehicle (n=10) underwent 180-min PET scans with arterial blood sampling. Kinetic analyses of tissue time-activity curves (TACs) were performed to validate invasive and non-invasive pharmacokinetic models. RESULTS: CsA application lead to a two- to threefold increase of [(18)F]altanserin uptake in different brain regions and showed a trend toward higher binding potentials (BP(ND)) of the radioligand. CONCLUSIONS: MDT inhibition led to an increased cerebral uptake of [(18)F]altanserin but did not improve the reliability of BP(ND) as a non-invasive estimate of 5-HT(2A)R. This finding is most probable caused by the heterogeneous distribution of P-gp in the rat brain and its incomplete blockade in the reference region (cerebellum). Differential MDT expressions in experimental animal models or pathological conditions are therefore likely to influence the applicability of imaging protocols and have to be carefully evaluated.

PET imaging of serotoninergic neurotransmission with [(11)C]DASB and [(18)F]altanserin after focal cerebral ischemia in rats.

The use of selective serotonin reuptake inhibitors has shown functional improvement after stroke. Despite this, the role of serotoninergic neurotransmission after cerebral ischemia evolution and its involvement in functional recovery processes are still largely unknown. For this purpose, we performed in parallel in vivo magnetic resonance imaging and positron emission tomography (PET) with [(11)C]DASB and [(18)F]altanserin at 1, 3, 7, 14, 21, and 28 days after middle cerebral artery occlusion (MCAO) in rats. In the ischemic territory, PET with [(11)C]DASB and [(18)F]altanserin showed a dramatic decline in serotonin transporter (SERT) and 5-HT2A binding potential in the cortex and striatum after cerebral ischemia. Interestingly, a slight increase in [(11)C]DASB binding was observed from days 7 to 21 followed by the uppermost binding at day 28 in the ipsilateral midbrain. In contrast, no changes were observed in the contralateral hemisphere by using both radiotracers. Likewise, both functional and behavior testing showed major impaired outcome at day 1 after ischemia onset followed by a recovery of the sensorimotor function and dexterity from day 21 to day 28 after cerebral ischemia. Taken together, these results might evidence that SERT changes in the midbrain could have a key role in the functional recovery process after cerebral ischemia.

Suitability of [18F]altanserin and PET to determine 5-HT2A receptor availability in the rat brain: in vivo and in vitro validation of invasive and non-invasive kinetic models.

PURPOSE: While the selective 5-hydroxytryptamine type 2a receptor (5-HT2AR) radiotracer [18F]altanserin is well established in humans, the present study evaluated its suitability for quantifying cerebral 5-HT2ARs with positron emission tomography (PET) in albino rats. PROCEDURES: Ten Sprague Dawley rats underwent 180 min PET scans with arterial blood sampling. Reference tissue methods were evaluated on the basis of invasive kinetic models with metabolite-corrected arterial input functions. In vivo 5-HT2AR quantification with PET was validated by in vitro autoradiographic saturation experiments in the same animals. RESULT: Overall brain uptake of [18F]altanserin was reliably quantified by invasive and non-invasive models with the cerebellum as reference region shown by linear correlation of outcome parameters. Unlike in humans, no lipophilic metabolites occurred so that brain activity derived solely from parent compound. PET data correlated very well with in vitro autoradiographic data of the same animals. CONCLUSION: [18F]Altanserin PET is a reliable tool for in vivo quantification of 5-HT2AR availability in albino rats. Models based on both blood input and reference tissue describe radiotracer kinetics adequately. Low cerebral tracer uptake might, however, cause restrictions in experimental usage.

Direct comparison of [(18) F]MH.MZ and [(18) F] altanserin for 5-HT2A receptor imaging with PET.

Imaging the cerebral serotonin 2A (5-HT2A ) receptors with positron emission tomography (PET) has been carried out in humans with [(11) C]MDL 100907 and [(18) F]altanserin. Recently, the MDL 100907 analogue [(18) F]MH.MZ was developed combining the selectivity profile of MDL 100907 and the favourable radiophysical properties of fluorine-18. Here, we present a direct comparison of [(18) F]altanserin and [(18) F]MH.MZ. 5-HT2A receptor binding in pig cortex and cerebellum was investigated by autoradiography with [(3) H]MDL 100907, [(18) F]MH.MZ, and [(18) F]altanserin. [(18) F]MH.MZ and [(18) F]altanserin were investigated in Danish Landrace pigs by brain PET scanning at baseline and after i.v. administration of blocking doses of ketanserin. Full arterial input function and high performance liquid chromatography (HPLC) analysis allowed for tissue-compartment kinetic modeling of PET data. In vitro autoradiography showed high binding in cortical regions with both [(18) F]MH.MZ and [(18) F]altanserin. Significant 5-HT2A receptor binding was also found in the pig cerebellum, thus making this region unsuitable as a reference region for in vivo data analysis in this species. The cortical binding of [(18) F]MH.MZ and [(18) F]altanserin was blocked by ketanserin supporting that both radioligands bind to 5-HT2A receptors in the pig brain. In the HPLC analysis of pig plasma, [(18) F]MH.MZ displayed a fast and reproducible metabolism resulting in hydrophilic radiometabolites only whereas the metabolic profile of [(18) F]altanserin as expected showed lipophilic radiometabolites. Due to the slow kinetics of [(18) F]MH.MZ in high-binding regions in vivo, we suggest that [(18) F]MH.MZ will be an appropriate tracer for low binding regions where kinetics will be faster, whereas [(18) F]altanserin is a suitable tracer for high-binding regions.

Trait aggression and trait impulsivity are not related to frontal cortex 5-HT2A receptor binding in healthy individuals.

Numerous studies indicate that the serotonergic (5-HT) transmitter system is involved in the regulation of impulsive aggression and there is from post-mortem, in vivo imaging and genetic studies evidence that the 5-HT2A receptor may be involved. We investigated 94 healthy individuals (60 men, mean age 47.0±18.7, range 23-86) to determine if trait aggression and trait impulsivity were related to frontal cortex 5-HT2A receptor binding (5-HT2AR) as measured with [18F]-altanserin PET imaging. Trait aggression and trait impulsivity were assessed with the Buss-Perry Aggression Questionnaire (AQ) and the Barratt Impulsiveness Scale 11 (BIS-11). Statistical analyses were conducted using a multiple linear regression model and internal consistency reliability of the AQ and BIS-11 was evaluated by Cronbach's alpha. Contrary to our hypothesis, results revealed no significant associations between 5-HT2AR and the AQ or BIS-11 total scores. Also, there was no significant interaction between gender and frontal cortex 5-HT2AR in predicting trait aggression and trait impulsivity. This is the first study to examine how 5-HT2AR relates to trait aggression and trait impulsivity in a large sample of healthy individuals. Our findings are not supportive of a selective role for 5-HT2AR in mediating the 5-HT related effects on aggression and impulsivity in psychiatrically healthy individuals.

Dysregulated 5-HT(2A) receptor binding in postmortem frontal cortex of schizophrenic subjects.

Previous postmortem and neuroimaging studies have repeatedly suggested alterations in serotonin 5-HT(2A) receptor (5-HT(2A)R) binding associated with the pathophysiology of schizophrenia. These studies were performed with ligands, such as ketanserin, altanserin and LSD, that may bind with high-affinity to different structural or functional conformations of the 5-HT(2A)R. Interpretation of results may also be confounded by chronic antipsychotic treatment and suicidal behavior in the schizophrenia group. We quantified 5-HT(2A)R density by radioligand binding assays in postmortem prefrontal cortex of antipsychotic-free (n=29) and antipsychotic-treated (n=16) schizophrenics, suicide victims with other psychiatric diagnoses (n=13), and individually matched controls. [³H]Ketanserin binding, and its displacement by altanserin or the LSD-like agonist DOI, was assayed. Results indicate that the number of [³H]ketanserin binding sites to the 5-HT(2A)R was increased in antipsychotic-free (128 ± 11%), but not in antipsychotic-treated (92 ± 12%), schizophrenic subjects. In suicide victims, [³H]ketanserin binding did not differ as compared to controls. Aging correlated negatively with [³H]ketanserin binding in schizophrenia, suicide victims and controls. The fraction of high-affinity sites of DOI displacing [³H]ketanserin binding to the 5-HT(2A)R was increased in antipsychotic-free schizophrenic subjects. Functional uncoupling of heterotrimeric G proteins led to increased fraction of high-affinity sites of altanserin displacing [³H]ketanserin binding to the 5-HT(2A)R in schizophrenic subjects, but not in controls. Together, these results suggest that the active conformation of the 5-HT(2A)R is up-regulated in prefrontal cortex of antipsychotic-free schizophrenic subjects, and may provide a pharmacological explanation for discordant findings previously obtained.

Sleep deprivation increases cerebral serotonin 2A receptor binding in humans.

STUDY OBJECTIVES: Serotonin and its cerebral receptors play an important role in sleep-wake regulation. The aim of the current study is to investigate the effect of 24-h total sleep deprivation on the apparent serotonin 2A receptor (5-HT(2A)R) binding capacity in the human brain to test the hypothesis that sleep deprivation induces global molecular alterations in the cortical serotonergic receptor system. DESIGN: Volunteers were tested twice with the subtype-selective radiotracer [(18)F]altanserin and positron emission tomography (PET) for imaging of 5-HT(2A)Rs at baseline and after 24 h of sleep deprivation. [(18)F]Altanserin binding potentials were analyzed in 13 neocortical regions of interest. The efficacy of sleep deprivation was assessed by questionnaires, waking electroencephalography, and cognitive performance measurements. SETTING: Sleep laboratory and neuroimaging center. PATIENTS OR PARTICIPANTS: Eighteen healthy volunteers. INTERVENTIONS: Sleep deprivation. MEASUREMENTS AND RESULTS: A total of 24 hours of sleep deprivation led to a 9.6% increase of [(18)F]altanserin binding on neocortical 5-HT(2A) receptors. Significant region-specific increases were found in the medial inferior frontal gyrus, insula, and anterior cingulate, parietal, sensomotoric, and ventrolateral prefrontal cortices. CONCLUSIONS: This study demonstrates that a single night of total sleep deprivation causes significant increases of 5-HT(2A)R binding potentials in a variety of cortical regions although the increase declines as sleep deprivation continued. It provides in vivo evidence that total sleep deprivation induces adaptive processes in the serotonergic system of the human brain.

Radiolabeling of [18F]altanserin - a microfluidic approach.

INTRODUCTION: Our aim was the optimization of radiochemical parameters for the microfluidic preparation of [(18)F]altanserin. The four main parameters evaluated were (1) precursor concentration, (2) reaction temperature, (3) bolus flow rate through the microreactor and (4) bolus volume. METHODS: For the determination of optimal reaction conditions within a flow-through microreactor synthesizer, 5-400 µL of precursor and dried [(18)F]fluoride solution were simultaneously pushed through the temperature-controlled reactor (180-220°C) with defined bolus flow rates of 10-60 µL/min. Radiochemical incorporation yields (RCIYs) were examined using a thin layer chromatography (TLC) set-up and radio- high-performance liquid chromatography (HPLC). RESULTS: Optimum reaction parameters for the microfluidic set-up were determined as following: 220°C, 5-10 µL/min pump rate per reactant (10-20 µL/min reaction overall flow rate) and 2mg/mL precursor concentration in the reaction mixture. Applying these optimized conditions, RCIYs of 53.7 ± 7.9 were observed for scaled-up preparations. A positive "bolus effect" was observed: applying higher reaction volume resulted in increased RCIYs. CONCLUSION: This study proved that the reaction bolus volume is an essential parameter influencing the RCIY of [(18)F]altanserin. A possible explanation is the inhomogeneous distribution within the reaction volume probably caused by diffusion at the bolus interface. This important finding should be considered an important variable for the evaluation of all novel radiotracers labeled using a flow-through reactor device.

Loss of serotonin 2A receptors exceeds loss of serotonergic projections in early Alzheimer's disease: a combined [11C]DASB and [18F]altanserin-PET study.

In patients with Alzheimer's disease (AD), postmortem and imaging studies have revealed early and prominent reductions in cerebral serotonin 2A (5-HT(2A)) receptors. To establish if this was due to a selective disease process of the serotonin system, we investigated the cerebral 5-HT(2A) receptor and the serotonin transporter binding, the latter as a measure of serotonergic projections and neurons. Twelve patients with AD (average Mini Mental State Examination [MMSE]: 24) and 11 healthy age-matched subjects underwent positron emission tomography (PET) scanning with [(18)F]altanserin and [(11)C]N,N-Dimethyl-2-(2-amino-4-cyanopheylthio)benzylamine ([(11)C]DASB). Overall [(18)F]altanserin binding was markedly reduced in AD by 28%-39% (p = 0.02), whereas the reductions in [(11)C]DASB binding were less prominent and mostly insignificant, except for a marked reduction of 33% in mesial temporal cortex (p = .0005). No change in [(11)C]DASB binding was found in the midbrain. We conclude that the prominent reduction in neocortical 5-HT(2A) receptor binding in early AD is not caused by a primary loss of serotonergic neurons or their projections.

Assessment of serotonin release capacity in the human brain using dexfenfluramine challenge and [18F]altanserin positron emission tomography.

Although alterations of serotonin (5-HT) system functioning have been proposed for a variety of psychiatric disorders, a direct method quantitatively assessing 5-HT release capacity in the living human brain is still lacking. Therefore, we evaluated a novel method to assess 5-HT release capacity in vivo using dexfenfluramine challenge and [(18)F]altanserin positron emission tomography (PET). Thirteen healthy male subjects received placebo and single oral doses of 40 mg (n = 6) or 60 mg (n = 7) of the potent 5-HT releaser dexfenfluramine separated by an interval of 14 days. Three further subjects received placebo on both days. Two hours after placebo/drug administration, 250 MBq of the 5-HT(2A) receptor selective PET-radiotracer [(18)F]altanserin was administered intravenously as a 30s bolus. Dynamic PET data were subsequently acquired over 90 min. Moreover, arterial blood samples were drawn for measurement of total activity and metabolite correction of the input function. Dexfenfluramine as well as cortisol and prolactin plasma concentration-time profiles was quantitatively determined. Tracer distribution volumes for five volumes-of-interest (prefrontal and occipital cortex, insula, thalamus, caudatum) were calculated by the Logan plot and a 2-tissue compartment model. Dexfenfluramine dose-dependently decreased the total distribution volume of [(18)F]altanserin in cortical regions independent of the PET modeling approach. Cortisol and prolactin plasma concentrations were dose-dependently increased by dexfenfluramine. The decrease in cortical [(18)F]altanserin receptor binding under dexfenfluramine was correlated with the increase of plasma prolactin. These data suggest that the combination of a dexfenfluramine-induced 5-HT release and subsequent assessment of 5-HT(2A) receptor availability with [(18)F]altanserin PET is suitable to measure cortical 5-HT release capacity in the human brain.

Validation and quantification of [18F]altanserin binding in the rat brain using blood input and reference tissue modeling.

The 5-hydroxytryptamine type 2a (5-HT(2A)) selective radiotracer [(18)F]altanserin has been subjected to a quantitative micro-positron emission tomography study in Lister Hooded rats. Metabolite-corrected plasma input modeling was compared with reference tissue modeling using the cerebellum as reference tissue. [(18)F]altanserin showed sufficient brain uptake in a distribution pattern consistent with the known distribution of 5-HT(2A) receptors. Full binding saturation and displacement was documented, and no significant uptake of radioactive metabolites was detected in the brain. Blood input as well as reference tissue models were equally appropriate to describe the radiotracer kinetics. [(18)F]altanserin is suitable for quantification of 5-HT(2A) receptor availability in rats.

Cognition-induced modulation of serotonin in the orbitofrontal cortex: a controlled cross-over PET study of a delayed match-to-sample task using the 5-HT2a receptor antagonist [18F]altanserin.

Behavioral and cellular studies indicate that serotonin interacting with the 5-HT2a receptor (5-HT2aR) is involved in cognitive processes supporting working memory (WM). However, 5-HT receptor neuroimaging studies directly relating WM-induced neuronal activations to concomitant changes in the availability of 5-HT receptors as a functional measure for serotonin release are lacking. This controlled cross-over PET study aimed to identify brain regions with WM-induced changes in the binding potential (BP(nd)) of the 5-HT2aR antagonist [(18)F]altanserin. Ten young males underwent a delayed match-to-sample task using photographs of faces and a control task. The BP(nd)s for both conditions were calculated by applying Ichise's noninvasive plot. Statistics were performed with the SPM toolbox statistical nonparametric mapping (SnPM3) particularly suited for analyzing whole-brain PET data in an exploratory way. A higher BP(nd) for [(18)F]altanserin during WM versus control was found in the orbitofrontal cortex (OFC) pointing towards an increased [(18)F]altanserin/5-HT2aR interaction in OFC while BP(nd) decreases during WM were not found. Furthermore, no BP(nd) changes in regions known from functional neuroimaging studies to be more specifically involved in WM were identified. These findings may suggest that the increased [(18)F]altanserin BP(nd) under WM challenge and hence the increased availability of 5-HT2aR reflects a decrease in local OFC serotonin. As the OFC plays a prominent role in decision-making and supports cognitive processes related to the central executive functions of WM it might be modulated by the serotoninergic system via the 5-HT2aR in order to support and optimize basic cognitive functions.