Test-retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

Neural Correlates of Early-Life Urbanization and Their Spatial Relationships with Gene Expression, Neurotransmitter, and Behavioral Domain Atlases

Previous neuroimaging research has established associations between urban exposure during early life and alterations in brain function and structure. However, the molecular mechanisms and behavioral relevance of these associations remain largely unknown. Here, we aimed to address this question using a combined analysis of multimodal data. Initially, we calculated amplitude of low-frequency fluctuations (ALFF) and gray matter volume (GMV) using resting-state functional and structural MRI to investigate their associations with early-life urbanization in a large sample of 511 healthy young adults. Then, we examined the spatial relationships of the identified neural correlates of early-life urbanization with gene expression, neurotransmitter, and behavioral domain atlases. Results showed that higher early-life urbanization scores were correlated with increased ALFF of the right fusiform gyrus and decreased GMV of the left dorsal medial prefrontal cortex and left precuneus. Remarkably, the identified neural correlates of early-life urbanization were spatially correlated with expression of gene categories primarily involving immune system process, signal transduction, and cellular metabolic process. Concurrently, there were significant associations between the neural correlates and specific neurotransmitter systems including dopamine, acetylcholine, and serotonin. Finally, we found that the ALFF correlates were associated with behavioral terms including "perception," "sensory," "cognitive control," and "reasoning." Apart from expanding existing knowledge of early-life urban environmental risk for mental disorders and health in general, our findings may contribute to an emerging framework for integrating social science, neuroscience, genetics, and public policy to respond to the major health challenge of world urbanization.

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 serotonin_2A 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 serotonin_2A receptors. This may be useful for investigating the elevation of endogenous serotonin levels caused by stimulations.

JuSpace: A tool for spatial correlation analyses of magnetic resonance imaging data with nuclear imaging derived neurotransmitter maps

Recent studies have shown that drug‐induced spatial alteration patterns in resting state functional activity as measured using magnetic resonance imaging (rsfMRI) are associated with the distribution of specific receptor systems targeted by respective compounds. Based on this approach, we introduce a toolbox (JuSpace) allowing for cross‐modal correlation of MRI‐based measures with nuclear imaging derived estimates covering various neurotransmitter systems including dopaminergic, serotonergic, noradrenergic, and GABAergic (gamma‐aminobutric acid) neurotransmission. We apply JuSpace to two datasets covering Parkinson's disease patients (PD) and risperidone‐induced changes in rsfMRI and cerebral blood flow (CBF). Consistently with the predominant neurodegeneration of dopaminergic and serotonergic system in PD, we find significant spatial associations between rsfMRI activity alterations in PD and dopaminergic (D2) and serotonergic systems (5‐HT1b). Risperidone induced CBF alterations were correlated with its main targets in serotonergic and dopaminergic systems. JuSpace provides a biologically meaningful framework for linking neuroimaging to underlying neurotransmitter information.

Quantification of 5-HT1A and 5-HT2A receptor Binding in Depressed Suicide Attempters and Non-Attempters

OBJECTIVE

To determine serotonin system abnormalities related to major depression or previous suicidal behavior.

METHODS

[¹¹C]WAY100635, [¹⁸F]altanserin and positron emission tomography were used to compare 5-HT_1A and 5-HT_2A binding in MDD patients divided into eight past suicide attempters (>4yrs prior to scanning) and eight lifetime non-attempters, and both groups were compared to eight healthy volunteers.

RESULTS

The two receptor types differed in binding pattern across brain regions from each other, but there were no differences in binding between healthy volunteers and the two depressed groups or between depressed suicide attempters and non-attempters. No effects of depression severity or lifetime aggression were observed for either receptor.

CONCLUSION

Limitations of this study include small sample size and absence of high lethality suicide attempts in the depressed attempter group. No trait-like binding correlations with past suicide attempt or current depression were observed. Given the heterogeneity of nonfatal suicidal behavior, a larger sample study emphasizing higher lethality suicide attempts may find the serotonin biological phenotype seen in suicide decedents.

Synthesis and Evaluation of Pyridyloxypyridyl Indole Carboxamides as Potential PET Imaging Agents for 5-HT2C Receptors

Nine pyridyloxypyridyl indole carboxamides were synthesized and displayed high affinities for 5-HT_2C receptors and high selectivity over 5-HT_2A and 5-HT_2B. Among them, 6-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]1H-indole-3-carboxamide (8) exhibits the highest 5-HT_2C binding affinity (K_i = 1.3 nM) and high selectivity over 5-HT_2A (∼1000 times) and 5-HT_2B (∼140 times). [¹¹C]8 was synthesized by palladium-catalyzed coupling reaction between pinacolboranate 16 and [¹¹C]CH₃I with an average radiochemical yield of 27 ± 4% (n = 8, decay-corrected from end of [¹¹C]CH₃I synthesis). MicroPET imaging studies in rhesus monkeys showed regional uptake of [¹¹C]8 in the choroid plexus, whereas the bindings in all other brain regions were low. The specific binding in the choroid plexus was confirmed by administration of a blocking dose of 0.1 mg/kg of the 5-HT_2C antagonist SB-242084.

Acute induction of anxiety in humans by delta-9-tetrahydrocannabinol related to amygdalar cannabinoid-1 (CB1) receptors

Use of Cannabis, the most widely used illicit drug worldwide, is associated with acute anxiety, and anxiety disorders following regular use. The precise neural and receptor basis of these effects have not been tested in man. Employing a combination of functional MRI (fMRI) and positron emission tomography (PET), we investigated whether the effects of delta-9-tetrahydrocannabinol (delta-9-THC), the main psychoactive ingredient of cannabis, on anxiety and on amygdala response while processing fearful stimuli were related to local availability of its main central molecular target, cannabinoid-1 (CB1) receptors in man. Fourteen healthy males were studied with fMRI twice, one month apart, following an oral dose of either delta-9-THC (10 mg) or placebo, while they performed a fear-processing task. Baseline availability of the CB1 receptor was studied using PET with [11C]MePPEP, a CB1 inverse agonist radioligand. Relative to the placebo condition, delta-9-THC induced anxiety and modulated right amygdala activation while processing fear. Both these effects were positively correlated with CB1 receptor availability in the right amygdala. These results suggest that the acute effects of cannabis on anxiety in males are mediated by the modulation of amygdalar function by delta-9-THC and the extent of these effects are related to local availability of CB1 receptors.

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.

Age, sex, and reproductive hormone effects on brain serotonin-1A and serotonin-2A receptor binding in a healthy population

There is a need for rigorous positron emission tomography (PET) and endocrine methods to address inconsistencies in the literature regarding age, sex, and reproductive hormone effects on central serotonin (5HT) 1A and 2A receptor binding potential (BP). Healthy subjects (n=71), aged 20-80 years, underwent 5HT1A and 2A receptor imaging using consecutive 90-min PET acquisitions with [(11)C]WAY100635 and [(18)F]altanserin. Logan graphical analysis was used to derive BP using atrophy-corrected distribution volume (V(T)) in prefrontal, mesiotemporal, occipital cortices, and raphe nucleus (5HT1A only). We used multivariate linear regression modeling to examine BP relationships with age, age(2), sex, and hormone concentrations, with post hoc regional significance set at p<0.008. There were small postsynaptic 5HT1A receptor BP increases with age and estradiol concentration in women (p=0.004-0.005) and a tendency for small 5HT1A receptor BP declines with age and free androgen index in men (p=0.05-0.06). Raphe 5HT1A receptor BP decreased 4.5% per decade of age (p=0.05), primarily in men. There was a trend for 15% receptor reductions in prefrontal cortical regions in women relative to men (post hoc p=0.03-0.10). The significant decline in 5HT2A receptor BP relative to age (8% per decade; p<0.001) was not related to sex or hormone concentrations. In conclusion, endocrine standardization minimized confounding introduced by endogenous hormonal fluctuations and reproductive stage and permitted us to detect small effects of sex, age, and endogenous sex steroid exposures upon 5HT1A binding. Reduced prefrontal cortical 5HT1A receptor BP in women vs men, but increased 5HT1A receptor BP with aging in women, may partially explain the increased susceptibility to affective disorders in women during their reproductive years that is mitigated in later life. 5HT1A receptor decreases with age in men might contribute to the known increased risk for suicide in men over age 75 years. Low hormone concentrations in adults <50 years of age may be associated with more extreme 5HT1A receptor BP values, but remains to be studied further. The 5HT2A receptor declines with age were not related to sex or hormone concentrations in this sample. Additional study in clinical populations is needed to further examine the affective role of sex-hormone-serotonin receptor relationships.

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.

Medial prefrontal cortex serotonin 1A and 2A receptor binding interacts to predict threat-related amygdala reactivity

BACKGROUND

The amygdala and medial prefrontal cortex (mPFC) comprise a key corticolimbic circuit that helps shape individual differences in sensitivity to threat and the related risk for psychopathology. Although serotonin (5-HT) is known to be a key modulator of this circuit, the specific receptors mediating this modulation are unclear. The colocalization of 5-HT1A and 5-HT2A receptors on mPFC glutamatergic neurons suggests that their functional interactions may mediate 5-HT effects on this circuit through top-down regulation of amygdala reactivity. Using a multimodal neuroimaging strategy in 39 healthy volunteers, we determined whether threat-related amygdala reactivity, assessed with blood oxygen level-dependent functional magnetic resonance imaging, was significantly predicted by the interaction between mPFC 5-HT1A and 5-HT2A receptor levels, assessed by positron emission tomography.

RESULTS

5-HT1A binding in the mPFC significantly moderated an inverse correlation between mPFC 5-HT2A binding and threat-related amygdala reactivity. Specifically, mPFC 5-HT2A binding was significantly inversely correlated with amygdala reactivity only when mPFC 5-HT1A binding was relatively low.

CONCLUSIONS

Our findings provide evidence that 5-HT1A and 5-HT2A receptors interact to shape serotonergic modulation of a functional circuit between the amygdala and mPFC. The effect of the interaction between mPFC 5-HT1A and 5-HT2A binding and amygdala reactivity is consistent with the colocalization of these receptors on glutamatergic neurons in the mPFC.

Positron emission tomography imaging of dopamine D₂/₃ receptors in the human cortex with [¹¹C]FLB 457: reproducibility studies

In a recent PET study, we demonstrated the ability to measure amphetamine-induced DA release in the human cortex with the relatively high affinity dopamine D₂/₃ radioligand [¹¹C]FLB 457 (Narendran et al., [2009] Synapse 63:447-461). The aim of this study was to evaluate the reproducibility and reliability of [¹¹C]FLB 457 in the same imaging paradigm we used to measure amphetamine-induced DA transmission. Six healthy human subjects (three males/three females)were studied twice with [¹¹C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D₂/₃ receptor binding parameters were estimated using a two-tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BP(P)), and binding potential relative to non-displaceable uptake (BP(ND)) of [¹¹C]FLB 457. The test-retest variability of [¹¹C]FLB 457 VT, BPP, and BP(ND) were ≤15%, consistent with the published test-retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215-1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666-1673). In addition, no significant decrease in [¹¹C]FLB457 BP(ND) was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [¹¹C]FLB 457 to the measured reduction in[¹¹C]FLB 457 BP(ND) following amphetamine was relatively low. These data support the further validation of [¹¹C]FLB 457 as a tool to measure amphetamine-induced dopamine release in the human cortex.

Gender, personality, and serotonin-2A receptor binding in healthy subjects

The vulnerability to mood disorders, impulsive-aggression, eating disorders, and suicidal behavior varies greatly with gender, and may reflect gender differences in central serotonergic function. We investigated the relationships of gender, mood, impulsivity, aggression and temperament to 5HT(2A) receptor binding in 21 healthy subjects using [18F]altanserin and PET neuroimaging. Binding potentials in pre-defined regions-of-interest (ROI) were calculated using the Logan graphical method, corrected for partial volume effects, and compared by gender with age co-varied. SPM analysis was used for voxel level comparisons. Altanserin binding (BP(P)) was greater in male than female subjects in the following nine ROIs: hippocampus (HIP) and Lt. HIP, lateral orbital frontal cortex (LOF) and Lt. LOF, left medial frontal cortex (Lt. MFC), left medial temporal cortex (Lt. MTC), left occipital cortex (Lt. OCC), thalamus (THL) and Lt. THL. Differences in Lt. HIP and Lt. MTL remained significant after Bonferroni correction. Gender differences were noted in the co-variation of psychological traits with BP(P) values in specific ROIs. Among males alone, aggression was negatively correlated with BP(P) values in Lt. LOF and Lt. MFC, and Suspiciousness positively correlated in LOF, Lt. LOF and Lt. MFC. Among female subjects alone, Negativism was positively correlated with BP(P) values in HIP, and Verbal Hostility in Lt. HIP. Altanserin binding in Lt. MTC was positively correlated with Persistence, with no significant gender effect. Gender differences in 5HT(2A) receptor function in specific ROIs may mediate expression of psychological characteristics such as aggression, suspiciousness and negativism. Future studies of 5HT(2A) receptor function and its relationship to behavior should control for gender.

Positron emission tomography imaging of D(2/3) agonist binding in healthy human subjects with the radiotracer [(11)C]-N-propyl-norapomorphine: preliminary evaluation and reproducibility studies

OBJECTIVE

(-)-N-[(11)C]-propyl-norapomorphine (NPA) is a full dopamine D(2/3) receptor agonist radiotracer suitable for imaging D(2/3) receptors configured in a state of high affinity for agonists using positron emission tomography. The aim of the present study was to define the optimal analytic method to derive accurate and reliable D(2/3) receptor parameters with [(11)C]NPA.

METHODS

Six healthy subjects (four females/two males) underwent two [(11)C]NPA scans in the same day. D(2/3) receptor-binding parameters were estimated using kinetic analysis (using one- and two-tissue compartment models) as well as simplified reference tissue method in the three functional subdivisions of the striatum (associative striatum, limbic striatum, and sensorimotor striatum). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (V(T)), binding potential relative to plasma concentration (BP(P)), and binding potential relative to nondisplaceable uptake (BP(ND)).

RESULTS

A two-tissue compartment kinetic model adequately described the functional subdivisions of the striatum as well as cerebellum time-activity data. The reproducibility of V(T) was excellent (<or=10%) in all regions, for this approach. The reproducibility of both BP(P) (<or=12%) and BP(ND) (<or=10%) was also excellent. The intraclass correlation coefficients of BP(P) and BP(ND) were acceptable as well (>0.75) in the three functional subdivisions of the striatum. Although SRTM led to an underestimation of BP(ND) values relative to that estimated by kinetic analysis by 8-13%, the values derived using both the methods were reasonably well correlated (r(2) = 0.89, n = 84). Both methods were similarly effective in detecting the differences in [(11)C]NPA BP(ND) between subjects.

CONCLUSION

The results of this study indicate that [(11)C]NPA can be used to measure D(2/3) receptors configured in a state of high affinity for the agonists with high reliability and reproducibility in the functional subdivisions of the human striatum.

Test-retest variability of high resolution positron emission tomography (PET) imaging of cortical serotonin (5HT2A) receptors in older, healthy adults

Background

Position emission tomography (PET) imaging using [¹⁸F]-setoperone to quantify cortical 5-HT_2A receptors has the potential to inform pharmacological treatments for geriatric depression and dementia. Prior reports indicate a significant normal aging effect on serotonin 5HT_2A receptor (5HT_2AR) binding potential. The purpose of this study was to assess the test-retest variability of [¹⁸F]-setoperone PET with a high resolution scanner (HRRT) for measuring 5HT_2AR availability in subjects greater than 60 years old. Methods: Six healthy subjects (age range = 65–78 years) completed two [¹⁸F]-setoperone PET scans on two separate occasions 5–16 weeks apart.

Results

The average difference in the binding potential (BP_ND) as measured on the two occasions in the frontal and temporal cortical regions ranged between 2 and 12%, with the lowest intraclass correlation coefficient in anterior cingulate regions.

Conclusion

We conclude that the test-retest variability of [¹⁸F]-setoperone PET in elderly subjects is comparable to that of [¹⁸F]-setoperone and other 5HT_2AR radiotracers in younger subject samples.

Preliminary in vivo and ex vivo evaluation of the 5-HT2A imaging probe [(18)F]MH.MZ

INTRODUCTION

The 5-HT(2A) receptor is one of the most interesting targets within the serotonergic system because it is involved in a number of important physiological processes and diseases.

METHODS

[(18)F]MH.MZ, a 5-HT(2A) antagonistic receptor ligand, is labeled by (18)F-fluoroalkylation of the corresponding desmethyl analogue MDL 105725 with 2-[(18)F]fluoroethyltosylate ([(18)F]FETos). In vitro binding experiments were performed to test selectivity toward a broad spectrum of neuroreceptors by radioligand binding assays. Moreover, first micro-positron emission tomography (microPET) experiments, ex vivo organ biodistribution, blood cell and protein binding and brain metabolism studies of [(18)F]MH.MZ were carried out in rats.

RESULTS

[(18)F]MH.MZ showed a K(i) of 3 nM toward the 5-HT(2A) receptor and no appreciable affinity for a variety of receptors and transporters. Ex vivo biodistribution as well as microPET showed highest brain uptake at approximately 5 min p.i. and steady state after approximately 30 min p.i. While [(18)F]MH.MZ undergoes extensive first-pass metabolism which significantly reduces its bioavailability, it is insignificantly metabolized within the brain. The binding potential in the rat frontal cortex is 1.45, whereas the cortex to cerebellum ratio was determined to be 2.7 after approximately 30 min.

CONCLUSION

Results from microPET measurements of [(18)F]MH.MZ are in no way inferior to data known for [(11)C]MDL 100907 at least in rats. [(18)F]MH.MZ appears to be a highly potent and selective serotonergic PET ligand in small animals.

Medial prefrontal cortex 5-HT(2A) density is correlated with amygdala reactivity, response habituation, and functional coupling

Feedback inhibition of the amygdala via medial prefrontal cortex (mPFC) is an important component in the regulation of complex emotional behaviors. The functional dynamics of this corticolimbic circuitry are, in part, modulated by serotonin (5-HT). Serotonin 2A (5-HT(2A)) receptors within the mPFC represent a potential molecular mechanism through which 5-HT can modulate this corticolimbic circuitry. We employed a multimodal neuroimaging strategy to explore the relationship between threat-related amygdala reactivity, assessed using blood oxygen level-dependent functional magnetic resonance imaging, and mPFC 5-HT(2A) density, assessed using [(18)F]altanserin positron emission tomography in 35 healthy adult volunteers. We observed a significant inverse relationship wherein greater mPFC 5-HT(2A) density was associated with reduced threat-related right amygdala reactivity. Remarkably, 25-37% of the variability in amygdala reactivity was explained by mPFC 5-HT(2A) density. We also observed a positive correlation between mPFC 5-HT(2A) density and the magnitude of right amygdala habituation. Furthermore, functional coupling between the amygdala and mPFC was positively correlated with 5-HT(2A) density suggesting that effective integration of emotionally salient information within this corticolimbic circuitry may be modulated, at least in part, by mPFC 5-HT(2A). Collectively, our results indicate that mPFC 5-HT(2A) is strongly associated with threat-related amygdala reactivity as well as its temporal habituation and functional coupling with prefrontal regulatory regions.

Synthesis and in vitro affinities of various MDL 100907 derivatives as potential 18F-radioligands for 5-HT2A receptor imaging with PET

Radiolabelled piperidine derivatives such as [(11)C]MDL 100907 and [(18)F]altanserin have played an important role in diagnosing malfunction in the serotonergic neurotransmission. A variety of novel piperidine MDL 100907 derivatives, possible to label with (18)F-fluorine, were synthesized to improve molecular imaging properties of [(11)C]MDL 100907. Their in vitro affinities to a broad spectrum of neuroreceptors and their lipophilicities were determined and compared to the clinically used reference compounds MDL 100907 and altanserin. The novel compounds MA-1 (53) and (R)-MH.MZ (56) show K(i)-values in the nanomolar range towards the 5-HT(2A) receptor and insignificant binding to other 5-HT receptor subtypes or receptors. Interestingly, compounds MA-1 (53), MH.MZ (55) and (R)-MH.MZ (56) provide a receptor selectivity profile similar to MDL 100907. These compounds could possibly be preferable antagonistic (18)F-tracers for visualization of the 5-HT(2A) receptor status. Medium affine compounds (VK-1 (32), (51), (52), (54)) were synthesized and have K(i) values between 30 and 120 nM. All promising compounds show logP values between 2 and 3, that is, within the range of those for the established radiotracers altanserin and MDL 100907. The novel compounds MA-1 (53) and (R)-MH.MZ (56) thus appear to be promising high affine and selective tracers of (18)F-labelled analogues for 5-HT(2A) imaging with PET.

Longitudinal assessment of cerebral 5-HT2A receptors in healthy elderly volunteers: an [18F]-altanserin PET study

PURPOSE

The serotonin 2A (5-HT(2A)) receptor is of interest in several psychiatric and neurological diseases. In the present study we investigated the longitudinal stability of 5-HT(2A) receptors and the stability of the quantification procedure in the elderly in order to be able to study elderly patients with neuropsychiatric diseases on a longitudinal basis.

METHODS

[(18)F]-Altanserin PET was used to quantify 5-HT(2A) receptors in 12 healthy elderly individuals at baseline and at 2 years in six volumes of interest. A bolus/infusion protocol was used to achieve the binding potential, BP(P). The reproducibility as assessed in terms of variability and the reliability as assessed in terms of intraclass correlation coefficient (ICC) were used to compare inter- and intraobserver stability and to evaluate the effects of increasing complexity of partial volume (PV) corrections. We also compared the stability of our measurements over 2 years with the stability of data from an earlier study with 2-week test-retest measurements.

RESULTS

BP(P) was unaltered at follow-up without the use of PV correction and when applying two-tissue PV correction, test-retest reproducibility was 12-15% and reliability 0.45-0.67 in the large bilateral regions such as the parietal, temporal, occipital and frontal cortices, while orbitofrontal and anterior cingulate cortical regions were less stable. The use of PV correction decreased the variability but also decreased the between-subject variation, thereby worsening the reliability.

CONCLUSION

In healthy elderly individuals, brain 5-HT(2A) receptor binding remains stable over 2 years, and acceptable reproducibility and reliability in larger regions and high intra- and interobserver stability allow the use of [(18)F]-altanserin in longitudinal studies of patients with neuropsychiatric disorders.

5HT2A receptor binding is increased in borderline personality disorder

BACKGROUND

Postmortem studies in suicide victims demonstrate an increase in the number of post-synaptic 5-HT(2A) receptor binding sites in ventral lateral and orbital frontal cortex. Diminished metabolic responses to serotonergic activation are noted in these areas in impulsive subjects with borderline personality disorder (BPD), a group at high risk for suicidal behaviors. We examined 5HT(2A) receptor binding potential (BP) in impulsive subjects with BPD, with positron emission tomography neuroimaging with [(18)F] altanserin.

METHODS

Fourteen female subjects with BPD were assessed for Axis I comorbidity, depressed mood, impulsivity, aggression, suicidality, childhood abuse, and compared with 11 healthy female control subjects. The 5HT(2A) receptor binding was evaluated in prefrontal cortex, anterior cingulate, hippocampus, temporal lobe, occipital cortex, and thalamus. Data were analyzed with Logan graphical analysis and a four-compartment (4C) model.

RESULTS

Hippocampal 5HT(2A) receptor binding was significantly increased in BPD subjects compared with control subjects in both Logan and 4C analyses, covarying for age. Hippocampal BP values were related to comorbid major depressive episode, with highest values found in non-depressed BPD subjects and lowest in healthy control subjects. The BP values were not related to depressed mood, impulsivity, aggression, suicidality, or childhood abuse.

CONCLUSIONS

5HT(2A) receptor binding is increased in the hippocampus of BPD subjects independent of depressed mood, impulsivity, aggression, suicidality, or childhood abuse. Dysregulation of serotonergic function in hippocampus might contribute to affective and behavioral symptoms in BPD.

PET imaging of neurokinin-1 receptors with [(18)F]SPA-RQ in human subjects: assessment of reference tissue models and their test-retest reproducibility

[(18)F]SPA-RQ (substance P antagonist receptor quantifier) labels the substance P-preferring (NK(1)) receptor in human brain. A prior study showed that [(18)F]SPA-RQ brain uptake can be quantified with a reference tissue method and thereby avoid invasive blood sampling. The purposes of this study were to compare three different reference tissue methods and to assess test-retest reproducibility. Eight healthy subjects underwent two [(18)F]SPA-RQ scans. We calculated the binding potential (BP), which is proportional to receptor density, from both regional volume of interest and voxel-wise data. We compared three reference tissue methods: simplified reference tissue model, multilinear reference tissue model (MRTM), and its two-parameter version (MRTM2). The three methods generated equivalent values of regional BP, but MRTM2 was the most resistant to noise. Temporally stable values of BP were obtained with 240 min of imaging data. MRTM2 had excellent test-retest reproducibility, with high reliability (intraclass correlation > 0.9) and low variability (< 10%). In addition to regional volume of interest analysis, we also created parametric images of BP, variability, and reliability based on voxel-wise time-activity data. The reproducibility of parametric BP was also good, with variability < 20% and reliability > 0.7 in gray matter regions. In conclusion, a two-parameter reference tissue method (MRTM2) provided reproducible and reliable measurements of [(18)F]SPA-RQ brain uptake using 240 min of both regional and voxel-wise data.

Test–retest stability of cerebral A1 adenosine receptor quantification using [18F]CPFPX and PET

PURPOSE

The goal of the present study was to evaluate the reproducibility of cerebral A1 adenosine receptor (A1AR) quantification using [18F]CPFPX and PET in a test-retest design.

METHODS

Eleven healthy volunteers were studied twice. Eight brain regions ranging from high to low receptor binding were examined. [18F]CPFPX was injected as a bolus with subsequent infusion over 120 min. Various outcome parameters were compared based on either metabolite-corrected venous blood sampling [e.g. apparent equilibrium total distribution volume (DVt')] or a reference region [ratio of specific to non-specific distribution volume (BP2)].

RESULTS

Test-retest variability was low in the outcome measure BP2 (on average 5.9%) and moderate in DVt' (on average 13.2%). Regarding reproducibility, the outcome parameter BP2 showed an intra-class correlation coefficient (ICC) of 0.94 +/- 0.1. For DVt' the between-subject coefficient of variation (%CV) was similar to the within-subject %CV (around 10%), resulting in a poor ICC of 0.06 +/- 0.2.

CONCLUSION

Our results suggest that quantification of [18F]CPFPX imaging is reproducible and reliable for PET studies of the cerebral A1AR. Among the outcome parameters the non-invasive measures were of superior test-retest stability over the invasive.

Reproducibility of 5-HT2A receptor measurements and sample size estimations with [18F]altanserin PET using a bolus/infusion approach

PURPOSE

To determine the reproducibility of measurements of brain 5-HT2A receptors with an [18F]altanserin PET bolus/infusion approach. Further, to estimate the sample size needed to detect regional differences between two groups and, finally, to evaluate how partial volume correction affects reproducibility and the required sample size.

METHODS

For assessment of the variability, six subjects were investigated with [18F]altanserin PET twice, at an interval of less than 2 weeks. The sample size required to detect a 20% difference was estimated from [18F]altanserin PET studies in 84 healthy subjects. Regions of interest were automatically delineated on co-registered MR and PET images.

RESULTS

In cortical brain regions with a high density of 5-HT2A receptors, the outcome parameter (binding potential, BP1) showed high reproducibility, with a median difference between the two group measurements of 6% (range 5-12%), whereas in regions with a low receptor density, BP1 reproducibility was lower, with a median difference of 17% (range 11-39%). Partial volume correction reduced the variability in the sample considerably. The sample size required to detect a 20% difference in brain regions with high receptor density is approximately 27, whereas for low receptor binding regions the required sample size is substantially higher.

CONCLUSION

This study demonstrates that [18F]altanserin PET with a bolus/infusion design has very low variability, particularly in larger brain regions with high 5-HT2A receptor density. Moreover, partial volume correction considerably reduces the sample size required to detect regional changes between groups.

Action by and sensitivity to neuroactive steroids in menstrual cycle related CNS disorders

Neuroactive steroids are a large group of substances having effect in the brain and on brain function. The steroids most studied are allopregnanolone (ALLO), tetrahydrodesoxycorticosterone (THDOC), pregnenolone sulfate (PS) dihydroepiandrosteronesulfate (DHEAS), and estradiol (E2). ALLO and THDOC are called gamma-aminobutyric acid (GABA) steroids as they are positive modulators of the GABAA receptor in a similar way as benzodiazepines, barbiturates, and alcohol. GABA steroids not only have similar behavioral effects as benzodiazepines and barbiturates but, possibly, also similar adverse effects as well. This review aims to elucidate the possible role that neuroactive steroids play in the development of mood disorders in women. One of the most clear-cut examples of the interaction between mood, neuroactive steroids, and the GABA system is premenstrual dysphoric disorder (PMDD), which is a cluster of negative mood symptoms occurring during the luteal phase of the menstrual cycle in 2-6% of reproductive women. Furthermore, certain women also experience adverse mood effects during sequential progestin addition to postmenopausal estrogen treatment, which is why the role of neuroactive steroids in postmenopausal women is also addressed in this review.

Synthesis and in vivo evaluation of [O-methyl-11C](2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine as a 5-HT2A receptor PET ligand

The serotonin2A (5-HT2A) receptor is implicated in the pathophysiology of schizophrenia and mood disorders, and in vivo studies of this receptor would be of value in studying the pathophysiology of these disorders and in measuring the relationship of clinical response to receptor occupancy for 5-HT2A antagonists such as atypical antipsychotics. Therefore, (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)-phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (MPM) (13), a selective and high-affinity (K(i)=0.79 nM) 5HT2A antagonist, has been radiolabeled with carbon-11 by O-methylation of the corresponding desmethyl analogue (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-hydroxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (12) with [11C]methyltriflate in order to determine the suitability of [11C]MPM to quantify 5-HT2A in living brain using PET. Desmethyl-MPM 12 and standard MPM were prepared, starting from 3-hydroxymethylphenol (2), in excellent yield. The yield obtained for radiolabeling was 40+/-5% (EOB), and the total synthesis time was 30 min at EOS. PET studies with [11C]MPM in baboon showed a distribution in the brain consistent with the known distribution of 5-HT2A receptors. The time-activity curves for the high-binding regions peaked at approximately 45 min after injection. Blocking studies with M100907 demonstrated not only 38-57% blocking of tracer binding in brain regions known to have 5-HT2A receptors but also 38% blocking in cerebellum, which has a low 5-HT2A receptor concentration. Although [11C]MPM exhibits appropriate kinetics in baboon for imaging 5-HT2A receptors, its specific binding in cerebellum and higher proportion of nonspecific binding limit its usefulness for the in vivo quantification of 5-HT2A receptors with PET.

Kinetic modeling of amyloid binding in humans using PET imaging and Pittsburgh Compound-B

A valid quantitative imaging method for the measurement of amyloid deposition in humans could improve Alzheimer's disease (AD) diagnosis and antiamyloid therapy assessment. Our group developed Pittsburgh Compound-B (PIB), an amyloid-binding radiotracer, for positron emission tomography (PET). The current study was aimed to further validate PIB PET through quantitative imaging (arterial input) and inclusion of subjects with mild cognitive impairment (MCI). Pittsburgh Compound-B studies were performed in five AD, five MCI, and five control subjects and five subjects were retested within 20 days. Magnetic resonance images were acquired for partial volume correction and region-of-interest definition (e.g., posterior cingulate: PCG; cerebellum: CER). Data were analyzed using compartmental and graphical approaches. Regional distribution volume (DV) values were normalized to the reference region (CER) to yield DV ratios (DVRs). Good agreement was observed between compartmental and Logan DVR values (e.g., PCG: r=0.89, slope=0.91); the Logan results were less variable. Nonspecific PIB retention was similar across subjects (n=15, Logan CER DV: 3.63+/-0.48). Greater retention was observed in AD cortical areas, relative to controls (P<0.05). The PIB retention in MCI subjects appeared either 'AD-like' or 'control-like'. The mean test/retest variation was approximately 6% in primary areas-of-interest. The Logan analysis was the method-of-choice for the PIB PET data as it proved stable, valid, and promising for future larger studies and voxel-based statistical analyses. This study also showed that it is feasible to perform quantitative PIB PET imaging studies that are needed to validate simpler methods for routine use across the AD disease spectrum.

Decreased hippocampal 5-HT(2A) receptor binding in older depressed patients using [18F]altanserin positron emission tomography

Serotonin receptor changes have been associated with the pathophysiology and treatment of major depression. Only one other study has investigated serotonin receptor changes in older depressed patients. We used positron emission tomography (PET) and [18F]altanserin, a ligand with high affinity for the 5-HT(2A) receptor, to examine the relationship between 5-HT(2A) receptor density and depression. Depressed subjects (n = 16), age > 50 years, were recruited as part of a larger study. Older depressed subjects consisted of early-onset recurrent depression (EORD, n = 11) and late-onset depression (LOD, n = 5). An age-matched control group (n = 9) was also recruited. All subjects were right-handed, nonsmokers and antidepressant-free. Regions of interest were determined on a summed MPRAGE scan transformed into Talairach space and coregistered with the PET images. Depressed subjects had less hippocampal 5-HT(2A) receptor binding than controls (p = 0.05). No significant differences in receptor binding were found between EORD and LOD subjects. Depressed subjects not previously treated for depression (n = 6) had less hippocampal 5-HT(2A) receptor binding (p = 0.04) than previously treated subjects (n = 10). It may be that prior medication treatment provides a compensatory upregulation of the 5-HT(2A) receptor.

[18F]altanserin binding to human 5HT2A receptors is unaltered after citalopram and pindolol challenge

The aim of the present study was to develop an experimental paradigm for the study of serotonergic neurotransmission in humans using positron emission tomography and the 5-HT2A selective radioligand [18F]altanserin. [18F]altanserin studies were conducted in seven subjects using the bolus/infusion approach designed for attaining steady state in blood and brain 2 hours after the initial [18F]altanserin administration. Three hours after commencement of radiotracer administration, 0.25 mg/kg of the selective serotonin reuptake inhibitor, citalopram (Lundbeck, Valby, Denmark), was administered to all subjects as a constant infusion for 20 minutes. To reduce 5-HT1A-mediated autoinhibition of cortical 5-HT release, four of the seven subjects were pretreated with the partial 5-HT1A agonist pindolol for 3 days at an increasing oral dose (25 mg on the day of scanning). In each subject, the baseline condition (120 to 180 minutes) was compared with the stimulated condition (195 to 300 minutes). Despite a pronounced increase in plasma prolactin and two subjects reporting hot flushes compatible with an 5-HT-induced adverse effect, cortical [18F]altanserin binding was insensitive to the citalopram challenge, even after pindolol pretreatment. The biochemical and cellular events possibly affecting the unsuccessful translation of the citalopram/pindolol challenge into a change in 5-HT2A receptor binding of [18F]altanserin are discussed.

A database of [18F]-altanserin binding to 5-HT2A receptors in normal volunteers: normative data and relationship to physiological and demographic variables

This study presents the results of an analysis of 5-hydroxytryptamine (5-HT)(2A) receptors in 52 healthy subjects. Thirty men and twenty-two women aged between 21 and 79 years were investigated with magnetic resonance imaging (MRI) and [(18)F]-altanserin positron emission tomography (PET). The distribution volumes of specific tracer binding (DV(3)') was calculated for 15 brain regions using either cerebellum or pons as reference regions and correlations between DV(3)' and physiological and demographic variables were made. The regional distribution of [(18)F]-altanserin binding in the healthy human brain was in agreement with existing in vitro post-mortem human 5-HT(2A) data. Apart from nonspecific cerebellar binding (DV(2)), there was no gender difference in 5-HT(2A) binding. A positive correlation between cerebellar binding and age was observed and negative correlations between age and DV(3)' were found in all cortical regions, except occipital cortex, corresponding to a decrease in DV(3)' of 6% or 4% per decade with cerebellum or pons as reference regions, respectively. In several temporal and frontal cortical regions, positive correlations were found between body mass index (BMI) and DV(3)'. Our findings provide a resource to aid design of clinical studies of the 5-HT(2A) receptors. [(18)F]-altanserin binding appears to be unaffected by gender, but the effects of ageing must be considered for clinical studies. The correlations between different cortical regions' 5-HT(2A) binding and BMI should be explored in future studies.

Cluster analysis in kinetic modelling of the brain: a noninvasive alternative to arterial sampling

In emission tomography, quantification of brain tracer uptake, metabolism or binding requires knowledge of the cerebral input function. Traditionally, this is achieved with arterial blood sampling. We propose a noninvasive alternative via the use of a blood vessel time-activity curve (TAC) extracted directly from dynamic positron emission tomography (PET) scans by cluster analysis. Five healthy subjects were injected with the 5HT(2A)-receptor ligand [(18)F]-altanserin and blood samples were subsequently taken from the radial artery and cubital vein. Eight regions-of-interest (ROI) TACs were extracted from the PET data set. Hierarchical K-means cluster analysis was performed on the PET time series to extract a cerebral vasculature ROI. The number of clusters was varied from K = 1 to 10 for the second of the two-stage method. Determination of the correct number of clusters was performed by the 'within-variance' measure and by 3D visual inspection of the homogeneity of the determined clusters. The cluster-determined input curve was then used in Logan plot analysis and compared with the arterial and venous blood samples, and additionally with one of the currently used alternatives to arterial blood sampling, the Simplified Reference Tissue Model (SRTM) and Logan analysis with cerebellar TAC as an input. There was a good agreement (P < 0.05) between the values of Distribution Volume (DV) obtained from the K-means-clustered input function and those from the arterial blood samples. This work acts as a proof-of-principle that the use of cluster analysis on a PET data set could obviate the requirement for arterial cannulation when determining the input function for kinetic modelling of ligand binding, and that this may be a superior approach as compared to the other noninvasive alternatives.

Decreased hippocampal 5-HT2A receptor binding in major depressive disorder: in vivo measurement with [18F]altanserin positron emission tomography

BACKGROUND

Serotonin 5-HT(2A) receptors play an important role in the regulation of many functions that are disturbed in patients with major depressive disorder. Postmortem and positron emission tomography studies have reported both increased and decreased 5-HT(2A) receptor binding in different limbic and paralimbic regions.

METHODS

We conducted a quantitative 5-HT(2A) receptor binding study using positron emission tomography and [(18)F]altanserin of four regions hypothesized to have altered levels of 5-HT(2A) receptors in major depressive disorder. Using a four-compartment model, the 5-HT(2A) receptor distribution was estimated by calculating the regional [(18)F]altanserin k(3)/k(4) ratio in which k(3) is the rate of binding to the receptor and k(4) is the rate of dissociation from the receptor. Forty-six antidepressant-free patients with major depressive disorder and 29 healthy control subjects were enrolled.

RESULTS

5-HT(2A) receptor binding in the hippocampus was reduced by 29% in depressed subjects (p =.004). In other regions, 5-HT(2A) receptor binding was decreased (averaging 15%) but not significantly. Both groups had similar age-dependent decreases in 5-HT(2A) receptors throughout all brain regions.

CONCLUSIONS

Altered serotoninergic function in the hippocampus is likely involved in the disturbances of mood regulation in major depressive disorder, although the specific role of the 5-HT(2A) receptor changes is still unclear.

Widespread increases of cortical serotonin type 2A receptor availability after hormone therapy in euthymic postmenopausal women

OBJECTIVE

To use statistical parametric mapping to determine the extent of previously reported serotonin type 2A (5HT(2A)) receptor binding potential (BP) increases in postmenopausal women following hormone therapy.

DESIGN

Repeated measures positron emission tomography (PET) study.

SETTING

Academic research environment.

PATIENT(S)

Five healthy, postmenopausal women.

INTERVENTION(S)

Serial PET images of [(18)F]altanserin uptake were acquired to measure 5HT(2A) receptor BP at menopausal baseline, following estradiol (E(2)), and following combined E(2) + micronized progesterone (P(4)).

MAIN OUTCOME MEASURE(S)

5HT(2A) receptor BP.

RESULT(S)

Combined E(2) + P(4) treatment was associated with significant increases in the 5HT(2A) receptor BP increases in widespread areas of cerebral cortex. Treatment with E(2) alone was also associated with widespread cortical BP increases, although these changes reached statistical significance in fewer regions. The rate of [(18)F]altanserin metabolism was significantly decreased in the E(2) + P(4) condition relative to menopausal baseline, but this difference did not appear to correlate with changes in 5HT(2A) receptor BP.

CONCLUSION(S)

Estradiol priming followed by combined E(2) + P(4) is associated with widespread increases in 5HT(2A) receptor BP in the cerebral cortex, consistent with the E(2)-associated increases in 5HT(2A) receptor density previously observed in experimental animals.

Quantification of 5-HT2A receptors in the human brain using [18F]altanserin-PET and the bolus/infusion approach

The aim of the present study is to describe and validate a method for accurate quantification of 5-hydroxytryptamine (5-HT)(2A) receptors using [18F]altanserin-positron emission tomography (PET) and the bolus/infusion approach. A bolus/infusion ratio of 1.75 h aimed at attaining rapid steady state in blood and brain was predicted from previous bolus studies performed in our laboratory. The infusion schedule was tested in normal subjects (n = 10) using dynamic PET and frequent plasma sampling for 6 h. Steady state was attained in brain and plasma within 2 h, and time-activity curves remained constant for another 3 h. To represent free and nonspecifically bound [18F]altanserin and its radiolabeled metabolites only, cerebellum must show no displacement in 5-HT(2A) displacement studies. To validate this, saturating doses of cold ketanserin were administered and it was found that specific binding of [18F]altanserin decreased uniformly to the level of the cerebellum and no change in the cerebellar time-activity curve was found after ketanserin administration. A shorter experimental setup was tested in a second group (n = 20) including patients with neuropsychiatric disorders. Dynamic PET (five frames of 8 minutes each) and venous blood sampling at midscan time started 2 h after [18F]altanserin administration. The mean percentage rate of change per hour in the outcome parameter, DV(3)', was low (mean -0.3% h-1; range -7.3-7.2% h-1) and no correlation of DV(3)' versus time was demonstrated. It is concluded that 5-HT(2A) receptor studies can be conducted within 2 h of [18F]altanserin infusion, yielding reliable results.

Imaging methods for evaluating brain function in man

Imaging of brain function and neurotransmission is an important bridge between basic and clinical research. Regional cerebral energy metabolism and blood flow are normally coupled to regional cerebral function. Positron tomography (PET) studies of cerebral glucose metabolism and blood flow, single photon tomography (SPECT) and MRI studies of cerebral perfusion, have been used to image cerebral development and aging in man. The sensitivity, temporal resolution, spatial resolution and lack of radiation have led to the widespread utilization of blood oxygen level dependent (BOLD) and MRI perfusion techniques. PET and SPECT methods for studying cerebral neurotransmission include studies of dopaminergic, serotonergic, cholinergic, opiate and GABAergic neurotransmission in man. Studies of cerebral neurotransmission in man have helped to delineate the mechanisms of action of antipsychotic and antidepressant drugs, the diagnosis and progression of Parkinson's disease, and to evaluate neuroprotective drugs. The strengths, limitations, and application of these modalities are reviewed. The application of these methods to cerebral development and aging are briefly discussed.

Influence of synaptic serotonin level on [18F]altanserin binding to 5HT2 receptors in man

The feasibility of in vivo serotonin 5HT(2) receptor binding measurement using [18F]altanserin as a radioligand has been well established. In this study, the postsynaptic receptor binding potential of this ligand was examined as a possible indicator of synaptic serotonin content after pharmacological challenge. Studies were performed in 11 subjects with a history of recurrent major depression. Six of them received serotonergic antidepressive treatment at the time of the experiment, the other five patients were untreated. Two PET measurements were carried out in each subject within 2 or 3 days. Before one of the measurements, 25 mg of the serotonin re-uptake inhibitor clomipramine were given intravenously, the other measurement was done without pharmacological challenge. The data were analyzed using non-linear least-square regression and Logan's graphical method. In the whole group of subjects, binding potential and distribution volume of altanserin decreased following clomipramine challenge. The decrease was between 14 (P=0.03) and 23% (P=0.004). This effect was mainly seen in subjects not on antidepressive medication. Clomipramine challenge probably increased the synaptic serotonin level, which competed with altanserin leading to the lowered binding potential. The paradigm might, thus, be useful to estimate serotonin release in vivo. Pretreatment with serotonergic antidepressants reduces the effect of clomipramine.

Reduced 5-HT2A receptor binding after recovery from anorexia nervosa

BACKGROUND

Several lines of evidence suggest that a disturbance of serotonin neuronal pathways may contribute to the pathogenesis of anorexia nervosa (AN). This study applied positron emission tomography (PET) to investigate the brain serotonin 2A (5HT2A) receptor, which could contribute to disturbances of appetite and behavior in AN.

METHODS

To avoid the confounding effects of malnutrition, we studied 16 women recovered from AN (REC AN, >1 year normal weight, regular menstrual cycles, no bingeing or purging) compared with 23 healthy control women (CW) using [18F]altanserin, a specific 5-HT2A receptor antagonist on PET imaging.

RESULTS

REC AN women had significantly reduced [18F]altanserin binding relative to CW in mesial temporal (amygdala and hippocampus), as well as cingulate cortical regions. In a subset of subjects (11 CW and 16 REC AN), statistical parametric mapping (SPM) confirmed reduced mesial temporal cortex 5HT2A receptor binding and, in addition, showed reduced occipital and parietal cortex binding.

CONCLUSIONS

This study extends research suggesting that altered 5-HT neuronal system activity persists after recovery from AN and may be related to disturbances of mesial temporal lobe function. Altered 5-HT neurotransmission after recovery also supports the possibility that this may be a trait-related disturbance that contributes to the pathophysiology of AN.

PET in psychopharmacology

Emission tomography techniques and, in particular, positron emission tomography (PET) enable the in vivo study of several physiological and neurochemical variables in human subjects using methods originally developed for quantitative autoradiography. In particular, PET allows one to evaluate in human subjects: (a) the effect of specific neurochemical challenges on regional brain function at rest or under activation; (b) the activity of neurotransmitters and the regional expression of specific molecular targets during pathology including their modulation by drug treatment; (c) the kinetics of drug disposition and activity directly in the target organ. This is of primary interest in the field of biological psychiatry and in psychoactive drugs development, where it is particularly difficult to reproduce human diseases using animal models in view of the peculiarity of this field and the large heterogeneity of each psychiatric illness also inside the same clinical definition. The aim of this paper is to review the principal strategies and the main results of the use of PET in psychopharmacology.

Analyses of [(18)F] altanserin bolus injection PET data. I: consideration of radiolabeled metabolites in baboons

Positron emission tomography (PET) has been used to study serotonin 2A (5-HT(2A)) receptor binding in human brain using the 5-HT(2A) antagonist, [(18)F]altanserin. Previous analyses of bolus injection [(18)F]altanserin data provided 5-HT(2A) specific binding measures that were highly correlated with the in vitro distribution of 5-HT(2A) receptors and reflected decreased binding after ketanserin (5-HT(2A) antagonist) administration. These observations were made in the presence of a nonspecific tissue component that was consistent with blood-brain barrier (BBB) passage of radiolabeled metabolites (radiometabolites). In this work, we evaluated the in vivo kinetics of [(18)F]altanserin and two major radiometabolites of [(18)F]altanserin, focusing on the kinetics of free and nonspecifically-bound radioactivity. PET studies were performed in baboons after the bolus injection of [(18)F]altanserin or one of its major radiometabolites, either [(18)F]altanserinol or [(18)F]4-(4-fluorobenzoyl)piperidine, at baseline and after pharmacologic receptor blockade (blocking data). The cerebellar and blocking data were analyzed using either single (parent radiotracer) or dual (parent radiotracer and radiometabolites) input function methods. After bolus injection of either [(18)F]altanserin metabolite, radioactivity crossed the BBB and localized nonspecifically. The radio- metabolites were found to contribute to nonspecific "background" radioactivity that was similar in receptor-poor and receptor-rich regions. After bolus injection in baboons, two of the major radiometabolites of [(18)F]altanserin crossed the BBB and contributed to a fairly uniform background of nonspecific radioactivity. This uniformity suggests that conventional analyses are appropriate for human bolus injection [(18)F]altanserin PET data, although these methods may overestimate [(18)F]altanserin nonspecific binding.

Reproducibility of in vivo brain measures of 5-HT2A receptors with PET and

The test/retest reproducibility of brain measures of 5-HT2A receptors with positron emission tomography (PET) and [18F]deuteroaltanserin was examined in a group of eight healthy human subjects. PET measures of 5-HT2A receptors were obtained under an equilibrium paradigm, with a 40-min PET acquisition starting approximately at 300 min (308+/-11 min) after bolus plus constant infusion of the radiotracer. Three brain outcome measures were obtained at equilibrium, V(3) (ratio of specific brain uptake to free parent plasma concentration of radiotracer), V(3)' (ratio of specific brain uptake to total parent plasma concentration) and RT (ratio of specific to non-displaceable brain uptakes). V(3)' and RT had high test/retest reproducibility, as measured by mean intra-subject% change for cortical brain areas of 14.1 and 11.0%, respectively. They also had high reliability, as measured by mean intra-class correlation coefficients (ICC) for cortical brain areas of 0.86 and 0.88, respectively. V(3) had low test/retest reproducibility, due to high variability in the measures of free parent tracer in plasma. This study supports the feasibility of equilibrium imaging of 5-HT2A receptors with PET and [18F]deuteroaltanserin. The equilibrium imaging method with [18F]deuteroaltanserin allows a single acquisition and blood measurement to provide an image whose pixel values equal a receptor volume of distribution. Since the single image pixel values are proportional to receptor densities, the images can be used in pixel-by-pixel statistical methods, such as SPM, to assess the distribution and density of 5-HT2A receptors in neuropsychiatric disorders.

Comparison of [(18)F]altanserin and [(18)F]deuteroaltanserin for PET imaging of serotonin(2A) receptors in baboon brain: pharmacological studies

The regional distribution in brain, distribution volumes, and pharmacological specificity of the PET 5-HT(2A) receptor radiotracer [(18)F]deuteroaltanserin were evaluated and compared to those of its non-deuterated derivative [(18)F]altanserin. Both radiotracers were administered to baboons by bolus plus constant infusion and PET images were acquired up to 8 h. The time-activity curves for both tracers stabilized between 4 and 6 h. The ratio of total and free parent to metabolites was not significantly different between radiotracers; nevertheless, total cortical R(T) (equilibrium ratio of specific to nondisplaceable brain uptake) was significantly higher (34-78%) for [(18)F]deuteroaltanserin than for [(18)F]altanserin. In contrast, the binding potential (Bmax/K(D)) was similar between radiotracers. [(18)F]Deuteroaltanserin cortical activity was displaced by the 5-HT(2A) receptor antagonist SR 46349B but was not altered by changes in endogenous 5-HT induced by fenfluramine. These findings suggest that [(18)F]deuteroaltanserin is essentially equivalent to [(18)F]altanserin for 5-HT(2A) receptor imaging in the baboon.

Amphetamine-induced dopamine release in human ventral striatum correlates with euphoria

BACKGROUND

Studies in experimental animals have implicated the mesolimbic dopaminergic projections into the ventral striatum in the neural processes underlying behavioral reinforcement and motivated behavior; however, understanding the relationship between subjective emotional experience and ventral striatal dopamine (DA) release has awaited human studies. Using positron emission tomography (PET), we correlated the change in endogenous dopamine concentrations following dextroamphetamine (AMPH) administration with the associated hedonic response in human subjects and compared the strength of this correlation across striatal subregions.

METHODS

We obtained PET measures of [(11)C]raclopride specific binding to DA D2/D3 receptors before and after AMPH injection (0.3 mg/kg IV) in seven healthy subjects. The change in [(11)C]raclopride binding potential (DeltaBP) induced by AMPH pretreatment and the correlation between DeltaBP and the euphoric response to AMPH were compared between the anteroventral striatum (AVS; comprised of accumbens area, ventromedial caudate, and anteroventral putamen) and the dorsal caudate (DCA) using an MRI-based region of interest analysis of the PET data.

RESULTS

The mean DeltaBP was greater in the AVS than in the DCA (p <.05). The AMPH-induced changes in euphoria analog scale scores correlated inversely with DeltaBP in the AVS (r = -.95; p <.001), but not in the DCA (r =.30, ns). Post hoc assessments showed that changes in tension-anxiety ratings correlated positively with DeltaBP in the AVS (r =.80; p [uncorrected] <.05) and that similar relationships may exist between DeltaBP and emotion ratings in the ventral putamen (as were found in the AVS).

CONCLUSIONS

The preferential sensitivity of the ventral striatum to the DA releasing effects of AMPH previously demonstrated in experimental animals extends to humans. The magnitude of ventral striatal DA release correlates positively with the hedonic response to AMPH.

Analyses of [18F]altanserin bolus injection PET data. II: Consideration of radiolabeled metabolites in humans

Imaging serotonin-2A (5-HT(2A)) neuroreceptors with positron emission tomography (PET) and [(18)F]altanserin has been the focus of a series of PET studies, as [(18)F]altanserin is one of the most selective 5-HT(2A) antagonist radiotracers. Previous animal studies showed that radiolabeled metabolites (radiometabolites) of [(18)F]altanserin crossed the blood-brain barrier (BBB) to localize nonspecifically in brain, consistent with a constant radioactivity "background." In this work, we evaluated human bolus injection [(18)F]altanserin PET data with detailed consideration of the impact of BBB-permeable metabolites on the specific binding parameters. Data were quantified using either single (parent radiotracer), dual (parent radiotracer and radiometabolites), or no arterial input function(s) (cerebellum as reference tissue input function). A step-gradient high-performance liquid chromatography (HPLC) analysis provided distinct separation of [(18)F]altanserin and four radiolabeled components in plasma. After [(18)F]altanserin injection, the step-gradient data showed that the major BBB-permeable radiometabolites approached constant levels in plasma (>50 min), consistent with a constant metabolite "background." The single-input Logan graphical results were highly correlated with the dual-input results and its bias was fairly constant across regions and subjects, as similarly observed for a nongraphical reference tissue method. The most comprehensive and quantitatively valid analysis for bolus [(18)F]altanserin PET data was the dual-input method that specifically accounted for BBB-permeable metabolites, although the Logan analysis was preferred because it provided a good compromise between validity, sensitivity, and reliability of implementation. Further study is needed to better understand how the cerebellar kinetics of [(18)F]altanserin and its radiometabolites impact the reference tissue measures.

Equilibrium modeling of 5-HT(2A) receptors with [18F]deuteroaltanserin and PET: feasibility of a constant infusion paradigm

[(18)F]Altanserin has emerged as a promising positron emission tomography (PET) ligand for serotonin-2A (5-HT(2A)) receptors. The deuterium substitution of both of the 2'-hydrogens of altanserin ([(18)F]deuteroaltanserin) yields a metabolically more stable radiotracer with higher ratios of parent tracer to radiometabolites and increased specific brain uptake than [(18)F]altanserin. The slower metabolism of the deuterated analog might preclude the possibility of achieving stable plasma and brain activities with a bolus plus constant infusion within a reasonable time frame for an (18)F-labeled tracer (T(1/2) 110 min). Thus, the purpose of this study was to test the feasibility in human subjects of a constant infusion paradigm for equilibrium modeling of [(18)F]deuteroaltanserin with PET. Seven healthy male subjects were injected with [(18)F]deuteroaltanserin as a bolus plus constant infusion lasting 10 h postinjection. PET acquisitions and venous blood sampling were performed throughout the infusion period. Linear regression analysis revealed that time-activity curves for both specific brain uptake and plasma [(18)F]deuteroaltanserin concentration stabilized after about 5 h. This permitted equilibrium modeling and estimation of V(')(3) (ratio of specific uptake to total plasma parent concentration) and the binding potential V(3) (ratio of specific uptake to free plasma parent concentration). Cortical/cerebellar ratios were increased by 26% relative to those we previously observed with [(18)F]altanserin using similar methodology in a somewhat older subject sample. These results demonstrate feasibility of equilibrium imaging with [(18)F]deuteroaltanserin and suggest that it may be superior to [(18)F]altanserin as a PET radioligand.

Effects of estradiol and progesterone administration on human serotonin 2A receptor binding: a PET study

BACKGROUND

Preclinical studies demonstrate that 17beta-estradiol (E(2)) increases serotonin-2A receptor (5-HT(2A)R) density in rat frontal cortex.

METHODS

We investigated the impact of hormone replacement therapy on 5-HT(2A)R binding potential (BP) using positron emission tomography and [(18)F]altanserin in five postmenopausal women. Subjects were imaged at baseline, following 8 to 14 weeks of transdermal E(2), 0.1 mg/d, and following 2 to 6 weeks of E(2) plus micronized progesterone (P) 100 mg per os twice daily. Regional BPs in the anterior cingulate cortex, dorsolateral prefrontal cortex, and lateral orbitofrontal cortex were calculated by Logan analysis.

RESULTS

There was a main effect of time (p = .017) for 5-HT(2A)R BP, which increased 21.2%+/-2.6% following combined E(2) and P administration relative to baseline. This effect was evident in all cerebral cortex regions examined.

CONCLUSIONS

5-HT(2A)R BP increased in widespread areas of the cerebral cortex following combined E(2) + P administration.

Effects of fluvoxamine treatment on the in vivo binding of [F-18]FESP in drug naive depressed patients: a PET study

This study investigates the effect of chronic treatment with Fluvoxamine, a potent and specific serotonin reuptake sites inhibitor (SSRI), on 5HT(2) serotonin and D(2) dopamine receptors in the brain of drug naive unipolar depressed patients. Drug effect was evaluated in different cortical areas and in the basal ganglia by positron emission tomography (PET) and fluoro-ethyl-spiperone ([(18)F]FESP), an high affinity 5HT(2) serotonin and D(2) dopamine receptors antagonist. Patients underwent a PET study at recruitment and after clinical response to Fluvoxamine treatment. Nine of the 15 patients recruited completed the study. Fluvoxamine treatment significantly improved clinical symptoms and modified [(18)F]FESP binding in the frontal and occipital cortex of all of the nine patients who completed the study; in these regions a mean 31% increase in the in vivo [(18)F]FESP binding was found (P < 0.01). On the contrary, no significant changes in the in vivo [(18)F]FESP binding were found in the basal ganglia where [(18)F]FESP binds mainly to D(2) dopamine receptors. Chronic treatment with Fluvoxamine significantly increases the in vivo binding of [(18)F]FESP in the frontal and occipital cortex of drug naive unipolar depressed patients. The increase of the in vivo binding of [(18)F]FESP may reflect a modification in 5HT(2) binding capacity secondary to changes in cortical serotonin activity.

5-HT(1A) and 5-HT(2A) serotonin receptor turnover in adult rat offspring prenatally exposed to cocaine

This study investigated the effects of prenatal exposure to cocaine on the intracellular kinetics (i.e. rate constant of receptor production and degradation) that govern the maintenance and regulation of cortical 5-HT(1A) and 5-HT(2A) receptor densities in offspring. Adult male rat offspring, prenatally exposed to saline or (-) cocaine (15 mg/kg, s.c., b.i.d, from gestational day 13 through 20), were injected with either vehicle or the irreversible receptor antagonist, EEDQ (10 mg/kg, s.c.), and sacrificed at various post-injection times to monitor the recovery of receptor densities in cerebral cortex. In both saline and cocaine exposed offspring, initial EEDQ-induced reductions (>80%) in 5-HT(1A) and 5-HT(2A) receptor densities were followed by a time-dependent repopulation that reached steady state ([B(max)](ss)) densities comparable to non-EEDQ treated controls by day 10 post-treatment. Calculation of 5-HT(1A) receptor kinetic parameters indicated that prenatal exposure to cocaine did not significantly alter: (1) the receptor production rate (saline: 0.809 fmol/mg protein/h; cocaine: 0.724 fmol/mg protein/h), (2) the receptor degradation rate constant (saline: 0.0063 h(-1); cocaine: 0.0062 h(-1)) or (3) the half-life (t(1/2)) of receptor repopulation (saline: 109.2 h; cocaine: 111.5 h). Similarly, 5-HT(2A) receptor rate constants for production (1. 550 fmol/mg protein/h) and degradation (0.0061 h(-1)) and consequently, t(1/2) (113.2 h), were not significantly altered by prenatal exposure to cocaine. These data suggest that within homogenates of cerebral cortex, prenatal exposure to cocaine did not alter the overall intracellular processes that underlie receptor production or degradation and determine steady state densities of 5-HT(1A) or 5-HT(2A) receptors.

Depression and insomnia: questions of cause and effect

Chronic insomnia is a risk factor for the development of psychiatric disorders, including depression, as well as a prodrome of major depressive episodes, a consequence or complication of depression that often persists beyond the clinical episode, and a prognostic indicator of long-term illness course and treatment response. In addition, sleep is physiologically abnormal in persons at risk for depression; for example, shortened REM sleep latency is present not only during clinical episodes of depression, but also before the clinical episode in subjects at risk for depressive illness. Although insomnia usually disappears as depression is treated, it may persist, indicating heightened vulnerability to depressive relapse or recurrence. Physiological changes in sleep related to depression correlate with the likelihood of response to psychotherapy alone and may also identify which patients are unlikely to do well with psychosocial treatment and, therefore, to need somatic therapy in order to preserve recovery. Electroencephalographic (EEG) sleep changes also correlate with the speed of response and with the brittleness or durability of response (i.eprobability of relapse or recurrence). These observations suggest a close relationship between the regulation of sleep and the regulation of mood. The importance of this relationship is further underscored by recent brain imaging studies of sleep and sleep deprivation in patients with major depression. For example, therapeutic sleep deprivation (TSD) may serve as both a catalyst of rapid antidepressant activity and as a probe of treatment resistance. TSD's effects on brain metabolic rates, especially in limbic areas, may correlate with a therapeutic response to a night of sleep loss and to antidepressant medication. Finally, treating chronic insomnia with newer selective serotonin reuptake inhibitor (SSRI) antidepressant medication may represent an opportunity for preventing complications of insomnia, including depressive illness.

Measurement of striatal and extrastriatal dopamine D1 receptor binding potential with [11C]NNC 112 in humans: validation and reproducibility

To evaluate the postulated role of extrastriatal D1 receptors in human cognition and psychopathology requires an accurate and reliable method for quantification of these receptors in the living human brain. [11C]NNC 112 is a promising novel radiotracer for positron emission tomography imaging of the D1 receptor. The goal of this study was to develop and evaluate methods to derive D1 receptor parameters in striatal and extrastriatal regions of the human brain with [11C]NNC 112. Six healthy volunteers were studied twice. Two methods of analysis (kinetic and graphical) were applied to 12 regions (neocortical, limbic, and subcortical regions) to derive four outcome measures: total distribution volume, distribution volume ratio, binding potential (BP), and specific-to-nonspecific equilibrium partition coefficient (k3/k4). Both kinetic and graphic analyses provided BP and k3/k4 values in good agreement with the known distribution of D1 receptors (striatum > limbic regions = neocortical regions > thalamus). The identifiability of outcome measures derived by kinetic analysis was excellent. Time-stability analysis indicated that 90 minutes of data collection generated stable outcome measures. Derivation of BP and k3/k4 by kinetic analysis was highly reliable, with intraclass correlation coefficients (ICCs) of 0.90+/-0.06 (mean +/- SD of 12 regions) and 0.84+/-0.11, respectively. The reliability of these parameters derived by graphical analysis was lower, with ICCs of 0.72+/-0.17 and 0.58+/-0.21, respectively. Noise analysis revealed a noise-dependent bias in the graphical but not the kinetic analysis. In conclusion, kinetic analysis of [11C]NNC 112 uptake provides an appropriate method with which to derive D1 receptor parameters in regions with both high (striatal) and low (extrastriatal) D1 receptor density.

PET imaging of serotonin type 2A receptors in late-life neuropsychiatric disorders

OBJECTIVE

To determine whether there are abnormalities in the in vivo status of the serotonin type 2A (5-HT2A) receptor in late-life depression and Alzheimer's disease, the authors used positron emission tomography (PET) to assess patients with these two conditions and healthy subjects.

METHOD

PET was performed by using [18F]altanserin to evaluate 5-HT2A receptor binding in 11 elderly patients with depression (four men, seven women; mean age = 65.0 years, SD = 5.5); nine Alzheimer's disease patients, including three with concurrent depression (two men, seven women; mean age = 69.7 years, SD = 5.0); and 10 age-matched healthy subjects (four men, six women; mean age = 69.8 years, SD = 5.0). Partial-volume correction of regional specific binding estimates was performed by using a method based on magnetic resonance imaging.

RESULTS

No significant abnormalities in [18F]altanserin binding (binding potential) were observed in the patients with late-life depression, and no effect of depression on binding potential was present within the Alzheimer's disease group. However, the patients with Alzheimer's disease had significantly lower binding than the normal subjects in several brain regions, including the anterior cingulate, prefrontal cortex, and sensorimotor cortex.

CONCLUSIONS

These results suggest that the 5-HT2A receptor is differentially affected in late-life depression and Alzheimer's disease, a finding that has implications for the etiological basis of mood and cognitive features of neuropsychiatric disorders of late life.