Tina Kroll

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

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

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.

Circadian variation of metabotropic glutamate receptor 5 availability in the rat brain

The metabotrophic subtype 5 glutamate receptor (mGluR5) plays a critical role in synaptic plasticity besides its involvement in numerous neurological disorders, such as depression. As mGluR5 availability in humans is altered in sleep deprivation, we hypothesized that mGluR5 availability underlies a circadian variation. To investigate whether mGluR5 underlies potential circadian changes we measured its density in a randomized fashion at six different daytimes in 11 adult Sprague–Dawley rats. mGluR5 density was quantified by positron emission tomography (PET) using the radioactive ligand [11C]ABP688. [11C]ABP688 uptake was quantified in nine regions of interest with a reference tissue model. Significant differences in the binding potential (BPND) and therefore mGluR5 availability between the different circadian times were found in cortex, cingulate cortex, amygdala, caudate putamen and nucleus accumbens. Further post‐hoc statistical analysis (Tukey–Kramer test) of the different time‐points revealed significant changes in BPND between 07:00 hours (start of light‐on phase) and 15:00 hours (last time‐point of the light‐on phase) in the caudate putamen. This study shows that mGluR5 availability is increased during the light‐on, or sleep phase, of rodents by approximately 10%. Given that altered mGluR5 densities play a role in psychiatric disorders, further investigation is warranted to evaluate their circadian involvement in mood changes in humans.

Chronic sleep restriction induces long-lasting changes in adenosine and noradrenaline receptor density in the rat brain.

Although chronic sleep restriction frequently produces long‐lasting behavioural and physiological impairments in humans, the underlying neural mechanisms are unknown. Here we used a rat model of chronic sleep restriction to investigate the role of brain adenosine and noradrenaline systems, known to regulate sleep and wakefulness, respectively. The density of adenosine A1 and A2a receptors and β‐adrenergic receptors before, during and following 5 days of sleep restriction was assessed with autoradiography. Rats (n = 48) were sleep‐deprived for 18 h day−1 for 5 consecutive days (SR1–SR5), followed by 3 unrestricted recovery sleep days (R1–R3). Brains were collected at the beginning of the light period, which was immediately after the end of sleep deprivation on sleep restriction days. Chronic sleep restriction increased adenosine A1 receptor density significantly in nine of the 13 brain areas analysed with elevations also observed on R3 (+18 to +32%). In contrast, chronic sleep restriction reduced adenosine A2a receptor density significantly in one of the three brain areas analysed (olfactory tubercle which declined 26–31% from SR1 to R1). A decrease in β‐adrenergic receptors density was seen in substantia innominata and ventral pallidum which remained reduced on R3, but no changes were found in the anterior cingulate cortex. These data suggest that chronic sleep restriction can induce long‐term changes in the brain adenosine and noradrenaline receptors, which may underlie the long‐lasting neurocognitive impairments observed in chronic sleep restriction.

Recovery sleep after extended wakefulness restores elevated A1 adenosine receptor availability in the human brain.

Significance Our study reveals that prolonged sleep deprivation is accompanied by an A1 adenosine receptor (A1AR) upregulation in the human brain. Recovery sleep quickly restores A1AR availability to control levels. High individual A1AR availability is related to a low sleep pressure and good cognitive performance. Sleep deprivation is an efficient but short-lasting therapeutic strategy in depression. A causal sleep–wake dysregulation has been proposed, possibly mediated by cerebral adenosine and its A1AR. The restoration of the A1AR availability after recovery from sleep deprivation mimics the rapid relapse following the end of therapeutic sleep deprivation. Understanding the adenosine regulation under sleep restriction, especially regarding individual characteristics, might improve the rationale for the individual indication and design of therapeutic sleep modulation in depression. Adenosine and functional A1 adenosine receptor (A1AR) availability are supposed to mediate sleep–wake regulation and cognitive performance. We hypothesized that cerebral A1AR availability after an extended wake period decreases to a well-rested state after recovery sleep. [18F]CPFPX positron emission tomography was used to quantify A1AR availability in 15 healthy male adults after 52 h of sleep deprivation and following 14 h of recovery sleep. Data were additionally compared with A1AR values after 8 h of baseline sleep from an earlier dataset. Polysomnography, cognitive performance, and sleepiness were monitored. Recovery from sleep deprivation was associated with a decrease in A1AR availability in several brain regions, ranging from 11% (insula) to 14% (striatum). A1AR availabilities after recovery did not differ from baseline sleep in the control group. The degree of performance impairment, sleepiness, and homeostatic sleep-pressure response to sleep deprivation correlated negatively with the decrease in A1AR availability. Sleep deprivation resulted in a higher A1AR availability in the human brain. The increase that was observed after 52 h of wakefulness was restored to control levels during a 14-h recovery sleep episode. Individuals with a large increase in A1AR availability were more resilient to sleep-loss effects than those with a subtle increase. This pattern implies that differences in endogenous adenosine and A1AR availability might be causal for individual responses to sleep loss.

Association of adenosine receptor gene polymorphisms and in vivo adenosine A1 receptor binding in the human brain.

Adenosine A1 receptors (A1ARs) and the interacting adenosine A2A receptors are implicated in neurological and psychiatric disorders. Variants within the corresponding genes ADORA1 and ADORA2A were shown associated with pathophysiologic alterations, particularly increased anxiety. It is unknown so far, if these variants might modulate the A1AR distribution and availability in different brain regions. In this pilot study, the influence of ADORA1 and ADORA2A variants on in vivo A1AR binding was assessed with the A1AR-selective positron emission tomography (PET) radioligand [18F]CPFPX in brains of healthy humans. Twenty-eight normal control subjects underwent PET procedures to calculate the binding potential BPND of [18F]CPFPX in cerebral regions and to assess ADORA1 and ADORA2A single nucleotide polymorphism (SNP) effects on regional BPND data. Our results revealed SNPs of both genes associated with [18F]CPFPX binding to the A1AR. The strongest effects that withstood even Bonferroni correction of multiple SNP testing were found in non-smoking subjects (N=22) for ADORA2A SNPs rs2236624 and rs5751876 (corr. Pall<0.05). SNP alleles previously identified at risk for increased anxiety like the rs5751876 T-allele corresponded to consistently higher A1AR availability in all brain regions. Our data indicate for the first time that variation of A1AR availability was associated with ADORA SNPs. The finding of increased A1AR availability in regions of the fear network, particularly in ADORA2A risk allele carriers, strongly warrants evaluation and replication in further studies including individuals with increased anxiety.

In Vivo Kinetic and Steady-State Quantification of 18F-CPFPX Binding to Rat Cerebral A1 Adenosine Receptors: Validation by Displacement and Autoradiographic Experiments

In vivo imaging of the A1 adenosine receptor (A1AR) using 18F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (18F-CPFPX) and PET has become an important tool for studying physiologic and pathologic states of the human brain. However, dedicated experimental settings for small-animal studies are still lacking. The aim of the present study was therefore to develop and evaluate suitable pharmacokinetic models for the quantification of the cerebral A1AR in high-resolution PET. Methods: On a dedicated animal PET scanner, 15 rats underwent 18F-CPFPX PET scans of 120-min duration. In all animals, arterial blood samples were drawn and corrected for metabolites. The radioligand was injected either as a bolus or as a bolus plus constant infusion. For the definition of unspecific binding, the A1AR selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was applied. After PET, the brains of 9 animals were dissected and in vitro saturation binding was performed using high-resolution 3H-DPCPX autoradiography. Results: The kinetics of 18F-CPFPX were well described by either compartmental or noncompartmental models based on arterial input function. The resulting distribution volume ratio correlated with a low bias toward identity with the binding potential derived from a reference region (olfactory bulb) approach. Furthermore, PET quantification correlated significantly with autoradiographic in vitro data. Blockade of the A1AR with DPCPX identified specific binding of about 45% in the reference region olfactory bulb. Conclusion: The present study provides evidence that 18F-CPFPX PET based on a reference tissue approach can be performed quantitatively in rodents in selected applications. Specific binding in the reference region needs careful consideration for quantitative investigations.

[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.

Cognitive impairments by alcohol and sleep deprivation indicate trait characteristics and a potential role for adenosine A1 receptors

Significance Modern work environments favor working around the clock, at the cost of insufficient sleep and increased risk for fatigue-related human error. Performance impairments caused by sleep loss show great variability among individuals and resemble the effects of ethanol intoxication. We found that there are individual trait characteristics that make individuals either vulnerable or resilient to both alcohol and to sleep deprivation. We also provide evidence by molecular brain imaging that ethanol induces an up-regulation of cerebral A1 adenosine receptors, reminiscent of the effects of sleep deprivation. Targeting the brain’s adenosine system might help to identify countermeasures for fatigue- and alcohol-related human error. Trait-like differences in cognitive performance after sleep loss put some individuals more at risk than others, the basis of such disparities remaining largely unknown. Similarly, interindividual differences in impairment in response to alcohol intake have been observed. We tested whether performance impairments due to either acute or chronic sleep loss can be predicted by an individual’s vulnerability to acute alcohol intake. Also, we used positron emission tomography (PET) to test whether acute alcohol infusion results in an up-regulation of cerebral A1 adenosine receptors (A1ARs), similar to the changes previously observed following sleep deprivation. Sustained attention in the psychomotor vigilance task (PVT) was tested in 49 healthy volunteers (26 ± 5 SD years; 15 females) (i) under baseline conditions: (ii) after ethanol intake, and after either (iii) total sleep deprivation (TSD; 35 hours awake; n = 35) or (iv) partial sleep deprivation (PSD; four nights with 5 hours scheduled sleep; n = 14). Ethanol- versus placebo-induced changes in cerebral A1AR availability were measured in 10 healthy male volunteers (31 ± 9 years) with [18F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX) PET. Highly significant correlations between the performance impairments induced by ethanol and sleep deprivation were found for various PVT parameters, including mean speed (TSD, r = 0.62; PSD, r = 0.84). A1AR availability increased up to 26% in several brain regions with ethanol infusion. Our studies revealed individual trait characteristics for being either vulnerable or resilient to both alcohol and to sleep deprivation. Both interventions induce gradual increases in cerebral A1AR availability, pointing to a potential common molecular response mechanism.

PET in Neurological and Psychiatric Disorders: Technologic Advances and Clinical Applications

This chapter resumes applications of PET in neurological and psychiatric disease and research. Routine applications and recent developments in [18F]FDG-PET are dealt with, followed by the role of amyloid PET as inclusion parameter in clinical trials. A large series of PET tracers do target neurotransmitter signaling at various steps. This serves as a tool to decipher the pathophysiology of disease as well as to conduct pharmacokinetic and dose-finding studies of new drug candidates. After presenting the different types of experimental approaches, an overview of targets is given that had been imaged thus far by PET in the human brain. Taken together, PET imaging always accompanied and catalyzed the emergence of new technologies – may it be deep brain stimulation or new biomarkers – at the threshold to routine.