Imaging Agonist-Induced D2/D3 Receptor Desensitization and Internalization In Vivo with PET/fMRI

PET-MRI Applications and Future Prospects in Psychiatry

This article reviews the synergistic application of positron emission tomography-magnetic resonance imaging (PET-MRI) in neuroscience with relevance for psychiatry, particularly examining neurotransmission, epigenetics, and dynamic imaging methodologies. We begin by discussing the complementary insights that PET and MRI modalities provide into neuroreceptor systems, with a focus on dopamine, opioids, and serotonin receptors, and their implications for understanding and treating psychiatric disorders. We further highlight recent PET-MRI studies using a radioligand that enables the quantification of epigenetic enzymes, specifically histone deacetylases, in the brain in vivo. Imaging epigenetics is used to exemplify the impact the quantification of novel molecular targets may have, including new treatment approaches for psychiatric disorders. Finally, we discuss innovative designs involving functional PET using [18F]FDG (fPET-FDG), which provides detailed information regarding dynamic changes in glucose metabolism. Concurrent acquisitions of fPET-FDG and functional MRI provide a time-resolved approach to studying brain function, yielding simultaneous metabolic and hemodynamic information and thereby opening new avenues for psychiatric research. Collectively, the review underscores the potential of a multimodal PET-MRI approach to advance our understanding of brain structure and function in health and disease, which could improve clinical care based on objective neurobiological features and treatment response monitoring. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 1.

Examining the role of dopamine in methylphenidate's effects on resting brain function

The amplitude of low-frequency fluctuations (ALFF) and global functional connectivity density (gFCD) are fMRI (Functional MRI) metrics widely used to assess resting brain function. However, their differential sensitivity to stimulant-induced dopamine (DA) increases, including the rate of DA rise and the relationship between them, have not been investigated. Here we used, simultaneous PET-fMRI to examine the association between dynamic changes in striatal DA and brain activity as assessed by ALFF and gFCD, following placebo, intravenous (IV), or oral methylphenidate (MP) administration, using a within-subject double-blind placebo-controlled design. In putamen, MP significantly reduced D2/3 receptor availability and strongly reduced ALFF and increased gFCD in the brain for IV-MP (Cohen's d > 1.6) but less so for oral-MP (Cohen's d < 0.6). Enhanced gFCD was associated with both the level and the rate of striatal DA increases, whereas decreased ALFF was only associated with the level of DA increases. These findings suggest distinct representations of neurovascular activation with ALFF and gFCD by stimulant-induced DA increases with differential sensitivity to the rate and the level of DA increases. We also observed an inverse association between gFCD and ALFF that was markedly enhanced during IV-MP, which could reflect an increased contribution from MP's vasoactive properties.

Amphetamine pretreatment blunts dopamine-induced D2/D3-receptor occupancy by an arrestin-mediated mechanism: A PET study in internalization compromised mice

While all reversible receptor-targeting radioligands for positron emission tomography (PET) can be displaced by competition with an antagonist at the receptor, many radiotracers show limited occupancies using agonists even at high doses. [11C]Raclopride, a D2/D3 receptor radiotracer with rapid kinetics, can identify the direction of changes in the neurotransmitter dopamine, but quantitative interpretation of the relationship between dopamine levels and radiotracer binding has proven elusive. Agonist-induced receptor desensitization and internalization, a homeostatic mechanism to downregulate neurotransmitter-mediated function, can shift radioligand-receptor binding affinity and confound PET interpretations of receptor occupancy. In this study, we compared occupancies induced by amphetamine (AMP) in drug-naive wild-type (WT) and internalization-compromised β-arrestin-2 knockout (KO) mice using a within-scan drug infusion to modulate the kinetics of [11C]raclopride. We additionally performed studies at 3 h following AMP pretreatment, with the hypothesis that receptor internalization should markedly attenuate occupancy on the second challenge, because dopamine cannot access internalized receptors. Without prior AMP treatment, WT mice exhibited somewhat larger binding potential than KO mice but similar AMP-induced occupancy. At 3 h after AMP treatment, WT mice exhibited binding potentials that were 15 % lower than KO mice. At this time point, occupancy was preserved in KO mice but suppressed by 60 % in WT animals, consistent with a model in which most receptors contributing to binding potential in WT animals were not functional. These results demonstrate that arrestin-mediated receptor desensitization and internalization produce large effects in PET [11C]raclopride occupancy studies using agonist challenges.

Striatal dopamine in anhedonia: A simultaneous [11C]raclopride positron emission tomography and functional magnetic resonance imaging investigation

BACKGROUND

Anhedonia is hypothesized to be associated with blunted mesocorticolimbic dopamine (DA) functioning in samples with major depressive disorder. The purpose of this study was to examine linkages between striatal DA, reward circuitry functioning, anhedonia, and, in an exploratory fashion, self-reported stress, in a transdiagnostic anhedonic sample.

METHODS

Participants with (n = 25) and without (n = 12) clinically impairing anhedonia completed a reward-processing task during simultaneous positron emission tomography and magnetic resonance (PET-MR) imaging with [11C]raclopride, a DA D2/D3 receptor antagonist that selectively binds to striatal DA receptors.

RESULTS

Relative to controls, the anhedonia group exhibited decreased task-related DA release in the left putamen, caudate, and nucleus accumbens and right putamen and pallidum. There were no group differences in task-related brain activation (fMRI) during reward processing after correcting for multiple comparisons. General functional connectivity (GFC) findings revealed blunted fMRI connectivity between PET-derived striatal seeds and target regions in the anhedonia group. Associations were identified between anhedonia severity and the magnitude of task-related DA release to rewards in the left putamen, but not mesocorticolimbic GFC.

CONCLUSIONS

Results provide evidence for reduced striatal DA functioning during reward processing and blunted mesocorticolimbic network functional connectivity in a transdiagnostic sample with clinically significant anhedonia.

Reduced cortical cerebral blood flow in antipsychotic-free first-episode psychosis and relationship to treatment response

BACKGROUND

Altered cerebral blood flow (CBF) has been found in people at risk for psychosis, with first-episode psychosis (FEP) and with chronic schizophrenia (SCZ). Studies using arterial spin labelling (ASL) have shown reduction of cortical CBF and increased subcortical CBF in SCZ. Previous studies have investigated CBF using ASL in FEP, reporting increased CBF in striatum and reduced CBF in frontal cortex. However, as these people were taking antipsychotics, it is unclear whether these changes are related to the disorder or antipsychotic treatment and how they relate to treatment response.

METHODS

We examined CBF in FEP free from antipsychotic medication (N = 21), compared to healthy controls (N = 22). Both absolute and relative-to-global CBF were assessed. We also investigated the association between baseline CBF and treatment response in a partially nested follow-up study (N = 14).

RESULTS

There was significantly lower absolute CBF in frontal cortex (Cohen's d = 0.84, p = 0.009) and no differences in striatum or hippocampus. Whole brain voxel-wise analysis revealed widespread cortical reductions in absolute CBF in large cortical clusters that encompassed occipital, parietal and frontal cortices (Threshold-Free Cluster Enhancement (TFCE)-corrected <0.05). No differences were found in relative-to-global CBF in the selected region of interests and in voxel-wise analysis. Relative-to-global frontal CBF was correlated with percentage change in total Positive and Negative Syndrome Scale after antipsychotic treatment (r = 0.67, p = 0.008).

CONCLUSIONS

These results show lower cortical absolute perfusion in FEP prior to starting antipsychotic treatment and suggest relative-to-global frontal CBF as assessed with magnetic resonance imaging could potentially serve as a biomarker for antipsychotic response.

Dietary fat restriction affects brain reward regions in a randomized crossover trial

BACKGROUNDWeight-loss diets often target dietary fat or carbohydrates, macronutrients that are sensed via distinct gut-brain pathways and differentially affect peripheral hormones and metabolism. However, the effects of such diet changes on the human brain are unclear. METHODSWe investigated whether selective isocaloric reductions in dietary fat or carbohydrates altered dopamine D2/3 receptor binding potential (D2BP) and neural activity in brain-reward regions in response to visual food cues in 17 inpatient adults with obesity as compared with a eucaloric baseline diet using a randomized crossover design. RESULTSOn the fifth day of dietary fat restriction, but not carbohydrate restriction, both D2BP and neural activity to food cues were decreased in brain-reward regions. After the reduced-fat diet, ad libitum intake shifted toward foods high in both fat and carbohydrates. CONCLUSIONThese results suggest that dietary fat restriction increases tonic dopamine in brain-reward regions and affects food choice in ways that may hamper diet adherence. TRIAL REGISTRATIONClinicalTrials.gov NCT00846040 FUNDING. NIDDK 1ZIADK013037.

Cerebral blood flow in striatum is increased by partial dopamine agonism in initially antipsychotic-naïve patients with psychosis

BACKGROUND

Resting cerebral blood flow (rCBF) in striatum and thalamus is increased in medicated patients with psychosis, but whether this is caused by treatment or illness pathology is unclear. Specifically, effects of partial dopamine agonism, sex, and clinical correlates on rCBF are sparsely investigated. We therefore assessed rCBF in antipsychotic-naïve psychosis patients before and after aripiprazole monotherapy and related findings to sex and symptom improvement.

METHODS

We assessed rCBF with the pseudo-Continuous Arterial Spin Labeling (PCASL) sequence in 49 first-episode patients (22.6 ± 5.2 years, 58% females) and 50 healthy controls (HCs) (22.3 ± 4.4 years, 63% females) at baseline and in 29 patients and 49 HCs after six weeks. RCBF in striatum and thalamus was estimated with a region-of-interest (ROI) approach. Psychopathology was assessed with the positive and negative syndrome scale.

RESULTS

Baseline rCBF in striatum and thalamus was not altered in the combined patient group compared with HCs, but female patients had lower striatal rCBF compared with male patients (p = 0.009). Treatment with a partial dopamine agonist increased rCBF significantly in striatum (p = 0.006) in the whole patient group, but not significantly in thalamus. Baseline rCBF in nucleus accumbens was negatively associated with improvement in positive symptoms (p = 0.046), but baseline perfusion in whole striatum and thalamus was not related to treatment outcome.

CONCLUSIONS

The findings suggest that striatal perfusion is increased by partial dopamine agonism and decreased in female patients prior to first treatment. This underlines the importance of treatment effects and sex differences when investigating the neurobiology of psychosis.

Combining CRISPR-Cas9 and brain imaging to study the link from genes to molecules to networks

Receptors, transporters, and ion channels are important targets for therapy development in neurological diseases, but their mechanistic role in pathogenesis is often poorly understood. Gene editing and in vivo imaging approaches will help to identify the molecular and functional role of these targets and the consequence of their regional dysfunction on the whole-brain level. We combine CRISPR-Cas9 gene editing with in vivo positron emission tomography (PET) and functional MRI (fMRI) to investigate the direct link between genes, molecules, and the brain connectome. The extensive knowledge of the Slc18a2 gene encoding the vesicular monoamine transporter (VMAT2), involved in the storage and release of dopamine, makes it an excellent target for studying the gene network relationships while structurally preserving neuronal integrity and function. We edited the Slc18a2 in the substantia nigra pars compacta of adult rats and used in vivo molecular imaging besides behavioral, histological, and biochemical assessments to characterize the CRISPR-Cas9-mediated VMAT2 knockdown. Simultaneous PET/fMRI was performed to investigate molecular and functional brain alterations. We found that stage-specific adaptations of brain functional connectivity follow the selective impairment of presynaptic dopamine storage and release. Our study reveals that recruiting different brain networks is an early response to the dopaminergic dysfunction preceding neuronal cell loss. Our combinatorial approach is a tool to investigate the impact of specific genes on brain molecular and functional dynamics, which will help to develop tailored therapies for normalizing brain function.

A reference tissue forward model for improved PET accuracy using within-scan displacement studies

We report a novel forward-model implementation of the full reference tissue model (fFTRM) that addresses the fast-exchange approximation employed by the simplified reference tissue model (SRTM) by incorporating a non-zero dissociation time constant from the specifically bound compartment. The forward computational approach avoided errors associated with noisy and nonorthogonal basis functions using an inverse linear model. Compared to analysis by a multilinear single-compartment reference tissue model (MRTM), fFTRM provided improved accuracy for estimation of binding potentials at early times in the scan, with no worse reproducibility across sessions. To test the model's ability to identify small focal changes in binding potential using a within-scan challenge, we employed a nonhuman primate model of focal dopamine release elicited by deep brain microstimulation remote to ventral striatum (VST) during imaging by simultaneous PET and fMRI. The new model reported an unambiguously lateralized response in VST consistent with fMRI, whereas the MRTM-derived response was not lateralized and was consistent with simulations of model bias. The proposed model enabled better accuracy in PET [11C]raclopride displacement studies and may also facilitate challenges sooner after injection, thereby recovering some sensitivity lost to radioactive decay of the PET tracer.

Cortical D1 and D2 dopamine receptor availability modulate methylphenidate-induced changes in brain activity and functional connectivity

Dopamine signaling plays a critical role in shaping brain functional network organization and behavior. Prominent theories suggest the relative expression of D1- to D2-like dopamine receptors shapes excitatory versus inhibitory signaling, with broad consequences for cognition. Yet it remains unknown how the balance between cortical D1R versus D2R signaling coordinates the activity and connectivity of functional networks in the human brain. To address this, we collected three PET scans and two fMRI scans in 36 healthy adults (13 female/23 male; average age 43 ± 12 years), including a baseline D1R PET scan and two sets of D2R PET scans and fMRI scans following administration of either 60 mg oral methylphenidate or placebo (two separate days, blinded, order counterbalanced). The drug challenge allowed us to assess how pharmacologically boosting dopamine levels alters network organization and behavior in association with D1R-D2R ratios across the brain. We found that the relative D1R-D2R ratio was significantly greater in high-level association cortices than in sensorimotor cortices. After stimulation with methylphenidate compared to placebo, brain activity (as indexed by the fractional amplitude of low frequency fluctuations) increased in association cortices and decreased in sensorimotor cortices. Further, within-network resting state functional connectivity strength decreased more in sensorimotor than association cortices following methylphenidate. Finally, in association but not sensorimotor cortices, the relative D1R-D2R ratio (but not the relative availability of D1R or D2R alone) was positively correlated with spatial working memory performance, and negatively correlated with age. Together, these data provide a framework for how dopamine-boosting drugs like methylphenidate alter brain function, whereby regions with relatively higher inhibitory D2R (i.e., sensorimotor cortices) tend to have greater decreases in brain activity and connectivity compared to regions with relatively higher excitatory D1R (i.e., association cortices). They also support the importance of a balanced interaction between D1R and D2R in association cortices for cognitive function and its degradation with aging.

Joint PET and MRI analyses of cortical D1 and D2 dopamine receptors in healthy adults provide a framework for understanding how dopamine-boosting drugs alter brain function.

Metabotropic Glutamate Receptor Trafficking and its Role in Drug-Induced Neurobehavioral Plasticity

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system that guides developmental and experience-dependent changes in many cellular substrates and brain circuits, through the process collectively referred to as neurobehavioral plasticity. Regulation of cell surface expression and membrane trafficking of glutamate receptors represents an important mechanism that assures optimal excitatory transmission, and at the same time, also allows for fine-tuning neuronal responses to glutamate. On the other hand, there is growing evidence implicating dysregulated glutamate receptor trafficking in the pathophysiology of several neuropsychiatric disorders. This review provides up-to-date information on the molecular determinants regulating trafficking and surface expression of metabotropic glutamate (mGlu) receptors in the rodent and human brain and discusses the role of mGluR trafficking in maladaptive synaptic plasticity produced by addictive drugs. As substantial evidence links glutamatergic dysfunction to the progression and the severity of drug addiction, advances in our understanding of mGluR trafficking may provide opportunities for the development of novel pharmacotherapies of addiction and other neuropsychiatric disorders.

A simultaneous [11C]raclopride positron emission tomography and functional magnetic resonance imaging investigation of striatal dopamine binding in autism

The social motivation hypothesis of autism posits that autism spectrum disorder (ASD) is characterized by impaired motivation to seek out social experience early in life that interferes with the development of social functioning. This framework suggests that impaired mesolimbic dopamine function underlies compromised responses to social rewards in ASD. Although this hypothesis is supported by functional magnetic resonance imaging (fMRI) studies, no molecular imaging study has evaluated striatal dopamine functioning in response to rewards in ASD. Here, we examined striatal functioning during monetary incentive processing in ASD and controls using simultaneous positron emission tomography (PET) and fMRI. Using a bolus + infusion protocol with the D2/D3 dopamine receptor antagonist [¹¹C]raclopride, voxel-wise binding potential (BP_ND) was compared between groups (controls = 12, ASD = 10) in the striatum. Striatal clusters showing significant between-group BP_ND differences were used as seeds in whole-brain fMRI general functional connectivity analyses. Relative to controls, the ASD group demonstrated decreased phasic dopamine release to incentives in the bilateral putamen and left caudate, as well as increased functional connectivity between a PET-derived right putamen seed and the precuneus and insula. Within the ASD group, decreased phasic dopamine release in the putamen was related to poorer theory-of-mind skills. Our findings that ASD is characterized by impaired striatal phasic dopamine release to incentives provide support for the social motivation hypothesis of autism. PET-fMRI may be a suitable tool to evaluate novel ASD therapeutics targeting the striatal dopamine system.

Dual Effect of 5-HT1B/1D Receptors on Dopamine Neurons in Ventral Tegmental Area: Implication for the Functional Switch After Chronic Cocaine Exposure

BACKGROUND

Serotonin (5-HT) 1B/1D receptor (5-HT_1B/1DR) agonists undergo an abstinence-induced switch in their effects on cocaine-related behaviors, which may involve changes in modulation of dopamine (DA) neurons in the ventral tegmental area (VTA). However, it is unclear how 5-HT_1B/1DRs affect VTA DA neuronal function and whether modulation of these neurons mediates the abstinence-induced switch after chronic cocaine exposure.

METHODS

We examined the ability of 5-HT_1B/1DRs to modulate D₂ autoreceptors (D₂ARs) and synaptic transmission in the VTA by slice recording and single unit recording in vivo in naïve mice and in mice with chronic cocaine treatment.

RESULTS

We report a bidirectional modulation of VTA DA neuronal firing through the interaction of VTA 5-HT_1B/1DRs and D₂ARs. In both VTA slices and the VTA of anesthetized mice, the 5-HT_1B/1DR agonist CP94253 decreased DA neuronal firing rate and evoked excitatory postsynaptic currents to DA neurons in slice. Paradoxically, CP94253 decreased quinpirole-induced inhibition of DA neurons by reducing D₂AR-mediated G protein-gated inwardly rectifying potassium current. This manifested decreased GABA_A (gamma-aminobutyric acid A) receptor-mediated evoked inhibitory postsynaptic currents in slice, resulting in disinhibition of DA neurons, in opposition to the 5-HT_1B/1DR-induced inhibition. This dual effect was verified in chronic cocaine-treated and mild stress-treated, male mice on days 1 and 20 posttreatment.

CONCLUSIONS

This study revealed dual effects of CP94253 on VTA DA neurons that are dependent on D₂AR sensitivity, with anti-inhibition under normal D₂AR sensitivity and inhibition under low D₂AR sensitivity. These dual effects may underlie the ability of CP94253 to both enhance and inhibit cocaine-induced behaviors.

Molecular imaging of schizophrenia: Neurochemical findings in a heterogeneous and evolving disorder

The past four decades have seen enormous efforts placed on a search for molecular markers of schizophrenia using positron emission tomography (PET) and single photon emission computed tomography (SPECT). In this narrative review, we cast a broad net to define and summarize what researchers have learned about schizophrenia from molecular imaging studies. Some PET studies of brain energy metabolism with the glucose analogue FDGhave have shown a hypofrontality defect in patients with schizophrenia, but more generally indicate a loss of metabolic coherence between different brain regions. An early finding of significantly increased striatal trapping of the dopamine synthesis tracer FDOPA has survived a meta-analysis of many replications, but the increase is not pathognomonic of the disorder, since one half of patients have entirely normal dopamine synthesis capacity. Similarly, competition SPECT studies show greater basal and amphetamine-evoked dopamine occupancy at post-synaptic dopamine D_2/3 receptors in patients with schizophrenia, but the difference is likewise not pathognomonic. We thus propose that molecular imaging studies of brain dopamine indicate neurochemical heterogeneity within the diagnostic entity of schizophrenia. Occupancy studies have established the relevant target engagement by antipsychotic medications at dopamine D_2/3 receptors in living brain. There is evidence for elevated frontal cortical dopamine D₁ receptors, especially in relation to cognitive deficits in schizophrenia. There is a general lack of consistent findings of abnormalities in serotonin markers, but some evidence for decreased levels of nicotinic receptors in patients. There are sparse and somewhat inconsistent findings of reduced binding of muscarinic, glutamate, and opioid receptors ligands, inconsistent findings of microglial activation, and very recently, evidence of globally reduced levels of synaptic proteins in brain of patients. One study reports a decline in histone acetylase binding that is confined to the dorsolateral prefrontal cortex. In most contexts, the phase of the disease and effects of past or present medication can obscure or confound PET and SPECT findings in schizophrenia.

Visual stimuli induce serotonin release in occipital cortex: A simultaneous positron emission tomography/magnetic resonance imaging study

Endogenous serotonin (5-HT) release can be measured noninvasively using positron emission tomography (PET) imaging in combination with certain serotonergic radiotracers. This allows us to investigate effects of pharmacological and nonpharmacological interventions on brain 5-HT levels in living humans. Here, we study the neural responses to a visual stimulus using simultaneous PET/MRI. In a cross-over design, 11 healthy individuals were PET/MRI scanned with the 5-HT_1B receptor radioligand [¹¹ C]AZ10419369, which is sensitive to changes in endogenous 5-HT. During the last part of the scan, participants either viewed autobiographical images with positive valence (n = 11) or kept their eyes closed (n = 7). The visual stimuli increased cerebral blood flow (CBF) in the occipital cortex, as measured with pseudo-continuous arterial spin labeling. Simultaneously, we found decreased 5-HT_1B receptor binding in the occipital cortex (-3.6 ± 3.6%), indicating synaptic 5-HT release. Using a linear regression model, we found that the change in 5-HT_1B receptor binding was significantly negatively associated with change in CBF in the occipital cortex (p = .004). For the first time, we here demonstrate how cerebral 5-HT levels change in response to nonpharmacological stimuli in humans, as measured with PET. Our findings more directly support a link between 5-HT signaling and visual processing and/or visual attention.

Methods for Quantifying Neurotransmitter Dynamics in the Living Brain With PET Imaging

Positron emission tomography (PET) neuroimaging in neuropsychiatry is a powerful tool for the quantification of molecular brain targets to characterize disease, assess disease subtype differences, evaluate short- and long-term effects of treatments, or even to measure neurotransmitter levels in healthy and psychiatric conditions. In this work, we present different methodological approaches (time-invariant models and models with time-varying terms) that have been used to measure dynamic changes in neurotransmitter levels induced by pharmacological or behavioral challenges in humans. The developments and potential use of hybrid PET/magnetic resonance imaging (MRI) for neurotransmission brain research will also be highlighted.

Human Positron Emission Tomography Neuroimaging

Neuroimaging with positron emission tomography (PET) is the most powerful tool for understanding pharmacology, neurochemistry, and pathology in the living human brain. This technology combines high-resolution scanners to measure radioactivity throughout the human body with specific, targeted radioactive molecules, which allow measurements of a myriad of biological processes in vivo. While PET brain imaging has been active for almost 40 years, the pace of development for neuroimaging tools, known as radiotracers, and for quantitative analytical techniques has increased dramatically over the past decade. Accordingly, the fundamental questions that can be addressed with PET have expanded in basic neurobiology, psychiatry, neurology, and related therapeutic development. In this review, we introduce the field of human PET neuroimaging, some of its conceptual underpinnings, and motivating questions. We highlight some of the more recent advances in radiotracer development, quantitative modeling, and applications of PET to the study of the human brain.

Pharmacological MRI responses of raclopride in rats: The relationship with D2 receptor occupancy and cataleptic behavior

Pharmacological magnetic resonance imaging (phMRI) allows the visualization of brain pharmacological effects of drugs using functional MRI (fMRI). phMRI can help us facilitate central nervous system (CNS) drug development. However, there have been few studies demonstrating the dose relationship of the fMRI response induced by CNS drugs to underlying target engagement or behavioral efficacy. To clarify these relationships, we examined receptor occupancy measurements using positron emission tomography (PET) (n = 3~5), fMRI (n = 5~8) and a cataleptic behavior (n = 6) with raclopride, a dopamine D2 receptor antagonist (8, 20, and 200 μg/kg) on Wistar rats. Dopamine D2 receptor occupancy was increased dose dependently by raclopride (41.8 ± 2.7%, 8 μg/kg; 64.9 ± 2.8%, 20 μg/kg; 83.1 ± 3.0%, 200 μg/kg). phMRI study revealed significant positive responses to raclopride at 200 μg/kg specifically in the striatum and nucleus accumbens, related to dopaminergic system. Slight fMRI responses were observed at 20 μg/kg in some areas corresponding to the striatum and nucleus accumbens. There were no noticeable fMRI responses at 8 μg/kg raclopride administration. Raclopride at 200 μg/kg significantly increased the cataleptic score, although, at 8 and 20 μg/kg, raclopride had no significant effects. These findings showed that raclopride-induced fMRI responses were observed at doses inducing cataleptic behavior and high D2 receptor occupancy, suggesting that phMRI can be useful for dose selection in clinical trial as an evaluation method of brain activity, which reflects behavioral responses induced by target engagements.

Advances in simultaneous PET/MR for imaging neuroreceptor function

Hybrid imaging using PET/MRI has emerged as a platform for elucidating novel neurobiology, molecular and functional changes in disease, and responses to physiological or pharmacological interventions. For the central nervous system, PET/MRI has provided insights into biochemical processes, linking selective molecular targets and distributed brain function. This review highlights several examples that leverage the strengths of simultaneous PET/MRI, which includes measuring the perturbation of multi-modal imaging signals on dynamic timescales during pharmacological challenges, physiological interventions or behavioral tasks. We discuss important considerations for the experimental design of dynamic PET/MRI studies and data analysis approaches for comparing and quantifying simultaneous PET/MRI data. The primary focus of this review is on functional PET/MRI studies of neurotransmitter and receptor systems, with an emphasis on the dopamine, opioid, serotonin and glutamate systems as molecular neuromodulators. In this context, we provide an overview of studies that employ interventions to alter the activity of neuroreceptors or the release of neurotransmitters. Overall, we emphasize how the synergistic use of simultaneous PET/MRI with appropriate study design and interventions has the potential to expand our knowledge about the molecular and functional dynamics of the living human brain. Finally, we give an outlook on the future opportunities for simultaneous PET/MRI.

Cell adhesion molecule close homolog of L1 binds to the dopamine receptor D2 and inhibits the internalization of its short isoform

Cell adhesion molecule close homolog of L1 (CHL1) and the dopamine receptor D2 (DRD2) are associated with psychiatric and mental disorders. We here show that DRD2 interacts with CHL1 in mouse brain, as evidenced by co-immunostaining, proximity ligation assay, co-immunoprecipitation, and pull-down assay with recombinant extracellular CHL1 domain fused to Fc (CHL1-Fc). Direct binding of CHL1-Fc to the first extracellular loop of DRD2 was shown by ELISA. Using HEK cells transfected to co-express CHL1 and the short (DRD2-S) or long (DRD2-L) DRD2 isoforms, co-localization of CHL1 and both isoforms was observed by immunostaining and proximity ligation assay. Moreover, CHL1 inhibited agonist-triggered internalization of DRD2-S. Proximity ligation assay showed close interaction between CHL1 and DRD2 in neurons expressing dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP32) or tyrosine hydroxylase (TH) in tissue sections of adult mouse striatum. In cultures of striatum or ventral midbrain, CHL1 was also closely associated with DRD2 in DARPP32- or TH-immunopositive cells, respectively. In the dorsal striatum of CHL1-deficient mice, lower levels of DRD2 and phosphorylated TH were observed, when compared to wild-type littermates. In the ventral striatum of CHL1-deficient mice, levels of phosphorylated DARPP32 were reduced. We propose that CHL1 regulates DRD2-dependent presynaptic and postsynaptic functions.

Alterations of cerebral perfusion and functional brain connectivity in medication-naïve male adults with attention-deficit/hyperactivity disorder

Aims

Functional brain abnormalities, including altered cerebral perfusion and functional connectivities, have been illustrated in adults with attention‐deficit/hyperactivity disorder (aADHD). The present study attempted to explore the alterations of cerebral blood flow (CBF) and resting‐state functional connectivity (RSFC) simultaneously to understand the neural mechanisms for adults with ADHD comprehensively.

Methods

Resting‐state arterial spin labeling (ASL) and blood oxygenation level‐dependent (BOLD) magnetic resonance imaging (MRI) data were acquired for 69 male aADHD and 69 matched healthy controls (HCs). The altered CBFs associated with aADHD were explored based on both categorical (aADHD vs HCs) and dimensional (correlation with aADHD core symptoms) perspectives. Then, the seed‐based RSFC analyses were developed for the regions showing significant alterations of CBF.

Results

Significantly decreased CBF in the large‐scale resting‐state networks regions (eg, ventral attentional network, somatomotor network, limbic network) and subcortical regions was indicated in aADHD compared with HCs. The correlation analyses indicated that the hypoperfusion in left putamen/global pallidum and left amygdala/hippocampus was correlated with ADHD inattentive and total symptoms, respectively. Further, weaker negative functional connectivity between left amygdala and bilateral supplementary motor area, bilateral superior frontal gyrus, and left medial frontal gyrus was found in adults with ADHD.

Conclusion

The present findings suggested alterations of both cerebral perfusion and functional connectivity for the left amygdala in aADHD. The combination of CBF and RSFCs may help to interpret the neuropathogenesis of ADHD more comprehensively.

Cerebral blood flow in striatal regions is associated with apathy in patients with schizophrenia

BACKGROUND

Striatal dysfunction has been proposed as a pathomechanism for negative symptoms in schizophrenia. There is consensus that negative symptoms can be grouped into 2 dimensions: apathy and diminished expression. Recent studies suggest that different neural mechanisms underlie these dimensions, but the relationship between regional resting-state cerebral blood flow (rCBF) and negative symptom dimensions has not been investigated.

METHODS

This study included 29 patients with schizophrenia and 20 healthy controls. We measured rCBF in the striatum using arterial spin labelling (ASL) MRI. We assessed negative symptoms using the Brief Negative Symptom Scale.

RESULTS

In the ventral and dorsal striatum, rCBF was not different between patients with schizophrenia and controls. However, we did find a positive association between the severity of apathy and increased rCBF in the ventral and dorsal striatum in patients with schizophrenia. This effect was not present for diminished expression.

LIMITATIONS

All patients were taking atypical antipsychotics, so an effect of antipsychotic medication on rCBF could not be excluded, although we did not find a significant association between rCBF and chlorpromazine equivalents.

CONCLUSION

The main finding of this study was a specific association between increased striatal rCBF and the negative symptom dimension of apathy. Our results further support the separate assessment of apathy and diminished expression when investigating the neural basis of negative symptoms. The ASL technique can provide a direct and quantitative approach to investigating the role of rCBF changes in the pathophysiology of negative symptoms.

Effects of flow changes on radiotracer binding: Simultaneous measurement of neuroreceptor binding and cerebral blood flow modulation

The potential effects of changes in blood flow on the delivery and washout of radiotracers has been an ongoing question in PET bolus injection studies. This study provides practical insight into this topic by experimentally measuring cerebral blood flow (CBF) and neuroreceptor binding using simultaneous PET/MRI. Hypercapnic challenges (7% CO₂) were administered to non-human primates in order to induce controlled increases in CBF, measured with pseudo-continuous arterial spin labeling. Simultaneously, dopamine D₂/D₃ receptor binding of [¹¹C]raclopride or [¹⁸F]fallypride was monitored with dynamic PET. Experiments showed that neither time activity curves nor quantification of binding through binding potentials ( BP_ND) were measurably affected by CBF increases, which were larger than two-fold. Simulations of experimental procedures showed that even large changes in CBF should have little effect on the time activity curves of radiotracers, given a set of realistic assumptions. The proposed method can be applied to experimentally assess the flow sensitivity of other radiotracers. Results demonstrate that CBF changes, which often occur due to behavioral tasks or pharmacological challenges, do not affect PET [¹¹C]raclopride or [¹⁸F]fallypride binding studies and their quantification. The results from this study suggest flow effects may have limited impact on many PET neuroreceptor tracers with similar properties.

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

In vivo biased agonism at 5-HT1A receptors: characterisation by simultaneous PET/MR imaging

In neuropharmacology, the recent concept of 'biased agonism' denotes the capacity of certain agonists to target-specific intracellular pathways of a given receptor in specific brain areas. In the context of serotonin pharmacotherapy, 5-HT_1A receptor-biased agonists could be of great interest in several neuropsychiatric disorders. The aim of this study was to determine whether biased agonists could be differentiated in terms of regional targeting by use of simultaneous functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) brain imaging. We compared two 5-HT_1A-biased agonists, NLX-112 and NLX-101, injected at three different doses in anaesthetised cats (n = 4). PET imaging was acquired for 90 min after bolus administration followed by constant infusion of the 5-HT_1A radiotracer, [¹⁸F]MPPF. Drug occupancy was evaluated after injection at  50 min and BOLD fMRI was simultaneously acquired to evaluate subsequent brain activation patterns. 5-HT_1A receptor occupancy was found to be dose-dependent for both agonists, but differed in magnitude and spatial distribution at equal doses with distinct BOLD patterns. Functional connectivity, as measured by BOLD signal temporal correlations between regions, was also differently modified by NLX-112 or NLX-101. Voxel-based correlation analyses between PET and fMRI suggested that NLX-112 stimulates both 5-HT_1A autoreceptors and post-synaptic receptors, whereas NLX-101 preferentially stimulates post-synaptic cortical receptors. In cingulate cortex, the agonists induced opposite BOLD signal changes in response to receptor occupancy. These data constitute the first simultaneous exploration of 5-HT_1A occupancy and its consequences in terms of brain activation, and demonstrates differential signalling by two 5-HT_1A-biased agonists. Combined PET/fMRI represents a powerful tool in neuropharmacology, and opens new ways to address the concept of biased agonism by translational approaches.

Advances in PET Methodology

Recent advances in disease understanding, instrumentation technology, and computationally demanding image analysis approaches are opening new frontiers in the investigation of movement disorders and brain disease in general. A key aspect is the recognition of the need to determine molecular correlates to early functional and metabolic connectivity alterations, which are increasingly recognized as useful signatures of specific clinical disease phenotypes. Such multi-modal approaches are highly likely to provide new information on pathogenic mechanisms and to help the identification of novel therapeutic targets. This chapter describes recent methodological developments in PET starting with a very brief overview of radiotracers relevant to movement disorders while emphasizing the development of instrumentation, algorithms and imaging analysis methods relevant to multi-modal investigation of movement disorders.

From simultaneous to synergistic MR-PET brain imaging: A review of hybrid MR-PET imaging methodologies

Simultaneous Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) scanning is a recent major development in biomedical imaging. The full integration of the PET detector ring and electronics within the MR system has been a technologically challenging design to develop but provides capacity for simultaneous imaging and the potential for new diagnostic and research capability. This article reviews state-of-the-art MR-PET hardware and software, and discusses future developments focusing on neuroimaging methodologies for MR-PET scanning. We particularly focus on the methodologies that lead to an improved synergy between MRI and PET, including optimal data acquisition, PET attenuation and motion correction, and joint image reconstruction and processing methods based on the underlying complementary and mutual information. We further review the current and potential future applications of simultaneous MR-PET in both systems neuroscience and clinical neuroimaging research. We demonstrate a simultaneous data acquisition protocol to highlight new applications of MR-PET neuroimaging research studies.

PET/MRI Hybrid Systems

Over the last decade, the combination of PET and MRI in one system has proven to be highly successful in basic preclinical research, as well as in clinical research. Nowadays, PET/MRI systems are well established in preclinical imaging and are progressing into clinical applications to provide further insights into specific diseases, therapeutic assessments, and biological pathways. Certain challenges in terms of hardware had to be resolved concurrently with the development of new techniques to be able to reach the full potential of both combined techniques. This review provides an overview of these challenges and describes the opportunities that simultaneous PET/MRI systems can exploit in comparison with stand-alone or other combined hybrid systems. New approaches were developed for simultaneous PET/MRI systems to correct for attenuation of 511 keV photons because MRI does not provide direct information on gamma photon attenuation properties. Furthermore, new algorithms to correct for motion were developed, because MRI can accurately detect motion with high temporal resolution. The additional information gained by the MRI can be employed to correct for partial volume effects as well. The development of new detector designs in combination with fast-decaying scintillator crystal materials enabled time-of-flight detection and incorporation in the reconstruction algorithms. Furthermore, this review lists the currently commercially available systems both for preclinical and clinical imaging and provides an overview of applications in both fields. In this regard, special emphasis has been placed on data analysis and the potential for both modalities to evolve with advanced image analysis tools, such as cluster analysis and machine learning.

The Relationship Between Dopamine Neurotransmitter Dynamics and the Blood-Oxygen-Level-Dependent (BOLD) Signal: A Review of Pharmacological Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (fMRI) is widely used in investigations of normal cognition and brain disease and in various clinical applications. Pharmacological fMRI (pharma-fMRI) is a relatively new application, which is being used to elucidate the effects and mechanisms of pharmacological modulation of brain activity. Characterizing the effects of neuropharmacological agents on regional brain activity using fMRI is challenging because drugs modulate neuronal function in a wide variety of ways, including through receptor agonist, antagonist, and neurotransmitter reuptake blocker events. Here we review current knowledge on neurotransmitter-mediated blood-oxygen-level dependent (BOLD) fMRI mechanisms as well as recently updated methodologies aimed at more fully describing the effects of neuropharmacologic agents on the BOLD signal. We limit our discussion to dopaminergic signaling as a useful lens through which to analyze and interpret neurochemical-mediated changes in the hemodynamic BOLD response. We also discuss the need for future studies that use multi-modal approaches to expand the understanding and application of pharma-fMRI.

Combined PET/MRI: Global Warming-Summary Report of the 6th International Workshop on PET/MRI, March 27-29, 2017, Tübingen, Germany

The 6th annual meeting to address key issues in positron emission tomography (PET)/magnetic resonance imaging (MRI) was held again in Tübingen, Germany, from March 27 to 29, 2017. Over three days of invited plenary lectures, round table discussions and dialogue board deliberations, participants critically assessed the current state of PET/MRI, both clinically and as a research tool, and attempted to chart future directions. The meeting addressed the use of PET/MRI and workflows in oncology, neurosciences, infection, inflammation and chronic pain syndromes, as well as deeper discussions about how best to characterise the tumour microenvironment, optimise the complementary information available from PET and MRI, and how advanced data mining and bioinformatics, as well as information from liquid biomarkers (circulating tumour cells and nucleic acids) and pathology, can be integrated to give a more complete characterisation of disease phenotype. Some issues that have dominated previous meetings, such as the accuracy of MR-based attenuation correction (AC) of the PET scan, were finally put to rest as having been adequately addressed for the majority of clinical situations. Likewise, the ability to standardise PET systems for use in multicentre trials was confirmed, thus removing a perceived barrier to larger clinical imaging trials. The meeting openly questioned whether PET/MRI should, in all cases, be used as a whole-body imaging modality or whether in many circumstances it would best be employed to give an in-depth study of previously identified disease in a single organ or region. The meeting concluded that there is still much work to be done in the integration of data from different fields and in developing a common language for all stakeholders involved. In addition, the participants advocated joint training and education for individuals who engage in routine PET/MRI. It was agreed that PET/MRI can enhance our understanding of normal and disrupted biology, and we are in a position to describe the in vivo nature of disease processes, metabolism, evolution of cancer and the monitoring of response to pharmacological interventions and therapies. As such, PET/MRI is a key to advancing medicine and patient care.

Increased Striatal and Reduced Prefrontal Cerebral Blood Flow in Clinical High Risk for Psychosis

Increased striatal dopaminergic activity and decreased prefrontal functioning have been reported in individuals at clinical high risk (CHR) for psychosis. Abnormal metabolic rate might affect resting-state cerebral blood flow (rCBF) in the respective regions. Here, we examined if striatal and prefrontal rCBF differ between patients with CHR, first-episode psychosis (FEP), chronic schizophrenia-spectrum disorder (SZ) and controls. Two cohorts with a total of 122 participants were included and analyzed separately: 32 patients with SZ and 31 healthy controls (HC) from the University Hospital of Psychiatry, and 59 patients from the Bern Early Recognition and Intervention Center (29 with CHR, 12 with FEP, and 18 clinical controls [CC]). Ultra-high risk criteria were assessed with the Structured Interview for Psychosis-Risk Syndromes, basic symptom criteria with the Schizophrenia Proneness Instrument. rCBF was measured with pseudo-continuous arterial spin labeling 3T-Magnetic Resonance Imaging. Striatal rCBF was significantly increased and prefrontal rCBF significantly decreased in the SZ group compared to HC group and in the CHR and FEP groups compared to CC group. Striatal rCBF correlated significantly with positive symptom scores in SZ and CHR. An inverse correlation between striatal and frontal rCBF was found in controls (HC, CC), but not in patient groups (SZ, FEP, CHR). This is the first study to demonstrate increased neuronal activity within the striatum, but reduced prefrontal activity in patients with CHR, FEP, and SZ compared to the respective controls. Our results indicate that alterations in striatal and prefrontal rCBF are reflecting metabolic abnormalities preceding the onset of frank psychosis.

News and views on in-vivo imaging of neurotransmission using PET and MRI

Molecular neuroimaging with PET is an integrated tool in psychiatry research and drug-development for as long as this modality has been available, in particular for studying neurotransmission and endogenous neurotransmitter release. Pharmacologic, behavioral and other types of challenges are currently applied to induce changes in neurochemical levels that can be inferred through their effects on changes in receptor binding and related outcome measures. Based on the availability of tracers that are sensitive for measuring neurotransmitter release these experiments have focused on the brain's dopamine system, while recent developments have extended those studies to other targets such as the serotonin or choline system. With the introduction of hybrid, truly simultaneous PET/MRI systems, in-vivo imaging of the dynamics of neuroreceptor signal transmission in the brain using PET and functional MRI (fMRI) has become possible. fMRI has the ability to provide information about the effects of receptor function that are complementary to the PET measurement. Dynamic acquisition of both PET and fMRI signals enables not only an in-vivo real-time assessment of neurotransmitter or drug binding to receptors but also dynamic receptor adaptations and receptor-specific neurotransmission. While fMRI temporal resolution is comparatively fast in relation to PET, the timescale of observable biological processes is highly dependent on the kinetics of radiotracers and study design. Overall, the combination of the specificity of PET radiotracers to neuroreceptors, fMRI signal as a functional readout and integrated study design promises to expand our understanding of the location, propagation and connections of brain activity in health and disease.

Principles of Simultaneous PET/MR Imaging

Combined PET/MR imaging scanners capable of acquiring simultaneously the complementary information provided by the 2 imaging modalities are now available for human use. After addressing the hardware challenges for integrating the 2 imaging modalities, most of the efforts in the field have focused on developing MR-based attenuation correction methods for neurologic and whole-body applications, implementing approaches for improving one modality by using the data provided by the other and exploring research and clinical applications that could benefit from the synergistic use of the multimodal data.

Functional Characterization of 5-HT1B Receptor Drugs in Nonhuman Primates Using Simultaneous PET-MR

In the present study, we used a simultaneous PET-MR experimental design to investigate the effects of functionally different compounds (agonist, partial agonist, and antagonist) on 5-HT_1B receptor (5-HT_1BR) occupancy and the associated hemodynamic responses. In anesthetized male nonhuman primates (n = 3), we used positron emission tomography (PET) imaging with the radioligand [¹¹C]AZ10419369 administered as a bolus followed by constant infusion to measure changes in 5-HT_1BR occupancy. Simultaneously, we measured changes in cerebral blood volume (CBV) as a proxy of drug effects on neuronal activity. The 5-HT_1BR partial agonist AZ10419369 elicited a dose-dependent biphasic hemodynamic response that was related to the 5-HT_1BR occupancy. The magnitude of the response was spatially overlapping with high cerebral 5-HT_1BR densities. High doses of AZ10419369 exerted an extracranial tissue vasoconstriction that was comparable to the less blood-brain barrier-permeable 5-HT_1BR agonist sumatriptan. By contrast, injection of the antagonist GR127935 did not elicit significant hemodynamic responses, even at a 5-HT_1BR cerebral occupancy similar to the one obtained with a high dose of AZ10419369. Given the knowledge we have of the 5-HT_1BR and its function and distribution in the brain, the hemodynamic response informs us about the functionality of the given drug: changes in CBV are only produced when the receptor is stimulated by the partial agonist AZ10419369 and not by the antagonist GR127935, consistent with low basal occupancy by endogenous serotonin.SIGNIFICANCE STATEMENT We here show that combined simultaneous positron emission tomography and magnetic resonance imaging uniquely enables the assessment of CNS active compounds. We conducted a series of pharmacological interventions to interrogate 5-HT_1B receptor binding and function and determined blood-brain barrier passage of drugs and demonstrate target involvement. Importantly, we show how the spatial and temporal effects on brain hemodynamics provide information about pharmacologically driven downstream CNS drug effects; the brain hemodynamic response shows characteristic dose-related effects that differ depending on agonistic or antagonistic drug characteristics and on local 5-HT_1B receptor density. The technique lends itself to a comprehensive in vivo investigation and understanding of drugs' effects in the brain.

Visualization of ligand-induced dopamine D2S and D2L receptor internalization by TIRF microscopy

G protein-coupled receptors (GPCRs), including the dopamine receptors, represent a group of important pharmacological targets. Upon agonist binding, GPCRs frequently undergo internalization, a process that is known to attenuate functional responses upon prolonged exposure to agonists. In this study, internalization was visualized by means of total internal reflection fluorescence (TIRF) microscopy at a level of discrete single events near the plasma membrane with high spatial resolution. A novel method has been developed to determine the relative extent of internalized fluorescent receptor-ligand complexes by comparative fluorescence quantification in living CHO cells. The procedure entails treatment with the reducing agent sodium borohydride, which converts cyanine-based fluorescent ligands on the membrane surface to a long-lived reduced form. Because the highly polar reducing agent is not able to pass the cell membrane, the fluorescent receptor-ligand complexes located in internalized compartments remain fluorescent under TIRF illumination. We applied the method to investigate differences of the short (D_2S) and the long (D_2L) isoforms of dopamine D₂ receptors in their ability to undergo agonist-induced internalization.

Neurotransmitters: The Critical Modulators Regulating Gut-Brain Axis

Neurotransmitters, including catecholamines and serotonin, play a crucial role in maintaining homeostasis in the human body. Studies on these neurotransmitters mainly revolved around their role in the "fight or flight" response, transmitting signals across a chemical synapse and modulating blood flow throughout the body. However, recent research has demonstrated that neurotransmitters can play a significant role in the gastrointestinal (GI) physiology. Norepinephrine (NE), epinephrine (E), dopamine (DA), and serotonin have recently been a topic of interest because of their roles in the gut physiology and their potential roles in GI and central nervous system pathophysiology. These neurotransmitters are able to regulate and control not only blood flow, but also affect gut motility, nutrient absorption, GI innate immune system, and the microbiome. Furthermore, in pathological states, such as inflammatory bowel disease (IBD) and Parkinson's disease, the levels of these neurotransmitters are dysregulated, therefore causing a variety of GI symptoms. Research in this field has shown that exogenous manipulation of catecholamine serum concentrations can help in decreasing symptomology and/or disease progression. In this review article, we discuss the current state-of-the-art research and literature regarding the role of neurotransmitters in regulation of normal GI physiology, their impact on several disease processes, and novel work focused on the use of exogenous hormones and/or psychotropic medications to improve disease symptomology. J. Cell. Physiol. 232: 2359-2372, 2017. © 2016 Wiley Periodicals, Inc.

Technical and instrumentational foundations of PET/MRI

This paper highlights the origins of combined positron emission tomography (PET) and magnetic resonance imaging (MRI) whole-body systems that were first introduced for applications in humans in 2010. This text first covers basic aspects of each imaging modality before describing the technical and methodological challenges of combining PET and MRI within a single system. After several years of development, combined and even fully-integrated PET/MRI systems have become available and made their way into the clinic. This multi-modality imaging system lends itself to the advanced exploration of diseases to support personalized medicine in a long run. To that extent, this paper provides an introduction to PET/MRI methodology and important technical solutions.

The role of beta-arrestin2 in shaping fMRI BOLD responses to dopaminergic stimulation

RATIONALE

The dopamine D₂ receptor (D₂R) couples to inhibitory G_i/o proteins and is targeted by antipsychotic and antiparkinsonian drugs. Beta-arrestin2 binds to the intracellular regions of the agonist-occupied D₂R to terminate G protein activation and promote internalization, but also to initiate downstream signaling cascades which have been implicated in psychosis. Functional magnetic resonance imaging (fMRI) has proven valuable for measuring dopamine receptor-mediated changes in neuronal activity, and might enable beta-arrestin2 function to be studied in vivo.

OBJECTIVES

The present study examined fMRI blood oxygenation level dependent (BOLD) signal changes elicited by a dopamine agonist in wild-type (WT) and beta-arrestin2 knockout (KO) mice, to investigate whether genetic deletion of beta-arrestin2 prolongs or otherwise modifies D₂R-dependent responses.

METHODS

fMRI BOLD data were acquired on a 9.4 T system. During scans, animals received 0.2 mg/kg apomorphine, i.v. In a subset of experiments, animals were pretreated with 2 mg/kg of the D₂R antagonist, eticlopride.

RESULTS

Following apomorphine administration, BOLD signal decreases were observed in caudate/putamen of WT and KO animals. The time course of response decay in caudate/putamen was significantly slower in KO vs. WT animals. In cingulate cortex, an initial BOLD signal decrease was followed by a positive response component in WT but not in KO animals. Eticlopride pretreatment significantly reduced apomorphine-induced BOLD signal changes.

CONCLUSIONS

The prolonged striatal response decay rates in KO animals might reflect impaired D₂R desensitization, consistent with the known function of beta-arrestin2. Furthermore, the apomorphine-induced positive response component in cingulate cortex may depend on beta-arrestin2 signaling downstream of D₂R.

Combined PET/MRI: from Status Quo to Status Go. Summary Report of the Fifth International Workshop on PET/MR Imaging; February 15-19, 2016; Tübingen, Germany

This article provides a collaborative perspective of the discussions and conclusions from the fifth international workshop of combined positron emission tomorgraphy (PET)/magnetic resonance imaging (MRI) that was held in Tübingen, Germany, from February 15 to 19, 2016. Specifically, we summarise the second part of the workshop made up of invited presentations from active researchers in the field of PET/MRI and associated fields augmented by round table discussions and dialogue boards with specific topics. This year, this included practical advice as to possible approaches to moving PET/MRI into clinical routine, the use of PET/MRI in brain receptor imaging, in assessing cardiovascular diseases, cancer, infection, and inflammatory diseases. To address perceived challenges still remaining to innovatively integrate PET and MRI system technologies, a dedicated round table session brought together key representatives from industry and academia who were engaged with either the conceptualisation or early adoption of hybrid PET/MRI systems. Discussions during the workshop highlighted that emerging unique applications of PET/MRI such as the ability to provide multi-parametric quantitative and visual information which will enable not only overall disease detection but also disease characterisation would eventually be regarded as compelling arguments for the adoption of PET/MR. However, as indicated by previous workshops, evidence in favour of this observation is only growing slowly, mainly due to the ongoing inability to pool data cohorts from independent trials as well as different systems and sites. The participants emphasised that moving from status quo to status go entails the need to adopt standardised imaging procedures and the readiness to act together prospectively across multiple PET/MRI sites and vendors.

A regularized full reference tissue model for PET neuroreceptor mapping

The full reference tissue model (FRTM) is a PET analysis framework that includes both free and specifically bound compartments within tissues, together with rate constants defining association and dissociation from the specifically bound compartment. The simplified reference tissue model (SRTM) assumes instantaneous exchange between tissue compartments, and this "1-tissue" approximation reduces the number of parameters and enables more robust mapping of non-displaceable binding potentials. Simulations based upon FRTM have shown that SRTM exhibits biases that are spatially dependent, because biases depend upon binding potentials. In this work, we describe a regularized model (rFRTM) that employs a global estimate of the dissociation rate constant from the specifically bound compartment (k₄). The model provides an internal calibration for optimizing k₄ through the reference-region outflow rate k₂', a model parameter that should be a global constant but varies regionally in SRTM. Estimates of k₄ by rFRTM are presented for four PET radioligands. We show that SRTM introduces bias in parameter estimates by assuming an infinite value for k₄, and that rFRTM ameliorates bias with an appropriate choice of k₄. Theoretical considerations and simulations demonstrate that rFRTM reduces bias in non-displaceable binding potentials. A two-parameter reduction of the model (rFRTM2) provides robust mapping at a voxel-wise level. With a structure similar to SRTM, the model is easily implemented and can be applied as a PET reference region analysis that reduces parameter bias without substantially altering parameter variance.

Dopamine D2 receptor gene variants and response to rasagiline in early Parkinson's disease: a pharmacogenetic study

The treatment of early Parkinson's disease with dopaminergic agents remains the mainstay of symptomatic therapy for this incurable neurodegenerative disorder. However, clinical responses to dopaminergic drugs vary substantially from person to person due to individual-, drug- and disease-related factors that may in part be genetically determined. Using clinical data and DNA samples ascertained through the largest placebo-controlled clinical trial of the monoamine oxidase B inhibitor, rasagiline (ClinicalTrials.gov number, NCT00256204), we examined how polymorphisms in candidate genes associate with the clinical response to rasagiline in early Parkinson's disease. Variants in genes that express proteins involved in the pharmacokinetics and pharmacodynamics of rasagiline, and genes previously associated with the risk to develop Parkinson's disease were genotyped. The LifeTechnologies OpenArray NT genotyping platform and polymerase chain reaction-based methods were used to analyse 204 single nucleotide polymorphisms and five variable number tandem repeats from 30 candidate genes in 692 available DNA samples from this clinical trial. The peak symptomatic response to rasagiline, the rate of symptom progression, and their relation to genetic variation were examined controlling for placebo effects using general linear and mixed effects models, respectively. Single nucleotide polymorphisms, rs2283265 and rs1076560, in the dopamine D2 receptor gene (DRD2) were found to be significantly associated with a favourable peak response to rasagiline at 12 weeks in early Parkinson's disease after controlling for multiple testing. From a linear regression, the betas were 2.5 and 2.38, respectively, with false discovery rate-corrected P-values of 0.032. These polymorphisms were in high linkage disequilibrium with each other (r(2) = 0.96) meaning that the same clinical response signal was identified by each of them. No polymorphisms were associated with slowing the rate of worsening in Parkinson symptoms from Weeks 12 to 36 after correction for multiple testing. This is the largest and most comprehensive pharmacogenetics study to date examining clinical response to an anti-parkinsonian drug and the first to be conducted in patients with early stage Parkinson's disease receiving monotherapy. The results indicate a clinically meaningful benefit to rasagiline in terms of the magnitude of improvement in parkinsonian symptoms for those with the favourable response genotypes. Future work is needed to elucidate the specific mechanisms through which these DRD2 variants operate in modulating the function of the nigrostriatal dopaminergic system.media-1vid110.1093/brain/aww109_video_abstractaww109_video_abstract.