Application of cross-species PET imaging to assess neurotransmitter release in brain

Restoring consciousness with pharmacologic therapy: Mechanisms, targets, and future directions

Severe brain injury impairs consciousness by disrupting a broad spectrum of neurotransmitter systems. Emerging evidence suggests that pharmacologic modulation of specific neurotransmitter systems, such as dopamine, promotes recovery of consciousness. Clinical guidelines now endorse the use of amantadine in individuals with traumatic disorders of consciousness (DoC) based on level 1 evidence, and multiple neurostimulants are used off-label in clinical practice, including methylphenidate, modafinil, bromocriptine, levodopa, and zolpidem. However, the relative contributions of monoaminergic, glutamatergic, cholinergic, GABAergic, and orexinergic neurotransmitter systems to recovery of consciousness after severe brain injury are unknown, and personalized approaches to targeted therapy have yet to be developed. This review summarizes the state-of-the-science in the neurochemistry and neurobiology of neurotransmitter systems involved in conscious behaviors, followed by a discussion of how pharmacologic therapies may be used to modulate these neurotransmitter systems and promote recovery of consciousness. We consider pharmacologic modulation of consciousness at the synapse, circuit, and network levels, with a focus on the mesocircuit model that has been proposed to explain the consciousness-promoting effects of various monoaminergic, glutamatergic, and paradoxically, GABAergic therapies. Though fundamental questions remain about neurotransmitter mechanisms, target engagement and optimal therapy selection for individual patients, we propose that pharmacologic therapies hold great promise to promote recovery and improve quality of life for patients with severe brain injuries.

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge

BACKGROUND

The serotonin hypothesis of depression proposes that diminished serotonergic (5-HT) neurotransmission is causal in the pathophysiology of the disorder. Although the hypothesis is over 50 years old, there is no firm in vivo evidence for diminished 5-HT neurotransmission. We recently demonstrated that the 5-HT_2A receptor agonist positron emission tomography (PET) radioligand [¹¹C]Cimbi-36 is sensitive to increases in extracellular 5-HT induced by an acute d-amphetamine challenge. Here we applied [¹¹C]Cimbi-36 PET to compare brain 5-HT release capacity in patients experiencing a major depressive episode (MDE) to that of healthy control subjects (HCs) without depression.

METHODS

Seventeen antidepressant-free patients with MDE (3 female/14 male, mean age 44 ± 13 years, Hamilton Depression Rating Scale score 21 ± 4 [range 16-30]) and 20 HCs (3 female/17 male, mean age 32 ± 9 years) underwent 90-minute dynamic [¹¹C]Cimbi-36 PET before and 3 hours after a 0.5-mg/kg oral dose of d-amphetamine. Frontal cortex (main region of interest) 5-HT_2A receptor nondisplaceable binding was calculated from kinetic analysis using the multilinear analysis-1 approach with the cerebellum as the reference region.

RESULTS

Following d-amphetamine administration, frontal nondisplaceable binding potential (BP_ND) was significantly reduced in the HC group (1.04 ± 0.31 vs. 0.87 ± 0.24, p < .001) but not in the MDE group (0.97 ± 0.25 vs. 0.92 ± 0.22, not significant). ΔBP_ND of the MDE group was significantly lower than that of the HC group (HC: 15% ± 14% vs. MDE: 6.5% ± 20%, p = .041).

CONCLUSIONS

This first direct assessment of 5-HT release capacity in people with depression provides clear evidence for dysfunctional serotonergic neurotransmission in depression by demonstrating reduced 5-HT release capacity in patients experiencing an MDE.

Letting the little light of mind shine: Advances and future directions in neurochemical detection

Synaptic transmission via neurochemical release is the fundamental process that integrates and relays encoded information in the brain to regulate physiological function, cognition, and emotion. To unravel the biochemical, biophysical, and computational mechanisms of signal processing, one needs to precisely measure the neurochemical release dynamics with molecular and cell-type specificity and high resolution. Here we reviewed the development of analytical, electrochemical, and fluorescence imaging approaches to detect neurotransmitter and neuromodulator release. We discussed the advantages and practicality in implementation of each technology for ease-of-use, flexibility for multimodal studies, and challenges for future optimization. We hope this review will provide a versatile guide for tool engineering and applications for recording neurochemical release.

PET as a Translational Tool in Drug Development for Neuroscience Compounds

In central nervous system drug discovery programs, early development of new chemical entities (NCEs) requires a multidisciplinary strategy and a translational approach to obtain proof of distribution, proof of occupancy, and proof of function in specific brain circuits. Positron emission tomography (PET) provides a way to assess in vivo the brain distribution of NCEs and their binding to the target of interest, provided that radiolabeling of the NCE is possible or that a suitable radioligand is available. PET is therefore a key tool for early phases of drug discovery programs. This review will summarize the main applications of PET in early drug development and discuss the usefulness of PET microdosing studies performed with direct labelling of the NCE and PET occupancy studies. The purpose of this review is also to propose an alignment of the nomenclatures used by drug metabolism and pharmacokinetic scientists and PET imaging scientists to indicate key pharmacokinetic parameters and to provide guidance in the performance and interpretation of PET studies.

Antidepressant efficacy is correlated with plasma levels: mega-analysis and further evidence

The debate around optimal target dose for first-line antidepressants (ADs) is still ongoing. Along this line, therapeutic drug monitoring (TDM) represents one of the most promising tools to improve clinical outcome. Nevertheless, a few data exist regarding the concentration-effect relationship of first-line ADs which limits TDM implementation in routine clinical practice. We conducted the first patient-level concentration-response mega-analysis including data acquired by us previously and explored the concentration dependency of first-line AD (206 subjects). Further, new data on mirtazapine are reported (18 subjects). Hamilton Depression Rating Scale-21 administered at baseline, at month 1 and month 3 was used as the measure of efficacy to assess antidepressant response (AR). When pooling all four first-line ADs together, normalized plasma levels and AR significantly fit a bell-shaped quadratic function with a progressive increase of AR up to around the upper normalized limit of the therapeutic reference range with a decrease of AR at higher serum levels. Our results complement the available evidence on the issue and the recent insights gained from dose-response studies. A concentration-dependent clinical efficacy, such as previously demonstrated for tricyclic compounds, also emerge for first-line ADs. Our study supports a role for TDM as a tool to optimize AD treatment to obtain maximum benefit.

The Synaptic Gene Study: Design and Methodology to Identify Neurocognitive Markers in Phelan-McDermid Syndrome and NRXN1 Deletions

Synaptic gene conditions, i.e., “synaptopathies,” involve disruption to genes expressed at the synapse and account for between 0.5 and 2% of autism cases. They provide a unique entry point to understanding the molecular and biological mechanisms underpinning autism-related phenotypes. Phelan-McDermid Syndrome (PMS, also known as 22q13 deletion syndrome) and NRXN1 deletions (NRXN1ds) are two synaptopathies associated with autism and related neurodevelopmental disorders (NDDs). PMS often incorporates disruption to the SHANK3 gene, implicated in excitatory postsynaptic scaffolding, whereas the NRXN1 gene encodes neurexin-1, a presynaptic cell adhesion protein; both are implicated in trans-synaptic signaling in the brain. Around 70% of individuals with PMS and 43–70% of those with NRXN1ds receive a diagnosis of autism, suggesting that alterations in synaptic development may play a crucial role in explaining the aetiology of autism. However, a substantial amount of heterogeneity exists between conditions. Most individuals with PMS have moderate to profound intellectual disability (ID), while those with NRXN1ds have no ID to severe ID. Speech abnormalities are common to both, although appear more severe in PMS. Very little is currently known about the neurocognitive underpinnings of phenotypic presentations in PMS and NRXN1ds. The Synaptic Gene (SynaG) study adopts a gene-first approach and comprehensively assesses these two syndromic forms of autism. The study compliments preclinical efforts within AIMS-2-TRIALS focused on SHANK3 and NRXN1. The aims of the study are to (1) establish the frequency of autism diagnosis and features in individuals with PMS and NRXN1ds, (2) to compare the clinical profile of PMS, NRXN1ds, and individuals with ‘idiopathic’ autism (iASD), (3) to identify mechanistic biomarkers that may account for autistic features and/or heterogeneity in clinical profiles, and (4) investigate the impact of second or multiple genetic hits on heterogeneity in clinical profiles. In the current paper we describe our methodology for phenotyping the sample and our planned comparisons, with information on the necessary adaptations made during the global COVID-19 pandemic. We also describe the demographics of the data collected thus far, including 25 PMS, 36 NRXN1ds, 33 iASD, and 52 NTD participants, and present an interim analysis of autistic features and adaptive functioning.

Carbon-Based Electrochemical Sensors for In Vivo and In Vitro Neurotransmitter Detection

As essential neurological chemical messengers, neurotransmitters play an integral role in the maintenance of normal mammalian physiology. Aberrant neurotransmitter activity is associated with a range of neurological conditions including Parkinson's disease, Alzheimer's disease, and Huntington's disease. Many studies to date have tested different approaches to detecting neurotransmitters, yet the detection of these materials within the brain, due to the complex environment of the brain and the rapid metabolism of neurotransmitters, remains challenging and an area of active research. There is a clear need for the development of novel neurotransmitter sensing technologies capable of rapidly and sensitively monitoring specific analytes within the brain without adversely impacting the local microenvironment in which they are implanted. Owing to their excellent sensitivity, portability, ease-of-use, amenability to microprocessing, and low cost, electrochemical sensors methods have been widely studied in the context of neurotransmitter monitoring. The present review, thus, surveys current progress in this research field, discussing developed electrochemical neurotransmitter sensors capable of detecting dopamine (DA), serotonin (5-HT), acetylcholine (Ach), glutamate (Glu), nitric oxide (NO), adenosine (ADO), and so on. Of these technologies, those based on carbon nanostructures-modified electrodes including carbon nanotubes (CNTs), graphene (GR), gaphdiyne (GDY), carbon nanofibers (CNFs), and derivatives thereof hold particular promise owing to their excellent biocompatibility and electrocatalytic performance. The continued development of these and related technologies is, thus, likely to lead to major advances in the clinical diagnosis of neurological diseases and the detection of novel biomarkers thereof.

Multi-scale neural decoding and analysis

Objective.

Complex spatiotemporal neural activity encodes rich information related to behavior and cognition. Conventional research has focused on neural activity acquired using one of many different measurement modalities, each of which provides useful but incomplete assessment of the neural code. Multi-modal techniques can overcome tradeoffs in the spatial and temporal resolution of a single modality to reveal deeper and more comprehensive understanding of system-level neural mechanisms. Uncovering multi-scale dynamics is essential for a mechanistic understanding of brain function and for harnessing neuroscientific insights to develop more effective clinical treatment.

Approach.

We discuss conventional methodologies used for characterizing neural activity at different scales and review contemporary examples of how these approaches have been combined. Then we present our case for integrating activity across multiple scales to benefit from the combined strengths of each approach and elucidate a more holistic understanding of neural processes.

Main results.

We examine various combinations of neural activity at different scales and analytical techniques that can be used to integrate or illuminate information across scales, as well the technologies that enable such exciting studies. We conclude with challenges facing future multi-scale studies, and a discussion of the power and potential of these approaches.

Significance.

This roadmap will lead the readers toward a broad range of multi-scale neural decoding techniques and their benefits over single-modality analyses. This Review article highlights the importance of multi-scale analyses for systematically interrogating complex spatiotemporal mechanisms underlying cognition and behavior.

Serotonin transporter availability increases in patients recovering from a depressive episode

Molecular imaging studies have shown low cerebral concentration of serotonin transporter in patients suffering from depression, compared to healthy control subjects. Whether or not this difference also is present before disease onset and after remission (i.e. a trait), or only at the time of the depressive episode (i.e. a state) remains to be explored. We examined 17 patients with major depressive disorder with positron emission tomography using [¹¹C]MADAM, a radioligand that binds to the serotonin transporter, before and after treatment with internet-based cognitive behavioral therapy. In all, 17 matched healthy control subjects were examined once. Cerebellum was used as reference to calculate the binding potential. Differences before and after treatment, as well as between patients and controls, were assessed in a composite cerebral region and in the median raphe nuclei. All image analyses and confirmatory statistical tests were preregistered. Depression severity decreased following treatment (p < 0.001). [¹¹C]MADAM binding in patients increased in the composite region after treatment (p = 0.01), while no change was observed in the median raphe (p = 0.51). No significant difference between patients at baseline and healthy controls were observed in the composite region (p = 0.97) or the median raphe (p = 0.95). Our main finding was that patients suffering from a depressive episode show an overall increase in cerebral serotonin transporter availability as symptoms are alleviated. Our results suggest that previously reported cross-sectional molecular imaging findings of the serotonin transporter in depression most likely reflect the depressive state, rather than a permanent trait. The finding adds new information on the pathophysiology of major depressive disorder.

Recent Advances in In Vivo Neurochemical Monitoring

The brain is a complex network that accounts for only 5% of human mass but consumes 20% of our energy. Uncovering the mysteries of the brain's functions in motion, memory, learning, behavior, and mental health remains a hot but challenging topic. Neurochemicals in the brain, such as neurotransmitters, neuromodulators, gliotransmitters, hormones, and metabolism substrates and products, play vital roles in mediating and modulating normal brain function, and their abnormal release or imbalanced concentrations can cause various diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease. A wide range of techniques have been used to probe the concentrations of neurochemicals under normal, stimulated, diseased, and drug-induced conditions in order to understand the neurochemistry of drug mechanisms and develop diagnostic tools or therapies. Recent advancements in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. However, direct in vivo measurements require a robust sensor that is highly sensitive and selective with minimal fouling and reduced inflammatory foreign body responses. Here, we review recent advances in neurochemical sensor development for in vivo studies, with a focus on electrochemical and optical probes. Other alternative methods are also compared. We discuss in detail the in vivo challenges for these methods and provide an outlook for future directions.

Age-related GABAergic differences in the primary sensorimotor cortex: A multimodal approach combining PET, MRS and TMS

Healthy aging is associated with mechanistic changes in gamma-aminobutyric acid (GABA), the most abundant inhibitory neurotransmitter in the human brain. While previous work mainly focused on magnetic resonance spectroscopy (MRS)-based GABA+ levels and transcranial magnetic stimulation (TMS)-based GABA_A receptor (GABA_AR) activity in the primary sensorimotor (SM1) cortex, the aim of the current study was to identify age-related differences in positron emission tomography (PET)-based GABA_AR availability and its relationship with GABA+ levels (i.e. GABA with the contribution of macromolecules) and GABA_AR activity. For this purpose, fifteen young (aged 20–28 years) and fifteen older (aged 65–80 years) participants were recruited. PET and MRS images were acquired using simultaneous time-of-flight PET/MR to evaluate age-related differences in GABA_AR availability (distribution volume ratio with pons as reference region) and GABA+ levels. TMS was applied to identify age-related differences in GABA_AR activity by measuring short-interval intracortical inhibition (SICI). Whereas GABA_AR availability was significantly higher in the SM cortex of older as compared to young adults (18.5%), there were neither age-related differences in GABA+ levels nor SICI. A correlation analysis revealed no significant associations between GABA_AR availability, GABA_AR activity and GABA+ levels. Although the exact mechanisms need to be further elucidated, it is possible that a higher GABA_AR availability in older adults is a compensatory mechanism to ensure optimal inhibitory functionality during the aging process.

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.

Inverse changes in raphe and cortical 5-HT1B receptor availability after acute tryptophan depletion in healthy human subjects

Serotonergic neurotransmission plays a key role in the pathophysiology and treatment of various neuropsychiatric diseases. The purpose of this study was to investigate changes in serotonergic neurotransmission after acute tryptophan depletion (ATD) using positron emission tomography (PET) with [¹¹ C]P943, a 5-HT_1B receptor radioligand previously shown to be sensitive to changes in 5-HT. Five healthy subjects were scanned on a high resolution PET scanner twice on the same day, before and approximately 5 hours after ingesting capsules containing an amino acid mixture that lacks tryptophan. For each scan, emission data were acquired for 120 min after intravenous bolus injection of [¹¹ C]P943. Binding potential (BP_ND ) values were estimated from parametric images using the second version of the multilinear reference tissue model (MRTM2, t* = 20 min) with cerebellar grey matter used as a reference region. The change in [¹¹ C]P943 binding (ΔBP_ND , %) was calculated as (BP_ND,post  - BP_ND,pre )/(BP_ND,pre ) × 100, and correlation analysis was performed to measure linear associations of ΔBP_ND between raphe and other regions of interest (ROIs). ΔBP_ND ranged from -6% to 45% in the raphe, with positive values indicating reduced competition from 5-HT. In cortical regions, ΔBP_ND ranged from -28% to 7%. While these changes did not reach significance, there were significant negative correlations of ΔBP_ND of the raphe with those of cerebral cortical regions and the thalamus (e.g., r = -.96, p = .011 for average cortex). These findings support the hypothesis that raphe serotonin is a critical modulator of cortical serotonin release via projecting neurons in healthy human subjects.

Serotonin release measured in the human brain: a PET study with [11C]CIMBI-36 and d-amphetamine challenge

Positron emission tomography (PET) enables non-invasive estimation of neurotransmitter fluctuations in the living human brain. While these methods have been applied to dopamine and some other transmitters, estimation of 5-hydroxytryptamine (5-HT; Serotonin) release has proved to be challenging. Here we demonstrate the utility of the novel 5-HT2A receptor agonist radioligand, [¹¹C]CIMBI-36, and a d-amphetamine challenge to evaluate synaptic 5-HT changes in the living human brain. Seventeen healthy male volunteers received [¹¹C]CIMBI-36 PET scans before and 3 h after an oral dose of d-amphetamine (0.5 mg/kg). Dynamic PET data were acquired over 90 min, and the total volume of distribution (V_T) in the frontal cortex and the cerebellum derived from a kinetic analysis using MA1. The frontal cortex binding potential (BP_ND^frontal) was calculated as (V_T^frontal/V_T^cerebellum) - 1. ∆BP_ND^frontal = 1 - (BP_ND^frontal post-dose/BP_ND^frontal baseline) was used as an index of 5-HT release. Statistical inference was tested by means of a paired Students t-test evaluating a reduction in post-amphetamine [¹¹C]CIMBI-36 BP_ND^frontal. Following d-amphetamine administration, [¹¹C]CIMBI-36 BP_ND^frontal was reduced by 14 ± 13% (p = 0.002). Similar effects were observed in other cortical regions examined in an exploratory analysis. [¹¹C]CIMBI-36 binding is sensitive to synaptic serotonin release in the human brain, and when combined with a d-amphetamine challenge, the evaluation of the human brain serotonin system in neuropsychiatric disorders, such as major depression and Parkinson's disease is enabled.

[PET imaging for better understanding of normal and pathological neurotransmission]

Positron emission tomography imaging is still an expanding field of preclinical and clinical investigations exploring the brain and its normal and pathological functions. In addition to technological improvements in PET scanners, the availability of suitable radiotracers for unexplored pharmacological targets is a key factor in this expansion. Many radiotracers (or radiopharmaceuticals, when administered to humans) have been developed by multidisciplinary teams to visualize and quantify a growing numbers of brain receptors, transporters, enzymes and other targets. The development of new PET radiotracers still represents an exciting challenge, given the large number of neurochemical functions that remain to be explored. In this article, we review the development context of the first preclinical radiotracers and their passage to humans. The main current contributions of PET radiotracers are described in terms of imaging neuronal metabolism, quantification of receptors and transporters, neurodegenerative and neuroinflammatory imaging. The different approaches to functional imaging of neurotransmission are also discussed. Finally, the contributions of PET imaging to the research and development of new brain drugs are described.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

A Review of Neurotransmitters Sensing Methods for Neuro-Engineering Research

Neurotransmitters as electrochemical signaling molecules are essential for proper brain function and their dysfunction is involved in several mental disorders. Therefore, the accurate detection and monitoring of these substances are crucial in brain studies. Neurotransmitters are present in the nervous system at very low concentrations, and they mixed with many other biochemical molecules and minerals, thus making their selective detection and measurement difficult. Although numerous techniques to do so have been proposed in the literature, neurotransmitter monitoring in the brain is still a challenge and the subject of ongoing research. This article reviews the current advances and trends in neurotransmitters detection techniques, including in vivo sampling and imaging techniques, electrochemical and nano-object sensing techniques for in vitro and in vivo detection, as well as spectrometric, analytical and derivatization-based methods mainly used for in vitro research. The document analyzes the strengths and weaknesses of each method, with the aim to offer selection guidelines for neuro-engineering research.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Effect of clinically relevant doses of vortioxetine and citalopram on serotonergic PET markers in the nonhuman primate brain

Vortioxetine is a multimodal antidepressant approved for treatment of major depressive disorder. Preclinical studies have demonstrated that the mechanism of action of vortioxetine might be different from selective serotonin reuptake inhibitors (SSRIs), including larger serotonin (5-HT) release and direct modulation of several 5-HT receptors. In the current positron emission tomography (PET) study, we evaluated the mechanism of action of vortioxetine by comparing its effect to the SSRI citalopram on the binding of [¹¹C]AZ10419369 to the 5-HT_1B receptor in the nonhuman primate brain. Initially, the 5-HT transporter (5-HTT) binding of vortioxetine was determined by [¹¹C]MADAM PET measurements before and after administration of vortioxetine (0.1-3.0 mg/kg) and data were used to confirm clinically relevant dosing in subsequent PET measurements with [¹¹C]AZ10419369. The 5-HT_1B receptor binding was significantly decreased after 0.3 mg/kg of citalopram in the dorsal raphe nucleus (5%), as well as after 0.3 mg/kg of vortioxetine in six brain regions (~25%) or 1.0 mg/kg of vortioxetine in all 12 examined regions (~48%). Moreover, there was no effect of 1.0 mg/kg of vortioxetine on the binding of [¹¹C]Cimbi-36 to the 5-HT_2A receptor, which has comparable sensitivity to 5-HT release as [¹¹C]AZ10419369 binding. In conclusion, at clinically relevant doses, vortioxetine induced larger reductions in [¹¹C]AZ10419369 binding than citalopram. These observations suggest that vortioxetine binds to the 5-HT_1B receptor at clinically relevant doses. Future studies are warranted to evaluate the role of the 5-HT_1B receptor in the therapeutic effects of vortioxetine and as a potential target for the development of novel antidepressant drugs.

Catecholamines: Knowledge and understanding in the 1960s, now, and in the future

The late 1960s was a heyday for catecholamine research. Technological developments made it feasible to study the regulation of sympathetic neuronal transmission and to map the distribution of noradrenaline and dopamine in the brain. At last, it was possible to explain the mechanism of action of some important drugs that had been used in the clinic for more than a decade (e.g. the first generation of antidepressants) and to contemplate the rational development of new treatments (e.g. l-dihydroxyphenylalanine therapy, to compensate for the dopaminergic neuropathy in Parkinson's disease, and β1-adrenoceptor antagonists as antihypertensives). The fact that drug targeting noradrenergic and/or dopaminergic transmission are still the first-line treatments for many psychiatric disorders (e.g. depression, schizophrenia, and attention deficit hyperactivity disorder) is a testament to the importance of these neurotransmitters and the research that has helped us to understand the regulation of their function. This article celebrates some of the highlights of research at that time, pays tribute to some of the subsequent landmark studies, and appraises the options for where it could go next.

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 Chemical Monitoring at High Spatiotemporal Resolution Using Microfabricated Sampling Probes and Droplet-Based Microfluidics Coupled to Mass Spectrometry

An essential approach for in vivo chemical monitoring is to use sampling probes coupled with analytical methods; however, this method traditionally has limited spatial and temporal resolution. To address this problem, we developed an analytical system that combines microfabricated push-pull sampling probes with droplet-based microfluidics. The microfabricated probe provides spatial resolution approximately 1000-fold better than that of common microdialysis probes. Microfabrication also facilitated integration of an extra channel into the probe for microinjection. We created microfluidic devices and interfaces that allowed manipulation of nanoliter droplet samples collected from the microfabricated probe at intervals of a few seconds. Use of droplet-based microfluidics prevented broadening of collected zones, yielding 6 s temporal resolution at 100 nL/min perfusion rates. Resulting droplets were analyzed by direct infusion nanoelectrospray ionization (nESI) mass spectrometry for simultaneous determination of glutamine, glutamate, γ-aminobutyric acid, and acetylcholine. Use of low infusion rates that enabled nESI (50 nL/min) was critical to allowing detection in the complex samples. Addition of ¹³C-labeled internal standards to the droplet samples was used for improved quantification. Utility of the overall system was demonstrated by monitoring dynamic chemical changes evoked by microinjection of high potassium concentrations into the brain of live rats. The results showed stimulated neurochemical release with rise times of 15 s. This work demonstrates the potential of coupling microfabricated sampling probes to droplet-based mass spectrometric assays for studying chemical dynamics in a complex microenvironment at high spatiotemporal resolution.

Positron emission tomography imaging of the γ-aminobutyric acid system

In this review, we summarize the recent development of positron emission tomography (PET) radioligands for γ-aminobutyric acid A (GABA_A) receptors and their potential to measure changes in endogenous GABA levels and highlight the clinical and translational applications of GABA-sensitive PET radioligands. We review the basic physiology of the GABA system with a focus on the importance of GABA_A receptors in the brain and specifically the benzodiazepine binding site. Challenges for the development of central nervous system radioligands and particularly for radioligands with increased GABA sensitivity are outlined, as well as the status of established benzodiazepine site PET radioligands and agonist GABA_A radioligands. We underline the challenge of using allosteric interactions to measure GABA concentrations and review the current state of PET imaging of changes in GABA levels. We conclude that PET tracers with increased GABA sensitivity are required to efficiently measure GABA release and that such a tool could be broadly applied to assess GABA transmission in vivo across several disorders.

Are cannabis-using and non-using patients different groups? Towards understanding the neurobiology of cannabis use in psychotic disorders

A substantial body of credible evidence has accumulated that suggest that cannabis use is an important potentially preventable risk factor for the development of psychotic illness and its worse prognosis following the onset of psychosis. Here we summarize the relevant evidence to argue that the time has come to investigate the neurobiological effects of cannabis in patients with psychotic disorders. In the first section we summarize evidence from longitudinal studies that controlled for a range of potential confounders of the association of cannabis use with increased risk of developing psychotic disorders, increased risk of hospitalization, frequent and longer hospital stays, and failure of treatment with medications for psychosis in those with established illness. Although some evidence has emerged that cannabis-using and non-using patients with psychotic disorders may have distinct patterns of neurocognitive and neurodevelopmental impairments, the biological underpinnings of the effects of cannabis remain to be fully elucidated. In the second and third sections we undertake a systematic review of 70 studies, including over 3000 patients with psychotic disorders or at increased risk of psychotic disorder, in order to delineate potential neurobiological and neurochemical mechanisms that may underlie the effects of cannabis in psychotic disorders and suggest avenues for future research.

Serotonin concentration enhancers at clinically relevant doses reduce [11C]AZ10419369 binding to the 5-HT1B receptors in the nonhuman primate brain

The serotonin (5-HT) system plays an important role in the pathophysiology and treatment of several major psychiatric disorders. Currently, no suitable positron emission tomography (PET) imaging paradigm is available to assess 5-HT release in the living human brain. [11C]AZ10419369 binds to 5-HT1B receptors and is one of the most 5-HT-sensitive radioligands available. This study applied 5-HT concentration enhancers which can be safely studied in humans, and examined their effect on [11C]AZ10419369 binding at clinically relevant doses, including amphetamine (1 mg/kg), 3,4-methylenedioxymethamphetamine (MDMA; 1 mg/kg) or 5-hydroxy-L-tryptophan (5-HTP; 5 mg/kg). Twenty-six PET measurements (14 for amphetamine, 6 for MDMA and 6 for 5-HTP) using a bolus and constant infusion protocol were performed in four cynomolgus monkeys before or after drug administration. Binding potential (BPND) values were determined with the equilibrium method (integral interval: 63-123 min) using cerebellum as the reference region. BPND values were significantly decreased in several examined brain regions after administration of amphetamine (range: 19-31%), MDMA (16-25%) or 5-HTP (13-31%). Reductions in [11C]AZ10419369 binding were greater in striatum than cortical regions after administration of 5-HTP, while no prominent regional differences were found for amphetamine and MDMA. In conclusion, [11C]AZ10419369 binding is sensitive to changes in 5-HT concentration induced by amphetamine, MDMA or 5-HTP. The robust changes in BPND, following pretreatment drugs administered at clinically relevant doses, indicate that the applied PET imaging paradigms hold promise to be successfully used in future human studies.

Cerebral serotonin release correlates with [11C]AZ10419369 PET measures of 5-HT1B receptor binding in the pig brain

Positron emission tomography (PET) can, when used with appropriate radioligands, non-invasively capture temporal and spatial information about acute changes in brain neurotransmitter systems. We here evaluate the 5-HT1B receptor partial agonist PET radioligand, [11C]AZ10419369, for its sensitivity to detect changes in endogenous cerebral serotonin levels, as induced by different pharmacological challenges. To enable a direct translation of PET imaging data to changes in brain serotonin levels, we compared the [11C]AZ10419369 PET signal in the pig brain to simultaneous measurements of extracellular serotonin levels with microdialysis after various acute interventions (saline, escitalopram, fenfluramine). The interventions increased the cerebral extracellular serotonin levels to two to six times baseline, with fenfluramine being the most potent pharmacological enhancer of serotonin release. The interventions induced a varying degree of decline in [11C]AZ10419369 binding in the brain, consistent with the occupancy competition model. The observed correlation between changes in the extracellular serotonin level in the pig brain and the 5-HT1B receptor occupancy indicates that [11C]AZ10419369 binding is sensitive to changes in endogenous serotonin levels to a degree equivalent to that reported of [11C]raclopride to dopamine, a much used approach to detect in vivo change in cerebral dopamine.

Neurotransmitter deficits from frontotemporal lobar degeneration

Frontotemporal lobar degeneration causes a spectrum of complex degenerative disorders including frontotemporal dementia, progressive supranuclear palsy and corticobasal syndrome, each of which is associated with changes in the principal neurotransmitter systems. We review the evidence for these neurochemical changes and propose that they contribute to symptomatology of frontotemporal lobar degeneration, over and above neuronal loss and atrophy. Despite the development of disease-modifying therapies, aiming to slow neuropathological progression, it remains important to advance symptomatic treatments to reduce the disease burden and improve patients' and carers' quality of life. We propose that targeting the selective deficiencies in neurotransmitter systems, including dopamine, noradrenaline, serotonin, acetylcholine, glutamate and gamma-aminobutyric acid is an important strategy towards this goal. We summarize the current evidence-base for pharmacological treatments and suggest strategies to improve the development of new, effective pharmacological treatments.

Discovery of a Novel Muscarinic Receptor PET Radioligand with Rapid Kinetics in the Monkey Brain

Positron emission tomography (PET), together with a suitable radioligand, is one of the more prominent methods for measuring changes in synaptic neurotransmitter concentrations in vivo. The radioligand of choice for such measurements on the cholinergic system is the muscarinic receptor antagonist N-[1-¹¹C]propyl-3-piperidyl benzilate (PPB). In an effort to overcome the shortcomings with the technically cumbersome synthesis of [¹¹C]PPB, we designed and synthesized four structurally related analogues of PPB, of which (S,R)-1-methylpiperidin-3-yl)2-cyclopentyl-2-hydroxy-2-phenylacetate (1) was found to bind muscarinic receptors with similar affinity as PPB (3.5 vs 7.9 nM, respectively). (S,R)-1 was radiolabeled via N-¹¹C-methylation at high radiochemical purity (>99%) and high specific radioactivity (>130 GBq/μmol). In vitro studies by autoradiography on human brain tissue and in vivo studies by PET in nonhuman primates demonstrated excellent signal-to-noise ratios and a kinetic profile in brain comparable to that of [¹¹C]PBB. (S,R)-[¹¹C]1 is a promising candidate for measuring changes in endogenous acetylcholine concentrations.

Astrocytic Regulation of Glutamate Transmission in Schizophrenia

According to the glutamate hypothesis of schizophrenia, the abnormality of glutamate transmission induced by hypofunction of NMDA receptors (NMDARs) is causally associated with the positive and negative symptoms of schizophrenia. However, the underlying mechanisms responsible for the changes in glutamate transmission in schizophrenia are not fully understood. Astrocytes, the major regulatory glia in the brain, modulate not only glutamate metabolism but also glutamate transmission. Here we review the recent progress in understanding the role of astrocytes in schizophrenia. We focus on the astrocytic mechanisms of (i) glutamate synthesis via the glutamate-glutamine cycle, (ii) glutamate clearance by excitatory amino acid transporters (EAATs), (iii) D-serine release to activate NMDARs, and (iv) glutamatergic target engagement biomarkers. Abnormality in these processes is highly correlated with schizophrenia phenotypes. These findings will shed light upon further investigation of pathogenesis as well as improvement of biomarkers and therapies for schizophrenia.

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.

[11C]AZ10419096 - a full antagonist PET radioligand for imaging brain 5-HT1B receptors

INTRODUCTION

The serotonergic system is widely present in all regions of the central nervous system (CNS) and plays a key modulatory role in many of its functions. Positron emission tomography (PET) is used to study several serotonin receptors in CNS in vivo. The G-protein coupled receptor 5-HT_1B is mostly present in the occipital cortex and in midbrain and is linked to several psychiatric disorders. There is evidence that agonist PET radioligands for neuroreceptors are more sensitive to endogenous neurotransmitters than antagonists. Our previously developed 5-HT_1B receptor PET radioligand, [¹¹C]AZ10419369, is now considered a partial agonist. In this work we are aiming to develop a full antagonist PET radioligand for imaging brain 5-HT_1B receptors, and evaluate its sensitivity to increased endogenous serotonin concentration.

MATERIALS

[¹¹C]AZ10419096 was synthesized by rapid methylation of the prepared corresponding N-desmethyl precursor with [¹¹C]methyl triflate. Five PET measurements were performed in cynomolgus monkeys, consisting of two at baseline, one after treatment of a monkey with a 5-HT_1B antagonist, AR-A000002, and two in which fenfluramine was administered during scanning to induce endogenous serotonin release.

RESULTS AND DISCUSSION

[¹¹C]AZ10419096 was synthesized in high yield and purity within 30 min, including purification, formulation and sterile filtration. The baseline PET measurements demonstrated [¹¹C]AZ10419096 to have favorable radioligand characteristics, including high specific binding in brain regions that have high 5-HT_1B density, such as occipital cortex and globus pallidus, as well as subsequent rapid elimination from brain and a minor abundance of lipophilic radiometabolites in plasma. AR-A00002 completely blocked radioligand receptor-specific binding. Fenfluramine produced a distinct displacement of radioligand consistent with an expected increase of synaptic endogenous serotonin concentration.

CONCLUSIONS

[¹¹C]AZ10419096, a full 5-HT_1B antagonist PET radioligand, demonstrates high specific binding in monkey brain that is sensitive to competition from a known 5-HT_1B antagonist as well as to putatively increased endogenous serotonin levels.

Novel CNS drug discovery and development approach: model-based integration to predict neuro-pharmacokinetics and pharmacodynamics

INTRODUCTION

CNS drug development has been hampered by inadequate consideration of CNS pharmacokinetic (PK), pharmacodynamics (PD) and disease complexity (reductionist approach). Improvement is required via integrative model-based approaches. Areas covered: The authors summarize factors that have played a role in the high attrition rate of CNS compounds. Recent advances in CNS research and drug discovery are presented, especially with regard to assessment of relevant neuro-PK parameters. Suggestions for further improvements are also discussed. Expert opinion: Understanding time- and condition dependent interrelationships between neuro-PK and neuro-PD processes is key to predictions in different conditions. As a first screen, it is suggested to use in silico/in vitro derived molecular properties of candidate compounds and predict concentration-time profiles of compounds in multiple compartments of the human CNS, using time-course based physiology-based (PB) PK models. Then, for selected compounds, one can include in vitro drug-target binding kinetics to predict target occupancy (TO)-time profiles in humans. This will improve neuro-PD prediction. Furthermore, a pharmaco-omics approach is suggested, providing multilevel and paralleled data on systems processes from individuals in a systems-wide manner. Thus, clinical trials will be better informed, using fewer animals, while also, needing fewer individuals and samples per individual for proof of concept in humans.

Quantitative positron emission tomography in brain research

The application of positron emission tomography (PET) in brain research has increased substantially during the past 20years, and is still growing. PET provides a unique insight into physiological and pathological processes in vivo. In this article we introduce the fundamentals of PET, and the methods available for acquiring quantitative estimates of the parameters of interest. A short introduction to different areas of application is also given, including basic research of brain function and in neurology, psychiatry, drug receptor occupancy studies, and its application in diagnostics of neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease. Our aim is to inform the unfamiliar reader of the underlying basics and potential applications of PET, hoping to inspire the reader into considering how the technique could be of benefit for his or her own research.

Fenfluramine Reduces [11C]Cimbi-36 Binding to the 5-HT2A Receptor in the Nonhuman Primate Brain

BACKGROUND

[11C]Cimbi-36 is a serotonin 2A receptor agonist positron emission tomography radioligand that has recently been examined in humans. The binding of agonist radioligand is expected to be more sensitive to endogenous neurotransmitter concentrations than antagonist radioligands. In the current study, we compared the effect of serotonin releaser fenfluramine on the binding of [11C]Cimbi-36, [11C]MDL 100907 (a serotonin 2A receptor antagonist radioligand), and [11C]AZ10419369 (a serotonin 1B receptor partial agonist radioligand with established serotonin sensitivity) in the monkey brain.

METHODS

Eighteen positron emission tomography measurements, 6 for each radioligand, were performed in 3 rhesus monkeys before or after administration of 5.0 mg/kg fenfluramine. Binding potential values were determined with the simplified reference tissue model using cerebellum as the reference region.

RESULTS

Fenfluramine significantly decreased [11C]Cimbi-36 (26-62%) and [11C]AZ10419369 (35-58%) binding potential values in most regions (P < 0.05). Fenfluramine-induced decreases in [11C]MDL 100907 binding potential were 8% to 30% and statistically significant in 3 regions. Decreases in [11C]Cimbi-36 binding potential were larger than for [11C]AZ10419369 in neocortical and limbic regions (~35%) but smaller in striatum and thalamus (~40%). Decreases in [11C]Cimbi-36 binding potential were 0.9 to 2.8 times larger than for [11C]MDL 100907, and the fraction of serotonin 2A receptor in the high-affinity state was estimated as 54% in the neocortex.

CONCLUSIONS

The serotonin sensitivity of serotonin 2A receptor agonist radioligand [11C]Cimbi-36 was higher than for antagonist radioligand [11C]MDL 100907. The serotonin sensitivity of [11C]Cimbi-36 was similar to [11C]AZ10419369, which is one of the most sensitive radioligands. [11C]Cimbi-36 is a promising radioligand to examine serotonin release in the primate brain.

18F-FPP: A PET Ligand for the 5-HT2C Receptor?

In the current issue of ACS Chemical Neuroscience, Kim et al. report on the early characterization of 4-(3-[¹⁸F] fluorophenethoxy)pyrimidine (¹⁸F-FPP) as a new positron emission tomography radiotracer for imaging brain 5-HT_2C receptors ( Kim, J., et al. ( 2017 ) A potential PET radiotracer for the 5-HT2c receptor: Synthesis and in vivo evaluation of 4-(3-[¹⁸F]Fluorophenethoxy)pyrimidine. ACS Chem. Neurosci.

, DOI

10.1021/acschemneuro.6b00445 ). At the present time, the tracer properties of ¹⁸F-FPP have only been reported in rats. If ¹⁸F-FPP is indeed shown to be suitable as a 5-HT_2C receptor PET tracer in humans, it will very likely have an important impact both in the development of any new chemical entities (NCEs) targeted to 5-HT_2C receptors, as well as a tool to advance understanding of 5-HT_2C receptor function both in normal and abnormal brain states.

Cerebral 5-HT release correlates with [11C]Cimbi36 PET measures of 5-HT2A receptor occupancy in the pig brain

Positron emission tomography (PET) can, when used with appropriate radioligands, non-invasively generate temporal and spatial information about acute changes in brain neurotransmitter systems. We for the first time evaluate the novel 5-HT_2A receptor agonist PET radioligand, [¹¹C]Cimbi-36, for its sensitivity to detect changes in endogenous cerebral 5-HT levels, as induced by different pharmacological challenges. To enable a direct translation of PET imaging data to changes in brain 5-HT levels, we calibrated the [¹¹C]Cimbi-36 PET signal in the pig brain by simultaneous measurements of extracellular 5-HT levels with microdialysis and [¹¹C]Cimbi-36 PET after various acute interventions (saline, citalopram, citalopram + pindolol, fenfluramine). In a subset of pigs, para-chlorophenylalanine pretreatment was given to deplete cerebral 5-HT. The interventions increased the cerebral extracellular 5-HT levels to 2-11 times baseline, with fenfluramine being the most potent pharmacological enhancer of 5-HT release, and induced a varying degree of decline in [¹¹C]Cimbi-36 binding in the brain, consistent with the occupancy competition model. The observed correlation between changes in the extracellular 5-HT level in the pig brain and the 5-HT_2A receptor occupancy indicates that [¹¹C]Cimbi-36 binding is sensitive to changes in endogenous 5-HT levels, although only detectable with PET when the 5-HT release is sufficiently high.

Neurochemical and metabolic effects of acute and chronic alcohol in the human brain: Studies with positron emission tomography

The use of Positron emission tomography (PET) to study the effects of acute and chronic alcohol on the human brain has enhanced our understanding of the mechanisms underlying alcohol's rewarding effects, the neuroadaptations from chronic exposure that contribute to tolerance and withdrawal, and the changes in fronto-striatal circuits that lead to loss of control and enhanced motivation to drink that characterize alcohol use disorders (AUD). These include studies showing that alcohol's reinforcing effects may result not only from its enhancement of dopaminergic, GABAergic and opioid signaling but also from its caloric properties. Studies in those suffering from an AUD have revealed significant alterations in dopamine (DA), GABA, cannabinoids, opioid and serotonin neurotransmission and in brain energy utilization (glucose and acetate metabolism) that are likely to contribute to compulsive alcohol taking, dysphoria/depression, and to alcohol-associated neurotoxicity. Studies have also evaluated the effects of abstinence on recovery of brain metabolism and neurotransmitter function and the potential value of some of these measures to predict clinical outcomes. Finally, PET studies have started to provide insights about the neuronal mechanisms by which certain genes contribute to the vulnerability to AUD. These findings have helped identify new strategies for prevention and treatment of AUD. This article is part of the Special Issue entitled "Alcoholism".

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Strategies for Utilizing Neuroimaging Biomarkers in CNS Drug Discovery and Development: CINP/JSNP Working Group Report

Despite large unmet medical needs in the field for several decades, CNS drug discovery and development has been largely unsuccessful. Biomarkers, particularly those utilizing neuroimaging, have played important roles in aiding CNS drug development, including dosing determination of investigational new drugs (INDs). A recent working group was organized jointly by CINP and Japanese Society of Neuropsychopharmacology (JSNP) to discuss the utility of biomarkers as tools to overcome issues of CNS drug development.The consensus statement from the working group aimed at creating more nuanced criteria for employing biomarkers as tools to overcome issues surrounding CNS drug development. To accomplish this, a reverse engineering approach was adopted, in which criteria for the utilization of biomarkers were created in response to current challenges in the processes of drug discovery and development for CNS disorders. Based on this analysis, we propose a new paradigm containing 5 distinct tiers to further clarify the use of biomarkers and establish new strategies for decision-making in the context of CNS drug development. Specifically, we discuss more rational ways to incorporate biomarker data to determine optimal dosing for INDs with novel mechanisms and targets, and propose additional categorization criteria to further the use of biomarkers in patient stratification and clinical efficacy prediction. Finally, we propose validation and development of new neuroimaging biomarkers through public-private partnerships to further facilitate drug discovery and development for CNS disorders.

Characterization of [11C]Lu AE92686 as a PET radioligand for phosphodiesterase 10A in the nonhuman primate brain

Purpose

[¹¹C]Lu AE92686 is a positron emission tomography (PET) radioligand that has recently been validated for examining phosphodiesterase 10A (PDE10A) in the human striatum. [¹¹C]Lu AE92686 has high affinity for PDE10A (IC₅₀ = 0.39 nM) and may also be suitable for examination of the substantia nigra, a region with low density of PDE10A. Here, we report characterization of regional [¹¹C]Lu AE92686 binding to PDE10A in the nonhuman primate (NHP) brain.

Methods

A total of 11 PET measurements, seven baseline and four following pretreatment with unlabeled Lu AE92686 or the structurally unrelated PDE10A inhibitor MP-10, were performed in five NHPs using a high resolution research tomograph (HRRT). [¹¹C]Lu AE92686 binding was quantified using a radiometabolite-corrected arterial input function and compartmental and graphical modeling approaches.

Results

Regional time-activity curves were best described with the two-tissue compartment model (2TCM). However, the distribution volume (V_T) values for all regions were obtained by the Logan plot analysis, as reliable cerebellar V_T values could not be derived by the 2TCM. For cerebellum, a proposed reference region, V_T values increased by ∼30 % with increasing PET measurement duration from 63 to 123 min, while V_T values in target regions remained stable. Both pretreatment drugs significantly decreased [¹¹C]Lu AE92686 binding in target regions, while no significant effect on cerebellum was observed. Binding potential (BP_ND) values, derived with the simplified reference tissue model (SRTM), were 13–17 in putamen and 3–5 in substantia nigra and correlated well to values from the Logan plot analysis.

Conclusions

The method proposed for quantification of [¹¹C]Lu AE92686 binding in applied studies in NHP is based on 63 min PET data and SRTM with cerebellum as a reference region. The study supports that [¹¹C]Lu AE92686 can be used for PET examinations of PDE10A binding also in substantia nigra.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-016-3544-9) contains supplementary material, which is available to authorized users.

PET Neurochemical Imaging Modes

PET has deep roots in neuroscience stemming from its first application in brain tumor and brain metabolism imaging. PET emerged over the past few decades and continues to play a prominent role in the study of neurochemistry in the living human brain. Over time, neurochemical imaging with PET has been expanded to address a host of research questions related to, among many others, protein density, drug occupancy, and endogenous neurochemical release. Each of these imaging modes has distinct design and analysis considerations that are critical for enabling quantitative measurements. The number of considerations required for a neurochemical PET study can make it unapproachable. This article aims to orient those interested in neurochemical PET imaging to three of the common imaging modes and to provide some perspective on needs that exist for expansion of neurochemical PET imaging.

Imaging of cerebral α4β2* nicotinic acetylcholine receptors with (-)-[(18)F]Flubatine PET: Implementation of bolus plus constant infusion and sensitivity to acetylcholine in human brain

The positron emission tomography (PET) radioligand (-)-[(18)F]flubatine is specific to α4β2(⁎) nicotinic acetylcholine receptors (nAChRs) and has promise for future investigation of the acetylcholine system in neuropathologies such as Alzheimer's disease, schizophrenia, and substance use disorders. The two goals of this work were to develop a simplified method for α4β2(⁎) nAChR quantification with bolus plus constant infusion (B/I) (-)-[(18)F]flubatine administration, and to assess the radioligand's sensitivity to acetylcholine fluctuations in humans. Healthy human subjects were imaged following either bolus injection (n=8) or B/I (n=4) administration of (-)-[(18)F]flubatine. The metabolite-corrected input function in arterial blood was measured. Free-fraction corrected distribution volumes (VT/fP) were estimated with modeling and graphical analysis techniques. Next, sensitivity to acetylcholine was assessed in two ways: 1. A bolus injection paradigm with two scans (n=6), baseline (scan 1) and physostigmine challenge (scan 2; 1.5mg over 60min beginning 5min prior to radiotracer injection); 2. A single scan B/I paradigm (n=7) lasting up to 240min with 1.5mg physostigmine administered over 60min beginning at 125min of radiotracer infusion. Changes in VT/fP were measured. Baseline VT/fP values were 33.8±3.3mL/cm(3) in thalamus, 12.9±1.6mL/cm(3) in cerebellum, and ranged from 9.8 to 12.5mL/cm(3) in other gray matter regions. The B/I paradigm with equilibrium analysis at 120min yielded comparable VT/fP values with compartment modeling analysis of bolus data in extrathalamic gray matter regions (regional means <4% different). Changes in VT/fP following physostigmine administration were small and most pronounced in cortical regions, ranging from 0.8 to 4.6% in the two-scan paradigm and 2.8 to 6.5% with the B/I paradigm. These results demonstrate the use of B/I administration for accurate quantification of (-)-[(18)F]flubatine VT/fP in 120min, and suggest possible sensitivity of (-)-[(18)F]flubatine binding to physostigmine-induced changes in acetylcholine levels.