Imaging of opioid receptors in the central nervous system

Two distinct profiles of fMRI and neurophysiological activity elicited by acetylcholine in visual cortex

fMRI changes are typically assumed to be due to changes in neural activity, although whether this remains valid under the influence of neuromodulators is relatively unknown. Here, we found evidence that intracortical acetylcholine elicits distinct profiles of fMRI and electrophysiological activity in visual cortex. Two patterns of cholinergic activity were observed, depending on the distance to the injection site, although neurovascular coupling was preserved. Our results illustrate the effects of neuromodulators on fMRI and electrophysiological responses and show that these depend on neuromodulator concentration and kinetics.

Cholinergic neuromodulation is involved in all aspects of sensory processing and is crucial for processes such as attention, learning and memory, etc. However, despite the known roles of acetylcholine (ACh), we still do not how to disentangle ACh contributions from sensory or task-evoked changes in functional magnetic resonance imaging (fMRI). Here, we investigated the effects of local injection of ACh on fMRI and neural signals in the primary visual cortex (V1) of anesthetized macaques by combining pharmaco-based MRI (phMRI) with electrophysiological recordings, using single electrodes and electrode arrays. We found that local injection of ACh elicited two distinct profiles of fMRI and neurophysiological activity, depending on the distance from the injector. Near the injection site, we observed an increase in the baseline blood oxygen-level-dependent (BOLD) and cerebral blood flow (CBF) responses, while their visual modulation decreased. In contrast, further from the injection site, we observed an increase in the visually induced BOLD and CBF modulation without changes in baseline. Neurophysiological recordings suggest that the spatial correspondence between fMRI responses and neural activity does not change in the gamma, high-gamma, and multiunit activity (MUA) bands. The results near the injection site suggest increased inhibitory drive and decreased metabolism, contrasting to the far region. These changes are thought to reflect the kinetics of ACh and its metabolism to choline.

PET imaging reveals lower kappa opioid receptor availability in alcoholics but no effect of age

Opioid receptors are implicated in alcoholism, other addictions, withdrawal, and depression, and are considered potential pharmacological targets for treatment. Our goal in the present study was to compare the availability of kappa opioid receptors (KOR) between an alcohol-dependent cohort (AD) and a healthy control cohort (HC). Sixty-four participants-36 AD and 28 HC-underwent PET scans with [11C]LY2795050, a selective kappa antagonist tracer. Partial-volume correction was applied to all PET data to correct for atrophy. Volume of distribution (VT) of the tracer was estimated regionally as a measure of KOR availability. VT values of AD versus HC were compared for 15 defined ROIs. Multivariate analysis showed a main effect of group on VT across these 15 ROIs. Post hoc tests showed that AD had significantly lower VT and thus a lower KOR availability than HC in amygdala and pallidum (corrected for multiple comparisons). Exploratory analysis of change in VT with age was conducted; VT was not found to vary significantly with age in any region. Our findings of lower VT in AD versus HC in multiple regions are in contrast to findings in the mu and delta opioid receptor systems of higher VT in AD versus HC. Although age-related decline in receptors has previously been observed in the mu opioid receptor system, we found that KOR availability does not change with age.

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.

Opioid modulation of cognitive impairment in depression

The failure of traditional antidepressant medications to adequately target cognitive impairment is associated with poor treatment response, increased risk of relapse, and greater lifetime disability. Opioid receptor antagonists are currently under development as novel therapeutics for major depressive disorder (MDD) and other stress-related illnesses. Although it is known that dysregulation of the endogenous opioid system is observed in patients diagnosed with MDD, the impact of opioidergic neurotransmission on cognitive impairment has not been systematically evaluated. Here we review the literature indicating that opioid manipulations can alter cognitive functions in humans. Furthermore, we detail the preclinical studies that demonstrate the ability of mu-opioid receptor and kappa-opioid receptor ligands to modulate several cognitive processes. Specifically, this review focuses on domains within higher order cognitive processing, including attention and executive functioning, which can differentiate cognitive processes influenced by motivational state.

Altered Signaling in the Descending Pain-modulatory System after Short-Term Infusion of the μ-Opioid Agonist Remifentanil

μ-Opioid receptor agonists are widely used within the contemporary treatment of pain, but abrupt opioid suspension, even after short-term infusion, can paradoxically increase the sensitivity to noxious stimuli, a phenomenon that has been, for example, reported after application of the fast-acting μ-opioid receptor agonist remifentanil. To investigate the mechanisms underlying the effects of discontinuation of remifentanil application on pain processing in the human CNS, we analyzed neuronal responses to thermal stimuli before and after a short-term infusion of remifentanil (30 min 0.1 μg/kg body weight/min) compared with control in the brain, brainstem, and spinal cord in drug-naive male volunteers using fMRI. Subsequent to remifentanil suspension, we observed reduced heat pain thresholds and increased neuronal responses in pain-encoding as well as in key regions of the descending pain-modulatory system, such as the periaqueductal gray matter, the nucleus cuneiformis, and the rostral ventromedial medulla. Moreover, the spinal pain-related multivoxel activity pattern showed an opioid-specific change after drug suspension. Importantly, remifentanil suspension increased the functional coupling between the nucleus cuneiformis and the rostral anterior cingulate cortex, and the coupling strength between the rostral anterior cingulate cortex and the nucleus cuneiformis correlated negatively with the individual pain threshold after opioid suspension. These findings demonstrate that, already subsequent to a short-term infusion of the μ-opioid receptor agonist remifentanil, signaling in the descending pain-modulatory system is fundamentally altered and that these changes are directly related to the behavioral sensitivity to pain.SIGNIFICANCE STATEMENT Opioids are widely used in modern medicine, but, in addition to their known side effects, it is increasingly recognized that opioids can also increase sensitivity to pain subsequent to their use. Using the fast-acting μ-opioid receptor agonist remifentanil and fMRI in healthy male volunteers, this study demonstrates how signaling changes occur along the entire descending pain-modulatory pathway after opioid discontinuation and how these alterations are closely linked to increased behavioral pain sensitivity. Particularly by revealing modified responses in pain-modulatory brainstem regions that have been previously demonstrated to be causally involved in acute opioid withdrawal effects in rodents, the data provide a plausible neuronal mechanism by which the increased sensitivity to pain after opioid suspension is mediated in humans.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Dopamine Is Signaled by Mid-frequency Oscillations and Boosts Output Layers Visual Information in Visual Cortex

Neural oscillations are ubiquitously observed in cortical activity, and are widely believed to be crucial for mediating transmission of information across the cortex. Yet, the neural phenomena contributing to each oscillation band, and their effect on information coding and transmission, are largely unknown. Here, we investigated whether individual frequency bands specifically reflect changes in the concentrations of dopamine, an important neuromodulator, and how dopamine affects oscillatory information processing. We recorded the local field potential (LFP) at different depths of primary visual cortex (V1) in anesthetized monkeys (Macaca mulatta) during spontaneous activity and during visual stimulation with Hollywood movie clips while pharmacologically mimicking dopaminergic neuromodulation by systemic injection of L-DOPA (a metabolic precursor of dopamine). We found that dopaminergic neuromodulation had marked effects on both spontaneous and movie-evoked neural activity. During spontaneous activity, dopaminergic neuromodulation increased the power of the LFP specifically in the [19-38 Hz] band, suggesting that the power of endogenous visual cortex oscillations in this band can be used as a robust marker of dopaminergic neuromodulation. Moreover, dopamine increased visual information encoding over all frequencies during movie stimulation. The information increase due to dopamine was prominent in the supragranular layers of cortex that project to higher cortical areas and in the gamma [50-100 Hz] band that has been previously implicated in mediating feedforward information transfer. These results thus individuate new neural mechanisms by which dopamine may promote the readout of relevant sensory information by strengthening the transmission of information from primary to higher areas.

Opioid Release after High-Intensity Interval Training in Healthy Human Subjects

Central opioidergic mechanisms may modulate the positive effects of physical exercise such as mood elevation and stress reduction. How exercise intensity and concomitant effective changes affect central opioidergic responses is unknown. We studied the effects of acute physical exercise on the cerebral μ-opioid receptors (MOR) of 22 healthy recreationally active males using positron emission tomography (PET) and the MOR-selective radioligand [¹¹C]carfentanil. MOR binding was measured in three conditions on separate days: after a 60-min aerobic moderate-intensity exercise session, after a high-intensity interval training (HIIT) session, and after rest. Mood was measured repeatedly throughout the experiment. HIIT significantly decreased MOR binding selectively in the frontolimbic regions involved in pain, reward, and emotional processing (thalamus, insula, orbitofrontal cortex, hippocampus, and anterior cingulate cortex). Decreased binding correlated with increased negative emotionality. Moderate-intensity exercise did not change MOR binding, although increased euphoria correlated with decreased receptor binding. These observations, consistent with endogenous opioid release, highlight the role of the μ-opioid system in mediating affective responses to high-intensity training as opposed to recreational moderate physical exercise.

Opioid Modulation of Value-Based Decision-Making in Healthy Humans

Modifying behavior to maximize reward is integral to adaptive decision-making. In rodents, the μ-opioid receptor (MOR) system encodes motivation and preference for high-value rewards. Yet it remains unclear whether and how human MORs contribute to value-based decision-making. We reasoned that if the human MOR system modulates value-based choice, this would be reflected by opposite effects of agonist and antagonist drugs. In a double-blind pharmacological cross-over study, 30 healthy men received morphine (10 mg), placebo, and the opioid antagonist naltrexone (50 mg). They completed a two-alternative decision-making task known to induce a considerable bias towards the most frequently rewarded response option. To quantify MOR involvement in this bias, we fitted accuracy and reaction time data with the drift-diffusion model (DDM) of decision-making. The DDM analysis revealed the expected bidirectional drug effects for two decision subprocesses. MOR stimulation with morphine increased the preference for the stimulus with high-reward probability (shift in starting point). Compared to placebo, morphine also increased, and naltrexone reduced, the efficiency of evidence accumulation. Since neither drug affected motor-coordination, speed-accuracy trade-off, or subjective state (indeed participants were still blinded after the third session), we interpret the MOR effects on evidence accumulation efficiency as a consequence of changes in effort exerted in the task. Together, these findings support a role for the human MOR system in value-based choice by tuning decision-making towards high-value rewards across stimulus domains.

Feeding Releases Endogenous Opioids in Humans

The endogenous opioid system supports a multitude of functions related to appetitive behavior in humans and animals, and it has been proposed to govern hedonic aspects of feeding thus contributing to the development of obesity. Here we used positron emission tomography to investigate whether feeding results in hedonia-dependent endogenous opioid release in humans. Ten healthy males were recruited for the study. They were scanned with the μ-opioid-specific ligand [¹¹C]carfentanil three times, as follows: after a palatable meal, a nonpalatable meal, and after an overnight fast. Subjective mood, satiety, and circulating hormone levels were measured. Feeding induced significant endogenous opioid release throughout the brain. This response was more pronounced following a nonpalatable meal versus a palatable meal, and independent of the subjective hedonic responses to feeding. We conclude that feeding consistently triggers cerebral opioid release even in the absence of subjective pleasure associated with feeding, suggesting that metabolic and homeostatic rather than exclusively hedonic responses play a role in the feeding-triggered cerebral opioid release.SIGNIFICANCE STATEMENT The endogenous opioid system supports both hedonic and homeostatic functions. It has been proposed that overeating and concomitant opioid release could downregulate opioid receptors and promote the development of obesity. However, it remains unresolved whether feeding leads to endogenous opioid release in humans. We used in vivo positron emission tomography to test whether feeding triggers cerebral opioid release and whether this response is associated with pleasurable sensations. We scanned volunteers using the μ-opioid receptor-specific radioligand [¹¹C]carfentanil three times, as follows: after an overnight fast, after consuming a palatable meal, and after consuming a nonpalatable meal. Feeding led to significant endogenous opioid release, and this occurred also in the absence of feeding-triggered hedonia. Feeding-triggered opioid release thus also reflects metabolic and homeostatic responses rather than hedonic responses exclusively.

Social Laughter Triggers Endogenous Opioid Release in Humans

The size of human social networks significantly exceeds the network that can be maintained by social grooming or touching in other primates. It has been proposed that endogenous opioid release after social laughter would provide a neurochemical pathway supporting long-term relationships in humans (Dunbar, 2012), yet this hypothesis currently lacks direct neurophysiological support. We used PET and the μ-opioid-receptor (MOR)-specific ligand [11C]carfentanil to quantify laughter-induced endogenous opioid release in 12 healthy males. Before the social laughter scan, the subjects watched laughter-inducing comedy clips with their close friends for 30 min. Before the baseline scan, subjects spent 30 min alone in the testing room. Social laughter increased pleasurable sensations and triggered endogenous opioid release in thalamus, caudate nucleus, and anterior insula. In addition, baseline MOR availability in the cingulate and orbitofrontal cortices was associated with the rate of social laughter. In a behavioral control experiment, pain threshold-a proxy of endogenous opioidergic activation-was elevated significantly more in both male and female volunteers after watching laughter-inducing comedy versus non-laughter-inducing drama in groups. Modulation of the opioidergic activity by social laughter may be an important neurochemical pathway that supports the formation, reinforcement, and maintenance of human social bonds.SIGNIFICANCE STATEMENT Social contacts are vital to humans. The size of human social networks significantly exceeds the network that can be maintained by social grooming in other primates. Here, we used PET to show that endogenous opioid release after social laughter may provide a neurochemical mechanism supporting long-term relationships in humans. Participants were scanned twice: after a 30 min social laughter session and after spending 30 min alone in the testing room (baseline). Endogenous opioid release was stronger after laughter versus the baseline scan. Opioid receptor density in the frontal cortex predicted social laughter rates. Modulation of the opioidergic activity by social laughter may be an important neurochemical mechanism reinforcing and maintaining social bonds between humans.

A hybrid deconvolution approach for estimation of in vivo non-displaceable binding for brain PET targets without a reference region

BACKGROUND AND AIM

Estimation of a PET tracer's non-displaceable distribution volume (VND) is required for quantification of specific binding to its target of interest. VND is generally assumed to be comparable brain-wide and is determined either from a reference region devoid of the target, often not available for many tracers and targets, or by imaging each subject before and after blocking the target with another molecule that has high affinity for the target, which is cumbersome and involves additional radiation exposure. Here we propose, and validate for the tracers [11C]DASB and [11C]CUMI-101, a new data-driven hybrid deconvolution approach (HYDECA) that determines VND at the individual level without requiring either a reference region or a blocking study.

METHODS

HYDECA requires the tracer metabolite-corrected concentration curve in blood plasma and uses a singular value decomposition to estimate the impulse response function across several brain regions from measured time activity curves. HYDECA decomposes each region's impulse response function into the sum of a parametric non-displaceable component, which is a function of VND, assumed common across regions, and a nonparametric specific component. These two components differentially contribute to each impulse response function. Different regions show different contributions of the two components, and HYDECA examines data across regions to find a suitable common VND. HYDECA implementation requires determination of two tuning parameters, and we propose two strategies for objectively selecting these parameters for a given tracer: using data from blocking studies, and realistic simulations of the tracer. Using available test-retest data, we compare HYDECA estimates of VND and binding potentials to those obtained based on VND estimated using a purported reference region.

RESULTS

For [11C]DASB and [11C]CUMI-101, we find that regardless of the strategy used to optimize the tuning parameters, HYDECA provides considerably less biased estimates of VND than those obtained, as is commonly done, using a non-ideal reference region. HYDECA test-retest reproducibility is comparable to that obtained using a VND determined from a non-ideal reference region, when considering the binding potentials BPP and BPND.

CONCLUSIONS

HYDECA can provide subject-specific estimates of VND without requiring a blocking study for tracers and targets for which a valid reference region does not exist.

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".

Objective methods for the assessment of the spinal and supraspinal effects of opioids

BACKGROUND AND PURPOSE

Opioids are potent analgesics. Opioids exert effects after interaction with opioid receptors. Opioid receptors are present in the peripheral- and central nervous system (CNS), but the analgesic effects are primarily mediated via receptors in the CNS. Objective methods for assessment of opioid effects may increase knowledge on the CNS processes responsible for analgesia. The aim of this review was to provide an overview of the most common objective methods for assessment of the spinal and supraspinal effects of opioids and discuss their advantages and limitations.

METHOD

The literature search was conducted in Pub Med (http://www.ncbi.nlm.nih.gov/pubmed) from November 2014 to June 2016, using free-text terms: "opioid", "morphine" and "oxycodone" combined with the terms "pupillometry," "magnetic resonance spectroscopy," "fMRI," "BOLD," "PET," "pharmaco-EEG", "electroencephalogram", "EEG," "evoked potentials," and "nociceptive reflex". Only original articles published in English were included.

RESULTS

For assessment of opioid effects at the supraspinal level, the following methods are evaluated: pupillometry, proton magnetic resonance spectroscopy, functional resonance magnetic imaging (fMRI), positron emission tomography (PET), spontaneous electroencephalogram (EEG) and evoked potentials (EPs). Pupillometry is a non-invasive tool used in research as well as in the clinical setting. Proton magnetic resonance spectroscopy has been used for the last decades and it is a non-invasive technique for measurement of in vivo brain metabolite concentrations. fMRI has been a widely used non-invasive method to estimate brain activity, where typically from the blood oxygen level-dependent (BOLD) signal. PET is a nuclear imaging technique based on tracing radio labeled molecules injected into the blood, where receptor distribution, density and activity in the brain can be visualized. Spontaneous EEG is typically quantified in frequency bands, power spectrum and spectral edge frequency. EPs are brain responses (assessed by EEG) to a predefined number of short phasic stimuli. EPs are quantified by their peak latencies and amplitudes, power spectrum, scalp topographies and brain source localization. For assessment of opioid effects at the spinal level, the following methods are evaluated: the nociceptive withdrawal reflex (NWR) and spinal EPs. The nociceptive withdrawal reflex can be recorded from all limbs, but it is standard to record the electromyography signal at the biceps femoris muscle after stimulation of the ipsilateral sural nerve; EPs can be recorded from the spinal cord and are typically recorded after stimulation of the median nerve at the wrist.

CONCLUSION AND IMPLICATIONS

The presented methods can all be used as objective methods for assessing the centrally mediated effects of opioids. Advantages and limitations should be considered before implementation in drug development, future experimental studies as well as in clinical settings. In conclusion, pupillometry is a sensitive measurement of opioid receptor activation in the CNS and from a practical and economical perspective it may be used as a biomarker for opioid effects in the CNS. However, if more detailed information is needed on opioid effects at different levels of the CNS, then EEG, fMRI, PET and NWR have the potential to be used. Finally, it is conceivable that information from different methods should be considered together for complementary information.

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.

Acute Morphine Exposure Increases the Brain Distribution of [18F]DPA-714, a PET Biomarker of Glial Activation in Nonhuman Primates

BACKGROUND

The neuroinflammatory response to morphine exposure modulates its antinociceptive effects, tolerance, and dependence. Positron emission tomography radioligands for translocator protein-18kDa such as [18F]DPA-714 are noninvasive biomarkers of glial activation, a hallmark of neuroinflammation.

METHODS

[18F]DPA-714 positron emission tomography imaging was performed in 5 baboons at baseline and 2 hours after i.m. morphine injection (1 mg/kg). Brain kinetics and metabolite-corrected input function were measured to estimate [18F]DPA-714 brain distribution.

RESULTS

Morphine significantly increased [18F]DPA-714 brain distribution by a 1.3 factor (P<.05; paired t test). The effect was not restricted to opioid receptor-rich regions. Differences in baseline [18F]DPA-714 binding were observed among baboons. The response to morphine predominated in animals with the highest baseline uptake.

CONCLUSIONS

[18F]DPA-714 positron emission tomography imaging may be useful to noninvasively investigate the brain immune component of morphine pharmacology. Correlation between baseline brain distribution and subsequent response to morphine exposure suggest a role for priming parameters in controlling the neuroinflammatory properties of opioids.

Synthesis and opioid receptor binding of indium (III) and [111In]-labeled macrocyclic conjugates of diprenorphine: novel ligands designed for imaging studies of peripheral opioid receptors

Radiolabeled diprenorphine (DPN) and analogs are widely used ligands for non-invasive brain imaging of opioid receptors. To develop complementary radioligands optimized for studies of the peripheral opioid receptors, we prepared a pair of hydrophilic DPN derivatives, conjugated to the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), for complexation with trivalent metals. The non-radioactive indium (III) complexes, tethered to the C6-oxygen position of the DPN scaffold by 6- to 9-atom spacers, displayed high affinities for binding to μ, δ and κ opioid receptors in vitro. Use of the 9-atom linker conferred picomolar affinities equipotent to those of the parent ligand DPN. The [¹¹¹In]-labeled complexes were prepared in good yield (>70%), with high radiochemical purity (~99%) and high specific radioactivity (>4000 mCi/μmol). Their log D_7.4 values were -2.21 to -1.66. In comparison, DPN is lipophilic, with a log D_7.4 of +2.25. Further study in vivo is warranted to assess the suitability of these [¹¹¹In]-labeled DPN-DOTA conjugates for imaging trials.

Pain tolerance predicts human social network size

Personal social network size exhibits considerable variation in the human population and is associated with both physical and mental health status. Much of this inter-individual variation in human sociality remains unexplained from a biological perspective. According to the brain opioid theory of social attachment, binding of the neuropeptide β-endorphin to μ-opioid receptors in the central nervous system (CNS) is a key neurochemical mechanism involved in social bonding, particularly amongst primates. We hypothesise that a positive association exists between activity of the μ-opioid system and the number of social relationships that an individual maintains. Given the powerful analgesic properties of β-endorphin, we tested this hypothesis using pain tolerance as an assay for activation of the endogenous μ-opioid system. We show that a simple measure of pain tolerance correlates with social network size in humans. Our results are in line with previous studies suggesting that μ-opioid receptor signalling has been elaborated beyond its basic function of pain modulation to play an important role in managing our social encounters. The neuroplasticity of the μ-opioid system is of future research interest, especially with respect to psychiatric disorders associated with symptoms of social withdrawal and anhedonia, both of which are strongly modulated by endogenous opioids.

Population pharmacokinetics of nalmefene in healthy subjects and its relation to μ-opioid receptor occupancy

AIMS

The aims of this study were to develop a population pharmacokinetic (PK) model to describe the PK of nalmefene in healthy subjects and to relate the exposure of nalmefene to the μ-opioid receptor occupancy by simulations in the target population.

METHODS

Data from nine phase I studies (243 subjects) with extensive blood sampling were pooled and used for the population PK model building. Data from four other phase I studies (85 subjects) were pooled and used as an external validation dataset. Eight subjects from an imaging study contributed occupancy data and the pharmacokinetic/pharmacodynamic (PK/PD) relationship was modelled. Combining the population PK model and the PK/PD relationship enabled simulations to predict μ-opioid occupancy.

RESULTS

A two compartment model with first order absorption best described the nalmefene PK data. The typical subject in the population was estimated to have a systemic clearance of 60.4 l h(-1) and a central volume of distribution of 266 l. Absolute oral bioavailability was estimated to 41% without food intake and with food about 53%. Simulation of the μ-opioid receptor occupancy shows that the 95% confidence bound is within or above 60-90% occupancy for up to 22-24 h after a single dose of 20 mg nalmefene.

CONCLUSIONS

A robust population PK model for nalmefene was developed. Based on the concentration-occupancy model the μ-opioid receptor occupancy after a single 20 mg dose of nalmefene is predicted to be above the target therapeutic occupancy for about 24 h in about 95% of the target population.

From unwitnessed fatality to witnessed rescue: Pharmacologic intervention in sudden unexpected death in epilepsy

The mechanisms of sudden unexpected death in epilepsy (SUDEP) have been difficult to define, as most cases occur unwitnessed, and physiologic recordings have been obtained in only a handful of cases. However, recent data obtained from human cases and experimental studies in animal models have brought us closer to identifying potential mechanisms. Theories of SUDEP should be able to explain how a seizure starting in the forebrain can sometimes lead to changes in brainstem cardiorespiratory control mechanisms. Herein we focus on three major themes of work on the causes of SUDEP. First, evidence is reviewed identifying postictal hypoventilation as a major contributor to the cause of death. Second, data are discussed that brainstem serotonin and adenosine pathways may be involved, as well as how they may contribute. Finally, parallels are drawn between SIDS and SUDEP, and we highlight similarities pointing to the possibility of shared pathophysiology involving combined failure of respiratory and cardiovascular control mechanisms. Knowledge about the causes of SUDEP may lead to potential pharmacologic approaches for prevention. We end by describing how translation of this work may result in future applications to clinical care.

The design of multitarget ligands for chronic and neuropathic pain

BACKGROUND

Monodrug therapy has been used with success to fight various pathologies. When one medicine fails, co-administration of two or more drugs at the same time may be successfully applied in the treatment of infections, hypertension, HIV and in many other fields.

DISCUSSION

This approach has some weakness related to the pharmacokinetic of the two different substances administered, side effects, possible drug-drug interaction. Bivalent ligand approach would maintain the strength of the multidrug therapy (synergistic effect, lower doses, and little side effects) and overcome the weakness of a co-administration.

CONCLUSION

In this review we have described the state-of-the-art of the multitarget approach for the control of pain. Several approaches adopted by different research groups and future perspectives have been discussed.

Organ-specific physiological responses to acute physical exercise and long-term training in humans

Virtually all tissues in the human body rely on aerobic metabolism for energy production and are therefore critically dependent on continuous supply of oxygen. Oxygen is provided by blood flow, and, in essence, changes in organ perfusion are also closely associated with alterations in tissue metabolism. In response to acute exercise, blood flow is markedly increased in contracting skeletal muscles and myocardium, but perfusion in other organs (brain and bone) is only slightly enhanced or is even reduced (visceral organs). Despite largely unchanged metabolism and perfusion, repeated exposures to altered hemodynamics and hormonal milieu produced by acute exercise, long-term exercise training appears to be capable of inducing effects also in tissues other than muscles that may yield health benefits. However, the physiological adaptations and driving-force mechanisms in organs such as brain, liver, pancreas, gut, bone, and adipose tissue, remain largely obscure in humans. Along these lines, this review integrates current information on physiological responses to acute exercise and to long-term physical training in major metabolically active human organs. Knowledge is mostly provided based on the state-of-the-art, noninvasive human imaging studies, and directions for future novel research are proposed throughout the review.

Aberrant mesolimbic dopamine-opiate interaction in obesity

Dopamine and opioid neurotransmitter systems share many functions such as regulation of reward and pleasure. μ-Opioid receptors (MOR) modulate the mesolimbic dopamine system in ventral tegmental area and striatum, key areas implicated in reward. We hypothesized that dopamine and opioid receptor availabilities correlate in vivo and that this correlation is altered in obesity, a disease with altered reward processing. Twenty lean females (mean BMI 22) and 25 non-binge eating morbidly obese females (mean BMI 41) underwent two positron emission tomography scans with [(11)C]carfentanil and [(11)C]raclopride to measure the MOR and dopamine D2 receptor (DRD2) availability, respectively. In lean subjects, the MOR and DRD2 availabilities were positively associated in the ventral striatum (r=0.62, p=0.003) and dorsal caudate nucleus (r=0.62, p=0.004). Moreover, DRD2 availability in the ventral striatum was associated with MOR availability in other regions of the reward circuitry, particularly in the ventral tegmental area. In morbidly obese subjects, this receptor interaction was significantly weaker in ventral striatum but unaltered in the caudate nucleus. Finally, the association between DRD2 availability in the ventral striatum and MOR availability in the ventral tegmental area was abolished in the morbidly obese. The study demonstrates a link between DRD2 and MOR availabilities in living human brain. This interaction is selectively disrupted in mesolimbic dopamine system in morbid obesity. We propose that interaction between the dopamine and opioid systems is a prerequisite for normal reward processing and that disrupted cross-talk may underlie altered reward processing in obesity.

Obesity is associated with decreased μ-opioid but unaltered dopamine D2 receptor availability in the brain

Neurochemical pathways involved in pathological overeating and obesity are poorly understood. Although previous studies have shown increased μ-opioid receptor (MOR) and decreased dopamine D2 receptor (D2R) availability in addictive disorders, the role that these systems play in human obesity still remains unclear. We studied 13 morbidly obese women [mean body mass index (BMI), 42 kg/m(2)] and 14 nonobese age-matched women, and measured brain MOR and D2R availability using PET with selective radioligands [(11)C]carfentanil and [(11)C]raclopride, respectively. We also used quantitative meta-analytic techniques to pool previous evidence on the effects of obesity on altered D2R availability. Morbidly obese subjects had significantly lower MOR availability than control subjects in brain regions relevant for reward processing, including ventral striatum, insula, and thalamus. Moreover, in these areas, BMI correlated negatively with MOR availability. Striatal MOR availability was also negatively associated with self-reported food addiction and restrained eating patterns. There were no significant differences in D2R availability between obese and nonobese subjects in any brain region. Meta-analysis confirmed that current evidence for altered D2R availability in obesity is only modest. Obesity appears to have unique neurobiological underpinnings in the reward circuit, whereby it is more similar to opioid addiction than to other addictive disorders. The opioid system modulates motivation and reward processing, and low μ-opioid availability may promote overeating to compensate decreased hedonic responses in this system. Behavioral and pharmacological strategies for recovering opioidergic function might thus be critical to curb the obesity epidemic.

Advances in CNS Imaging Agents: Focus on PET and SPECT Tracers in Experimental and Clinical Use

The physiological functioning of the brain is not well-known in current day medicine and the pathologies of many neuropsychiatric disorders are still not yet fully understood. With our aging population and better life expectancies, it has become imperative to find better biomarkers for disease progression as well as receptor target engagements. In the last decade, these major advances in the field of molecular CNS imaging have been made available with tools such as functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT), and neuroreceptor-targeted positron emission tomography (PET). These tools have given researchers, pharmaceutical companies, and clinical physicians a better method of understanding CNS dysfunctions, and the ability to employ improved therapeutic agents. This review is intended to provide an update on brain imaging agents that are currently used in clinical and translational research toward treatment of CNS disorders. The review begins with amyloid and tau imaging, the former of which has at least three [(18)F] agents that have been recently approved and will soon be available for clinical use for specific indications in the USA and elsewhere. Other prevalent PET and SPECT neurotransmitter system agents, including those newly US FDA-approved imaging agents related to the dopaminergic system, are included. A review of both mature and potentially growing PET imaging agents, including those targeting serotonin and opiate receptor systems, is also provided.

Liquid chromatography-mass spectrometry platform for both small neurotransmitters and neuropeptides in blood, with automatic and robust solid phase extraction

Neurons communicate via chemical signals called neurotransmitters (NTs). The numerous identified NTs can have very different physiochemical properties (solubility, charge, size etc.), so quantification of the various NT classes traditionally requires several analytical platforms/methodologies. We here report that a diverse range of NTs, e.g. peptides oxytocin and vasopressin, monoamines adrenaline and serotonin, and amino acid GABA, can be simultaneously identified/measured in small samples, using an analytical platform based on liquid chromatography and high-resolution mass spectrometry (LC-MS). The automated platform is cost-efficient as manual sample preparation steps and one-time-use equipment are kept to a minimum. Zwitter-ionic HILIC stationary phases were used for both on-line solid phase extraction (SPE) and liquid chromatography (capillary format, cLC). This approach enabled compounds from all NT classes to elute in small volumes producing sharp and symmetric signals, and allowing precise quantifications of small samples, demonstrated with whole blood (100 microliters per sample). An additional robustness-enhancing feature is automatic filtration/filter back-flushing (AFFL), allowing hundreds of samples to be analyzed without any parts needing replacement. The platform can be installed by simple modification of a conventional LC-MS system.

[Integration of pharmacokinetics and pharmacodynamics based on the in vivo analysis of drug-receptor binding]

  As I was deeply interested in the effects of drugs on the human body, I chose pharmacology as the subject of special study when I became a 4th year student at Shizuoka College of Pharmacy. I studied abroad as a postdoctoral fellow for two years, from 1978, under the tutelage of Professor Henry I. Yamamura (pharmacology) in the College of Medicine at the University of Arizona, USA. He taught me a variety of valuable skills such as the radioreceptor binding assay, which represented the most advanced technology developed in the US at that time. After returning home, I engaged in clarifying receptor abnormalities in pathological conditions, as well as in drug action mechanisms, by making the best use of this radioreceptor binding assay. In 1989, following the founding of the University of Shizuoka, I was invited by Professor Ryohei Kimura to join the Department of Pharmacokinetics. This switch in discipline provided a good opportunity for me to broaden my perspectives in pharmaceutical sciences. I worked on evaluating drug-receptor binding in vivo as a combined index for pharmacokinetics and pharmacological effect manifestation, with the aim of bridging pharmacology and pharmacokinetics. In fact, by focusing on data from in vivo receptor binding, it became possible to clearly rationalize the important consideration of drug dose-concentration-action relationships, and to study quantitative and kinetic analyses of relationships among pharmacokinetics, receptor binding and pharmacological effects. Based on this concept, I was able to demonstrate the utility of dynamic analyses of drug-receptor binding in drug discovery, drug fostering, and the proper use of pharmacokinetics with regard to many drugs.

Objective markers of the analgesic response to morphine in experimental pain research

INTRODUCTION

In experimental pain research the effect of opioids is normally assessed by verbal subjective response to analgesia. However, as many confounders in pain assessment exist, objective bed-side assessment of the effect is highly warranted. Therefore, we aimed to assess the effect of morphine on three objective pharmacodynamic markers (pupil diameter, prolactin concentration and resting electroencephalography (EEG)) and compare the changes from placebo with subjective analgesia on experimental muscle pain for convergent validation.

METHODS

Fifteen healthy male participants received placebo or 30 mg rectal morphine at two separate sessions. At baseline and several time points after drug administration, the central effects of morphine were assessed by experimental muscle pain, pupil diameter, prolactin concentration and resting EEG.

RESULTS

Morphine increased tolerance to muscle pain, together with significant reductions in pupil diameter and increase in prolactin concentration (all P < 0.001). Miosis was induced simultaneously with the onset of analgesic effect 30 min after dosing, while a significant increase in prolactin concentration was seen after 45 min. The change in pupil diameter was negatively correlated to change in tolerated muscle pressure (r = -0.40, P < 0.001), whereas the increase in prolactin concentration was positively correlated (r = 0.32, P = 0.001). The effect of morphine on EEG was seen as a decrease in the relative theta (4-7.5 Hz) activity (P = 0.03), but was not significant until 120 min after dosing and did not correlate to the increase in tolerated muscle pressure (r = -0.1, P=0.43).

DISCUSSION

Prolactin concentration and pupil diameter showed similar temporal development, had good dynamic ranges and were sensitive to morphine. Thus, both measures proved to be sensitive measures of morphine effects. EEG may give additive information on the brain's response to pain, however more advanced analysis may be necessary. We therefore recommend using pupil diameter in studies where a simple and reliable objective measure of the morphine-induced central activation is needed.

Dynamic measurement of extracellular opioid activity: status quo, challenges, and significance in rewarded behaviors

Opioid peptides are the endogenous ligands of opioid receptors, which are also the molecular target of naturally occurring and synthetic opiates, such as morphine and heroin. Since their discovery in the 1970s, opioid peptides, which are found widely throughout the central nervous system and the periphery, have been intensely studied because of their involvement in pain and pleasure. Over the years, our understanding of opioid peptides has widened to cover a multitude of functions, including learning and memory, affective state, gastrointestinal transit, feeding, immune function, and metabolism. Unsurprisingly, aberrant opioid activity is implicated in numerous pathologies, including drug addiction, overeating, pain, depression, and obesity. To date, virtually all preclinical and clinical studies aimed at understanding the function of endogenous opioids have relied upon manipulating endogenous opioid fluxes using opioid receptor ligands or genetic manipulations of opioid receptors and endogenous opioids. Difficulties in directly monitoring endogenous opioid fluxes, particularly in the central nervous system, have presented a major obstacle to fully understanding endogenous opioid function. This review summarizes these challenges and offers suggestions for future goals while focusing on the neurobiology of reward, specifically drawing attention to studies that have succeeded in dynamically measuring opioid peptides.

Test-retest reproducibility of binding parameters in humans with 11C-LY2795050, an antagonist PET radiotracer for the κ opioid receptor

(11)C-LY2795050 is a new antagonist PET radioligand for the κ opioid receptor (KOR). In this study, we assessed the reproducibility of the binding parameters of (11)C-LY2795050 in healthy human subjects.

METHODS

Sixteen healthy subjects (11 men and 5 women) underwent 2 separate 90-min PET scans with arterial input function and plasma free fraction (fP) measurements. The 2-tissue-compartment model and multilinear analysis-1 were applied to calculate 5 outcome measures in 14 brain regions: distribution volume (VT), VT normalized by fP (VT/fP), and 3 binding potentials (nondisplaceable binding potential, binding potential relative to total plasma concentration, and binding potential relative to free plasma concentration: BPND, BPP, BPF, respectively). Since KOR is distributed ubiquitously throughout the brain, there are no suitable reference regions. We used a fixed fraction of individual cerebellar VT value (VT,CER) as the nondisplaceable VT (VND) (VND = VT,CER/1.17). The relative and absolute test-retest variability and intraclass correlation coefficient were evaluated for the outcome measures of (11)C-LY2795050.

RESULTS

The test-retest variability of (11)C-LY2795050 for VT was no more than 10% in any region and was 12% in the amygdala. For binding potential (BPND and BPP), the test-retest variability was good in regions of moderate and high KOR density (BPND > 0.4) and poor in regions of low density. Correction by fP (VT/fP or BPF) did not improve the test-retest performance.

CONCLUSION

Our results suggest that quantification of (11)C-LY2795050 imaging is reproducible and reliable in regions with moderate and high KOR density. Therefore, we conclude that this first antagonist radiotracer is highly useful for PET studies of KOR.

Noninvasive blood-free full quantification of positron emission tomography radioligand binding

Full quantification of a positron emission tomography (PET) radioligand binding to its target is preferred because it requires the fewest assumptions, but generally involves measuring the concentration of free radioligand in the arterial plasma by collecting blood samples from the subject's radial artery during the scan, and performing metabolite analysis. This invasive, costly procedure deters subjects' participation, and requires specialized staff and equipment. Simultaneous estimation (SIME) can fully quantify binding using only PET data from multiple brain regions and one individual anchor value, which is based on a single arterial blood sample. Drawing this sample can still be challenging in clinical settings, particularly when using simultaneous PET/magnetic resonance scanners. Here we propose a methodology for full quantification of binding that does not require any blood samples. The methodology substitutes the SIME blood-based anchor with a value predicted using multiple linear regression of noninvasive, easy-to-collect variables related to the radioligand blood concentration, and individual metabolism, such as injected dose, body mass index, or body surface area. As a study case, we show here the methodology in comparison to analysis with full arterial-line blood sampling in a cohort of 23 available scans with [(11)C]CUMI-101, a partial agonist of the serotonin 5-HT1A receptors.

Simultaneous fMRI-PET of the opioidergic pain system in human brain

MRI and PET provide complementary information for studying brain function. While the potential use of simultaneous MRI/PET for clinical diagnostic and disease staging has been demonstrated recently; the biological relevance of concurrent functional MRI-PET brain imaging to dissect neurochemically distinct components of the blood oxygenation level dependent (BOLD) fMRI signal has not yet been shown. We obtained sixteen fMRI-PET data sets from eight healthy volunteers. Each subject participated in randomized order in a pain scan and a control (nonpainful pressure) scan on the same day. Dynamic PET data were acquired with an opioid radioligand, [(11)C]diprenorphine, to detect endogenous opioid releases in response to pain. BOLD fMRI data were collected at the same time to capture hemodynamic responses. In this simultaneous human fMRI-PET imaging study, we show co-localized responses in thalamus and striatum related to pain processing, while modality specific brain networks were also found. Co-localized fMRI and PET signal changes in the thalamus were positively correlated suggesting that pain-induced changes in opioid neurotransmission contribute a significant component of the fMRI signal change in this region. Simultaneous fMRI-PET provides unique opportunities allowing us to relate specific neurochemical events to functional hemodynamic activation and to investigate the impacts of neurotransmission on neurovascular coupling of the human brain in vivo.

Association of in vivo κ-opioid receptor availability and the transdiagnostic dimensional expression of trauma-related psychopathology

IMPORTANCE

Exposure to trauma increases the risk for developing threat (ie, fear) symptoms, such as reexperiencing and hyperarousal symptoms, and loss (ie, dysphoria) symptoms, such as emotional numbing and depressive symptoms. While preclinical data have implicated the activated dynorphin/κ-opioid receptor (KOR) system in relation to these symptoms, the role of the KOR system in mediating these phenotypes in humans is unknown. Elucidation of molecular targets implicated in threat and loss symptoms is important because it can help inform the development of novel, mechanism-based treatments for trauma-related psychopathology.

OBJECTIVE

To use the newly developed [11C]LY2795050 radiotracer and high-resolution positron emission tomography to evaluate the relation between in vivo KOR availability in an amygdala-anterior cingulate cortex-ventral striatal neural circuit and the severity of threat and loss symptoms. We additionally evaluated the role of 24-hour urinary cortisol levels in mediating this association.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional positron emission tomography study under resting conditions was conducted at an academic medical center. Thirty-five individuals representing a broad transdiagnostic and dimensional spectrum of trauma-related psychopathology, ranging from nontrauma-exposed psychiatrically healthy adults to trauma-exposed adults with severe trauma-related psychopathology (ie, posttraumatic stress disorder, major depressive disorder, and/or generalized anxiety disorder).

MAIN OUTCOMES AND MEASURES

[11C]LY2795050 volume of distribution values in amygdala-anterior cingulate cortex-ventral striatal neural circuit; composite measures of threat (ie, reexperiencing, avoidance, and hyperarousal symptoms) and loss (ie, emotional numbing, major depressive disorder, and generalized anxiety disorder symptoms) symptoms as assessed using the Clinician-Administered PTSD Scale, Hamilton Depression Rating Scale, and Hamilton Rating Scale for Anxiety; and 24-hour urinary cortisol levels.

RESULTS

[11C]LY2795050 volume of distribution values in an amygdala-anterior cingulate cortex-ventral striatal neural circuit were negatively associated with severity of loss (r = -0.39; 95% CI, -0.08 to -0.66), but not threat (r = -0.03; 95% CI, -0.30 to 0.27), symptoms; this association was most pronounced for dysphoria symptoms (r = -0.45; 95% CI, -0.10 to -0.70). Path analysis revealed that lower [11C]LY2795050 volume of distribution values in this circuit was directly associated with greater severity of loss symptoms and indirectly mediated by 24-hour urinary cortisol levels.

CONCLUSIONS AND RELEVANCE

Results of this study suggest that KOR availability in an amygdala-anterior cingulate cortex-ventral striatal neural circuit mediates the phenotypic expression of trauma-related loss (ie, dysphoria) symptoms. They further suggest that an activated corticotropin-releasing factor/hypothalamic-pituitary-adrenal axis system, as assessed by 24-hour urinary cortisol levels, may indirectly mediate this association. These results may help inform the development of more targeted, mechanism-based transdiagnostic treatments for loss (ie, dysphoric) symptoms.

Synthesis and evaluation of three structurally related ¹⁸F-labeled orvinols of different intrinsic activities: 6-O-[¹⁸F]fluoroethyl-diprenorphine ([¹⁸F]FDPN), 6-O-[¹⁸F]fluoroethyl-buprenorphine ([¹⁸F]FBPN), and 6-O-[¹⁸F]fluoroethyl-phenethyl-orvinol ([¹⁸F]FPEO)

We report the synthesis and biological evaluation of a triplet of 6-O-(18)F-fluoroethylated derivatives of structurally related orvinols that span across the full range of intrinsic activities, the antagonist diprenorphine, the partial agonist buprenorphine, and the full agonist phenethyl-orvinol. [(18)F]fluoroethyl-diprenorphine, [(18)F]fluoroethyl-buprenorphine, and [(18)F]fluoroethyl-phenethyl-orvinol were prepared in high yields and quality from their 6-O-desmethyl-precursors. The results indicate suitable properties of the three 6-O-(18)F-fluoroethylated derivatives as functional analogues to the native carbon-11 labeled versions with similar pharmacological properties.

Automated radiochemical synthesis and biodistribution of [¹¹C]l-α-acetylmethadol ([¹¹C]LAAM)

Long-acting opioid agonists methadone and l-α-acetylmethadol (LAAM) prevent withdrawal in opioid-dependent persons. Attempts to synthesize [(11)C]-methadone for PET evaluation of brain disposition were unsuccessful. Owing, however, to structural and pharmacologic similarities, we aimed to develop [(11)C]LAAM as a PET ligand to probe the brain exposure of long-lasting opioids in humans. This manuscript describes [(11)C]LAAM synthesis and its biodistribution in mice. The radiochemical synthetic strategy afforded high radiochemical yield, purity and specific activity, thereby making the synthesis adaptable to automated modules.

An Improved Antagonist Radiotracer for the κ-Opioid Receptor: Synthesis and Characterization of (11)C-LY2459989

The κ-opioid receptors (KORs) are implicated in several neuropsychiatric diseases and addictive disorders. PET with radioligands provides a means to image the KOR in vivo and investigate its function in health and disease. The purpose of this study was to develop the selective KOR antagonist (11)C-LY2459989 as a PET radioligand and characterize its imaging performance in nonhuman primates.

METHODS

LY2459989 was synthesized and assayed for in vitro binding to opioid receptors. Ex vivo studies in rodents were conducted to assess its potential as a tracer candidate. (11)C-LY2459989 was synthesized by reaction of its iodophenyl precursor with (11)C-cyanide, followed by partial hydrolysis of the resulting (11)C-cyanophenyl intermediate. Imaging experiments with (11)C-LY2459989 were performed in rhesus monkeys with arterial input function measurement. Imaging data were analyzed with kinetic models to derive in vivo binding parameters.

RESULTS

LY2459989 is a full antagonist with high binding affinity and selectivity for KOR (0.18, 7.68, and 91.3 nM, respectively, for κ, μ, and δ receptors). Ex vivo studies in rats indicated LY2459989 as an appropriate tracer candidate with high specific binding signals and confirmed its KOR binding selectivity in vivo. (11)C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, (11)C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, (11)C-LY2459989 displayed fast uptake kinetics in the brain and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity in the brain. Further, specific binding of (11)C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential values derived from the multilinear analysis-1 (MA1) method, as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, and 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus.

CONCLUSION

The novel PET radioligand (11)C-LY2459989 displayed favorable pharmacokinetic properties, a specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR.

Low- and high-frequency transcutaneous electrical acupoint stimulation induces different effects on cerebral μ-opioid receptor availability in rhesus monkeys

Although systematic studies have demonstrated that acupuncture or electroacupuncture (EA) analgesia is based on their accelerating endogenous opioid release to activate opioid receptors and that EA of different frequencies is mediated by different opioid receptors in specific areas of the central nervous system, there is little direct, real-time evidence to confirm this in vivo. The present study was designed to investigate the effects of transcutaneous electrical acupoint stimulation (TEAS), an analogue of EA, at low and high frequencies on μ-opioid receptor (MOR) availability in the brain of rhesus monkeys. Monkeys underwent 95-min positron emission tomography (PET) with (11) C-carfentanil three times randomly while receiving 0, 2, or 100 Hz TEAS, respectively. Each TEAS was administered in the middle 30 min during the 95-min PET scan, and each session of PET and TEAS was separated by at least 2 weeks. The results revealed that 2 Hz but not 100 Hz TEAS evoked a significant increase in MOR binding potential in the anterior cingulate cortex, the caudate nucleus, the putamen, the temporal lobe, the somatosensory cortex, and the amygdala compared with 0 Hz TEAS. The effect remained after the end of TEAS in the anterior cingulate cortex and the temporal lobe. The selective increase in MOR availability in multiple brain regions related to pain and sensory processes may play a role in mediating low-frequency TEAS efficacy.

Comparative evaluation of opioid-induced changes in immune reactivity of CBA mice

Comparison of IgM and IgG-humoral immune response of CBA mice immunized with SE (5 × 10(8) cells) to activation of μ- (DAGO, 100 μg/kg) and δ2- (DSLET, 100 μg/kg) opioid receptors was studied. Different effects of opioids on IgM and IgG antibody production and peripheral blood and spleen levels of CD4(+) and CD8(+) subpopulations were detected at the peak of immune response under these conditions.

The role of the opioid system in alcohol dependence

The role of the brain opioid system in alcohol dependence has been the subject of much research for over 25 years. This review explores the evidence: firstly describing the opioid receptors in terms of their individual subtypes, neuroanatomy, neurophysiology and ligands; secondly, summarising emerging data from specific neurochemical, behavioural and neuroimaging studies, explaining the characteristics of addiction with a focus on alcohol dependence and connecting the opioid system with alcohol dependence; and finally reviewing the known literature regarding opioid antagonists in clinical use for alcohol dependence. Further interrogation of how modulation of the opioid system, via use of MOP (mu), DOP (delta) and KOP (kappa) agents, restores the balance of a dysregulated system in alcohol dependence should increase our insight into this disease process and therefore guide better methods for understanding and treating alcohol dependence in the future.