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

Acute Stress Increases Striatal Connectivity With Cortical Regions Enriched for μ and κ Opioid Receptors

BACKGROUND

Understanding the neurobiological effects of stress is critical for addressing the etiology of major depressive disorder (MDD). Using a dimensional approach involving individuals with differing degree of MDD risk, we investigated 1) the effects of acute stress on cortico-cortical and subcortical-cortical functional connectivity (FC) and 2) how such effects are related to gene expression and receptor maps.

METHODS

Across 115 participants (37 control, 39 remitted MDD, 39 current MDD), we evaluated the effects of stress on FC during the Montreal Imaging Stress Task. Using partial least squares regression, we investigated genes whose expression in the Allen Human Brain Atlas was associated with anatomical patterns of stress-related FC change. Finally, we correlated stress-related FC change maps with opioid and GABAA (gamma-aminobutyric acid A) receptor distribution maps derived from positron emission tomography.

RESULTS

Results revealed robust effects of stress on global cortical connectivity, with increased global FC in frontoparietal and attentional networks and decreased global FC in the medial default mode network. Moreover, robust increases emerged in FC of the caudate, putamen, and amygdala with regions from the ventral attention/salience network, frontoparietal network, and motor networks. Such regions showed preferential expression of genes involved in cell-to-cell signaling (OPRM1, OPRK1, SST, GABRA3, GABRA5), similar to previous genetic MDD studies.

CONCLUSIONS

Acute stress altered global cortical connectivity and increased striatal connectivity with cortical regions that express genes that have previously been associated with imaging abnormalities in MDD and are rich in μ and κ opioid receptors. These findings point to overlapping circuitry underlying stress response, reward, and MDD.

Kappa-opioid receptor stimulation in the nucleus accumbens shell and ethanol drinking: Differential effects by rostro-caudal location and level of drinking

Although the kappa-opioid receptor (KOR) and its endogenous ligand, dynorphin, are believed to be involved in ethanol drinking, evidence on the direction of their effects has been mixed. The nucleus accumbens (NAc) shell densely expresses KORs, but previous studies have not found KOR activation to influence ethanol drinking. Using microinjections into the NAc shell of male and female Long-Evans rats that drank under the intermittent-access procedure, we found that the KOR agonist, U50,488, had no effect on ethanol drinking when injected into the middle NAc shell, but that it promoted intake in males and high-drinking females in the caudal NAc shell and high-drinking females in the rostral shell, and decreased intake in males and low-drinking females in the rostral shell. Conversely, injection of the KOR antagonist, nor-binaltorphimine, stimulated ethanol drinking in low-drinking females when injected into the rostral NAc shell and decreased drinking in high-drinking females when injected into the caudal NAc shell. These effects of KOR activity were substance-specific, as U50,488 did not affect sucrose intake. Using quantitative real-time PCR, we found that baseline gene expression of the KOR was higher in the rostral compared to caudal NAc shell, but that this was upregulated in the rostral shell with a history of ethanol drinking. Our findings have important clinical implications, demonstrating that KOR stimulation in the NAc shell can affect ethanol drinking, but that this depends on NAc subregion, subject sex, and ethanol intake level, and suggesting that this may be due to differences in KOR expression.

Voluntary Exercise Ameliorates Chronic Ethanol Withdrawal-Induced Adaptations of Opioid Receptor Expression in the Nucleus Accumbens, Dopamine Release, and Ethanol Consumption

Exercise has increasingly been recognized as an adjunctive therapy for alcohol-use disorder (AUD), yet our understanding of its underlying neurological mechanisms remains limited. This knowledge gap impedes the development of evidence-based exercise guidelines for AUD treatment. Chronic ethanol (EtOH) exposure has been shown to upregulate and sensitize kappa opioid receptors (KORs) in the nucleus accumbens (NAc), which is innervated by dopamine (DA) neurons in the midbrain ventral tegmental area (VTA), which may contribute to AUD-related behaviors. In this study, we investigated the impact of voluntary exercise in EtOH-dependent mice on EtOH consumption, KOR and delta opioid receptor (DOR) expression in the NAc and VTA, and functional effects on EtOH-induced alterations in DA release in the NAc. Our findings reveal that voluntary exercise reduces EtOH consumption, reduces KOR and enhances DOR expression in the NAc, and modifies EtOH-induced adaptations in DA release, suggesting a competitive interaction between exercise-induced and EtOH-induced alterations in KOR expression. We also found changes to DOR expression in the NAc and VTA with voluntary exercise but no significant changes to DA release. These findings elucidate the complex interplay of AUD-related neurobiological processes, highlighting the potential for exercise as a therapeutic intervention for AUD.

Opioid modulation of prefrontal cortex cells and circuits

Several neurochemical systems converge in the prefrontal cortex (PFC) to regulate cognitive and motivated behaviors. A rich network of endogenous opioid peptides and receptors spans multiple PFC cell types and circuits, and this extensive opioid system has emerged as a key substrate underlying reward, motivation, affective behaviors, and adaptations to stress. Here, we review the current evidence for dysregulated cortical opioid signaling in the pathogenesis of psychiatric disorders. We begin by providing an introduction to the basic anatomy and function of the cortical opioid system, followed by a discussion of endogenous and exogenous opioid modulation of PFC function at the behavioral, cellular, and synaptic level. Finally, we highlight the therapeutic potential of endogenous opioid targets in the treatment of psychiatric disorders, synthesizing clinical reports of altered opioid peptide and receptor expression and activity in human patients and summarizing new developments in opioid-based medications. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Controlling the human connectome with spatially diffuse input signals

The human brain is never at "rest"; its activity is constantly fluctuating over time, transitioning from one brain state-a whole-brain pattern of activity-to another. Network control theory offers a framework for understanding the effort - energy - associated with these transitions. One branch of control theory that is especially useful in this context is "optimal control", in which input signals are used to selectively drive the brain into a target state. Typically, these inputs are introduced independently to the nodes of the network (each input signal is associated with exactly one node). Though convenient, this input strategy ignores the continuity of cerebral cortex - geometrically, each region is connected to its spatial neighbors, allowing control signals, both exogenous and endogenous, to spread from their foci to nearby regions. Additionally, the spatial specificity of brain stimulation techniques is limited, such that the effects of a perturbation are measurable in tissue surrounding the stimulation site. Here, we adapt the network control model so that input signals have a spatial extent that decays exponentially from the input site. We show that this more realistic strategy takes advantage of spatial dependencies in structural connectivity and activity to reduce the energy (effort) associated with brain state transitions. We further leverage these dependencies to explore near-optimal control strategies such that, on a per-transition basis, the number of input signals required for a given control task is reduced, in some cases by two orders of magnitude. This approximation yields network-wide maps of input site density, which we compare to an existing database of functional, metabolic, genetic, and neurochemical maps, finding a close correspondence. Ultimately, not only do we propose a more efficient framework that is also more adherent to well-established brain organizational principles, but we also posit neurobiologically grounded bases for optimal control.

Dynamic regulation of CeA gene expression during acute and protracted abstinence from chronic binge drinking of male and female C57BL/6J mice

Binge alcohol consumption is a major risk factor for developing Alcohol Use Disorder (AUD) and is associated with alcohol-related problems like accidental injury, acute alcohol poisoning, and black-outs. While there are numerous brain regions that have been shown to play a role in this AUD in humans and animal models, the central nucleus of the amygdala (CeA) has emerged as a critically important locus mediating binge alcohol consumption. In this study, we sought to understand how relative gene expression of key signaling molecules in the CeA changes during different periods of abstinence following bouts of binge drinking. To test this, we performed drinking in the dark (DID) on two separate cohorts of C57BL/6J mice and collected CeA brain tissue at one day (acute) and 7 days (protracted) abstinence after DID. We used qRTPCR to evaluate relative gene expression changes of 25 distinct genes of interest related to G protein-coupled receptors (GPCRs), neuropeptides, ion channel subunits, and enzymes that have been previously implicated in AUD. Our findings show that during acute abstinence CeA punches collected from female mice had upregulated relative mRNA expression of the gamma-aminobutyric acid receptor subunit alpha 2 (Gabra2), and the peptidase, angiotensinase c (Prcp). CeA punches from male mice at the same time point in abstinence had upregulated relative mRNA encoding for neuropeptide-related molecules, neuropeptide Y (Npy) and somatostatin (Sst), as well as the neuropeptide Y receptor Y2 (Npyr2) but downregulated, Glutamate ionotropic receptor NMDA type subunit 1 (Grin1). After protracted abstinence CeA punches collected from female mice had increased mRNA expression of corticotropin releasing hormone (Crh) and Npy. While CeA punches collected from male mice at the same timepoint had upregulated relative mRNA expression of Npy2r and downregulated mRNA expression of Gabra2, Grin1 and opioid receptor kappa 1 (Oprk1). Our findings support that there are differences in how the CeA of male and female respond to binge-alcohol exposure, highlighting the need to understand the implications of such differences in the context of AUD and binge drinking behavior.

Naltrexone engages a brain reward network in the presence of reward-predictive distractor stimuli in males

The non-selective opioid receptor antagonist, naltrexone is one of the most prescribed medications for treating alcohol and opioid addiction. Despite decades of clinical use, the mechanism(s) by which naltrexone reduces addictive behavior remains unclear. Pharmaco-fMRI studies to date have largely focused on naltrexone's impact on brain and behavioral responses to drug or alcohol cues or on decision-making circuitry. We hypothesized that naltrexone's effects on reward-associated brain regions would associate with reduced attentional bias (AB) to non-drug, reward-conditioned cues. Twenty-three adult males, including heavy and light drinkers, completed a two-session, placebo-controlled, double-blind study testing the effects of acute naltrexone (50 mg) on AB to reward-conditioned cues and neural correlates of such bias measured via fMRI during a reward-driven AB task. While we detected significant AB to reward-conditioned cues, naltrexone did not reduce this bias in all participants. A whole-brain analysis found that naltrexone significantly altered activity in regions associated with visuomotor control regardless of whether a reward-conditioned distractor was present. A region-of-interest analysis of reward-associated areas found that acute naltrexone increased BOLD signal in the striatum and pallidum. Moreover, naltrexone effects in the pallidum and putamen predicted individual reduction in AB to reward-conditioned distractors. These findings suggest that naltrexone's effects on AB primarily reflect not reward processing per se, but rather top-down control of attention. Our results suggest that the therapeutic actions of endogenous opioid blockade may reflect changes in basal ganglia function enabling resistance to distraction by attractive environmental cues, which could explain some variance in naltrexone's therapeutic efficacy.

Insula Dynorphin and Kappa Opioid Receptor Systems Regulate Alcohol Drinking in a Sex-Specific Manner in Mice

Alcohol use disorder is complex and multifaceted, involving the coordination of multiple signaling systems across numerous brain regions. Previous work has indicated that both the insular cortex and dynorphin (DYN)/kappa opioid receptor (KOR) systems contribute to excessive alcohol use. More recently, we identified a microcircuit in the medial aspect of the insular cortex that signals through DYN/KOR. Here, we explored the role of insula DYN/KOR circuit components on alcohol intake in a long-term intermittent access (IA) procedure. Using a combination of conditional knock-out strategies and site-directed pharmacology, we discovered distinct and sex-specific roles for insula DYN and KOR in alcohol drinking and related behavior. Our findings show that insula DYN deletion blocked escalated consumption and decreased the overall intake of and preference for alcohol in male and female mice. This effect was specific to alcohol in male mice, as DYN deletion did not impact sucrose intake. Further, insula KOR antagonism reduced alcohol intake and preference during the early phase of IA in male mice only. Alcohol consumption was not affected by insula KOR knockout in either sex. In addition, we found that long-term IA decreased the intrinsic excitability of DYN and deep layer pyramidal neurons (DLPNs) in the insula of male mice. Excitatory synaptic transmission was also impacted by IA, as it drove an increase in excitatory synaptic drive in both DYN neurons and DLPNs. Combined, our findings suggest there is a dynamic interplay between excessive alcohol consumption and insula DYN/KOR microcircuitry.SIGNIFICANCE STATEMENT The insular cortex is a complex region that serves as an integratory hub for sensory inputs. In our previous work, we identified a microcircuit in the insula that signals through the kappa opioid receptor (KOR) and its endogenous ligand dynorphin (DYN). Both the insula and DYN/KOR systems have been implicated in excessive alcohol use and alcohol use disorder (AUD). Here, we use converging approaches to determine how insula DYN/KOR microcircuit components contribute to escalated alcohol consumption. Our findings show that insula DYN/KOR systems regulate distinct phases of alcohol consumption in a sex-specific manner, which may contribute to the progression to AUD.

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Clustering of KOR PET images separates people with AUD into distinct responses to naltrexone

Striatal kappa opioid receptor (KOR) availability in 48 subjects with Alcohol Use Disorder (AUD) was previously found to be associated with degree of drinking following a week of naltrexone treatment (de Laat et al. Biological Psychiatry, 86(11), 864-871, 2019). The purpose of the current study was to determine if spectral clustering applied to previously acquired KOR images (with [11C]LY2795050 PET) could identify meaningful groupings of different responses to naltrexone and to assess the robustness of the finding. Spectral clustering was applied to 6 features (regional volume of distribution values, V_T) per AUD subject to produce 3 classes of subjects with different mean responses to naltrexone. Response to naltrexone was quantified as the difference in drinks consumed in an established lab-based alcohol drinking paradigm (Krishnan-Sarin et al. Biological Psychiatry, 62(6), 694-697, 2007) prior to, and after a week of naltrexone treatment. Clustering was applied exclusively to features of the image data with no a priori knowledge of the subjects' responses. Separation of classes was tested using a 1-way analysis of variance (ANOVA) with drink reduction as the outcome of interest. To assess robustness of the result, the size of the training set was varied by using successively reduced subsets of the data. Clustering resulted in significantly different groupings of drink reduction. The finding was robust to initialization of the spectral clustering procedure and was replicable for different random subsets of training subjects. Finding: Spectral clustering of kappa PET images separates AUD subjects into behaviorally distinct groups expressing distinct responses to naltrexone.

Molecular Imaging Studies of Alcohol Use Disorder

Alcohol use disorder (AUD) is a serious public health problem in many countries, bringing a gamut of health risks and impairments to individuals and a great burden to society. Despite the prevalence of a disease model of AUD, the current pharmacopeia does not present reliable treatments for AUD; approved treatments are confined to a narrow spectrum of medications engaging inhibitory γ-aminobutyric acid (GABA) neurotransmission and possibly excitatory N-methyl-D-aspartate (NMDA) receptors, and opioid receptor antagonists. Molecular imaging with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can open a window into the living brain and has provided diverse insights into the pathology of AUD. In this narrative review, we summarize the state of molecular imaging findings on the pharmacological action of ethanol and the neuropathological changes associated with AUD. Laboratory and preclinical imaging results highlight the interactions between ethanol and GABA A-type receptors (GABAAR), but the interpretation of such results is complicated by subtype specificity. An abundance of studies with the glucose metabolism tracer fluorodeoxyglucose (FDG) concur in showing cerebral hypometabolism after ethanol challenge, but there is relatively little data on long-term changes in AUD. Alcohol toxicity evokes neuroinflammation, which can be tracked using PET with ligands for the microglial marker translocator protein (TSPO). Several PET studies show reversible increases in TSPO binding in AUD individuals, and preclinical results suggest that opioid-antagonists can rescue from these inflammatory responses. There are numerous PET/SPECT studies showing changes in dopaminergic markers, generally consistent with an impairment in dopamine synthesis and release among AUD patients, as seen in a number of other addictions; this may reflect the composite of an underlying deficiency in reward mechanisms that predisposes to AUD, in conjunction with acquired alterations in dopamine signaling. There is little evidence for altered serotonin markers in AUD, but studies with opioid receptor ligands suggest a specific up-regulation of the μ-opioid receptor subtype. Considerable heterogeneity in drinking patterns, gender differences, and the variable contributions of genetics and pre-existing vulnerability traits present great challenges for charting the landscape of molecular imaging in AUD.

Differences in the association between kappa opioid receptors and pain among Black and White adults with alcohol use disorders

BACKGROUND

The relationship between alcohol and pain is complex. Associations between pain and alcohol use disorder (AUD) vary by race, but the underlying biological basis is not understood. We examined the association of the kappa opioid receptor (KOR) with responses to the cold-pressor test (CPT), before and after treatment with the opioid antagonist naltrexone, among individuals with AUD who self-identified as Black or White.

METHODS

Thirty-seven individuals (12 Black, 24 White, and 1 Multiracial) with AUD participated in two CPTs, separated by 1 week during which they received naltrexone 100 mg daily. During each CPT, pain reporting threshold (PRT), average pain increase rate (APIR), relative pain recovery (RPR), and alcohol craving were recorded. KOR availability was measured using [¹¹ C]-LY2795050 positron emission tomography (PET) prior to treatment with naltrexone.

RESULTS

Black participants reported higher PRT and APIR than White participants during the CPT before, but not after, naltrexone treatment. Among Black participants, KOR availability was positively associated with PRT and APIR before, but not after naltrexone. Greater KOR availability was associated with faster RPR for White, but not Black, participants. The CPT induced more alcohol craving in Black than White participants, particularly in individuals with low KOR availability, an effect that was not attenuated by naltrexone.

CONCLUSIONS

KOR involvement and naltrexone effects on responses to the CPT were different between Black and White participants. These preliminary findings suggest that further exploration of the differences in the opioid system and pain among Black and White individuals with AUD and their relationship with naltrexone's effects is warranted.

Multimodal neuroimaging of metabotropic glutamate 5 receptors and functional connectivity in alcohol use disorder

BACKGROUND

People recovering from alcohol use disorder (AUD) show altered resting brain connectivity. The metabotropic glutamate 5 (mGlu5) receptor is an important regulator of synaptic plasticity potentially linked with synchronized brain activity and a target of interest in treating AUD. The goal of this work was to assess potential relationships of brain connectivity at rest with mGlu5 receptor availability in people with AUD at two time points early in abstinence.

METHODS

Forty-eight image data sets were acquired with a multimodal neuroimaging battery that included resting-state functional magnetic resonance imaging (fMRI) and mGlu5 receptor positron emission tomography (PET) with the radiotracer [¹⁸ F]FPEB. Participants with AUD (n = 14) were scanned twice, at approximately 1 and 4 weeks after beginning supervised abstinence. [¹⁸ F]FPEB PET results were published previously. Primary comparisons of fMRI outcomes were performed between the AUD group and healthy controls (HCs; n = 23) and assessed changes over time within the AUD group. Relationships between resting-state connectivity measures and mGlu5 receptor availability were explored within groups.

RESULTS

Compared to HCs, global functional connectivity of the orbitofrontal cortex was higher in the AUD group at 4 weeks of abstinence (p = 0.003), while network-level functional connectivity within the default mode network (DMN) was lower (p < 0.04). Exploratory multimodal analyses showed that mGlu5 receptor availability was correlated with global connectivity across all brain regions (HCs, r = 0.41; AUD group at 1 week of abstinence, r = 0.50 and at 4 weeks, r = 0.46; all p < 0.0001). Furthermore, a component of cortical and striatal mGlu5 availability was correlated with connectivity between the DMN and salience networks in HCs (r = 0.60, p = 0.003) but not in the AUD group (p > 0.3).

CONCLUSIONS

These preliminary findings of altered global and network connectivity during the first month of abstinence from drinking may reflect the loss of efficient network function, while exploratory relationships with mGlu5 receptor availability suggest a potential glutamatergic relationship with network coherence.

Diels-Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels-Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

Clinical moderators of response to nalmefene in a randomized-controlled trial for alcohol dependence: An exploratory analysis

BACKGROUND

Nalmefene is the only medication marketed to reduce the consumption of alcohol in patients with alcohol dependence, but it remains unclear which patients could most benefit from it. This study aimed to identify clinical moderators that affect treatment response to nalmefene in patients with alcohol dependence.

METHODS

In a multicenter, randomized, controlled, double-blind, phase 3 study of nalmefene on Japanese patients with alcohol dependence, the relationship between the reduction of heavy drinking days (HDD) and total alcohol consumption (TAC) at 12 and 24 weeks of treatment and baseline variables of the participants were analyzed in a linear regression and multiple adjusted analysis.

RESULTS

Age < 65, no family history of problem drinking, age at onset of problem drinking ≥ 25, and not currently smoking were possible positive moderators. Nalmefene showed a significant HDD reduction in patients with age < 65 or no family history of problem drinking, and a significant TAC reduction in patients with age at onset of problem drinking ≥ 25 or who were not currently smoking. After multiple adjusted analyses, age < 65 (p = .028), no family history of problem drinking (p = .047), and age at onset of problem drinking ≥ 25 (p = .030) were statistically significant. Not currently smoking (p = .071) was marginally significant. In combination, these moderators indicated synergistic effects.

CONCLUSIONS

Alcohol-dependent patients with favorable prognostic factors such as non-smoking status, no family history of problem drinking, and a late-onset of problem drinking selectively benefit from nalmefene. Further research is needed to validate these exploratory results.

Evaluation of Specific Binding of [11C]RTI-97 to Kappa Opioid Receptor by Autoradiography and PET Imaging in Rat

Kappa opioid receptor (KOR) PET imaging remains attractive to understand the role of KOR in health and diseases and to help the development of drugs especially for psychiatric disorders such as depression, anxiety, and addiction. The potent and selective KOR antagonist RTI-97 labeled with carbon-11 was previously demonstrated to display specific KOR binding in mouse brain by ex vivo autoradiography studies. Herein, we evaluated [¹¹C]RTI-97 in rat by in vitro autoradiography and by in vivo PET imaging. The radiosynthesis of [¹¹C]RTI-97 was optimized to obtain high molar activities. Despite a low cerebral uptake, the overall results showed a heterogeneous repartition and specific KOR binding of [¹¹C]RTI-97 in brain and a high and specific accumulation of [¹¹C]RTI-97 in pituitary in accordance with KOR expression.

Occupancy of the kappa opioid receptor by naltrexone predicts reduction in drinking and craving

The efficacy of naltrexone to treat alcohol use disorder (AUD) is modest. A better understanding of the neurobiology underlying naltrexone effects could optimize treatments. We evaluated the occupancy of the kappa opioid receptor (KOR) by naltrexone measured with [¹¹C]-LY2795050 positron emission tomography (PET) as a predictor of response to naltrexone. Response to naltrexone was defined as the difference in craving and the difference between the number of drinks consumed during an alcohol drinking paradigm (ADP) before and after 1 week of supervised 100 mg daily oral naltrexone. Forty-four (14 F) nontreatment seeking heavy drinkers meeting criteria for AUD were enrolled. Participants drank 47 ± 16 drinks per week and were balanced in family history of alcoholism (FH, 26 positive). High KOR occupancy (92 ± 1%) was achieved. Occupancy was negatively associated with number of years drinking (YOD) in FH positive, but not FH negative, participants (t_3,42 = 4.00, p = 0.0003). Higher KOR occupancy by naltrexone was associated with higher alcohol craving during the ADP (F_1,81 = 4.88, p = 0.030). The reduction in drinking after naltrexone was negatively associated with KOR occupancy, with significant effects of FH status (t_1,43 = -2.08, p = 0.044). A logistic regression model including KOR occupancy, YOD, and FH variables achieved an 84% prediction accuracy for ≥50% reduction in drinking. These results confirm that naltrexone binds at the KOR site and suggest that KOR occupancy by naltrexone may be related to clinical response. Based on our results, we propose that differential affinities for the mu and KOR could explain why lower doses of naltrexone can have greater clinical efficacy.

Chronic Pain in the Elderly: Mechanisms and Distinctive Features

Background: Chronic pain is a major issue affecting more than 50% of the older population and up to 80% of nursing homes residents. Research on pain in the elderly focuses mainly on the development of clinical tools to assess pain in patients with dementia and cognitive impairment or on the efficacy and tolerability of medications. In this review, we searched for evidence of specific pain mechanisms or modifications in pain signals processing either at the cellular level or in the central nervous system. Methods: Narrative review. Results: Investigation on pain sensitivity led to conflicting results, with some studies indicating a modest decrease in age-related pain sensitivity, while other researchers found a reduced pain threshold for pressure stimuli. Areas of the brain involved in pain perception and analgesia are susceptible to pathological changes such as gliosis and neuronal death and the effectiveness of descending pain inhibitory mechanisms, particularly their endogenous opioid component, also appears to deteriorate with advancing age. Hyperalgesia is more common at older age and recovery from peripheral nerve injury appears to be delayed. In addition, peripheral nociceptors may contribute minimally to pain sensation at either acute or chronic time points in aged populations. Conclusions: Elderly subjects appear to be more susceptible to prolonged pain development, and medications acting on peripheral sensitization are less efficient. Pathologic changes in the central nervous system are responsible for different pain processing and response to treatment. Specific guidelines focusing on specific pathophysiological changes in the elderly are needed to ensure adequate treatment of chronic pain conditions.

Imaging Kappa Opioid Receptors in the Living Brain with Positron Emission Tomography

Kappa opioid receptor (KOR) neuroimaging using positron emission tomography (PET) has been immensely successful in all phases of discovery and validation in relation to radiotracer development from preclinical imaging to human imaging. There are now several KOR-specific PET radiotracers that can be utilized for neuroimaging, including agonist and antagonist ligands, as well as C-11 and F-18 variants. These technologies will increase KOR PET utilization by imaging centers around the world and have provided a foundation for future studies. In this chapter, I review the advances in KOR radiotracer discovery, focusing on ligands that have been translated into human imaging, and highlight key attributes unique to each KOR PET radiotracer. The utilization of these radiotracers in KOR PET neuroimaging can be subdivided into three major investigational classes: the first, measurement of KOR density; the second, measurement of KOR drug occupancy; the third, detecting changes in endogenous dynorphin following activation or deactivation. Given the involvement of the KOR/dynorphin system in a number of brain disorders including, but not limited to, pain, itch, mood disorders and addiction, measuring KOR density in the living brain will offer insight into the chronic effects of these disorders on KOR tone in humans. Notably, KOR PET has been successful at measuring drug occupancy in the human brain to guide dose selection for maximal therapeutic efficacy while avoiding harmful side effects. Lastly, we discuss the potential of KOR PET to detect changes in endogenous dynorphin in the human brain, to elucidate neural mechanisms and offer critical insight into disease-modifying therapeutics. We conclude with comments on other translational neuroimaging modalities such as MRI that could be used to study KOR-dynorphin tone in the living human brain.

Kappa opioid receptor and dynorphin signaling in the central amygdala regulates alcohol intake

Excessive alcohol drinking has been shown to modify brain circuitry to predispose individuals for future alcohol abuse. Previous studies have implicated the central nucleus of the amygdala (CeA) as an important site for mediating the somatic symptoms of withdrawal and for regulating alcohol intake. In addition, recent work has established a role for both the Kappa Opioid Receptor (KOR) and its endogenous ligand dynorphin in mediating these processes. However, it is unclear whether these effects are due to dynorphin or KOR arising from within the CeA itself or other input brain regions. To directly examine the role of preprodynorphin (PDYN) and KOR expression in CeA neurons, we performed region-specific conditional knockout of these genes and assessed the effects on the Drinking in the Dark (DID) and Intermittent Access (IA) paradigms. Conditional gene knockout resulted in sex-specific responses wherein PDYN knockout decreased alcohol drinking in both male and female mice, whereas KOR knockout decreased drinking in males only. We also found that neither PDYN nor KOR knockout protected against anxiety caused by alcohol drinking. Lastly, a history of alcohol drinking did not alter synaptic transmission in PDYN neurons in the CeA of either sex, but excitability of PDYN neurons was increased in male mice only. Taken together, our findings indicate that PDYN and KOR signaling in the CeA plays an important role in regulating excessive alcohol consumption and highlight the need for future studies to examine how this is mediated through downstream effector regions.

A balancing act: the role of pro- and anti-stress peptides within the central amygdala in anxiety and alcohol use disorders

The central nucleus of the amygdala (CeA) is widely implicated as a structure that integrates both appetitive and aversive stimuli. While intrinsic CeA microcircuits primarily consist of GABAergic neurons that regulate amygdala output, a notable feature of the CeA is the heterogeneity of neuropeptides and neuropeptide/neuromodulator receptors that it expresses. There is growing interest in the role of the CeA in mediating psychopathologies, including stress and anxiety states and their interactions with alcohol use disorders. Within the CeA, neuropeptides and neuromodulators often exert pro- or anti- stress actions, which can influence anxiety and alcohol associated behaviours. In turn, alcohol use can cause adaptions within the CeA, which may render an individual more vulnerable to stress which is a major trigger of relapse to alcohol seeking. This review examines the neurocircuitry, neurochemical phenotypes and how pro- and anti-stress peptide systems act within the CeA to regulate anxiety and alcohol seeking, focusing on preclinical observations from animal models. Furthermore, literature exploring the targeting of genetically defined populations or neuronal ensembles and the role of the CeA in mediating sex differences in stress x alcohol interactions are explored.

Targeting the Opioid Receptors: A Promising Therapeutic Avenue for Treatment in “Heavy Drinking Smokers”

Aims

Despite a general decline in tobacco use in the last decades, the prevalence of tobacco smoking in individuals with alcohol use disorder (AUD) remains substantial (45–50%). Importantly, the co-use of both substances potentiates the adverse effects, making it a significant public health problem. Substantial evidence suggests that AUD and Tobacco use disorder (TUD) may share common mechanisms. Targeting these mechanisms may therefore provide more effective therapy. Numerous studies describe a potential role of the endogenous opioid system in both AUD and TUD. Reviewing this literature, we aim to evaluate the efficacy of molecules that target the opioid system as promising therapeutic interventions for treating alcohol and tobacco co-use disorders.

Methods

We provide a synthesis of the current epidemiological knowledge of alcohol and tobacco co-use disorders. We evaluate clinical and preclinical research that focuses on the regulation of the endogenous opioid system in alcohol, nicotine, and their interactions.

Results

The epidemiological data confirm that smoking stimulates heavy drinking and facilitates alcohol craving. Pharmacological findings suggest that treatments that are efficacious in the dual addiction provide a beneficial treatment outcome in comorbid AUD and TUD. In this regard, MOP, DOP and NOP-receptor antagonists show promising results, while the findings prompt caution when considering KOP-receptor antagonists as a treatment option in alcohol and tobacco co-use disorders.

Conclusions

Existing literature suggests a role of the opioid system in sustaining the high comorbidity rates of AUD and TUD. Molecules targeting opioid receptors may therefore represent promising therapeutic interventions in ‘heavy drinking smokers.’

Kappa Opioid Receptors in the Pathology and Treatment of Major Depressive Disorder

The kappa opioid receptor (KOR) is thought to regulate neural systems associated with anhedonia and aversion and mediate negative affective states that are associated with a number of psychiatric disorders, but especially major depressive disorder (MDD). Largely because KOR antagonists mitigate the effects of stress in preclinical studies, KOR antagonists have been recommended as novel drugs for treating MDD. The purpose of this review is to examine the role of KORs and its endogenous ligand dynorphins (DYNs) in the pathology and treatment of MDD derived from different types of clinical studies. Evidence pertaining to the role of KOR and MDD will be reviewed from (1) post mortem mRNA expression patterns in MDD, (2) the utility of KOR neuroimaging agents and serum biomarkers in MDD, and (3) evidence from the recent Fast Fail clinical trial that established KOR antagonism as a potential therapeutic strategy for the alleviation of anhedonia, a core feature of MDD. These findings are compared with a focused evaluation of stress-induced alterations in OPRK and PDYN mRNA expression. Finally, the current status of the effects of KOR antagonists on behavioral phenotypes of stress in preclinical studies related to MDD is summarized.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Sex/gender differences in brain function and structure in alcohol use: A narrative review of neuroimaging findings over the last 10 years

Over the last 10 years, rates of alcohol use disorder (AUD) have increased in women by 84% relative to a 35% increase in men. Rates of alcohol use and high-risk drinking have also increased in women by 16% and 58% relative to a 7% and 16% increase in men, respectively, over the last decade. This robust increase in drinking among women highlights the critical need to identify the underlying neural mechanisms that may contribute to problematic alcohol consumption across sex/gender (SG), especially given that many neuroimaging studies are underpowered to detect main or interactive effects of SG on imaging outcomes. This narrative review aims to explore the recent neuroimaging literature on SG differences in brain function and structure as it pertains to alcohol across positron emission tomography, magnetic resonance imaging, and functional magnetic resonance imaging modalities in humans. Additional work using magnetic resonance spectroscopy, diffusion tensor imaging, and event-related potentials to examine SG differences in AUD will be covered. Overall, current research on the neuroimaging of AUD, alcohol consumption, or risk of AUD is limited, and findings are mixed regarding the effect of SG on neurochemical, structural, and functional mechanisms associated with AUD. We address SG disparities in the neuroimaging of AUD and propose a call to action to include women in brain imaging research. Future studies are crucial to our understanding of the neurobiological underpinnings of AUD across neural systems and the vulnerability for AUD among women and men.

The kappa opioid receptor modulates GABA neuron excitability and synaptic transmission in midbrainprojections from the insular cortex

As an integrative hub, the insular cortex (IC) translates external cues into interoceptive states that generate complex physiological, affective, and behavioral responses. However, the precise circuit and signaling mechanisms in the IC that modulate these processes are unknown. Here, we describe a midbrain-projecting microcircuit in the medial aspect of the agranular IC that signals through the Gαi/o-coupled kappa opioid receptor (KOR) and its endogenous ligand dynorphin (Dyn). Within this microcircuit, Dyn is robustly expressed in layer 2/3, while KOR is localized to deep layer 5, which sends a long-range projection to the substantia nigra (SN). Using ex vivo electrophysiology, we evaluated the functional impact of KOR signaling in layer 5 of the IC. We found that bath application of dynorphin decreased GABA release and increased glutamate release on IC-SN neurons, but did not alter their excitability. Conversely, dynorphin decreased the excitability of GABA neurons without altering synaptic transmission. Pretreatment with the KOR antagonist nor-BNI blocked the effects of dynorphin in IC-SN neurons and GABA neurons, indicating that the changes in synaptic transmission and excitability were selectively mediated through KOR. Selective inhibition of IC GABA neurons using a KOR-derived DREADD recapitulated these effects. This work provides insight into IC microcircuitry and indicates that Dyn/KOR signaling may act to directly reduce activity of layer 5 GABA neurons. In turn, KOR-driven inhibition of GABA promotes disinhibition of IC-SN neurons, which can modulate downstream circuits. Our findings present a potential mechanism whereby chronic upregulation of IC Dyn/KOR signaling can lead to altered subcortical function and downstream activity.

The Kappa Opioid Receptor Is Associated With Naltrexone-Induced Reduction of Drinking and Craving

BACKGROUND

Naltrexone is a nonselective opioid receptor antagonist used as a treatment for alcohol use disorder. However, only modest clinical effects have been observed, possibly because of limited knowledge about the biological variables affecting the efficacy of naltrexone. We investigated the potential role of the kappa opioid receptor (KOR) in the therapeutic effect of naltrexone.

METHODS

A total of 48 non-treatment-seeking heavy drinkers (16 women) who met DSM-IV criteria for alcohol dependence participated in two alcohol drinking paradigms (ADPs) separated by a week of open-label naltrexone (100 mg daily). Craving, assessed with the Alcohol Urge Questionnaire and the Yale Craving Scale, and drinking behavior were recorded in each ADP. Prior to naltrexone initiation, KOR availability was determined in the amygdala, hippocampus, pallidum, striatum, cingulate cortex, and prefrontal cortex using positron emission tomography with [¹¹C]LY2795050.

RESULTS

Participants reported lower levels of craving (Yale Craving Scale: -11 ± 1, p < .0001; Alcohol Urge Questionnaire: -6 ± 0.6, p < .0001) and consumed fewer drinks (-3.7 ± 4, p < .0001) during the second ADP following naltrexone therapy. The observed reduction in drinking was negatively associated with baseline KOR availability in the striatum (p = .005), pallidum (p = .023), and cingulate cortex (p = .018). Voxelwise analysis identified clusters in the bilateral insula, prefrontal, and cingulate cortex associated with the reduction in drinking (p < .0001). In addition, KOR availability in all evaluated brain regions was associated with craving measured in both ADPs.

CONCLUSIONS

The KOR is implicated in drinking and craving following naltrexone therapy in alcohol use disorder.

A Survey of Molecular Imaging of Opioid Receptors

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as μ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-¹¹C]morphine, -codeine and -heroin did not show significant binding in vivo. [¹¹C]Diprenorphine ([¹¹C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the μ-preferring agonist [¹¹C]carfentanil ([¹¹C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [¹¹C]DPN or [¹¹C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate μ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of μORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [¹¹C]GR103545 is validated for studies of κORs. Structures such as [¹¹C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

Clinically utilized kappa-opioid receptor agonist nalfurafine combined with low-dose naltrexone prevents alcohol relapse-like drinking in male and female mice

Alcohol relapse is a treatment goal for alcohol dependence and the target for medications' development. Clinically utilized nalfurafine (NFF) is a potent and selective kappa- opioid receptor (KOP-r) agonist, with fewer side effects (e.g., sedation or anhedonia) than classic KOP-r full agonists. We have recently found that NFF reduces excessive alcohol drinking in mice via a KOP-r-mediated mechanism. Here, we further investigated whether NFF alone (1-10 μg/kg) or in combination with naltrexone (NTX, mu-opioid receptor [MOP-r] antagonist) altered alcohol relapse-like drinking using a mouse alcohol deprivation effect (ADE) paradigm to mimic the relapse episodes in human alcoholics. Nalmefene (NMF, clinically utilized KOP-r partial agonist with MOP-r antagonism) was used as a reference compound for the effects on mouse ADE of new NFF + NTX combination. After exposed to 3-week intermittent- access alcohol drinking (two-bottle choice, 24-h access every other day), both male and female mice displayed excessive alcohol intake and then pronounced ADE after 1-week abstinence. NFF prevented the ADE in a dose-dependent manner in both male and female mice. A combination of NFF with NTX reduced the ADE without sex differences at doses lower than those individual effective ones, suggesting synergistic effects between the two compounds. NMF prevented the ADE in both sexes, while selective KOP-r antagonist nor-BNI had no effect. Our new study suggests that a combination of clinically-utilized, potent KOP-r agonist NFF with low-dose NTX has therapeutic potential in alcohol "relapse" treatment.

Dopaminergic cellular and circuit contributions to kappa opioid receptor mediated aversion

Neural circuits that enable an organism to protect itself by promoting escape from immediate threat and avoidance of future injury are conceptualized to carry an "aversive" signal. One of the key molecular elements of these circuits is the kappa opioid receptor (KOR) and its endogenous peptide agonist, dynorphin. In many cases, the aversive response to an experimental manipulation can be eliminated by selective blockade of KOR function, indicating its necessity in transmitting this signal. The dopamine system, through its contributions to reinforcement learning, is also involved in processing of aversive stimuli, and KOR control of dopamine in the context of aversive behavioral states has been intensely studied. In this review, we have discussed the multiple ways in which the KORs regulate dopamine dynamics with a central focus on dopamine neurons and projections from the ventral tegmental area. At the neuronal level, KOR agonists inhibit dopamine neurons both in the somatodendritic region as well as at terminal release sites, through various signaling pathways and ion channels, and these effects are specific to different synaptic sites. While the dominant hypotheses are that aversive states are driven by decreases in dopamine and increases in dynorphin, reported exceptions to these patterns indicate these ideas require refinement. This is critical given that KOR is being considered as a target for development of new therapeutics for anxiety, depression, pain, and other psychiatric disorders.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.