Opioid receptors: drivers to addiction?

Navacaprant, a novel and selective kappa opioid receptor antagonist, has no agonist properties implicated in opioid-related abuse

Kappa opioid receptors (KORs) are implicated in the pathophysiology of various psychiatric and neurological disorders creating interest in targeting the KOR system for therapeutic purposes. Accordingly, navacaprant (NMRA-140) is a potent, selective KOR antagonist being evaluated as a treatment for major depressive disorder. In the present report, we have extended the pharmacological characterization of navacaprant by further demonstrating its selective KOR antagonist properties and confirming its lack of agonist activity at KORs and related targets involved in opioid-related abuse. Using CHO-K1 cells expressing human KOR, mu (MOR), or delta (DOR) opioid receptors, navacaprant demonstrated selective antagonist properties at KOR (IC50 = 0.029 μM) versus MOR (IC50 = 3.3 μM) and DOR (IC50 > 10 μM) in vitro. In vivo, navacaprant (10-30 mg/kg, i.p.) dose-dependently abolished KOR-agonist induced analgesia in the mouse tail-flick assay. Additionally, navacaprant (10, 30 mg/kg, p.o.) significantly reduced KOR agonist-stimulated prolactin release in mice and rats, confirming KOR antagonism in vivo. Navacaprant showed no agonist activity at any opioid receptor subtype (EC50 > 10 μM) in vitro and exhibited no analgesic effect in the tail-flick assays at doses ≤100 mg/kg, p.o. thereby confirming a lack of opioid receptor agonist activity in vivo. Importantly, navacaprant did not alter extracellular dopamine concentrations in the nucleus accumbens shell of freely-moving rats following doses ≤100 mg/kg, p.o., whereas morphine (10, 20 mg/kg, i.p.) significantly increased dopamine levels. These results demonstrate that navacaprant is a KOR-selective antagonist with no pharmacological properties implicated in opioid-related abuse.

Decoding the κ Opioid Receptor (KOR): Advancements in Structural Understanding and Implications for Opioid Analgesic Development

The opioid crisis in the United States is a significant public health issue, with a nearly threefold increase in opioid-related fatalities between 1999 and 2014. In response to this crisis, society has made numerous efforts to mitigate its impact. Recent advancements in understanding the structural intricacies of the κ opioid receptor (KOR) have improved our knowledge of how opioids interact with their receptors, triggering downstream signaling pathways that lead to pain relief. This review concentrates on the KOR, offering crucial structural insights into the binding mechanisms of both agonists and antagonists to the receptor. Through comparative analysis of the atomic details of the binding site, distinct interactions specific to agonists and antagonists have been identified. These insights not only enhance our understanding of ligand binding mechanisms but also shed light on potential pathways for developing new opioid analgesics with an improved risk-benefit profile.

Distinct µ-opioid ensembles trigger positive and negative fentanyl reinforcement

Fentanyl is a powerful painkiller that elicits euphoria and positive reinforcement1. Fentanyl also leads to dependence, defined by the aversive withdrawal syndrome, which fuels negative reinforcement2,3 (that is, individuals retake the drug to avoid withdrawal). Positive and negative reinforcement maintain opioid consumption, which leads to addiction in one-fourth of users, the largest fraction for all addictive drugs4. Among the opioid receptors, µ-opioid receptors have a key role5, yet the induction loci of circuit adaptations that eventually lead to addiction remain unknown. Here we injected mice with fentanyl to acutely inhibit γ-aminobutyric acid-expressing neurons in the ventral tegmental area (VTA), causing disinhibition of dopamine neurons, which eventually increased dopamine in the nucleus accumbens. Knockdown of µ-opioid receptors in VTA abolished dopamine transients and positive reinforcement, but withdrawal remained unchanged. We identified neurons expressing µ-opioid receptors in the central amygdala (CeA) whose activity was enhanced during withdrawal. Knockdown of µ-opioid receptors in CeA eliminated aversive symptoms, suggesting that they mediate negative reinforcement. Thus, optogenetic stimulation caused place aversion, and mice readily learned to press a lever to pause optogenetic stimulation of CeA neurons that express µ-opioid receptors. Our study parses the neuronal populations that trigger positive and negative reinforcement in VTA and CeA, respectively. We lay out the circuit organization to develop interventions for reducing fentanyl addiction and facilitating rehabilitation.

A synthetic peptide exerts nontolerance-forming antihyperalgesic and antidepressant effects in mice

Chronic pain is a prevalent and persistent ailment that affects individuals worldwide. Conventional medications employed in the treatment of chronic pain typically demonstrate limited analgesic effectiveness and frequently give rise to debilitating side effects, such as tolerance and addiction, thereby diminishing patient compliance with medication. Consequently, there is an urgent need for the development of efficacious novel analgesics and innovative methodologies to address chronic pain. Recently, a growing body of evidence has suggested that multireceptor ligands targeting opioid receptors (ORs) are favorable for improving analgesic efficacy, decreasing the risk of adverse effects, and occasionally yielding additional advantages. In this study, the intrathecal injection of a recently developed peptide (VYWEMEDKN) at nanomolar concentrations decreased pain sensitivity in naïve mice and effectively reduced pain-related behaviors in nociceptive pain model mice with minimal opioid-related side effects. Importantly, the compound exerted significant rapid-acting antidepressant effects in both the forced swim test and tail suspension test. It is possible that the rapid antihyperalgesic and antidepressant effects of the peptide are mediated through the OR pathway. Overall, this peptide could both effectively provide pain relief and alleviate depression with fewer side effects, suggesting that it is a potential agent for chronic pain and depression comorbidities from the perspective of pharmaceutical development.

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".

Genetic mouse models in opioid research: current status and future directions

Synthetic and semi-synthetic opioids are prescribed for the management of severe pain conditions, but their long-term use is often leading to physical dependence and addiction disorders. Understanding the complex neurobiology of the opioid system in preclinical models will be essential for the development of safe and efficacious analgesics. With rising numbers of synthetic opioid users and overdose cases, a better understanding of the neuroanatomical and cellular pathways associated with physical dependence and addiction is expected to guide treatment approaches for opioid use disorders. In this commentary, we highlight the importance of advanced genetic mouse models for studying the regional effects of opioid receptors, and we discuss the need of genetic mouse models for the investigation of the regional, circuit and cell compartment-specific role of intracellular mediators of opioid actions.

Neuronal Panx1 drives peripheral sensitization in experimental plantar inflammatory pain

BACKGROUND

The channel-forming protein Pannexin1 (Panx1) has been implicated in both human studies and animal models of chronic pain, but the underlying mechanisms remain incompletely understood.

METHODS

Wild-type (WT, n = 24), global Panx1 KO (n = 24), neuron-specific Panx1 KO (n = 20), and glia-specific Panx1 KO (n = 20) mice were used in this study at Albert Einstein College of Medicine. The von Frey test was used to quantify pain sensitivity in these mice following complete Freund's adjuvant (CFA) injection (7, 14, and 21 d). The qRT-PCR was employed to measure mRNA levels of Panx1, Panx2, Panx3, Cx43, Calhm1, and β-catenin. Laser scanning confocal microscopy imaging, Sholl analysis, and electrophysiology were utilized to evaluate the impact of Panx1 on neuronal excitability and morphology in Neuro2a and dorsal root ganglion neurons (DRGNs) in which Panx1 expression or function was manipulated. Ethidium bromide (EtBr) dye uptake assay and calcium imaging were employed to investigate the role of Panx1 in adenosine triphosphate (ATP) sensitivity. β-galactosidase (β-gal) staining was applied to determine the relative cellular expression levels of Panx1 in trigeminal ganglia (TG) and DRG of transgenic mice.

RESULTS

Global or neuron-specific Panx1 deletion markedly decreased pain thresholds after CFA stimuli (7, 14, and 21 d; P < 0.01 vs. WT group), indicating that Panx1 was positively correlated with pain sensitivity. In Neuro2a, global Panx1 deletion dramatically reduced neurite extension and inward currents compared to the WT group (P < 0.05), revealing that Panx1 enhanced neurogenesis and excitability. Similarly, global Panx1 deletion significantly suppressed Wnt/β-catenin dependent DRG neurogenesis following 5 d of nerve growth factor (NGF) treatment (P < 0.01 vs. WT group). Moreover, Panx1 channels enhanced DRG neuron response to ATP after CFA injection (P < 0.01 vs. Panx1 KO group). Furthermore, ATP release increased Ca2+ responses in DRGNs and satellite glial cells surrounding them following 7 d of CFA treatment (P < 0.01 vs. Panx1 KO group), suggesting that Panx1 in glia also impacts exaggerated neuronal excitability. Interestingly, neuron-specific Panx1 deletion was found to markedly reduce differentiation in cultured DRGNs, as evidenced by stunted neurite outgrowth (P < 0.05 vs. Panx1 KO group; P < 0.01 vs. WT group or GFAP-Cre group), blunted activation of Wnt/β-catenin signaling (P < 0.01 vs. WT, Panx1 KO and GFAP-Cre groups), and diminished cell excitability (P < 0.01 vs. GFAP-Cre group) and response to ATP stimulation (P < 0.01 vs. WT group). Analysis of β-gal staining showed that cellular expression levels of Panx1 in neurons are significantly higher (2.5-fold increase) in the DRG than in the TG.

CONCLUSIONS

The present study revealed that neuronal Panx1 is a prominent driver of peripheral sensitivity in the setting of inflammatory pain through cell-autonomous effects on neuronal excitability. This hyperexcitability dependence on neuronal Panx1 contrasts with inflammatory orofacial pain, where similar studies revealed a prominent role for glial Panx1. The apparent differences in Panx1 expression in neuronal and non-neuronal TG and DRG cells are likely responsible for the distinct impact of these cell types in the two pain models.

Disruption of positive- and negative-feature morphine interoceptive occasion setters by dopamine receptor agonism and antagonism in male and female rats

RATIONALE

Internally perceived stimuli evoked by morphine administration can form Pavlovian associations such that they can function as occasion setters (OSs) for externally perceived reward cues in rats, coming to modulate reward-seeking behaviour. Though much research has investigated mechanisms underlying opioid-related reinforcement and analgesia, neurotransmitter systems involved in the functioning of opioids as Pavlovian interoceptive discriminative stimuli remain to be disentangled despite documented differences in the development of tolerance to analgesic versus discriminative stimulus effects.

OBJECTIVES

Dopamine has been implicated in many opioid-related behaviours, so we aimed to investigate the role of this neurotransmitter in expression of morphine occasion setting.

METHODS

Male and female rats were assigned to positive- (FP) or negative-feature (FN) groups and received an injection of morphine or saline before each training session. A 15-s white noise conditioned stimulus (CS) was presented 8 times during every training session; offset of this stimulus was followed by 4-s access to liquid sucrose on morphine, but not saline, sessions for FP rats. FN rats learned the reverse contingency. Following stable discrimination, rats began generalization testing for expression of morphine-guided sucrose seeking after systemic pretreatment with different doses of the non-selective dopamine receptor antagonist, flupenthixol, and the non-selective dopamine receptor agonist, apomorphine, combined with training doses of morphine or saline in a Latin-square design.

RESULTS

The morphine discrimination was acquired under both FP and FN contingencies by males and females. Neither flupenthixol nor apomorphine at any dose substituted for morphine, but both apomorphine and flupenthixol disrupted expression of the morphine OS. This inhibition was specific to sucrose seeking during CS presentations rather than during the period before CS onset and, in the case of apomorphine more so than flupenthixol, to trials on which access to sucrose was anticipated.

CONCLUSIONS

Our findings lend support to a mechanism of occasion setting involving gating of CS-induced dopamine release rather than by direct dopaminergic modulation by the morphine stimulus.

Inhibition of adenylyl cyclase by GTPase-deficient Gαi is mechanistically different from that mediated by receptor-activated Gαi

Signal transduction through G protein-coupled receptors (GPCRs) has been a major focus in cell biology for decades. Numerous disorders are associated with GPCRs that utilize Gi proteins to inhibit adenylyl cyclase (AC) as well as regulate other effectors. Several early studies have successfully defined the AC-interacting domains of several members of Gαi by measuring the loss of activity upon homologous replacements of putative regions of constitutive active Gαi mutants. However, whether such findings can indeed be translated into the context of a receptor-activated Gαi have not been rigorously verified. To address this issue, an array of known and new chimeric mutations was introduced into GTPase-deficient Q204L (QL) and R178C (RC) mutants of Gαi1, followed by examinations on their ability to inhibit AC. Surprisingly, most chimeras failed to abolish the constitutive activity brought on by the QL mutation, while some were able to eliminate the inhibitory activity of RC mutants. Receptor-mediated inhibition of AC was similarly observed in the same chimeric constructs harbouring the pertussis toxin (PTX)-resistant C351I mutation. Moreover, RC-bearing loss-of-function chimeras appeared to be hyper-deactivated by endogenous RGS protein. Molecular docking revealed a potential interaction between AC and the α3/β5 loop of Gαi1. Subsequent cAMP assays support a cooperative action of the α3/β5 loop, the α4 helix, and the α4/β6 loop in mediating AC inhibition by Gαi1-i3. Our results unveiled a notable functional divergence between constitutively active mutants and receptor-activated Gαi1 to inhibit AC, and identified a previously unknown AC-interacting domain of Gαi subunits. These results collectively provide valuable insights on the mechanism of AC inhibition in the cellular environment.

In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

The resting-state brain activity signatures for addictive disorders

BACKGROUND

Addiction is a chronic and relapsing brain disorder. Despite numerous neuroimaging and neurophysiological studies on individuals with substance use disorder (SUD) or behavioral addiction (BEA), currently a clear neural activity signature for the addicted brain is lacking.

METHODS

We first performed systemic coordinate-based meta-analysis and partial least-squares regression to identify shared or distinct brain regions across multiple addictive disorders, with abnormal resting-state activity in SUD and BEA based on 46 studies (55 contrasts), including regional homogeneity (ReHo) and low-frequency fluctuation amplitude (ALFF) or fractional ALFF. We then combined Neurosynth, postmortem gene expression, and receptor/transporter distribution data to uncover the potential molecular mechanisms underlying these neural activity signatures.

FINDINGS

The overall comparison between addiction cohorts and healthy subjects indicated significantly increased ReHo and ALFF in the right striatum (putamen) and bilateral supplementary motor area, as well as decreased ReHo and ALFF in the bilateral anterior cingulate cortex and ventral medial prefrontal cortex, in the addiction group. On the other hand, neural activity in cingulate cortex, ventral medial prefrontal cortex, and orbitofrontal cortex differed between SUD and BEA subjects. Using molecular analyses, the altered resting activity recapitulated the spatial distribution of dopaminergic, GABAergic, and acetylcholine system in SUD, while this also includes the serotonergic system in BEA.

CONCLUSIONS

These results indicate both common and distinctive neural substrates underlying SUD and BEA, which validates and supports targeted neuromodulation against addiction.

FUNDING

This work was supported by the National Natural Science Foundation of China and Intramural Research Program of the National Institute on Drug Abuse, National Institutes of Health.

Novel dual-target FAAH and TRPV1 ligands as potential pharmacotherapeutics for pain management

Dual-acting drugs that simultaneously inhibit fatty acid amide hydrolase (FAAH) and antagonize the transient receptor potential vanilloid 1 (TRPV1) is a promising stronger therapeutic approach for pain management without side effects associated with single-target agents. Here, several series of dual FAAH/TRPV1 blockers were designed and synthesized through rational molecular hybridization between the pharmacophore of classical TRPV1 antagonists and FAAH inhibitors. The studies resulted in compound 2r, which exhibited strong dual FAAH/TRPV1 inhibition/antagonism in vitro, exerted powerful analgesic effects in formalin-induced pain test (phase II, in mice), desirable anti-inflammatory activity in carrageenan-induced paw edema in rats, no TRPV1-related hyperthermia side effect, and favorable pharmacokinetic properties. Meanwhile, the contributions of TRPV1 and FAAH to its antinociceptive effects were verified by target engagement and molecular docking studies. Overall, compound 2r can serve as a new scaffold for developing FAAH/TRPV1 dual-activie ligands to counteract pain.

Peptide-Drug Conjugates: An Emerging Direction for the Next Generation of Peptide Therapeutics

Building on recent advances in peptide science, medicinal chemists have developed a hybrid class of bioconjugates, called peptide-drug conjugates, that demonstrate improved efficacy compared to peptides and small molecules independently. In this Perspective, we discuss how the conjugation of synergistic peptides and small molecules can be used to overcome complex disease states and resistance mechanisms that have eluded contemporary therapies because of their multi-component activity. We highlight how peptide-drug conjugates display a multi-factor therapeutic mechanism similar to that of antibody-drug conjugates but also demonstrate improved therapeutic properties such as less-severe off-target effects and conjugation strategies with greater site-specificity. The many considerations that go into peptide-drug conjugate design and optimization, such as peptide/small-molecule pairing and chemo-selective chemistries, are discussed. We also examine several peptide-drug conjugate series that demonstrate notable activity toward complex disease states such as neurodegenerative disorders and inflammation, as well as viral and bacterial targets with established resistance mechanisms.

A Combination of a Dopamine Receptor 2 Agonist and a Kappa Opioid Receptor Antagonist Synergistically Reduces Weight in Diet-Induced Obese Rodents

As the global obesity rate increases, so does the urgency to find effective anti-obesity drugs. In the search for therapeutic targets, central nervous system (CNS) mechanisms engaged in the regulation of energy expenditure and food intake, such as the opioid and dopamine systems, are crucial. In this study, we examined the effect on body weight of two drugs: bromocriptine (BC), a D2R receptor agonist, and PF-04455242, a selective κ opioid receptor (KOR) antagonist. Using diet-induced obese (DIO) rats, we aimed to ascertain whether the administration of BC and PF-04455242, independently or in combination, could enhance body weight loss. Furthermore, the present work demonstrates that the peripheral coadministration of BC and PF-04455242 enhances the reduction of weight in DIO rats and leads to a decrease in adiposity in a food-intake-independent manner. These effects were based on heightened energy expenditure, particularly through the activation of brown adipose tissue (BAT) thermogenesis. Overall, our findings indicate that the combination of BC and PF-04455242 effectively induces body weight loss through increased energy expenditure by increasing thermogenic activity and highlight the importance of the combined use of drugs to combat obesity.

Nalmefene attenuates reinstatement of methamphetamine-seeking behavior in rats through group II metabotropic glutamate receptors (mGluR2/3)

Nalmefene, an analog to naltrexone, is an antagonist at the μ opioid receptor and a partial agonist at the κ opioid receptor. Both agents are approved for the treatment of alcohol use disorder and opioid addiction. Here, we evaluated the potential of nalmefene for treating psychostimulant dependence using a methamphetamine (METH) self-administration rat model. Rats were trained to press a lever for 0.02-mg intravenous METH infusions paired with drug-associated cues (light and tone) under a fixed ratio 1 schedule. After a 10-day METH self-administration acquisition phase, rats underwent extinction training. A reinstatement test was conducted after fulfilment of the extinction criterion under saline infusions. Re-exposure to METH-associated cues or a priming injection of METH (1.0 mg/kg, i.p.) significantly reinstated METH-seeking behaviors. Pretreatment with nalmefene (10 mg/kg, i.p.) immediately before reinstatement tests significantly attenuated the METH-seeking behaviors induced by both cues and METH priming injection. To investigate the mechanism of effect of nalmefene, we also tested the ability of a group II metabotropic glutamate receptors (mGluR2/3) antagonist, LY341495, to the ameliorating effects of nalmefene. Pretreatment with LY341495 (1.0 mg/kg, i.p.) before nalmefene administration antagonized the effect of nalmefene on reinstatement. LY341495 alone did not affect the reinstatement of lever pressing. We found that nalmefene attenuates METH-seeking behaviors during withdrawal, and this attenuation of reinstatement is mediated by the activation of mGluR2/3. The present findings suggest that nalmefene could decrease incentive motivation for drug use in psychostimulant dependence.

IUPHAR review: Recent progress in the development of Mu opioid receptor modulators to treat opioid use disorders

Opioid Use Disorder (OUD) can be described as intense preoccupation with using or obtaining opioids despite the negative consequences associated with their use. As the number of OUD cases in the U.S. increase, so do the number of opioid-related overdose deaths. In 2022, opioid-related overdose became the No. 1 cause of death for individuals in the U.S. between the ages of 25 and 64 years of age. Because of the introduction of highly potent synthetic opioids (e.g. fentanyl) to the illicit drug market, there is an urgent need for therapeutics that successfully reduce the number of overdoses and can help OUD patients maintain sobriety. Most abused opioids stimulate the mu-opioid receptor (MOR) and activation of this receptor can lead to positive (e.g., euphoria) consequences. However, the negative side effects of MOR stimulation can be fatal (e.g., sedation, respiratory depression). Therefore, the MOR is an attractive target for developing medications to treat OUD. Current FDA drugs include MOR agonists that aid in detoxification and relapse prevention, and MOR antagonists that also serve as maintenance therapies or reverse overdose. These medications are limited by their abuse potential, adverse effects, or pharmacological profiles which leaves ample room for research into designing new chemical entities with optimal physiological effects. These includes, orthosteric ligands that target the primary binding site of the MOR, allosteric ligands that positively, negatively, or "silently" modulate receptor function, and lastly, bitopic ligands target both the orthosteric and allosteric sites simultaneously.

Allosteric modulation of G protein-coupled receptors as a novel therapeutic strategy in neuropathic pain

Neuropathic pain is a debilitating pathological condition that presents significant therapeutic challenges in clinical practice. Unfortunately, current pharmacological treatments for neuropathic pain lack clinical efficacy and often lead to harmful adverse reactions. As G protein-coupled receptors (GPCRs) are widely distributed throughout the body, including the pain transmission pathway and descending inhibition pathway, the development of novel neuropathic pain treatments based on GPCRs allosteric modulation theory is gaining momentum. Extensive research has shown that allosteric modulators targeting GPCRs on the pain pathway can effectively alleviate symptoms of neuropathic pain while reducing or eliminating adverse effects. This review aims to provide a comprehensive summary of the progress made in GPCRs allosteric modulators in the treatment of neuropathic pain, and discuss the potential benefits and adverse factors of this treatment. We will also concentrate on the development of biased agonists of GPCRs, and based on important examples of biased agonist development in recent years, we will describe universal strategies for designing structure-based biased agonists. It is foreseeable that, with the continuous improvement of GPCRs allosteric modulation and biased agonist theory, effective GPCRs allosteric drugs will eventually be available for the treatment of neuropathic pain with acceptable safety.

Endogenous Opioids in the Homeostatic Regulation of Hunger, Satiety, and Hedonic Eating: Neurobiological Foundations

This chapter (part one of a trilogy) summarizes the neurobiological foundations of endogenous opioids in the regulation of energy balance and eating behavior, dysregulation of which translates to maladaptive dietary responses in individuals with obesity and eating disorders, including anorexia, bulimia, and binge eating disorder. Knowledge of these neurobiological foundations is vital to researchers' and clinicians' understanding of pathophysiology as well as the science-based development of multidisciplinary diagnoses and treatments for obesity and eating disorders. We highlight mechanisms of endogenous opioids in both homeostatic and hedonic feeding behavior, review research on the dysregulation of food reward that plays a role in a wide array of obesity and disordered eating, and the clinical implications of neurobiological responses to food for current science-based treatments for obesity and eating disorders.

CE: Reducing Bias Against People with Substance Use Disorders

ABSTRACT

Addiction is a highly misunderstood and stigmatized chronic illness frequently encountered by health care providers during routine medical care. People with substance use disorders, in particular, face extraordinary stigma and bias when interacting with health care providers, including nurses. Stigma associated with addiction contributes to health inequities and is recognized as a significant barrier to people seeking and receiving necessary health care. Since patients often spend the most time with nurses in the clinical setting, nurses are ideally positioned to address addiction stigma. However, many nurses lack knowledge about addiction, stigma, and the impact of the words they use, whether in conversation or in clinical documentation. This article reviews the consequences of addiction stigma (labeling, stereotyping, or discrimination) and the steps nurses can take to reduce biases related to substance use. A case scenario based on our experience will be used to guide a discussion of opportunities for nurses to intervene and improve care.

Chronic Morphine Leaves a Durable Fingerprint on Whole-Brain Functional Connectivity

BACKGROUND

Opioid use disorder is a chronic relapsing disorder. The brain adapts to opioids that are taken for pain treatment or recreational use so that abstinence becomes a true challenge for individuals with opioid use disorder. Studying brain dysfunction at this stage is difficult, and human neuroimaging has provided highly heterogeneous information.

METHODS

Here, we took advantage of an established mouse model of morphine abstinence together with functional magnetic resonance imaging to investigate whole-brain functional connectivity (FC) first at rest and then in response to an acute morphine challenge during image acquisition.

RESULTS

Hierarchical clustering of seed pair correlation coefficients showed modified FC in abstinent animals, brainwide and regardless of the condition. Seed-to-voxel analysis and random forest classification, performed on data at rest, indicated that the retrosplenial cortex (a core component of the default mode network) and the amygdala (a major aversion center) are the best markers of abstinence, thus validating the translatability of the study. Seed pair network clustering confirmed disruption of a retrosplenial cortex-centered network, reflecting major reorganization of brain FC. The latter analysis also identified a persistent but unreported morphine signature in abstinent mice at rest, which involves cortical and midbrain components and characterizes the enduring morphine footprint. Finally, dynamic FC analysis revealed that the intrascanner acute morphine challenge modified FC faster and more broadly in abstinent animals, demonstrating brainwide adaptations of FC reactivity to an acute opioid challenge.

CONCLUSIONS

This study used a unique experimental design to demonstrate that a prior history of chronic opioid exposure leaves a durable pharmacological signature on brain communication, with implications for pain management and recovery from opioid use disorder.

Phylogenetic Analysis Provides Insight Into the Molecular Evolution of Nociception and Pain-Related Proteins

Nociception and pain sensation are important neural processes in humans to avoid injury. Many proteins are involved in nociception and pain sensation in humans; however, the evolution of these proteins in animals is unknown. Here, we chose nociception- and pain-related proteins, including G protein-coupled receptors (GPCRs), ion channels (ICs), and neuropeptides (NPs), which are reportedly associated with nociception and pain in humans, and identified their homologs in various animals by BLAST, phylogenetic analysis and protein architecture comparison to reveal their evolution from protozoans to humans. We found that the homologs of transient receptor potential channel A 1 (TRPA1), TRAPM, acid-sensing IC (ASIC), and voltage-dependent calcium channel (VDCC) first appear in Porifera. Substance-P receptor 1 (TACR1) emerged from Coelenterata. Somatostatin receptor type 2 (SSTR2), TRPV1 and voltage-dependent sodium channels (VDSC) appear in Platyhelminthes. Calcitonin gene-related peptide receptor (CGRPR) was first identified in Nematoda. However, opioid receptors (OPRs) and most NPs were discovered only in vertebrates and exist from agnatha to humans. The results demonstrated that homologs of nociception and pain-related ICs exist from lower animal phyla to high animal phyla, and that most of the GPCRs originate from low to high phyla sequentially, whereas OPRs and NPs are newly evolved in vertebrates, which provides hints of the evolution of nociception and pain-related proteins in animals and humans.

Design and structural validation of peptide-drug conjugate ligands of the kappa-opioid receptor

Despite the increasing number of GPCR structures and recent advances in peptide design, the development of efficient technologies allowing rational design of high-affinity peptide ligands for single GPCRs remains an unmet challenge. Here, we develop a computational approach for designing conjugates of lariat-shaped macrocyclized peptides and a small molecule opioid ligand. We demonstrate its feasibility by discovering chemical scaffolds for the kappa-opioid receptor (KOR) with desired pharmacological activities. The designed De Novo Cyclic Peptide (DNCP)-β-naloxamine (NalA) exhibit in vitro potent mixed KOR agonism/mu-opioid receptor (MOR) antagonism, nanomolar binding affinity, selectivity, and efficacy bias at KOR. Proof-of-concept in vivo efficacy studies demonstrate that DNCP-β-NalA(1) induces a potent KOR-mediated antinociception in male mice. The high-resolution cryo-EM structure (2.6 Å) of the DNCP-β-NalA-KOR-Gi1 complex and molecular dynamics simulations are harnessed to validate the computational design model. This reveals a network of residues in ECL2/3 and TM6/7 controlling the intrinsic efficacy of KOR. In general, our computational de novo platform overcomes extensive lead optimization encountered in ultra-large library docking and virtual small molecule screening campaigns and offers innovation for GPCR ligand discovery. This may drive the development of next-generation therapeutics for medical applications such as pain conditions.

Mu Opioid Receptor-Expressing Neurons in the Dorsal Raphe Nucleus Are Involved in Reward Processing and Affective Behaviors

BACKGROUND

Mu opioid receptors (MORs) are key for reward processing, mostly studied in dopaminergic pathways. MORs are also expressed in the dorsal raphe nucleus (DRN), which is central for the modulation of reward and mood, but MOR function in the DRN remains underexplored. Here, we investigated whether MOR-expressing neurons of the DRN (DRN-MOR neurons) participate in reward and emotional responses.

METHODS

We characterized DRN-MOR neurons anatomically using immunohistochemistry and functionally using fiber photometry in responses to morphine and rewarding/aversive stimuli. We tested the effect of opioid uncaging on the DRN on place conditioning. We examined the effect of DRN-MOR neuron optostimulation on positive reinforcement and mood-related behaviors. We mapped their projections and selected DRN-MOR neurons projecting to the lateral hypothalamus for a similar optogenetic experimentation.

RESULTS

DRN-MOR neurons form a heterogeneous neuronal population essentially composed of GABAergic (gamma-aminobutyric acidergic) and glutamatergic neurons. Calcium activity of DRN-MOR neurons was inhibited by rewarding stimuli and morphine. Local photo-uncaging of oxymorphone in the DRN produced conditioned place preference. DRN-MOR neuron optostimulation triggered real-time place preference and was self-administered, promoted social preference, and reduced anxiety and passive coping. Finally, specific optostimulation of DRN-MOR neurons projecting to the lateral hypothalamus recapitulated the reinforcing effects of total DRN-MOR neuron stimulation.

CONCLUSIONS

Our data show that DRN-MOR neurons respond to rewarding stimuli and that their optoactivation has reinforcing effects and promotes positive emotional responses, an activity which is partially mediated by their projections to the lateral hypothalamus. Our study also suggests a complex regulation of DRN activity by MOR opioids, involving mixed inhibition/activation mechanisms that fine-tune DRN function.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Insula Connectivity Abnormalities Predict Impulsivity in Chronic Heroin Use Disorder: A Cross-Sectional Resting-State fMRI Study

Patients with heroin use disorder (HUD) often exhibit trait impulsivity, which may be an important factor in and a good predictor of addiction. However, the factor structure of HUD trait impulsivity (motor, attentional, and nonplanning) and its neural correlates are not yet known. A total of 24 male volunteers with HUD and 16 healthy control volunteers were recruited for this cross-sectional study. The Barratt Impulsiveness Scale (BIS-11) and resting-state functional magnetic resonance imaging (rs-fMRI) were employed using the insula as a seed point in an effort to understand the association between trait impulsivity and its intrinsic factors and functional connectivity (FC) between the insula and the whole brain. The HUD group in this study exhibited higher total trait impulsivity scores, motor impulsivity, and nonplanning impulsivity than the control group. Changes in FC between the right insula and the lateral occipital cortex and the right angular gyrus were significantly positively correlated with total trait impulsivity scores, motor impulsivity, and nonplanning impulsivity, whereas changes in the FC between the left insula and the left superior frontal gyrus and left frontopolar brain region were significantly negatively correlated with trait impulsivity. Thus, the insula may serve as an important biomarker for identifying trait impulsivity and its intrinsic factor structure in patients with HUDs.

The temporal relationship between antibiotic and opioid prescription on the risk of developing an opioid use disorder: A national database study

Background: Previously, we discovered that subjects co-prescribed both antibiotics and opioids on the same day in a hospital setting displayed an increased risk of developing an opioid use disorder (OUD) 12 months following hospital discharge. The goal of this study was to examine whether prescribing antibiotics in the inpatient or emergency department setting at various time points before or after an opioid prescription impacted the risk OUD.Methods: A propensity score matched cohort study was conducted to identify subjects (18-65 years old) with no previous history of OUD. Two cohorts were defined: subjects who were prescribed antibiotics 0-1, 2-4, 5-7, 8-10, 11-12 months before or after the date of an opioid prescription while in the emergency department or inpatient setting, from the years 2010-2019. The diagnosis of an Opioid Related Disorder (F11.10-F11.20) 12 months following discharge from the emergency department or inpatient unit was then observed.Results: Primary analysis showed that subjects prescribed an antibiotic 0-1 month or 8-10 months before an opioid prescription showed a modest risk of developing an OUD 12 months following an opioid prescription (0.04% and 0.20%, respectively). Similarly, subjects prescribed an antibiotic 0-1 month, 5-7 months, or 8-10 months after an opioid prescription displayed a modest risk of developing OUD 12 months after an opioid prescription (0.02% risk, 0.14% risk, and 0.16% risk, respectively).Conclusions: These findings suggest that there is little to no effect on the risk of developing OUD when antibiotics are prescribed at various time points before or after opioid prescription.

Development of a Selective Peptide κ-Opioid Receptor Antagonist by Late-Stage Functionalization with Cysteine Staples

The κ-opioid receptor (KOR) is an attractive target for the development of novel drugs. KOR agonists are potentially safer pain medications, whereas KOR antagonists are promising drug candidates for the treatment of neuropsychiatric disorders. Hitherto, the vast majority of selective drug leads that have been developed for KOR are small molecules. In this study, novel peptide probes were designed by using an endogenous dynorphin A1-13 sequence as a template for peptide stapling via late-stage cysteine functionalization. Leveraging this strategy, we developed a stable and potent KOR antagonist, CSD-CH2(1,8)-NH2, with approximately 1000-fold improved selectivity for KOR over μ- and δ-opioid receptors. Its potent competitive KOR antagonism was verified in KOR-expressing cells, peripheral dorsal root ganglion neurons, and using the tail-flick and rotarod tests in mice. This work highlights the value of cysteine stapling to develop selective peptide probes to modulate central KOR function, as innovative peptide drug candidates for the treatment of KOR-related illnesses.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

Allele frequency and genotype distribution of the opioid receptor μ-1 (OPRM1) A118G polymorphism in the Western Saudi population

The single nucleotide polymorphism (SNP) A118G (rs1799971) in the Mu Opioid Receptor 1 (OPRM1) gene is associated with significant variations in analgesic doses and adverse effects of opioids. The A118G OPRM1 allele distributions vary significantly between different populations worldwide. The study aimed to assess the allele frequency and genotype distribution of OPRM1 A118G SNP in Saudis. This cross-sectional study included 124 healthy Saudis (62 males and 62 females) visiting the King Abdulaziz University Hospital in Jeddah, Saudi Arabia. The Oragene®-DISCOVER (OGR-600) kits were used to collect saliva samples from the participants. Polymerase chain reaction-restriction fragment length polymorphism was utilized to assess the SNP. Among the tested population, 79.03% (95% C.I. 70.81-85.82) were homozygous wild-type A118A, 16.13% (95% C.I. 10.14-23.80) were heterozygous A118G, and 4.84% (95% C.I. 1.80-10.23) were homozygous mutant G118G. OPRM1 A118G polymorphism allele frequencies were 87% (95% C.I. 79.89-92.44) and 13% (95% C.I. 7.56-20.11) for the 118A and 118G alleles, respectively. A higher frequency of the OPRM1 118G allele was present in females, 21% (95% C.I. 11.66-33.17) compared to males, 5% (95% C.I. 1.01-13.50). Relative to other Asian countries, the Saudi population showed a low prevalence of the OPRM1 A118G polymorphism, with a higher frequency of the 118G allele in females. Our research will contribute to the existing knowledge on the prevalence of OPRM1 A118G polymorphism, which could be considered for the personalized prescribing of opioid analgesics.

Stress-Induced Changes in the Endogenous Opioid System Cause Dysfunction of Pain and Emotion Regulation

Early life stress, such as child abuse and neglect, and psychosocial stress in adulthood are risk factors for psychiatric disorders, including depression and anxiety. Furthermore, exposure to these stresses affects the sensitivity to pain stimuli and is associated with the development of chronic pain. However, the mechanisms underlying the pathogenesis of stress-induced depression, anxiety, and pain control remain unclear. Endogenous opioid signaling is reportedly associated with analgesia, reward, addiction, and the regulation of stress responses and anxiety. Stress alters the expression of various opioid receptors in the central nervous system and sensitivity to opioid receptor agonists and antagonists. μ-opioid receptor-deficient mice exhibit attachment disorders and autism-like behavioral expression patterns, while those with δ-opioid receptor deficiency exhibit anxiety-like behavior. In contrast, deficiency and antagonists of the κ-opioid receptor suppress the stress response. These findings strongly suggest that the expression and dysfunction of the endogenous opioid signaling pathways are involved in the pathogenesis of stress-induced psychiatric disorders and chronic pain. In this review, we summarize the latest basic and clinical research studies on the effects of endogenous opioid signaling on early-life stress, psychosocial stress-induced psychiatric disorders, and chronic pain.

Oxycodone withdrawal induces HDAC1/HDAC2-dependent transcriptional maladaptations in the reward pathway in a mouse model of peripheral nerve injury

The development of physical dependence and addiction disorders due to misuse of opioid analgesics is a major concern with pain therapeutics. We developed a mouse model of oxycodone exposure and subsequent withdrawal in the presence or absence of chronic neuropathic pain. Oxycodone withdrawal alone triggered robust gene expression adaptations in the nucleus accumbens, medial prefrontal cortex and ventral tegmental area, with numerous genes and pathways selectively affected by oxycodone withdrawal in mice with peripheral nerve injury. Pathway analysis predicted that histone deacetylase (HDAC) 1 is a top upstream regulator in opioid withdrawal in nucleus accumbens and medial prefrontal cortex. The novel HDAC1/HDAC2 inhibitor, Regenacy Brain Class I HDAC Inhibitor (RBC1HI), attenuated behavioral manifestations of oxycodone withdrawal, especially in mice with neuropathic pain. These findings suggest that inhibition of HDAC1/HDAC2 may provide an avenue for patients with chronic pain who are dependent on opioids to transition to non-opioid analgesics.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Peripheralization Strategies Applied to Morphinans and Implications for Improved Treatment of Pain

Opioids are considered the most effective analgesics for the treatment of moderate to severe acute and chronic pain. However, the inadequate benefit/risk ratio of currently available opioids, together with the current 'opioid crisis', warrant consideration on new opioid analgesic discovery strategies. Targeting peripheral opioid receptors as effective means of treating pain and avoiding the centrally mediated side effects represents a research area of substantial and continuous attention. Among clinically used analgesics, opioids from the class of morphinans (i.e., morphine and structurally related analogues) are of utmost clinical importance as analgesic drugs activating the mu-opioid receptor. In this review, we focus on peripheralization strategies applied to N-methylmorphinans to limit their ability to cross the blood-brain barrier, thus minimizing central exposure and the associated undesired side effects. Chemical modifications to the morphinan scaffold to increase hydrophilicity of known and new opioids, and nanocarrier-based approaches to selectively deliver opioids, such as morphine, to the peripheral tissue are discussed. The preclinical and clinical research activities have allowed for the characterization of a variety of compounds that show low central nervous system penetration, and therefore an improved side effect profile, yet maintaining the desired opioid-related antinociceptive activity. Such peripheral opioid analgesics may represent alternatives to presently available drugs for an efficient and safer pain therapy.

G protein-coupled receptor modulation of striatal dopamine transmission: Implications for psychoactive drug effects

Dopamine transmission in the striatum is a critical mediator of the rewarding and reinforcing effects of commonly misused psychoactive drugs. G protein-coupled receptors (GPCRs) that bind a variety of neuromodulators including dopamine, endocannabinoids, acetylcholine and endogenous opioid peptides regulate dopamine release by acting on several components of dopaminergic circuitry. Striatal dopamine release can be driven by both somatic action potential firing and local mechanisms that depend on acetylcholine released from striatal cholinergic interneurons. GPCRs that primarily regulate somatic firing of dopamine neurons via direct effects or modulation of synaptic inputs are likely to affect distinct aspects of behaviour and psychoactive drug actions compared with those GPCRs that primarily regulate local acetylcholine-dependent dopamine release in striatal regions. This review will highlight mechanisms by which GPCRs modulate dopaminergic transmission and the relevance of these findings to psychoactive drug effects on physiology and behaviour.

Loss of Astrocytic µ Opioid Receptors Exacerbates Aversion Associated with Morphine Withdrawal in Mice: Role of Mitochondrial Respiration

Astrocytes express mu/µ opioid receptors, but the function of these receptors remains poorly understood. We evaluated the effects of astrocyte-restricted knockout of µ opioid receptors on reward- and aversion-associated behaviors in mice chronically exposed to morphine. Specifically, one of the floxed alleles of the Oprm1 gene encoding µ opioid receptor 1 was selectively deleted from brain astrocytes in Oprm1 inducible conditional knockout (icKO) mice. These mice did not exhibit changes in locomotor activity, anxiety, or novel object recognition, or in their responses to the acute analgesic effects of morphine. Oprm1 icKO mice displayed increased locomotor activity in response to acute morphine administration but unaltered locomotor sensitization. Oprm1 icKO mice showed normal morphine-induced conditioned place preference but exhibited stronger conditioned place aversion associated with naloxone-precipitated morphine withdrawal. Notably, elevated conditioned place aversion lasted up to 6 weeks in Oprm1 icKO mice. Astrocytes isolated from the brains of Oprm1 icKO mice had unchanged levels of glycolysis but had elevated oxidative phosphorylation. The basal augmentation of oxidative phosphorylation in Oprm1 icKO mice was further exacerbated by naloxone-precipitated withdrawal from morphine and, similar to that for conditioned place aversion, was still present 6 weeks later. Our findings suggest that µ opioid receptors in astrocytes are linked to oxidative phosphorylation and they contribute to long-term changes associated with opioid withdrawal.

Shared Mechanisms of GABAergic and Opioidergic Transmission Regulate Corticolimbic Reward Systems and Cognitive Aspects of Motivational Behaviors

The functional interplay between the corticolimbic GABAergic and opioidergic systems plays a crucial role in regulating the reward system and cognitive aspects of motivational behaviors leading to the development of addictive behaviors and disorders. This review provides a summary of the shared mechanisms of GABAergic and opioidergic transmission, which modulate the activity of dopaminergic neurons located in the ventral tegmental area (VTA), the central hub of the reward mechanisms. This review comprehensively covers the neuroanatomical and neurobiological aspects of corticolimbic inhibitory neurons that express opioid receptors, which act as modulators of corticolimbic GABAergic transmission. The presence of opioid and GABA receptors on the same neurons allows for the modulation of the activity of dopaminergic neurons in the ventral tegmental area, which plays a key role in the reward mechanisms of the brain. This colocalization of receptors and their immunochemical markers can provide a comprehensive understanding for clinicians and researchers, revealing the neuronal circuits that contribute to the reward system. Moreover, this review highlights the importance of GABAergic transmission-induced neuroplasticity under the modulation of opioid receptors. It discusses their interactive role in reinforcement learning, network oscillation, aversive behaviors, and local feedback or feedforward inhibitions in reward mechanisms. Understanding the shared mechanisms of these systems may lead to the development of new therapeutic approaches for addiction, reward-related disorders, and drug-induced cognitive impairment.

Public health and medical preparedness for mass casualties from the deliberate release of synthetic opioids

The large amounts of opioids and the emergence of increasingly potent illicitly manufactured synthetic opioids circulating in the unregulated drug supply in North America and Europe are fueling not only the ongoing public health crisis of overdose deaths but also raise the risk of another type of disaster: deliberate opioid release with the intention to cause mass harm. Synthetic opioids are highly potent, rapidly acting, can cause fatal ventilatory depression, are widely available, and have the potential to be disseminated for mass exposure, for example, if effectively formulated, via inhalation or ingestion. As in many other chemical incidents, the health consequences of a deliberate release of synthetic opioid would manifest quickly, within minutes. Such an incident is unlikely, but the consequences could be grave. Awareness of the risk of this type of incident and preparedness to respond are required to save lives and reduce illness. Coordinated planning across the entire local community emergency response system is also critical. The ability to rapidly recognize the opioid toxidrome, education on personal protective actions, and training in medical management of individuals experiencing an opioid overdose are key components of preparedness for an opioid mass casualty incident.

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

GPCR interactions involving metabotropic glutamate receptors and their relevance to the pathophysiology and treatment of CNS disorders

Cellular responses to metabotropic glutamate (mGlu) receptor activation are shaped by mechanisms of receptor-receptor interaction. mGlu receptor subtypes form homodimers, intra- or inter-group heterodimers, and heteromeric complexes with other G protein-coupled receptors (GPCRs). In addition, mGlu receptors may functionally interact with other receptors through the βγ subunits released from G proteins in response to receptor activation or other mechanisms. Here, we discuss the interactions between (i) mGlu₁ and GABA_B receptors in cerebellar Purkinje cells; (ii) mGlu₂ and 5-HT_2Aserotonergic receptors in the prefrontal cortex; (iii) mGlu₅ and A_2A receptors or mGlu₅ and D₁ dopamine receptors in medium spiny projection neurons of the indirect and direct pathways of the basal ganglia motor circuit; (iv) mGlu₅ and A_2A receptors in relation to the pathophysiology of Alzheimer's disease; and (v) mGlu₇ and A₁ adenosine or α- or β₁ adrenergic receptors. In addition, we describe in detail a novel form of non-heterodimeric interaction between mGlu₃ and mGlu₅ receptors, which appears to be critically involved in mechanisms of activity-dependent synaptic plasticity in the prefrontal cortex and hippocampus. Finally, we highlight the potential implication of these interactions in the pathophysiology and treatment of cerebellar disorders, schizophrenia, Alzheimer's disease, Parkinson's disease, l-DOPA-induced dyskinesias, stress-related disorders, and cognitive dysfunctions.

Genetic and epigenetic studies of opioid abuse disorder - the potential for future diagnostics

INTRODUCTION

Opioid use disorder (OUD) is a global problem that often begins with prescribed medications. The available treatment and maintenance plans offer solutions for the consumption rate by individuals leaving the outstanding problem of relapse, which is a major factor hindering the long-term efficacy of treatments.

AREAS COVERED

Understanding the neurobiology of addiction and relapse would help identifying the core causes of relapse and distinguish vulnerable from resilient individuals, which would lead to more targeted and effective treatment and provide diagnostics to screen individuals who have a propensity to OUD. In this review, we cover the neurobiology of the reward system highlighting the role of multiple brain regions and opioid receptors in the development of the disorder. We also review the current knowledge of the epigenetics of addiction and the available screening tools for aberrant use of opioids.

EXPERT OPINION

Relapse remains an anticipated limitation in the way of recovery even after long period of abstinence. This highlights the need for diagnostic tools that identify vulnerable patients and prevent the cycle of addiction. Finally, we discuss the limitations of the available screening tools and propose possible solutions for the discovery of addiction diagnostics.

Opioid Receptors Modulate Firing and Synaptic Transmission in the Paraventricular Nucleus of the Thalamus

The paraventricular nucleus of the thalamus (PVT) is involved in drug addiction-related behaviors, and morphine is a widely used opioid for the relief of severe pain. Morphine acts via opioid receptors, but the function of opioid receptors in the PVT has not been fully elucidated. Here, we used in vitro electrophysiology to study neuronal activity and synaptic transmission in the PVT of male and female mice. Activation of opioid receptors suppresses the firing and inhibitory synaptic transmission of PVT neurons in brain slices. On the other hand, the involvement of opioid modulation is reduced after chronic morphine exposure, probably because of desensitization and internalization of opioid receptors in the PVT. Overall, the opioid system is essential for the modulation of PVT activities.SIGNIFICANCE STATEMENT Opioid receptors modulate the activities and synaptic transmission in the PVT by suppressing the firing rate and inhibitory synaptic inputs. These modulations were largely diminished after chronic morphine exposure.

Three-Dimensional Structural Insights Have Revealed the Distinct Binding Interactions of Agonists, Partial Agonists, and Antagonists with the µ Opioid Receptor

The United States is experiencing the most profound and devastating opioid crisis in history, with the number of deaths involving opioids, including prescription and illegal opioids, continuing to climb over the past two decades. This severe public health issue is difficult to combat as opioids remain a crucial treatment for pain, and at the same time, they are also highly addictive. Opioids act on the opioid receptor, which in turn activates its downstream signaling pathway that eventually leads to an analgesic effect. Among the four types of opioid receptors, the µ subtype is primarily responsible for the analgesic cascade. This review describes available 3D structures of the µ opioid receptor in the protein data bank and provides structural insights for the binding of agonists and antagonists to the receptor. Comparative analysis on the atomic details of the binding site in these structures was conducted and distinct binding interactions for agonists, partial agonists, and antagonists were observed. The findings in this article deepen our understanding of the ligand binding activity and shed some light on the development of novel opioid analgesics which may improve the risk benefit balance of existing opioids.

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Neural circuits controlling choice behavior in opioid addiction

As the opioid epidemic presents an ever-expanding public health threat, there is a growing need to identify effective new treatments for opioid use disorder (OUD). OUD is characterized by a behavioral misallocation in choice behavior between opioids and other rewards, as opioid use leads to negative consequences, such as job loss, family neglect, and potential overdose. Preclinical models of addiction that incorporate choice behavior, as opposed to self-administration of a single drug reward, are needed to understand the neural circuits governing opioid choice. These choice models recapitulate scenarios that humans suffering from OUD encounter in their daily lives. Indeed, patients with substance use disorders (SUDs) exhibit a propensity to choose drug under certain conditions. While most preclinical addiction models have focused on relapse as the outcome measure, our data suggest that choice is an independent metric of addiction severity, perhaps relating to loss of cognitive control over choice, as opposed to excessive motivational drive to seek drugs during relapse. In this review, we examine both preclinical and clinical literature on choice behavior for drugs, with a focus on opioids, and the neural circuits that mediate drug choice versus relapse. We argue that preclinical models of opioid choice are needed to identify promising new avenues for OUD therapy that are translationally relevant. Both forward and reverse translation will be necessary to identify novel treatment interventions. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Effect of Selective Lesions of Nucleus Accumbens µ-Opioid Receptor-Expressing Cells on Heroin Self-Administration in Male and Female Rats: A Study with Novel Oprm1-Cre Knock-in Rats

The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to MOR-expressing cells. After performing anatomic and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to study the involvement of NAc MOR-expressing cells in heroin self-administration in male and female rats. Using RNAscope, autoradiography, and FISH chain reaction (HCR-FISH), we found no differences in Oprm1 expression in NAc, dorsal striatum, and dorsal hippocampus, or MOR receptor density (except dorsal striatum) or function between Oprm1-Cre knock-in rats and wildtype littermates. HCR-FISH assay showed that iCre is highly coexpressed with Oprm1 (95%-98%). There were no genotype differences in pain responses, morphine analgesia and tolerance, heroin self-administration, and relapse-related behaviors. We used the Cre-dependent vector AAV1-EF1a-Flex-taCasp3-TEVP to lesion NAc MOR-expressing cells. We found that the lesions decreased acquisition of heroin self-administration in male Oprm1-Cre rats and had a stronger inhibitory effect on the effort to self-administer heroin in female Oprm1-Cre rats. The validation of an Oprm1-Cre knock-in rat enables new strategies for understanding the role of MOR-expressing cells in rat models of opioid addiction, pain-related behaviors, and other opioid-mediated functions. Our initial mechanistic study indicates that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in male and female rats.SIGNIFICANCE STATEMENT The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to brain MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to show that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in males and females. The new Oprm1-Cre rats can be used to study the role of brain MOR-expressing cells in animal models of opioid addiction, pain-related behaviors, and other opioid-mediated functions.

Novel dual-target μ‑opioid and TRPV1 ligands as potential pharmacotherapeutics for pain management

Currently, the development of effective analgesic drugs with few side effects remains a great challenge. Studies have suggested that multi-target drug treatments show high efficacy and reduced side effects compared to single-target drug therapies. In this work, we designed and synthesized two series of novel MOR/TRPV1 dual active ligands in which the phenylpiperidine group or the N-phenyl-N-(piperidin-4-yl) propionamide group as the MOR pharmacophore was fused to the benzylpiperazinyl urea-based TRPV1 pharmacophore. In particular, compound 5a exhibited promising dual pharmacological activity for MOR (EC₅₀ = 53.7 nM) and TRPV1 (IC₅₀ = 32.9 nM) in vitro. In formalin tests, compound 5a showed potent, dose-dependent in vivo analgesic activity in both the 1st and 2nd phases. Gratifyingly, compound 5a did not cause the side effects of hyperthermia and analgesic tolerance. Consistent with its in vitro activity, compound 5a also simultaneously agonized MOR and antagonized TRPV1 in vivo. Further studies on compound 5a showed acceptable pharmacokinetic properties and brain permeability. Furthermore, molecular docking studies showed that compound 5a tightly bound to the active pockets of hMOR and hTRPV1, respectively. Overall, this work shows the promise in discovering new analgesic treatments through the strategy of simultaneously targeting MOR and TRPV1 with a single molecule.