Molecular mechanisms of opioid receptor-dependent signaling and behavior

Blocking potential metabolic sites on NAT to improve its safety profile while retaining the pharmacological profile

The number of opioid-related overdose deaths and individuals that have suffered from opioid use disorders have significantly increased over the last 30 years. FDA approved maintenance therapies to treat opioid use disorder may successfully curb drug craving and prevent relapse but harbor adverse effects that reduce patient compliance. This has created a need for new chemical entities with improved patient experience. Previously our group reported a novel lead compound, NAT, a mu-opioid receptor antagonist that potently antagonized the antinociception of morphine and showed significant blood-brain barrier permeability. However, NAT belongs to thiophene containing compounds which are known structural alerts for potential oxidative metabolism. To overcome this, 15 NAT derivatives with various substituents at the 5'-position of the thiophene ring were designed and their structure-activity relationships were studied. These derivatives were characterized for their binding affinity, selectivity, and functional activity at the mu opioid receptor and assessed for their ability to antagonize the antinociceptive effects of morphine in vivo. Compound 12 showed retention of the basic pharmacological attributes of NAT while improving the withdrawal effects that were experienced in opioid-dependent mice. Further studies will be conducted to fully characterize compound 12 to examine whether it would serve as a new lead for opioid use disorder treatment and management.

The influence of drug class on reward in substance use disorders

In the United States, the societal costs associated with drug use surpass $500 billion annually. The rewarding and reinforcing properties that drive the use of these addictive substances are typically examined concerning the neurobiological effects responsible for their abuse potential. In this review, terms such as "abuse potential," "drug," and "addictive properties" are used due to their relevance to the methodological, theoretical, and conceptual framework for understanding the phenomenon of drug-taking behavior and the associated body of preclinical and clinical literature. The use of these terms is not intended to cast aspersions on individuals with substance use disorders (SUD). Understanding what motivates substance use has been a focus of SUD research for decades. Much of this corpus of work has focused on the shared effects of each drug class to increase dopaminergic transmission within the central reward pathways of the brain, or the "reward center." However, the precise influence of each drug class on dopamine signaling, and the extent thereof, differs considerably. Furthermore, the aforementioned substances have effects on several neurobiological targets that mediate and modulate their addictive properties. The current manuscript sought to review the influence of drug class on the rewarding effects of each of the major pharmacological classes of addictive drugs (i.e., psychostimulants, opioids, nicotine, alcohol, and cannabinoids). Our review suggests that even subtle differences in drug effects can result in significant variability in the subjective experience of the drug, altering rewarding and other reinforcing effects. Additionally, this review will argue that reward (i.e., the attractive and motivational property of a stimulus) alone is not sufficient to explain the abuse liability of these substances. Instead, abuse potential is best examined as a function of both positive and negative reinforcing drug effects (i.e., stimuli that the subject will work to attain and stimuli that the subject will work to end or avoid, respectively). Though reward is central to drug use, the factors that motivate and maintain drug taking are varied and complex, with much to be elucidated.

Impact of opioid analgesics on survival in cancer patients receiving immune checkpoint inhibitors

PURPOSE

This study aimed to assess the effects of concurrent opioid analgesic (OA) use with immune checkpoint inhibitors (ICIs) on progression-free survival (PFS) and overall survival (OS).

METHODS

In this observational retrospective study, we included advanced cancer patients who received ICIs at Hacettepe University Hospital's Department of Medical Oncology between June 2018 and January 2023.

RESULTS

Our study included 375 recurrent or metastatic cancer patients treated with ICIs in the first, second line, or beyond. There were no significant differences between the OA-treated and OA-untreated groups regarding median age, age group, gender, primary tumor location, ICI type, or the presence of baseline liver and lung metastases. However, the OA-treated group exhibited a significantly higher proportion of patients who had received three or more prior treatments before initiating ICIs (p = 0.015). OA-Untreatment was significantly correlated with prolonged mPFS (6.83 vs. 4.30 months, HR 0.59, 95% CI 0.44-0.79, p < 0.001) and mOS (17.05 vs. 7.68 months, HR 0.60, 95% CI 0.45-0.80, p < 0.001).

CONCLUSIONS

Our study demonstrates an association between the concurrent use of OAs and reduced OS and PFS in patients treated with ICIs. While OA treatment serves as a surrogate marker for higher disease burden, it may also suggest a potential biological relationship between opioids and immunotherapy efficacy.

Acute Effects of Hallucinogens on Functional Connectivity: Psilocybin and Salvinorin-A

The extent of changes in functional connectivity (FC) within functional networks as a common feature across hallucinogenic drug classes is under-explored. This work utilized fMRI to assess the dissociative hallucinogens Psilocybin, a classical serotonergic psychedelic, and Salvinorin-A, a kappa-opioid receptor (KOR) agonist, on resting-state FC in nonhuman primates. We highlight overlapping and differing influence of these substances on FC relative to the thalamus, claustrum, prefrontal cortex (PFC), default mode network (DMN), and DMN subcomponents. Analysis was conducted on a within-subject basis. Findings support the cortico-claustro-cortical network model for probing functional effects of hallucinogens regardless of serotonergic potential, with a potential key paradigm centered around the claustrum, PFC, anterior cingulate cortices (ACC), and angular gyrus relationship. Thalamo-cortical networks are implicated but appear dependent on 5-HT2AR activation. Acute desynchronization relative to the DMN for both drugs was also shown. Our findings provide a framework to understand broader mechanisms at which hallucinogens in differing classes may impact subjects regardless of the target receptor.

Inhibition of the protective effects of preconditioning in ischemia-reperfusion injury by chronic methadone: the role of pAkt and pSTAT3

Cardiac ischemic preconditioning (Pre) reduces cardiac ischemia-reperfusion injury (IRI) by stimulating opioid receptors. Chronic use of opioids can alter the signaling pathways. We investigated the effects of chronic methadone use on IRI and Pre. The experiments were performed on isolated hearts of male Wistar rats in four groups: IRI, Methadone + IRI (M-IRI), Pre + IRI (Pre-IRI), Methadone + Pre + IRI (M-Pre-IRI). The infarct size (IS) in the Pre-IRI group was smaller than the IRI group (26.8% vs. 47.8%, P < 0.05). In the M-IRI and M-Pre-IRI groups, the infarct size was similar to the IRI group. Akt (Ak strain transforming) phosphorylation in the Pre-IRI, M-IRI, and M-Pre-IRI groups was significantly higher than in the IRI group (0.56 ± 0.15, 0.63 ± 0.20, and 0.93 ± 0.18 vs 0.28 ± 0.17 respectively). STAT3 (signal transducer and activator of transcription 3) phosphorylation in the Pre-IRI and M-Pre-IRI groups (1.38 ± 0.14 and 1.46 ± 0.33) was significantly higher than the IRI and M-IRI groups (0.99 ± 0.1 and 0.98 ± 0.2). Thus, chronic use of methadone not only has no protective effect against IRI but also destroys the protective effects of ischemic preconditioning. This may be due to the hyperactivation of Akt and changes in signaling pathways.

A novel strategy to elicit enduring anti-morphine immunity and relief from addiction by targeting Acr1 protein nano vaccine through TLR-2 to dendritic cells

Morphine addiction poses a significant challenge to global healthcare. Current opioid substitution therapies, such as buprenorphine, naloxone and methadone are effective but often lead to dependence. Thus, exploring alternative treatments for opioid addiction is crucial. We have developed a novel vaccine that presents morphine and Pam3Cys (a TLR-2 agonist) on the surface of Acr1 nanoparticles. This vaccine has self-adjuvant properties and targets TLR-2 receptors on antigen-presenting cells, particularly dendritic cells. Our vaccination strategy promotes the proliferation and differentiation of morphine-specific B-cells and Acr1-reactive CD4 T-cells. Additionally, the vaccine elicited the production of high-affinity anti-morphine antibodies, effectively eliminating morphine from the bloodstream and brain in mice. It also reduced the expression of addiction-associated μ-opioid receptor and dopamine genes. The significant increase in memory CD4 T-cells and B-cells indicates the vaccine's ability to induce long-lasting immunity against morphine. This vaccine holds promise as a prophylactic measure against morphine addiction.

Systematic Structure-Activity Relationship Study of Nalfurafine Analogues toward Development of Potentially Nonaddictive Pain Management Treatments

Despite the availability of numerous pain medications, the current array of Food and Drug Administration-approved options falls short in adequately addressing pain states for numerous patients and consequently worsens the opioid crisis. Thus, it is imperative for basic research to develop novel and nonaddictive pain medications. Toward addressing this clinical goal, nalfurafine (NLF) was chosen as a lead and its structure-activity relationship (SAR) systematically studied through design, syntheses, and in vivo characterization of 24 analogues. Two analogues, 21 and 23, showed longer durations of action than NLF in a warm-water tail immersion assay, produced in vivo effects primarily mediated by KOR and DOR, penetrated the blood-brain barrier, and did not function as reinforcers. Additionally, 21 produced fewer sedative effects than NLF. Taken together, these results aid the understanding of NLF SAR and provide insights for future endeavors in developing novel nonaddictive therapeutics to treat pain.

Neuroprotective effects of naltrexone in a mouse model of post-traumatic seizures

Traumatic Brain Injury (TBI) induces neuroinflammatory response that can initiate epileptogenesis, which develops into epilepsy. Recently, we identified anti-convulsive effects of naltrexone, a mu-opioid receptor (MOR) antagonist, used to treat drug addiction. While blocking opioid receptors can reduce inflammation, it is unclear if post-TBI seizures can be prevented by blocking MORs. Here, we tested if naltrexone prevents neuroinflammation and/or seizures post-TBI. TBI was induced by a modified Marmarou Weight-Drop (WD) method on 4-week-old C57BL/6J male mice. Mice were placed in two groups: non-telemetry assessing the acute effects or in telemetry monitoring for interictal events and spontaneous seizures both following TBI and naltrexone. Molecular, histological and neuroimaging techniques were used to evaluate neuroinflammation, neurodegeneration and fiber track integrity at 8 days and 3 months post-TBI. Peripheral immune responses were assessed through serum chemokine/cytokine measurements. Our results show an increase in MOR expression, nitro-oxidative stress, mRNA expression of inflammatory cytokines, microgliosis, neurodegeneration, and white matter damage in the neocortex of TBI mice. Video-EEG revealed increased interictal events in TBI mice, with 71% mice developing post-traumatic seizures (PTS). Naltrexone treatment ameliorated neuroinflammation, neurodegeneration, reduced interictal events and prevented seizures in all TBI mice, which makes naltrexone a promising candidate against PTS, TBI-associated neuroinflammation and epileptogenesis in a WD model of TBI.

Neural circuit-selective, multiplexed pharmacological targeting of prefrontal cortex-projecting locus coeruleus neurons drives antinociception

Selective manipulation of neural circuits using optogenetics and chemogenetics holds great translational potential but requires genetic access to neurons. Here, we demonstrate a general framework for identifying genetic tool-independent, pharmacological strategies for neural circuit-selective modulation. We developed an economically accessible calcium imaging-based approach for large-scale pharmacological scans of endogenous receptor-mediated neural activity. As a testbed for this approach, we used the mouse locus coeruleus due to the combination of its widespread, modular efferent neural circuitry and its wide variety of endogenously expressed GPCRs. Using machine learning-based action potential deconvolution and retrograde tracing, we identified an agonist cocktail that selectively inhibits medial prefrontal cortex-projecting locus coeruleus neurons. In vivo, this cocktail produces synergistic antinociception, consistent with selective pharmacological blunting of this neural circuit. This framework has broad utility for selective targeting of other neural circuits under different physiological and pathological states, facilitating non-genetic translational applications arising from cell type-selective discoveries.

Opioid Maintenance Therapy: A Review of Methadone, Buprenorphine, and Naltrexone Treatments for Opioid Use Disorder

The rates of opioid use and opioid related deaths are escalating in the United States. Despite this, evidence-based treatments for Opioid Use Disorder are underutilized. There are three medications FDA approved for treatment of Opioid Use Disorder: Methadone, Buprenorphine, and Naltrexone. This article reviews the history, criteria, and mechanisms associated with Opioid Use Disorder. Pertinent pharmacology considerations, treatment strategies, efficacy, safety, and challenges of Methadone, Buprenorphine, and Naltrexone are outlined. Lastly, a practical decision making algorithm is discussed to address pertinent psychiatric and medical comorbidities when prescribing pharmacology for Opioid Use Disorder.

[Appropriate use of strong opioids in medical units: Recommendations and action to be taken in daily practice]

The use of strong opioids in medical units is recurrent, mainly for analgesic purposes. The risk of occurrence of an overdose or an opioid use disorders causes very legitimate concerns for the physician, which may limit the use of opioid treatment or the adaptation of the doses necessary to relieve the patient. We provide a summary of the literature aimed at defining the indications, the adverse effects and the risks involved, the prescribing methods in order to reassure professionals and promote the safe use of these molecules.

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.

Chronic Opioid Treatment Arrests Neurodevelopment and Alters Synaptic Activity in Human Midbrain Organoids

Understanding the impact of long-term opioid exposure on the embryonic brain is critical due to the surging number of pregnant mothers with opioid dependency. However, this has been limited by human brain inaccessibility and cross-species differences in animal models. Here, a human midbrain model is established that uses hiPSC-derived midbrain organoids to assess cell-type-specific responses to acute and chronic fentanyl treatment and fentanyl withdrawal. Single-cell mRNA sequencing of 25,510 cells from organoids in different treatment groups reveals that chronic fentanyl treatment arrests neuronal subtype specification during early midbrain development and alters synaptic activity and neuron projection. In contrast, acute fentanyl treatment increases dopamine release but does not significantly alter gene expression related to cell lineage development. These results provide the first examination of the effects of opioid exposure on human midbrain development at the single-cell level.

Molecular mechanisms of morphine tolerance and dependence; novel insights and future perspectives

Drug addiction is a devastating condition that poses a serious burden on the society. The use of some drugs like morphine for their tremendous analgesic properties is also accompanied with developing tolerance, dependence and the withdrawal symptoms. These symptoms are frequently severe enough to reinforce the person in recovery to start over the use of drug again and hinder the clinical use of drugs like morphine for chronic pain. Research into opioid receptors and related molecular pathways has seen resurgence in the wake of the growing opioid epidemic. The current study provides a comprehensive scientific exploration of the molecular mechanisms and underlying signalling in morphine tolerance and dependence. It also critically evaluates current therapeutic approaches, shedding light on their efficacy and limitations, and future prospects.

Neuroimaging of opioid effects in humans across conditions of acute administration, chronic pain therapy, and opioid use disorder

Evidence of central nervous system (CNS) exogenous opioid effects in humans has been primarily gained through neuroimaging of three participant populations: individuals after acute opioid administration, those with opioid use disorder (OUD), and those with chronic pain receiving opioid therapy. In both the brain and spinal cord, opioids alter processes of pain, cognition, and reward. Opioid-related CNS effects may persist and accumulate with longer opioid use duration. Meanwhile, opioid-induced benefits versus risks to brain health remain unclear. This review article highlights recent accumulating evidence for how exogenous opioids impact the CNS in humans. While investigation of CNS opioid effects has remained largely disparate across contexts of opioid acute administration, OUD, and chronic pain opioid therapy, integration across these contexts may enable advancement toward effective interventions.

Role of antioxidants in the neurobiology of drug addiction: An update

Relationships between protective enzymatic and non-enzymatic pro-antioxidant mechanisms and addictive substances use disorders (SUDs) are analyzed here, based on the results of previous research, as well as on the basis of our current own studies. This review introduces new aspects of comparative analysis of associations of pro-antixidant and neurobiological effects in patients taking psychoactive substances and complements very limited knowledge about relationships with SUDs from different regions, mainly Europe. In view of the few studies on relations between antioxidants and neurobiological processes acting in patients taking psychoactive substances, this review is important from the point of view of showing the state of knowledge, directions of diagnosis and treatment, and further research needed explanation. We found significant correlations between chemical elements, pro-antioxidative mechanisms, and lipoperoxidation in the development of disorders associated with use of addictive substances, therefore elements that show most relations (Pr, Na, Mn, Y, Sc, La, Cr, Al, Ca, Sb, Cd, Pb, As, Hg, Ni) may be significant factors shaping SUDs. The action of pro-antioxidant defense and lipid peroxidation depends on the pro-antioxidative activity of ions. We explain the strongest correlations between Mg and Sb, and lipoperoxidation in addicts, which proves their stimulating effect on lipoperoxidation and on the induction of oxidative stress. We discussed which mechanisms and neurobiological processes change susceptibility to SUDs. The innovation of this review is to show that addicted people have lower activity of dismutases and peroxidases than healthy ones, which indicates disorders of antioxidant system and depletion of enzymes after long-term tolerance of stressors. We explain higher level of catalases, reductases, ceruloplasmin, bilirubin, retinol, α-tocopherol and uric acid of addicts. In view of poorly understood factors affecting addiction, analysis of interactions allows for more effective understanding of pathogenetic mechanisms leading to formation of addiction and development the initiation of directed, more effective treatment (pharmacological, hormonal) and may be helpful in the diagnosis of psychoactive changes.

Neurocircuitry underlying the actions of glucagon-like peptide 1 and peptide YY3-36 in the suppression of food, drug-seeking, and anxiogenesis

Obesity is a critical health condition worldwide that increases the risks of comorbid chronic diseases, but it can be managed with weight loss. However, conventional interventions relying on diet and exercise are inadequate for achieving and maintaining weight loss, thus there is significant market interest for pharmaceutical anti-obesity agents. For decades, receptor agonists for the gut peptide glucagon-like peptide 1 (GLP-1) featured prominently in anti-obesity medications by suppressing appetite and food reward to elicit rapid weight loss. As the neurocircuitry underlying food motivation overlaps with that for drugs of abuse, GLP-1 receptor agonism has also been shown to decrease substance use and relapse, thus its therapeutic potential may extend beyond weight management to treat addictions. However, as prolonged use of anti-obesity drugs may increase the risk of mood-related disorders like anxiety and depression, and individuals taking GLP-1-based medication commonly report feeling demotivated, the long-term safety of such drugs is an ongoing concern. Interestingly, current research now focuses on dual agonist approaches that include GLP-1 receptor agonism to enable synergistic effects on weight loss or associated functions. GLP-1 is secreted from the same intestinal cells as the anorectic gut peptide, Peptide YY3-36 (PYY3-36), thus this review assessed the therapeutic potential and underlying neural circuits targeted by PYY3-36 when administered independently or in combination with GLP-1 to curb the appetite for food or drugs of abuse like opiates, alcohol, and nicotine. Additionally, we also reviewed animal and human studies to assess the impact, if any, for GLP-1 and/or PYY3-36 on mood-related behaviors in relation to anxiety and depression. As dual agonists targeting GLP-1 and PYY3-36 may produce synergistic effects, they can be effective at lower doses and offer an alternative approach for therapeutic benefits while mitigating undesirable side effects.

The fentanyl-specific antibody FenAb024 can shield against carfentanil effects

The surge in opioid-related deaths, driven predominantly by fentanyl and its synthetic derivatives, has become a critical public health concern, which is particularly evident in the United States. While the situation is less severe in Europe, the European Monitoring Centre for Drugs and Drug Addiction reports a rise in drug overdose deaths, with emerging concerns about the impact of fentanyl-related molecules. Synthetic opioids, initially designed for medical use, have infiltrated illicit markets due to their low production costs and high potency, with carfentanil posing additional threats, including potential chemical weaponization. Existing overdose mitigation heavily relies on naloxone, requiring timely intervention and caregiver presence, while therapeutic prevention strategies face many access challenges. To provide an additional treatment option, we propose the use of a fentanyl-specific monoclonal antibody (mAb), as a non-opioid method of prophylaxis against fentanyl and carfentanil. This mAb shows protection from opioid effects in a pre-clinical murine model. mAbs could emerge as a versatile countermeasure in civilian and biodefense settings, offering a novel approach to combat opioid-associated mortality.

Fentanyl and its derivatives: Pain-killers or man-killers?

Fentanyl is a synthetic μ-opioid receptor agonist approved to treat severe to moderate pain with faster onset of action and about 100 times more potent than morphine. Over last two decades, abuse of fentanyl and its derivatives has an increased trend, globally. Currently, the United States (US) faces the most serious situation related to fentanyl overdose, commonly referred to as the opioid epidemic. Nowadays, fentanyl is considered as the number one cause of death for adults aged 18-45 in the US. Synthesis and derivatization of fentanyl is inexpensive to manufacture and easily achievable. Indeed, more than 1400 fentanyl derivatives have been described in the scientific literature and patents. In addition, accessibility and efficacy of fentanyl and its derivatives can play a potential role in misuse of these compounds as a chemical weapon. In this review, the properties, general pharmacology, and overdose death cases associated with fentanyl and selected derivatives are presented. Moreover, current opioid epidemic in the US, Moscow theatre hostage crisis, and potential misuse of fentanyl and its derivatives as a chemical weapon are disclosed.

Fentanyl activates opposing opioid and non-opioid receptor systems that control breathing

Fentanyl elicits profound disturbances in ventilatory control processes in humans and experimental animals. The traditional viewpoint with respect to fentanyl-induced respiratory depression is that once the effects on the frequency of breathing (Freq), tidal volume (TV), and minute ventilation (MV = Freq × TV) are resolved, then depression of breathing is no longer a concern. The results of the present study challenge this concept with findings, as they reveal that while the apparent inhibitory effects of fentanyl (75 μg/kg, IV) on Freq, TV, and MV in adult male rats were fully resolved within 15 min, many other fentanyl-induced responses were in full effect, including opposing effects on respiratory timing parameters. For example, although the effects on Freq were resolved at 15 min, inspiratory duration (Ti) and end inspiratory pause (EIP) were elevated, whereas expiratory duration (Te) and end expiratory pause (EEP) were diminished. Since the effects of fentanyl on TV had subsided fully at 15 min, it would be expected that the administration of an opioid receptor (OR) antagonist would have minimal effects if the effects of fentanyl on this and other parameters had resolved. We now report that the intravenous injection of a 1.0 mg/kg dose of the peripherally restricted OR antagonist, methyl-naloxone (naloxone methiodide, NLXmi), did not elicit arousal but elicited some relatively minor changes in Freq, TV, MV, Te, and EEP but pronounced changes in Ti and EIP. In contrast, the injection of a 2.5 mg/kg dose of NLXmi elicited pronounced arousal and dramatic changes in many variables, including Freq, TV, and MV, which were not associated with increases in non-apneic breathing events such as apneas. The two compelling conclusions from this study are as follows: 1) the blockade of central ORs produced by the 2.5 mg/kg dose of NLXmi elicits pronounced increases in Freq, TV, and MV in rats in which the effects of fentanyl had apparently resolved, and 2) it is apparent that fentanyl had induced the activation of two systems with counter-balancing effects on Freq and TV: one being an opioid receptor inhibitory system and the other being a non-OR excitatory system.

Evaluating Sedation Strategies for Magnetic Resonance Imaging: A Comprehensive Review of Intravenous Fentanyl, Butorphanol, and Midazolam in Adult and Pediatric Populations

Magnetic resonance imaging (MRI) is a critical diagnostic tool that often requires patient sedation to ensure optimal image quality and patient comfort, particularly in those with anxiety or an inability to remain still. This comprehensive review examines the efficacy, safety, and practical considerations of three commonly used intravenous sedatives, namely, fentanyl, butorphanol, and midazolam, in adult populations undergoing MRI procedures. This review highlights the pharmacological profiles, advantages, and limitations associated with each sedative agent through a detailed analysis of current literature, clinical guidelines, and practice-based evidence. Fentanyl is noted for its potent analgesic properties and rapid onset of action, making it suitable for painful procedures. Butorphanol, with its unique opioid agonist-antagonist activity, presents an alternative with a balance between analgesia and sedation, potentially offering a safer profile for certain patient populations. Midazolam, widely recognized for its anxiolytic and amnestic effects, remains a staple in managing procedure-related anxiety. The review further discusses patient selection criteria, dosing strategies, and the importance of individualized sedation planning to enhance patient experience and procedural outcomes. Future directions highlight the potential of emerging sedation agents and non-pharmacological approaches to improve patient comfort and compliance. The findings underscore the necessity for healthcare providers to adapt sedation practices to the specific needs of each patient, considering both the clinical context and the inherent characteristics of the sedative agents. This review aims to guide clinicians in selecting the most appropriate sedation strategy for adult patients undergoing MRI, optimizing patient care and diagnostic efficacy.

Exploring HD-tDCS Effect on μ-opioid Receptor and Pain Sensitivity in Temporomandibular Disorder: A Pilot Randomized Clinical Trial Study

This study explored the association between experimentally-induced pain sensitivity and µ-opioid receptor (μOR) availability in patients with temporomandibular disorder (TMD) and further investigated any changes in the pain and μOR availability following high-definition transcranial direct current stimulation (HD-tDCS) over the primary motor cortex (M1) with pilot randomized clinical trials. Seven patients with TMD completed either active (n = 3) or sham treatment (n = 4) for 10 daily sessions and underwent positron emission tomography (PET) scans with [11C]carfentanil, a selective μOR agonist, a week before and after treatment. PET imaging consisted of an early resting and late phase with the sustained masseteric pain challenge by computer-controlled injection of 5% hypertonic saline. We also included 12 patients with TMD, obtained from our previous study, for baseline PET analysis. We observed that patients with more sensitivity to pain, indicated by lower infusion rate, had less μOR availability in the right amygdala during the late phase. Moreover, active M1 HD-tDCS, compared to sham, increased μOR availability post-treatment in the thalamus during the early resting phase and the amygdala, hippocampus, and parahippocampal gyrus during the late pain challenge phase. Importantly, increased μOR availability post-treatment in limbic structures including the amygdala and hippocampus was associated with decreased pain sensitivity. The findings underscore the role of the μOR system in pain regulation and the therapeutic potential of HD-tDCS for TMD. Nonetheless, large-scale studies are necessary to establish the clinical significance of these results. TRIAL REGISTRATION: ClinicalTrial.gov (NCT03724032) PERSPECTIVE: This study links pain sensitivity and µ-opioid receptors in patients with TMD. HD-tDCS over M1 improved µOR availability, which was associated with reduced pain sensitivity. Implications for TMD pain management are promising, but larger clinical trials are essential for validation.

Astrocyte PERK and IRE1 Signaling Contributes to Morphine Tolerance and Hyperalgesia through Upregulation of Lipocalin-2 and NLRP3 Inflammasome in the Rodent Spinal Cord

BACKGROUND

Endoplasmic reticulum stress plays a crucial role in the pathogenesis of neuroinflammation and chronic pain. This study hypothesized that PRKR-like endoplasmic reticulum kinase (PERK) and inositol-requiring enzyme type 1 (IRE1) regulate lipocalin-2 (LCN2) and Nod-like receptor family pyrin domain containing 3 (NLRP3) expression in astrocytes, thereby contributing to morphine tolerance and hyperalgesia.

METHODS

The study was performed in Sprague-Dawley rats and C57/Bl6 mice of both sexes. The expression of LCN2 and NLRP3 was assessed by Western blotting. The tail-flick, von Frey, and Hargreaves tests were used to evaluate nociceptive behaviors. Chromatin immunoprecipitation was conducted to analyze the binding of activating transcription factor 4 (ATF4) to the promoters of LCN2 and TXNIP. Whole-cell patch-clamp recordings were used to evaluate neuronal excitability.

RESULTS

Pharmacologic inhibition of PERK and IRE1 attenuated the development of morphine tolerance and hyperalgesia in male (tail latency on day 7, 8.0 ± 1.13 s in the morphine + GSK2656157 [10 μg] group vs. 5.8 ± 0.65 s in the morphine group; P = 0.04; n = 6 rats/group) and female (tail latency on day 7, 6.0 ± 0.84 s in the morphine + GSK2656157 [10 μg] group vs. 3.1 ± 1.09 s in the morphine group; P = 0.0005; n = 6 rats/group) rats. Activation of PERK and IRE1 upregulated expression of LCN2 and NLRP3 in vivo and in vitro. Chromatin immunoprecipitation analysis showed that ATF4 directly bound to the promoters of the LCN2 and TXNIP. Lipocalin-2 induced neuronal hyperexcitability in the spinal cord and dorsal root ganglia via melanocortin-4 receptor.

CONCLUSIONS

Astrocyte endoplasmic reticulum stress sensors PERK and IRE1 facilitated morphine tolerance and hyperalgesia through upregulation of LCN2 and NLRP3 in the spinal cord.

EDITOR’S PERSPECTIVE

Modulators of nicotine reward and reinforcement

Nicotine has been well-characterized for its ability to alter neurophysiology to promote rewarding and reinforcing properties. However, several exogenous chemicals possess properties that modulate or enhance nicotine's ability to alter neurophysiology. This chapter focuses on nicotine's impact on behavior through changes in neurophysiology and several chemical entities that in-turn modulate nicotine's ability to act as a neuromodulator.

An insight review on the neuropharmacological effects, mechanisms of action, pharmacokinetics and toxicity of mitragynine

Mitragynine is one of the main psychoactive alkaloids in Mitragyna speciosa Korth. (kratom). It has opium-like effects by acting on μ-, δ-, and κ-opioid receptors in the brain. The compound also interacts with other receptors, such as adrenergic and serotonergic receptors and neuronal Ca2+ channels in the central nervous system to have its neuropharmacological effects. Mitragynine has the potential to treat diseases related to neurodegeneration such as Alzheimer's disease and Parkinson's disease, as its modulation on the opioid receptors has been reported extensively. This review aimed to provide an up-to-date and critical overview on the neuropharmacological effects, mechanisms of action, pharmacokinetics and safety of mitragynine as a prospective psychotropic agent. Its multiple neuropharmacological effects on the brain include antinociceptive, anti-inflammatory, antidepressant, sedative, stimulant, cognitive, and anxiolytic activities. The potential of mitragynine to manage opioid withdrawal symptoms related to opioid dependence, its pharmacokinetics and toxic effects were also discussed. The interaction of mitragynine with various receptors in the brain produce diverse neuropharmacological effects, which have beneficial properties in neurological disorders. However, further studies need to be carried out on mitragynine to uncover its complex mechanisms of action, pharmacokinetics, pharmacodynamic profiles, addictive potential, and safe dosage to prevent harmful side effects.

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

Drug-induced atrial fibrillation. A narrative review of a forgotten adverse effect

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with an increased morbidity and mortality. There is clinical evidence that an increasing number of cardiovascular and non-cardiovascular drugs, mainly anticancer drugs, can induce AF either in patients with or without pre-existing cardiac disorders, but drug-induced AF (DIAF) has not received the attention that it might deserve. In many cases DIAF is asymptomatic and paroxysmal and patients recover sinus rhythm spontaneously, but sometimes, DIAF persists, and it is necessary to perform a cardioversion. Furthermore, DIAF is not mentioned in clinical guidelines on the treatment of AF. The risk of DIAF increases in elderly and in patients treated with polypharmacy and with risk factors and comorbidities that commonly coexist with AF. This is the case of cancer patients. Under these circumstances ascribing causality of DIAF to a given drug often represents a clinical challenge. We review the incidence, the pathophysiological mechanisms, risk factors, clinical relevance, and treatment of DIAF. Because of the limited information presently available, further research is needed to obtain a deeper insight into DIAF. Meanwhile, it is important that clinicians are aware of the problem that DIAF represents, recognize which drugs may cause DIAF, and consider the possibility that a drug may be responsible for a new-onset AF episode.

Caffeine and naloxone treated mesenchymal stem cells improve symptoms and reduce inflammation in a mouse model of ulcerative colitis

BACKGROUND AND OBJECTIVE

Ulcerative colitis (UC) causes ulcers in the colon and rectum, leading to abdominal pain, diarrhea, and rectal bleeding, and if left untreated, can lead to serious complications. The therapeutic effects of mesenchymal stem cells (MSCs) on experimental models of UC have been proven. Since the microenvironment around these cells is crucial in maintaining cell proliferation, differentiation, metabolism, and overall function, this study aims to evaluation the role of caffeine and naloxone as a new microenvironment for MSCs in reducing inflammation and improving symptoms in an experimental model of UC.

MATERIAL AND METHOD

A group of 40 outbred NMRI mice were studied and divided randomly into four equal groups (N = 10 each group). UC was induced in all groups using acetic acid. The first group (control) was treated with phosphate buffer saline (PBS), the second group with MSCs-Caffeine, the third with MSCs-Naloxone, and the fourth with Mesalazine. The disease activity index (DAI), tissue damage, myeloperoxidase (MPO) activity, nitric oxide (NO) levels, and the production of IL-1, IL-6, and TNF-α cytokines were evaluated.

RESULT

Our research demonstrated that all treatments were effective in improving the symptoms and reducing inflammatory markers in mice with colitis. Among the two MSCs treatments, the MSCs-Caffeine was found to be the most potent in reducing the levels of NO, IL-1, IL-6, tissue damage (P < 0.001) and as well as TNF-α (P < 0.0001) in compared to the control group.

CONCLUSION

MSCs treated with caffeine and naloxone can enhance the immunoregulatory potential of these. As a result, treated MSCs can lead to improved clinical signs and reduced inflammatory parameters in mice with UC, making this approach a useful way for controlling and treating the disease. However, additional research is needed to access the mechanism behind the stronger immune system regulatory effects of treated MSCs in UC treatment.

The Role of Pro-Opiomelanocortin Derivatives in the Development of Type 2 Diabetes-Associated Myocardial Infarction: Possible Links with Prediabetes

Myocardial infarction is a major contributor to CVD-related mortality. T2DM is a risk factor for MI. Stress activates the HPA axis, SNS, and endogenous OPS. These POMC derivatives increase the blood glucose and cardiovascular response by inhibiting the PI3K/AkT insulin signaling pathway and increasing cardiac contraction. Opioids regulate the effect of the HPA axis and SNS and they are cardioprotective. The chronic activation of the stress response may lead to insulin resistance, cardiac dysfunction, and MI. Stress and T2DM, therefore, increase the risk of MI. T2DM is preceded by prediabetes. Studies have shown that prediabetes is associated with an increased risk of MI because of inflammation, hyperlipidemia, endothelial dysfunction, and hypertension. The HPA axis is reported to be dysregulated in prediabetes. However, the SNS and the OPS have not been explored during prediabetes. The effect of prediabetes on POMC derivatives has yet to be fully explored and understood. The impact of stress and prediabetes on the cardiovascular response needs to be investigated. This study sought to review the potential impact of prediabetes on the POMC derivatives and pathways that could lead to MI.

Opioid Analgesic as a Positive Allosteric Modulator of Acid-Sensing Ion Channels

Tafalgin (Taf) is a tetrapeptide opioid used in clinical practice in Russia as an analgesic drug for subcutaneous administration as a solution (4 mg/mL; concentration of 9 mM). We found that the acid-sensing ion channels (ASICs) are another molecular target for this molecule. ASICs are proton-gated sodium channels that mediate nociception in the peripheral nervous system and contribute to fear and learning in the central nervous system. Using electrophysiological methods, we demonstrated that Taf could increase the integral current through heterologically expressed ASIC with half-maximal effective concentration values of 0.09 mM and 0.3 mM for rat and human ASIC3, respectively, and 1 mM for ASIC1a. The molecular mechanism of Taf action was shown to be binding to the channel in the resting state and slowing down the rate of desensitization. Taf did not compete for binding sites with both protons and ASIC3 antagonists, such as APETx2 and amiloride (Ami). Moreover, Taf and Ami together caused an unusual synergistic effect, which was manifested itself as the development of a pronounced second desensitizing component. Thus, the ability of Taf to act as a positive allosteric modulator of these channels could potentially cause promiscuous effects in clinical practice. This fact must be considered in patients' treatment.

Opioidergic signaling contributes to food-mediated suppression of AgRP neurons

Opioids are generally known to promote hedonic food consumption. Although much of the existing evidence is primarily based on studies of the mesolimbic pathway, endogenous opioids and their receptors are widely expressed in hypothalamic appetite circuits as well; however, their role in homeostatic feeding remains unclear. Using a fluorescent opioid sensor, deltaLight, here we report that mediobasal hypothalamic opioid levels increase by feeding, which directly and indirectly inhibits agouti-related protein (AgRP)-expressing neurons through the μ-opioid receptor (MOR). AgRP-specific MOR expression increases by energy surfeit and contributes to opioid-induced suppression of appetite. Conversely, its antagonists diminish suppression of AgRP neuron activity by food and satiety hormones. Mice with AgRP neuron-specific ablation of MOR expression have increased fat preference without increased motivation. These results suggest that post-ingestion release of endogenous opioids contributes to AgRP neuron inhibition to shape food choice through MOR signaling.

Divergent opioid-mediated suppression of inhibition between hippocampus and neocortex across species and development

Opioid receptors within the CNS regulate pain sensation and mood and are key targets for drugs of abuse. Within the adult rodent hippocampus (HPC), μ-opioid receptor agonists suppress inhibitory parvalbumin-expressing interneurons (PV-INs), thus disinhibiting the circuit. However, it is uncertain if this disinhibitory motif is conserved in other cortical regions, species, or across development. We observed that PV-IN mediated inhibition is robustly suppressed by opioids in HPC but not neocortex in mice and nonhuman primates, with spontaneous inhibitory tone in resected human tissue also following a consistent dichotomy. This hippocampal disinhibitory motif was established in early development when immature PV-INs and opioids already influence primordial network rhythmogenesis. Acute opioid-mediated modulation was partially occluded with morphine pretreatment, with implications for the effects of opioids on hippocampal network activity during circuit maturation as well as learning and memory. Together, these findings demonstrate that PV-INs exhibit a divergence in opioid sensitivity across brain regions that is remarkably conserved across evolution and highlights the underappreciated role of opioids acting through immature PV-INs in shaping hippocampal development.

Pharmacokinetics, metabolite profiling, safety and tolerability of YZJ-4729 tartrate, a novel G protein-biased μ-opioid receptor agonist, in healthy Chinese subjects

Objective: YZJ-4729 is a novel G protein-biased μ-opioid receptor agonist for the treatment of acute pain in adult patients who require intravenous opioid analgesic therapy. The aim of this study was to assess the pharmacokinetics, metabolite profiling, safety and tolerability of YZJ-4729 in healthy Chinese subjects following the single intravenous doses ranged from 0.2 mg to 6 mg. Methods: This single-center, randomized, double-blind, placebo-controlled clinical study was conducted in 54 healthy male and female Chinese subjects after single ascending doses of YZJ-4729 tartrate (0.2, 0.5, 1.5, 3, 4.5, and 6 mg). Subjects in each cohort were assigned randomly to receive a single intravenous dose of YZJ-4729 tartrate injection or placebo at a ratio of 4:1. Pharmacokinetic characteristics, metabolite profiling, safety and tolerability profiles of the study drug were evaluated. Results: Overall, YZJ-4729 was safe and well tolerated in healthy Chinese subjects. The study drug reached peak plasma concentrations nearly at the end of the infusion. After administration, YZJ-4729 was eliminated rapidly with a terminal elimination half-life of 0.862-2.50 h, and excreted little in human excreta. The maximum drug concentration and area under the plasma concentration-time curve increased with dose escalation across the entire dose range. YZJ-4729 experienced extensive metabolism in human body. A total of 19 metabolites were identified and the characteristic metabolic pathways involved hydroxylation, ketone formation, N-dealkylation and glucuronide conjugation. Metabolite M10 was the most abundant circulating metabolite, and represented over 10% of total drug-related systemic exposure. Further PK and safety evaluation of M10 was necessary. Conclusion: The clinical study results laid a foundation for the further clinical studies of YZJ-4729 in patients. Clinical Trial Registration: http://www.chinadrugtrials.org.cn, identifier CTR20222574.

The Role of Estrogen Signaling and Exercise in Drug Abuse: A Review

BACKGROUND

Discovering how sex differences impact the efficacy of exercise regimens used for treating drug addiction is becoming increasingly important. Estrogen is a hormone believed to explain a large portion of sex differences observed during drug addiction, and why certain exercise regimens are not equally effective between sexes in treatment. Addiction is currently a global hindrance to millions, many of whom are suffering under the influence of their brain's intrinsic reward system coupled with external environmental factors. Substance abuse disorders in the U.S. alone cost billions of dollars annually.

REVIEW SUMMARY

Studies involving the manipulation of estrogen levels in female rodents, primarily via ovariectomy, highlight its impact regarding drug addiction. More specifically, female rodents with higher estrogen levels during the estrus phase increase cocaine consumption, whereas those in the non-estrus phase (low estrogen levels) decrease cocaine consumption. If estrogen is reintroduced, self-administration increases once again. Exercise has been proven to decrease relapse tendency, but its effect on estrogen levels is not fully understood.

CONCLUSIONS

Such findings and results discussed in this review suggest that estrogen influences the susceptibility of females to relapse. Therefore, to improve drug-abuse-related treatment, exercise regimens for females should be generated based on key sex differences with respect to males.

The Impact of UFP-512 in Mice with Osteoarthritis Pain: The Role of Hydrogen Sulfide

The pain-relieving properties of opioids in inflammatory and neuropathic pain are heightened by hydrogen sulfide (H2S). However, whether allodynia and functional and/or emotional impairments related to osteoarthritis (OA) could be reduced by activating δ-opioid receptors (DOR) and the plausible influence of H2S on these actions has not been completely established. In female C57BL/6J mice with OA pain generated via monosodium acetate (MIA), we analyze: (i) the effects of UFP-512 (a DOR agonist), given alone and co-administered with two H2S donors, on the symptoms of allodynia, loss of grip strength (GS), and anxiodepressive-like comportment; (ii) the reversion of UFP-512 actions with naltrindole (a DOR antagonist), and (iii) the impact of UFP-512 on the expression of phosphorylated NF-kB inhibitor alpha (p-IKBα) and the antioxidant enzymes superoxide dismutase 1 (SOD-1) and glutathione sulfur transferase M1 (GSTM1); and the effects of H2S on DOR levels in the dorsal root ganglia (DRG), amygdala (AMG), and hippocampus (HIP) of MIA-injected animals. Results showed that systemic and local administration of UFP-512 dose-dependently diminished the allodynia and loss of GS caused by MIA, whose effects were potentiated by H2S and reversed by naltrindole. UFP-512 also inhibited anxiodepressive-like behaviors, normalized the overexpression of p-IKBα in DRG and HIP, and enhanced the expression of SOD-1 and GSTM1 in DRG, HIP, and/or AMG. Moreover, the increased expression of DOR triggered by H2S might support the improved analgesic actions of UFP-512 co-administered with H2S donors. This study proposes the use of DOR agonists, alone or combined with H2S donors, as a new treatment for OA pain.

The effect of acetic acid-induced pain in Parkinson's disease model in zebrafish

Parkinson's disease (PD) is the second most common neurodegenerative disease caused by the degeneration of dopaminergic neurons and the accumulation of Lewy bodies. Pain is one of the most common non-motor symptoms in PD, but the molecular mechanism of pain in PD is not fully understood, which prevents early diagnosis of PD. We aimed to determine the changes in opioidergic pathways when external pain is inflicted by inducing pain intraperitoneally in zebrafish, for which we generated a rotenone-induced PD model. After behavioural analyses in control(C), acetic acid (AA), rotenone (ROT), and rotenone+ acetic acid (ROT+AA) groups, catecholamine levels in brain tissue were determined by LC-MS/MS, expression of opioid peptides and their receptors by RT-PCR, expression of tyrosine hydroxylase by immunohistochemical method, and analyses of oxidant-antioxidant parameters by spectrophotometric methods. In the ROT group, distance travelled, average speed, and brain dopamine levels decreased, while LPO (lipid peroxidation) and NO (nitric oxide) increased as indicators of oxidative damage, and the SOD activity decreased. The mRNA expression of lrrk, pink1, and park7 genes associated with PD increased, while the mRNA expression of park2 decreased. This indicates that rotenone exposure is a suitable means to induce PD in zebrafish. The fact that body curvature was higher in the AA group than in the ROT and ROT+AA groups, as well as the decreased expression of penka, pdyn, and ion channels associated with the perception of peripheral pain in the ROT+AA group, suggest that mechanisms associated with pain are impaired in the rotenone-induced PD model in zebrafish.

The bidirectional relationship between opioids and the gut microbiome: Implications for opioid tolerance and clinical interventions

Opioids are widely used in treating patients with acute and chronic pain; however, this class of drugs is also commonly abused. Opioid use disorder and associated overdoses are becoming more prevalent as the opioid crisis continues. Chronic opioid use is associated with tolerance, which decreases the efficacy of opioids over time, but also puts individuals at risk of fatal overdoses. Therefore, it is essential to identify strategies to reduce opioid tolerance in those that use these agents. The gut microbiome has been found to play a critical role in opioid tolerance, with opioids causing dysbiosis of the gut, and changes in the gut microbiome impacting opioid tolerance. These changes in turn have a detrimental effect on the gut microbiome, creating a positive feedback cycle. We review the bidirectional relationship between the gut microbiome and opioid tolerance, discuss the role of modulation of the gut microbiome as a potential therapeutic option in opioid-induced gut dysbiosis, and suggest opportunities for further research and clinical interventions.

Long-Term Opiate Therapy-Induced Secondary Adrenal Insufficiency: A Distinct Differential Diagnosis That Should Be Considered

Pain management with opioid medication is associated with several side effects. Opioid-induced adrenal insufficiency by suppression of the hypothalamic-pituitary-adrenal (HPA) axis is one of them that needs to be considered. The possible effects of opioid use on adrenal function are addressed in this case report. This is a case of a 21-year-old female patient with sickle cell disease who started, for the last year, on extended-release morphine sulfate 45mg daily in an attempt to control the severity of her pain and frequent admission with the vaso-occlusive crisis. She presented with a sepsis-like presentation and received vasopressor, empiric antibiotics, and glucocorticoid. She experienced low blood pressure and low blood glucose after weaning off of steroids. A diagnosis of secondary adrenal insufficiency was established after comprehensive reevaluation and confirmed by morning cortisol value and ACTH stimulation test. Her long-term use of opioids was considered the underlying cause of her secondary adrenal insufficiency after the exclusion of other causes and the normal pituitary gland on the brain magnetic resonance image. She received maintenance hydrocortisone. On follow-up, the patient showed effective improvement, and her adrenal function recovered after discontinuation of the morphine over the following six months. In conclusion, OIAI is an under-recognized condition of adrenal insufficiency secondary to long-term exposure to opioids. OIAI can cause symptoms and may result in potentially life-threatening adrenal crises, but it can be managed. A direct detrimental impact on the hypothalamus and pituitary gland mostly causes the suppression of cortisol secretion by opioids. Understanding how to diagnose and treat OIAI is crucial, particularly since opioids are widely used. To determine the frequency and clinical importance of opioid-induced adrenal insufficiency and if hormone replacement therapy is necessary, more research is required.

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.

The effect of morphine on rat microglial phagocytic activity: An in vitro study of brain region-, plating density-, sex-, morphine concentration-, and receptor-dependency

Opioids have long been used for clinical pain management, but also have addictive properties that have contributed to the ongoing opioid epidemic. While opioid activation of opioid receptors is well known to contribute to reward and reinforcement, data now also suggest that opioid activation of immune signaling via toll-like receptor 4 (TLR4) may also play a role in addiction-like processes. TLR4 expression is enriched in immune cells, and in the nervous system is primarily expressed in microglia. Microglial phagocytosis is important for developmental, homeostatic, and pathological processes. To examine how morphine impacts microglial phagocytosis, we isolated microglia from adult male and female rat cortex and striatum and plated them in vitro at 10,000 (10K) or 50,000 cells/well densities. Microglia were incubated with neutral fluorescent microbeads to stimulate phagocytosis in the presence of one of four morphine concentrations. We found that the brain region from which microglia are isolated and plating density, but not morphine concentration, impacts cell survival in vitro. We found that 10-12 M morphine, but not higher concentrations, increases phagocytosis in striatal microglia in vitro independent of sex and plating density, while 10-12 M morphine increased phagocytosis in cortical microglia in vitro independent of sex, but contingent on a plating density. Finally, we demonstrate that the effect of 10-12 M morphine in striatal microglia plated at 10 K density is mediated via TLR4, and not μORs. Overall, our data suggest that in rats, a morphine-TLR4 signaling pathway increases phagocytic activity in microglia independent of sex. This may is useful information for better understanding the possible neural outcomes associated with morphine exposures.

Topical analgesics for acute corneal pain: current options and emerging therapeutics

Acute corneal pain is a common complaint that causes significant distress to patients and continues to challenge therapeutic avenues for pain management. Current topical treatment options have marked limitations in terms of both efficacy and safety, thus often prompting the adjunctive use of systemic analgesics, including opioids. In general, there have not been extensive advancements in pharmacologic options for the management of corneal pain over the past several decades. Despite this, multiple promising therapeutic avenues exist which hold the potential to transform the ocular pain landscape, including druggable targets within the endocannabinoid system. This review will summarize the current evidence base for topical nonsteroidal anti-inflammatory drugs, anticholinergic agents, and anesthetics before focusing on several potential avenues in the setting of acute corneal pain management, including autologous tear serum, topical opioids and endocannabinoid system modulators.

Functional genomic mechanisms of opioid action and opioid use disorder: a systematic review of animal models and human studies

In the past two decades, over-prescription of opioids for pain management has driven a steep increase in opioid use disorder (OUD) and death by overdose, exerting a dramatic toll on western countries. OUD is a chronic relapsing disease associated with a lifetime struggle to control drug consumption, suggesting that opioids trigger long-lasting brain adaptations, notably through functional genomic and epigenomic mechanisms. Current understanding of these processes, however, remain scarce, and have not been previously reviewed systematically. To do so, the goal of the present work was to synthesize current knowledge on genome-wide transcriptomic and epigenetic mechanisms of opioid action, in primate and rodent species. Using a prospectively registered methodology, comprehensive literature searches were completed in PubMed, Embase, and Web of Science. Of the 2709 articles identified, 73 met our inclusion criteria and were considered for qualitative analysis. Focusing on the 5 most studied nervous system structures (nucleus accumbens, frontal cortex, whole striatum, dorsal striatum, spinal cord; 44 articles), we also conducted a quantitative analysis of differentially expressed genes, in an effort to identify a putative core transcriptional signature of opioids. Only one gene, Cdkn1a, was consistently identified in eleven studies, and globally, our results unveil surprisingly low consistency across published work, even when considering most recent single-cell approaches. Analysis of sources of variability detected significant contributions from species, brain structure, duration of opioid exposure, strain, time-point of analysis, and batch effects, but not type of opioid. To go beyond those limitations, we leveraged threshold-free methods to illustrate how genome-wide comparisons may generate new findings and hypotheses. Finally, we discuss current methodological development in the field, and their implication for future research and, ultimately, better care.

IUPHAR themed review: Opioid efficacy, bias, and selectivity

Drugs acting at the opioid receptor family are clinically used to treat chronic and acute pain, though they represent the second line of treatment behind GABA analogs, antidepressants and SSRI's. Within the opioid family mu and kappa opioid receptor are commonly targeted. However, activation of the mu opioid receptor has side effects of constipation, tolerance, dependence, euphoria, and respiratory depression; activation of the kappa opioid receptor leads to dysphoria and sedation. The side effects of mu opioid receptor activation have led to mu receptor drugs being widely abused with great overdose risk. For these reasons, newer safer opioid analgesics are in high demand. For many years a focus within the opioid field was finding drugs that activated the G protein pathway at mu opioid receptor, without activating the β-arrestin pathway, known as biased agonism. Recent advances have shown that this may not be the way forward to develop safer analgesics at mu opioid receptor, though there is still some promise at the kappa opioid receptor. Here we discuss recent novel approaches to develop safer opioid drugs including efficacy vs bias and fine-tuning receptor activation by targeting sub-pockets in the orthosteric site, we explore recent works on the structural basis of bias, and we put forward the suggestion that Gα subtype selectivity may be an exciting new area of interest.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

IUPHAR Review - Bivalent and bifunctional opioid receptor ligands as novel analgesics

Though efficacious in managing chronic, severe pain, opioid analgesics are accompanied by significant adverse effects including constipation, tolerance, dependence, and respiratory depression. The life-threatening risks associated with µ opioid receptor agonist-based analgesics challenges their use in clinic. A rational approach to combatting these adverse effects is to develop agents that incorporate activity at a second pharmacologic target in addition to µ opioid receptor activation. The promise of such bivalent or bifunctional ligands is the development of an analgesic with an improved side effect profile. In this review, we highlight ongoing efforts in the development of bivalent and bifunctional analgesics that combine µ agonism with efficacy at κ and δ opioid receptors, the nociceptin opioid peptide (NOP) receptor, σ receptors, and cannabinoid receptors. Several examples of bifunctional analgesics in preclinical and clinical development are highlighted, as are strategies being employed toward the rational design of novel agents.

Nerve terminals in the tumor microenvironment as targets for local infiltration analgesia

Nerve terminals within the tumor microenvironment as potential pain-mitigating targets for local infiltration analgesia is relatively less explored. In this study, we examine the role of key analgesics administered as local infiltration analgesia in a model of cancer-induced bone pain (CIBP). CIBP was induced by administration of allogenic MRMT1 breast cancer cells in the proximal tibia of rats, and tumor mass characterized using radiogram, micro-CT, and histological analysis. In vitro responsiveness to key analgesics δ-opioid receptor agonist (DOPr), Ca2+ channel and TRPV1 antagonists was assessed using ratiometric Ca2+ imaging in sensory neurons innervating the tumor site. Effectiveness of locally infiltrated analgesics administered independently or in combination was assessed by quantifying evoked limb withdrawal thresholds at two distinct sites for up to 14 days. CIBP animals demonstrated DOPr, N-, and L-type and TRPV1 expression in lumbar dorsal root ganglion neurons (DRG), comparable to controls. Evoked Ca2+ transients in DRG neurons from CIBP animals were significantly reduced in response to treatment with compounds targeting DOPr, N-, L-type Ca2+ channels and TRPV1 proteins. Behaviourally, evoked hyperalgesia at the tumor site was strongly mitigated by peritumoral injection of the DOPr agonist and T-type calcium antagonist, via its activity on bone afferents. Results from this study suggest that nerve terminals at tumor site could be utilized as targets for specific analgesics, using local infiltration analgesia.

Optical Approaches for Investigating Neuromodulation and G Protein-Coupled Receptor Signaling

Despite the fact that roughly 40% of all US Food and Drug Administration (FDA)-approved pharmacological therapeutics target G protein-coupled receptors (GPCRs), there remains a gap in our understanding of the physiologic and functional role of these receptors at the systems level. Although heterologous expression systems and in vitro assays have revealed a tremendous amount about GPCR signaling cascades, how these cascades interact across cell types, tissues, and organ systems remains obscure. Classic behavioral pharmacology experiments lack both the temporal and spatial resolution to resolve these long-standing issues. Over the past half century, there has been a concerted effort toward the development of optical tools for understanding GPCR signaling. From initial ligand uncaging approaches to more recent development of optogenetic techniques, these strategies have allowed researchers to probe longstanding questions in GPCR pharmacology both in vivo and in vitro. These tools have been employed across biologic systems and have allowed for interrogation of everything from specific intramolecular events to pharmacology at the systems level in a spatiotemporally specific manner. In this review, we present a historical perspective on the motivation behind and development of a variety of optical toolkits that have been generated to probe GPCR signaling. Here we highlight how these tools have been used in vivo to uncover the functional role of distinct populations of GPCRs and their signaling cascades at a systems level. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) remain one of the most targeted classes of proteins for pharmaceutical intervention, yet we still have a limited understanding of how their unique signaling cascades effect physiology and behavior at the systems level. In this review, we discuss a vast array of optical techniques that have been devised to probe GPCR signaling both in vitro and in vivo.

From Plant to Chemistry: Sources of Active Opioid Antinociceptive Principles for Medicinal Chemistry and Drug Design

Pain continues to be an enormous global health challenge, with millions of new untreated or inadequately treated patients reported annually. With respect to current clinical applications, opioids remain the mainstay for the treatment of pain, although they are often associated with serious side effects. To optimize their tolerability profiles, medicinal chemistry continues to study novel ligands and innovative approaches. Among them, natural products are known to be a rich source of lead compounds for drug discovery, and they hold potential for pain management. Traditional medicine has had a long history in clinical practice due to the fact that nature provides a rich source of active principles. For instance, opium had been used for pain management until the 19th century when its individual components, such as morphine, were purified and identified. In this review article, we conducted a literature survey aimed at identifying natural products interacting either directly with opioid receptors or indirectly through other mechanisms controlling opioid receptor signaling, whose structures could be interesting from a drug design perspective.

Loss of ATP-sensitive channel expression and function decreases opioid sensitivity in a mouse model of type 2 diabetes

During diabetes, β-cell dysfunction due to loss of potassium channels sensitive to ATP, known as KATP channels occurs progressively over time contributing to hyperglycemia. KATP channels are additionally present in the central and peripheral nervous systems and are downstream targets of opioid receptor signaling. The aim of this study is to investigate if KATP channel expression or activity in the nervous system changes in diabetic mice and if morphine antinociception changes in mice fed a high fat diet (HFD) for 16 weeks compared to controls. Mechanical thresholds were also monitored before and after administration of glyburide or nateglinide, KATP channel antagonists, for four weeks. HFD mice have decreased antinociception to systemic morphine, which is exacerbated after systemic treatment with glyburide or nateglinide. HFD mice also have lower rotarod scores, decreased mobility in an open field test, and lower burrowing behavior compared to their control diet counterparts, which is unaffected by KATP channel antagonist delivery. Expression of KATP channel subunits, Kcnj11 (Kir6.2) and Abcc8 (SUR1), were decreased in the peripheral and central nervous system in HFD mice, which is significantly correlated with baseline paw withdrawal thresholds. Upregulation of SUR1 through an adenovirus delivered intrathecally increased morphine antinociception in HFD mice, whereas Kir6.2 upregulation improved morphine antinociception only marginally. Perspective: This article presents the potential link between KATP channel function and neuropathy during diabetes. There is a need for increased knowledge in how diabetes affects structural and molecular changes in the nervous system to lead to the progression of chronic pain and sensory issues.

The therapeutic potential of opioids in narcolepsy type 1: A systematic literature review and questionnaire study

OBJECTIVE

Narcolepsy type 1 is a primary sleep disorder caused by deficient hypocretin transmission leading to excessive daytime sleepiness and cataplexy. Opioids have been suggested to increase the number of hypocretin-producing neurons. We aimed to assess opioid use and its self-reported effect on narcolepsy type 1 symptom severity through a literature review and questionnaire study.

METHODS

We systematically reviewed literature on opioid use in narcolepsy. We also recruited 100 people with narcolepsy type 1 who completed an online questionnaire on opioid use in the previous three years. The main questionnaire topics were the indication for use, and the possible effects on narcolepsy symptom severity. Structured follow-up interviews were conducted when opioid use was reported.

RESULTS

The systematic literature review mainly showed improvements in narcolepsy symptom severity. Recent opioid use was reported by 16/100 questionnaire respondents, who had used 20 opioids (codeine: 7/20, tramadol: 6/20, oxycodone: 6/20, fentanyl: 1/20). Narcolepsy symptom changes were reported in 11/20. Positive effects on disturbed nocturnal sleep (9/20), excessive daytime sleepiness (4/20), hypnagogic hallucinations (3/17), cataplexy (2/18), and sleep paralysis (1/13) were most pronounced for oxycodone (4/6) and codeine (4/7).

CONCLUSIONS

Opioids were relatively frequently used compared to a similarly young general Dutch sample. Oxycodone and, to a lesser extent, codeine were associated with self-reported narcolepsy symptom severity improvements. Positive changes in disturbed nocturnal sleep and daytime sleepiness were most frequently reported, while cataplexy effects were less pronounced. Randomised controlled trials are now needed to verify the potential of opioids as therapeutic agents for narcolepsy.