Search Results (757)

Background/Objectives: Chronic pain is a significant clinical challenge, in part due to the absence of reliable objective biomarkers for its evaluation and treatment. Growing evidence indicates that immune cells, including natural killer (NK) cells, are involved in the regulation of pain processes. NK cells are innate cytotoxic lymphocytes whose functional status may mirror underlying pathological pain states. In this study, we investigated μ-opioid receptor (MOR) expression and functional alterations of NK cells in a murine model of neuropathic pain induced by chronic constriction injury (CCI). Methods: Mice were divided into three groups: Sham (sciatic nerve exposure without ligation), CCI 14-day, and CCI 21-day groups. At the respective time points, animals were sacrificed and spleens were collected for analysis. Splenocytes were isolated by mechanical dissociation followed by centrifugation and erythrocyte lysis. Lymphocytes were analyzed by flow cytometry to evaluate MOR expression in NK cells and their degranulation activity (CD107a assay). Cells were incubated with fluorochrome-conjugated antibodies against NK cell markers (NK1.1, CD3, Ly49A, Ly49C/I) in combination with anti-MOR and anti-Interferon γ antibody (IFN-γ). Immunofluorescence and confocal microscopy analyses were performed to assess MOR localization and granzyme localization, supporting CD107a-mediated degranulation. Results: Flow cytometry analysis revealed a significant reduction in surface MOR expression on total NK cells from CCI mice compared with sham controls at 14 and 21 days post-injury, a finding corroborated by immunofluorescence evidence of MOR cellular internalization. Functionally, CCI induced a marked decrease in CD107a expression and impaired IFN-γ production both under basal conditions and following PMA/ionomycin stimulation, indicating a hyporesponsive state of NK cells. Consistently, confocal microscopy revealed extracellular release of Granzyme A following CCI, suggesting dysregulated degranulation. Conclusions: Neuropathic pain is associated with a remodeling of NK cell phenotype and effector functions, characterized by impaired cytotoxic activity and cytokine production, along with modulation of inhibitory receptor expression. Notably, MOR-reduced surface expression in NK cells emerges as a potential biomarker of neuropathic pain. Further studies are needed to elucidate the molecular mechanisms regulating MOR expression and its relationship with NK cell hyporesponsiveness and degranulation in chronic pain conditions. Full article

Opioids comprise a class of substances characterized by a narrow therapeutic window and significant euphoric effects. Opioid-related mortality remains a critical global health issue, primarily resulting from respiratory depression associated with full μ-opioid receptor agonists. Buprenorphine is distinguished by its pharmacological profile as a partial μ-opioid receptor agonist, exhibiting a ceiling effect on both respiratory depression and euphoria. These characteristics render buprenorphine particularly valuable in opioid replacement therapy for opioid use disorder and in the management of both chronic and acute pain. Current literature indicates that, due to these properties, buprenorphine possesses a superior safety profile compared to other opioids. This narrative review evaluates whether buprenorphine can cause fatal overdose despite its protective ceiling effect and examines cases of buprenorphine-related fatalities. The review synthesizes data from pre-clinical, clinical, pharmacological, and toxicological studies. Current literature supports the consensus that the established ceiling effect on respiratory depression does not fully eliminate the risk of fatal overdose. Polysubstance use, particularly co-ingestion of other central nervous system depressants, significantly increases this risk, and most buprenorphine-related deaths involve such combinations. Additionally, buprenorphine’s safety is reduced in vulnerable populations, including opioid-naive individuals, children, and patients with pre-existing respiratory compromise. While buprenorphine alone presents a substantially lower fatality risk compared to other opioids, the ceiling effect is pharmacodynamic rather than absolute, and toxicity or fatal overdose may still occur. Current data gathered from clinical and pre-clinical studies suggest that buprenorphine is safer than full μ-opioid receptor agonists, but it is not without risk. The ceiling effect’s protective benefits depend on context. Therefore, despite its improved safety profile, buprenorphine should not be regarded as risk-free. Full article

Background: Impulse control disorders (ICDs) are a common non-motor complication in Parkinson’s disease (PD) patients with multiple negative consequences for the individual and caregivers. Although ICDs are strongly linked to dopaminergic therapy, particularly dopamine agonists, only a percentage of patients develop these behaviors, suggesting the involvement of additional susceptibility factors, including genetic variability. This review aims to analyze current knowledge on the genetic background of ICDs. Methods: A literature search was conducted in the PubMed and Scopus databases for peer-reviewed research regarding the role of genetics in ICDs, published in the English language from 1996 to 2026. References of the selected articles for possible additional articles were also screened in order to include most of the key recent evidence. Genes that are involved in the dopaminergic system play a central role in ICD susceptibility, although the findings in studies are often inconsistent and not replicated. Moreover, variants in genes related to the glutamatergic (e.g., GRIN2B), serotonergic (e.g., HTR2A and TPH2), and opioid systems (e.g., OPRK1 and OPRM1) have been implicated, supporting a multi-system contribution to ICD pathophysiology. Conclusions: Early recognition of genetic factors that increase susceptibility to ICDs in PD patients is awaited to increase diagnostic accuracy and expedite individualized treatment. Full article

Background: The 2-benzylbenzimidazoles or nitazenes are an evolving class of highly potent mu-opioid receptor agonists. Nitazenes were originally developed in the late 1950s for pharmaceutical use as analgesics; however, due to their extreme potency and the risk of adverse health outcomes, pharmaceutical research was discontinued. Since 2019, nitazenes have emerged as illicit drugs of abuse, causing significant concern. From 2021, they have been detected in both coronial and clinical casework in Victoria, Australia. This study examined nitazene-related coronial casework in Victoria from 2021 to 2025 to explore the trends and characteristics of nitazene-related deaths. Methods: Relevant cases were identified from the Victorian Institute of Forensic Medicine’s (VIFM’s) case management system. Data were collated and analysed from all coronial cases where a nitazene was detected by a toxicological analysis between 1 January 2021 and 31 December 2025. Trend comparisons were made with nitazene detections reported in other countries. Results: Nitazenes were detected in 23 deaths from a total of approximately 33,108 coronial cases admitted to the VIFM for investigation over the time period. The age range was 17–45 years, with a median of 32 and with 87% of the deaths being male. The nitazenes detected were protonitazene (n = 14), metonitazene (n = 5), isotonitazene (n = 2), N-pyrrolidino etonitazene (n = 2), N-desethyl isotonitazene (n = 1), methylenedioxynitazene (n = 1) and etodesnitazene (n = 1). Two cases contained more than one nitazene; both involved protonitazene, one involved metonitazene, and the other involved N-desethyl isotonitazene and methylenedioxynitazene. The timeline of detection of these nitazenes displays similarities with emergence trends in other countries. The nitazene concentrations ranged from 0.1 to 33 ng/mL. Broad polydrug usage was evident in all cases, with other drugs co-detected in the blood including stimulants (particularly, methylamphetamine (48%) and cocaine (44%)) as well as pharmaceutical benzodiazepines (43%) and pharmaceutical opioids (22%), and 13% had 6-monoacetylmorphine detected in either blood or urine. Novel benzodiazepines (39%) were also common, including bromazolam, which was co-detected in 35% of cases. Nineteen deaths were attributed solely to nitazene-related mixed-drug toxicity, while the remaining four cases were attributed to cardiac- and pulmonary-related disease, with polydrug use deemed a contributing factor. Conclusions: This novel case series adds comprehensive toxicological information to the body of evidence reinforcing the high risk of harm associated with the use of nitazenes. It is imperative that toxicology services continue to monitor for nitazenes to promote community awareness against nitazene-related harm. Full article

The overdose crisis in North America is increasingly driven by illicitly manufactured fentanyl and other high-potency synthetic opioids, which are associated with severe and unpredictable opioid-induced respiratory depression (OIRD). Current pharmacologic strategies to prevent fatal overdose have largely emphasized downstream rescue through opioid antagonism (e.g., naloxone), leaving limited attention to upstream pharmacologic modification of respiratory risk. In this Opinion article, we argue that buprenorphine should be reframed not only as a treatment for opioid use disorder (OUD), but also as a pharmacologic modifier of OIRD risk in fentanyl-dominant drug markets. Drawing on its partial μ opioid receptor agonism, ceiling effect on respiratory depression, and exceptionally high receptor affinity, we describe how buprenorphine can displace full agonists while limiting respiratory suppression. We further situate this pharmacology within emerging population-level observations from North American fentanyl contexts, suggesting reduced overdose mortality among individuals receiving opioid agonist therapy, particularly buprenorphine. In fentanyl-dominant drug markets, reframing buprenorphine as a modifier of respiratory risk has direct implications for clinical messaging about overdose protection, medication selection for individuals with ongoing illicit opioid use, and policy approaches aimed at reducing opioid-related mortality. Full article

Cardiac safety assessment is an integral part of drug discovery and development. Drug candidates that adversely affect cardiac or hemodynamic function should be discontinued early unless a favorable benefit-risk ratio for patients can be justified. In this hypothesis-generating work, we aimed to develop a conceptual framework for informing early safety risk assessment based on in vitro drug affinities to pharmacological targets. For illustration, we used the drug-induced cardiotoxicity rank (DICTrank) data comprising 1318 drugs with cardiac safety concerns according to FDA labeling. The data was enriched with information on affinity to the most plausible mechanistic targets, clinical drug exposure, and human plasma protein binding. We descriptively identified 18 target classes potentially associated with elevated cardiovascular risk: potassium channels (accounting alone for 20% of the ‘most concern’ safety group); adrenergic, dopamine, serotonin, androgen, sex hormone, and opioid receptors; cyclooxygenase; sodium and calcium channels; muscarinic and glucocorticoid receptors; phosphodiesterase; topoisomerase; angiotensin-converting enzyme; angiotensin II type 1 receptor; monoamine transporters, and acetylcholinesterase. Overall, 80% of the ‘most concern’ drugs compared with only 12% of the ‘no concern’ drugs were associated with these targets in this exploratory descriptive analysis. Concentration–response analyses revealed differences in target potency and free drug exposure that appeared associated with variability in the severity of cardiotoxicity among drugs acting on the same target. This framework demonstrates how in vitro data can be used to benchmark new compounds early in development, enabling the timely discontinuation of candidates associated with substantial risk. Full article

The hybrid approach remains a compelling strategy for designing molecules that combine enhanced biological activity with a favorable safety profile. Marine peptides, in particular, have attracted significant attention due to their well-documented broad spectrum of biological activities. Peptides derived from rays have been recognized for their diverse biological activities. Notably, physicochemical properties of these peptides support practical application without requiring further refinement of the mature molecule or specialized formulations. In this study, we present two new chimeric peptides, PK01# and PK02#, which incorporate an opioid pharmacophore linked to a short amino acid sequence derived from the skate Raja porosa. Those compounds interact with the opioidergic system, specifically targeting the mu-opioid receptor (MOR). Furthermore, the compounds were evaluated for their effects on cancer cell viability through in vitro MTT assays (as an exploratory endpoint) and for their binding compatibility with EGFR via in silico docking. Both compounds showed limited effects on cell viability in HeLa, SAS, and PANC-1 cells, while PK02# induced a minor reduction in metabolic activity in glioblastoma cells without reaching IC50 values or significant cytotoxic thresholds. Interestingly, the structures of these hybrid compounds offer valuable insights into the role of phenylalanine residues within their sequences, which appear to be critical for both biological activity and receptor interaction. Moreover, these findings may support future structural optimization of peptide hybrids focused on receptor modulation and biological profiling. Full article

Background/Objectives: Opioid use disorder (OUD) is caused by a complex interplay between genetic and non-genetic factors. DNA methylation is an epigenetic mechanism that modulates gene expression. Data on DNA methylation and opioid addiction and treatment are limited. This association study was designed to assess the difference in genome-wide methylation patterns between individuals with OUD in methadone maintenance treatment (MMT) (n = 114) and those with OUD who achieved long-term abstinence (>10 years) without mu opioid receptor agonist treatment (n = 136). Methods: Differential DNA methylation analysis was performed in whole blood using the Illumina EPIC array. Results: A total of 135 differentially methylated probes (DMPs) reached epigenome-wide significance (p < 1 × 10⁻⁷), controlling for sex, age, estimates of blood cell proportions, and the first two principal components based on genome-wide SNP genotypes. The methylation sites were annotated to 157 genes, including 32% long non-coding RNAs. These genes are related to several systems, including cell adhesion (e.g., SAXO4), immune system and inflammation (e.g., UBTF, USP39, C10orf90, PRKCA), stress response (e.g., CRHR1, GPR19), and spermatogenesis (e.g., SPATA16, COX7B2). DMP cg11641410 is located in lncRNA ENSG00000254687, an antisense to OPRK1. Six of the DMPs were also identified in a related longitudinal study of MMT. Conclusions: At this point, it is not possible to determine whether the minor methylation differences observed in this study cause clinically relevant changes in gene expression. However, these findings have the potential to identify biomarkers and to provide new targets for treatment optimization. Full article

Recent Phase 3 clinical trials of selective kappa-opioid (KOP) receptor antagonists aticaprant and navacaprant failed to demonstrate sufficient clinical efficacy in treatment-resistant depression (TRD). This highlights a critical gap in current strategies that target opioid-mediated hedonic suppression. We propose two hypotheses to explain these setbacks: (1) neutral antagonists are inherently ineffective in blocking constitutively active KOP receptor hyperactivation and (2) the nociceptin opioid (NOP) receptor provides functional redundancy that compensates for KOP receptor blockade. Gaining insights from paralogous compensation in drug-resistant tumors, we argue for shifting from selective opioid antagonists to dual KOP/NOP receptor blockers to meaningfully improve reward function. This concept provides a theoretical framework for overcoming clinical resistance where selective KOP targeting with neutral antagonists has failed. Thus, we advocate for the development of opioid inverse agonists (such as nor-BNI, CAS: 105618-26-6), pan-antagonists (such as AT-076, CAS: 1657028-64-2), and combinations of selective blockers. Full article

Venlafaxine, a serotonin–norepinephrine reuptake inhibitor, shows analgesic effects in rodents, but its efficacy and pharmacological profile in acute stimulus-evoked nociception may depend on the nociceptive test used and the pharmacological context. The aim of this review was to identify the receptors implicated in venlafaxine antinociceptive effects and to examine which molecular processes most consistently explain its acute antinociceptive profile. We reviewed in vivo rodent studies testing venlafaxine in acute nociceptive assays (writhing, tail-flick, hot-plate, and other eligible acute tests) as monotherapy or associated with other pharmacologically active substances. PubMed/MEDLINE and Web of Science were searched from 1993 to 5 January 2026, and reference lists were also screened. Outcomes were synthesized and stratified by type of nociceptive test and interaction class. Fourteen studies were identified as relevant to the scope of this review. Venlafaxine produced dose-dependent antinociception across tests, reducing writhing and increasing thermal withdrawal latency. Central administration generally yielded effects at lower absolute doses than systemic routes. Interaction studies most consistently supported modulation of opioid receptors (e.g., leftward opioid dose–response shifts and attenuation of morphine tolerance in repeated-exposure designs), with convergent evidence implicating opioid and α₂-adrenergic mechanisms and context-dependent serotonergic contributions. Additional pathways were variably implicated, including nitric oxide—cyclic guanosine monophosphate (NO–cGMP) signaling and oxidative/mitochondrial processes in opioid tolerance paradigms. Preclinical evidence supports venlafaxine as a modulator of acute nociceptive control with notable opioid-interaction potential. Standardized pharmacodynamic reporting and translationally oriented studies are needed. Full article

Background/Objectives: Opioids paradoxically induce hypersensitivity that typically resolves after discontinuation. Despite normalization of sensitivity, the nociceptive system may remain primed. Descending pathways contribute to opioid-induced hyperalgesia (OIH), and the following latently maintained sensitization. The mechanisms underlying the latter vulnerability remain unknown. We previously showed that the dorsal reticular nucleus (DRt), a key brainstem area involved in pain facilitation, drives OIH through a maladaptation in μ-opioid receptor (MOR) signaling. Whether cellular alterations in the DRt persist after opioids are discontinued and hyperalgesia resolves is unknown. Here, we investigated the long-term effects of morphine on DRt MOR expression, signaling, and function after hyperalgesia has been resolved. Methods: Male Wistar rats received morphine for 7 days either subcutaneously or continuously. Nociceptive sensitivity was evaluated by von Frey and hot-plate tests. MOR and phosphorylated CREB (pCREB) expression in the DRt were quantified during OIH and after hyperalgesia resolution. In the post-OIH phase, we evaluated the effects of MOR activation by DAMGO at DRt, postoperative pain behavior, and systemic morphine dose–response curves. Results: Both morphine regimens induced hypersensitivity that subsided within two weeks. MOR expression in the DRt increased during OIH and normalized in the post-OIH phase, whereas pCREB levels remained elevated in both phases. In the post-OIH phase, DAMGO microinjection at the DRt reinstated robust hypersensitivity and systemic morphine showed reduced antiallodynic potency in postoperative pain. Conclusions: Chronic morphine leaves a lasting molecular imprint in the DRt, sustaining excitatory MOR signaling that can reinstate hyperalgesia and likely diminishes later opioid analgesic efficacy. Full article

Background: Endogenous opioids, such as endomorphin-2, are key regulators of the body’s pain pathways and mediate analgesia by engaging the $\mu$-opioid receptor. This class of opioids are distinguished by their transient activation of the $\mu$-opioid receptor, which is attributed to rapid enzymatic degradation. Methods: To understand how degradation of endomorphin-2 by the enzyme DPP IV affects its interaction with the $\mu$-opioid receptor, we analyzed the ligand–receptor conformational dynamics and interaction patterns of molecular dynamics simulations data of morphine, fentanyl and endomorphin-2 and one degradation product Phe-Phe-NH₂, using molecular fingerprints and the mathematical framework ISOKANN. Results: Our analyses revealed that both the clinically relevant opioids, morphine and fentanyl, as well as the endogenous opioid endomorphin-2, adopt a set of recurring binding conformations within the $\mu$-opioid receptor binding pocket, maintaining overlapping interaction motifs throughout the simulations. In contrast, Phe-Phe-NH₂ failed to maintain a persistent binding mode over the simulated timescale. This instability arises from the dipeptidyl peptidase IV mediated cleavage of endomorphin-2, which generates Phe-Phe-NH₂ and removes critical proline and tyrosine residues, thereby leading to the loss of stabilizing hydrophobic contacts with receptor residues Tyr150^3,33, Val238^5,43 and Val302^6,55. Conclusion: By mapping structural interaction motifs essential for stable $\mu$-opioid receptor binding, this study provides mechanistic insights into how endogenous degradation reshapes ligand–receptor interactions. Full article

Opioid-induced constipation (OIC) represents a prevalent adverse effect of opioid analgesics, affecting 60–90% of patients and significantly compromising quality of life. This review delineates the multifactorial pathogenesis of OIC. Peripheral μ-opioid receptor (MOR) activation suppresses enteric neuronal excitability, inhibits intestinal motility and secretion, and impairs rectoanal function. Notably, the colon appears to exhibit a distinctive lack of tolerance to opioids. Enteric glial cell activation has been implicated in neuroinflammation, while interstitial cells of Cajal show impaired pacemaker function. Central mechanisms are increasingly recognized to involve the brain–gut axis. Furthermore, opioid-induced barrier disruption, microbiota dysbiosis, and LPS/TLR4-mediated inflammation are proposed to interact and may contribute to a self-reinforcing cycle. Animal models have been instrumental in dissecting these mechanisms. However, they present limitations in reproducibility, clinical phenotype fidelity, and translational validity, particularly regarding microbiome composition and neuroimmune responses. Future research should prioritize the development of standardized, physiologically relevant animal models incorporating multi-omics approaches, and validate mechanism-based therapeutic strategies, including peripherally acting MOR antagonists and microbiota-targeted interventions, for precision management of OIC. Full article

Kratom (Mitragyna speciosa) is a botanical candidate for pain management with potentially reduced opioid-related risks, partly through modulation of neuroimmune pathways involving Toll-Like Receptor 4 (TLR4). This study aimed to characterize the phytochemical profile of kratom ethanol extract and evaluate its effects on TLR4 signalling, neuroinflammatory cytokines, analgesic activity, withdrawal behaviours, and organ safety in morphine-dependent mice. Metabolite profiling was conducted using UHPLC–Q-Exactive Orbitrap HRMS, followed by molecular docking of major constituents to the TLR4 complex. In vivo assessments included flow cytometry and gene expression analyses of TLR4-mediated cytokines (NF-κB, IL-1β, IL-6), behavioural assays for antinociception, endurance, and withdrawal symptoms, and histopathological and biochemical evaluation of liver, kidney, and spleen tissues. More than 100 metabolites were identified, including mitragynine and flavonoids such as rutin and isoquercetin, which showed interactions with key TLR4 residues. Selected fractions suppressed pro-inflammatory cytokine expression, increased tail-pinch latency comparable to morphine, reduced withdrawal manifestations, and demonstrated nephroprotective and immunomodulatory effects, although mild reversible hepatic alterations were observed in specific fractions. Overall, kratom ethanol extract exhibited fraction-dependent analgesic and anti-neuroinflammatory activities associated with TLR4 modulation, supporting its potential as a botanical analgesic candidate while emphasizing the importance of safety optimization and standardized fraction development. Full article

δ-opioid receptor (DOR) is a promising therapeutic target for developing safer treatments for pain and neuroprotection. In this study, we applied a structure-guided drug-repurposing workflow to identify FDA-approved drugs with predicted DOR-binding and agonist-like structural features. Using a validated GNINA-based docking protocol with an active-state DOR model (PDB ID: 6PT3), we screened 2342 approved compounds and identified 39 candidates with predicted submicromolar binding affinities. These hits were further evaluated through molecular dynamics simulations, binding pocket volume analysis, and principal component analysis, which enabled the prioritization of two leading candidates, aprepitant and mavorixafor. Both compounds formed stable receptor-ligand complexes, maintained persistent interactions with Asp128, promoted contraction of the orthosteric pocket, and retained favorable redocking scores on the MD-refined receptor conformations. Overall, these results identify aprepitant and mavorixafor as promising putative DOR agonists and provide a rational foundation for their experimental validation through binding, functional, and in vivo pain studies in the future. Full article