Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Mechanisms of constitutive and agonist-induced 5-HT2B internalization, persistent endosomal signaling and paradoxical regulation of agonist pharmacology

Certain ergot derivatives, particularly cabergoline, produce wash-resistant signaling through the 5-HT2B receptor persisting for many hours without loss of potency or efficacy. Previously, we reported that this signaling may be mediated by sequestered or internalized receptors. Here, we evaluated numerous mechanistic aspects of 5-HT2B internalization and wash-resistant signaling and directly addressed the role of internalization. In the absence of an agonist, 5-HT2B undergoes robust, constitutive internalization and recycling and is distributed at equilibrium between cell surface and intracellular compartments. Both constitutive and agonist-induced internalization are mediated through dynamin-dependent clathrin-mediated endocytosis. Constitutive internalization is unaffected by application of 5-HT2B inverse agonists. We identified two, adjacent di-leucine motifs followed by a di-acidic cluster in the C-terminal tail of 5-HT2B that are responsible for constitutive internalization of the receptor. Mutations in either of the leucine clusters or in the di-acidic motif partially inhibit constitutive 5-HT2B internalization. A 5-HT2B mutant in which both di-leucine clusters are disrupted, displays no constitutive internalization while undergoing robust agonist induced internalization. We demonstrate that wash-resistant signaling of ergots is mediated by persistently/irreversibly internalized signaling receptor complexes. Paradoxically, the potencies of ergot agonists are influenced by receptor internalization; measured potencies are reduced upon inhibition of receptor internalization, while potencies for 5-HT or other conventional agonists are unaffected. This phenomenon represents a novel mechanism by which agonist-dependent kinetics of receptor internalization and recycling affects not only the duration of receptor signaling, but also a basic pharmacological parameter such as agonist potency.

Carfentanil stabilizes µ opioid receptor conformations that are ultra-efficient in inhibiting cAMP, resistant to naloxone or nalmefene but sensitive to naltrexone

The highly potent opioid carfentanil (CAR) represents a growing health risk. CAR acts via Gi/o-coupled µ opioid receptors (µOR) and exhibits ultra-high toxicity. So far, no clear association between pharmacodynamics and toxicity of CAR has been described. We created a HEK-293 cell line stably expressing the µOR and, determined ligand binding affinity (Ki) and potency (EC50) of CAR, fentanyl, remifentanil, morphine or the endogenous ligand endomorphin-1. We found that µOR bind CAR with ~ 10-times higher affinity than fentanyl or remifentanil and with ~ 70-times higher affinity than morphine. Potency of CAR to inhibit cAMP was ~ 85-times higher compared to the fentanyl's and ~ 620 higher compared to morphine. Thus, CAR's toxicity rather associates with receptor potency than affinity. When receptor occupancy at EC50-values was calculated, it appeared that CAR is ~ 8-times more efficient to inhibit cAMP in comparison to morphine, fentanyl or remifentanil. Hence, we postulate that CAR stabilizes µOR conformations that are ultra-efficient in inhibiting cAMP. The OR antagonists naloxone and nalmefene are used as antidotes against opioid intoxication. Both antagonists revealed 10 to 100-times higher IC50-values against CAR-mediated cAMP inhibition compared to the other opioids, indicating that µOR conformations stabilized by CAR are rather resistant towards clinically used antidotes. Of note, when the long acting OR antagonist naltrexone was tested, it exhibited a ~ 65-times higher potency to inhibit CAR but not fentanyl compared to naloxone. Our data highlight the unique nature of CAR's interactions with µOR and provide first pharmacodynamic indication that naltrexone might be a superior antidote.

Mu-opioid receptor activation potentiates excitatory transmission at the habenulo-peduncular synapse

Identifying how opioids modulate signaling in relevant neurocircuitry is essential for developing new therapeutic strategies for opioid addiction. The medial habenula (MHb) is a mu-opioid receptor (MOR) hotspot that projects predominantly to the interpeduncular nucleus (IPN); however, little is known about MOR function in this pathway. Using reporter mice, we observed MOR expression in a subset of MHb and IPN neurons, where its activation induces inhibitory effects on neuronal activity. However, stimulation of MOR+ axons at the habenulo-peduncular (HP) synapse leads to excitatory currents that are significantly potentiated by MOR agonism. These facilitatory effects were also observed at cholinergic-defined HP synapses, depend on a monosynaptic mechanism, and are disrupted by genetic disruption of MOR in the presynaptic MHb. Thus, MORs induce a canonical inhibitory effect in somatodendritic compartments but non-canonical facilitatory effects on evoked glutamate transmission at the HP synapse, establishing a distinct mode by which MORs can modulate neuronal function.

Serotonin release mediates analgesia via opioidergic system and withdrawal symptoms in chronic kratom extract-treated mice

BACKGROUND

Kratom alleviates pain by activating μu-opioid receptors (MOR), which trigger serotonin release to produce analgesia. Serotonin also interferes drug abuse effect. This study aimed to determine the role of serotonin in kratom-induced pain relief and withdrawal symptoms in mice.

METHODS

The analgesic effect was assessed using the hot-plate test. To induce withdrawal symptoms, mice received naloxone after being treated with kratom extracts for five days at increasing doses. Another group of morphine-dependent mice was treated with kratom extracts to ameliorate their withdrawal symptoms. A molecular docking study and molecular dynamics were conducted to predict the binding target of alkaloid kratom for increasing serotonin levels.

RESULTS

Chronic administration of kratom alkaloid extract (20 mg/kg) produced analgesic effects comparable to morphine (10 mg/kg). In contrast, kratom crude extracts (10 mg/kg and 20 mg/kg) demonstrated lower analgesia activity. This analgesic effect was mediated by MOR activation, leading to decreased intracellular cAMP and increased serotonin transmission. Repeated and increasing doses of crude or alkaloid kratom extracts (8 mg/kg to 45 mg/kg) produced less severe withdrawal symptoms than morphine. Increased dopamine and serotonin levels contributed to the onset of withdrawal symptoms. In the morphine group, treatment with kratom extracts increased serotonin levels while reducing dopamine. Molecular docking and molecular dynamics result revealed that kratom alkaloids interacts more readily with tryptophan hydroxylase, the enzyme responsible for serotonin biosynthesis.

CONCLUSIONS

Kratom extracts have the potential to provide analgesic effects and withdrawal symptoms, both of which are mediated by elevated serotonin release.

Tolerance in Thalamic Paraventricular Nucleus Neurons Following Chronic Treatment of Animals with Morphine

Neurons in the paraventricular nucleus of the thalamus (PVT) integrate visceral and limbic inputs and project to multiple brain regions to bias behavior toward aversive or defensive states. This study examines MOR signaling in anterior PVT neurons in brain slices from untreated and morphine-treated animals. Imaging in a MOR-Cre reporter rat revealed extensive expression in aPVT cells, and the application of [Met]5- enkephalin (ME) induced outward currents which were abolished by the MOR-selective antagonist CTAP. A saturating concentration of ME resulted in desensitization that was blocked by compound 101, indicating a phosphorylation-dependent process. The opioid sensitivity of amygdala-, nucleus accumbens-, and prefrontal cortex-projecting neurons was then examined. Neurons that projected to the amygdala were more sensitive to ME than cortical- and accumbal-projecting cells. Following chronic treatment, tolerance to morphine was found in neurons projecting to the amygdala and nucleus accumbens with a trend toward tolerance observed in neurons projecting to the prefrontal cortex. The results reveal that adaptations to chronic opioid exposure are in the aPVT circuits contribute to affective pain processing and may provide specific insights into the etiology of withdrawal following the cessation of opioid use.

Persistent Transcriptome Alterations in Zebrafish Embryos After Discontinued Opioid Exposure

Much attention has been paid to the public health crisis that has resulted from the opioid epidemic. Given the high number of opioid users that are of childbearing age, the impact of utero exposure is a serious concern. Unfortunately, there is little knowledge regarding the consequences of opioid exposure during early development. While neurobehavioral effects of opioid exposure are well-documented, effects of exposure on embryogenesis remain largely unexplored. To address this gap in knowledge, we investigated the effects of oxycodone and fentanyl exposure on gene expression in zebrafish (Danio rerio) embryos using whole embryo RNA sequencing. Embryos were exposed to environmentally relevant (oxycodone HCl 10.6 ng/L and fentanyl citrate 0.629 ng/L) and therapeutically relevant doses (oxycodone HCl 35.14 μg/L and fentanyl citrate 3.14 μg/L) from 2 to 24 h post-fertilization (hpf), followed by another 24 h of opioid-free development. mRNA profiling at 48 hpf revealed dose- and drug-specific gene expression changes. Lower doses of oxycodone and fentanyl both induced more differentially expressed transcripts (DETs) than higher doses, potentially indicative of opioid receptor desensitization occurring at higher concentrations. In total, 892 DETs (corresponding to 866 genes) were identified across all conditions suggesting continued differential gene expression well after cessation of opioid exposure. Gene ontology analysis revealed changes in gene expression relating to extracellular matrix (ECM) organization, cell adhesion, and visual and nervous system formation. Key pathways include those involved in axon guidance, synapse formation, and ECM biosynthesis/remodeling, all of which have potential implications on neural connectivity and sensory development. These findings demonstrate that very early developmental exposure to opioids induces persistent transcriptomic changes which may have lasting implications for vertebrate cellular functions. Overall, these data provide insights into the molecular mechanisms of opioid-induced alterations during development.

Contributions of synaptic energetic dysfunction by microtubule dynamics and microtubule-based mitochondrial transport disorder to morphine tolerance

BACKGROUND AND PURPOSE

Morphine is among the most powerful analgesic, but its long-term use can cause tolerance. Synaptic ATP supply is critical for maintaining synaptic transmission. Microtubule-based mitochondrial transport ensures synaptic energy supply. How synaptic energy changes with morphine and the role of microtubule tracks in synaptic mitochondrial energy supply remain elusive. Chronic morphine treatment can destroy microtubule cytoskeletons. We investigated the effect of the microtubule cytoskeleton on synaptic mitochondrial energy supply and the mechanism of microtubule dynamics after morphine exposure.

EXPERIMENTAL APPROACH

Rats were treated with long-term morphine and the effect on thermal pain thresholds was evaluated by the tail-flick latency test. Various antagonists and agonists were used elucidated the role and mechanism of synaptic mitochondrial energy supply and microtubules in morphine tolerance in vivo and in SH-SY5Y cells.

KEY RESULTS

Chronic morphine treatment reduced synaptic mitochondrial ATP production. Improving mitochondrial oxidative phosphorylation (OXPHOS) alleviated the downregulation of synaptic ATP levels. Microtubule-stabilizing agents prevented microtubule disruption and ameliorated synaptic energy deficit via microtubule-based microtubule transport. In SH-SY5Y cells, morphine exposure reduced microtubule expression. And re-opening the synaptic Ca2+ channel by agonist alleviated microtubule decrease by calcium/calmodulin-dependent protein kinase 2 (CAMKK2)/AMP-activated protein kinase (AMPK) pathway.

CONCLUSION AND IMPLICATIONS

This study demonstrates that the microtubule cytoskeleton regulated by the Ca2+-CAMKK2-AMPK axis is critical for synaptic mitochondrial transport and ATP production, explaining an interplay between chronic morphine-induced abnormal neuroadaptation and synaptic energetic dysfunction. These findings implicated a potential clinical strategy for prolonging the opioid antinociceptive effect during long-term pain control.

Comparative Efficacy of Dexmedetomidine and Remifentanil in Reducing Postoperative Pain and Opioid Use: A Systematic Review

Postoperative pain management is a critical component of patient care following surgical procedures. Opioid analgesics, particularly Remifentanil, have been traditionally used to manage pain, but these drugs come with significant risks, including opioid-induced hyperalgesia (OIH) and the potential for dependence on opioids. Dexmedetomidine, an alpha-2 adrenergic receptor agonist, is a promising non-opioid alternative. This systematic review aims to compare the efficacy of Dexmedetomidine and Remifentanil in reducing postoperative pain and opioid use, as well as to evaluate their safety profiles. A systematic literature review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, utilizing PubMed and ClinicalTrials.gov. The findings revealed that both agents provided effective intraoperative analgesia. However, Dexmedetomidine consistently outperformed Remifentanil in reducing postoperative pain and opioid consumption. Additionally, Dexmedetomidine was associated with lower pain scores in the immediate postoperative period, particularly in spinal and gynecological surgeries. Despite its advantages in pain control and opioid-sparing effects, Dexmedetomidine was associated with slower recovery times, such as delayed eye-opening and longer extubation periods. In contrast, Remifentanil, with its rapid onset and offset, facilitated quicker recovery but resulted in increased postoperative opioid requirements and a higher incidence of OIH. Dexmedetomidine's side effects include bradycardia and hypotension, which were generally manageable and less severe than those associated with Remifentanil. Dexmedetomidine represents a promising alternative to Remifentanil for postoperative pain management. Its ability to reduce opioid consumption and mitigate the risk of OIH makes it an attractive option, particularly in patients at risk for opioid misuse or dependency. Nevertheless, further large-scale studies with diverse patient populations are needed to confirm these findings and refine the optimal dosing regimens for Dexmedetomidine.

Non-canonical signaling initiated by hormone-responsive G protein-coupled receptors from subcellular compartments

G protein-coupled receptors (GPCRs), the largest family of membrane receptors in the mammalian genomes, regulate almost all known physiological processes by transducing numerous extracellular stimuli including almost two-thirds of endogenous hormones and neurotransmitters. The traditional view held that GPCR signaling occurs exclusively at the cell surface, where the receptors bind with the ligands and undergo conformational changes to recruit and activate heterotrimeric G proteins. However, with the application of advanced biochemical and biophysical techniques, this conventional model is challenged by the elucidation of spatiotemporal GPCR activation with the evidence that receptors can signal from subcellular compartments to exhibit various molecular and cellular responses with physiological and pathophysiological relevance. Thus, this 'location bias' of GPCR signaling has become another layer of complexity of GPCR signal transduction. In this review, we generally introduce the development of the concept of compartmentalized GPCR signaling and comprehensively summarize the receptors reported to be localized on the membranes of different intracellular organelles. We review the physiological functions of these compartmentalized GPCRs with emphasis on some well-characterized prototypical hormone/neurotransmitter-binding receptors, including β2-adrenergic receptor, opioid receptors, parathyroid hormone type 1 receptor, thyroid-stimulating hormone receptor, cannabinoid receptor type 1, and metabotropic glutamate receptor 5, as examples. In addition, the therapeutic implications of compartmentalized GPCR signaling by introducing lipophilic or hydrophilic ligands for intracellular targeting, lipid conjugation anchor drugs, and strategy to modulate receptor internalization/resensitization, are highlighted and open new avenues in GPCR pharmacology and therapeutics.

Unraveling morphine tolerance: CCL2 induces spinal cord apoptosis via inhibition of Nrf2 signaling pathway and PGC-1α-mediated mitochondrial biogenesis

BACKGROUND

Morphine effectively relieves severe pain but leads to analgesic tolerance with long-term use.The molecular mechanisms underlying morphine tolerance remain incompletely understood. Existing literature suggests that chemokine CCL2, present in the spinal cord, plays a role in central nervous system inflammation, including neuropathic pain. Nevertheless, the precise mechanism through which CCL2 mediates morphine tolerance has yet to be elucidated. Consequently, this study aims to investigate the molecular pathways by which CCL2 contributes to the development of morphine analgesic tolerance.

METHODS

Rats were administered intrathecal morphine (10 μg/5 μl) twice a day for seven consecutive days to induce a model of morphine nociceptive tolerance. Western blotting and quantitative real-time polymerase chain reaction (qRT-PCR) were used to detect the expression levels of CCL2 and its related mechanism molecules. Immunofluorescence was used to detect the localization of CCL2 in the spinal cord. Intrathecal injections of inhibitors or agonists to artificially regulate the expression of relevant molecules. The thermal tail-flick experiment was performed to evaluate morphine tolerance in rats.

RESULTS

Morphine-induced CCL2 expression was significantly increased in spinal cord, while conversely, the expressions of Nrf2 and PGC-1a were downregulated. Immunofluorescence showed that the enhanced immune response of CCL2 mainly co-localized with neurons. In vivo, we confirmed that intrathecally injection of CCL2 inhibitor Bindarit could effectively alleviate the occurrence of apoptosis and alleviate morphine tolerance. Similarly, pretreatment with Nrf2 signaling pathway agonist Oltipraz and PGC-1α agonist ZLN005 also achieved similar results, respectively. ROS Fluorescence Assay Kit indicated that increasing the expression of PGC-1α could alleviate the occurrence of apoptosis by reducing the level of ROS.

CONCLUSION

Our data emphasize that chemokine CCL2 inhibited the Nrf2 signaling pathway and PGC-1α-mediated mitochondrial biogenesis, alleviating the occurrence of apoptosis in spinal cord, thereby participating in morphine tolerance. This may provide new targets for the treatment of morphine tolerance.

A pH-sensitive opioid does not exhibit analgesic tolerance in a mouse model of colonic inflammation

BACKGROUND AND PURPOSE

Tolerance to the analgesic effects of opioids and resultant dose escalation is associated with worsening of side effects and greater addiction risk. Here, we compare the development of tolerance to the conventional opioid fentanyl with a novel pH-sensitive μ-opioid receptor (MOR) agonist, (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide (NFEPP) that is active only in acidic inflammatory microenvironments.

EXPERIMENTAL APPROACH

An opioid tolerance model was developed in male C57BL/6 mice, with and without dextran sulphate sodium colitis, using increasing doses of either fentanyl or NFEPP over 5 days. Visceral nociception was assessed in vivo by measuring visceromotor responses (VMRs) to noxious colorectal distensions and in vitro measuring colonic afferent nerve activity of mesenteric nerves and performing patch-clamp recordings from isolated dorsal root ganglia neurons. Somatic thermal nociception was tested using a tail immersion assay. Cardiorespiratory effects were analysed by pulse oximeter experiments.

KEY RESULTS

VMRs and tail immersion tests demonstrated tolerance to fentanyl, but not to NFEPP in colitis mice. Cross-tolerance also occurred to fentanyl, but not to NFEPP. The MOR agonist DAMGO inhibited colonic afferent nerve activity in colitis mice exposed to chronic NFEPP, but not those from fentanyl-treated mice. Similarly, in patch-clamp recordings from isolated dorsal root ganglia neurons, DAMGO inhibited neurons from NFEPP-, but not fentanyl-treated mice.

CONCLUSION AND IMPLICATIONS

NFEPP did not exhibit tolerance in an inflammatory pain model, unlike fentanyl. Consequently, dose escalation to maintain analgesia during an evolving inflammation could be avoided, mitigating the potential risk of side effects.

Positive allosteric modulation of µ-opioid receptor - A new possible approach in the pain management?

The antinociceptive effect of the opioid drugs is achieved through activation of the µ-opioid receptor (MOP). The orthosteric and allosteric sites of opioid receptors may be modulated, orthosteric site by endogenous i.e.β-endorphin and exogenous opioids (morphine, oxycodone, fentanyl); whereas BMS-986121, BMS-986122, Comp5, MS1, Ignavine or even oxytocin act on the allosteric site of the MOP. Opioid therapy is associated with numerous side effects, such as: respiratory depression, sedation, constipation, and importantly, prolonged therapy can influence the development of tolerance, overdose, and addiction. Opioid tolerance is a result of MOP internalization and desensitization, preceded by MOP phosphorylation, performed by protein kinases such as: PKA, PKC, GRKs or CaMKII. In vitro and in vivo data suggest that positive allosteric modulators may enhance antinociception triggered by orthosteric ligands and reduce side effects, which would allow the dose of opioids to be reduced and thus provide a more effective therapy. In this review, we present that positive modulation of the allosteric sites of MOP may constitute a new strategy for pain therapy.

Decreased opioid receptor availability and impaired neurometabolic coupling as signatures of morphine tolerance in male rats: A positron emission tomography study

Translational neuroimaging techniques are needed to address the impact of opioid tolerance on brain function and quantitatively monitor the impaired neuropharmacological response to opioids at the CNS level. A multiparametric PET study was conducted in rats. Rats received morphine daily to induce tolerance (15 mg/kg/day for 5 days), followed by 2-day withdrawal. Then, opioid effects were precipitated using a buprenorphine challenge (0.1 mg/kg, s.c, BUP-challenge), which safely enables full occupancy of available mu-opioid receptors (MOR). The impact of the BUP-challenge on the pain threshold was estimated using the hot-plate test. The corresponding availability of MOR was estimated using [11C]buprenorphine PET imaging (n = 4). The brain glucose metabolism was investigated using [18F]2-fluoro-D-deoxy-glucose ([18F]FDG) PET imaging after the BUP-challenge or saline (n = 5-6). Opioid tolerance was confirmed by the attenuated antinociceptive response to the BUP-challenge in morphine-treated rats compared to saline controls (p < 0.001). In tolerant rats, [11C]buprenorphine binding was decreased in MOR-rich regions (p < 0.01), and the baseline uptake of [18F]FDG was decreased (p < 0.05). The BUP-challenge decreased [18F]FDG uptake to a lower extent in tolerant rats compared with opioid-naive animals (p < 0.05), suggesting impaired neurometabolic coupling. Moreover, the impact of the BUP-challenge on the neurometabolic connectivity across brain regions was disrupted by opioid tolerance. PET imaging enables the study of the decreased availability of MOR and impaired neurometabolic coupling as molecular signatures of opioid tolerance in rats. Combining molecular neuroimaging with a suitable pharmacological challenge may provide a translational and quantitative paradigm to explore opioid tolerance at the CNS level in parallel to pharmacodynamics.

Insurmountable antagonism of human mu opioid receptors by buprenorphine is due to hemi-equilibrium

Buprenorphine is an FDA approved drug for the treatment of opioid use disorder and is a long-lasting, low efficacy (partial) agonist of the μ opioid receptor. As a partial agonist, buprenorphine can act as either an agonist or an antagonist depending on the efficiency of the cellular signaling system. Here we investigated the antagonist properties of buprenorphine using a genetically-encoded biosensor to monitor cAMP levels in real time in HEK293 cells expressing a relatively low density of the human μ receptor. Pre-treatment of cells with buprenorphine decreased the maximal response (Emax) of the μ receptor agonists DAMGO and fentanyl, consistent with expectations of a long-lasting partial agonist in these cells. However, buprenorphine pretreatment also reduced the potency of these agonists. In a Schild analysis, the reduction in Emax saturated with increasing concentrations of buprenorphine, while the reduction in potency did not. This unexpected behavior of buprenorphine on potency was mediated through the orthosteric site, as pretreatment with the opioid receptor antagonist naloxone prevented buprenorphine antagonism. Computation simulations using a model of hemi-equilibrium indicated that buprenorphine's slow receptor dissociation could account for both effects of antagonism. In support of this kinetic explanation, buprenorphine antagonism became surmountable with additional agonist incubation time. In this system, buprenorphine appeared to behave both as a competitive and non-competitive (insurmountable) antagonist due to its slow dissociation resulting in partial re-equilibration (hemi-equilibrium). As fast-acting biosensors become more prevalent, it will be important to consider kinetically-mediated effects such as hemi-equilibrium when characterizing the pharmacological properties of antagonists.

A Comprehensive Analysis of Fibromyalgia and the Role of the Endogenous Opioid System

Fibromyalgia represents a chronic pain disorder characterized by musculoskeletal pain, fatigue, and cognitive impairments. The exact mechanisms underlying fibromyalgia remain undefined; as a result, diagnosis and treatment present considerable challenges. On the other hand, the endogenous opioid system is believed to regulate pain intensity and emotional responses; hence, it might be expected to play a key role in the enhanced sensitivity experienced by fibromyalgia patients. One explanation for the emergence of disrupted pain modulation in individuals with fibromyalgia is a significant reduction in opioid receptor activity or an imbalance in the levels of endogenous opioid peptides. Further research is essential to clarify the complex details of the mechanisms underlying this abnormality. This complexity arises from the notion that an improved understanding could contribute to the development of innovative therapeutic strategies aimed at targeting the endogenous opioid system in the context of fibromyalgia. Although progress is being made, a complete understanding of these complexities remains a significant challenge. This paradigm has the potential to revolutionize the complex management of fibromyalgia, although its implementation may experience challenges. The effectiveness of this approach depends on multiple factors, but the implications could be profound. Despite the challenges involved in this transformation, the potential for improving patient care is considerable, as this condition has long been inadequately treated.

Role of the G protein-coupled receptor kinase 2/3 N terminus in discriminating the endocytic effects of opioid agonist drugs

Endocytosis of the μ-type opioid receptor (MOR) is a fundamentally important cellular regulatory process that is characteristically driven less effectively by partial relative to full agonist ligands. Such agonist-selective endocytic discrimination depends on how strongly drugs promote MOR binding to β-arrestins, and this, in turn, depends on how strongly they stimulate phosphorylation of the MOR cytoplasmic tail by G protein-coupled receptor kinases (GRKs) from the GRK2/3 subfamily. While these relatively "downstream" steps in the agonist selective endocytic pathway are now well defined, it remains unclear how agonist-bound receptors are distinguished "upstream" by GRKs. Focusing on GRK2 as a prototype, we show that this single GRK subtype can distinguish the endocytic activities of different MOR agonists in cells lacking other GRKs and that agonist selectivity is introduced at the most upstream step of GRK2 binding to MOR. This interaction requires prior membrane recruitment of GRK2 by its conserved Pleckstrin homology domain and is enhanced by phosphorylation of the MOR tail, but neither reaction can explain the high degree of agonist selectivity in the observed interaction of GRK2 with MOR. We identify the N-terminal domain (NTD) of GRK2, which is identical in GRK3, as a discrete element required for the full agonist selectivity of MOR-GRK2 interaction and show that the NTD is also required for GRK2 to promote MOR endocytosis after it is bound. We propose a simple cellular mechanism of upstream agonist discrimination that is organized as a series of biochemical checkpoints and uses the NTD as an agonist-selective sensor. SIGNIFICANCE STATEMENT: This study investigates how G protein-coupled receptor kinases (GRKs) distinguish the effects of opioid agonist drugs on regulated endocytosis of the μ-type opioid receptor (MOR). It shows that a single GRK subtype is sufficient to determine the agonist selectivity of MOR internalization, agonists are distinguished by how strongly they promote GRK2 recruitment by MOR, and the GRK2/3 N terminus is a key determinant of agonist discrimination.

Retromer Opposes Opioid-Induced Downregulation of the Mu Opioid Receptor

The mu opioid receptor (MOR) is protected from opioid-induced trafficking to lysosomes and proteolytic downregulation by its ability to access the endosomal recycling pathway through its C-terminal recycling motif, LENL. MOR sorting towards the lysosome results in downregulation of opioid signaling while recycling of MOR to the plasma membrane preserves signaling function. However, the mechanisms by which LENL promotes MOR recycling are unknown, and this sequence does not match any known consensus recycling motif. Here we took a functional genomics approach with a comparative genome-wide screen design to identify genes which control opioid receptor expression and downregulation. We identified 146 hits including all three subunits of the endosomal Retromer complex. We show that the LENL motif in MOR is a novel Retromer recycling motif and that LENL is a necessary, sufficient, and conserved mechanism to give MOR access to the Retromer recycling pathway and protect MOR from agonist-induced downregulation to multiple clinically relevant opioids including fentanyl and methadone.

Gene expression in the dorsal root ganglion and the cerebrospinal fluid metabolome in polyneuropathy and opioid tolerance in rats

First-line pharmacotherapy for peripheral neuropathic pain (NP) of diverse pathophysiology consists of antidepressants and gabapentinoids, but only a minority achieve sufficient analgesia with these drugs. Opioids are considered third-line analgesics in NP due to potential severe and unpredictable adverse effects in long-term use. Also, opioid tolerance and NP may have shared mechanisms, raising further concerns about opioid use in NP. We set out to further elucidate possible shared and separate mechanisms after chronic morphine treatment and oxaliplatin-induced and diabetic polyneuropathies, and to identify potential diagnostic markers and therapeutic targets. We analysed thermal nociceptive behaviour, the transcriptome of dorsal root ganglia (DRG) and the metabolome of cerebrospinal fluid (CSF) in these three conditions, in rats. Several genes were differentially expressed, most following oxaliplatin and least after chronic morphine treatment, compared with saline-treated rats. A few genes were differentially expressed in the DRGs in all three models (e.g. Csf3r and Fkbp5). Some, e.g. Alox15 and Slc12a5, were differentially expressed in both diabetic and oxaliplatin models. Other differentially expressed genes were associated with nociception, inflammation, and glial cells. The CSF metabolome was most significantly affected in the diabetic rats. Interestingly, we saw changes in nicotinamide metabolism, which has been associated with opioid addiction and withdrawal, in the CSF of morphine-tolerant rats. Our results offer new hypotheses for the pathophysiology and treatment of NP and opioid tolerance. In particular, the role of nicotinamide metabolism in opioid addiction deserves further study.

Effect of NLRP3 inflammasome induced astrocyte phenotype alteration in morphine tolerance

Introduction

Morphine is a widely used analgesic, but its prolonged use often leads to tolerance, limiting its therapeutic efficacy. Research implicates the NLRP3 inflammasome and reactive astrocytes in the development of morphine tolerance, with reactive astrocytes classified into A1 neurotoxic and A2 neuroprotective phenotypes. This study explores the role of the NLRP3 inflammasome and the transformation of astrocyte phenotypes in the progression of morphine tolerance.

Methods

A model of morphine tolerance was established by administering morphine intrathecally for seven consecutive days. To inhibit NLRP3 inflammasome activation, we coadministered MCC950, a selective NLRP3 inhibitor. Thermal withdrawal latency was used to assess tolerance development. Protein and mRNA levels of GFAP, IL-18, NLRP3, C3 (A1 marker), and S100A10 (A2 marker) in the spinal cord were measured using Western blotting (WB) and real-time quantitative polymerase chain reaction (RT-qPCR). Immunofluorescence was employed to assess the colocalization of C3 and GFAP.

Results

Seven days of morphine administration induced tolerance, which was associated with increased levels of GFAP, IL-18, NLRP3, and C3, and a decreased level of S100A10. Coadministration of morphine and MCC950 significantly slowed the development of morphine tolerance and reversed changes in NLRP3, IL-18, GFAP, C3, and S100A10 protein levels.

Discussion

Our findings indicate a significant link between NLRP3 inflammasome activation and morphine tolerance, suggesting that NLRP3 contributes to the transformation of astrocytes to the A1 phenotype. Inhibiting NLRP3 inflammasome activation holds promise in reversing astrocyte phenotype changes, potentially mitigating morphine tolerance.

FLT3 signaling inhibition abrogates opioid tolerance and hyperalgesia while preserving analgesia

Navigating the duality of opioids' potent analgesia and side effects, including tolerance and hyperalgesia, is a significant challenge in chronic pain management, often prompting hazardous dose escalation to maintain analgesic effects. The peripheral mu-opioid receptor (MOR) is known to mediate these contradictory effects. Here, we show that the fms-like tyrosine kinase receptor 3 (FLT3) in peripheral somatosensory neurons drives morphine tolerance and hyperalgesia in a male rodent model. We found that chronic morphine treatment increases FLT3 and MOR co-expression, and that inhibiting FLT3 represses MOR-induced hyperactivation of the cyclic adenosine monophosphate (cAMP) signaling pathway, mitigating maladaptive excitatory processes engaged after chronic morphine treatment. Furthermore, in postsurgical or inflammatory models of chronic pain, co-administering morphine with a FLT3-specific inhibitor not only prevents or suppresses tolerance and hyperalgesia but also potentiates the analgesic efficacy of morphine, without aggravating other morphine-induced adverse effects. Our findings suggest that pairing morphine with FLT3 inhibitors could become a promising avenue for chronic pain management to safely harness the power of opioids, without the risk of dose escalation. By enhancing morphine analgesic potency through FLT3 inhibition, this approach could minimize opioid dosage, thereby curtailing the risk of addiction and other opioid-related side effects.

Involvement of spinal NADPH oxidase 4 and endoplasmic reticulum stress in morphine-tolerant rats

Morphine tolerance (MT) is currently a challenging issue related to intractable pain treatment. Studies have shown that reactive oxygen species (ROSs) derived from NADPH oxidase (NOX) and produced in response to endoplasmic reticulum (ER) stress participate in MT development. However, which NOX subtype initiates ER stress during MT development is unclear. NOX4 is mainly expressed on intracellular membranes, such as the ER and mitochondrial membranes, and its sole function is to produce ROS. Whether NOX4 is activated during MT development and the mechanisms underlying the association between NOX4 and ER stress during this process still need to be confirmed. In our study, we used the classic morphine-tolerant rat model and evaluated the analgesic effect of intrathecally injected morphine through a hot water tail-flick assay. Our research demonstrated for the first time that chronic morphine administration upregulates NOX4 expression in the spinal cord by activating three ER stress sensors, protein kinase RNA-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6 (ATF6), subsequently leading to the activation of microtubule-associated protein 1 light chain 3 b (LC3B) and P62 (a well-known autophagy marker) in GABAergic neurons. Our results may suggest that regulating NOX4 and the key mechanism underlying ER stress or autophagy may be a promising strategy to treat and prevent MT development.

Respiratory Depression Associated with Opioids: A Narrative Review

OPINION

All opioids have a risk of causing respiratory depression and reduced cerebral circulation. Fentanyl has the greatest risk of causing both. This is particularly a concern when combined with illicit opioids such as diamorphine (also known as heroin). Fentanyl should not be used as a frontline potent opioid due its significant risks. Buprenorphine, a schedule III opioid, morphine, or hydromorphone is preferred, followed by oxycodone, which has a significant risk of abuse relative to buprenorphine and morphine. Although all opioids were equally effective in producing analgesia, the relative safety of each opioid is no longer a secondary concern when prescribing. In the face of an international opioid epidemic, clinicians need to choose opioid analgesics safely, wisely, and carefully.

Transactivation of the EGF receptor as a novel desensitization mechanism for G protein-coupled receptors, illustrated by dopamine D2-like and β2 adrenergic receptors

Transactivation of epidermal growth factor receptors (EGFR) provides intricate control over multiple regulatory cellular processes that merge the diversity of G protein-coupled receptors (GPCRs) with the robust signaling capacities of receptor tyrosine kinases. Contrary to the typical assertions, our findings demonstrate that EGFR transactivation contributes to the desensitization of GPCRs. Repeated agonist stimulation of certain GPCRs enhanced EGFR transactivation, triggering a series of cellular events associated with GPCR desensitization. This effect was observed in receptors undergoing desensitization (D3R, K149C-D2R, β2AR) but not in those resistant to desensitization (D2R, C147K-D3R, D4R, β2AR mutants lacking GRK2 or GRK6 phosphorylation sites). The EGFR inhibitor AG1478 prevented both desensitization and the associated cellular events. Similarly, these cellular events were also observed when cells were treated with EGF, but only in GPCRs that undergo desensitization. These findings suggest that EGFR transactivation diversifies pathways involved in ERK activation through the EGFR signaling system and also mediates GPCR desensitization. Alongside the widely accepted steric hindrance model, these findings offer new insights into understanding the mechanisms of GPCR desensitization, which occurs through complex cellular processes.

Glycine Transporter 1 Inhibitors Minimize the Analgesic Tolerance to Morphine

Opioid analgesic tolerance (OAT), among other central side effects, limits opioids' indispensable clinical use for managing chronic pain. Therefore, there is an existing unmet medical need to prevent OAT. Extrasynaptic N-methyl D-aspartate receptors (NMDARs) containing GluN2B subunit blockers delay OAT, indicating the involvement of glutamate in OAT. Glycine acts as a co-agonist on NMDARs, and glycine transporters (GlyTs), particularly GlyT-1 inhibitors, could affect the NMDAR pathways related to OAT. Chronic subcutaneous treatments with morphine and NFPS, a GlyT-1 inhibitor, reduced morphine antinociceptive tolerance (MAT) in the rat tail-flick assay, a thermal pain model. In spinal tissues of rats treated with a morphine-NFPS combination, NFPS alone, or vehicle-comparable changes in µ-opioid receptor activation, protein and mRNA expressions were seen. Yet, no changes were observed in GluN2B mRNA levels. An increase was observed in glycine and glutamate contents of cerebrospinal fluids from animals treated with a morphine-NFPS combination and morphine, respectively. Finally, GlyT-1 inhibitors are likely to delay MAT by mechanisms relying on NMDARs functioning rather than an increase in opioid efficacy. This study, to the best of our knowledge, shows for the first time the impact of GlyT-1 inhibitors on MAT. Nevertheless, future studies are required to decipher the exact mechanisms.

Longitudinal associations between beta-endorphin, nonsuicidal self-injury and comorbid psychopathology

Homeostasis models posit that nonsuicidal self-injury (NSSI) serves, in part, to upregulate the endogenous opioid system in order to compensate for an opioid deficiency. A few studies have demonstrated lower basal levels of beta-endorphin (BE), an endogenous opioid, in individuals with NSSI. However, longitudinal studies are missing. Hence, the present study aimed to investigate the longitudinal associations between NSSI, comorbid psychopathology (i.e., borderline personality disorder and depressive symptoms), pain sensitivity and basal BE levels in adolescents with NSSI. N = 53 adolescents with NSSI disorder undergoing specialized treatment participated in baseline and one-year follow-up assessments. BE was measured in plasma; pain sensitivity was assessed with a heat pain stimulation paradigm. Associations between BE and change in NSSI, borderline personality disorder and depressive symptoms as well as pain sensitivity were examined using negative binomial and linear regression analyses. We found that an increase in basal BE was significantly associated with a decrease in depressive symptoms. No associations between BE and NSSI, borderline personality disorder symptoms or pain sensitivity were observed. Our findings may confirm a role of plasma BE in the etiology of depressive symptoms but challenge current models of endogenous opioid homeostasis in NSSI.

Positive allosteric modulation of the cannabinoid CB1 receptor potentiates endocannabinoid signalling and changes ERK1/2 phosphorylation kinetics

BACKGROUND AND PURPOSE

Activation of CB1 by exogenous agonists causes adverse effects in vivo. Positive allosteric modulation may offer improved therapeutic potential and a reduced on-target adverse effect profile compared with orthosteric agonists, due to reduced desensitisation/tolerance, but this has not been directly tested. This study investigated the ability of PAMs/ago-PAMs to induce receptor regulation pathways, including desensitisation and receptor internalisation.

EXPERIMENTAL APPROACH

Bioluminescence resonance energy transfer (BRET) assays in HEK293 cells were performed to investigate G protein dissociation, ERK1/2 phosphorylation and β-arrestin 2 translocation, while immunocytochemistry was performed to measure internalisation of CB1 in response to the PAMs ZCZ011, GAT229 and ABD1236 alone and in combination with the orthosteric agonists AEA, 2-AG, and AMB-FUBINACA.

KEY RESULTS

ZCZ011, GAT229 and ABD1236 were allosteric agonists in all pathways tested. The ago-PAM ZCZ011 induced a biphasic ERK1/2 phosphorylation time course compared to transient activation by orthosteric agonists. In combination with 2-AG but not AEA or AMB-FUBINACA, ZCZ011 and ABD1236 caused the transient peak of ERK1/2 phosphorylation to become sustained. All PAMs increased the potency and efficacy of AEA-induced signalling in all pathways tested; however, no notable potentiation of 2-AG or AMB-FUBINACA was observed.

CONCLUSION AND IMPLICATIONS

Ago-PAMs can potentiate endocannabinoid CB1 agonism by AEA to a larger extent compared with 2-AG. However, all compounds were found to be allosteric agonists and induce activation of CB1 in the absence of endocannabinoid, including β-arrestin 2 recruitment and internalisation. Thus, the spatiotemporal signalling of endogenous cannabinoids will not be retained in vivo.

Non-linear blood-brain barrier transport and dosing strategies influence receptor occupancy ratios of morphine and its metabolites in pain matrix

BACKGROUND AND PURPOSE

Morphine is important for treatment of acute and chronic pain. However, there is high interpatient variability and often inadequate pain relief and adverse effects. To better understand variability in the dose-effect relationships of morphine, we investigated the effects of its non-linear blood-brain barrier (BBB) transport on μ-receptor occupancy in different CNS locations, in conjunction with its main metabolites that bind to the same receptor.

EXPERIMENTAL APPROACH

CNS exposure profiles for morphine, M3G and M6G for clinically relevant dosing regimens based on intravenous, oral immediate- and extended-release formulations were generated using a physiology-based pharmacokinetic model of the CNS, with non-linear BBB transport of morphine. The simulated CNS exposure profiles were then used to derive corresponding μ-receptor occupancies at multiple CNS pain matrix locations.

KEY RESULTS

Simulated CNS exposure profiles for morphine, M3G and M6G, associated with non-linear BBB transport of morphine resulted in varying μ-receptor occupancies between different dose regimens, formulations and CNS locations. At lower doses, the μ-receptor occupancy of morphine was relatively higher than at higher doses of morphine, due to the relative contribution of M3G and M6G. At such higher doses, M6G showed higher occupancy than morphine, whereas M3G occupancy was low throughout the dose ranges.

CONCLUSION AND IMPLICATIONS

Non-linear BBB transport of morphine affects the μ-receptor occupancy ratios of morphine with its metabolites, depending on dose and route of administration, and CNS location. These predictions need validation in animal or clinical experiments, to understand the clinical implications.

Intrathecal administration of MCRT produced potent antinociception in chronic inflammatory pain models via μ-δ heterodimer with limited side effects

An important goal in the opioid field is to discover effective analgesic drugs with minimal side effects. MCRT demonstrated potent antinociceptive effects with limited side effects, making it a promising candidate. However, its pharmacological properties and how it minimizes side effects remain unknown. Various mouse pain and opioid side effect models were used to evaluate the antinociceptive properties and safety at the spinal level. The targets of MCRT were identified through cAMP measurement, isolated tissue assays, and pharmacological experiments. Immunofluorescence was employed to visualize protein expression. MCRT displayed distinct antinociceptive effects between acute and chronic inflammatory pain models due to its multifunctional properties at the μ opioid receptor (MOR), µ-δ heterodimer (MDOR), and neuropeptide FF receptor 2 (NPFFR2). Activation of NPFFR2 reduced MOR-mediated antinociception, leading to bell-shaped response curves in acute pain models. However, activation of MDOR produced more effective antinociception in chronic inflammatory pain models. MCRT showed limited tolerance and opioid-induced hyperalgesia in both acute and chronic pain models and did not develop cross-tolerance to morphine. Additionally, MCRT did not exhibit addictive properties, gastrointestinal inhibition, and effects on motor coordination. Mechanistically, peripheral chronic inflammation or repeated administration of morphine and MCRT induced an increase in MDOR in the spinal cord. Chronic administration of MCRT had no apparent effect on microglial activation in the spinal cord. These findings suggest that MCRT is a versatile compound that provides potent antinociception with minimal opioid-related side effects. MDOR could be a promising target for managing chronic inflammatory pain and addressing the opioid crisis.

NMDA Receptors: Distribution, Role, and Insights into Neuropsychiatric Disorders

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are members of the ionotropic glutamate receptor family. These ligand-gated channels are entwined with numerous fundamental neurological functions within the central nervous system (CNS), and numerous neuropsychiatric disorders may arise from their malfunction.

METHODS

The purpose of the present review is to provide a detailed description of NMDARs by addressing their molecular structures, activation mechanisms, and physiological roles in the mammalian brain. In the second part, their role in various neuropsychiatric disorders including stroke, epilepsy, anti-NMDA encephalitis, Alzheimer's and Huntington's diseases, schizophrenia, depression, neuropathic pain, opioid-induced tolerance, and hyperalgesia will be covered.

RESULTS

Finally, through a careful exploration of the main non-competitive NMDARs antagonists (channel-blockers).

CONCLUSION

We discuss the strengths and limitations of the various molecular structures developed for diagnostic or therapeutic purposes.

A Review of Toxicological Profile of Fentanyl-A 2024 Update

Fentanyl and its analogues are synthetic opioids of varying potencies that are unfortunately heavily abused. Over the last 15 years, fentanyl and its analogues have contributed to the increasing prominence of hospitalisation and numerous deaths due to drug overdose. In this comprehensive literature review, the mechanism of toxicity of the drug in humans is evaluated. A systematic approach was used whereby the relevant literature has been detailed where the toxicity of fentanyl and/or its analogues to different organs/systems were investigated. Furthermore, the review covers the post-mortem toxicological data and demographic information from past fatal cases where fentanyl was believed to be involved. Such insight into fentanyl toxicity is useful as an aid to better understand the toxic doses of the drug and the suspected mechanism of action and the unexpected complications associated with overdose incidences involving the drug. Finally, the review offers an overview of the traditional and emerging test systems used to investigate the adverse effects of fentanyl on human health.

Impact of opioids and mu-opioid receptors on oncologic metastasis

Opioids are the most effective and widely used treatments for acute and chronic pain in patients with cancer. This review focuses on the impact of opioids and mu-opioid receptors (MORs) on the stages of oncologic metastasis. Studies have shown that opioids can facilitate tumor progression and are related to a poor prognosis in patients with cancer. As the primary receptor for opioids, MORs play a significant role in regulating malignant tumor transformation and are involved in processes, such as proliferation, angiogenesis, epithelial-mesenchymal transition (EMT), circulating tumor cells (CTCs) and the tumor microenvironment (TME). While clinical trials have investigated the relationship between opioids and patient prognosis, further research is needed to clarify the relationship between opioids, MORs and metastasis.

Chronic morphine treatment induces sex- and synapse-specific cellular tolerance on thalamo-cortical mu opioid receptor signaling

How cellular adaptations give rise to opioid analgesic tolerance to opioids like morphine is not well understood. For one, pain is a complex phenomenon comprising both sensory and affective components, largely mediated through separate circuits. Glutamatergic projections from the medial thalamus (MThal) to the anterior cingulate cortex (ACC) are implicated in processing of affective pain, a relatively understudied component of the pain experience. The goal of this study was to determine the effects of chronic morphine exposure on mu-opioid receptor (MOR) signaling on MThal-ACC synaptic transmission within the excitatory and feedforward inhibitory pathways. Using whole cell patch-clamp electrophysiology and optogenetics to selectively target these projections, we measured morphine-mediated inhibition of optically evoked postsynaptic currents in ACC layer V pyramidal neurons in drug-naïve and chronically morphine-treated mice. We found that morphine perfusion inhibited the excitatory and feedforward inhibitory pathways similarly in females but caused greater inhibition of the inhibitory pathway in males. Chronic morphine treatment robustly attenuated morphine presynaptic inhibition within the inhibitory pathway in males, but not females, and mildly attenuated presynaptic inhibition within the excitatory pathway in both sexes. These effects were not observed in MOR phosphorylation-deficient mice. This study indicates that chronic morphine treatment induces cellular tolerance to morphine within a thalamo-cortical circuit relevant to pain and opioid analgesia. Furthermore, it suggests this tolerance may be driven by MOR phosphorylation. Overall, these findings improve our understanding of how chronic opioid exposure alters cellular signaling in ways that may contribute to opioid analgesic tolerance.NEW & NOTEWORTHY Opioid signaling within the anterior cingulate cortex (ACC) is important for opioid modulation of affective pain. Glutamatergic medial thalamus (MThal) neurons synapse in the ACC and opioids, acting through mu opioid receptors (MORs), acutely inhibit synaptic transmission from MThal synapses. However, the effect of chronic opioid exposure on MThal-ACC synaptic transmission is not known. Here, we demonstrate that chronic morphine treatment induces cellular tolerance at these synapses in a sex-specific and phosphorylation-dependent manner.

Parental Exposure to Morphine Before Conception Decreases Morphine and Cocaine-Induced Locomotor Sensitization in Male Offspring

Repeated exposure to abused drugs leads to reorganizing synaptic connections in the brain, playing a pivotal role in the relapse process. Additionally, recent research has highlighted the impact of parental drug exposure before gestation on subsequent generations. This study aimed to explore the influence of parental morphine exposure 10 days prior to pregnancy on drug-induced locomotor sensitization. Adult male and female Wistar rats were categorized into morphine-exposed and control groups. Ten days after their last treatment, they were mated, and their male offspring underwent morphine, methamphetamine, cocaine, and nicotine-induced locomotor sensitization tests. The results indicated increased locomotor activity in both groups after drug exposure, although the changes were attenuated in morphine and cocaine sensitization among the offspring of morphine-exposed parents (MEPs). Western blotting analysis revealed altered levels of D2 dopamine receptors (D2DRs) in the prefrontal cortex and nucleus accumbens of the offspring from MEPs. Remarkably, despite not having direct in utero drug exposure, these offspring exhibited molecular alterations affecting morphine and cocaine-induced sensitization. The diminished sensitization to morphine and cocaine suggested the development of a tolerance phenotype in these offspring. The changes in D2DR levels in the brain might play a role in these adaptations.

Key phosphorylation sites for robust β-arrestin2 binding at the MOR revisited

Desensitisation of the mu-opioid receptor (MOR) is proposed to underlie the initiation of opioid analgesic tolerance and previous work has shown that agonist-induced phosphorylation of the MOR C-tail contributes to this desensitisation. Moreover, phosphorylation is important for β-arrestin recruitment to the receptor, and ligands of different efficacies induce distinct phosphorylation barcodes. The C-tail 370TREHPSTANT379 motif harbours Ser/Thr residues important for these regulatory functions. 375Ser is the primary phosphorylation site of a ligand-dependent, hierarchical, and sequential process, whereby flanking 370Thr, 376Thr and 379Thr get subsequently and rapidly phosphorylated. Here we used GRK KO cells, phosphosite specific antibodies and site-directed mutagenesis to evaluate the contribution of the different GRK subfamilies to ligand-induced phosphorylation barcodes and β-arrestin2 recruitment. We show that both GRK2/3 and GRK5/6 subfamilies promote phosphorylation of 370Thr and 375Ser. Importantly, only GRK2/3 induce phosphorylation of 376Thr and 379Thr, and we identify these residues as key sites to promote robust β-arrestin recruitment to the MOR. These data provide insight into the mechanisms of MOR regulation and suggest that the cellular complement of GRK subfamilies plays an important role in determining the tissue responses of opioid agonists.

Moxie begets MOXI: The journey to a novel hypothesis about Mu-opioid and OXytocin system Interactions

This narrative review summarizes the early life of the author, Khalin E. Nisbett, and highlights the factors that led to her career in research and her development of two novel research hypotheses: the Mu-opioid and OXytocin system Interaction (MOXI) hypothesis and Mu-Opioid receptor antagonist and OXytocin receptor Agonist In Combination (MOXAIC) treatment hypothesis. Notably, Nisbett's career began in the era after countless studies demonstrated that oxytocin is not just a female neurotransmitter and not just a female reproductive hormone, an era in which researchers are exploring the role of oxytocin in emotion regulation, social interaction, and cognitive processing across both sexes. As such, the previously held perspective that oxytocin is "just a female hormone" did not impede Nisbett's ideas. Intrigued by science, emotion regulation, and social interaction, she began to explore the role of oxytocin and opioids in emotion regulation. On the heels of earlier theories, such as the Tend-and-Befriend theory and Opioid Theory of Social Attachment, she began to develop the MOXI hypothesis, which postulates that the μ-opioid receptor and oxytocin systems interact to mediate social interaction and emotion regulation. In this narrative review, Nisbett summarizes two studies that explored (i) the role of oxytocin in anxiety- and depression-like behavior and (ii) the effect of opioid receptor blockade on the anxiolytic-like effect of oxytocin, which led to a revision of the MOXI hypothesis and postulation of the Mu-Opioid receptor antagonist and OXytocin receptor Agonist In Combination (MOXAIC) treatment hypothesis. Nisbett also discusses several limitations of these hypotheses and her current research interests and aspirations.

Emerging modes of regulation of neuromodulatory G protein-coupled receptors

In the nervous system, G protein-coupled receptors (GPCRs) control neuronal excitability, synaptic transmission, synaptic plasticity, and, ultimately, behavior through spatiotemporally precise initiation of a variety of signaling pathways. However, despite their critical importance, there is incomplete understanding of how these receptors are regulated to tune their signaling to specific neurophysiological contexts. A deeper mechanistic picture of neuromodulatory GPCR function is needed to fully decipher their biological roles and effectively harness them for the treatment of neurological and psychiatric disorders. In this review, we highlight recent progress in identifying novel modes of regulation of neuromodulatory GPCRs, including G protein- and receptor-targeting mechanisms, receptor-receptor crosstalk, and unique features that emerge in the context of chemical synapses. These emerging principles of neuromodulatory GPCR tuning raise critical questions to be tackled at the molecular, cellular, synaptic, and neural circuit levels in the future.

Biasing Gβγ Downstream Signaling with Gallein Inhibits Development of Morphine Tolerance and Potentiates Morphine-Induced Nociception in a Tolerant State

Opioid analgesics are widely used as a treatment option for pain management and relief. However, the misuse of opioid analgesics has contributed to the current opioid epidemic in the United States. Prescribed opioids such as morphine, codeine, oxycodone, and fentanyl are mu-opioid receptor (MOR) agonists primarily used in the clinic to treat pain or during medical procedures, but development of tolerance limits their utility for treatment of chronic pain. Here we explored the effects of biasing Gβγ signaling on tolerance development after chronic morphine treatment in vivo. We hypothesized that biasing Gβγ signaling with gallein could prevent activation of regulatory signaling pathways that result in tolerance to antinociceptive effects of MOR agonists. Gallein has been shown to bind to Gβγ and inhibit interactions of Gβγ with phospholipase-Cβ3 (PLCβ3) or G-protein-coupled receptor kinase 2 (GRK2) but not G-protein inwardly rectifying potassium (GIRK) channels. In mice, morphine-induced antinociception was evaluated in the 55°C warm water tail withdrawal assay. We used two paradigms for gallein treatment: administration during and after three times-daily morphine administration. Our results show that gallein cotreatment during repeated administration of morphine decreased opioid tolerance development and that gallein treatment in an opioid-tolerant state enhanced the potency of morphine. Mechanistically, our data suggest that PLCβ3 is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. These studies demonstrate that small molecules that target Gβγ signaling could reduce the need for large doses of opioid analgesics to treat pain by producing an opioid-sparing effect. SIGNIFICANCE STATEMENT: Biasing Gβγ signaling prevents tolerance to repeated morphine administration in vivo and potentiates the antinociceptive effects of morphine in an opioid-tolerant state. Mechanistically, phospholipase-Cβ is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. This study identifies a novel treatment strategy to decrease the development of tolerance to the analgesic effects of mu-opioid receptor agonists, which are necessary to improve pain treatment and decrease the incidence of opioid use disorder.

Differential activation of rhodopsin triggers distinct endocytic trafficking and recycling in vivo via differential phosphorylation

Activated GPCRs are phosphorylated and internalized mostly via clathrin-mediated endocytosis (CME), which are then sorted for recycling or degradation. We investigated how differential activation of the same GPCR affects its endocytic trafficking in vivo using rhodopsin as a model in pupal photoreceptors of flies expressing mCherry-tagged rhodopsin 1 (Rh1-mC) or GFP-tagged arrestin 1 (Arr1-GFP). Upon blue light stimulation, activated Rh1 recruited Arr1-GFP to the rhabdomere, which became co-internalized and accumulated in cytoplasmic vesicles of photoreceptors. This internalization was eliminated in shits1 mutants affecting dynamin. Moreover, it was blocked by either rdgA or rdgB mutations affecting the PIP2 biosynthesis. Together, the blue light-initiated internalization of Rh1 and Arr1 belongs to CME. Green light stimulation also triggered the internalization and accumulation of activated Rh1-mC in the cytoplasm but with faster kinetics. Importantly, Arr1-GFP was also recruited to the rhabdomere but not co-internalized with Rh1-mC. This endocytosis was not affected in shits1 nor rdgA mutants, indicating it is not CME. We explored the fate of internalized Rh1-mC following CME and observed it remained in cytoplasmic vesicles following 30 min of dark adaptation. In contrast, in the non-CME Rh1-mC appeared readily recycled back to the rhabdomere within five min of dark treatment. This faster recycling may be regulated by rhodopsin phosphatase, RdgC. Together, we demonstrate two distinct endocytic and recycling mechanisms of Rh1 via two light stimulations. It appears that each stimulation triggers a distinct conformation leading to different phosphorylation patterns of Rh1 capable of recruiting Arr1 to rhabdomeres. However, a more stable interaction leads to the co-internalization of Arr1 that orchestrates CME. A stronger Arr1 association appears to impede the recycling of the phosphorylated Rh1 by preventing the recruitment of RdgC. We conclude that conformations of activated rhodopsin determine the downstream outputs upon phosphorylation that confers differential protein-protein interactions.

Exploring neuroadaptive cellular pathways in chronic morphine exposure: An in-vitro analysis of cabergoline and Mdivi-1 co-treatment effects on the autophagy-apoptosis axis

The complex impacts of prolonged morphine exposure continue to be a significant focus in the expanding area of addiction studies. This research investigates the effectiveness of a combined treatment using Cabergoline and Mdivi-1 to counteract the neuroadaptive changes caused by in vitro morphine treatment. The impact of Methadone, Cabergoline, and a combination of Cabergoline and Mdivi-1 on the cellular and molecular responses associated with Morphine-induced changes was studied in human Neuroblastoma (SK-N-MC) and Glioblastoma (U87-MG) cell lines that were exposed to prolong Morphine treatment. Cabergoline and Mdivi-1 combined treatment effectively influenced the molecular alterations associated with neuroadaptation in chronic morphine-exposed neural cells. This combination therapy normalized autophagy and reduced oxidative stress by enhancing total-antioxidant capacity, mitigating apoptosis, restoring BDNF expression, and balancing apoptotic elements. Our research outlines morphine's dual role in modulating mitochondrial dynamics via the dysregulation of the autophagy-apoptosis axis. This emphasizes the significant involvement of DRP1 activity in neurological adaptation processes, as well as disturbances in the dopaminergic pathway during in vitro chronic exposure to morphine in neural cells. This study proposes a novel approach by recommending the potential effectiveness of combining Cabergoline and Mdivi-1 to modulate the neuroadaptations caused by morphine. Additionally, we identified BDNF and PCNA in neural cells as potential neuroprotective markers for assessing the effectiveness of drugs against opioid toxicity, emphasizing the need for further validation. The study uncovers diverse effects observed in pretreated morphine glioblastoma cells under treatment with Cabergoline and methadone. This highlights the potential for new treatments in the DRD2 pathway and underscores the importance of investigating the interplay between autophagy and apoptosis to advance research in managing cancer-related pain. The study necessitates an in-depth investigation into the relationship between autophagy and apoptosis, with a specific emphasis on protein interactions and the dynamics of cell signaling.

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.

Distinct µ-opioid ensembles trigger positive and negative fentanyl reinforcement

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

Selective targeting of mu opioid receptors to primary cilia

Opioid receptors are therapeutically important G protein-coupled receptors (GPCRs) with diverse neuromodulatory effects. The functional consequences of opioid receptor activation are known to depend on receptor location in the plasma membrane, but mechanisms mediating selective localization of receptors to any particular membrane domain remain elusive. Here, we demonstrate the targeting of the mu opioid receptor (MOR) to the primary cilium, a discrete microdomain of the somatic plasma membrane, both in vivo and in cultured cells. We further show that ciliary targeting is specific to MORs, requires a 17-residue sequence unique to the MOR cytoplasmic tail, and additionally requires the Tubby-like protein 3 (TULP3) ciliary adaptor protein. Our results reveal the potential for opioid receptors to undergo selective localization to the primary cilium. We propose that ciliary targeting is mediated through an elaboration of the recycling pathway, directed by a specific C-terminal recycling sequence in cis and requiring TULP3 in trans.

The role of mu-opioid receptors in pancreatic islet alpha cells

30% of people in the United States have diabetes or pre-diabetes. Many of these individuals will develop diabetic neuropathy as a comorbidity, which is often treated with exogenous opioids like morphine, oxycodone, or tramadol. Although these opioids are effective analgesics, growing evidence indicates that they may directly impact the endocrine pancreas function in human and preclinical models. One common feature of these exogenous opioid ligands is their preference for the mu opioid receptor (MOPR), so we aimed to determine if endogenous MOPRs directly regulate pancreatic islet metabolism and hormone secretion. We show that pharmacological antagonism of MOPRs enhances glucagon secretion, but not insulin secretion, from human islets under high glucose conditions. This increased secretion is accompanied by increased cAMP signaling. mRNA expression of MOPRs is enriched in human islet α-cells, but downregulated in T2D islet donors, suggesting a link between metabolism and MOPR expression. Conditional genetic knockout of MOPRs in murine α-cells increases glucagon secretion in high glucose conditions without increasing glucagon content. Consistent with downregulation of MOPRs during metabolic disease, conditional MOPR knockout mice treated with a high fat diet show impaired glucose tolerance, increased glucagon secretion, increased insulin content, and increased islet size. Finally, we show that MOPR-mediated changes in glucagon secretion are driven, in part, by KATP channel activity. Together, these results demonstrate a direct mechanism of action for endogenous opioid regulation of endocrine pancreas.

The Role of Orexin Receptor Antagonists in Inhibiting Drug Addiction: A Review Article

The orexinergic system and its receptors are involved in many physiological processes. Their functions in energy homeostasis, arousal, cognition, stress processing, endocrine functions, and pain modulation have been investigated. Many studies have shown that the orexinergic system cooperates with the dopaminergic system in the addiction process. Emerging evidence suggests that the orexinergic system can be effective in the induction of drug dependence and tolerance. Therefore, several researches have been conducted on the effect of orexin receptor (OXR) antagonists on reducing tolerance and dependence caused by drug abuse. Due to the significant growth of the studies on the orexinergic system, the current literature was conducted to collect the findings of previous studies on orexin and its receptors in the induction of drug addiction. In addition, cellular and molecular mechanisms of the possible role of orexin in drug tolerance and dependence are discussed. The findings indicate that the administration of OXR antagonists reduces drug dependence. OXR blockers seem to counteract the addictive effects of drugs through multiple mechanisms, such as preventing neuronal adaptation. This review proposes the potential clinical use of OXR antagonists in the treatment of drug dependence.

The blockade of neuropeptide FF receptor 1 and 2 differentially contributed to the modulating effects on fentanyl-induced analgesia and hyperalgesia in mice

Neuropeptide FF (NPFF) plays a critical role in various physiological processes through the activation of neuropeptide FF receptor 1 and 2 (NPFFR1 and NPFFR2). Numerous evidence has indicated that NPFF exhibits opposite opioid-modulating effects on opioid-induced analgesia after supraspinal and spinal administrations, while the detailed role of NPFFR1 and NPFFR2 remains unclear. In this study, we employed pharmacological and genetic inhibition of NPFFR to investigate the modulating roles of central NPFFR1 and NPFFR2 in opioid-induced analgesia and hyperalgesia, using a male mouse model of acute fentanyl-induced analgesia and secondary hyperalgesia. Our findings revealed that intrathecal (i.t.) injection of the nonselective NPFFR antagonist RF9 significantly enhanced fentanyl-induced analgesia, whereas intracerebroventricular (i.c.v.) injection did not show the same effect. Moreover, NPFFR2 deficient (npffr2-/-) mice exhibited stronger analgesic responses to fentanyl compared to wild type (WT) or NPFFR1 knockout (npffr1-/-) mice. Intrathecal injection of RF9 in npffr1-/- mice also significantly enhanced fentanyl-induced analgesia. These results indicate a crucial role of spinal NPFFR2 in the enhancement of opioid analgesia. Contrastingly, hyperalgesia induced by fentanyl was markedly reversed in npffr1-/- mice but remained unaffected in npffr2-/- mice. Similarly, i.c.v. injection of the selective NPFFR1 antagonist RF3286 effectively prevented fentanyl-induced hyperalgesia in WT or npffr2-/- mice. Notably, co-administration of i.c.v. RF3286 and i.t. RF9 augmented fentanyl-induced analgesia while reducing hyperalgesia. Collectively, these findings highlight the modulating effects of blocking spinal NPFFR2 and supraspinal NPFFR1 on fentanyl-induced analgesia and hyperalgesia, respectively, which shed a light on understanding the pharmacological function of NPFF system in future studies.

Managing Opioid Withdrawal Symptoms During the Fentanyl Crisis: A Review

Illicitly manufactured fentanyl (IMF) is a significant contributor to the increasing rates of overdose-related deaths. Its high potency and lipophilicity can complicate opioid withdrawal syndromes (OWS) and the subsequent management of opioid use disorder (OUD). This scoping review aimed to collate the current OWS management of study populations seeking treatment for OWS and/or OUD directly from an unregulated opioid supply, such as IMF. Therefore, the focus was on therapeutic interventions published between January 2010 and November 2023, overlapping with the period of increasing IMF exposure. A health science librarian conducted a systematic search on November 13, 2023. A total of 426 studies were screened, and 173 studies were reviewed at the full-text level. Forty-nine studies met the inclusion criteria. Buprenorphine and naltrexone were included in most studies with the goal of transitioning to a long-acting injectable version. Various augmenting agents were tested (buspirone, memantine, suvorexant, gabapentin, and pregabalin); however, the liberal use of adjunctive medication and shortened timelines to initiation had the most consistently positive results. Outside of FDA-approved medications for OUD, lofexidine, gabapentin, and suvorexant have limited evidence for augmenting opioid agonist initiation. Trials often have low retention rates, particularly when opioid agonist washout is required. Neurostimulation strategies were promising; however, they were developed and studied early. Precipitated withdrawal is a concern; however, the rates were low and adequately mitigated or managed with low- or high-dose buprenorphine induction. Maintenance treatment continues to be superior to detoxification without continued management. Shorter induction protocols allow patients to initiate evidence-based treatment more quickly, reducing the use of illicit or non-prescribed substances.

TRAF6-mediated ubiquitination of AKT in the nucleus is a critical event underlying the desensitization of G protein-coupled receptors

BACKGROUND

Desensitization of G protein-coupled receptors (GPCRs) refers to the attenuation of receptor responsiveness by prolonged or intermittent exposure to agonists. The binding of β-arrestin to the cytoplasmic cavity of the phosphorylated receptor, which competes with the G protein, has been widely accepted as an extensive model for explaining GPCRs desensitization. However, studies on various GPCRs, including dopamine D2-like receptors (D2R, D3R, D4R), have suggested the existence of other desensitization mechanisms. The present study employed D2R/D3R variants with different desensitization properties and utilized loss-of-function approaches to uncover the mechanisms underlying GPCRs homologous desensitization, focusing on the signaling cascade that regulates the ubiquitination of AKT.

RESULTS

AKT undergoes K8/14 ubiquitination by TRAF6, which occurs in the nucleus and promotes its membrane recruitment, phosphorylation and activation under receptor desensitization conditions. The nuclear entry of TRAF6 relies on the presence of the importin complex. Src regulates the nuclear entry of TRAF6 by mediating the interaction between TRAF6 and importin β1. Ubiquitinated AKT translocates to the plasma membrane where it associates with Mdm2 to phosphorylate it at the S166 and S186 residues. Thereafter, phosphorylated Mdm2 is recruited to the nucleus, resulting in the deubiquitination of β-Arr2. The deubiquitinated β-Arr2 then forms a complex with Gβγ, which serves as a biomarker for GPCRs desensitization. Like in D3R, ubiquitination of AKT is also involved in the desensitization of β2 adrenoceptors.

CONCLUSION

Our study proposed that the property of a receptor that causes a change in the subcellular localization of TRAF6 from the cytoplasm to the nucleus to mediate AKT ubiquitination could initiate the desensitization of GPCRs.

Dysphagia as a Missing Link Between Post-surgical- and Opioid-Related Pneumonia

PURPOSE

Postoperative pneumonia remains a common complication of surgery, despite increased attention. The purpose of our study was to determine the effects of routine surgery and post-surgical opioid administration on airway protection risk.

METHODS

Eight healthy adult cats were evaluated to determine changes in airway protection status and for evidence of dysphagia in two experiments. (1) In four female cats, airway protection status was tracked following routine abdominal surgery (spay surgery) plus low-dose opioid administration (buprenorphine 0.015 mg/kg, IM, q8-12 h; n = 5). (2) Using a cross-over design, four naive cats (2 male, 2 female) were treated with moderate-dose (0.02 mg/kg) or high-dose (0.04 mg/kg) buprenorphine (IM, q8-12 h; n = 5).

RESULTS

Airway protection was significantly affected in both experiments, but the most severe deficits occurred post-surgically as 75% of the animals exhibited silent aspiration.

CONCLUSION

Oropharyngeal swallow is impaired by the partial mu-opioid receptor agonist buprenorphine, most remarkably in the postoperative setting. These findings have implications for the prevention and management of aspiration pneumonia in vulnerable populations.

Progress in the study of intestinal microbiota involved in morphine tolerance

Morphine is a widely used opioid for treatment of pain. The attendant problems including morphine tolerance and morphine dependence pose a major public health challenge. In recent years, there has been increasing interest in the gastrointestinal microbiota in many physiological and pathophysiological processes. The connectivity network between the gut microbiota and the brain is involved in multiple biological systems, and bidirectional communication between them is critical in gastrointestinal tract homeostasis, the central nervous system, and the microbial system. Many research have previously shown that morphine has a variety of effects on the gastrointestinal tract, but none have determined the function of intestinal microbiota in morphine tolerance. This study reviewed the mechanisms of morphine tolerance from the perspective of dysregulation of microbiota-gut-brain axis homeostasis, by summarizing the possible mechanisms originating from the gut that may affect morphine tolerance and the improvement of morphine tolerance through the gut microbiota.