Morphine promotes non-small cell lung cancer progression by downregulating E-cadherin via the PI3K/AKT/mTOR pathway.

Morphine has been suggested to affect cancer cell dynamics and decrease survival rates in lung cancer patients at specific doses, but the precise mechanisms poorly understood. In this study, we aimed to investigate the molecular mechanisms by which morphine modulates the malignant characteristics of non-small cell lung cancer. Cell proliferation was assessed via the Cell Counting Kit-8 assay, and cell migration and invasion were examined via wound healing and Transwell assays. We employed immunofluorescence staining to evaluate E-cadherin expression in A549 and Lewis lung cancer (LLC) cell lines and immunohistochemistry to evaluate E-cadherin expression in nude mice tumours. Additionally, the in vivo effects of morphine on lung cancer progression were explored in a xenograft tumour experiments, in which naloxone was used as a morphine antagonist. Western blot analysis was performed to detect E-cadherin, phosphorylated mTOR (p-mTOR), mTOR, phosphorylated AKT (p-AKT), AKT, phosphorylated PI3K (p-PI3K), and PI3K protein levels in A549 and LLC cells as well as in tumour samples. Morphine (10 µM) significantly increased the proliferation of A549 and LLC cells in vitro (p < 0.05). It also enhanced the migratory and invasive capacities of these cell lines (p < 0.01). Mechanistically, morphine treatment (10 µM) led to a reduction in the expression of E-cadherin, and an increase in the phosphorylation of PI3K, AKT, and mTOR in A549 and LLC cells (p < 0.01). Morphine treatment (1.5 mg/kg) also reduced E-cadherin expression in xenograft tumours and promoted tumour growth in vivo (p < 0.05). This effect was reversed by naloxone (0.1 mg/kg). The results demonstrated that morphine stimulates the malignant proliferation of A549 and LLC cell lines and promotes xenograft tumour growth. Perhaps by specifically targeting MOR, morphine triggers a signalling cascade that activates the PI3K/AKT/mTOR pathway while inhibiting the EMT marker E-cadherin, which may consequently promote the progression of lung cancer.

Safety and Efficacy of Combined Injection of Pure-µ-Opioid Agonist with Tramadol as an Opioid Induction Agent for Opioid-Naïve Cancer Patients.

Background: Tramadol is known to provide synergistic analgesia when used in combination with morphine. Objectives: The aims of this study were: (1) to introduce an opioid combination therapy using pure-µ-opioid receptor agonist (OPI) + tramadol injections (OPI + tramadol) and (2) to elucidate safety and efficacy of this combination therapy for opioid-naïve cancer pain patients. Methods: Opioid-naïve patients referred to our palliative care team (in Japan) who were unable to take oral medications and received OPI + tramadol as opioid induction agents were retrospectively investigated on the electric medical chart. OPI + tramadol dosage was adjusted to achieve the patient's pain as Numerical Rating Scale ≤4/10 or Support Team Assessment Schedule-Japanese ≤1. Patients' demography, doses of OPI and tramadol administered, and adverse events were analyzed. Results: A total of 44 patients were included. The primary organs of malignancy were pancreas (11), stomach (5), lung (4), breast (4), liver (4), and others (13). OPI injections administered were hydromorphone (39), morphine (6), oxycodone (1), and fentanyl (1). The starting doses of OPI (morphine equivalent) and tramadol were 6.05 ± 1.63 and 67.8 ± 13.6 mg/day, respectively, and the final doses of OPI (morphine equivalent) and tramadol were 8.14 ± 3.85 and 80.0 ± 28.5 mg/day, respectively. Treatment goals were achieved in all patients. There were three patients in whom OPI was switched owing to inadequate analgesia and no new side effects other than those known to occur when OPI or tramadol is administered appeared. Conclusion: The results suggest that this innovative and unique opioid therapy can be safely and effectively introduced to opioid-naïve cancer patients who are relatively close to the end of life.

Naldemedine-induced perforation of a diverticulum in the sigmoid colon of a patient with opioid-related constipation: a case report.

BACKGROUND: Naldemedine is an orally available peripherally acting µ-opioid receptor antagonist approved to treat opioid-induced constipation (OIC). It is contraindicated for patients with known or suspected gastrointestinal obstruction to protect against naldemedine-induced perforation. Here, we report a clinical case of suspected perforation of a diverticulum in the sigmoid colon associated with naldemedine. CASE PRESENTATION: The patient was a 65-year-old man with a history of oral cancer who had been prescribed oxycodone (20 mg/day) for cancer pain. On day 0, the patient started naldemedine 0.2 mg once daily before bedtime for OIC. The dose of oxycodone was increased for pain control up to 60 mg/day. On day 35 of naldemedine treatment, the patient developed fever and abdominal pain, and his frequency of defecation had decreased. Initial laboratory results showed a C-reactive protein (CRP) level of 28.5 mg/dL and white blood cell (WBC) count of 13,500/µL. On day 37, the patient still had tenderness in his lower abdomen. Abdominal computed tomography revealed free air in the abdominal cavity suggesting an intestinal perforation. A Hartmann procedure was performed. Histopathological findings showed numerous diverticula in the sigmoid colon, some of which were perforated. CONCLUSIONS: These results suggest that the effects of OIC may have compressed the intestinal tract, which was followed by naldemedine-activation of peristalsis, which led to the onset of intestinal perforation. In patients with pre-existing diverticular disease, we should monitor for increased WBC counts and CRP levels after the initiation of treatment with naldemedine, and consider performing appropriate tests early in the event of abdominal complaints.

Elucidating the harm potential of brorphine analogues as new synthetic opioids: Synthesis, in vitro, and in vivo characterization.

The emergence of new synthetic opioids (NSOs) has added complexity to recreational opioid markets worldwide. While NSOs with diverse chemical structures have emerged, brorphine currently remains the only NSO with a piperidine benzimidazolone scaffold. However, the emergence of new generations of NSOs, including brorphine analogues, can be anticipated. This study explored the pharmaco-toxicological, opioid-like effect profile of brorphine alongside its non-brominated analogue (orphine) and three other halogenated analogues (fluorphine, chlorphine, iodorphine). In vitro, radioligand binding assays in rat brain tissue indicated that all analogues bind to the µ-opioid receptor (MOR) with nM affinity. While analogues with smaller-sized substituents showed the highest MOR affinity, further in vitro characterization via two cell-based (HEK 293T) MOR activation (ß-arrestin 2 and mini-Gαi recruitment) assays indicated that chlorphine, brorphine, and iodorphine were generally the most active MOR agonists. None of the compounds showed significant in vitro biased agonism compared to hydromorphone. In vivo, we investigated the effects of intraperitoneal (IP) administration of the benzimidazolones (0.01-15 mg/kg) on mechanical and thermal antinociception in male CD-1 mice. Chlorphine and brorphine overall induced the highest levels of antinociception. Furthermore, the effects on respiratory changes induced by a fixed dose (15 mg/kg IP) of the compounds were investigated using non-invasive plethysmography. Fluorphine-, chlorphine-, and brorphine-induced respiratory depressant effects were the most pronounced. For some compounds, pretreatment with naloxone (6 mg/kg IP) could not reverse respiratory depression. Taken together, brorphine-like piperidine benzimidazolones are opioid agonists that have the potential to cause substantial harm to users should they emerge as NSOs. This article is part of the Special Issue on "Novel Synthetic Opioids (NSOs)".

Dopamine enhances GABAA receptor-mediated current amplitude in a subset of intrinsically photosensitive retinal ganglion cells.

Neuromodulation in the retina is crucial for effective processing of retinal signal at different levels of illuminance. Intrinsically photosensitive retinal ganglion cells (ipRGCs), the neurons that drive nonimage-forming visual functions, express a variety of neuromodulatory receptors that tune intrinsic excitability as well as synaptic inputs. Past research has examined actions of neuromodulators on light responsiveness of ipRGCs, but less is known about how neuromodulation affects synaptic currents in ipRGCs. To better understand how neuromodulators affect synaptic processing in ipRGC, we examine actions of opioid and dopamine agonists have on inhibitory synaptic currents in ipRGCs. Although µ-opioid receptor (MOR) activation had no effect on γ-aminobutyric acid (GABA) currents, dopamine [via the D1-type dopamine receptor (D1R)]) amplified GABAergic currents in a subset of ipRGCs. Furthermore, this D1R-mediated facilitation of the GABA conductance in ipRGCs was mediated by a cAMP/PKA-dependent mechanism. Taken together, these findings reinforce the idea that dopamine's modulatory role in retinal adaptation affects both nonimage-forming and image-forming visual functions.NEW & NOTEWORTHY Neuromodulators such as dopamine are important regulators of retinal function. Here, we demonstrate that dopamine increases inhibitory inputs to intrinsically photosensitive retinal ganglion cells (ipRGCs), in addition to its previously established effect on intrinsic light responsiveness. This indicates that dopamine, in addition to its ability to intrinsically modulate ipRGC activity, can also affect synaptic inputs to ipRGCs, thereby tuning retina circuits involved in nonimage-forming visual functions.

Structure-Activity Relationship Study of CYM51010, an agonist for the µ-δ Opioid Receptor Heterodimer.

Although opioid analgesics are indispensable in treating pain, these drugs are accompanied by life-threatening side effects. While clinically relevant opioid drugs target the µ opioid receptor (MOR), a heterodimer between the MOR and the δ opioid receptor (DOR) has emerged as another target to develop safer analgesics. Although some heterodimer-preferring agonists have been reported so far, it is still difficult to activate the MOR/DOR heterodimer selectively in the presence of MOR or DOR monomers/homodimers. To gain insights to develop selective agonists for MOR/DOR, herein we prepared analogs of CYM51010, one of the reported heterodimer-preferring agonists, and collected structure-activity relationship information. We found that the ethoxycarbonyl group was needed for the activity for the heterodimer, although this group could be substituted with functional groups with similar sizes, such as an ethoxycarbonyl group. As for the acetylaminophenyl group, not a type of substituent, but rather a substituent located at a specific position (para-position) was essential for the activity. Changing the linker length between the acetylaminophenyl group and the piperidine moiety also had deleterious effects on the activity. On the other hand, the substitution of the acetylamino group with a trifluoroacetylamino group and the substitution of the phenethyl group with a benzyl group diminished the activities for the monomers/homodimers while keeping the activity for MOR/DOR, which enhanced the selectivity. Our findings herein will play an important role in developing selective agonists for MOR/DOR and for elucidating the physiological roles of this heterodimer in analgesic processes and in the establishment of side effects.

A peripherally acting µ-opioid receptor antagonist for treating opioid-associated tinnitus: A case report.

BACKGROUND: The use of opioids occasionally causes tinnitus. However, there is a paucity of data regarding the use of peripherally acting µ-opioid receptor antagonists for opioid-associated tinnitus in patients with cancer. ACTUAL CASE: A 74-year-old male with pancreatic cancer complained of abdominal pain. Two days after initiating oxycodone therapy, the patient experienced tinnitus during body movements. Although peripheral tinnitus disappeared after discontinuing oxycodone, it reappeared with hydromorphone or tapentadol administration. POSSIBLE COURSES OF ACTION: Drug cessation is a preferred intervention for drug-induced tinnitus; however, the cessation of opioids may not be feasible in patients with cancer who are already taking opioids. FORMULATION OF A PLAN: Based on the presumed mechanism of peripheral tinnitus, the use of peripherally acting µ-opioid receptor antagonists was planned, and 200 µg/day of naldemedine was prescribed for tinnitus relief. OUTCOME: Tinnitus disappeared immediately after initiating naldemedine, and the pain was well-controlled. The effect was preserved after increasing or switching opioids. LESSONS: The use of peripherally acting µ-opioid receptor antagonists may be an option to treat opioid-associated tinnitus without compromising the analgesic effects. VIEW: Further clinical data regarding the secondary effect of peripherally acting µ-opioid receptor antagonists on opioid-associated complications other than constipation are required.

In vitro structure-activity relationships and forensic case series of emerging 2-benzylbenzimidazole 'nitazene' opioids.

2-Benzylbenzimidazole 'nitazene' opioids are presenting a growing threat to public health. Although various nitazenes were previously studied, systematic comparisons of the effects of different structural modifications to the 2-benzylbenzimidazole core structure on µ-opioid receptor (MOR) activity are limited. Here, we assessed in vitro structure-activity relationships of 9 previously uncharacterized nitazenes alongside known structural analogues. Specifically, we focused on MOR activation by 'ring' substituted analogues (i.e., N-pyrrolidino and N-piperidinyl modifications), 'desnitazene' analogues (lacking the 5-nitro group), and N-desethyl analogues. The results from two in vitro MOR activation assays (ß-arrestin 2 recruitment and inhibition of cAMP accumulation) showed that 'ring' modifications overall yield highly active drugs. With the exception of 4'-OH analogues (which are metabolites), N-pyrrolidino substitutions were generally more favorable for MOR activation than N-piperidine substitutions. Furthermore, removal of the 5-nitro group on the benzimidazole ring consistently caused a pronounced decrease in potency. The N-desethyl modifications showed important MOR activity, and generally resulted in a slightly lowered potency than comparator nitazenes. Intriguingly, N-desethyl isotonitazene was the exception and was consistently more potent than isotonitazene. Complementing the in vitro findings and demonstrating the high harm potential associated with many of these compounds, we describe 85 forensic cases from North America and the United Kingdom involving etodesnitazene, N-desethyl etonitazene, N-desethyl isotonitazene, N-pyrrolidino metonitazene, and N-pyrrolidino protonitazene. The low-to-sub ng/mL blood concentrations observed in most cases underscore the drugs' high potencies. Taken together, by bridging pharmacology and case data, this study may aid to increase awareness and guide legislative and public health efforts.

Strategies for developing µ opioid receptor agonists with reduced adverse effects.

Opioids are currently the most effective and widely used painkillers in the world. Unfortunately, the clinical use of opioid analgesics is limited by serious adverse effects. Many researchers have been working on designing and optimizing structures in search of novel µ opioid receptor(MOR) agonists with improved analgesic activity and reduced incidence of adverse effects. There are many strategies to develop MOR drugs, mainly focusing on new low efficacy agonists (potentially G protein biased agonists), MOR agonists acting on different Gα subtype, targeting opioid receptors in the periphery, acting on multiple opioid receptor, and targeting allosteric sites of opioid receptors, and others. This review summarizes the design methods, clinical applications, and structure-activity relationships of small-molecule agonists for MOR based on these different design strategies, providing ideas for the development of safer novel opioid ligands with therapeutic potential.

Modulation of Nicotine-Associated Behaviour in Rats By µ-Opioid Signals from the Medial Prefrontal Cortex to the Nucleus Accumbens Shell.

Nicotine addiction is a concern worldwide. Most mechanistic investigations are on nicotine substance dependence properties based on its pharmacological effects. However, no effective therapeutic treatment has been established. Nicotine addiction is reinforced by environments or habits. We demonstrate the neurobiological basis of the behavioural aspect of nicotine addiction. We utilized the conditioned place preference to establish nicotine-associated behavioural preferences (NABP) in rats. Brain-wide neuroimaging analysis revealed that the medial prefrontal cortex (mPFC) was activated and contributed to NABP. Chemogenetic manipulation of µ-opioid receptor positive (MOR+) neurons in the mPFC or the excitatory outflow to the nucleus accumbens shell (NAcShell) modulated the NABP. Electrophysiological recording confirmed that the MOR+ neurons directly regulate the mPFC-NAcShell circuit via GABAA receptors. Thus, the MOR+ neurons in the mPFC modulate the formation of behavioural aspects of nicotine addiction via direct excitatory innervation to the NAcShell, which may provide new insight for the development of effective therapeutic strategies.

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.

Opioids and Immunosuppression: Clinical Evidence, Mechanisms of Action, and Potential Therapies.

Background: In addition to the more well-known adverse effects of opioids, such as constipation, mounting evidence supports underlying immunosuppressive effects as well. Methods: In this study, we provide a narrative review of preclinical and clinical evidence of opioid suppression of the immune system as well as possible considerations for therapies. Results: In vitro and animal studies have shown clear effects of opioids on inflammatory cytokine expression, immune cell activity, and pathogen susceptibility. Observational data in humans have so far supported preclinical findings, with multiple reports of increased rates of infections in various settings of opioid use. However, the extent to which this risk is due to the impact of opioids on the immune system compared with other risk factors associated with opioid use remains uncertain. Considering the data showing immunosuppression and increased risk of infection with opioid use, measures are needed to mitigate this risk in patients who require ongoing treatment with opioids. In preclinical studies, administration of opioid receptor antagonists blocked the immunomodulatory effects of opioids. Conclusions: As selective antagonists of peripheral opioid receptors, peripherally acting mu-opioid receptor (MOR) antagonists may be able to protect against immune impairment while still allowing for opioid analgesia. Future research is warranted to further investigate the relationship between opioids and infection risk as well as the potential application of peripherally acting MOR antagonists to counteract these risks.

Stereoselective recognition of morphine enantiomers by µ-opioid receptor.

Stereospecific recognition of chiral molecules plays a crucial role in biological systems. The µ-opioid receptor (MOR) exhibits binding affinity towards (-)-morphine, a well-established gold standard in pain management, while it shows minimal binding affinity for the (+)-morphine enantiomer, resulting in a lack of analgesic activity. Understanding how MOR stereoselectively recognizes morphine enantiomers has remained a puzzle in neuroscience and pharmacology for over half-a-century due to the lack of direct observation techniques. To unravel this mystery, we constructed the binding and unbinding processes of morphine enantiomers with MOR via molecular dynamics simulations to investigate the thermodynamics and kinetics governing MOR's stereoselective recognition of morphine enantiomers. Our findings reveal that the binding of (-)-morphine stabilizes MOR in its activated state, exhibiting a deep energy well and a prolonged residence time. In contrast, (+)-morphine fails to sustain the activation state of MOR. Furthermore, the results suggest that specific residues, namely D1142.50 and D1473.32, are deprotonated in the active state of MOR bound to (-)-morphine. This work highlights that the selectivity in molecular recognition goes beyond binding affinities, extending into the realm of residence time.

In Vitro Functional Profiling of Fentanyl and Nitazene Analogs at the µ-Opioid Receptor Reveals High Efficacy for Gi Protein Signaling.

Novel synthetic opioids (NSOs), including both fentanyl and non-fentanyl analogs that act as µ-opioid receptor (MOR) agonists, are associated with serious intoxication and fatal overdose. Previous studies proposed that G-protein-biased MOR agonists are safer pain medications, while other evidence indicates that low intrinsic efficacy at MOR better explains the reduced opioid side effects. Here, we characterized the in vitro functional profiles of various NSOs at the MOR using adenylate cyclase inhibition and ß-arrestin2 recruitment assays, in conjunction with the application of the receptor depletion approach. By fitting the concentration-response data to the operational model of agonism, we deduced the intrinsic efficacy and affinity for each opioid in the Gi protein signaling and ß-arrestin2 recruitment pathways. Compared to the reference agonist [d-Ala2,N-MePhe4,Gly-ol5]enkephalin, we found that several fentanyl analogs were more efficacious at inhibiting cAMP production, whereas all fentanyl analogs were less efficacious at recruiting ß-arrestin2. In contrast, the non-fentanyl 2-benzylbenzimidazole (i.e., nitazene) analogs were highly efficacious and potent in both the cAMP and ß-arrestin2 assays. Our findings suggest that the high intrinsic efficacy of the NSOs in Gi protein signaling is a common property that may underlie their high risk of intoxication and overdose, highlighting the limitation of using in vitro functional bias to predict the adverse effects of opioids. In addition, the extremely high potency of many NSOs now infiltrating illicit drug markets further contributes to the danger posed to public health.

Towards Multitargeted Ligands as Pain Therapeutics: Dual Ligands of the Cavα2δ-1 Subunit of Voltage-Gated Calcium Channel and the µ-Opioid Receptor.

The synthesis and pharmacological activity of a new series of dual ligands combining activities towards the α2δ-1 subunit of voltage-gated calcium channels (Cavα2δ-1) and the µ-opioid receptor (MOR) as novel pain therapeutics are reported. A careful exploration of the pharmacophores related to both targets, which in principle had few common characteristics, led to the design of novel compounds exhibiting both activities. The construction of the dual ligands started from published Cavα2δ-1 ligands, onto which MOR ligand pharmacophoric elements were added. This exercise led to new amino-acidic substances with good affinities on both targets as well as good metabolic and physicochemical profiles and low potential for drug-drug interactions. A representative compound, (2S,4S)-4-(4-chloro-3-(((cis)-4-(dimethylamino)-4-phenylcyclohexyl)methyl)-5-fluorophenoxy)pyrrolidine-2-carboxylic acid, displayed promising analgesic activities in several in vivo pain models as well as a reduced side-effect profile in relation to morphine.

The µ-opioid receptor-mediated Gi/o protein and ß-arrestin2 signaling pathways both contribute to morphine-induced side effects.

The µ-opioid receptor-biased agonist theory holds that Gio protein signaling mediates the analgesic effect of opioids and the related side effects via the ß-arrestin2 signaling pathway. A series of µ-opioid-biased agonists have been developed in accordance with this theory, and the FDA has approved TRV130 (as a representative of biased agonists) for marketing. However, several reports have raised the issue of opioid side effects associated with the use of agonists. In this study, five permeable peptides were designed to emulate 11 S/T phosphorylation sites at the µ-opioid receptor (MOR) carboxyl-terminal. In vitro experiments were performed to detect the activation level of G proteins from the cAMP inhibition assay and the ß-arrestin2 recruitment by the BRET assay. Designed peptides might effectively interfere with the activation of the Gio and ß-arrestin2 pathways when combined with morphine. The resulting morphine-induced tolerance, respiratory inhibition, and constipation in mice showed that the ß-arrestin2 pathway was responsible for morphine tolerance while the Gio signaling pathway was involved with respiratory depression and constipation and that these side effects were significantly related to phosphorylation sites S363 and T370. This study may provide new directions for the development of safer and more effective opioid analgesics, and the designed peptides may be an effective tool for exploring the mechanism by which µ-opioid receptors function, with the potential of reducing the side effects that are associated with clinical opioid treatment.

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

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

Opioid-Induced Hyperalgesia and Tolerance Are Driven by HCN Ion Channels.

Prolonged exposure to opioids causes an enhanced sensitivity to painful stimuli (opioid-induced hyperalgesia, OIH) and a need for increased opioid doses to maintain analgesia (opioid-induced tolerance, OIT), but the mechanisms underlying both processes remain obscure. We found that pharmacological block or genetic deletion of HCN2 ion channels in primary nociceptive neurons of male mice completely abolished OIH but had no effect on OIT. Conversely, pharmacological inhibition of central HCN channels alleviated OIT but had no effect on OIH. Expression of C-FOS, a marker of neuronal activity, was increased in second-order neurons of the dorsal spinal cord by induction of OIH, and the increase was prevented by peripheral block or genetic deletion of HCN2, but block of OIT by spinal block of HCN channels had no impact on C-FOS expression in dorsal horn neurons. Collectively, these observations show that OIH is driven by HCN2 ion channels in peripheral nociceptors, while OIT is driven by a member of the HCN family located in the CNS. Induction of OIH increased cAMP in nociceptive neurons, and a consequent shift in the activation curve of HCN2 caused an increase in nociceptor firing. The shift in HCN2 was caused by expression of a constitutively active µ-opioid receptor (MOR) and was reversed by MOR antagonists. We identified the opioid-induced MOR as a six-transmembrane splice variant, and we show that it increases cAMP by coupling constitutively to Gs HCN2 ion channels therefore drive OIH, and likely OIT, and may be a novel therapeutic target for the treatment of addiction.

Repurposing EGFR Inhibitors for Oral Cancer Pain and Opioid Tolerance.

Oral cancer pain remains a significant public health concern. Despite the development of improved treatments, pain continues to be a debilitating clinical feature of the disease, leading to reduced oral mobility and diminished quality of life. Opioids are the gold standard treatment for moderate-to-severe oral cancer pain; however, chronic opioid administration leads to hyperalgesia, tolerance, and dependence. The aim of this review is to present accumulating evidence that epidermal growth factor receptor (EGFR) signaling, often dysregulated in cancer, is also an emerging signaling pathway critically involved in pain and opioid tolerance. We presented preclinical and clinical data to demonstrate how repurposing EGFR inhibitors typically used for cancer treatment could be an effective pharmacological strategy to treat oral cancer pain and to prevent or delay the development of opioid tolerance. We also propose that EGFR interaction with the µ-opioid receptor and glutamate N-methyl-D-aspartate receptor could be two novel downstream mechanisms contributing to pain and morphine tolerance. Most data presented here support that repurposing EGFR inhibitors as non-opioid analgesics in oral cancer pain is promising and warrants further research.