Distinct Profiles of Desensitization of µ-Opioid Receptors Caused by Remifentanil or Fentanyl: In Vitro Assay with Cells and Three-Dimensional Structural Analyses.

Remifentanil (REM) and fentanyl (FEN) are commonly used analgesics that act by activating a µ-opioid receptor (MOR). Although optimal concentrations of REM can be easily maintained during surgery, it is sometimes switched to FEN for optimal pain regulation. However, standards for this switching protocol remain unclear. Opioid anesthetic efficacy is decided in part by MOR desensitization; thus, in this study, we investigated the desensitization profiles of REM and FEN to MOR. The efficacy and potency during the 1st administration of REM or FEN in activating the MOR were almost equal. Similarly, in ß arrestin recruitment, which determines desensitization processes, they showed no significant differences. In contrast, the 2nd administration of FEN resulted in a stronger MOR desensitization potency than that of REM, whereas REM showed a higher internalization potency than FEN. These results suggest that different ß arrestin-mediated signaling caused by FEN or REM led to their distinct desensitization and internalization processes. Our three-dimensional analysis, with in silico binding of REM and FEN to MOR models, highlighted that REM and FEN bound to similar but distinct sites of MOR and led to distinct ß arrestin-mediated profiles, suggesting that distinct binding profiles to MOR may alter ß arrestin activity, which accounts for MOR desensitization and internalization.

Combination of asleep and awake craniotomy as a novel strategy for resection in patients with butterfly glioblastoma: Two case reports.

Background: Several studies have reported that gross total resection contributes to improved prognosis in patients with butterfly glioblastoma (bGBM). However, it sometimes damages the corpus callosum and cingulate gyrus, leading to severe neurological complications. Case Description: We report two cases of bGBM that was safely and maximally resected using brief and exact awake mapping after general anesthesia. Two patients had butterfly tumors in both the frontal lobes and the genu of the corpus callosum. Tumor resection was first performed on the nondominant side under general anesthesia to shorten the resection time and maintain patient concentration during awake surgery. After that, awake surgery was performed for the lesions in the dominant frontal lobe and genu of the corpus callosum. Tumor resection was performed through minimal cortical incisions in both frontal lobes. Postoperative magnetic resonance imaging showed gross total resection, and the patients had no chronic neurological sequelae, such as akinetic mutism and abulia. Conclusion: bGBM could be safely and maximally resected by a combination of asleep and brief awake resection, which enabled patients to maintain their attention to the task without fatigue, somnolence, or decreased attention. The bilateral approach from a small corticotomy can avoid extensive damage to the cingulate gyrus.

Ketamine Improves Desensitization of µ-Opioid Receptors Induced by Repeated Treatment with Fentanyl but Not with Morphine.

The issue of tolerance to continuous or repeated administration of opioids should be addressed. The ability of ketamine to improve opioid tolerance has been reported in clinical studies, and its mechanism of tolerance may involve improved desensitization of µ-opioid receptors (MORs). We measured changes in MOR activity and intracellular signaling induced by repeated fentanyl and morphine administration and investigated the effects of ketamine on these changes with human embryonic kidney 293 cells expressing MOR using the CellKey™, cADDis cyclic adenosine monophosphate, and PathHunter® ß-arrestin recruitment assays. Repeated administration of fentanyl or morphine suppressed the second MOR responses. Administration of ketamine before a second application of opioids within clinical concentrations improved acute desensitization and enhanced ß-arrestin recruitment elicited by fentanyl but not by morphine. The effects of ketamine on fentanyl were suppressed by co-treatment with an inhibitor of G-protein-coupled receptor kinase (GRK). Ketamine may potentially reduce fentanyl tolerance but not that of morphine through modulation of GRK-mediated pathways, possibly changing the conformational changes of ß-arrestin to MOR.

Oxytocin Is a Positive Allosteric Modulator of κ-Opioid Receptors but Not δ-Opioid Receptors in the G Protein Signaling Pathway.

Oxytocin (OT) influences various physiological functions such as uterine contractions, maternal/social behavior, and analgesia. Opioid signaling pathways are involved in one of the analgesic mechanisms of OT. We previously showed that OT acts as a positive allosteric modulator (PAM) and enhances µ-opioid receptor (MOR) activity. In this study, which focused on other opioid receptor (OR) subtypes, we investigated whether OT influences opioid signaling pathways as a PAM for δ-OR (DOR) or κ-OR (KOR) using human embryonic kidney-293 cells expressing human DOR or KOR, respectively. The CellKeyTM results showed that OT enhanced impedance induced by endogenous/exogenous KOR agonists on KOR-expressing cells. OT did not affect DOR activity induced by endogenous/exogenous DOR agonists. OT potentiated the KOR agonist-induced Gi/o protein-mediated decrease in intracellular cAMP, but did not affect the increase in KOR internalization caused by the KOR agonists dynorphin A and (-)-U-50488 hydrochloride (U50488). OT did not bind to KOR orthosteric binding sites and did not affect the binding affinities of dynorphin A and U50488 for KOR. These results suggest that OT is a PAM of KOR and MOR and enhances G protein signaling without affecting ß-arrestin signaling. Thus, OT has potential as a specific signaling-biased PAM of KOR.

The G Protein Signal-Biased Compound TRV130; Structures, Its Site of Action and Clinical Studies.

Opioid agonists elicit their analgesic action mainly via µ opioid receptors; however, their use is limited because of adverse events including constipation and respiratory depression. It has been shown that analgesic action is transduced by the G protein-mediated pathway whereas adverse events are by the ß-arrestin-mediated pathway through µ opioid receptor signaling. The first new-generation opioid TRV130, which preferentially activates G protein- but not ß-arrestin-mediated signal, was constructed and developed to reduce adverse events. TRV130 and other G protein-biased compounds tend to elicit desirable analgesic action with less adverse effects. In clinical trials, the intravenous TRV130 (oliceridine) was evaluated in Phase I, II and III clinical studies. Here we review the discovery and synthesis of TRV130, its main action as a novel analgesic having less adverse events, its up-to-date status in clinical trials, and additional concerns about TRV130 as demonstrated in the literature.

Possible biased analgesic of hydromorphone through the G protein-over ß-arrestin-mediated pathway: cAMP, CellKey™, and receptor internalization analyses.

Morphine, fentanyl, and oxycodone are widely used as analgesics, and recently hydromorphone has been approved in Japan. Although all of these are selective for µ-opioid receptors (MORs) and have similar structures, their analgesic potencies and adverse effects (AEs) are diverse. Recent molecular analyses of MOR signaling revealed that the G protein-mediated signaling pathway causes analgesic effects and the ß-arrestin-mediated signaling pathway is responsible for AEs. We used several cell-based analyses that selectively measure cellular responses activated by either G protein- or ß-arrestin-mediated pathways. GloSensor™ cAMP, CellKey™, and receptor internalization assays were performed with four different types of cells stably expressing differentially labelled MOR. EC50 values measured by cAMP and CellKey™ assays had potencies in the order fentanyl ≤ hydromorphone < morphine ≤ oxycodone, all also exhibiting full agonist responses. However, in the internalization assay, only fentanyl elicited a full agonist response. Hydromorphone had the strongest potency next to fentanyl; however, contribution of the ß-arrestin-mediated pathway was small, suggesting that its effect could be biased toward the G protein-mediated pathway. Based on these properties, hydromorphone could be chosen as an effective analgesic.

Prolonged Tachycardia with Higher Heart Rate Is Associated with Higher ICU and In-hospital Mortality.

Tachycardia is common in intensive care units (ICUs). It is unknown whether tachycardia or prolonged tachycardia affects patient outcomes. We investigated the association between tachycardia and mortality in critically ill patients. This retrospective cohort study's primary outcome was patient mortality in the ICU and the hospital. We stratified the patients (n=476) by heart rate (HR) as LowHR, MediumHR, and HighHR groups. We also stratified them by their durations of HR >100 (prolonged HR; tachycardia): MildT, ModerateT, and SevereT groups. We determined the six groups' mortality. The ICU mortality rates of the LowHR, MediumHR, and HighHR groups were 1.0%, 1.5%, and 7.9%, respectively; significantly higher in the HighHR vs. LowHR group. The in-hospital mortality rates of these groups were 1%, 4.5%, and 14.6%, respectively; significantly higher in the HighHR vs. LowHR group. The ICU mortality rates of the MildT, ModerateT, and SevereT groups were 0.9%, 5.6%, and 57.1%, respectively. The mortality of the HRT=0 (i.e., all HR ≤ 100) patients was 0%. The in-hospital mortality rates of the MildT, ModerateT, and SevereT groups were 1.8%, 16.7%, and 85.7%, respectively; that of the HRT=0 patients was 0.5%. Both higher HR and prolonged tachycardia were associated with poor outcomes.

Neuropeptide oxytocin enhances µ opioid receptor signaling as a positive allosteric modulator.

Oxytocin (OT) is a 9-amine neuropeptide that plays an essential role in mammalian labor, lactation, maternal bonding, and social affiliation. OT has been reported to exert an analgesic effect in both humans and animals, and the results of certain animal experiments have shown that the analgesic effect of OT is partially blocked by opioid receptor antagonists. To investigate the relationship between OT and µ opioid receptor (MOR), we evaluated how OT affects MOR in vitro by performing an electrical impedance-based receptor biosensor assay (CellKey™ assay), an intracellular cAMP assay, and a competitive receptor-binding analysis by using cells stably expressing human MOR and OT receptor. In both the CellKey™ assay and the intracellular cAMP assay, OT alone exerted no direct agonistic effect on human MOR, but treatment with 10-6 M OT markedly enhanced the MOR signaling induced by 10-6 M endomorphin-1, ß-endorphin, morphine, fentanyl, and DAMGO. Moreover, in the competitive receptor-binding assay, 10-6 M OT did not alter the affinity of endomorphin-1 or morphine for MOR. These results suggest that OT could function as a positive allosteric modulator that regulates the efficacy of MOR signaling, and thus OT might represent a previously unrecognized candidate analgesic agent.