Opioid Receptor Agonist Research Articles

The Kappa Opioid Receptor Agonist 16-Bromo Salvinorin A Has Anti-Cocaine Effects without Significant Effects on Locomotion, Food Reward, Learning and Memory, or Anxiety and Depressive-like Behaviors.

Kappa opioid receptor (KOR) agonists have preclinical antipsychostimulant effects; however, adverse side effects have limited their therapeutic development. In this preclinical study, conducted in Sprague Dawley rats, B6-SJL mice, and non-human primates (NHPs), we evaluated the G-protein-biased analogue of salvinorin A (SalA), 16-bromo salvinorin A (16-BrSalA), for its anticocaine effects, side effects, and activation of cellular signaling pathways. 16-BrSalA dose-dependently decreased the cocaine-primed reinstatement of drug-seeking behavior in a KOR-dependent manner. It also decreased cocaine-induced hyperactivity, but had no effect on responding for cocaine on a progressive ratio schedule. Compared to SalA, 16-BrSalA had an improved side effect profile, with no significant effects in the elevated plus maze, light-dark test, forced swim test, sucrose self-administration, or novel object recognition; however, it did exhibit conditioned aversive effects. 16-BrSalA increased dopamine transporter (DAT) activity in HEK-293 cells coexpressing DAT and KOR, as well as in rat nucleus accumbens and dorsal striatal tissue. 16-BrSalA also increased the early phase activation of extracellular-signal-regulated kinases 1 and 2, as well as p38 in a KOR-dependent manner. In NHPs, 16-BrSalA caused dose-dependent increases in the neuroendocrine biomarker prolactin, similar to other KOR agonists, at doses without robust sedative effects. These findings highlight that G-protein-biased structural analogues of SalA can have improved pharmacokinetic profiles and fewer side effects while maintaining their anticocaine effects.

Opioid suppression of an excitatory pontomedullary respiratory circuit by convergent mechanisms.

Opioids depress breathing by inhibition of interconnected respiratory nuclei in the pons and medulla. Mu opioid receptor (MOR) agonists directly hyperpolarize a population of neurons in the dorsolateral pons, particularly the Kölliker-Fuse (KF) nucleus, that are key mediators of opioid-induced respiratory depression. However, the projection target and synaptic connections of MOR-expressing KF neurons are unknown. Here, we used retrograde labeling and brain slice electrophysiology to determine that MOR-expressing KF neurons project to respiratory nuclei in the ventrolateral medulla, including the preBötzinger complex (preBötC) and rostral ventral respiratory group (rVRG). These medullary-projecting, MOR-expressing dorsolateral pontine neurons express FoxP2 and are distinct from calcitonin gene-related peptide-expressing lateral parabrachial neurons. Furthermore, dorsolateral pontine neurons release glutamate onto excitatory preBötC and rVRG neurons via monosynaptic projections, which is inhibited by presynaptic opioid receptors. Surprisingly, the majority of excitatory preBötC and rVRG neurons receiving MOR-sensitive glutamatergic synaptic input from the dorsolateral pons are themselves hyperpolarized by opioids, suggesting a selective opioid-sensitive circuit from the KF to the ventrolateral medulla. Opioids inhibit this excitatory pontomedullary respiratory circuit by three distinct mechanisms-somatodendritic MORs on dorsolateral pontine and ventrolateral medullary neurons and presynaptic MORs on dorsolateral pontine neuron terminals in the ventrolateral medulla-all of which could contribute to opioid-induced respiratory depression.

Iridoids from Morinda lucida, (Benth.) Rubiaceae, produced analgesic and anti-inflammatory activities via agonism at the kappa and delta opioid receptors, inhibition of COX-2 besides elevation of CAT and SOD activities.

Pain and inflammation are the major symptoms of almost every human disease. Herbal preparations from Morinda lucida are used to treat pain and inflammation in traditional medicine. However, the analgesic and anti-inflammatory activities of some of its chemical constituents are not known. The aim of this study is to evaluate the analgesic and anti-inflammatory activities and possible mechanisms of these activities of iridoids from Morinda lucida. The compounds were isolated using column chromatography and characterized by NMR spectroscopy and LC-MS. Anti-inflammatory activity was evaluated using carrageenan-induced paw edema. Whereas, the analgesic activity was assessed in the hot plate and acetic acid-induced writhing assays. Mechanistic studies were conducted using pharmacological blockers, determination of antioxidant enzymes, lipid peroxidation, and docking studies. The iridoid, ML2-2 exhibited inverse dose-dependent anti-inflammatory activity (42.62% maximum at 2 mg/kg p. o). ML2-3 produced dose-dependent anti-inflammatory activity (64.52% maximum at 10 mg/kg p. o.). Anti-inflammatory activity of diclofenac sodium was 58.60% at 10 mg/kg p. o. Furthermore, ML2-2 and ML2-3 produced analgesic activity (P < 0.01) of 44.44 ± 5.84 and 54.18 ± 19.01%. at 10 mg/kg p. o. respectively in the hot plate assay and 64.88 and 67.44% in the writhing assay. ML2-2 significantly elevated catalase activity. However, ML2-3 elevated SOD and catalase activity significantly. In the docking studies, both iridoids formed stable crystal complexes with delta and kappa opioid receptors, and the COX-2 enzyme with very low free binding energies (ΔG) from -11.2 to -14.0 kcal/mol. However, they did not bind with the mu opioid receptor. The lower bound RMSD of most of the poses were found to be ≤ 2. Several amino acids were involved in the interactions through various inter molecular forces. These results indicate that ML2-2 and ML2-3 possessed very significant analgesic and anti-inflammatory activities via acting as both delta and kappa opioid receptor agonist, elevation of anti-oxidant activity and inhibition of COX-2.

A pharmacologically pre-contracted smooth muscle bowel model for the study of highly-potent opioid receptor agonists and antagonists

Isolated organ models are a versatile tool for pharmacological and toxicological research. Small bowel has been used to assess the inhibition of smooth muscle contraction by opioids. In the present study, we set out to establish a pharmacologically stimulated rat bowel model. The effects of carfentanil, remifentanil and the new synthetic opioid U-48800 and their respective antagonists naloxone, nalmefene and naltrexone were studied in a small bowel model in rats. The IC50 values of the tested opioids were as follows: carfentanil (IC50 = 0.02 µmol/L, CI 0.02–0.03 µmol/L) ≫ remifentanil (IC50 = 0.51 µmol/L, CI 0.40–0.66 µmol/L) ≫ U-48800 (IC50 = 1.36 µmol/L, CI 1.20–1.54 µmol/L). The administration of the opioid receptor antagonists naloxone, naltrexone and nalmefene led to progressive, parallel rightward shifts of the dose-response curves. Naltrexone was most potent in antagonizing the effects of U-48800, whereas naltrexone and nalmefene were most effective in antagonizing the effects of carfentanil. In summary, the current model seems to be a robust tool to study opioid effects in a small bowel model without the necessity of using electrical stimulation.

(185) Over the Counter Kratom Use and Its Implications on Ejaculatory Dysfunction

Abstract Introduction The United States opioid epidemic remains responsible for 75,673 overdose deaths in 2021. Self-medication with kratom, a Southeastern Asian plant containing the mu opioid receptor agonists mitragynine and 7-hydroxymitragynine, is becoming increasingly prevalent. Kratom is legally available in most states with purported benefits in the self-treatment of opiate withdrawal and mood disorders. Anecdotal data linking kratom to male sexual health enhancement have not been formally investigated. Objective To objectively investigate the potential effect of kratom use on men's sexual health including ejaculatory and erectile function. Methods Under IRB approval, Twitter and Reddit (r/Kratom) were accessed to disseminate our survey featuring validated instruments (IIEF and the premature ejaculation diagnostic tool, PEDT). Sexual health prior to and after four weeks of kratom use were assessed. Descriptive statistics were used to ascertain differences between ejaculatory and erectile function with kratom use. Results Most males surveyed (n =165) were 18-40 years old (84.9%) and were white (86.7%). When examining kratom use, 82.4% (n= 136) of males used kratom for 1 or more years, with 95.8% using it at least weekly. Reasons for using kratom included an illicit drug dependency (17.6%), a prescription medicine dependency (14.5%), acute or chronic pain (39.4%), a mental health condition (63.6%), and issues with sexual health (4.8%). Of the 156 sexual health responses, 37.7% (n=59) and 20.5% (n=32) reported kratom having a positive and negative impact on their sexual health, respectively. Kratom use lead to increased time to ejaculation in 104 (66.6%) patients which was perceived as positive by 62 (59.6%). A subset of patients (n=78) answered questions about premature ejaculation prior to and after 4 weeks of kratom use. The mean pre and post kratom premature ejaculation diagnostic tool (PEDT) scores were 13.0 +/− 0.6 and 8.3 +/− 0.4 respectively (p &amp;lt; 0.001). Ejaculation prior to 5 minutes was improved after kratom use (n=40; 51.3% vs. n=10; 12.8%) (p &amp;lt; 0.0001). Post kratom use, more patients reported lack of frustration with ejaculation prior to desire (n=17; 21.8 % vs. n=48; 61.5%) (p &amp;lt; 0.001). Kratom users were unlikely to report concerns regarding fertility (n=1, 6.1%), testosterone levels (n=25, 15.2%), libido (n=38, 23.0%), or erectile dysfunction (ED) (n=30, 18.2%). The mean International Index of Erectile Function score was decreased with kratom use (26.6 +/− 0.6 vs. 25.9 +/− 0.5) (p &amp;lt; 0.05). Users rarely stopped kratom due to concerns regarding delayed ejaculation (n=5, 10%), low libido (n=8, 7.5 %), or ED (n=6, 7.9%). Conclusions Kratom users report delayed ejaculation, overall benefits to their sexual health, and a statistically significant impact on erectile function with kratom use. Clinicians treating male sexual health should be aware of kratom and its potential effect on ejaculatory and erectile function. Disclosure Any of the authors act as a consultant, employee or shareholder of an industry for: Boston Scientific.

Carfentanil is a β-arrestin-biased agonist at the μ opioid receptor.

The illicit use of fentanyl-like drugs (fentanyls), which are μ opioid receptor agonists, and the many overdose deaths that result, has become a major problem. Fentanyls are very potent in vivo, leading to respiratory depression and death. However, the efficacy and possible signalling bias of different fentanyls is not clearly known. Here, we compared the relative efficacy and bias of a series of fentanyls. For agonist signalling bias and efficacy measurements, Bioluminescence Resonance Energy Transfer experiments were undertaken in HEK293T cells transiently transfected with μ opioid receptors, to assess Gi protein activation and β-arrestin 2 recruitment. Agonist-induced cell surface receptor loss was assessed using an enzyme-linked immunosorbent assay, whilst agonist-induced G protein-coupled inwardly rectifying potassium channel current activation was measured electrophysiologically from rat locus coeruleus slices. Ligand poses in the μ opioid receptor were determined in silico using molecular dynamics simulations. Relative to the reference ligand DAMGO, carfentanil was β-arrestin-biased, whereas fentanyl, sufentanil and alfentanil did not display bias. Carfentanil induced potent and extensive cell surface receptor loss, whilst the marked desensitisation of G protein-coupled inwardly rectifying potassium channel currents in the continued presence of carfentanil in neurones was prevented by a GRK2/3 inhibitor. Molecular dynamics simulations suggested unique interactions of carfentanil with the orthosteric site of the receptor that could underlie the bias. Carfentanil is a β-arrestin-biased opioid drug at the μ receptor. It is uncertain how such bias influences in vivo effects of carfentanil relative to other fentanyls.

Assessment of the physical dependence potential of difelikefalin: randomized placebo-controlled study in patients receiving hemodialysis.

Difelikefalin is a selective kappa opioid receptor agonist approved for treating moderate-to-severe pruritus in adults undergoing hemodialysis (HD). Difelikefalin is not a controlled substance under the Controlled Substances Act. This study assessed the potential for developing physical dependence on difelikefalin in patients undergoing HD. Eligible patients received open-label difelikefalin after each dialysis session for 3 weeks before entering a 2-week double-blind phase, when they were randomized to either continue difelikefalin or to switch to receiving placebo. Signs of physical withdrawal were assessed using the Clinical Opiate Withdrawal Scale (COWS), several patient-reported scales, and physiological measures. The primary endpoint was the between-group difference in mean maximum COWS total scores during the double-blind phase; the mean difference (placebo - difelikefalin) was compared against a pre-defined noninferiority limit (+4). Thirty-five patients (57.1% male; 91.4% Black or African American; median [range] age 58 [28-77] years) were included, of which 30 were randomized (placebo, n=14; difelikefalin, n=16). The least square mean difference in maximum COWS total scores was 0.52 (95% confidence interval [CI]: -0.56, 1.59). The upper CI limit (1.59) was below +4, indicating that patients who discontinued difelikefalin (placebo group) had similar withdrawal scores to patients who continued difelikefalin. Additional assessments supported the COWS results, showing no meaningful differences between groups in physiological measures or in patient-reported measures of sleep or physical withdrawal. These results demonstrate that abruptly discontinuing chronic difelikefalin treatment in patients undergoing HD does not produce signs or symptoms of physical withdrawal.

The Role of NO Synthase, MPT pore, and Protein Kinase A in the Cardioprotective Effect of the Opioid Receptor Agonist Deltorphin II.

In male Wistar rats, coronary occlusion (45 min) and reperfusion (120 min) were modeled. Selective δ2-opioid receptor agonist (deltorphin II, 0.12 mg/kg) was administered intravenously 5 min before reperfusion; NO synthase inhibitor (L-NAME, 10 mg/kg), MPT pore blocker (atractyloside, 5 mg/kg), and protein kinase A inhibitor (H-89, 10 μg/kg) were administered intravenously 10 min before reperfusion. Deltorphin II administered before reperfusion led to a 2-fold decrease in the infarct size. The infarct-limiting effect of deltorphin II was associated with blockade of MPT pore. Protein kinase A and NO synthase were not involved in the cardioprotective effect of deltorphin II.

The Role of Opioid Receptors in Cancer.

Opioids are one of the most potent drugs for treating moderate to severe pain. Despite irrefutable clinical application in chronic pain management, the long-term use of opioids has been increasingly questioned due to undesired side effects that demand attention. Opioids such as morphine mediate clinically relevant effects primarily through the µ-opioid receptor that go beyond their classical role as analgesics, causing potentially fatal side effects such as tolerance, dependence, and addiction. Furthermore, there is growing evidence that opioids affect immune system function, cancer progression, metastasis, and recurrence. Though a biological plausibility, the clinical evidence for the action of opioids on cancer is mixed, revealing a more complex picture as researchers struggle to establish a vital link between opioid receptor agonists, cancer progression, and suppression, or both. Thus, in light of the uncertainty of opioid effects on cancer, in this review, a focused overview of the role of opioid receptors in modulating cancer progression, their underlying signaling mechanisms, and the biological activity of opioid receptor agonists and antagonists is provided.

Regional differences of extrinsic and intrinsic cholinergic innervation and alterations by chronic opioid-induced constipation in the porcine colonic enteric nerve system

The colon is richly innervated by extrinsic (ext) and intrinsic (int) cholinergic nerves from central and enteric nervous system (CNS and ENS) modulating colonic motility, secretion and inflammation. However, a challenge lies in our inability to anatomically distinguish such innervation in the colon. The porcine colon is regarded as a useful translational research model due to many similarities with human. The present study developed an approach to differentiate ext- and int-cholinergic innervation of the porcine colon using double immunolabeling with a novel mouse antibody against human peripheral choline acetyltransferase (hpChAT) combined with a rabbit antibody anti-common ChAT (cChAT) respectively. The spatial configuration of ext- and int- cholinergic innervation in the ENS was assessed on 3D images generated from CLARITY-cleared porcine colonic samples. Their densities were computationally quantitated using Imaris 9 within the inner and outer submucosal and myenteric plexuses (ISP, OSP, MP) along the proximal, transverse and distal colonic regions (pC, tC, dC) collected from 18 adult Yucatan minipigs including 6 naïve (3 of each sex), 4 vehicle controls (2 of each sex), 8 treated with loperamide, a μ opioid receptor agonist (LOP, 0.4 or 3 mg/kg/day, po, 4 weeks, 4/group, 2 of each sex). The specificities of cChAT and hpChAT antibodies were confirmed with porcine cervical vagal trunk (CVT), sacral spinal cord (SSC, S2-S4) and colonic ENS. The double labeling showed the strong cChAT immunoreactive (+) fibers in the CVT and neuronal somata and fibers in the SSC, but no hpChAT+ labeling was visualized in these structures. In the colonic MP, the dense hpChAT+ neurons and fibers and varicose cChAT+ fibers were simultaneously visualized. 3D images demonstrated that int- cholinergic neurons (hpChAT+) were closely surrounded by ext- cholinergic varicose fibers and dot like structures (cChAT+), presumably nerve terminals. The density of cChAT+ but not hpChAT+ fibers+somata showed significant differences among three segments (dC&gt;tC&gt;pC) within three enteric plexuses (MP&gt;OSP&gt;ISP) in naïve pigs. Compared to control groups, the high dose LOP induced a 1.3-fold decrease of cChAT+ fiber density in males (p&lt;0.05) and 1.5-fold in females (p&lt;0.05), and 2.2-fold decrease of hpChAT+ fibers+somata density in males (p&lt;0.05) and 3.6-fold in female (p&lt;0.01) with a 1.5-fold greater decrease in females than males (p&lt;0.05) in the MP of the dC. This study demonstrates for the first time the ext- and int- cholinergic innervation, the spatial configuration in 3D structure, regional variations in the porcine colonic ENS and the reduction by chronic opiate use with greater in females than males. These findings provide neuroanatomical evidence of CNS-gut cross-talk via cholinergic innervation of the colon and point to the neuromodulation of this pathway to alleviate functional colonic disorders like chronic opioid-induced constipation. Supported by NIH/SPARC OT2OD024899. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The effect of neutrophils on limited hepatitis C virus vaccine immunogenicity in people who inject drugs

Abstract In the only efficacy trial of a hepatitis C virus (HCV) vaccine, the vector-based vaccine lowered peak HCV RNA levels in people who inject drugs (PWIDs) but did not prevent chronic HCV infection. The same vaccine regimen tested in healthy controls (HCs) resulted in a larger percentage of individuals responding to vaccination (100 vs. 78%) and a 3-fold higher magnitude of T cell responses compared to PWIDs. We evaluated baseline immune characteristics of PWIDs to understand the reduced responses, ruling out differences in baseline anti-vector immunity, subclinical HCV exposure, and increased T regs. However, we found markedly elevated neutrophil counts in PWIDs at baseline compared to HCs. Elevated neutrophils were previously linked to nonresponse to hepatitis B vaccination in HCs, yet this phenomenon has been largely understudied. We analyzed baseline neutrophil phenotypes using flow cytometry and identified elevated PD-L1, CD11b, and CD63 on neutrophils from PWIDs compared to HCs. Stimulation of HC neutrophils with opioid receptor (OR) agonist DAMGO significantly upregulated PD-L1 and CD11b, an effect partially reversed with an OR antagonist. We also observed metabolic reprogramming of neutrophils in PWIDs compared to HCs, with higher expression of glycolytic machinery (GLUT1/HK2) and mitochondrial membrane protein VDAC1, indicative of increased mitochondrial mass and use of oxidative phosphorylation. Elevated baseline CD11b and CD63 suggest dysregulation and activation of neutrophils, with increased PD-L1 potentially contributing to poor T cell responses in PWIDs. Mechanistic studies of dysregulated neutrophil interactions with T cells may provide insights into vaccine-induced responses and future vaccine efforts. Supported by NIH grant U19 AI159822.

Climbing behavior by mice as an endpoint for preclinical assessment of drug effects in the absence and presence of pain.

This study evaluated climbing in mice as a tool to assess the expression and treatment of pain-related behavioral depression in male and female ICR mice. Mice were videotaped during 10-min sessions in a vertical plexiglass cylinder with wire mesh walls, and "Time Climbing" was scored by observers blind to treatments. Initial validation studies demonstrated that baseline climbing was stable across repeated days of testing and depressed by intraperitoneal injection of dilute lactic acid (IP acid) as an acute pain stimulus. Additionally, IP acid-induced depression of climbing was blocked by the positive-control non-steroidal anti-inflammatory drug (NSAID) ketoprofen but not by the negative control kappa opioid receptor agonist U69593. Subsequent studies examined effects of single-molecule opioids (fentanyl, buprenorphine, naltrexone) and of fixed-proportion fentanyl/naltrexone mixtures (10:1, 3.2:1, and 1:1) that vary in their efficacy at the mu opioid receptor (MOR). Opioids administered alone produced a dose- and efficacy-dependent decrease in climbing, and fentanyl/naltrexone-mixture data indicated that climbing in mice is especially sensitive to disruption by even low-efficacy MOR activation. Opioids administered as a pretreatment to IP acid failed to block IP acid-induced depression of climbing. Taken together, these findings support the utility of climbing in mice as an endpoint to evaluate candidate-analgesic effectiveness both to (a) produce undesirable behavioral disruption when the test drug is administered alone, and (b) produce a therapeutic blockade of pain-related behavioral depression. The failure of MOR agonists to block IP acid-induced depression of climbing likely reflects the high sensitivity of climbing to disruption by MOR agonists.

Difelikefalin for the treatment of moderate-to-severe pruritus associated with chronic kidney disease on hemodialysis

ABSTRACT Introduction Chronic kidney disease-associated pruritus (CKD-aP) is often experienced by patients with CKD receiving dialysis. Approximately 40% of hemodialysis patients are ‘moderately’ to ‘extremely bothered’ by itching, associated with reduced quality of life, poor sleep quality, and depression as well as worse clinical outcomes, including increased medication use, infections, hospitalizations, and mortality. Areas covered This review covers the pathophysiology and treatment landscape of CKD-aP, and the development, clinical efficacy, and safety profile of difelikefalin. We summarize the existing evidence, and discuss both the position of difelikefalin in the treatment pathway and potential future developments. Expert opinion Difelikefalin is a kappa opioid receptor agonist, with a primary mode of action that is outside of the central nervous system and provides an improved safety profile compared with other opioid agonists, with limited potential for abuse and dependency. Difelikefalin has demonstrated efficacy, tolerability, and safety profile in several large-scale clinical trials in more than 1,400 hemodialysis patients with CKD-aP treated for up to 64 weeks. Difelikefalin is the only approved treatment for CKD-aP in the U.S.A and Europe; other treatments are used off-label, have limited proof of efficacy in large-scale clinical trials in this patient population, and may present an increased risk of toxicity in patients with CKD.

Pharmacokinetic and Pharmacodynamic Consequences of Cytochrome P450 3A Inhibition on Mitragynine Metabolism in Rats.

Mitragynine, an opioidergic alkaloid present in Mitragyna speciosa (kratom), is metabolized by cytochrome P450 3A (CYP3A) to 7-hydroxymitragynine, a more potent opioid receptor agonist. The extent to which conversion to 7-hydroxymitragynine mediates the in vivo effects of mitragynine is unclear. The current study examined how CYP3A inhibition (ketoconazole) modifies the pharmacokinetics of mitragynine in rat liver microsomes in vitro. The study further examined how ketoconazole modifies the discriminative stimulus and antinociceptive effects of mitragynine in rats. Ketoconazole [30 mg/kg, oral gavage (o.g.)] increased systemic exposure to mitragynine (13.3 mg/kg, o.g.) by 120% and 7-hydroxymitragynine exposure by 130%. The unexpected increase in exposure to 7-hydroxymitragynine suggested that ketoconazole inhibits metabolism of both mitragynine and 7-hydroxymitragynine, a finding confirmed in rat liver microsomes. In rats discriminating 3.2 mg/kg morphine from vehicle under a fixed-ratio schedule of food delivery, ketoconazole pretreatment increased the potency of both mitragynine (4.7-fold) and 7-hydroxymitragynine (9.7-fold). Ketoconazole did not affect morphine's potency. Ketoconazole increased the antinociceptive potency of 7-hydroxymitragynine by 4.1-fold. Mitragynine (up to 56 mg/kg, i.p.) lacked antinociceptive effects both in the presence and absence of ketoconazole. These results suggest that both mitragynine and 7-hydroxymitragynine are cleared via CYP3A and that 7-hydroxymitragynine is formed as a metabolite of mitragynine by other routes. These results have implications for kratom use in combination with numerous medications and citrus juices that inhibit CYP3A. SIGNIFICANCE STATEMENT: Mitragynine is an abundant kratom alkaloid that exhibits low efficacy at the μ-opioid receptor (MOR). Its metabolite, 7-hydroxymitragynine, is also an MOR agonist but with higher affinity and efficacy than mitragynine. Our results in rats demonstrate that cytochrome P450 3A (CYP3A) inhibition can increase the systematic exposure of both mitragynine and 7-hydroxymitragynine and their potency to produce MOR-mediated behavioral effects. These data highlight potential interactions between kratom and CYP3A inhibitors, which include numerous medications and citrus juices.

G Protein-Biased Mu Opioid Receptor Agonist SR-17018 Has Low In Vivo Efficacy In Non-Human Primates

G Protein-Biased Mu Opioid Receptor Agonist SR-17018 Has Low In Vivo Efficacy In Non-Human Primates

KNT-127, a selective delta opioid receptor agonist, shows beneficial effects in the hippocampal dentate gyrus of a chronic vicarious social defeat stress mouse model

Delta opioid receptors (DOPs) play an important role in depression and other mood disorders. However, little is known about the underlying physiological mechanisms. The hypothalamic–pituitary–adrenal axis, adult hippocampal neurogenesis, and neuroinflammation are regarded as key pathophysiological factors in depression. In this study, we investigated the influence of DOP activation on those factors in a valid animal model of depression, chronic vicarious social defeat stress (cVSDS) mice. cVSDS mice (male C57BL/6J mice) were produced following a 10-day exposure to witness of social defeat stress, and each evaluation was performed more than 28 days after the stress period. Repeated administrations to cVSDS mice with a selective DOP agonist, KNT-127, both during (10 days) and after (28 days) the stress period respectively improved their decreased social interaction behaviors and increased serum corticosterone levels. When administered during the stress period, KNT-127 suppressed decreases in the hippocampal newborn neuron survival rate in cVSDS mice. Moreover, in both administration paradigms, KNT-127 reduced the number of Iba-1- and CD11b-positive cells in the subgranular zone and the granule cell layer of the hippocampal dentate gyrus, indicating a suppression of cVSDS-induced microglial overactivation. These results suggest that KNT-127 acts over the hypothalamic–pituitary–adrenal axis and regulates neurogenesis and neuroinflammation resulting in anti-stress effects, and the antidepressant-like effects of the DOP agonist are implicated in the suppression of the neuroinflammation. This study presents a new finding on the effects of repeated DOP activations on the pathophysiological states of depression.

In vivo photopharmacology with a caged mu opioid receptor agonist drives rapid changes in behavior.

Photoactivatable drugs and peptides can drive quantitative studies into receptor signaling with high spatiotemporal precision, yet few are compatible with behavioral studies in mammals. We developed CNV-Y-DAMGO-a caged derivative of the mu opioid receptor-selective peptide agonist DAMGO. Photoactivation in the mouse ventral tegmental area produced an opioid-dependent increase in locomotion within seconds of illumination. These results demonstrate the power of in vivo photopharmacology for dynamic studies into animal behavior.

Opioids in the Treatment of Chronic Idiopathic Diarrhea in Humans—A Systematic Review and Treatment Guideline

In patients with chronic idiopathic diarrhea resistant to standard treatment, opioids are often used as rescue therapy. This systematic review investigated opioid effects on gut function in chronic diarrhea. PubMed and Embase were searched regarding effects of opioid agonists on the gastrointestinal tract in humans with chronic or experimentally induced diarrhea. A total of 1472 relevant articles were identified and, after thorough evaluation, 11 clinical trials were included. Generally, studies reported a reduction in stool frequency and an increase in transit time during treatment with the opioid receptor agonists loperamide, asimadoline, casokefamide, and codeine compared with placebo. Loperamide and diphenoxylate significantly improved stool consistency compared with placebo, whereas asimadoline showed no such effects. Compared with placebo, loperamide treatment caused less abdominal pain and urgency. Asimadoline showed no significant subjective improvements, but fedotozine was superior to placebo in reducing abdominal pain and bloating in selected patients. Only two relevant studies were published within the last 20 years, and standardized endpoint measures are lacking. Most trials included few participants, and further evidence is needed from larger, prospective studies. Likewise, consensus is needed to standardize endpoints for stool frequency, transit time, and consistency to conduct future meta-analyses on opioids in management of chronic idiopathic diarrhea.

Slowed gastrointestinal transit is associated with an altered caecal microbiota in an aged rat model.

Gastrointestinal (GI) motility is largely dependent upon activity within the enteric nervous system (ENS) and is an important part of the digestive process. Dysfunction of the ENS can impair GI motility as is seen in the case of constipation where gut transit time is prolonged. Animal models mimicking symptoms of constipation have been developed by way of pharmacological manipulations. Studies have reported an association between altered GI motility and gut microbial population. Little is known about the changes in gut microbiota profile resulting specifically from pharmacologically induced slowed GI motility in rats. Moreover, the relationship between gut microbiota and altered intestinal motility is based on studies using faecal samples, which are easier to obtain but do not accurately reflect the intestinal microbiome. The aim of this study was to examine how delayed GI transit due to opioid receptor agonism in the ENS modifies caecal microbiota composition. Differences in caecal microbial composition of loperamide-treated or control male Sprague Dawley rats were determined by 16S rRNA gene amplicon sequencing. The results revealed that significant differences were observed at both genus and family level between treatment groups. Bacteroides were relatively abundant in the loperamide-induced slowed GI transit group, compared to controls. Richness and diversity of the bacterial communities was significantly lower in the loperamide-treated group compared to the control group. Understanding the link between specific microbial species and varying transit times is crucial to design interventions targeting the microbiome and to treat intestinal motility disorders.

A2 ACUTE COLITIS CAUSES CHANGES TO THE EFFICACY OF CANNABINOID-1 AND MU-OPIOID RECEPTOR AGONISTS ON INHIBITING ABDOMINAL PAIN

Abstract Background Abdominal pain is a debilitating symptom in patients with inflammatory bowel disease. Previously we have shown that both cannabinoid-1 receptor (CB1R) and mu opioid receptor (MOR) agonists inhibit mechanosensitivity of colonic nociceptive nerves in healthy mice. However, it is unknown whether CBR and MOR agonists continue to have effects during colitis. Purpose To determine the effects of CBR and MOR agonists on colonic nociceptive nerves during acute colitis. Method Colitis was induced in male and female C57BL/6 mice using 2.5% dextran sodium sulfate in drinking water. Visceromotor response (VMR) to colorectal distention (CRD) (volume range 20-80 µL) was measured using telemetric transmitters. Mice were injected intraperitoneally with vehicle, ACEA, a selective CB1R agonist and/or morphine, a MOR agonist, 30-minutes prior to distention. Extracellular afferent nerve recordings were obtained from ex vivo flat sheet preparations of mouse distal colon. Mechanosensitivity of single afferent axons was assessed via mechanical probing of the colon with a 1g von Frey hair before and after superfusion of ACEA and/or DAMGO (MOR agonist). Data were analyzed using a one- or two-way ANOVA with Bonferroni test. N denotes number of mice; n denotes number of single afferent axons. Result(s) ACEA (3 mg/kg), a dose that significantly inhibited VMR in healthy mice, did not inhibit VMR in mice with colitis (p=0.55, N=6). At a dose that previously had no effect in healthy controls, morphine (0.3 mg/kg) significantly inhibited VMR to CRD, when compared to vehicle (34% reduction, p&amp;lt;0.01, N = 8). A combination of a sub-analgesic dose of ACEA (0.3 mg/kg) with morphine (0.3 mg/kg) significantly reduced VMR (p&amp;lt;0.01, N=5); at 60 µL and 80 µL distention there was a 44% and 49% reduction, respectively (p&amp;lt;0.05 for both). Interestingly, the effect of the combination of ACEA and morphine was larger than that of morphine alone (0.3 mg/kg), but this did not reach statistical significance (-62% vs. -34%; p=0.06, N=5-8). In extracellular afferent nerve recordings, compared to the previous findings in healthy mice, a higher concentration of ACEA (10 µM) was required to inhibit mechanosensitivity (15.2 vs. 11.6 Hz; p&amp;lt;0.05, n=11, N=6) whereas 100nM (p&amp;gt;0.99, n=7, N=5) and 1µM (p=0.25, n=10, N=6) had no effect. DAMGO (1 nM), which previously had no effect in healthy controls, had a tendency to reduce colonic mechanosensitivity, but this did not reach statistical significance (p=0.12, n=7 units, N=5). Interestingly, a combination of sub-analgesic concentrations of ACEA (100 nM) and DAMGO (1 nM) significantly reduced colonic mechanosensitivity (18.4 vs. 12.0 Hz; p&amp;lt;0.05, n=7, N=5). Conclusion(s) At doses that previously inhibited nociception in healthy mice, CB1R agonists may have lost their analgesic effect during acute colitis. Conversely, less of MOR agonists may be needed to achieve analgesia. Interestingly, sub-analgesic doses of CB1R agonists potentiate the analgesic effect of the MOR agonist during colitis. Please acknowledge all funding agencies by checking the applicable boxes below NRC Disclosure of Interest None Declared