Role of Efficacy as a Determinant of Locomotor Activation by Mu Opioid Receptor Ligands in Female and Male Mice

Neurocircuitry underlying the actions of glucagon-like peptide 1 and peptide YY3-36 in the suppression of food, drug-seeking, and anxiogenesis

Obesity is a critical health condition worldwide that increases the risks of comorbid chronic diseases, but it can be managed with weight loss. However, conventional interventions relying on diet and exercise are inadequate for achieving and maintaining weight loss, thus there is significant market interest for pharmaceutical anti-obesity agents. For decades, receptor agonists for the gut peptide glucagon-like peptide 1 (GLP-1) featured prominently in anti-obesity medications by suppressing appetite and food reward to elicit rapid weight loss. As the neurocircuitry underlying food motivation overlaps with that for drugs of abuse, GLP-1 receptor agonism has also been shown to decrease substance use and relapse, thus its therapeutic potential may extend beyond weight management to treat addictions. However, as prolonged use of anti-obesity drugs may increase the risk of mood-related disorders like anxiety and depression, and individuals taking GLP-1-based medication commonly report feeling demotivated, the long-term safety of such drugs is an ongoing concern. Interestingly, current research now focuses on dual agonist approaches that include GLP-1 receptor agonism to enable synergistic effects on weight loss or associated functions. GLP-1 is secreted from the same intestinal cells as the anorectic gut peptide, Peptide YY3-36 (PYY3-36), thus this review assessed the therapeutic potential and underlying neural circuits targeted by PYY3-36 when administered independently or in combination with GLP-1 to curb the appetite for food or drugs of abuse like opiates, alcohol, and nicotine. Additionally, we also reviewed animal and human studies to assess the impact, if any, for GLP-1 and/or PYY3-36 on mood-related behaviors in relation to anxiety and depression. As dual agonists targeting GLP-1 and PYY3-36 may produce synergistic effects, they can be effective at lower doses and offer an alternative approach for therapeutic benefits while mitigating undesirable side effects.

Unique pharmacodynamic properties and low abuse liability of the µ-opioid receptor ligand (S)-methadone

(R,S)-methadone ((R,S)-MTD) is a µ-opioid receptor (MOR) agonist comprised of (R)-MTD and (S)-MTD enantiomers. (S)-MTD is being developed as an antidepressant and is considered an N-methyl-D-aspartate receptor (NMDAR) antagonist. We compared the pharmacology of (R)-MTD and (S)-MTD and found they bind to MORs, but not NMDARs, and induce full analgesia. Unlike (R)-MTD, (S)-MTD was a weak reinforcer that failed to affect extracellular dopamine or induce locomotor stimulation. Furthermore, (S)-MTD antagonized motor and dopamine releasing effects of (R)-MTD. (S)-MTD acted as a partial agonist at MOR, with complete loss of efficacy at the MOR-galanin Gal1 receptor (Gal1R) heteromer, a key mediator of the dopaminergic effects of opioids. In sum, we report novel and unique pharmacodynamic properties of (S)-MTD that are relevant to its potential mechanism of action and therapeutic use. One-sentence summary: (S)-MTD, like (R)-MTD, binds to and activates MORs in vitro, but (S)-MTD antagonizes the MOR-Gal1R heteromer, decreasing its abuse liability.

Alkoxy chain length governs the potency of 2-benzylbenzimidazole 'nitazene' opioids associated with human overdose

RATIONALE

Novel synthetic opioids (NSOs) are emerging in recreational drug markets worldwide. In particular, 2-benzylbenzimidazole 'nitazene' compounds are problematic NSOs associated with serious clinical consequences, including fatal respiratory depression. Evidence from in vitro studies shows that alkoxy chain length can influence the potency of nitazenes at the mu-opioid receptor (MOR). However, structure-activity relationships (SARs) of nitazenes for inducing opioid-like effects in animal models are not well understood compared to relevant opioids contributing to the ongoing opioid crisis (e.g., fentanyl).

OBJECTIVES

Here, we examined the in vitro and in vivo effects of nitazene analogues with varying alkoxy chain lengths (i.e., metonitazene, etonitazene, isotonitazene, protonitazene, and butonitazene) as compared to reference opioids (i.e., morphine and fentanyl).

METHODS AND RESULTS

Nitazene analogues displayed nanomolar affinities for MOR in rat brain membranes and picomolar potencies to activate MOR in transfected cells. All compounds induced opioid-like effects on locomotor activity, hot plate latency, and body temperature in male mice, and alkoxy chain length markedly influenced potency. Etonitazene, with an ethoxy chain, was the most potent analogue in MOR functional assays (EC50 = 30 pM, Emax = 103%) and across all in vivo endpoints (ED50 = 3-12 μg/kg). In vivo SARs revealed that ethoxy, isopropoxy, and propoxy chains engendered higher potencies than fentanyl, whereas methoxy and butoxy analogues were less potent. MOR functional potencies, but not MOR affinities, were positively correlated with in vivo potencies to induce opioid effects.

CONCLUSIONS

Overall, our data show that certain nitazene NSOs are more potent than fentanyl as MOR agonists in mice, highlighting concerns regarding the high potential for overdose in humans who are exposed to these compounds.

Endogenous opiates and behavior: 2022

This paper is the forty-fifth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2022 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Role of mu opioid receptor (MOR) agonist efficacy as a determinant of opioid antinociception in a novel assay of pain-depressed behavior in female and male mice

Introduction

Intermediate efficacy mu opioid receptor (MOR) agonists have potential to retain analgesic effectiveness while improving safety, but the optimal MOR efficacy for effective and safe opioid analgesia is unknown. Preclinical assays of pain-depressed behavior can assess effects of opioids and other candidate analgesics on pain-related behavioral depression, which is a common manifestation of clinically relevant pain and target of pain treatment. Accordingly, the present study goal was to validate a novel assay of pain-depressed locomotor behavior in mice and evaluate the role of MOR efficacy as a determinant of opioid analgesic effects and related safety measures.

Methods

Male and female ICR mice were tested in a locomotor chamber consisting of 2 compartments connected by a doorway that contained a 1-inch-tall barrier. Dependent measures during 15-min behavioral sessions included crosses between compartments (which required vertical activity to surmount the barrier) and total movement counts (which required horizontal activity to break photobeams in each compartment).

Results and Discussion

Intraperitoneal injection of lactic acid (IP acid) produced a concentration- and time-dependent depression of both endpoints. Optimal blockade of IP acid-induced behavioral depression with minimal motor impairment was achieved with intermediate-efficacy MOR treatments that also produced less gastrointestinal-transit inhibition and respiratory depression than the high-efficacy MOR agonist fentanyl. Sex differences in treatment effects were rare. Overall, these findings validate a novel procedure for evaluating opioids and other candidate analgesic effects on pain-related behavioral depression in mice and support continued research with intermediate-efficacy MOR agonists as a strategy to retain opioid analgesic effectiveness with improved safety.

Hyperactivity in Mice Induced by Opioid Agonists with Partial Intrinsic Efficacy and Biased Agonism Administered Alone and in Combination with Morphine

Opioid analgesics such as morphine and fentanyl induce mu-opioid receptor (MOR)-mediated hyperactivity in mice. Herein, we show that morphine, fentanyl, SR-17018, and oliceridine have submaximal intrinsic efficacy in the mouse striatum using ³⁵S-GTPγS binding assays. While all of the agonists act as partial agonists for stimulating G protein coupling in striatum, morphine, fentanyl, and oliceridine are fully efficacious in stimulating locomotor activity; meanwhile, the noncompetitive biased agonists SR-17018 and SR-15099 produce submaximal hyperactivity. Moreover, the combination of SR-17018 and morphine attenuates hyperactivity while antinociceptive efficacy is increased. The combination of oliceridine with morphine increases hyperactivity, which is maintained over time. These findings provide evidence that noncompetitive agonists at MOR can be used to suppress morphine-induced hyperactivity while enhancing antinociceptive efficacy; moreover, they demonstrate that intrinsic efficacy measured at the receptor level is not directly proportional to drug efficacy in the locomotor activity assay.

Toxicity and behavioural effects of ocfentanil and 2-furanylfentanyl in zebrafish larvae and mice

The introduction of the so-called New Psychoactive Substances represents a problem of global concern due to several factors, including multiplicity of structures, poorly known activity, short half-life in the market, lack of pure standards etc. Among these problems, of the highest relevance is also the lack of information about metabolism and adverse effects, which must be faced using simple and low-cost animal models. On these grounds, the present work has been carried out on 5 days post fertilization zebrafish (Danio rerio) larvae in comparison with adult mice (Mus musculus). Ocfentanil and 2-furanylfentanyl were administered at different concentrations to zebrafish larvae (1, 10 µM) and mice (0.1, 1, 6, 15 mg/kg). The behavioural assay showed a decrease in basal locomotor activity in zebrafish, whereas in mice this effect was evident only after the mechanical stimulus. Larva extracts and mice urine were analysed by using liquid chromatography coupled to high resolution mass spectrometry to identify the metabolic pathways of the fentanyl analogs. For 2-furanylfentanyl, the most common biotransformations observed were hydroxylation, hydration and oxidation in zebrafish larvae, whereas mice produced mainly the dihydrodiol metabolite. Hydroxylation was the major route of metabolism for ocfentanil in zebrafish larvae, while in mice the O-demethylated derivative was the main metabolite. In addition, a study was conducted to evaluate morphological effects of the two drugs on zebrafish larvae. Malformations were noticeable only at the highest concentration of 2-furanylfentanyl, whereas no significant damage was observed with ocfentanil. In conclusion, the two animal models show similarities in behavioral response and in metabolism, considering the different biological investigated.

Structurally diverse fentanyl analogs yield differential locomotor activities in mice

Synthetic narcotics have been implicated as the single greatest contributor to increases in opioid-related fatalities in recent years. This study evaluated the effects of nine fentanyl-related substances that have emerged in the recreational drug marketplace, and for which there are no existing or only limited in vivo data. Adult male Swiss Webster mice were administered fentanyl-related substances and their effects on locomotion as compared to MOR agonist standards were recorded. In locomotor activity tests, morphine (100, 180 mg/kg), buprenorphine (1, 10 mg/kg), fentanyl (1, 10 mg/kg), cyclopropylfentanyl (1, 10 mg/kg), cyclopentylfentanyl (10 mg/kg), (±)-cis-3-methylbutyrylfentanyl (0.1, 1, 10 mg/kg), ortho-methylacetylfentanyl (10 mg/kg), para-chloroisobutyrylfentanyl (100 mg/kg), ocfentanil (1, 10 mg/kg), and ortho-fluoroacrylfentanyl (0.1, 1, 10 mg/kg) elicited significant (p ≤ 0.05) dose-dependent increases in locomotion. However, 2,2,3,3-tetramethylcyclopropylfentanyl did not have any effects on locomotion, even when tested up to 100 mg/kg, and 4'-methylacetylfentanyl (10, 100 mg/kg) significantly decreased locomotion. The rank order of efficacy for stimulating locomotion (maximum effect as a % of fentanyl's maximum effect) for fentanyl-related substances relative to MOR agonist standards was cyclopropylfentanyl (108.84 ± 20.21) > fentanyl (100 ± 15.3) > ocfentanil (79.27 ± 16.92) > morphine (75.9 ± 14.5) > (±)-cis-3-methylbutyrylfentanyl (68.04 ± 10.08) > ortho-fluoroacrylfentanyl (63.56 ± 19.88) > cyclopentylfentanyl (56.46 ± 8.54) > para-chloroisobutyrylfentanyl (22.44 ± 8.51) > buprenorphine (11.26 ± 2.30) > ortho-methylacetylfentanyl (9.45 ± 2.92) > 2,2,3,3-tetramethylcyclopropylfentanyl (6.75 ± 1.43) > 4'-methylacetylfentanyl (3.47 ± 0.43). These findings extend in vivo results from previous reports documenting additional fentanyl related-related substances that stimulate locomotion similar to known abused opioids while also identifying some anomalies.

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.