Chemogenetic Manipulation of Amygdala Kappa Opioid Receptor Neurons Modulates Amygdala Neuronal Activity and Neuropathic Pain Behaviors.

Neuroplasticity in the central nucleus of the amygdala (CeA) plays a key role in the modulation of pain and its aversive component. The dynorphin/kappa opioid receptor (KOR) system in the amygdala is critical for averse-affective behaviors in pain conditions, but its mechanisms are not well understood. Here, we used chemogenetic manipulations of amygdala KOR-expressing neurons to analyze the behavioral consequences in a chronic neuropathic pain model. For the chemogenetic inhibition or activation of KOR neurons in the CeA, a Cre-inducible viral vector encoding Gi-DREADD (hM4Di) or Gq-DREADD (hM3Dq) was injected stereotaxically into the right CeA of transgenic KOR-Cre mice. The chemogenetic inhibition of KOR neurons expressing hM4Di with a selective DREADD actuator (deschloroclozapine, DCZ) in sham control mice significantly decreased inhibitory transmission, resulting in a shift of inhibition/excitation balance to promote excitation and induced pain behaviors. The chemogenetic activation of KOR neurons expressing hM3Dq with DCZ in neuropathic mice significantly increased inhibitory transmission, decreased excitability, and decreased neuropathic pain behaviors. These data suggest that amygdala KOR neurons modulate pain behaviors by exerting an inhibitory tone on downstream CeA neurons. Therefore, activation of these interneurons or blockade of inhibitory KOR signaling in these neurons could restore control of amygdala output and mitigate pain.

Discovery of an Ortho-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaine Derivative as a Selective and Potent Kappa Opioid Receptor Agonist with Subsided Sedative Effect.

Research into kappa opioid receptor (KOR) agonists with attenuated central-nervous-system side effects is a critical focus for developing productive and safe analgesics. Herein, a series of ortho-substituted N-cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaines were designed, synthesized, and subjected to bioassays. Compound 7a exhibited high subtype selectivity and potent agonistic activity toward KOR (KOR, Ki = 3.9 nM, MOR/KOR = 270, DOR/KOR = 1075; [35S]GTPγS binding, EC50 = 3.4 nM). Additionally, this compound exhibited robust and persistent antinociceptive effects in rodent models with different animal strains (hot plate test, ED50 = 0.20-0.30 mg/kg, i.p.; abdominal constriction test, ED50 = 0.20-0.60 mg/kg, i.p.), with its KOR-mediated mechanism for antinociception firmly established. Notably, compound 7a, unlike conventional KOR agonists, displayed minimal sedation and aversion at the antinociceptive ED50 dose. This feature addresses a crucial limitation in existing KOR agonists, positioning compound 7a as a promising novel therapeutic agent.

Single-chain fluorescent integrators for mapping G-protein-coupled receptor agonists.

G protein-coupled receptors (GPCRs) transduce the effects of many neuromodulators including dopamine, serotonin, epinephrine, acetylcholine, and opioids. The localization of synthetic or endogenous GPCR agonists impacts their action on specific neuronal pathways. In this paper, we show a series of single-protein chain integrator sensors that are highly modular and could potentially be used to determine GPCR agonist localization across the brain. We previously engineered integrator sensors for the mu- and kappa-opioid receptor agonists called M- and K-Single-chain Protein-based Opioid Transmission Indicator Tool (SPOTIT), respectively. Here, we engineered red versions of the SPOTIT sensors for multiplexed imaging of GPCR agonists. We also modified SPOTIT to create an integrator sensor design platform called SPOTIT for all GPCRs (SPOTall). We used the SPOTall platform to engineer sensors for the beta 2-adrenergic receptor (B2AR), the dopamine receptor D1, and the cholinergic receptor muscarinic 2 agonists. Finally, we demonstrated the application of M-SPOTIT and B2AR-SPOTall in detecting exogenously administered morphine, isoproterenol, and epinephrine in the mouse brain via locally injected viruses. The SPOTIT and SPOTall sensor design platform has the potential for unbiased agonist detection of many synthetic and endogenous neuromodulators across the brain.

Effects of sex and hydration status on kappa opioid receptor-mediated diuresis in rats.

Understanding the function of the kappa opioid receptor (KOP) is crucial for the development of novel therapeutic interventions that target KOP for the treatment of pain, stress-related disorders and other indications. Activation of KOP produces diuretic effects in rodents and man. Sex is a vital factor to consider when assessing drug response in pre-clinical and clinical studies. In this study, the diuretic effect of the KOP agonist, U50488 (1-10 mg/kg), was investigated in both adult female and male Wistar rats that were either normally hydrated or water-loaded. The KOP antagonist norbinaltorphimine (norBNI, 10 mg/kg) was administered 24 h prior to U50488 to confirm the involvement of KOP. U50488 elicited a significant diuretic response at doses ≥ 3 mg/kg in both female and male rats independent of hydration status. U50488 diuretic effects were inhibited by norBNI pre-administration. Water-loading reduced data variability for urine volume in males, but not in females, compared with normally hydrated rats. Sex differences were also evident in U50488 eliciting a significant increase in sodium and potassium ion excretion only in males. This may suggest different mechanisms of U50488 diuretic action in males where renal excretion mechanisms are directly affected more than in females.

LY2444296, a κ-opioid receptor antagonist, selectively reduces alcohol drinking in male and female Wistar rats with a history of alcohol dependence.

Alcohol use disorder (AUD) remains a major public health concern. The dynorphin (DYN)/κ-opioid receptor (KOP) system is involved in actions of alcohol, particularly its withdrawal-associated negative affective states. This study tested the ability of LY2444296, a selective, short-acting, KOP antagonist, to decrease alcohol self-administration in dependent male and female Wistar rats at 8 h abstinence. Animals were trained to orally self-administer 10% alcohol (30 min/day for 21 sessions) and were made dependent via chronic intermittent alcohol vapor exposure for 6 weeks or exposed to air (nondependent). After 6 weeks, the effect of LY2444296 (0, 3, and 10 mg/kg, p.o.) was tested on alcohol self-administration at 8 h of abstinence. A separate cohort of rats was prepared in parallel, and their somatic withdrawal signs and alcohol self-administration were measured after LY2444296 administration at 8 h, 2 weeks, and 4 weeks abstinence. LY2444296 at 3 and 10 mg/kg significantly reduced physical signs of withdrawal in dependent rats at 8 h abstinence, only. Furthermore, 3 and 10 mg/kg selectively decreased alcohol self-administration in dependent rats at only 8 h abstinence. These results highlight the DYN/KOP system in actions of alcohol during acute abstinence, suggesting KOP antagonism could be beneficial for mitigating acute withdrawal signs and, in turn, significantly reduce excessive alcohol consumption associated with AUD.

The Role of Opioid Receptor Antagonists in Regulation of Blood Pressure and T-Cell Activation in Mice Selected for High Analgesia Induced by Swim Stress.

Opioid peptides and their G protein-coupled receptors are important regulators within the cardiovascular system, implicated in the modulation of both heart and vascular functions. It is known that naloxone-an opioid antagonist-may exert a hypertensive effect. Recent experimental and clinical evidence supports the important role of inflammatory mechanisms in hypertension. Since opioids may play a role in the regulation of both blood pressure and immune response, we studied these two processes in our model. We aimed to evaluate the effect of selective and non-selective opioid receptor antagonists on blood pressure and T-cell activation in a mouse model of high swim stress-induced analgesia. Blood pressure was measured before and during the infusion of opioid receptor antagonists using a non-invasive tail-cuff measurement system. To assess the activation of T-cells, flow cytometry was used. We discovered that the non-selective antagonism of the opioid system by naloxone caused a significant elevation of blood pressure. The selective antagonism of µ and κ but not δ opioid receptors significantly increased systolic blood pressure. Subsequently, a brief characterization of T-cell subsets was performed. We found that the blockade of µ and δ receptors is associated with the increased expression of CD69 on CD4 T-cells. Moreover, we observed an increase in the central memory CD4 and central memory CD8 T-cell populations after the δ opioid receptor blockade. The antagonism of the µ opioid receptor increased the CD8 effector and central memory T-cell populations.

Co-targeting the kappa opioid receptor and dopamine transporter reduces motivation to self-administer cocaine and partially reverses dopamine system dysregulation.

Cocaine disrupts dopamine (DA) and kappa opioid receptor (KOR) system activity, with long-term exposure reducing inhibiton of DA uptake by cocaine and increasing KOR system function. Single treatment therapies have not been successful for cocaine use disorder; therefore, this study focuses on a combination therapy targeting the dopamine transporter (DAT) and KOR. Sprague Dawley rats self-administered 5 days of cocaine (1.5 mg/kg/inf, max 40 inf/day, FR1), followed by 14 days on a progressive ratio (PR) schedule (0.19 mg/kg/infusion). Behavioral effects of individual and combined administration of phenmetrazine and nBNI were then examined using PR. Additionally, ex vivo fast scan cyclic voltammetry was then used to assess alterations in DA and KOR system activity in the nucleus accumbens before and after treatments. Chronic administration of phenmetrazine as well as the combination of phenmetrazine and nBNI-but not nBNI alone-significantly reduced PR breakpoints. In addition, the combination of phenmetrazine and nBNI partially reversed cocaine-induced neurodysregulations of the KOR and DA systems, indicating therapeutic benefits of targeting the DA and KOR systems in tandem. These data highlight the potential benefits of the DAT and KOR as dual-cellular targets to reduce motivation to administer cocaine and reverse cocaine-induced alterations of the DA system.

Pharmacologic Characterization of Substituted Nitazenes at µ, κ, and Δ Opioid Receptors Suggests High Potential for Toxicity.

The benzimidazole opioids (substituted nitazenes) are highly potent µ opiod receptor (MOR) agonists with heroin- or fentanyl-like effects. These compounds have caused hospitalizations and fatal overdoses. We characterized the in vitro pharmacology and structure-activity relationships of 19 nitazenes with substitutions at three positions of the benzimidazole core. Affinities were assessed using agonist radioligand binding assays at human µ, κ, and Δ opioid receptors (MOR, KOR, and DOR, respectively) heterologously expressed in CHO cells. Notably, for MOR binding, nine substituted nitazenes had significantly higher affinities than fentanyl including N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, and N-desethyl isotonitazene; 13 had subnanomolar affinities. Only metodesnitazene and flunitazene had significantly lower affinities than fentanyl. Affinities for the substituted nitazenes at KOR and DOR relative to MOR were 46- to 2580-fold and 180- to 1280-fold lower, respectively. Functional activities were assessed using [35S]GTPγS binding assays. Four nitazenes had subnanomolar potencies at MOR: N-pyrrolidino etonitazene, N-pyrrilidino isonitazene, N-pyrrilidino protonitazene and N-desethyl isotonitazene. Ten substituted nitazenes had significantly higher potencies than fentanyl. All tested nitazenes were full MOR agonists. Potencies at KOR and DOR relative to MOR were 7.3- to 7920-fold and 24- to 9400-fold lower, respectively. Thus, many of these compounds are high affinity/high potency MOR agonists with elevated potential to elicit toxicity and overdose at low doses. SIGNIFICANCE STATEMENT: Substituted nitazenes are a growing public health threat. Although the 19 nitazenes tested vary in their opioid receptor pharmacology, a number are very high affinity, high potency, and high efficacy compounds- higher than fentanyl. Their pharmacology suggests high potential for harm.

A Combination of a Dopamine Receptor 2 Agonist and a Kappa Opioid Receptor Antagonist Synergistically Reduces Weight in Diet-Induced Obese Rodents.

As the global obesity rate increases, so does the urgency to find effective anti-obesity drugs. In the search for therapeutic targets, central nervous system (CNS) mechanisms engaged in the regulation of energy expenditure and food intake, such as the opioid and dopamine systems, are crucial. In this study, we examined the effect on body weight of two drugs: bromocriptine (BC), a D2R receptor agonist, and PF-04455242, a selective κ opioid receptor (KOR) antagonist. Using diet-induced obese (DIO) rats, we aimed to ascertain whether the administration of BC and PF-04455242, independently or in combination, could enhance body weight loss. Furthermore, the present work demonstrates that the peripheral coadministration of BC and PF-04455242 enhances the reduction of weight in DIO rats and leads to a decrease in adiposity in a food-intake-independent manner. These effects were based on heightened energy expenditure, particularly through the activation of brown adipose tissue (BAT) thermogenesis. Overall, our findings indicate that the combination of BC and PF-04455242 effectively induces body weight loss through increased energy expenditure by increasing thermogenic activity and highlight the importance of the combined use of drugs to combat obesity.

Coexpressed δ-, µ-, and κ-Opioid Receptors Modulate Voltage-Gated Ca2+ Channels in Gastric-Projecting Vagal Afferent Neurons.

Opioid analgesics are frequently associated with gastrointestinal side effects, including constipation, nausea, dysphagia, and reduced gastric motility. Though it has been shown that stimulation of opioid receptors expressed in enteric motor neurons contributes to opioid-induced constipation, it remains unclear whether activation of opioid receptors in gastric-projecting nodose ganglia neurons contributes to the reduction in gastric motility and emptying associated with opioid use. In the present study, whole-cell patch-clamp recordings were performed to determine the mechanism underlying opioid receptor-mediated modulation of Ca2+ currents in acutely isolated gastric vagal afferent neurons. Our results demonstrate that CaV2.2 channels provide the majority (71% ± 16%) of Ca2+ currents in gastric vagal afferent neurons. Furthermore, we found that application of oxycodone, U-50488, or deltorphin II on gastric nodose ganglia neurons inhibited Ca2+ currents through a voltage-dependent mechanism by coupling to the Gα i/o family of heterotrimeric G-proteins. Because previous studies have demonstrated that the nodose ganglia expresses low levels of δ-opioid receptors, we also determined the deltorphin II concentration-response relationship and assessed deltorphin-mediated Ca2+ current inhibition following exposure to the δ-opioid receptor antagonist ICI 174,864 (0.3 µM). The peak mean Ca2+ current inhibition following deltorphin II application was 47% ± 24% (EC50 = 302.6 nM), and exposure to ICI 174,864 blocked deltorphin II-mediated Ca2+ current inhibition (4% ± 4% versus 37% ± 20%). Together, our results suggest that analgesics targeting any opioid receptor subtype can modulate gastric vagal circuits. SIGNIFICANCE STATEMENT: This study demonstrated that in gastric nodose ganglia neurons, agonists targeting all three classical opioid receptor subtypes (µ, δ, and κ) inhibit voltage-gated Ca2+ channels in a voltage-dependent mechanism by coupling to Gαi/o. These findings suggest that analgesics targeting any opioid receptor subtype would modulate gastric vagal circuits responsible for regulating gastric reflexes.

Identification and In Vivo Evaluation of Myelination Agent PIPE-3297, a Selective Kappa Opioid Receptor Agonist Devoid of ß-Arrestin-2 Recruitment Efficacy.

Structure-activity relationship studies led to the discovery of PIPE-3297, a fully efficacious and selective kappa opioid receptor (KOR) agonist. PIPE-3297, a potent activator of G-protein signaling (GTPγS EC50 = 1.1 nM, 91% Emax), did not elicit a ß-arrestin-2 recruitment functional response (Emax < 10%). Receptor occupancy experiments performed with the novel KOR radiotracer [3H]-PIPE-3113 revealed that subcutaneous (s.c.) administration of PIPE-3297 at 30 mg/kg in mice achieved 90% occupancy of the KOR in the CNS 1 h post dose. A single subcutaneous dose of PIPE-3297 in healthy mice produced a statistically significant increase of mature oligodendrocytes (P < 0.0001) in the KOR-enriched striatum, an effect that was not observed in animals predosed with the selective KOR antagonist norbinaltorphimine. An equivalent dose given to mice in an open-field activity-monitoring system revealed a small KOR-independent decrease in total locomotor activity versus vehicle measured between 60 and 75 min post dose. Daily doses of PIPE-3297 at both 3 and 30 mg/kg s.c. reduced the disease score in the mouse experimental autoimmune encephalomyelitis (EAE) model. Visually evoked potential (VEP) N1 latencies were also significantly improved versus vehicle in both dose groups, and latencies matched those of untreated animals. Taken together, these findings highlight the potential therapeutic value of functionally selective G-protein KOR agonists in demyelinating disease, which may avoid the sedating side effects typically associated with classical nonbiased KOR agonists.

Nalbuphine-6-glucuronide is a potent analgesic with superior safety profiles by altering binding affinity and selectivity for mu-/kappa-opioid receptors.

Although nalbuphine, a semi-synthetic analgesic compound, is less potent than morphine in terms of alleviating severe pain, our recent findings have revealed that nalbuphine-6-glucuronide (N6G), one of the glucuronide metabolites of nalbuphine, promotes a significantly more robust analgesic effect than its parent drug. Nevertheless, despite these promising observations, the precise mechanisms underlying the analgesic effects of nalbuphine glucuronides have yet to be determined. In this study, we aim to elucidate the mechanisms associated with the analgesic effects of nalbuphine glucuronides. Pharmacokinetic and pharmacodynamic studies were conducted to investigate the relationship between the central and peripheral compartments of nalbuphine and its derivatives. The analgesic responses of these compounds were evaluated based on multiple behavioral tests involving thermal and mechanical stimuli. Radioligand binding assays were also performed to determine the binding affinity and selectivity of these compounds for different opioid receptors. The results of these tests consistently confirmed that the heightened analgesic effects of N6G are mediated through its enhanced binding affinity for both mu- and kappa-opioid receptors, even comparable to those of morphine. Notably, N6G exhibited fewer side effects and did not induce sudden death, thereby highlighting its superior safety profile. Additionally, pharmacokinetic studies indicated that N6G could cross the blood-brain barrier when administered peripherally, offering pain relief. Overall, N6G provides great analgesic efficacy and enhanced safety. These findings highlight the potential value of nalbuphine glucuronides, particularly N6G, as promising candidates for the development of novel analgesic drugs.

Endomorphin-2 analogs with C-terminal esterification display potent antinociceptive effects in the formalin pain test in mice.

Previously, we have investigated three C-terminal esterified endomorphin-2 (EM-2) analogs EM-2-Me, EM-2-Et and EM-2-Bu with methyl, ethyl and tert-butyl ester modifications, respectively. These analogs produced significant antinociception in acute pain at the spinal and supraspinal levels, with reduced tolerance and gastrointestinal side effects. The present study was undertaken to determine the analgesic effects and opioid mechanisms of these three analogs in the formalin pain test. Our results demonstrated that intracerebroventricular (i.c.v.) administration of 0.67-20 nmol EM-2 analogs EM-2-Me, EM-2-Et and EM-2-Bu produced dose-dependent antinociceptive effects in both phase Ⅰ and phase Ⅱ of formalin pain. EM-2-Me and EM-2-Bu displayed more potent antinociception than morphine. Especially, EM-2-Bu exhibited the highest antinociception in phase Ⅱ of formalin pain, with the ED50 value being 2.1 nmol. Naloxone (80 nmol, i.c.v.) completely antagonized the antinociceptive effects of EM-2-Me, EM-2-Et and EM-2-Bu (20 nmol, i.c.v.) in both phase I and phase Ⅱ of formalin pain, suggesting a central opioid mechanism. Nevertheless, the antinociception induced by EM-2-Me might be involved in the release of dynorphin A, which subsequently acted on κ- opioid receptor. EM-2-Bu produced the antinociception probably by the direct activation of both µ- and δ-opioid receptors. EM-2-Me, EM-2-Et and EM-2-Bu also produced significant analgesic effects after peripheral administration, and the central opioid receptors were involved. Furthermore, EM-2-Bu had no influence on the locomotor activity after i.c.v. injection. The present investigation demonstrated that C-terminal esterified modifications of EM-2 will be beneficial for developing novel therapeutics in formalin pain.

Intravenous difelikefalin for the treatment of hemodialysis pruritus.

INTRODUCTION: Patients with chronic kidney disease (CKD) undergoing hemodialysis often experience significant itch secondary to their condition and a subsequent reduction in their overall quality of life. Current treatments are underwhelming, necessitating the search for new, effective therapeutic options to combat itch in this population. AREAS COVERED: The purpose of this review is to explore the available data for the use of intravenous difelikefalin in patients with CKD undergoing hemodialysis. The pathophysiology of CKD-associated itch is multifactorial, with one proposed mechanism involving an imbalance in the endogenous opioid system, favoring upregulation of itch-activating µ-opioid receptors (MORs) and downregulation of itch-inhibiting κ-opioid receptors (KORs). Dysregulation of the immune system is also involved. Difelikefalin is a recent FDA approved treatment that functions as peripherally acting KOR agonist, targeting this imbalance in the endogenous opioid system seen in CKD patients with itch and having an anti-inflammatory effect on immune cells. Clinical data on intravenous difelikefalin is promising regarding its ability to reduce itch in CKD patients on hemodialysis and improve patient quality of life, with few, mild adverse side effects. EXPERT OPINION: As intravenous difelikefalin becomes more widely used in the clinical setting, further studies assessing long-term efficacy and safety will be needed.

Opposing Effects on Descending Control of Nociception by µ and κ Opioid Receptors in the Anterior Cingulate Cortex.

BACKGROUND: The efficiency of descending pain modulation, commonly assessed with the conditioned pain modulation procedure, is diminished in patients with chronic pain. The authors hypothesized that the efficiency of pain modulation is controlled by cortical opioid circuits. METHODS: This study evaluated the effects of µ opioid receptor activation in the anterior cingulate cortex on descending control of nociception, a preclinical correlate of conditioned pain modulation, in male Sprague-Dawley rats with spinal nerve ligation-induced chronic pain or in sham-operated controls. Additionally, the study explored the consequences of respective activation or inhibition of κ opioid receptor in the anterior cingulate cortex of naive rats or animals with neuropathic pain. Descending control of nociception was measured as the hind paw withdrawal response to noxious pressure (test stimulus) in the absence or presence of capsaicin injection in the forepaw (conditioning stimulus). RESULTS: Descending control of nociception was diminished in the ipsilateral, but not contralateral, hind paw of rats with spinal nerve ligation. Bilateral administration of morphine in the anterior cingulate cortex had no effect in shams but restored diminished descending control of nociception without altering hypersensitivity in rats with neuropathic pain. Bilateral anterior cingulate cortex microinjection of κ opioid receptor antagonists, including nor-binaltorphimine and navacaprant, also re-established descending control of nociception in rats with neuropathic pain without altering hypersensitivity and with no effect in shams. Conversely, bilateral injection of a κ opioid receptor agonist, U69,593, in the anterior cingulate cortex of naive rats inhibited descending control of nociception without altering withdrawal thresholds. CONCLUSIONS: Anterior cingulate cortex κ opioid receptor activation therefore diminishes descending control of nociception both in naive animals and as an adaptive response to chronic pain, likely by enhancing net descending facilitation. Descending control of nociception can be restored by activation of µ opioid receptors in the anterior cingulate cortex, but also by κ opioid receptor antagonists, providing a nonaddictive alternative to opioid analgesics. Navacaprant is now in advanced clinical trials.

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors.

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

κ-Opioid receptor activation attenuates osteoarthritis synovitis by regulating macrophage polarization through the NF-κB pathway.

Osteoarthritis (OA) is a prevalent and chronic joint disease that affects the aging population, causing pain and disability. Macrophages in synovium are important mediators of synovial inflammatory activity and pathological joint pain. Previous studies have demonstrated the significant involvement of κ-opioid receptor (KOR) in the regulation of pain and inflammation. Our study reveals a significant reduction in synovial KOR expression among patients and mice with OA. Here, we find that KOR activation effectively inhibits the expressions of the LPS-induced-inflammatory cytokines TNF-α and IL-6 by inhibiting macrophage M1 phenotype. Mechanistically, KOR activation effectively suppresses the proinflammatory factor secretion of macrophages by inhibiting the translocation of NF-κB into the nucleus. Our animal experiments reveal that activation of KOR effectively alleviates knee pain and prevents synovitis progression in OA mice. Consistently, KOR administration suppresses the expressions of M1 macrophage markers and the NF-κB pathway in the synovium of the knee. Collectively, our study suggests that targeting KOR may be a viable strategy for treating OA by inhibiting synovitis and improving joint pain in affected patients.

Contingent administration of typical and biased kappa opioid agonists reduces cocaine and oxycodone choice in a drug vs. food choice procedure in male rhesus monkeys.

RATIONALE: Combinations of mu and kappa-opioid receptor (KOR) agonists have been proposed as analgesic formulations with reduced abuse potential. The feasibility of this approach has been increased by the development of KOR agonists with biased signaling profiles that produce KOR-typical antinociception with fewer KOR-typical side effects. OBJECTIVE: The present study determined if the biased KOR agonists, nalfurafine and triazole 1.1, could reduce choice for oxycodone in rhesus monkeys as effectively as the typical KOR agonist, salvinorin A. METHODS: Adult male rhesus monkeys (N = 5) responded under a concurrent schedule of food delivery and intravenous cocaine injections (0.018 mg/kg/injection). Once trained, cocaine (0.018 mg/kg/injection) or oxycodone (0.0056 mg/kg/injection) was tested alone or in combination with contingent injections of salvinorin A (0.1-3.2 µg/kg/injection), nalfurafine (0.0032-0.1 µg/kg/injection), triazole 1.1 (3.2-100.0 µg/kg/injection), or vehicle. In each condition, the cocaine or oxycodone dose, as well as the food amount, was held constant across choice components, while the dose of the KOR agonist was increased across choice components. RESULTS: Cocaine and oxycodone were chosen over food on more than 80% of trials when administered alone or contingently with vehicle. When KOR agonists were administered contingently with either cocaine or oxycodone, drug choice decreased in a dose-dependent manner. Salvinorin A and triazole 1.1 decreased drug-reinforcer choice without altering total trials completed (i.e., choice allocation shifted to food), while nalfurafine dose dependently decreased total trials completed. CONCLUSIONS: These results demonstrate that salvinorin A and triazole 1.1, but not nalfurafine, selectively reduce cocaine and oxycodone self-administration independent of nonspecific effects on behavior, suggesting that G-protein bias does not appear to be a moderating factor in this outcome. Triazole 1.1 represents an important prototypical compound for developing novel KOR agonists as deterrents for prescription opioid abuse.

Structure-Activity Relationships of a Novel Cyclic Hexapeptide That Exhibits Multifunctional Opioid Agonism and Produces Potent Antinociceptive Activity.

The cyclic peptide c[d-Lys2, Asp5]-DN-9 has recently been identified as a multifunctional opioid/neuropeptide FF receptor agonist, displaying potent analgesic activity with reduced side effects. This study utilized Tyr-c[d-Lys-Gly-Phe-Asp]-d-Pro-NH2 (0), a cyclic hexapeptide derived from the opioid pharmacophore of c[d-Lys2, Asp5]-DN-9, as a chemical template. We designed, synthesized, and characterized 22 analogs of 0 with a single amino acid substitution to investigate its structure-activity relationship. Most of these cyclic hexapeptide analogs exhibited multifunctional activity at µ and δ opioid receptors (MOR and DOR, respectively) and produced antinociceptive effects following subcutaneous administration. The lead compound analog 15 showed potent agonistic activities at the MOR, κ opioid receptor (KOR), and DOR in vitro and produced a strong and long-lasting analgesic effect through peripheral MOR and KOR in the tail-flick test. Further biological evaluation identified that analog 15 did not cause significant side effects such as tolerance, withdrawal, or reward liability.