STING controls nociception via type I interferon signalling in sensory neurons.

The innate immune regulator STING is a critical sensor of self- and pathogen-derived DNA. DNA sensing by STING leads to the induction of type-I interferons (IFN-I) and other cytokines, which promote immune-cell-mediated eradication of pathogens and neoplastic cells1,2. STING is also a robust driver of antitumour immunity, which has led to the development of STING activators and small-molecule agonists as adjuvants for cancer immunotherapy3. Pain, transmitted by peripheral nociceptive sensory neurons (nociceptors), also aids in host defence by alerting organisms to the presence of potentially damaging stimuli, including pathogens and cancer cells4,5. Here we demonstrate that STING is a critical regulator of nociception through IFN-I signalling in peripheral nociceptors. We show that mice lacking STING or IFN-I signalling exhibit hypersensitivity to nociceptive stimuli and heightened nociceptor excitability. Conversely, intrathecal activation of STING produces robust antinociception in mice and non-human primates. STING-mediated antinociception is governed by IFN-Is, which rapidly suppress excitability of mouse, monkey and human nociceptors. Our findings establish the STING-IFN-I signalling axis as a critical regulator of physiological nociception and a promising new target for treating chronic pain.

Epithelia-Sensory Neuron Cross Talk Underlies Cholestatic Itch Induced by Lysophosphatidylcholine.

BACKGROUND & AIMS: Limited understanding of pruritus mechanisms in cholestatic liver diseases hinders development of antipruritic treatments. Previous studies implicated lysophosphatidic acid (LPA) as a potential mediator of cholestatic pruritus. METHODS: Pruritogenicity of lysophosphatidylcholine (LPC), LPA's precursor, was examined in naïve mice, cholestatic mice, and nonhuman primates. LPC's pruritogenicity involving keratinocyte TRPV4 was studied using genetic and pharmacologic approaches, cultured keratinocytes, ion channel physiology, and structural computational modeling. Activation of pruriceptor sensory neurons by microRNA-146a (miR-146a), secreted from keratinocytes, was identified by in vitro and ex vivo Ca2+ imaging assays. Sera from patients with primary biliary cholangitis were used for measuring the levels of LPC and miR-146a. RESULTS: LPC was robustly pruritic in mice. TRPV4 in skin keratinocytes was essential for LPC-induced itch and itch in mice with cholestasis. Three-dimensional structural modeling, site-directed mutagenesis, and channel function analysis suggested a TRPV4 C-terminal motif for LPC binding and channel activation. In keratinocytes, TRPV4 activation by LPC induced extracellular release of miR-146a, which activated TRPV1+ sensory neurons to cause itch. LPC and miR-146a levels were both elevated in sera of patients with primary biliary cholangitis with itch and correlated with itch intensity. Moreover, LPC and miR-146a were also increased in sera of cholestatic mice and elicited itch in nonhuman primates. CONCLUSIONS: We identified LPC as a novel cholestatic pruritogen that induces itch through epithelia-sensory neuron cross talk, whereby it directly activates skin keratinocyte TRPV4, which rapidly releases miR-146a to activate skin-innervating TRPV1+ pruriceptor sensory neurons. Our findings support the new concept of the skin, as a sensory organ, playing a critical role in cholestatic itch, beyond liver, peripheral sensory neurons, and central neural pathways supporting pruriception.

Therapeutic potentials of NOP and MOP receptor coactivation for the treatment of pain and opioid abuse.

Following the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) as an endogenous ligand for the NOP receptor, ample evidence has revealed unique functional profiles of the N/OFQ-NOP receptor system. NOP receptors are expressed in key neural substrates involved in pain and reward modulation. In nonhuman primates (NHPs), NOP receptor activation effectively exerts antinociception and anti-hypersensitivity at the spinal and supraspinal levels. Moreover, NOP receptor activation inhibits dopaminergic transmission and synergistically enhances mu-opioid peptide (MOP) receptor-mediated analgesia. In this article, we have discussed the functional profiles of ligands with dual NOP and MOP receptor agonist activities and highlight their optimal functional efficacy for pain relief and drug abuse treatment. Through coactivation of NOP and MOP receptors, bifunctional NOP/MOP receptor "partial" agonists (e.g., AT-121, BU08028, and BU10038) reveal a wider therapeutic window with fewer side effects. These newly developed ligands potently induce antinociception without MOP receptor agonist-associated side effects such as abuse potential, respiratory depression, itching sensation, and physical dependence. In addition, in both rodent and NHP models, bifunctional NOP/MOP receptor agonists can attenuate reward processing and/or the reinforcing effects of opioids and other abused drugs. While a mixed NOP/opioid receptor "full" agonist cebranopadol is undergoing clinical trials, bifunctional NOP/MOP "partial" agonists exhibit promising therapeutic profiles in translational NHP models for the treatment of pain and opioid abuse. This class of drugs demonstrates the therapeutic advantage of NOP and MOP receptor coactivation, indicating a greater potential for future development.

Pleiotropic Effects of Kappa Opioid Receptor-Related Ligands in Non-human Primates.

The kappa opioid receptor (KOR)-related ligands have been demonstrated in preclinical studies for several therapeutic potentials. This chapter highlights (1) how non-human primates (NHP) studies facilitate the research and development of ligands targeting the KOR, (2) effects of the endogenous opioid peptide, dynorphin A-(1-17), and its analogs in NHP, and (3) pleiotropic effects and therapeutic applications of KOR-related ligands. In particular, synthetic ligands targeting the KOR have been extensively studied in NHP in three therapeutic areas, i.e., the treatment for itch, pain, and substance use disorders. As the KORs are widely expressed in the peripheral and central nervous systems, pleiotropic effects of KOR-related ligands, such as discriminative stimulus effects, neuroendocrine effects (e.g., prolactin release and stimulation of hypothalamic-pituitary-adrenal axis), and diuresis, in NHP are discussed. Centrally acting KOR agonists are known to produce adverse effects including dysphoria, hallucination, and sedation. Nonetheless, with strategic advances in medicinal chemistry, three classes of KOR-related agonists, i.e., peripherally restricted KOR agonists, mixed KOR/mu opioid receptor partial agonists, and G protein-biased KOR agonists, warrant additional NHP studies to improve our understanding of their functional efficacy, selectivity, and tolerability. Pharmacological studies in NHP which carry high translational significance will facilitate future development of KOR-based medications.

Functional Profile of Systemic and Intrathecal Cebranopadol in Nonhuman Primates.

BACKGROUND: Cebranopadol, a mixed nociceptin/opioid receptor full agonist, can effectively relieve pain in rodents and humans. However, it is unclear to what degree different opioid receptor subtypes contribute to its antinociception and whether cebranopadol lacks acute opioid-associated side effects in primates. The authors hypothesized that coactivation of nociceptin receptors and µ receptors produces analgesia with reduced side effects in nonhuman primates. METHODS: The antinociceptive, reinforcing, respiratory-depressant, and pruritic effects of cebranopadol in adult rhesus monkeys (n = 22) were compared with µ receptor agonists fentanyl and morphine using assays, including acute thermal nociception, IV drug self-administration, telemetric measurement of respiratory function, and itch-scratching responses. RESULTS: Subcutaneous cebranopadol (ED50, 2.9 [95% CI, 1.8 to 4.6] µg/kg) potently produced antinociception compared to fentanyl (15.8 [14.6 to 17.1] µg/kg). Pretreatment with antagonists selective for nociceptin and µ receptors, but not δ and κ receptor antagonists, caused rightward shifts of the antinociceptive dose-response curve of cebranopadol with dose ratios of 2 and 9, respectively. Cebranopadol produced reinforcing effects comparable to fentanyl, but with decreased reinforcing strength, i.e., cebranopadol (mean ± SD, 7 ± 3 injections) versus fentanyl (12 ± 3 injections) determined by a progressive-ratio schedule of reinforcement. Unlike fentanyl (8 ± 2 breaths/min), systemic cebranopadol at higher doses did not decrease the respiratory rate (17 ± 2 breaths/min). Intrathecal cebranopadol (1 µg) exerted full antinociception with minimal scratching responses (231 ± 137 scratches) in contrast to intrathecal morphine (30 µg; 3,009 ± 1,474 scratches). CONCLUSIONS: In nonhuman primates, the µ receptor mainly contributed to cebranopadol-induced antinociception. Similar to nociceptin/µ receptor partial agonists, cebranopadol displayed reduced side effects, such as a lack of respiratory depression and pruritus. Although cebranopadol showed reduced reinforcing strength, its detectable reinforcing effects and strength warrant caution, which is critical for the development and clinical use of cebranopadol.

Antinociceptive, reinforcing, and pruritic effects of the G-protein signalling-biased mu opioid receptor agonist PZM21 in non-human primates.

BACKGROUND: A novel G-protein signalling-biased mu opioid peptide (MOP) receptor agonist, PZM21, was recently developed with a distinct chemical structure. It is a potent Gi/o activator with minimal ß-arrestin-2 recruitment. Despite intriguing activity in rodent models, PZM21 function in non-human primates is unknown. The aim of this study was to investigate PZM21 actions after systemic or intrathecal administration in primates. METHODS: Antinociceptive, reinforcing, and pruritic effects of PZM21 were compared with those of the clinically used MOP receptor agonists oxycodone and morphine in assays of acute thermal nociception, capsaicin-induced thermal allodynia, itch scratching responses, and drug self-administration in gonadally intact, adult rhesus macaques (10 males, six females). RESULTS: After subcutaneous administration, PZM21 (1.0-6.0 mg kg-1) and oxycodone (0.1-0.6 mg kg-1) induced dose-dependent thermal antinociceptive effects (P<0.05); PZM21 was 10 times less potent than oxycodone. PZM21 exerted oxycodone-like reinforcing effects and strength as determined by two operant schedules of reinforcement in the intravenous drug self-administration assay. After intrathecal administration, PZM21 (0.03-0.3 mg) dose-dependently attenuated capsaicin-induced thermal allodynia (P<0.05). Although intrathecal PZM21 and morphine induced MOP receptor-mediated antiallodynic effects, both compounds induced robust, long-lasting itch scratching. CONCLUSIONS: PZM21 induced antinociceptive, reinforcing, and pruritic effects similar to clinically used MOP receptor agonists in primates. Although structure-based discovery of PZM21 identified a novel avenue for studying G-protein signalling-biased ligands, biasing an agonist towards G-protein signalling pathways did not determine or alter reinforcing (i.e. abuse potential) or pruritic effects of MOP receptor agonists in a translationally relevant non-human primate model.

BU10038 as a safe opioid analgesic with fewer side-effects after systemic and intrathecal administration in primates.

BACKGROUND: The marked increase in mis-use of prescription opioids has greatly affected our society. One potential solution is to develop improved analgesics which have agonist action at both mu opioid peptide (MOP) and nociceptin/orphanin FQ peptide (NOP) receptors. BU10038 is a recently identified bifunctional MOP/NOP partial agonist. The aim of this study was to determine the functional profile of systemic or spinal delivery of BU10038 in primates after acute and chronic administration. METHODS: A series of behavioural and physiological assays have been established specifically to reflect the therapeutic (analgesia) and side-effects (abuse potential, respiratory depression, itch, physical dependence, and tolerance) of opioid analgesics in rhesus monkeys. RESULTS: After systemic administration, BU10038 (0.001-0.01 mg kg-1) dose-dependently produced long-lasting antinociceptive and antihypersensitive effects. Unlike the MOP agonist oxycodone, BU10038 lacked reinforcing effects (i.e. little or no abuse liability), and BU10038 did not compromise the physiological functions of primates including respiration, cardiovascular activities, and body temperature at antinociceptive doses and a 10-30-fold higher dose (0.01-0.1 mg kg-1). After intrathecal administration, BU10038 (3 µg) exerted morphine-comparable antinociception and antihypersensitivity without itch scratching responses. Unlike morphine, BU10038 did not cause the development of physical dependence and tolerance after repeated and chronic administration. CONCLUSIONS: These in vivo findings demonstrate the translational potential of bifunctional MOP/NOP receptor agonists such as BU10038 as a safe, non-addictive analgesic with fewer side-effects in primates. This study strongly supports that bifunctional MOP/NOP agonists may provide improved analgesics and an alternative solution for the ongoing prescription opioid crisis.

Neuromedin B Induces Acute Itch in Mice via the Activation of Peripheral Sensory Neurons.

Neuromedin B is expressed in nociceptive and itch-sensitive dorsal root ganglia neurons, but its peripheral pruritogenic potential is not well described. The potential of neuromedin B as a pruritogen and pro-inflammatory peptide in the skin was tested in vivo in an acute model in mice and monkeys as well as an allergic dermatitis model in mice. To identify the underlying mechanisms in vitro real time PCR analysis for neuromedin B and its receptor expression in murine mast cells and dorsal root ganglia as well as functional calcium imaging in the ganglia was applied. Neuromedin B induces itch when injected intradermally, and the peripheral signal is likely transmitted through the activation of dorsal root ganglia. Thus, neuromedin B could be an interesting new therapeutic target for peripheral processing of itch at the level of sensory neurons.

Effects of NOP-Related Ligands in Nonhuman Primates.

The nociceptin/orphanin FQ peptide (NOP) receptor-related ligands have been demonstrated in preclinical studies for several therapeutic applications. This article highlights (1) how nonhuman primates (NHP) were used to facilitate the development and application of positron emission tomography tracers in humans; (2) effects of an endogenous NOP ligand, nociceptin/orphanin FQ, and its interaction with mu opioid peptide (MOP) receptor agonists; and (3) promising functional profiles of NOP-related agonists in NHP as analgesics and treatment for substance use disorders. NHP models offer the most phylogenetically appropriate evaluation of opioid and non-opioid receptor functions and drug effects. Based on preclinical and clinical data of ligands with mixed NOP/MOP receptor agonist activity, several factors including their intrinsic efficacies for activating NOP versus MOP receptors and different study endpoints in NHP could contribute to different pharmacological profiles. Ample evidence from NHP studies indicates that bifunctional NOP/MOP receptor agonists have opened an exciting avenue for developing safe, effective medications with fewer side effects for treating pain and drug addiction. In particular, bifunctional NOP/MOP partial agonists hold a great potential as (1) effective spinal analgesics without itch side effects; (2) safe, nonaddictive analgesics without opioid side effects such as respiratory depression; and (3) effective medications for substance use disorders.

A novel orvinol analog, BU08028, as a safe opioid analgesic without abuse liability in primates.

Despite the critical need, no previous research has substantiated safe opioid analgesics without abuse liability in primates. Recent advances in medicinal chemistry have led to the development of ligands with mixed mu opioid peptide (MOP)/nociceptin-orphanin FQ peptide (NOP) receptor agonist activity to achieve this objective. BU08028 is a novel orvinol analog that displays a similar binding profile to buprenorphine with improved affinity and efficacy at NOP receptors. The aim of this preclinical study was to establish the functional profile of BU08028 in monkeys using clinically used MOP receptor agonists for side-by-side comparisons in various well-honed behavioral and physiological assays. Systemic BU08028 (0.001-0.01 mg/kg) produced potent long-lasting (i.e., >24 h) antinociceptive and antiallodynic effects, which were blocked by MOP or NOP receptor antagonists. More importantly, the reinforcing strength of BU08028 was significantly lower than that of cocaine, remifentanil, or buprenorphine in monkeys responding under a progressive-ratio schedule of drug self-administration. Unlike MOP receptor agonists, BU08028 at antinociceptive doses and ∼10- to 30-fold higher doses did not cause respiratory depression or cardiovascular adverse events as measured by telemetry devices. After repeated administration, the monkeys developed acute physical dependence on morphine, as manifested by precipitated withdrawal signs, such as increased respiratory rate, heart rate, and blood pressure. In contrast, monkeys did not show physical dependence on BU08028. These in vivo findings in primates not only document the efficacy and tolerability profile of bifunctional MOP/NOP receptor agonists, but also provide a means of translating such ligands into therapies as safe and potentially abuse-free opioid analgesics.

Synthesis of Enantiopure PZM21, A Novel Biased Agonist of the Mu-Opioid Receptor.

PZM21 (1) was recently reported as a biased agonist of the mu-opioid receptor (MOR) with improved antinociceptive effects but reduced side effects than traditional opioid-based analgesics. The original synthesis of PZM21 with the desired (S,S) configuration required the separation of diastereomeric mixture in the final step using chiral HPLC. We have designed a concise synthesis of 1 in the enantiomeric pure form starting with commercially available L-alanine and via a chiral aziridine as a key intermediate. The final product 1 as the (S,S) diastereomer was obtained in 7 steps in 22.5% yield from L-alanine. This synthetic strategy could be readily applied to the development of PZM21 analogs at the thiophenyl position.

Altered expression of glial markers, chemokines, and opioid receptors in the spinal cord of type 2 diabetic monkeys.

Neuroinflammation is a pathological condition that underlies diabetes and affects sensory processing. Given the high prevalence of pain in diabetic patients and crosstalk between chemokines and opioids, it is pivotal to know whether neuroinflammation-associated mediators are dysregulated in the central nervous system of diabetic primates. Therefore, the aim of this study was to investigate whether mRNA expression levels of glial markers, chemokines, and opioid receptors are altered in the spinal cord and thalamus of naturally occurring type 2 diabetic monkeys (n=7) compared with age-matched non-diabetic monkeys (n=6). By using RT-qPCR, we found that mRNA expression levels of both GFAP and IBA1 were up-regulated in the spinal dorsal horn (SDH) of diabetic monkeys compared with non-diabetic monkeys. Among all chemokines, expression levels of three chemokine ligand-receptor systems, i.e., CCL2-CCR2, CCL3-CCR1/5, and CCL4-CCR5, were up-regulated in the SDH of diabetic monkeys. Moreover, in the SDH, seven additional chemokine receptors, i.e., CCR4, CCR6, CCR8, CCR10, CXCR3, CXCR5, and CXCR6, were also up-regulated in diabetic monkeys. In contrast, expression levels of MOP, KOP, and DOP, but not NOP receptors, were down-regulated in the SDH of diabetic monkeys, and the thalamus had fewer changes in the glial markers, chemokines and opioids. These findings indicate that neuroinflammation, manifested as glial activation and simultaneous up-regulation of multiple chemokine ligands and receptors, seems to be permanent in type 2 diabetic monkeys. As chemokines and opioids are important pain modulators, this first-in-primate study provides a translational bridge for determining the functional efficacy of spinal drugs targeting their signaling cascades.

Spinal Functions of B-Type Natriuretic Peptide, Gastrin-Releasing Peptide, and Their Cognate Receptors for Regulating Itch in Mice.

B-type natriuretic peptide (BNP)-natriuretic peptide receptor A (NPRA) and gastrin-releasing peptide (GRP)-GRP receptor (GRPR) systems contribute to spinal processing of itch. However, pharmacological and anatomic evidence of these two spinal ligand-receptor systems are still not clear. The aim of this study was to determine the spinal functions of BNP-NPRA and GRP-GRPR systems for regulating scratching activities in mice by using pharmacological and immunohistochemical approaches. Our results showed that intrathecal administration of BNP (0.3-3 nmol) dose dependently elicited scratching responses, which could be blocked by the NPRA antagonist (Arg6,ß-cyclohexyl-Ala8,D-Tic16,Arg17,Cys18)-atrial natriuretic factor(6-18) amide (A71915). However, A71915 had no effect on intrathecal GRP-induced scratching. In contrast, pretreatment with a GRPR antagonist (D-Tpi6,Leu13ψ(CH2-NH)-Leu14)bombesin(6-14) (RC-3095) inhibited BNP-induced scratching. Immunostaining revealed that NPRA proteins colocalize with GRP, but not GRPR, in the superficial area of dorsal horn, whereas BNP proteins do not colocalize with either GRP or GRPR in the dorsal horn. Intradermal administration of ligands including endothelin-1, U-46619, bovine adrenal medulla 8-22, and Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SLIGRL) increased scratching bouts at different levels of magnitude. Pretreatment with intrathecal A71915 did not affect scratching responses elicited by all four pruritogens, whereas pretreatment with RC-3095 only inhibited SLIGRL-induced scratching. Interestingly, immunostaining showed that RC-3095, but not A71915, inhibited SLIGRL-elicited c-Fos activation in the spinal dorsal horn, which was in line with behavioral outcomes. These findings demonstrate that: 1) BNP-NPRA system may function upstream of the GRP-GRPR system to regulate itch in the mouse spinal cord, and 2) both NPRA and GRPR antagonists may have antipruritic efficacy against centrally, but not peripherally, elicited itch.

TC-2559, an α4ß2 nicotinic acetylcholine receptor agonist, suppresses the expression of CCL3 and IL-1ß through STAT3 inhibition in cultured murine macrophages.

The anti-inflammatory properties of TC-2559, an α4ß2 nicotinic acetylcholine receptor (nAChR) agonist, on cultured murine macrophages was investigated. TC-2559 suppressed the upregulation of CC-chemokine ligand 3 (CCL3) and interleukin-1ß (IL-1ß) following lipopolysaccharide (LPS) treatment in J774A.1 cells. TC-2559 inhibited the phosphorylation of signal transducer and activator of transcription 3 (pSTAT3) but not nuclear factor-κB p65 after LPS. Blockade of pSTAT3 by AG490 inhibited the upregulation of CCL3 and IL-1ß after LPS. In conclusion, TC-2559-driven α4ß2 nAChR signaling suppressed the upregulation of CCL3 and IL-1ß by inhibiting pSTAT3 in inflammatory macrophages, resulting in the suppression of neuropathic pain.

Neuraxial opioid-induced itch and its pharmacological antagonism.

Given its profound analgesic nature, neuraxial opioids are frequently used for pain management. Unfortunately, the high incident rate of itch/pruritus after spinal administration of opioid analgesics reported in postoperative and obstetric patients greatly diminishes patient satisfaction and thus the value of the analgesics. Many endeavors to solve the mystery behind neuraxial opioid-induced itch had not been successful, as the pharmacological antagonism other than the blockade of mu opioid receptors remains elusive. Nevertheless, as the characteristics of all opioid receptor subtypes have become more understood, more studies have shed light on the potential effective treatments. This review discusses the mechanisms underlying neuraxial opioid-induced itch and compares pharmacological evidence in nonhuman primates with clinical findings across diverse drugs. Both nonhuman primate and human studies corroborate that mixed mu/kappa opioid partial agonists seem to be the most effective drugs in ameliorating neuraxial opioid-induced itch while retaining neuraxial opioid-induced analgesia.

Effects of spinally administered bifunctional nociceptin/orphanin FQ peptide receptor/µ-opioid receptor ligands in mouse models of neuropathic and inflammatory pain.

Nociceptin/orphanin FQ peptide receptor (NOP) agonists produce antinociceptive effects in animal models after spinal administration and potentiate µ-opioid receptor (MOP)-mediated antinociception. This study determined the antinociceptive effects of spinally administered bifunctional NOP/MOP ligands and the antinociceptive functions of spinal NOP and MOP receptors in mice. Antinociceptive effects of bifunctional NOP/MOP ligands BU08028 [(2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6-methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol] and SR16435 [1-(1-(2,3,3α,4,5,6-hexahydro-1H-phenalen-1-yl)piperidin-4-yl)-indolin-2-one] were pharmacologically compared with the putative bifunctional ligand buprenorphine, selective NOP agonist SCH221510 [3-endo-8-[bis(2-methylphenyl)methyl]-3-phenyl-8-azabicyclo[3.2.1]octan-3-ol] and selective MOP agonist morphine in neuropathic and inflammatory pain models. Additionally, the degree of tolerance development to the antiallodynic effects of SR16435 and buprenorphine were determined after repeated intrathecal administration. Our data indicated that BU08028 and SR16435 were more potent than morphine and SCH221510 in attenuating nerve injury-induced tactile allodynia and inflammation-induced thermal hyperalgesia. Coadministration of receptor-selective antagonists further revealed that both NOP and MOP in the spinal cord mediated the antiallodynic effects of BU08028 and SR16435, but intrathecal buprenorphine-induced antiallodynic effects were primarily mediated by MOP. Repeated intrathecal administration of SR16435 resulted in reduced and slower development of tolerance to its antiallodynic effects compared with buprenorphine. In conclusion, both NOP and MOP receptors in the spinal cord independently drive antinociception in mice. Spinally administered bifunctional NOP/MOP ligands not only can effectively attenuate neuropathic and inflammatory pain, but also have higher antinociceptive potency with reduced tolerance development to analgesia. Such ligands therefore display a promising profile as spinal analgesics.

Functional consequences of short-term exposure to opioids versus cannabinoids in nonhuman primates.

Opioids provide pain relief but are associated with several adverse effects. Researchers are exploring cannabis-based medicine as an alternative. However, little is known about the tendency for physical dependence on cannabinoids in comparison with that on opioids in primates. The aim of this study was to compare the potency of heroin and delta-9-tetrahydrocannabinol (THC) in eliciting analgesic effects and the development of physical dependence between opioids and cannabinoids in both male and female rhesus monkeys. Systemic administration of either heroin (0.03-0.18 mg/kg) or THC (0.3-1.8 mg/kg) in a dose-dependent manner produced antinociceptive effects against an acute thermal nociceptive stimulus. The µ-opioid receptor antagonist naltrexone (0.01 mg/kg) and the cannabinoid receptor antagonist SR141716A (0.3 mg/kg) produced the same degree of rightward shift in the dose-response curves for heroin- and THC-induced antinociception, respectively. Monkeys implanted with telemetry devices were subjected to short-term repeated administrations (two injections per day for 1-3 days) of either heroin (0.18 mg/kg), morphine (1.8 mg/kg), THC (1.8 mg/kg), or CP 55,940 (0.032 mg/kg). Administration of naltrexone (0.01 mg/kg) increased respiration, heart rate, and blood pressure in heroin- or morphine-treated monkeys. In contrast, administration of SR141716A (0.3 mg/kg) did not cause a significant change in these physiological parameters in THC- or CP 55,940-treated monkeys. Additionally, morphine, but not CP 55,940, enhanced the monkeys' hypersensitivity to the algogen capsaicin. Collectively, these results demonstrate that in nonhuman primates, both opioids and cannabinoids exert comparable antinociception; however, physical dependence on opioids, but not cannabinoids, at their antinociceptive doses, occurs following short-term exposures.

Nociceptin Receptor-Related Agonists as Safe and Non-addictive Analgesics.

As clinical use of currently available opioid analgesics is often impeded by dose-limiting adverse effects, such as abuse liability and respiratory depression, new approaches have been pursued to develop safe, effective, and non-addictive pain medications. After the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor more than 25 years ago, NOP receptor-related agonists have emerged as a promising target for developing novel and effective opioids that modulate the analgesic and addictive properties of mu-opioid peptide (MOP) receptor agonists. In this review, we highlight the effects of the NOP receptor-related agonists compared with those of MOP receptor agonists in experimental rodent and more translational non-human primate (NHP) models and the development status of key NOP receptor-related agonists as potential safe and non-addictive analgesics. Several lines of evidence demonstrated that peptidic and non-peptidic NOP receptor agonists produce potent analgesic effects by intrathecal delivery in NHPs. Moreover, mixed NOP/MOP receptor partial agonists (e.g., BU08028, BU10038, and AT-121) display potent analgesic effects when administered intrathecally or systemically, without eliciting adverse effects, such as respiratory depression, itch behavior, and signs of abuse liability. More importantly, cebranopadol, a mixed NOP/opioid receptor agonist with full efficacy at NOP and MOP receptors, produces robust analgesic efficacy with reduced adverse effects, conferring promising outcomes in clinical studies. A balanced coactivation of NOP and MOP receptors is a strategy that warrants further exploration and refinement for the development of novel analgesics with a safer and effective profile.

PD-L1/PD-1 checkpoint pathway regulates hippocampal neuronal excitability and learning and memory behavior.

Programmed death protein 1 (PD-1) and its ligand PD-L1 constitute an immune checkpoint pathway. We report that neuronal PD-1 signaling regulates learning/memory in health and disease. Mice lacking PD-1 (encoded by Pdcd1) exhibit enhanced long-term potentiation (LTP) and memory. Intraventricular administration of anti-mouse PD-1 monoclonal antibody (RMP1-14) potentiated learning and memory. Selective deletion of PD-1 in excitatory neurons (but not microglia) also enhances LTP and memory. Traumatic brain injury (TBI) impairs learning and memory, which is rescued by Pdcd1 deletion or intraventricular PD-1 blockade. Conversely, re-expression of Pdcd1 in PD-1-deficient hippocampal neurons suppresses memory and LTP. Exogenous PD-L1 suppresses learning/memory in mice and the excitability of mouse and NHP hippocampal neurons through PD-1. Notably, neuronal activation suppresses PD-L1 secretion, and PD-L1/PD-1 signaling is distinctly regulated by learning and TBI. Thus, conditions that reduce PD-L1 levels or PD-1 signaling could promote memory in both physiological and pathological conditions.

Roles of µ-opioid receptors and nociceptin/orphanin FQ peptide receptors in buprenorphine-induced physiological responses in primates.

Buprenorphine is known as a µ-opioid peptide (MOP) receptor agonist, but its antinociception is compromised by the activation of nociceptin/orphanin FQ peptide (NOP) receptors in rodents. The aim of this study was to investigate the roles of MOP and NOP receptors in regulating buprenorphine-induced physiological responses in primates (rhesus monkeys). The effects of MOP antagonist (naltrexone), NOP antagonist [(±)-1-[(3R*,4R*)-1-(cyclooctylmethyl)-3-(hydroxymethyl)-4-piperidinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J-113397)], and NOP agonists [(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5] decan-4-one (Ro 64-6198) and 3-endo-8-[bis(2-methylphenyl)methyl]-3-phenyl-8-azabicyclo[3.2.1]octan-3-ol (SCH 221510)] on buprenorphine were studied in three functional assays for measuring analgesia, respiratory depression, and itch in primates. Over the dose range of 0.01 to 0.1 mg/kg, buprenorphine dose-dependently produced antinociception, respiratory depression, and itch/scratching responses, and there was a ceiling effect at higher doses (0.1-1 mg/kg). Naltrexone (0.03 mg/kg) produced similar degrees of rightward shifts of buprenorphine's dose-response curves for all three endpoints. Mean pK(B) values of naltrexone (8.1-8.3) confirmed that MOP receptors mediated mainly buprenorphine-induced antinociception, respiratory depression, and itch/scratching. In contrast, J-113397 (0.1 mg/kg) did not change buprenorphine-induced physiological responses, indicating that there were no functional NOP receptors in buprenorphine-induced effects. More importantly, both NOP agonists, Ro 64-6198 and SCH 221510, enhanced buprenorphine-induced antinociception without respiratory depression and itch/ scratching. The dose-addition analysis revealed that buprenorphine in combination with the NOP agonist synergistically produced antinociceptive effects. These findings provided functional evidence that the activation of NOP receptors did not attenuate buprenorphine-induced antinociception in primates; instead, the coactivation of MOP and NOP receptors produced synergistic antinociception without other side effects. This study strongly supports the therapeutic potential of mixed MOP/NOP agonists as innovative analgesics.