An NPY Y1 receptor antagonist unmasks latent sensitization and reveals the contribution of protein kinase A and Epac to chronic inflammatory pain

Engineering peptide drug therapeutics through chemical conjugation and implication in clinics

The development of peptide drugs has made tremendous progress in the past few decades because of the advancements in modification chemistry and analytical technologies. The novel-designed peptide drugs have been modified through various biochemical methods with improved diagnostic, therapeutic, and drug-delivery strategies. Researchers found it a helping hand to overcome the inherent limitations of peptides and bring continued advancements in their applications. Furthermore, the emergence of peptide-drug conjugates (PDCs)-utilizes target-oriented peptide moieties as a vehicle for cytotoxic payloads via conjugation with cleavable chemical agents, resulting in the key foundation of the new era of targeted peptide drugs. This review summarizes the various classifications of peptide drugs, suitable chemical modification strategies to improve the ADME (adsorption, distribution, metabolism, and excretion) features of peptide drugs, and recent (2015-early 2024) progress/achievements in peptide-based drug delivery systems as well as their fruitful implication in preclinical and clinical studies. Furthermore, we also summarized the brief description of other types of PDCs, including peptide-MOF conjugates and peptide-UCNP conjugates. The principal aim is to provide scattered and diversified knowledge in one place and to help researchers understand the pinching knots in the science of PDC development and progress toward a bright future of novel peptide drugs.

Ghrelin Induces the Production of Hypothalamic NPY Through the AMPK-mTOR Pathway to Alleviate Cancer-induced Bone Pain

BACKGROUND/AIM

Cancer-induced bone pain (CIBP) is one of the most common symptoms of bone metastasis of tumor cells. The hypothalamus may play a pivotal role in the regulation of CIBP. However, little is known about the exact mechanisms.

MATERIALS AND METHODS

First, we established a CIBP model to explore the relationship among hypothalamic ghrelin, NPY and CIBP. Then, we exogenously administered NPY and NPY receptor antagonists to investigate whether hypothalamic NPY exerted an antinociceptive effect through binding to NPY receptors. Finally, we exogenously administered ghrelin to investigate whether ghrelin alleviated CIBP by inducing the production of hypothalamic NPY through the AMPK-mTOR pathway. Body weight, food intake and behavioral indicators of CIBP were measured every 3 days. Hypothalamic ghrelin, NPY and the AMPK-mTOR pathway were also measured.

RESULTS

The expression of hypothalamic ghrelin and NPY was simultaneously decreased in cancer-bearing rats, which was accompanied by CIBP. Intracerebroventricular (i.c.v.) administration of NPY significantly alleviated CIBP in the short term. The antinociceptive effect of NPY was reversed with the i.c.v. administration of the Y1R and Y2R antagonists. The administration of ghrelin activated the AMPK-mTOR pathway and induced hypothalamic NPY production to alleviate CIBP. This effect of ghrelin on NPY and antinociception was reversed with the administration of a GHS-R1α antagonist.

CONCLUSION

Ghrelin could induce the production of hypothalamic NPY through the AMPK-mTOR pathway to alleviate CIBP, which can provide a novel therapeutic mechanism for CIBP.

Role of EPAC1 in chronic pain

Chronic pain usually lasts over three months and commonly occurs in chronic diseases (cancer, arthritis, and diabetes), injuries (herniated discs, torn ligaments), and many major pain disorders (neuropathic pain, fibromyalgia, chronic headaches). Unfortunately, there is currently a lack of effective treatments to help people with chronic pain to achieve complete relief. Therefore,it is particularly important to understand the mechanism of chronic pain and find new therapeutic targets. The exchange protein directly activated by cyclic adenosine monophosphate(cAMP) (EPAC) has been recognized for its functions in nerve regeneration, stimulating insulin release, controlling vascular pressure, and controlling other metabolic activities. In recent years, many studies have found that the subtype of EPAC, EPAC1 is involved in the regulation of neuroinflammation and plays a crucial role in the regulation of pain, which is expected to become a new therapeutic target for chronic pain. This article reviews the major contributions of EPAC1 in chronic pain.

Peptide-Drug Conjugates: Design, Chemistry, and Drug Delivery System as a Novel Cancer Theranostic

The emergence of peptide-drug conjugates (PDCs) that utilize target-oriented peptide moieties as carriers of cytotoxic payloads, interconnected with various cleavable/noncleavable linkers, resulted in the key-foundation of the new era of targeted therapeutics. They are capable of retaining the integrity of conjugates in the blood circulatory system as well as releasing the drugs at the tumor microenvironment. Other valuable advantages are specificity and selectivity toward targeted-receptors, higher penetration ability, and drug-loading capacity, making them a suitable candidate to play their vital role as promising carrier agents. In this review, we summarized the types of cell-targeting (CTPs) and cell-penetrating peptides (CPPs) that have broad applications in the advancement of targeted drug-delivery systems (DDS). Moreover, the techniques to overcome the limitations of peptide-chemistry for their extensive implementation to construct the PDCs. Besides this, the diversified breakthrough of linker chemistry, and ample knowledge of various cytotoxic payloads used in PDCs in recent years, as well as the mechanism of action of PDCs was critically discussed. The principal aim is to provide scattered and diversified knowledge in one place and to help researchers understand the pinching knots in the science of PDC development, also their progression toward a bright future for PDCs as novel theranostics in clinical practice.

The plasticity of neuropeptide Y-Y1 receptor system on Tac2 neurons contributes to mechanical hyperknesis during chronic itch

Rationale: In the physiological states, the act of scratching protects the person from harmful substances, while in certain pathological conditions, the patient suffers from chronic itch, both physically and mentally. Chronic itch sufferers are more sensitive to mechanical stimuli, and mechanical hyperknesis relief is essential for chronic itch treatment. While neuropeptide Y-Y1 receptor (NPY-Y1R) system is known to play a crucial role in modulating mechanical itch in physiological conditions, it is elusive how they are altered during chronic itch. We hypothesize that the negative regulatory effect of Y1Rs on Tac2 neurons, the key neurons that transmit mechanical itch, declines during chronic itch. Methods: We combined transgenic mice, chemogenetic manipulation, immunofluorescence, rabies virus circuit tracing, and electrophysiology to investigate the plasticity of Y1Rs on Tac2 neurons during chronic itch. Results: We found that Tac2 neurons receive direct input from Npy neurons and that inhibition of Npy neurons induces activation of Tac2 neurons. Moreover, the expression of Y1Rs on Tac2 neurons is reduced, and the regulatory effect is also reduced during chronic itch. Conclusion: Our study clarifies the plasticity of Y1Rs on Tac2 neurons during chronic itch and further elucidates the mechanism by which NPY-Y1R system is responsible for modulating mechanical itch. We highlight Y1Rs as a promising therapeutic target for mechanical hyperknesis during chronic itch.

CCL2 Potentiates Inflammation Pain and Related Anxiety-Like Behavior Through NMDA Signaling in Anterior Cingulate Cortex

Previous studies have shown that the C-C motif chemokine ligand 2 (CCL2) is widely expressed in the nervous system and involved in regulating the development of chronic pain and related anxiety-like behaviors, but its precise mechanism is still unclear. This paper provides an in-depth examination of the involvement of CCL2-CCR2 signaling in the anterior cingulate cortex (ACC) in intraplantar injection of complete Freund's adjuvant (CFA) leading to inflammatory pain and its concomitant anxiety-like behaviors by modulation of glutamatergic N-methyl-D-aspartate receptor (NMDAR). Our findings suggest that local bilateral injection of CCR2 antagonist in the ACC inhibits CFA-induced inflammatory pain and anxiety-like behavior. Meanwhile, the expression of CCR2 and CCL2 was significantly increased in ACC after 14 days of intraplantar injection of CFA, and CCR2 was mainly expressed in excitatory neurons. Whole-cell patch-clamp recordings showed that the CCR2 inhibitor RS504393 reduced the frequency of miniature excitatory postsynaptic currents (mEPSC) in ACC, and CCL2 was involved in the regulation of NMDAR-induced current in ACC neurons in the pathological state. In addition, local injection of the NR2B inhibitor of NMDAR subunits, Ro 25-6981, attenuated the effects of CCL2-induced hyperalgesia and anxiety-like behavior in the ACC. In summary, CCL2 acts on CCR2 in ACC excitatory neurons and participates in the regulation of CFA-induced pain and related anxiety-like behaviors through upregulation of NR2B. CCR2 in the ACC neuron may be a potential target for the treatment of chronic inflammatory pain and pain-related anxiety.

Alleviation of neuropathic pain with neuropeptide Y requires spinal Npy1r interneurons that coexpress Grp

Neuropeptide Y targets the Y1 receptor (Y1) in the spinal dorsal horn (DH) to produce endogenous and exogenous analgesia. DH interneurons that express Y1 (Y1-INs; encoded by Npy1r) are necessary and sufficient for neuropathic hypersensitivity after peripheral nerve injury. However, as Y1-INs are heterogenous in composition in terms of morphology, neurophysiological characteristics, and gene expression, we hypothesized that a more precisely defined subpopulation mediates neuropathic hypersensitivity. Using fluorescence in situ hybridization, we found that Y1-INs segregate into 3 largely nonoverlapping subpopulations defined by the coexpression of Npy1r with gastrin-releasing peptide (Grp/Npy1r), neuropeptide FF (Npff/Npy1r), and cholecystokinin (Cck/Npy1r) in the superficial DH of mice, nonhuman primates, and humans. Next, we analyzed the functional significance of Grp/Npy1r, Npff/Npy1r, and Cck/Npy1r INs to neuropathic pain using a mouse model of peripheral nerve injury. We found that chemogenetic inhibition of Npff/Npy1r-INs did not change the behavioral signs of neuropathic pain. Further, inhibition of Y1-INs with an intrathecal Y1 agonist, [Leu31, Pro34]-NPY, reduced neuropathic hypersensitivity in mice with conditional deletion of Npy1r from CCK-INs and NPFF-INs but not from GRP-INs. We conclude that Grp/Npy1r-INs are conserved in higher order mammalian species and represent a promising and precise pharmacotherapeutic target for the treatment of neuropathic pain.

Endogenous μ-opioid-Neuropeptide Y Y1 receptor synergy silences chronic postoperative pain in mice

Tissue injury creates a delicate balance between latent pain sensitization (LS) and compensatory endogenous analgesia. Inhibitory G-protein-coupled receptor (GPCR) interactions that oppose LS, including μ-opioid receptor (MOR) or neuropeptide Y Y1 receptor (Y1R) activity, persist in the spinal cord dorsal horn (DH) for months, even after the resolution of normal pain thresholds. Here, we demonstrate that following recovery from surgical incision, a potent endogenous analgesic synergy between MOR and Y1R activity persists within DH interneurons to reduce the intensity and duration of latent postoperative hypersensitivity and ongoing pain. Failure of such endogenous GPCR signaling to maintain LS in remission may underlie the transition from acute to chronic pain states.

Kinin B2 and B1 Receptors Activation Sensitize the TRPA1 Channel Contributing to Anastrozole-Induced Pain Symptoms

Aromatase inhibitors (AIs) cause symptoms of musculoskeletal pain, and some mechanisms have been proposed to explain them. However, signaling pathways downstream from kinin B2 (B2R) and B1 (B1R) receptor activation and their possible sensitizing of the Transient Receptor Potential Ankyrin 1 (TRPA1) remain unknown. The interaction between the kinin receptor and the TRPA1 channel in male C57BL/6 mice treated with anastrozole (an AI) was evaluated. PLC/PKC and PKA inhibitors were used to evaluate the signaling pathways downstream from B2R and B1R activation and their effect on TRPA1 sensitization. Anastrozole caused mechanical allodynia and muscle strength loss in mice. B2R (Bradykinin), B1R (DABk), or TRPA1 (AITC) agonists induced overt nociceptive behavior and enhanced and prolonged the painful parameters in anastrozole-treated mice. All painful symptoms were reduced by B2R (Icatibant), B1R (DALBk), or TRPA1 (A967079) antagonists. We observed the interaction between B2R, B1R, and the TRPA1 channel in anastrozole-induced musculoskeletal pain, which was dependent on the activation of the PLC/PKC and PKA signaling pathways. TRPA1 seems to be sensitized by mechanisms dependent on the activation of PLC/PKC, and PKA due to kinin receptors stimulation in anastrozole-treated animals. Thus, regulating this signaling pathway could contribute to alleviating AIs-related pain symptoms, patients’ adherence to therapy, and disease control.

CAV-1 Overexpression Exacerbates the Manifestation in EPAC-1-Induced Chronic Postsurgical Pain in Rats

Purpose. Caveolae (CAV) are an invaginated microcapsule with the shape of Ω on the surface of the cell membrane. Caveolin-1 (CAV-1) is involved in neuropathic pain, and adenosine monophosphate (AMP)-exchange protein directly activated by cAMP1 (EPAC-1) is a potential therapeutic target for chronic pain. However, whether EPAC-1 promotes chronic postsurgical pain (CPSP) through CAV-1 has not been reported. Here, we aim to investigate the underlying mechanism of CAV in CPSP. Methods. All the rats were divided into 9 groups, including the Naive group, Sham group, skin/muscle incision and retraction (SMIR) group, SMIR + CAV-1 siRNA group, SMIR + control siRNA group, SMIR (7 days)+Saline group, SMIR (7 days)+CE3F4 group, 8-PCPT group, and Saline group. The CPSP rat model was established after SMIR. A mechanical withdrawal threshold (MWT) was recorded to evaluate the animal’s behavior. Western blotting and immunofluorescent were performed to detect the protein expression levels of EPAC-1 and P-CAV-1. Results. EPAC-1 and CAV-1 were both overexpressed after operation, particularly in astrocytes, microglia, and neurons of spinal marrow (all $P<0.05$ ). Interestingly, CAV-1 siRNA can partly reverse the SMIR-induced hypersensitivity, but there was no effect on EPAC-1. Besides, EPAC-1 blockage partly reversed the SMIR-induced hypersensitivity and CAV-1 overexpression, and EPAC-1 activation promoted CAV-1 overexpression and hypersensitivity in normal rats (all $P<0.05$ ). Conclusion. CAV-1 mediates the functional coupling of microglia, astrocytes, and neurons, and thus EPAC-1/CAV-1 plays an important role in CPSP exacerbation.

Postsurgical Latent Pain Sensitization Is Driven by Descending Serotonergic Facilitation and Masked by µ-Opioid Receptor Constitutive Activity in the Rostral Ventromedial Medulla

Following tissue injury, latent sensitization (LS) of nociceptive signaling can persist indefinitely, kept in remission by compensatory µ-opioid receptor constitutive activity (MORCA) in the dorsal horn of the spinal cord. To demonstrate LS, we conducted plantar incision in mice and then waited 3-4 weeks for hypersensitivity to resolve. At this time (remission), systemic administration of the opioid receptor antagonist/inverse agonist naltrexone reinstated mechanical and heat hypersensitivity. We first tested the hypothesis that LS extends to serotonergic neurons in the rostral ventral medulla (RVM) that convey pronociceptive input to the spinal cord. We report that in male and female mice, hypersensitivity was accompanied by increased Fos expression in serotonergic neurons of the RVM, abolished on chemogenetic inhibition of RVM 5-HT neurons, and blocked by intrathecal injection of the 5-HT3R antagonist ondansetron; the 5-HT2AR antagonist MDL-11 939 had no effect. Second, to test for MORCA, we microinjected the MOR inverse agonist d-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) and/or neutral opioid receptor antagonist 6β-naltrexol. Intra-RVM CTAP produced mechanical hypersensitivity at both hindpaws; 6β-naltrexol had no effect by itself, but blocked CTAP-induced hypersensitivity. This indicates that MORCA, rather than an opioid ligand-dependent mechanism, maintains LS in remission. We conclude that incision establishes LS in descending RVM 5-HT neurons that drives pronociceptive 5-HT3R signaling in the dorsal horn, and this LS is tonically opposed by MORCA in the RVM. The 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic postsurgical pain.SIGNIFICANCE STATEMENT Surgery leads to latent pain sensitization and a compensatory state of endogenous pain control that is maintained long after tissue healing. Here, we show that either chemogenetic inhibition of serotonergic neuron activity in the RVM or pharmacological inhibition of 5-HT3 receptor signaling at the spinal cord blocks behavioral signs of postsurgical latent sensitization. We conclude that MORCA in the RVM opposes descending serotonergic facilitation of LS and that the 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic postsurgical pain.

Estrogen metabolites increase nociceptor hyperactivity in a mouse model of uterine pain

Pain emanating from the female reproductive tract is notoriously difficult to be treated and the prevalence of transient pelvic pain has been placed as high as 70-80% in women surveyed. Although sex hormones, especially estrogen, are thought to underlie enhanced pain perception in females, the underlying molecular and cellular mechanisms are not completely understood. Here we show that the pain-initiating TRPA1 channel is required for pain-related behaviors in a mouse model of estrogen-induced uterine pain in ovariectomized female mice. Surprisingly, 2- and 4-hydroxylated estrogen metabolites (HEMs) in the estrogen hydroxylation pathway, but not estrone, estradiol and 16-HEMs, directly increase nociceptor hyperactivity through TRPA1 and TRPV1 channels, and picomolar concentrations of 2- and 4-hydroxylation estrone (OHE1) can sensitize TRPA1 channel function. Moreover, both TRPA1 and TRPV1 are expressed in uterine-innervating primary nociceptors and their expressions are increased in the estrogen-induced uterine pain model. Importantly, pretreatment of 2- or 4-OHE1 recapitulates estrogen-induced uterine pain-like behaviors and intraplantar injections of 2- and 4-OHE1 directly produce a TRPA1-dependent mechanical hypersensitivity. Our findings demonstrate that TRPA1 is critically involved in estrogen-induced uterine pain-like behaviors, which may provide a potential drug target for treating female reproductive tract pain.

Endogenous µ-opioid receptor activity in the lateral and capsular subdivisions of the right central nucleus of the amygdala prevents chronic postoperative pain

Tissue injury induces a long-lasting latent sensitization (LS) of spinal nociceptive signaling that is kept in remission by an opposing µ-opioid receptor (MOR) constitutive activity. To test the hypothesis that supraspinal sites become engaged, we induced hindpaw inflammation, waited 3 weeks for mechanical hypersensitivity to resolve, and then injected the opioid receptor inhibitors naltrexone, CTOP or β-funaltrexamine subcutaneously, and/or into the cerebral ventricles. Intracerebroventricular injection of each inhibitor reinstated hypersensitivity and produced somatic signs of withdrawal, indicative of LS and endogenous opioid dependence, respectively. In naïve or sham controls, systemic naloxone (3 mg/kg) produced conditioned place aversion, and systemic naltrexone (3 mg/kg) increased Fos expression in the central nucleus of the amygdala (CeA). In LS animals tested 3 weeks after plantar incision, systemic naltrexone reinstated mechanical hypersensitivity and produced an even greater increase in Fos than in sham controls, particularly in the capsular subdivision of the right CeA. One third of Fos+ profiles co-expressed protein kinase C delta (PKCδ), and 35% of PKCδ neurons co-expressed tdTomato+ in Oprm1^Cre ::tdTomato transgenic mice. CeA microinjection of naltrexone (1 µg) reinstated mechanical hypersensitivity only in male mice and did not produce signs of somatic withdrawal. Intra-CeA injection of the MOR-selective inhibitor CTAP (300 ng) reinstated hypersensitivity in both male and female mice. We conclude that MORs in the capsular subdivision of the right CeA prevent the transition from acute to chronic postoperative pain.

Behavioral characterization, potential clinical relevance and mechanisms of latent pain sensitization

Chronic pain is a debilitating disorder that can occur as painful episodes that alternates with bouts of remission and occurs despite healing of the primary insult. Those episodes are often triggered by stressful events. In the last decades, a similar situation has been evidenced in a wide variety of rodent models (including inflammatory pain, neuropathy and opioid-induced hyperalgesia) where animals develop a chronic latent hyperalgesia that silently persists after behavioral signs of pain resolution. This state, referred as latent pain sensitization, is due to the compensatory activation of antinociceptive systems, such as the opioid system or NPY and its receptors. A transitory phase of hyperalgesia can then be reinstated by pharmacological or genetic blockade of these antinociceptive systems or by submitting animals to acute stress. Those observations reveal that there is a constant endogenous analgesia responsible for chronic pain inhibition that might paradoxically contribute to maintain this maladaptive state and could then participate to the transition from acute to chronic pain. Thus, demonstration of the existence of this phenomenon in humans and a better understanding of the mechanisms by which latent pain sensitization develops and maintains over long periods of time will be of particular interest to help identifying new therapeutic strategies and targets for chronic pain treatment. The present review aims to recapitulate behavioral expression, potential clinical relevance, cellular mechanisms and intracellular signaling pathways involved so far in latent pain sensitization.

Anxiolytic effect of GABAergic neurons in the anterior cingulate cortex in a rat model of chronic inflammatory pain

Chronic pain easily leads to concomitant mood disorders, and the excitability of anterior cingulate cortex (ACC) pyramidal neurons (PNs) is involved in chronic pain-related anxiety. However, the mechanism by which PNs regulate pain-related anxiety is still unknown. The GABAergic system plays an important role in modulating neuronal activity. In this paper, we aimed to study how the GABAergic system participates in regulating the excitability of ACC PNs, consequently affecting chronic inflammatory pain-related anxiety. A rat model of CFA-induced chronic inflammatory pain displayed anxiety-like behaviors, increased the excitability of ACC PNs, and reduced inhibitory presynaptic transmission; however, the number of GAD65/67 was not altered. Interestingly, intra-ACC injection of the GABA_AR agonist muscimol relieved anxiety-like behaviors but had no effect on chronic inflammatory pain. Intra-ACC injection of the GABA_AR antagonist picrotoxin induced anxiety-like behaviors but had no effect on pain in normal rats. Notably, chemogenetic activation of GABAergic neurons in the ACC alleviated chronic inflammatory pain and pain-induced anxiety-like behaviors, enhanced inhibitory presynaptic transmission, and reduced the excitability of ACC PNs. Chemogenetic inhibition of GABAergic neurons in the ACC led to pain-induced anxiety-like behaviors, reduced inhibitory presynaptic transmission, and enhanced the excitability of ACC PNs but had no effect on pain in normal rats. We demonstrate that the GABAergic system mediates a reduction in inhibitory presynaptic transmission in the ACC, which leads to enhanced excitability of pyramidal neurons in the ACC and is associated with chronic inflammatory pain-related anxiety.

Neuropeptide Y prevents nucleus pulposus cells from cell apoptosis and IL‑1β‑induced extracellular matrix degradation

Intervertebral disc degeneration (IDD) is characterized by excessive inflammatory reaction, and neuropeptide Y (NPY) was reported to have anti-inflammatory effect. However, the effect of NPY on NP cells has not been investigated up to date. This study aimed to clarify the role of NPY on the process of IDD. Fourteen fresh human lumbar intervertebral discs were harvested, and degeneration-related proteins were examined. Pfirrmann grading system was used to evaluate IDD. Rat nucleus pulposus (NP) cells were used to investigate the effect of NPY on the proliferation, apoptosis, and extracellular matrix (ECM) in NP cell induced by IL-1βin vitro. The expression levels of NPY and its receptors (type 1 receptor, Y1R, and type 2 receptor, Y2R) were detected via immunohistochemical analysis, western blot, and quantitative real-time polymerase chain reaction (qRT-PCR). Cell viability and proliferation were explored using cell counting kit-8 assay, western blot, and immunofluorescence analysis. Cell apoptosis was investigated by Hoechst staining, JC-1 Staining, annexin V-FITC/PI double staining, and western blot. The secretion of NPY from NP cells was determined via enzyme-linked immunosorbent assay (ELISA). The expression of anabolic and catabolic gene was analyzed by qRT-PCR, western blot, immunofluorescence analysis, and ELISA. The expression of Y2R was significantly increased in both human degenerative intervertebral discs and IL-1β-induced NP cells. Although no positive results for NPY indicated by western blot both in vivo and in vitro, ELISA results demonstrated that the secretion of NPY from NP cells was increased by low-concentration IL-1β, but was decreased when the concentration of IL-1β was 30 ng/ml and above. In addition, NPY could promote NP cells proliferation and protect NP cells against IL‑1β‑induced apoptosis via suppressing mitochondrial-mediated apoptosis pathway. What's more, NPY can suppress the expression of catabolic gene and ameliorate IL-1β- induced matrix degeneration in NP cells. In conclusion, NPY could promote NP cell proliferation and alleviate IL‑1β‑induced cell apoptosis via mitochondrial pathway. In addition, NPY can suppress the expression of ECM‑catabolic proteinases and ameliorate IL-1β- induced ECM degeneration in vitro.

The µ-δ opioid heteromer masks latent pain sensitization in neuropathic and inflammatory pain in male and female mice

The episodic nature of chronic pain can be studied in the rodent model of latent pain sensitization. After remission, central sensitization is opposed by activation of opioid receptors. At the behavioral level, latent pain sensitization is unmasked when pain hypersensitivity is reinstated by opioid receptor (OR) antagonism. Previous studies have focused on inflammatory pain and male rodents. Whether latent pain sensitization occurs in models of chemotherapy-induced neuropathic pain in female and male mice is unknown. The first aim of this study was to investigate whether μ- and δ-OR suppress latent pain sensitization in our model of chemotherapy-induced neuropathic pain in both sexes. Mounting evidence suggests that μ-and δ-ORs form a heteromer and that the heteromer modulates pain sensitivity. Potential implications of the μ-δ OR heteromer in latent pain sensitization have not been fully explored due to a lack of tools to effectively modulate the heteromer. To specifically target the μ-δ OR heteromer, we used a specific interfering peptide blocking the heteromerization. The second aim of this study was to investigate whether disruption of the μ-δOR heteromer, after remission, reinstates pain hypersensitivity. After remission from cisplatin-induced neuropathic pain, antagonism of µ-OR and δOR reinstates pain hypersensitivity in both sexes. After remission from cisplatin-induced neuropathic pain and postoperative pain, disruption of the μ-δOR heteromer reinstates pain hypersensitivity in both sexes. Taken together our findings suggest that the μ-δOR heteromer plays a crucial role in remission in various pain models and may represent a novel therapeutic target to prevent the relapse to pain and the transition to chronic pain.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur - prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT₄ receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT₄ agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT₄-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the G_s-coupled 5-HT₄ receptor and downstream cAMP signaling involving both PKA and EPAC.

High-dose naloxone: Effects by late administration on pain and hyperalgesia following a human heat injury model. A randomized, double-blind, placebo-controlled, crossover trial with an enriched enrollment design

Severe chronic postsurgical pain has a prevalence of 4–10% in the surgical population. The underlying nociceptive mechanisms have not been well characterized. Following the late resolution phase of an inflammatory injury, high-dose μ-opioid-receptor inverse agonists reinstate hypersensitivity to nociceptive stimuli. This unmasking of latent pain sensitization has been a consistent finding in rodents while only observed in a limited number of human volunteers. Latent sensitization could be a potential triggering venue in chronic postsurgical pain. The objective of the present trial was in detail to examine the association between injury-induced secondary hyperalgesia and naloxone-induced unmasking of latent sensitization. Healthy volunteers (n = 80) received a cutaneous heat injury (47°C, 420 s, 12.5 cm²). Baseline secondary hyperalgesia areas were assessed 1 h post-injury. Utilizing an enriched enrollment design, subjects with a magnitude of secondary hyperalgesia areas in the upper quartile (‘high-sensitizers’ [n = 20]) and the lower quartile (‘low-sensitizers’ [n = 20]) were selected for further study. In four consecutive experimental sessions (Sessions 1 to 4), the subjects at two sessions (Sessions 1 and 3) received a cutaneous heat injury followed 168 h later (Sessions 2 and 4) by a three-step target-controlled intravenous infusion of naloxone (3.25 mg/kg), or normal saline. Assessments of secondary hyperalgesia areas were made immediately before and stepwise during the infusions. Simple univariate statistics revealed no significant differences in secondary hyperalgesia areas between naloxone and placebo treatments (P = 0.215), or between ‘high-sensitizers’ and ‘low-sensitizers’ (P = 0.757). In a mixed-effects model, secondary hyperalgesia areas were significantly larger following naloxone as compared to placebo for ‘high-sensitizers’ (P < 0.001), but not ‘low-sensitizers’ (P = 0.651). Although we could not unequivocally demonstrate naloxone-induced reinstatement of heat injury-induced hyperalgesia, further studies in clinical postsurgical pain models are warranted.

A Src family kinase maintains latent sensitization in rats, a model of inflammatory and neuropathic pain

Latent sensitization is a long-term model of chronic pain in which hyperalgesia is continuously suppressed by opioid receptors, as demonstrated by the induction of mechanical allodynia by opioid antagonists. Different intracellular signals may mediate the initiation, maintenance and expression of latent sensitization. Our criterion for the involvement of a signal in the maintenance of latent sensitization is that inhibitors should permanently eliminate the allodynia produced by an opioid antagonist. We hypothesized that Src family kinases (SFKs) maintain latent sensitization and tested this hypothesis by inducing latent sensitization in rats with complete Freund's adjuvant (CFA) or spared nerve injury. After measures of mechanical allodynia returned to baseline, vehicle or the SFK inhibitor PP2 were injected intrathecally. The opioid antagonist naltrexone injected intrathecally 15 min later produced allodynia in control rats but not in rats injected with PP2. Vehicle or PP2 were injected daily for two more days and naltrexone was injected five days later. Again, naltrexone induced allodynia in the control rats but not in the rats injected with PP2. Results were similar when latent sensitization was induced with CFA or spared nerve injury. We concluded that an SFK, likely Fyn, maintains latent sensitization induced by inflammation or nerve injury.

Neurokinin 1 receptor activation in the rat spinal cord maintains latent sensitization, a model of inflammatory and neuropathic chronic pain

Latent sensitization is a model of chronic pain in which a persistent state of pain hypersensitivity is suppressed by opioid receptors, as evidenced by the ability of opioid antagonists to induce a period of mechanical allodynia. Our objective was to determine if substance P and its neurokinin 1 receptor (NK1R) mediate the maintenance of latent sensitization. Latent sensitization was induced by injecting rats in the hindpaw with complete Freund's adjuvant (CFA), or by tibial spared nerve injury (SNI). When responses to von Frey filaments returned to baseline (day 28), the rats were injected intrathecally with saline or the NK1R antagonist RP67580, followed 15 min later by intrathecal naltrexone. In both pain models, the saline-injected rats developed allodynia for 2 h after naltrexone, but not the RP67580-injected rats. Saline or RP67580 were injected daily for two more days. Five days later (day 35), naltrexone was injected intrathecally. Again, the saline-injected rats, but not the RP67580-injected rats, developed allodynia in response to naltrexone. To determine if there is sustained activation of NK1Rs during latent sensitization, NK1R internalization was measured in lamina I neurons in rats injected in the paw with saline or CFA, and then injected intrathecally with saline or naltrexone on day 28. The rats injected with CFA had a small amount of NK1R internalization that was significantly higher than in the saline-injected rats. Naltrexone increased NK1R internalization in the CFA-injected rats but nor in the saline-injected rats. Therefore, sustained activation of NK1Rs maintains pain hypersensitivity during latent sensitization.

Targeting spinal neuropeptide Y1 receptor-expressing interneurons to alleviate chronic pain and itch

An accelerating basic science literature is providing key insights into the mechanisms by which spinal neuropeptide Y (NPY) inhibits chronic pain. A key target of pain inhibition is the G_i-coupled neuropeptide Y1 receptor (Y1). Y1 is located in key sites of pain transmission, including the peptidergic subpopulation of primary afferent neurons and a dense subpopulation of small, excitatory, glutamatergic/somatostatinergic interneurons (Y1-INs) that are densely expressed in the dorsal horn, particularly in superficial lamina I-II. Selective ablation of spinal Y1-INs with an NPY-conjugated saporin neurotoxin attenuates the development of peripheral nerve injury-induced mechanical and cold hypersensitivity. Conversely, conditional knockdown of NPY expression or intrathecal administration of Y1 antagonists reinstates hypersensitivity in models of chronic latent pain sensitization. These and other results indicate that spinal NPY release and the consequent inhibition of pain facilitatory Y1-INs represent an important mechanism of endogenous analgesia. This mechanism can be mimicked with exogenous pharmacological approaches (e.g. intrathecal administration of Y1 agonists) to inhibit mechanical and thermal hypersensitivity and spinal neuron activity in rodent models of neuropathic, inflammatory, and postoperative pain. Pharmacological activation of Y1 also inhibits mechanical- and histamine-induced itch. These immunohistochemical, pharmacological, and cell type-directed lesioning data, in combination with recent transcriptomic findings, point to Y1-INs as a promising therapeutic target for the development of spinally directed NPY-Y1 agonists to treat both chronic pain and itch.

Neuropeptide Y tonically inhibits an NMDAR➔AC1➔TRPA1/TRPV1 mechanism of the affective dimension of chronic neuropathic pain

Transection of the sural and common peroneal branches of the sciatic nerve produces cutaneous hypersensitivity at the tibial innervation territory of the mouse hindpaw that resolves within a few weeks. We report that interruption of endogenous neuropeptide Y (NPY) signaling during remission, with either conditional NPY knockdown in NPY^tet/tet mice or intrathecal administration of the Y1 receptor antagonist BIBO3304, reinstated hypersensitivity. These data indicate that nerve injury establishes a long-lasting latent sensitization of spinal nociceptive neurons that is masked by spinal NPY-Y1 neurotransmission. To determine whether this mechanism extends beyond the sensory component of nociception, we used conditioned place aversion and preference assays to evaluate the affective component of pain. We found that BIBO3304 produced place aversion in mice when administered during remission. Furthermore, the analgesic drug gabapentin produced place preference after NPY knockdown in NPY^tet/tet but not control mice. We then used pharmacological agents and deletion mutant mice to investigate the cellular mechanisms of neuropathic latent sensitization. BIBO3304-induced reinstatement of mechanical hypersensitivity and conditioned place aversion could be prevented with intrathecal administration of an N-methyl-d-aspartate receptor antagonist (MK-801) and was absent in adenylyl cyclase type 1 (AC1) deletion mutant mice. BIBO3304-induced reinstatement could also be prevented with intrathecal administration an AC1 inhibitor (NB001) or a TRPV1 channel blocker (AMG9801), but not vehicle. Intrathecal administration of a TRPA1 channel blocker (HC030031) prevented the reinstatement of neuropathic hypersensitivity produced either by BIBO3304, or by NPY knockdown in NPY^tet/tet but not control mice. Our results confirm new mediators of latent sensitization: TRPA1 and TRPV1. We conclude that NPY acts at spinal Y1 to tonically inhibit a molecular NMDAR➔AC1 intracellular signaling pathway in the dorsal horn that is induced by peripheral nerve injury and drives both the sensory and affective components of chronic neuropathic pain.

SAP102 contributes to hyperalgesia formation in the cancer induced bone pain rat model by anchoring NMDA receptors

The pathogenesis of cancer induced bone pain (CIBP) is extremely complex, and glutamate receptor dysfunction plays an important role in the formation of CIBP. Synapse-associated protein 102 (SAP102) anchors glutamate receptors in the postsynaptic membrane. However, its effect on hyperalgesia formation in CIBP has not been clarified. This study investigated the role of SAP102 in the formation of hyperalgesia in rats with CIBP SAP102 is present in spinal dorsal horn neurons, but not in astrocytes or microglia. NMDAR-NR2B is localized with neurons. In addition, SAP102 and NMDAR-NR2B expression levels in spinal dorsal horn tissues were detected by Western blot and co-immunoprecipitation. Intrathecal injection of lentiviral vector of RNAi to knockdown SAP102 expression in the spinal dorsal horn significantly attenuated abnormal mechanic pain when compared to non-coding lentiviral vector. These findings indicate that SAP102 can anchor NMDA receptors to affect hyperalgesia formation in bone cancer pain.

Sex differences in kappa opioid receptor inhibition of latent postoperative pain sensitization in dorsal horn

Tissue injury produces a delicate balance between latent pain sensitization (LS) and compensatory endogenous opioid receptor analgesia that continues for months, even after re-establishment of normal pain thresholds. To evaluate the contribution of mu (MOR), delta (DOR), and/or kappa (KOR) opioid receptors to the silencing of chronic postoperative pain, we performed plantar incision at the hindpaw, waited 21 days for the resolution of hyperalgesia, and then intrathecally injected subtype-selective ligands. We found that the MOR-selective inhibitor CTOP (1-1000 ng) dose-dependently reinstated mechanical hyperalgesia. Two DOR-selective inhibitors naltrindole (1-10 μg) and TIPP[Ψ] (1-20 μg) reinstated mechanical hyperalgesia, but only at the highest dose that also produced itching, licking, and tail biting. Both the prototypical KOR-selective inhibitors nor-BNI (0.1-10 μg) and the newer KOR inhibitor with more canonical pharmocodynamic effects, LY2456302 (0.1-10 μg), reinstated mechanical hyperalgesia. Furthermore, LY2456302 (10 μg) increased the expression of phosphorylated signal-regulated kinase (pERK), a marker of central sensitization, in dorsal horn neurons but not glia. Sex studies revealed that LY2456302 (0.3 μg) reinstated hyperalgesia and pERK expression to a greater degree in female as compared to male mice. Our results suggest that spinal MOR and KOR, but not DOR, maintain LS within a state of remission to reduce the intensity and duration of postoperative pain, and that endogenous KOR but not MOR analgesia is greater in female mice.