Hypothalamic GPR40 signaling activated by free long chain fatty acids suppresses CFA-induced inflammatory chronic pain

Metallothionein-3-mediated intracellular zinc mediates antioxidant and anti-inflammatory responses in the complete Freund's adjuvant-induced inflammatory pain mouse model

Chronic inflammatory pain is often caused by peripheral tissue damage and persistent inflammation. This disease substantially affects patients' physical and social well-being. We investigated the role of metallothionein-3 (MT3) in modulating complete Freund's adjuvant (CFA)-induced intracellular Zn2+ activity in an MT3 knockout mouse model of inflammatory pain in the hind paw. The results demonstrated that increasing intracellular Zn2+ levels ameliorate deficits in motor behavior, as well as inflammation in the paw, spleen, and thymus. Furthermore, intracellular Zn2+ was crucial in regulating oxidative stress markers (glutathione, superoxide dismutase, catalase, and malondialdehyde) and inflammatory cytokines, such as tumor necrosis factor-α and interleukin-6, in MT3 knockout mice induced with CFA. This study highlights the critical role of MT3 in coordinating the intracellular interaction with Zn2+, which is vital for the immune systems's protective functions. These interactions are fundamental for maintaining metal ion homeostasis and regulating the synthesis of various biomolecules in the body.

Moxibustion Alleviates Inflammation via SIRT5-mediated Post-translational Modification and Macrophage Polarization

Macrophage polarisation is influenced by Sirtuin5 (SIRT5), which is crucial for regulating anti-inflammatory processes. Moxibustion, a traditional Chinese medicine therapy, exerts anti-inflammatory effects by altering the succinate/α-ketoglutarate (α-KG) ratio, an indicator of the M1 to M2 macrophage shift. Glutamate dehydrogenase 1 (GLUD1), a key enzyme involved in α-KG production, is desuccinylated by SIRT5. Currently, the potential influence of moxibustion on SIRT5-GLUD1-α-KG-mediated macrophage polarization in inflammatory diseases remains unexplored. C57BL/6 J and Sirt5 knockout mice were used as complete Freund's adjuvant (CFA)-induced adjuvant arthritis models. Moxibustion and acupoint injections of MC3482 were administered. Paw capacity asssays and ELISA were performed to quantify inflammatory effects and the expression of succinate, and α-KG expressions. Flow cytometry (FCM) and immunofluorescence were used to assesss the expression of M1- and M2-like macrophages. LC-MS/MS-based proteomic analysis was performed, and GLUD1 was identified desuccinylated protein associated with SIRT5. Western blotting and immunoprecipitation (IP) were used to detect SIRT5, GLUD1, and succinylated GLUD1expressions. Moxibustion and the SIRT5-mediated desuccinylation inhibitor MC3482 decreased inflammation by increasing the number of M2 macrophages and reducing the number of M1 macrophage in the CFA model. The potential mechanism may be related to the effects of moxibustion and SIRT5 inhibition, which inverted succinate and α-KG levels in the CFA group, resulting in low succinate, high α-KG, and increased GLUD1 succinylation after treatment. These findings suggest that the anti-inflammatory effects moxibustion are related to the impact of macrophage conversion after SIRT5-mediated post-translational modification.

Metabolite-Sensing Receptors: Emerging Targets for Modulating Chronic Pain Pathways

Chronic pain is a debilitating condition affecting millions worldwide, often resulting from complex interactions between the nervous and immune systems. Recent advances highlight the critical role of metabolite-sensing G protein-coupled receptors (GPCRs) in various chronic pain types. These receptors link metabolic changes with cellular responses, influencing inflammatory and degenerative processes. Receptors such as free fatty acid receptor 1 (FFAR1/GPR40), free fatty acid receptor 4 (FFAR4/GPR120), free fatty acid receptor 2 (FFAR2/GPR43), and Takeda G protein-coupled receptor 5 (TGR5/GPR131/GPBAR1) are key modulators of nociceptive signaling. GPR40, activated by long-chain fatty acids, exhibits strong anti-inflammatory effects by reducing cytokine expression. Butyrate-activated GPR43 inhibits inflammatory mediators like nitric oxide synthase-2 and cyclooxygenase-2, mitigating inflammation. TGR5, activated by bile acids, regulates inflammation and cellular senescence through pathways like NF-κB and p38. These receptors are promising therapeutic targets in chronic pain, addressing the metabolic and inflammatory factors underlying nociceptive sensitization and tissue degeneration. This review explores the molecular mechanisms of metabolite-sensing receptors in chronic pain, their therapeutic potential, and challenges in clinical application. By uncovering these mechanisms, metabolite-sensing receptors could lead to safer, more effective pain management strategies.

4-Hydroxynonenal from Mitochondrial and Dietary Sources Causes Lysosomal Cell Death for Lifestyle-Related Diseases

Excessive consumption of vegetable oils such as soybean and canolla oils containing ω-6 polyunsaturated fatty acids is considered one of the most important epidemiological factors leading to the progression of lifestyle-related diseases. However, the underlying mechanism of vegetable-oil-induced organ damage is incompletely elucidated. Since proopiomelanocortin (POMC) neurons in the hypothalamus are related to the control of appetite and energy expenditure, their cell degeneration/death is crucial for the occurrence of obesity. In patients with metabolic syndrome, saturated fatty acids, especially palmitate, are used as an energy source. Since abundant reactive oxygen species are produced during β-oxidation of the palmitate in mitochondria, an increased amount of 4-hydroxy-2-nonenal (4-HNE) is endogenously generated from linoleic acids constituting cardiolipin of the inner membranes. Further, due to the daily intake of deep-fried foods and/or high-fat diets cooked using vegetable oils, exogenous 4-HNE being generated via lipid peroxidation during heating is incorporated into the blood. By binding with atheromatous and/or senile plaques, 4-HNE inactivates proteins via forming hybrid covalent chemical addition compounds and causes cellular dysfunction and tissue damage by the specific oxidation carbonylation. 4-HNE overstimulates G-protein-coupled receptors to induce abnormal Ca2+ mobilization and µ-calpain activation. This endogenous and exogenous 4-HNE synergically causes POMC neuronal degeneration/death and obesity. Then, the resultant metabolic disorder facilitates degeneration/death of hippocampal neurons, pancreatic β-cells, and hepatocytes. Hsp70.1 is a molecular chaperone which is crucial for both protein quality control and the stabilization of lysosomal limiting membranes. Focusing on the monkey hippocampus after ischemia, previously we formulated the 'calpain-cathepsin hypothesis', i.e., that calpain-mediated cleavage of carbonylated Hsp70.1 is a trigger of programmed neuronal death. This review aims to report that in diverse organs, lysosomal cell degeneration/death occurs via the calpain-cathepsin cascade after the consecutive injections of synthetic 4-HNE in monkeys. Presumably, 4-HNE is a root substance of lysosomal cell death for lifestyle-related diseases.

Anti-Inflammatory and Antinociceptive Properties of the Quercetin-3-Oleate AV2, a Novel FFAR1 Partial Agonist

Free fatty acid receptor 1 (FFAR1) has emerged as the most targeted isoform of the free fatty acid receptors because of its involvement in the modulation of energy balance and its potential role in the control of inflammatory and pain conditions. Quercetin-3-oleate (AV2), recognized as a new FFAR1 partial agonist, was investigated for its ability to modulate inflammation and nociception. Human immortal neuroblastoma SH and the murine macrophagic RAW 264.7 cells were used to evaluate cell viability, the potential cytoprotective activity, and the anti-inflammatory properties of AV2 in vitro. Paw edema, caused by zymosan-A, and the formalin test were used to assess the in vivo anti-inflammatory and antinociceptive effects in CD-1 mice. In vitro, AV2 was devoid of cytotoxicity, significantly reduced ROS in both cell types, and protected RAW 264.7 cells from lipopolysaccharide damage by reducing tumor necrosis factor-α production. Interestingly, AV2 induced a transient elevation of intracellular calcium that was reduced in cells, pre-incubated with the FFAR1 antagonist DC260126. In vivo, AV2 reduced formalin-induced nociception and zymosan A-induced paw edema, and both effects were reversed by the FFAR1 antagonist GW1100. In conclusion, these data strongly support the AV2-mediated antioxidant, anti-inflammatory, and antinociceptive activity. AV2 represents a promising molecule for the clinical management of inflammatory-related pain conditions.

Specific inhibition of TET1 in the spinal dorsal horn alleviates inflammatory pain in mice by regulating synaptic plasticity

DNA demethylation mediated by ten-eleven translocation 1 (TET1) is a critical epigenetic mechanism in which gene expression is regulated via catalysis of 5-methylcytosine to 5-hydroxymethylcytosine. Previously, we demonstrated that TET1 is associated with the genesis of chronic inflammatory pain. However, how TET1 participates in enhanced nociceptive responses in chronic pain remains poorly understood. Here, we report that conditional knockout of Tet1 in dorsal horn neurons via intrathecal injection of rAAV-hSyn-Cre in Tet1fl/fl mice not only reversed the inflammation-induced upregulation of synapse-associated proteins (post-synaptic density protein 95 (PSD95) and synaptophysin (SYP)) in the dorsal horn but also ameliorated abnormalities in dendritic spine morphology and alleviated pain hypersensitivities. Pharmacological blockade of TET1 by intrathecal injection of a TET1-specific inhibitor-Bobcat 339-produced similar results, as did knockdown of Tet1 by intrathecal injection of siRNA. Thus, our data strongly suggest that increased TET1 expression during inflammatory pain upregulates the expression of multiple synapse-associated proteins and dysregulates synaptic morphology in dorsal horn neurons, suggesting that Tet1 may be a potential target for analgesic strategies.

HCN2 Channels in the Ventral Hippocampal CA1 Regulate Nociceptive Hypersensitivity in Mice

Chronic pain is a significant health problem worldwide. Recent evidence has suggested that the ventral hippocampus is dysfunctional in humans and rodents, with decreased neuronal excitability and connectivity with other brain regions, parallel pain chronicity, and persistent nociceptive hypersensitivity. But the molecular mechanisms underlying hippocampal modulation of pain remain poorly elucidated. In this study, we used ex vivo whole-cell patch-clamp recording, immunofluorescence staining, and behavioral tests to examine whether hyperpolarization-activated cyclic nucleotide-gated channels 2 (HCN2) in the ventral hippocampal CA1 (vCA1) were involved in regulating nociceptive perception and CFA-induced inflammatory pain in mice. Reduced sag potential and firing rate of action potentials were observed in vCA1 pyramidal neurons from CFA-injected mice. Moreover, the expression of HCN2, but not HCN1, in vCA1 decreased in mice injected with CFA. HCN2 knockdown in vCA1 pyramidal neurons induced thermal hypersensitivity, whereas overexpression of HCN2 alleviated thermal hyperalgesia induced by intraplantar injection of CFA in mice. Our findings suggest that HCN2 in the vCA1 plays an active role in pain modulation and could be a promising target for the treatment of chronic pain.

Role of SIRT5 in the analgesic effectiveness of moxibustion at ST36 in mice with inflammatory pain

Sirtuine5 (SIRT5) is an important molecule involved in the pathology of inflammatory diseases. To investigate the impact of SIRT5 on the analgesic effectiveness of moxibustion, we established a complete Freund's adjuvant- (CFA-) induced inflammatory pain in mice model. Moxibustion was applied at the Zusanli (ST36) acupoint in mice with inflammatory pain. The analgesic effectiveness was evaluated by thermal hyperalgesia and mechanical allodynia tests in the right paws after CFA injection. The expression of inflammatory cytokines, including the pro-inflammatory factors IL-1β and TNF-α, and the anti-inflammatory factors IL-4 and TGF-β expressions, was evaluated using by ELISA. Furthermore, SIRT5 was evaluated by immunofluorescence and western blotting. The results showed that, compared with the CFA group, both thermal and mechanical pain thresholds increased with moxibustion and the SIRT5 inhibitor MC3482 intervention at ST36. Additionally, compared to the CFA-induced group, the inflammatory mediators, including IL-1β and TNF-α, decreased, while the anti-inflammatory cytokines IL-4 and TGF-β increased with moxibustion and MC3482 ST36 acupoint injection. Western blot results showed a decreased expression of SIRT5 at the ST36 site with moxibustion and MC3482 injection, compared to the CFA-induced group. SIRT5 expression in the right paw of mice injected with moxibustion and MC3482 was higher than that in the CFA-induced group. This study revealed that SIRT5 expression is involved in moxibustion analgesia and may be a potential mediator in the regulation of analgesia.

GPRC5B protects osteoarthritis by regulation of autophagy signaling

Osteoarthritis (OA) is one of the most common chronic diseases in the world. However, current treatment modalities mainly relieve pain and inhibit cartilage degradation, but do not promote cartilage regeneration. In this study, we show that G protein-coupled receptor class C group 5 member B (GPRC5B), an orphan G-protein-couple receptor, not only inhibits cartilage degradation, but also increases cartilage regeneration and thereby is protective against OA. We observed that Gprc5b deficient chondrocytes had an upregulation of cartilage catabolic gene expression, along with downregulation of anabolic genes in vitro. Furthermore, mice deficient in Gprc5b displayed a more severe OA phenotype in the destabilization of the medial meniscus (DMM) induced OA mouse model, with upregulation of cartilage catabolic factors and downregulation of anabolic factors, consistent with our in vitro findings. Overexpression of Gprc5b by lentiviral vectors alleviated the cartilage degeneration in DMM-induced OA mouse model by inhibiting cartilage degradation and promoting regeneration. We also assessed the molecular mechanisms downstream of Gprc5b that may mediate these observed effects and identify the role of protein kinase B (AKT)-mammalian target of rapamycin (mTOR)-autophagy signaling pathway. Thus, we demonstrate an integral role of GPRC5B in OA pathogenesis, and activation of GPRC5B has the potential in preventing the progression of OA.

Discovery of a structurally novel, potent, and once-weekly free fatty acid receptor 1 agonist for the treatment of diabetes

Type 2 diabetes mellitus (T2DM) is a lifelong disease that requires long-term medication to control glucose levels, and thereby long-acting drug has been clinically needed for improving medical adherence. The free fatty acid receptor 1 (FFA1) was considered as a promising target for several diseases, such as T2DM, pain and fatty liver. However, no once-weekly FFA1 agonist has been reported until now. Herein, we report the successful discovery of ZLY50, the first once-weekly FFA1 agonist with a completely new chemotype, highly agonistic activity and selectivity on FFA1. Moreover, ZLY50 has enough brain exposure to activate FFA1 in brain, and it is the first orally available FFA1 agonist with analgesic activity. Notably, the long-term anti-diabetic and anti-fatty liver effects of ZLY50 (once-weekly) were better than those of HWL-088 (once-daily), a highly potent FFA1 agonist with far stronger glucose-lowering effect than Phase 3 clinical candidate TAK-875. Further mechanism studies suggested that ZLY50 alleviates fatty liver by regulating the expressions of genes related to lipid metabolism, mitochondrial function, and oxidative stress in liver.

[Possible involvement of FFAR1 signaling in mouse emotional behaviors through the regulation of brain monoamine releases]

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor for medium- to long-chain free fatty acids. We have previously shown that FFAR1 signaling pathway plays an important suppressive role in spinal nociceptive processing after peripheral inflammation and nerve injury, and that FFAR1 agonists might serve as a new class of analgesics for treating inflammatory and neuropathic pain. To further pursue the functional significance of central FFAR1 signaling, we investigated the possible involvement of FFAR1 in endogenous pain modulation, depressive-like behavior, and aberrant behavior induced by addictive drugs using FFAR1 agonist (GW9508), FFAR1 antagonist (GW1100), and FFAR1 gene-deficient mice. As a result, FFAR1-deficient mice were found to exhibit stronger inflammatory and peripheral neuropathic pain-like behavior as well as depressive-like behavior. In particular, we noticed that peripheral nerve injury-induced depressive-like behavior was insensitive to imipramine. Next, we employed in vivo microdialysis to investigate whether FFAR1 is actually involved in the regulation of brain monoamines (dopamine and serotonin) releases. Our findings suggest that FFAR1 indirectly regulates dopamine release by promoting serotonin release. Thus, we are currently investigating how FFAR1 is involved in behavioral changes induced by addictive drugs such as cocaine and morphine. In this review, we briefly discuss about the possible involvement of FFAR1 in cocaine-induced locomotor hyperactivity.

TRAF6 Contributes to CFA-Induced Spinal Microglial Activation and Chronic Inflammatory Pain in Mice

Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been reported to be expressed in spinal astrocytes and is involved in neuropathic pain. In this study, we investigated the role and mechanism of TRAF6 in complete Freund's adjuvant (CFA)-evoked chronic inflammatory hypersensitivity and the effect of docosahexaenoic acid (DHA) on TRAF6 expression and inflammatory pain. We found that TRAF6 was dominantly increased in microglia at the spinal level after intraplantar injection of CFA. Intrathecal TRAF6 siRNA alleviated CFA-triggered allodynia and reversed the upregulation of IBA-1 (microglia marker). In addition, intrathecal administration of DHA inhibited CFA-induced upregulation of TRAF6 and IBA-1 in the spinal cord and attenuated CFA-evoked mechanical allodynia. Furthermore, DHA prevented lipopolysaccharide (LPS)-caused increase of TRAF6 and IBA-1 in both BV2 cell line and primary cultured microglia. Finally, intrathecal DHA reduced LPS-induced upregulation of spinal TRAF6 and IBA-1, and alleviated LPS-induced mechanical allodynia. Our findings indicate that TRAF6 contributes to pain hypersensitivity via regulating microglial activation in the spinal dorsal horn. Direct inhibition of TRAF6 by siRNA or indirect inhibition by DHA may have therapeutic effects on chronic inflammatory pain.

Changes in median eminence of fatty acid-binding protein 3 in a mouse model of pain

AIMS

Fatty acid-binding protein (FABP) regulates polyunsaturated fatty acid (PUFA) intracellular trafficking and signal transduction. Our previous studies demonstrated that the alteration of PUFA in the hypothalamus is involved in pain process. However, how FABP subtypes change during pain remain unclear. Here, we examined the expression changes and localization in the hypothalamic FABP subtype in postoperative pain model mice.

METHODS

Paw incision-induced postoperative methods were adopted as a pain model in male ddY mice. Mechanical allodynia was examined using the von Frey test. The analysis of several FABPs mRNA was measured by real-time PCR, and cellular localization of its protein level was measured by immunofluorescent study.

RESULTS

Postoperative pain mouse elicited mechanical allodynia on Day 2 after paw incision, and mRNA expression of FABP3 increased significantly in the hypothalamus in the postoperative pain mouse model compared to that in control mice. FABP3 protein expressed in the median eminence and the arcuate nucleus, and colocalized with Iba-1, which is a microglial cell marker. Its protein level significantly increased in the median eminence on Day 2 after incision and returned to the control level on Day 4 after incision.

CONCLUSIONS

Our findings indicate that FABP3 in the median eminence may change in pain stimuli and may represent a molecular link controlling pain.

FFAR1/GPR40 Contributes to the Regulation of Striatal Monoamine Releases and Facilitation of Cocaine-Induced Locomotor Activity in Mice

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor (GPR40) for medium-to-long-chain free fatty acids. Previous studies on the expression of FFAR1 revealed that the nigrostriatal region is one of the areas which express abundant FFAR1 mRNA/protein in the central nervous system (CNS). However, the role of FFAR1 in the CNS has been still largely unclarified. Here, we examined a possible functional role of FFAR1 in the control of extracellular concentrations of striatal monoamines and cocaine-induced locomotor activity. Microdialysis analysis revealed that the basal level of extracellular dopamine (DA) was significantly elevated, while the basal serotonin (5-HT) level tended to be reduced in the striatum of FFAR1 knockout (-/-) mice. Interestingly, local application of a FFAR1 agonist, GW9508, markedly augmented the striatal 5-HT release in FFAR1 wild-type (+/+) mice, whereas topical application of a FFAR1 antagonist, GW1100, significantly reduced the 5-HT release. However, the enhanced 5-HT release was completely lost in -/- mice. Although acute administration of cocaine enhanced the locomotor activity in both +/+ and -/- mice, the magnitude of the enhancement was significantly reduced in -/- mice. In addition, intraperitoneal injection of GW1100 significantly decreased the cocaine-induced locomotor enhancement. These results suggest that FFAR1 has a facilitatory role in striatal 5-HT release, and the evoked 5-HT release might contribute to enhance cocaine-induced locomotor activity.

Pituitary hormones are specifically expressed in trigeminal sensory neurons and contribute to pain responses in the trigeminal system

Trigeminal (TG), dorsal root (DRG), and nodose/jugular (NG/JG) ganglia each possess specialized and distinct functions. We used RNA sequencing of two-cycle sorted Pirt-positive neurons to identify genes exclusively expressing in L3-L5 DRG, T10-L1 DRG, NG/JG, and TG mouse ganglion neurons. Transcription factor Phox2b and Efcab6 are specifically expressed in NG/JG while Hoxa7 is exclusively present in both T10-L1 and L3-L5 DRG neurons. Cyp2f2, Krt18, and Ptgds, along with pituitary hormone prolactin (Prl), growth hormone (Gh), and proopiomelanocortin (Pomc) encoding genes are almost exclusively in TG neurons. Immunohistochemistry confirmed selective expression of these hormones in TG neurons and dural nerves; and showed GH expression in subsets of TRPV1+ and CGRP+ TG neurons. We next examined GH roles in hypersensitivity in the spinal versus trigeminal systems. Exogenous GH produced mechanical hypersensitivity when injected intrathecally, but not intraplantarly. GH-induced thermal hypersensitivity was not detected in the spinal system. GH dose-dependently generated orofacial and headache-like periorbital mechanical hypersensitivity after administration into masseter muscle and dura, respectively. Periorbital mechanical hypersensitivity was reversed by a GH receptor antagonist, pegvisomant. Overall, pituitary hormone genes are selective for TG versus other ganglia somatotypes; and GH has distinctive functional significance in the trigeminal versus spinal systems.

GPR40 receptor agonist TAK-875 improves cognitive deficits and reduces β-amyloid production in APPswe/PS1dE9 mice

RATIONALE

Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by progressive cognitive dysfunction and memory impairment. G protein-coupled receptor 40 (GPR40) is expressed in brain in addition to periphery and is associated with cognitive function such as space orientation, memory, and learning. However, the effects and mechanisms of GPR40 agonist in improving the AD progression remain largely unknown.

OBJECTIVES

The present study aimed to investigate the therapeutic effects and mechanisms of a potent and selective GPR40 agonist TAK-875 on the APPswe/PS1dE9 mice.

RESULTS

The results showed that intracerebroventricular administration of TAK-875 significantly rescued cognitive deficits in APPswe/PS1dE9 mice, and these effects may be mediated by the regulation of phospholipase C/protein kinase C signaling pathway, which enhanced α-secretase ADAM10 activity, promoted amyloid precursor protein non-amyloidogenic processing pathway, and reduced β-amyloid production.

CONCLUSIONS

These results suggest that GPR40 may be a potential therapeutic target for AD, and GPR40 agonists may become promising AD drugs in the future.

Activation of GPR40 attenuates neuroinflammation and improves neurological function via PAK4/CREB/KDM6B pathway in an experimental GMH rat model

BACKGROUND

Germinal matrix hemorrhage (GMH) is defined by the rupture of immature blood vessels in the germinal matrix, where subsequent hemorrhage enters the subependymal zone and the cerebral lateral ventricles. The consequent blood clot has been identified as the causative factor of secondary brain injury, which triggers a series of complex parallel and sequential harmful mechanisms, including neuroinflammation. The orphan G-protein-coupled receptor 40 (GPR40), a free fatty acid (FFA) receptor 1, has been shown to exert anti-inflammatory effects when activated and improved outcomes in animal models of stroke. We aimed to investigate the anti-inflammatory effects of GPR40 and its underlying mechanisms after GMH.

METHODS

GMH model was induced in 7-day-old rat pups by an intraparenchymal injection of bacterial collagenase. GPR40 agonist, GW9508, was administered intranasally 1 h, 25 h, and 49 h after GMH induction. CRISPR targeting GPR40, PAK4, and KDM6B were administered through intracerebroventricular injection 48 h before GMH induction. Neurologic scores, microglia polarization, and brain morphology were evaluated by negative geotaxis, right reflex, rotarod test, foot fault test, Morris water maze, immunofluorescence staining, Western blots, and nissl staining respectfully.

RESULTS

The results demonstrated that GW9508 improved neurological and morphological outcomes after GMH in the short (24 h, 48 h, 72h) and long-term (days 21-27). However, the neuroprotective effects of treatment were abolished by GW1100, a selective GPR40 antagonist. GW9508 treatment increased populations of M2 microglia and decreased M1 microglia in periventricular areas 24 h after GMH induction. GW9508 upregulated the phosphorylation of PAK4, CREB, and protein level of KDM6B, CD206, IL-10, which was also met with the downregulation of inflammatory markers IL-1β and TNF-α. The mechanism study demonstrated that the knockdown of GPR40, PAK4, and KDM6B reversed the neuroprotective effects brought on by GW9508. This evidence suggests that GPR40/PAK4/CREB/KDM6B signaling pathway in microglia plays a role in the attenuation of neuroinflammation after GMH.

CONCLUSIONS

In conclusion, the present study demonstrates that the activation of GPR40 attenuated GMH-induced neuroinflammation through the activation of the PAK4/CREB/KDM6B signaling pathway, and M2 microglia may be a major mediator of this effect. Thus, GPR40 may serve as a potential target in the reduction of the inflammatory response following GMH, thereby improving neurological outcomes in the short- and long-term.

The role of STIM1/ORAI1 channel in the analgesic effect of grain-sized moxibustion on inflammatory pain mice model

The therapeutic effect of grain-sized moxibustion (GS-Moxi) on inflammatory pain has been well recognized clinically, but the mechanism remains unclear. STIM1/ORAI1 is a sensible temperature channel, therefore; this study aimed to investigate the analgesic effect of GS-Moxi and the association with STIM1/ORAI1 expression. CFA-induced inflammatory pain model was established and was treated with GS-Moxi after 3 days of CFA injection. The behavioral test was measured after the GS-Moxi; then, serum was prepared for IL-1β, IL-6, and TNF-α, and the stimulated skin was used for measuring STIM1 and ORAI1 expression. The results indicated GS-Moxi had an analgesic effect on inflammatory pain and the heat variation was significant for the analgesia. GS-Moxi decreased the expression of IL-1β, IL-6, and TNF-α. Immunofluorescence and western blot analysis illustrated that heat change was associated with the stimulation of STIM1 and ORAI1. Suggesting that heat variation created by GS-Moxi could be crucial in this therapy and STIM1 and ORAI1 were potential enhancers in regulating analgesia of GS-Moxi.

Understanding the appetite modulation pathways: The role of the FFA1 and FFA4 receptors

Pharmaconutrition is an area of current interest, especially concerning the advances in the pharmacology of nutrient-sensing receptors, as have been accomplished in the last 20 years. The family of free fatty acid (FFA) receptors is composed of four members, sequentially named as FFA1 to FFA4, which are activated by the short to long-chain fatty acids. The affinity of the FFA1 and FFA4 receptors for the omega-3 polyunsaturated fatty acids prompted pre-clinical and clinical investigations regarding their involvement in metabolic diseases. The main studies have been focused on the receptors' expression analyses, the featuring of knockout mice, and the assessment of selective synthetic ligands. These clearly have indicated a relevant role for FFA1 and FFA4 in the peripheral and central circuits for the regulation of energetic metabolism. This review article aimed to discuss the relevance of the FFA1 and FFA4 receptors in appetite-related complications, mainly related to obesity, cancer cachexia, and anorexia in the elderly, emphasizing whether their pharmacological modulation might be useful for the management of these disorders.

Targeting FFA1 and FFA4 receptors in cancer-induced cachexia

Free fatty acid (FFA) receptors FFA1 and FFA4 are omega-3 molecular targets in metabolic diseases; however, their function in cancer cachexia remains unraveled. We assessed the role of FFA1 and FFA4 receptors in the mouse model of cachexia induced by Lewis lung carcinoma (LLC) cell implantation. Naturally occurring ligands such as α-linolenic acid (ALA) and docosahexaenoic acid (DHA), the synthetic FFA1/FFA4 agonists GW9508 and TUG891, or the selective FFA1 GW1100 or FFA4 AH7614 antagonists were tested. FFA1 and FFA4 expression and other cachexia-related parameters were evaluated. GW9508 and TUG891 decreased tumor weight in LLC-bearing mice. Regarding cachexia-related end points, ALA, DHA, and the preferential FFA1 agonist GW9508 rescued body weight loss. Skeletal muscle mass was reestablished by ALA treatment, but this was not reflected in the fiber cross-sectional areas (CSA) measurement. Otherwise, TUG891, GW1100, or AH7614 reduced the muscle fiber CSA. Treatments with ALA, GW9508, GW1100, or AH7614 restored white adipose tissue (WAT) depletion. As for inflammatory outcomes, ALA improved anemia, whereas GW9508 reduced splenomegaly. Concerning behavioral impairments, ALA and GW9508 rescued locomotor activity, whereas ALA improved motor coordination. Additionally, DHA improved grip strength. Notably, GW9508 restored abnormal brain glucose metabolism in different brain regions. The GW9508 treatment increased leptin levels, without altering uncoupling protein-1 downregulation in visceral fat. LLC-cachectic mice displayed FFA1 upregulation in subcutaneous fat, but not in visceral fat or gastrocnemius muscle, whereas FFA4 was unaltered. Overall, the present study shed new light on FFA1 and FFA4 receptors' role in metabolic disorders, indicating FFA1 receptor agonism as a promising strategy in mitigating cancer cachexia.

Effect of omega-3 fatty acids on glucose homeostasis: role of free fatty acid receptor 1

Insulin resistance is a worldwide health problem. This study investigated the acute effects of eicosapentanoic acid (EPA) on glucose homeostasis focusing on the role of free fatty acid receptor 1 (FFAR1) and the chronic effects of fish oil omega-3 fatty acids on insulin resistance. Insulin resistance was induced by feeding mice high-fructose, high-fat diet (HFrHFD) for 16 weeks. In the first part, the acute effects of EPA alone and in combination with GW1100 and DC260126 (FFAR1 blockers) on glucose homeostasis and hepatic phosphatidyl-inositol 4,5-bisphosphate (PIP2) and diacylglycerol (DAG) were investigated in standard chow diet (SCD)- and HFrHFD-fed mice. In the second part, mice were treated with fish oil omega-3 fatty acids for 4 weeks starting at the week 13 of feeding HFrHFD. Changes in the blood- and liver tissue-insulin resistance markers and FFAR1 downstream signals were recorded at the end of experiment. Results showed that EPA increased 0 and 30 min blood glucose levels after glucose load in SCD-fed mice but improved glucose tolerance in HFrHFD-fed mice. Moreover, FFAR1 blockers reduced EPA effects on glucose tolerance and hepatic PIP2 and DAG levels. On the other hand, chronic use of fish oil omega-3 fatty acids increased FBG levels and decreased serum insulin and triglycerides levels without improving the index of insulin resistance. Also, they increased hepatic β-arrestin-2, PIP2, and pS473 Akt levels but decreased DAG levels. In conclusion, EPA acutely improved glucose homeostasis in HFrHFD-fed mice by modulating the activity of FFAR1. However, the chronic use of fish oil omega-3 fatty acids did not improve the insulin resistance.

Long Chain Fatty Acids Differentially Regulate Sub-populations of Arcuate POMC and NPY Neurons

Long chain fatty acids (LCFAs) have been suggested to influence the activity of hypothalamic neurons, however, limited studies have attempted to identify the neurochemical phenotype of these neurons. We aimed to determine if physiological levels of LCFAs alter the electrical excitability of pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus. We utilised whole-cell patch-clamp electrophysiology on brain slice preparations from genetic mouse models where green fluorescent protein was expressed in either POMC or NPY expressing cells. All animals had undergone an overnight fast to replicate conditions in which fatty acids would usually increase. Bath application of LCFAs were found to predominantly inhibit POMC neurons and predominantly excite NPY neurons. Differences between oleic and palmitic acid were not observed. These results suggest that LCFAs in the cerebrospinal fluid exert an underlying orexigenic tone to key hypothalamic neurons known to regulate energy homeostasis.

Intake of ω-6 Polyunsaturated Fatty Acid-Rich Vegetable Oils and Risk of Lifestyle Diseases

Although excessive consumption of deep-fried foods is regarded as 1 of the most important epidemiological factors of lifestyle diseases such as Alzheimer's disease, type 2 diabetes, and obesity, the exact mechanism remains unknown. This review aims to discuss whether heated cooking oil-derived peroxidation products cause cell degeneration/death for the occurrence of lifestyle diseases. Deep-fried foods cooked in ω-6 PUFA-rich vegetable oils such as rapeseed (canola), soybean, sunflower, and corn oils, already contain or intrinsically generate "hydroxynonenal" by peroxidation. As demonstrated previously, hydroxynonenal promotes carbonylation of heat-shock protein 70.1 (Hsp70.1), with the resultant impaired ability of cells to recycle damaged proteins and stabilize the lysosomal membrane. Until now, the implication of lysosomal/autophagy failure due to the daily consumption of ω-6 PUFA-rich vegetable oils in the progression of cell degeneration/death has not been reported. Since the "calpain-cathepsin hypothesis" was formulated as a cause of ischemic neuronal death in 1998, its relevance to Alzheimer's neuronal death has been suggested with particular attention to hydroxynonenal. However, its relevance to cell death of the hypothalamus, liver, and pancreas, especially related to appetite/energy control, is unknown. The hypothalamus senses information from both adipocyte-derived leptin and circulating free fatty acids. Concentrations of circulating fatty acid and its oxidized form, especially hydroxynonenal, are increased in obese and/or aged subjects. As overactivation of the fatty acid receptor G-protein coupled receptor 40 (GPR40) in response to excessive or oxidized fatty acids in these subjects may lead to the disruption of Ca2+ homeostasis, it should be evaluated whether GPR40 overactivation contributes to diverse cell death. Here, we describe the molecular implication of ω-6 PUFA-rich vegetable oil-derived hydroxynonenal in lysosomal destabilization leading to cell death. By oxidizing Hsp70.1, both the dietary PUFA- (exogenous) and the membrane phospholipid- (intrinsic) peroxidation product "hydroxynonenal," when combined, may play crucial roles in the occurrence of diverse lifestyle diseases including Alzheimer's disease.

Activation of GPR40 Suppresses AGE-Induced Reduction of Type II Collagen and Aggrecan in Human SW1353 Chondrocytes

Introduction

Osteoarthritis (OA) is an age-related chronic degenerative disease. Accumulation of advanced glycation end products (AGEs) induces degradation of the articular extracellular matrix (ECM) and is considered a critical step toward the development and progression of OA. GPR40 is a well-known free fatty acid receptor, which possesses pleiotropic effects in different types of diseases. However, the biological function of GPR40 in OA is indistinct. The purpose of the present study was to determine the impact of the GPR40 agonist GW9508 on AGEs-treated chondrocytes.

Materials and Methods

Cultures of human SW1353 chondrocytes were stimulated with GW9508, followed by exposure to 100 µg/mL AGEs. Gene and protein expression of TNF-α, IL-6, MMP-3, MMP-13, ADAMTS-4, and ADAMTS-5 were measured by real-time PCR and ELISA analysis. The levels of type II collagen, aggrecan, and nuclear NF-κB p65 were measured by Western blot analysis. A luciferase assay measured the transcriptional activity of NF-κB.

Results

The results show that treatment with AGEs decreased the expression of GPR40 in human SW1353 chondrocytes. Treatment with GW9508 plays a beneficial role in protecting type II Collagen and aggrecan from degeneration by attenuating the expression of MMP-3, MMP-13, ADAMTS-4, and ADAMTS-5. Additionally, GW9508 reduces the appearance of pro-inflammatory cytokines and suppresses NF-κB activation in AGEs-induced chondrocytes. Notably, co-treatment with GW1100, a specific antagonist of GPR40, abolishes the beneficial role of GW9508 against AGEs, implying that GPR40 mediates these effects of GW9508.

Conclusion

Our results suggest that GPR40 is a novel therapeutic target for OA and that GPR40 agonists, including GW9508, may have therapeutic potential in preventing and slowing the progression of OA.

Docosahexaenoic acid inhibits zymogen activation by suppressing vacuolar ATPase activation in cerulein-stimulated pancreatic acinar cells

BACKGROUND

The premature activation of digestive enzyme zymogens within pancreatic acinar cells is an important early feature of acute pancreatitis. Supraphysiological concentrations of cholecystokinin (CCK) cause intrapancreatic zymogen activation and acute pancreatitis. Stimulation of vacuolar ATPase (vATPase) activity is required for zymogen activation in pancreatic acinar cells. Parkin, a multiprotein E3 ubiquitin ligase complex, promotes vATPase ubiquitination and degradation, which inhibits vATPase activity. Docosahexaenoic acid (DHA), an omega-3 fatty acid, exerts anti-inflammatory effects. It is reported to bind to G-protein coupled receptor 120 (GPR120) and GPR40. DHA induces the degradation of certain proteins by activating ubiquitin-proteasome system in various cells. This study aimed to investigate whether DHA induces Parkin and inhibits vATPase activity, resulting in zymogen inactivation in pancreatic acinar AR42J cells stimulated with cerulein, a CCK analog.

RESULTS

Cerulein induced the translocation of the cytosolic V1 domain (E subunit) of vATPase to the membrane, which indicated vATPase activation, and zymogen activation in AR42J cells. DHA suppressed the association of the vATPase with membranes, and zymogen activation (increased trypsin activity and amylase release) induced by cerulein. Pretreatment with a GPR120 antagonist AH-7614, a GPR40 antagonist DC260126, or an ubiquitination inhibitor PYR-41 reduced the effect of DHA on cerulein-induced zymogen activation. Treatment with PYR-41 reversed the DHA-induced decrease in vATPase activation in cerulein-treated cells. Furthermore, DHA increased the level of Parkin in membranes of cerulein-treated cells.

CONCLUSIONS

DHA upregulates Parkin which inhibits vATPase-mediated zymogen activation, via GPR120 and GPR40, in cerulein-stimulated pancreatic acinar cells.

Ginseng Gintonin Contains Ligands for GPR40 and GPR55

Gintonin, a novel ginseng-derived glycolipoprotein complex, has an exogenous ligand for lysophosphatidic acid (LPA) receptors. However, recent lipid analysis of gintonin has shown that gintonin also contains other bioactive lipids besides LPAs, including linoleic acid and lysophosphatidylinositol (LPI). Linoleic acid, a free fatty acid, and LPI are known as ligands for the G-protein coupled receptors (GPCR), GPR40, and GPR55, respectively. We, herein, investigated whether gintonin could serve as a ligand for GPR40 and GPR55, using the insulin-secreting beta cell-derived cell line INS-1 and the human prostate cancer cell line PC-3, respectively. Gintonin dose-dependently enhanced insulin secretion from INS-1 cells. Gintonin-stimulated insulin secretion was partially inhibited by a GPR40 receptor antagonist but not an LPA1/3 receptor antagonist and was down-regulated by small interfering RNA (siRNA) against GPR40. Gintonin dose-dependently induced [Ca²⁺]_i transients and Ca²⁺-dependent cell migration in PC-3 cells. Gintonin actions in PC-3 cells were attenuated by pretreatment with a GPR55 antagonist and an LPA1/3 receptor antagonist or by down-regulating GPR55 with siRNA. Taken together, these results demonstrated that gintonin-mediated insulin secretion by INS-1 cells and PC-3 cell migration were regulated by the respective activation of GPR40 and GPR55 receptors. These findings indicated that gintonin could function as a ligand for both receptors. Finally, we demonstrated that gintonin contained two more GPCR ligands, in addition to that for LPA receptors. Gintonin, with its multiple GPCR ligands, might provide the molecular basis for the multiple pharmacological actions of ginseng.

A Neural basis for Octanoic acid regulation of energy balance

Objectives

Nutrient sensing by hypothalamic neurons is critical for the regulation of food intake and energy expenditure. We aimed to identify long- and medium-chain fatty acid species transported into the brain, their effects on energy balance, and the mechanisms by which they regulate activity of hypothalamic neurons.

Methods

Simultaneous blood and cerebrospinal fluid (CSF) sampling was undertaken in rats and metabolic analyses using radiolabeled fatty acid tracers were performed on mice. Electrophysiological recording techniques were used to investigate signaling mechanisms underlying fatty acid-induced changes in activity of pro-opiomelanocortin (POMC) neurons.

Results

Medium-chain fatty acid (MCFA) octanoic acid (C8:0), unlike long-chain fatty acids, was rapidly transported into the hypothalamus of mice and almost exclusively oxidized, causing rapid, transient reductions in food intake and increased energy expenditure. Octanoic acid differentially regulates the excitability of POMC neurons, activating these neurons directly via GPR40 and inducing inhibition via an indirect non-synaptic, purine, and adenosine receptor-dependent mechanism.

Conclusions

MCFA octanoic acid is a central signaling nutrient that targets POMC neurons via distinct direct and indirect signal transduction pathways to instigate changes in energy status. These results could explain the beneficial health effects that accompany MCFA consumption.

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Octanoic acid (C8:0) is rapidly transported from blood to the cerebrospinal fluid.

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Octanoic acid rapidly reduces food intake and increases energy expenditure.

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Octanoic acid targets POMC neurons through direct and indirect signaling pathways.

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Activation of POMC neurons occurs directly through GPR40.

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Inhibition occurs through a nonsynaptic, purine and adenosine receptor-dependent mechanism.

The Molecular Mechanisms Associated with the Effects of Propofol in a Rat Model of Pain Due to Inflammation Following Injection with Complete Freund's Adjuvant

Background

This study aimed to investigate the molecular mechanisms associated with the effects of propofol in a rat model of pain due to inflammation following subcutaneous injection with complete Freund’s adjuvant (CFA).

Material/Methods

Sprague-Dawley rats were injected subcutaneously in the paw with CFA. Propofol or saline was administered by tail vein injection. After CFA treatment for 0 hours, 4 hours, 1 day, 4 days, 7 days, and 14 days, the behavior of the rats was assessed. An enzyme-linked immunosorbent assay (ELISA) measured serum levels of proinflammatory cytokines, including tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6. Western blot and the quantitative reverse-transcription polymerase chain reaction (qRT-PCR) were used to detect levels of p38MAPK and NF-κB related mRNA and proteins, including p-p38, p38, p65, p-p65, NOD-like receptor family protein 3 (NLRP3), apoptosis-associated speck-like protein (ASC) and caspase-1 in rat spinal cord tissues.

Results

Injection of CFA significantly reduced the mechanical withdrawal threshold (MWT), thermal withdrawal latency (TWL), and frequency responses to cold stimulation. Propofol treatment significantly reduced serum levels of TNF-α, IL-1β, and IL-6. Protein expression levels of p-p38 and p-p65 were upregulated in the rat model, which were inhibited by propofol treatment. CFA injection increased the expression levels of NLRP3, ASC, and caspase-1 in the spinal cord tissues of rats, which were reduced by propofol treatment.

Conclusions

In a rat model of pain following subcutanous injection with CFA, propofol reduced CFA-induced pain and inhibited the inflammatory response through the p38MAPK-nuclear factor-κB (NF-κB) pathway and the NLRP3 inflammasome.

Resolvin D1 suppresses inflammation-induced hyperexcitability of nociceptive trigeminal neurons associated with mechanical hyperalgesia

7S,8R,17S-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid (resolvin D1 [RvD1]) is biosynthesized from docosahexaenoic acid (DHA), and belongs to a novel family of lipid mediators showing remarkable anti-inflammatory effects; however, the effect of RvD1 on inflammation-induced hyperexcitability of nociceptive neurons under in vivo conditions remains to be determined. The present study, therefore, investigated whether under in vivo conditions, systemic administration of RvD1 could attenuate the inflammation-induced hyperexcitability of spinal trigeminal nucleus caudalis (SpVc) wide-dynamic range (WDR) neurons associated with hyperalgesia in rats. The threshold of escape from mechanical stimulation applied to the orofacial area in rats with complete Freund's adjuvant-induced inflammation was significantly lower than in naïve rats. The lowered mechanical threshold in rats with inflammation was returned to control levels following administration of RvD1 (3 ng/kg, i.p.) for 3 days. The mean discharge frequency of SpVc WDR neurons in rats with inflammation was significantly decreased after RvD1 administration for both non-noxious and noxious mechanical stimuli. Increased spontaneous discharge of SpVc WDR neurons in rats with inflammation was also significantly decreased after RvD1 administration. Noxious pinch-evoked afterdischarge frequency and occurrence in rats with inflammation was significantly diminished after RvD1 administration. Expansion of the receptive field in rats with inflammation also returned to control levels after RvD1 administration. These results suggest that administration of RvD1 attenuates inflammation-induced hyperexcitability of SpVc WDR neurons associated with inflammatory hyperalgesia. These findings support the idea that RvD1, derived from DHA, as well as DHA itself, are potential complementary or alternative therapeutic agents for the alleviation of inflammatory hyperalgesia.

A Selectivity Study of FFAR4/FFAR1 Agonists by Molecular Modeling

FFAR4 has been considered as a potential target for metabolic diseases, including diabetes. Some compounds with biphenyl scaffold, represented by compound SR13 reported by our group, showed significant FFAR4 selectivity. However, the molecular basis for their selectivity has not been definitely disclosed. This study provided insights into the protein-ligand interactions between agonists and FFAR4/FFAR1 by molecular modeling. The important residues identified were consistent with those found in experimental studies. Moreover, the results proposed that the selectivity of SR13 between FFAR4 and FFAR1 depended on whether it can enter the ligand-binding site through the entrance region by adopting its preferential conformation. The big difference between the preferential conformation of SR13 and the narrow entrance region determined its poor agonist activity against FFAR1. These findings will facilitate the further development of selective FFAR4 agonists.

Activation of GPR40 produces mechanical antiallodynia via the spinal glial interleukin-10/β-endorphin pathway

Background

The G protein-coupled receptor 40 (GPR40), broadly expressed in various tissues such as the spinal cord, exerts multiple physiological functions including pain regulation. This study aimed to elucidate the mechanisms underlying GPR40 activation-induced antinociception in neuropathic pain, particularly related to the spinal glial expression of IL-10 and subsequent β-endorphin.

Methods

Spinal nerve ligation-induced neuropathic pain model was used in this study. β-Endorphin and IL-10 levels were measured in the spinal cord and cultured primary microglia, astrocytes, and neurons. Double immunofluorescence staining of β-endorphin with glial and neuronal cellular biomarkers was also detected in the spinal cord and cultured primary microglia, astrocytes, and neurons.

Results

GPR40 was expressed on microglia, astrocytes, and neurons in the spinal cords and upregulated by spinal nerve ligation. Intrathecal injection of the GPR40 agonist GW9508 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia in neuropathic rats, with E_max values of 80% and 100% MPE and ED₅₀ values of 6.7 and 5.4 μg, respectively. Its mechanical antiallodynia was blocked by the selective GPR40 antagonist GW1100 but not GPR120 antagonist AH7614. Intrathecal GW9508 significantly enhanced IL-10 and β-endorphin immunostaining in spinal microglia and astrocytes but not in neurons. GW9508 also markedly stimulated gene and protein expression of IL-10 and β-endorphin in cultured primary spinal microglia and astrocytes but not in neurons, originated from 1-day-old neonatal rats. The IL-10 antibody inhibited GW9508-stimulated gene expression of the β-endorphin precursor proopiomelanocortin (POMC) but not IL-10, whereas the β-endorphin antibody did not affect GW9508-stimulated IL-10 or POMC gene expression. GW9508 increased phosphorylation of mitogen-activated protein kinases (MAPKs) including p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK), and its stimulatory effects on IL-10 and POMC expression were blocked by each MAPK isoform inhibitor. Spinal GW9508-induced mechanical antiallodynia was completely blocked by intrathecal minocycline, IL-10 neutralizing antibody, β-endorphin antiserum, and μ-opioid receptor-preferred antagonist naloxone.

Conclusions

Our results illustrate that GPR40 activation produces antinociception via the spinal glial IL-10/β-endorphin antinociceptive pathway.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1457-9) contains supplementary material, which is available to authorized users.

Ipsi- and Contralateral Moxibustion Generate Similar Analgesic Effect on Inflammatory Pain

The aim of this study was to investigate whether contralateral moxibustion would generate a similar analgesic effect with ipsilateral moxibustion. Contra- and ipsilateral moxibustion were separately applied to Zusanli (ST36) acupoints of inflammatory pain mice. The analgesic effect was evaluated, respectively, by licking/biting time (LBT) of formalin-induced inflammatory pain and thermal withdrawal latency (TWL) of complete Freund's adjuvant- (CFA-) induced inflammatory pain. For formalin-induced pain, compared with formalin group, the total LBT of ipsi- and contralateral moxibustion reduced in both phase I and phase II, but there was no significant difference between ipsi- and contralateral moxibustion. For CFA-induced inflammatory pain, compared with CFA group, TWL of ipsi- and contra-Moxi groups increased immediately after moxibustion intervention; however there was no obvious difference between ipsi- and contralateral moxibustion at any timepoint. It indicated that contralateral moxibustion had a similar analgesic effect with ipsilateral moxibustion in both formalin- and CFA-induced pain. These results suggest that both ipsi- and contralateral moxibustion could be applied for pain relief.

Palmitic Acid Reduces the Autophagic Flux and Insulin Sensitivity Through the Activation of the Free Fatty Acid Receptor 1 (FFAR1) in the Hypothalamic Neuronal Cell Line N43/5

Chronic consumption of high fat diets (HFDs), rich in saturated fatty acids (SatFAs) like palmitic acid (PA), is associated with the development of obesity and obesity-related metabolic diseases such as type II diabetes mellitus (T2DM). Previous studies indicate that PA accumulates in the hypothalamus following consumption of HFDs; in addition, HFDs consumption inhibits autophagy and reduces insulin sensitivity. Whether malfunction of autophagy specifically in hypothalamic neurons decreases insulin sensitivity remains unknown. PA does activate the Free Fatty Acid Receptor 1 (FFAR1), also known as G protein-coupled receptor 40 (GPR40); however, whether FFAR1 mediates the effects of PA on hypothalamic autophagy and insulin sensitivity has not been shown. Here, we demonstrate that exposure to PA inhibits the autophagic flux and reduces insulin sensitivity in a cellular model of hypothalamic neurons (N43/5 cells). Furthermore, we show that inhibition of autophagy and the autophagic flux reduces insulin sensitivity in hypothalamic neuronal cells. Interestingly, the inhibition of the autophagic flux, and the reduction in insulin sensitivity are prevented by pharmacological inhibition of FFAR1. Our findings show that dysregulation of autophagy reduces insulin sensitivity in hypothalamic neuronal cells. In addition, our data suggest FFAR1 mediates the ability of PA to inhibit autophagic flux and reduce insulin sensitivity in hypothalamic neuronal cells. These results reveal a novel cellular mechanism linking PA-rich diets to decreased insulin sensitivity in the hypothalamus and suggest that hypothalamic autophagy might represent a target for future T2DM therapies.

Phoenixin Expression Is Regulated by the Fatty Acids Palmitate, Docosahexaenoic Acid and Oleate, and the Endocrine Disrupting Chemical Bisphenol A in Immortalized Hypothalamic Neurons

Phoenixin (PNX) is a newly identified reproductive peptide required for the estrous cycle. It is most highly expressed in the hypothalamus, where it is a positive regulator of gonadotropin-releasing hormone (GnRH) and kisspeptin. However, it is unknown what signals lie upstream of Pnx to coordinate its effects on GnRH and kisspeptin. We investigated the effects of the hormones, estrogen and leptin; the fatty acids, palmitate, docosahexaenoic acid (DHA), oleate and palmitoleate; and the endocrine disrupting chemical BPA on Pnx mRNA levels. We also examined whether the signaling pathways of nitric oxide, lipopolysaccharide, cAMP and protein kinase C could alter Pnx expression. Immortalized hypothalamic neurons were treated from 2 to 24 h with these compounds and Pnx mRNA levels were measured with RT-qPCR. Unexpectedly, only BPA as well as the fatty acids, palmitate, DHA and oleate, could alter Pnx expression; therefore suggesting that Pnx may fulfill a nutrient-sensing role in the hypothalamus. Our study is the first to delineate potential regulators of this novel neuropeptide, and our findings provide some insight into the functional role of PNX in the hypothalamus.

Spinal interleukin-10 produces antinociception in neuropathy through microglial β-endorphin expression, separated from antineuroinflammation

Interleukin 10 (IL-10) is antinociceptive in various animal models of pain without induction of tolerance, and its mechanism of action was generally believed to be mediated by inhibition of neuroinflammation. Here we reported that intrathecal IL-10 injection dose dependently attenuated mechanical allodynia and thermal hyperalgesiain male and female neuropathic rats, with ED₅₀ values of 40.8 ng and 24 ng, and E_max values of 61.5% MPE and 100% MPE in male rats. Treatment with IL-10 specifically increased expression of the β-endorphin (but not prodynorphin) gene and protein in primary cultures of spinal microglia but not in astrocytes or neurons. Intrathecal injection of IL-10 stimulated β-endorphin expression from microglia but not neurons or astrocytes in both contralateral and ipsilateral spinal cords of neuropathic rats. However, intrathecal injection of the β-endorphin neutralizing antibody, opioid receptor antagonist naloxone, or μ-opioid receptor antagonist CTAP completely blocked spinal IL-10-induced mechanical antiallodynia, while the microglial inhibitor minocycline and specific microglia depletor reversed spinal IL-10-induced β-endorphin overexpression and mechanical antiallodynia. IL-10 treatment increased spinal microglial STAT3 phosphorylation, and the STAT3 inhibitor NSC74859 completely reversed IL-10-increased spinal expression of β-endorphin and neuroinflammatory cytokines and mechanical antiallodynia. Silence of the Bcl3 and Socs3 genes nearly fully reversed IL-10-induced suppression of neuroinflammatory cytokines (but not expression of β-endorphin), although it had no effect on mechanical allodynia. In contrast, disruption of the POMC gene completely blocked IL-10-stimulated β-endorphin expression and mechanical antiallodynia, but had no effect on IL-10 inhibited expression of neuroinflammatory cytokines. Thus this study revealed that IL-10 produced antinociception through spinal microglial β-endorphin expression, but not inhibition of neuroinflammation.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

GPR40 receptor activation promotes tight junction assembly in airway epithelial cells via AMPK-dependent mechanisms

Tight junctions play key roles in the regulation of airway epithelial barrier function and promotion of tight junction integrity is beneficial to lung health. G-protein coupled receptor (GPR) 40 has been identified as a receptor of polyunsaturated fatty acids. This study aimed to investigate the function of GPR40 in regulating tight junction assembly in human airway epithelial cells (Calu-3 cells) using GW9508, a GPR40 agonist. Immunoblotting and immunofluorescence analyses showed that Calu-3 cells expressed both types of polyunsaturated fatty acid receptors including GPR40 and GPR120. Intracellular Ca²⁺ measurements confirmed that GW9508 stimulated GPR40, but not GPR120. In Ca²⁺ switch assays, GW9508 promoted the recovery of transepithelial electrical resistance and re-localization of zonula occludens (ZO)-1 to intercellular areas. These effects were suppressed by inhibitors of GPR40 and phospholipase C (PLC). Interestingly, GW9508 enhanced tight junction assembly in an AMP-activated protein kinase (AMPK)-dependent manner. The effect of GW9508 on inducing tight junction assembly was also confirmed in 16HBE14o- cells. Our results indicate that GPR40 stimulation by GW9508 leads to AMPK activation via calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ). Collectively, this study reveals an unprecedented role of GPR40 in facilitating airway epithelial tight junction assembly via PLC-CaMKKβ-AMPK pathways. GPR40 represents a novel regulator of airway epithelial integrity and its stimulation may be beneficial in the treatment of airway diseases.

The involvement of free fatty acid-GPR40/FFAR1 signaling in chronic social defeat stress-induced pain prolongation in C57BL/6J male mice

RATIONALE

Depression and anxiety can cause the development of chronic pain. However, the mechanism of chronic pain induced by emotional dysfunction is still unknown. Previously, we demonstrated that the G protein-coupled receptor 40/free fatty acid receptor 1 (GPR40/FFAR1) signaling in the brain is related to regulation of both pain and emotion. In the present study, we proved that the role of GPR40/FFAR1 signaling in the development of chronic pain is induced by emotional dysfunction.

RESULTS

Repeated social defeat (SD)-stressed mice showed the impairment of social interaction and anxiety behavior. These mice also caused pain prolongation after paw-incision comparison with non-SD mice. This pain prolongation was markedly continued by infusion of the GPR40/FFAR1 antagonist, GW1100 during SD stress but not non-SD stress. Although, infusion of the GW1100 during SD stress did not cause deterioration of the emotional behavior. Furthermore, GW1100-treated SD-mice showed strong tendency of emotional dysfunction after paw incision.

CONCLUSION

Our findings indicate that the dysfunction of fatty acids-GPR40/FFAR1 signaling in the brain underlying stress condition might be related to the development of chronic pain.

The Role of Metabolite-Sensing G Protein-Coupled Receptors in Inflammation and Metabolic Disease

SIGNIFICANCE

Great attention has been placed on the link between metabolism and immune function giving rise to the term "immunometabolism." It is widely accepted that inflammation and oxidative stress are key processes that underlie metabolic complications during obesity, diabetes, and atherosclerosis. Therefore, identifying the mechanisms and mediators that are involved in the regulation of both inflammation and metabolic homeostasis is of high scientific and therapeutic interest. Recent Advances: G protein-coupled receptors (GPCRs) that signal in response to metabolites have emerged as attractive therapeutic targets in inflammatory disease. Critical Issues and Future Directions: In this review, we discuss recent findings about the physiological role of the main metabolite-sensing GPCRs, their implication in immunometabolic disorders, their principal endogenous and synthetic ligands, and their potential as drug targets in inflammation and metabolic disease. Antioxid. Redox Signal. 29, 237-256.

Food pyramid for subjects with chronic pain: foods and dietary constituents as anti-inflammatory and antioxidant agents

Emerging literature suggests that diet constituents may play a modulatory role in chronic pain (CP) through management of inflammation/oxidative stress, resulting in attenuation of pain. We performed a narrative review to evaluate the existing evidence regarding the optimum diet for the management of CP, and we built a food pyramid on this topic. The present review also describes the activities of various natural compounds contained in foods (i.e. phenolic compounds in extra-virgin olive oil (EVO)) listed on our pyramid, which have comparable effects to drug management therapy. This review included 172 eligible studies. The pyramid shows that carbohydrates with low glycaemic index should be consumed every day (three portions), together with fruits and vegetables (five portions), yogurt (125 ml), red wine (125 ml) and EVO; weekly: legumes and fish (four portions); white meat, eggs and fresh cheese (two portions); red or processed meats (once per week); sweets can be consumed occasionally. The food amounts are estimates based on nutritional and practical considerations. At the top of the pyramid there is a pennant: it means that CP subjects may need a specific customised supplementation (vitamin B12, vitamin D, n-3 fatty acids, fibre). The food pyramid proposal will serve to guide dietary intake with to the intent of alleviating pain in CP patients. Moreover, a targeted diet can also help to solve problems related to the drugs used to combat CP, i.e. constipation. However, this paper would be an early hypothetical proposal due to the limitations of the studies.

[The role of brain n-3 fatty acids-GPR40/FFAR1 signaling in pain]

G-protein-coupled receptor 40 (GPR40)/free fatty acid receptor (FFAR) 1 is activated by long-chain fatty acids such as docosahexaenoic acid (DHA). Its receptor is expressed predominantly in the central nervous system (CNS) and in β-cells in the pancreatic Islets. We have already demonstrated that the intracerebroventricular administration of DHA or GW9508, a GPR40/FFAR1 agonist, suppresses formalin-induced pain behavior. It also attenuates complete Freund's adjuvant-induced mechanical allodynia and thermal hyperalgesia, suggesting that these effects occur by increasing β-endorphin release from propiomelanocortin neurons. Furthermore, we found that the brain GPR40/FFAR1 signaling may involve in the regulation of the descending pain control system, whereas the deletion of GPR40/FFAR1 might exacerbate mechanical allodynia in postoperative pain. Therefore, it is possible that the brain n-3 fatty acid-GPR40/FFAR1 signaling may play a key role in the modulation of the endogenous pain control system and emotional function. Here, we discuss the role of brain n-3 fatty acids-GPR40/FFAR1 signaling in a pain, and we review the current status and future prospects of the brain GPR40/FFAR1.

Bromodomain-containing Protein 4 Activates Voltage-gated Sodium Channel 1.7 Transcription in Dorsal Root Ganglia Neurons to Mediate Thermal Hyperalgesia in Rats

Background

Bromodomain-containing protein 4 binds acetylated promoter histones and promotes transcription; however, the role of bromodomain-containing protein 4 in inflammatory hyperalgesia remains unclear.

Methods

Male Sprague–Dawley rats received hind paw injections of complete Freund’s adjuvant to induce hyperalgesia. The dorsal root ganglia were examined to detect changes in bromodomain-containing protein 4 expression and the activation of genes involved in the expression of voltage-gated sodium channel 1.7, which is a key pain-related ion channel.

Results

The intraplantar complete Freund’s adjuvant injections resulted in thermal hyperalgesia (4.0 ± 1.5 s; n = 7). The immunohistochemistry and immunoblotting results demonstrated an increase in the bromodomain-containing protein 4–expressing dorsal root ganglia neurons (3.78 ± 0.38 fold; n = 7) and bromodomain-containing protein 4 protein levels (2.62 ± 0.39 fold; n = 6). After the complete Freund’s adjuvant injection, histone H3 protein acetylation was enhanced in the voltage-gated sodium channel 1.7 promoter, and cyclin-dependent kinase 9 and phosphorylation of RNA polymerase II were recruited to this area. Furthermore, the voltage-gated sodium channel 1.7–mediated currents were enhanced in neurons of the complete Freund’s adjuvant rats (55 ± 11 vs. 19 ± 9 pA/pF; n = 4 to 6 neurons). Using bromodomain-containing protein 4–targeted antisense small interfering RNA to the complete Freund’s adjuvant–treated rats, the authors demonstrated a reduction in the expression of bromodomain-containing protein 4 (0.68 ± 0.16 fold; n = 7), a reduction in thermal hyperalgesia (7.5 ± 1.5 s; n = 7), and a reduction in the increased voltage-gated sodium channel 1.7 currents (21 ± 4 pA/pF; n = 4 to 6 neurons).

Conclusions

Complete Freund’s adjuvant triggers enhanced bromodomain-containing protein 4 expression, ultimately leading to the enhanced excitability of nociceptive neurons and thermal hyperalgesia. This effect is likely mediated by the enhanced expression of voltage-gated sodium channel 1.7.

Dysfunctional GPR40/FFAR1 signaling exacerbates pain behavior in mice

We previously showed that activation of G protein-coupled receptor 40/free fatty acid receptor 1 (GPR40/FFAR1) signaling modulates descending inhibition of pain. In this study, we investigated the involvement of fatty acid-GPR40/FFAR1 signaling in the transition from acute to chronic pain. We used GPR40/FFAR1-knockout (GPR40KO) mice and wild-type (WT) mice. A plantar incision was performed, and mechanical allodynia and thermal hyperalgesia were evaluated with a von Frey filament test and plantar test, respectively. Immunohistochemistry was used to localize GPR40/FFAR1, and the levels of free fatty acids in the hypothalamus were analyzed with liquid chromatography-tandem mass spectrometry. The repeated administration of GW1100, a GPR40/FFAR1 antagonist, exacerbated the incision-induced mechanical allodynia and significantly increased the levels of phosphorylated extracellular signal-regulated kinase in the spinal cord after low-threshold touch stimulation in the mice compared to vehicle-treated mice. The levels of long-chain free fatty acids, such as docosahexaenoic acid, oleic acid, and palmitate, which are GPR40/FFAR1 agonists, were significantly increased in the hypothalamus two days after the surgery compared to levels in the sham group. Furthermore, the incision-induced mechanical allodynia was exacerbated in the GPR40KO mice compared to the WT mice, while the response in the plantar test was not changed. These findings suggested that dysfunction of the GPR40/FFAR1 signaling pathway altered the endogenous pain control system and that this dysfunction might be associated with the development of chronic pain.

A New Pain Regulatory System via the Brain Long Chain Fatty Acid Receptor GPR40/FFA1 Signal

An increasingly large number of pharmacological and physiological works on fatty acids have shown that the functional properties of fatty acids are regulated by the amount of individual fatty acid intake and the distribution of fatty acids among organs. Recently, it has been determined that G-protein-coupled receptor 40/free fatty acid receptor 1 (GPR40/FFA1) is activated by long-chain fatty acids, such as docosahexaenoic acid (DHA). GPR40/FFA1 is mainly expressed in the β cell of the pancreas, spinal cord and brain. It is reported that this receptor has a functional role in controlling blood glucose levels via the modulation of insulin secretion. However, its physiological function in the brain remains unknown. Our previous studies have shown that GPR40/FFA1 is expressed in pro-opiomelanocortin (POMC)-positive neurons of the arcuate nucleus, serotonergic neurons in the nucleus raphe magnus, and in noradrenergic neurons in the locus coeruleus. Furthermore, the intracerebroventricular injection of DHA or GW9508, which is a selective GPR40/FFA1 agonist, attenuates formalin-induced inflammatory pain behavior through increasing β-endorphin release in the hypothalamus. It also suppresses complete Freund's adjuvant-induced mechanical allodynia and thermal hyperalgesia. Our findings suggest that brain free long-chain fatty acids-GPR40/FFA1 signaling might have an important role in the modulation of endogenous pain control systems. In this review, I discuss the current status and our recent study regarding a new pain regulatory system via the brain long chain fatty acid receptor GPR40/FFA1 signal.