Activation of spinal dorsal horn P2Y13 receptors can promote the expression of IL-1β and IL-6 in rats with diabetic neuropathic pain

P2Y13 receptor involved in HIV-1 gp120 induced neuropathy in superior cervical ganglia through NLRP3 inflammasome activation

Cardiac autonomic neuropathy resulting from human immunodeficiency virus (HIV) infection is common; however, its mechanism remains unknown. The current work attempted to explore the function and mechanism of the P2Y13 receptor in HIV-glycoprotein 120 (gp120)-induced neuropathy in cervical sympathetic ganglion. The superior cervical ganglion (SCG) of the male SD rat was coated with HIV-gp120 to establish a model of autonomic neuropathy. In each group, we measured heart rate, blood pressure, heart rate variability, sympathetic nerve discharge and cardiac function. The expression of P2Y13 mRNA and protein in the SCG was tested by real-time polymerase chain reaction and western blotting. Additionally, this study focused on identifying the protein levels of NOD-like receptor family pyrin domain-containing 3 (NLRP3), Caspase-1, Gasdermin D (GSDMD), interleukin (IL)-1β and IL-18 in the SCG using western blotting and immunofluorescence. In gp120 rats, increased blood pressure, heart rate, cardiac sympathetic nerve activity, P2Y13 receptor levels and decreased cardiac function could be found. P2Y13 shRNA or MRS2211 inhibited the above mentioned changes induced by gp120, suggesting that the P2Y13 receptor may be engaged in gp120-induced sympathetic nerve injury. Moreover, the levels of NLRP3, Caspase-1, GSDMD, IL-1β and IL-18 in the gp120 group were increased, while significantly decreased by P2Y13 shRNA or MRS2211. Therefore, the P2Y13 receptor is involved in gp120-induced sympathetic neuropathy, and its molecular mechanism shows an association with the activation of the NLRP3 inflammasome, followed by GSDMD formation along with the release of inflammatory factors including IL-1β and IL-18. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Exploring the molecular mechanism of Xuebifang in the treatment of diabetic peripheral neuropathy based on bioinformatics and network pharmacology

Background

Xuebifang (XBF), a potent Chinese herbal formula, has been employed in managing diabetic peripheral neuropathy (DPN). Nevertheless, the precise mechanism of its action remains enigmatic.

Purpose

The primary objective of this investigation is to employ a bioinformatics-driven approach combined with network pharmacology to comprehensively explore the therapeutic mechanism of XBF in the context of DPN.

Study design and Methods

The active chemicals and their respective targets of XBF were sourced from the TCMSP and BATMAN databases. Differentially expressed genes (DEGs) related to DPN were obtained from the GEO database. The targets associated with DPN were compiled from the OMIM, GeneCards, and DrugBank databases. The analysis of GO, KEGG pathway enrichment, as well as immuno-infiltration analysis, was conducted using the R language. The investigation focused on the distribution of therapeutic targets of XBF within human organs or cells. Subsequently, molecular docking was employed to evaluate the interactions between potential targets and active compounds of XBF concerning the treatment of DPN.

Results

The study successfully identified a total of 122 active compounds and 272 targets associated with XBF. 5 core targets of XBF for DPN were discovered by building PPI network. According to GO and KEGG pathway enrichment analysis, the mechanisms of XBF for DPN could be related to inflammation, immune regulation, and pivotal signalling pathways such as the TNF, TLR, CLR, and NOD-like receptor signalling pathways. These findings were further supported by immune infiltration analysis and localization of immune organs and cells. Moreover, the molecular docking simulations demonstrated a strong binding affinity between the active chemicals and the carefully selected targets.

Conclusion

In summary, this study proposes a novel treatment model for XBF in DPN, and it also offers a new perspective for exploring the principles of traditional Chinese medicine (TCM) in the clinical management of DPN.

Suppression of HCN channels in the spinal dorsal horn restores KCC2 expression and attenuates diabetic neuropathic pain

Previous studies have shown that the hyperpolarized cyclic nucleotide gated (HCN) ion channels in the spinal dorsal horn (SDH) might be involved in the development of diabetic neuropathic pain (DNP). Additionally, other studies have shown that the decreased potassium-chloride cotransporter 2 (KCC2) expression in the SDH promotes pain hypersensitivity. Both HCN channels and KCC2 were highly expressed in spinal substantia gelatinosa neurons. However, whether the K+ efflux induced by the activation of HCN channels in DNP modulate KCC2 function and subsequently affect the role of γ-aminobutyric acid (GABA)/GABA-A receptors of neurons in the SDH remains to be clarified. The purpose of this work was to investigate the underlying mechanisms of KCC2 participating in HCN channels to promote DNP. Here, we found that the analgesic role of HCN channels blocker ZD7288 was associated with the up-regulated KCC2 expression and could be prevented by DIOA, a KCC2 blocker. Furthermore, the level of GABA in DNP rats significantly increased, which was decreased by ZD72288. Moreover, DIOA pretreatment could partly block the inhibitory effect of ZD7288 on the cyclic adenosine monophosphate-protein kinase A (cAMP-PKA) signaling activation of DNP rats. Finally, inhibition of cAMP-PKA signaling alleviated allodynia and elevated KCC2 expression in DNP rats. Altogether, this study reveals that the role of cAMP-PKA signaling-regulated HCN channels in DNP associated with decreased KCC2 expression in the spinal cord and altered GABA nature.

Targeting the JAK2/STAT3 signaling pathway for chronic pain

Chronic pain is a notable health concern because of its prevalence, persistence, and associated mental stress. Drugs targeting chronic pain with potent abirritation, and minimal side effects remain unidentified. Substantial evidence indicates that the Janus Kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway plays a distinct and critical role in different stages of chronic pain. Aberrant activation of the JAK2/STAT3 signaling pathway is evident in multiple chronic pain models. Moreover, an increasing number of studies have demonstrated that the downregulation of JAK2/STAT3 can attenuate chronic pain in different animal models. In this review, we investigated the mechanism and role of the JAK2/STAT3 signaling pathway in modulating chronic pain. The aberrant activation of JAK2/STAT3 can trigger chronic pain by interacting with microglia and astrocytes, releasing proinflammatory cytokines, inhibiting anti-inflammatory cytokines, and regulating synaptic plasticity. We also retrospectively reviewed current reports on JAK2/STAT3 pharmacological inhibitors that demonstrated their significant therapeutic potential in different types of chronic pain. In summary, our results provide strong evidence that the JAK2/STAT3 signaling pathway is a promising therapeutic target for chronic pain.

Exercise, Spinal Microglia and Neuropathic Pain: Potential Molecular Mechanisms

As one of the most common neuropathic disorders, neuropathic pain often has a negative impact on patients with persistent pain, mood disorders and sleep disturbances. Currently, neuropathic pain is not treated with any specific drug, instead, drugs for other diseases are used as replacements in clinics, but most have adverse effects. In recent years, the role of spinal cord microglia in the pathogenesis of neuropathic pain has been widely recognized, and they are being explored as potential therapeutic targets. Spinal microglia are known to be involved in the pathogenic mechanisms of neuropathic pain through purine signaling, fractalkine signaling, and p38 MAPK signaling. Exercise is a safe and effective treatment, and numerous studies have demonstrated its effectiveness in improving neurological symptoms. Nevertheless, it remains unclear what the exact molecular mechanism is. This review summarized the specific molecular mechanisms of exercise in alleviating neuropathic pain by mediating the activity of spinal microglia and maintaining the phenotypic homeostasis of spinal microglia through purine signaling, fractalkine signaling and p38 MAPK signaling. In addition, it has been proposed that different intensities and types of exercise affect the regulation of the above-mentioned signaling pathways differently, providing a theoretical basis for the improvement of neuropathic pain through exercise.

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Electroacupuncture may alleviate diabetic neuropathic pain by inhibiting the microglia P2X4R and neuroinflammation

Diabetic neuropathic pain (DNP) is a common and destructive complication of diabetes mellitus. The discovery of effective therapeutic methods for DNP is vitally imperative because of the lack of effective treatments. Although 2 Hz electroacupuncture (EA) was a successful approach for relieving DNP, the mechanism underlying the effect of EA on DNP is still poorly understood. Here, we established a rat model of DNP that was induced by streptozotocin (STZ) injection. P2X4R was upregulated in the spinal cord after STZ-injection. The upregulation of P2X4R was mainly expressed on activated microglia. Intrathecal injection of a P2X4R antagonist or microglia inhibitor attenuated STZ-induced nociceptive thermal hyperalgesia and reduced the overexpression of brain-derived neurotrophic factor (BDNF), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the spinal cord. We also assessed the effects of EA treatment on the pain hypersensitivities of DNP rats, and further investigated the possible mechanism underlying the analgesic effect of EA. EA relieved the hyperalgesia of DNP. In terms of mechanism, EA reduced the upregulation of P2X4R on activated microglia and decreased BDNF, IL-1β and TNF-α in the spinal cord. Mechanistic research of EA's analgesic impact would be beneficial in ensuring its prospective therapeutic effect on DNP as well as in extending EA's applicability.

G protein-coupled P2Y12 receptor is involved in the progression of neuropathic pain

The pathological mechanism of neuropathic pain is complex, which seriously affects the physical and mental health of patients, and its treatment is also difficult. The role of G protein-coupled P2Y12 receptor in pain has been widely recognized and affirmed. After nerve injury, stimulated cells can release large amounts of nucleotides into the extracellular matrix, act on P2Y12 receptor. Activated P2Y12 receptor activates intracellular signal transduction and is involved in the development of pain. P2Y12 receptor activation can sensitize primary sensory neurons and receive sensory information. By transmitting the integrated information through the dorsal root of the spinal cord to the secondary neurons of the posterior horn of the spinal cord. The integrated information is then transmitted to the higher center through the ascending conduction tract to produce pain. Moreover, activation of P2Y12 receptor can mediate immune cells to release pro-inflammatory factors, increase damage to nerve cells, and aggravate pain. While inhibits the activation of P2Y12 receptor can effectively relieve pain. Therefore, in this article, we described P2Y12 receptor antagonists and their pharmacological properties. In addition, we explored the potential link between P2Y12 receptor and the nervous system, discussed the intrinsic link of P2Y12 receptor and neuropathic pain and as a potential pharmacological target for pain suppression.

P2RY13 Exacerbates Intestinal Inflammation by Damaging the Intestinal Mucosal Barrier via Activating IL-6/STAT3 Pathway

The pathogenesis of ulcerative colitis (UC) is unclear, while genetic factors have been confirmed to play an important role in its development. P2RY13 is a G protein-coupled receptor (GPCRs), which are involved in the pathogenesis of inflammation and immune disorders. According to GEO database analysis, we first observed that the expression of P2Y13 was increased in UC patients. Therefore, we sought to determine the role of P2Y13 in the development of colitis. Our data showed that P2RY13 was highly expressed in the inflamed intestinal tissues of UC patients. In mice, pharmacological antagonism of P2Y13 can significantly attenuate the intestinal mucosal barrier disruption. In LPS-induced NCM460 cell, knockdown or pharmacological inhibition of P2RY13 increased the expression of intestinal tight junction protein and reduced apoptosis. In addition, we found that the effect of P2Y13 on colitis is related to the activation of the IL-6/STAT3 pathway. Activation of P2Y13 increases IL-6 expression and promotes STAT3 phosphorylation and nuclear transport. Deletion of the STAT3 gene in the intestinal epithelial cells of mice significantly mitigated the exacerbation of colitis due to P2Y13 activation. Thus, P2Y13 can aggravate intestinal mucosal barrier destruction by activating the IL-6/STAT3 pathway. P2Y13 might be a potential drug target for UC.

Blockage of HCN Channels Inhibits the Function of P2X Receptors in Rat Dorsal Root Ganglion Neurons

Hyperpolarization-activated cyclic nucleotide-gated channels and purinergic P2X receptors play critical roles in the nerve injury-induced pain hypersensitivity. Both HCN channels and P2XR are expressed in dorsal root ganglia sensory neurons. However, it is not clear whether the expression and function of P2X2 and P2X3 receptors can be modulated by HCN channel activity. For this reason, in rats with chronic constriction injury of sciatic nerve, we evaluated the effect of intrathecal administration of HCN channel blocker ZD7288 on nociceptive behavior and the expression of P2X2 and P2X3 in rat DRG. The mechanical withdrawal threshold was measured to evaluate pain behavior in rats. The protein expression of P2X2 and P2X3 receptor in rat DRG was observed by using Western Blot. The level of cAMP in rat DRG was measured by ELISA. As a result, decreased MWT was observed in CCI rats on 1 d after surgery, and the allodynia was sustained throughout the experimental period. In addition, CCI rats presented increased expression of P2X2 and P2X3 receptor in the ipsilateral DRG at 7 d and 14 d after CCI operation. Intrathecal injection of ZD7288 significantly reversed CCI-induced mechanical hyperalgesia, and attenuated the increased expression of P2X2 and P2X3 receptor in rat DRG, which open up the possibility that the expression of P2X2 and P2X3 receptor in DRG is down-regulated by HCN channel blocker ZD7288 in CCI rats. Furthermore, the level of cAMP in rat DRG significantly increased after nerve injury. Intrathecal administration of ZD7288 attenuated the increase of cAMP in DRG caused by nerve injury. Subsequently, effects of HCN channel activity on ATP-induced current (I_ATP) in rat DRG neurons were explored by using whole-cell patch-clamp techniques. ATP (100 μM) elicited three types of currents (fast, slow and mixed I_ATP) in cultured DRG neurons. Pretreatment with ZD7288 concentration-dependently inhibited three types of ATP-activated currents. On the other hand, pretreatment with 8-Br-cAMP (a cell-permeable cAMP analog, also known as an activator of PKA) significantly increased the amplitude of fast, slow and mixed I_ATP in DRG neurons. The enhanced effect of 8-Br-cAMP on ATP-activated currents could be reversed by ZD7288. In a summary, our observations suggest that the opening of HCN channels could enhance the expression and function of P2X2 and P2X3 receptor via the cAMP-PKA signaling pathway. This may be important for pathophysiological events occurring within the DRG, for where it is implicated in nerve injury-induced pain hypersensitivity.

[Quantative evaluation and analysis of the central mechanisms involved in analgesic effect of Alflutop in patients with chronic lower back pain]

OBJECTIVE

Evaluation of Alfultop impact on nociceptive afferentation central mechanisms in patients with chronic lower back pain.

MATERIALS AND METHODS

The study involved 40 patients with CLBP. The therapy included Alflutop, 2 ml once a day for 10 days. Mean VAS-pain, LANSS, Roland-Morris questionnaire, Global Rating of Change Scale (GRC), pressure pain thresholds data were analyzed. Follow-up duration was 3 months.

RESULTS

Most of the patients were females (f:m=1.3:1); average age - 60.5 [54.2; 67.3] years with a mean disease duration of 14.3±4.2 mo. At visit 1, the VAS score was 63.2±9.4, LANSS 14.2±2.1 points, and the Roland-Morris questionnaire 9.9±3.5 points. There was a significant decrease in the pain threshold both in the zone of maximum pain, located in the lower back (3.97±0.9 kg/sm²), and in suprasegmental area (5.22±1.7 kg/sm²), and a pathological change in the temporal summation of pain (789.2±45.6 mm). After Alflutop therapy, significant changes in the parameters of the VAS, LANSS and Roland-Morris scores were recorded after 30 days of observation and maximum changes in 3 months. The pain threshold has significantly increased after 3 months of observation. The majority of patients rated the improvement from the therapy as «moderate» and «pronounced» (33/82.5%) according to the GRC scale. A significant relationship was established between the level of pain threshold and the intensity of pain on the VAS scale (R=0.714), its duration (R=0.799) and disability of the patients (R=0.706).

CONCLUSIONS

Central sensitization develops in patients with CLBP, which correlates with VAS score of pain intensity, its duration and the degree of disability. Alflutop significantly reduces the intensity of the pain syndrome, its neuropathic component, significantly increases the level of pain threshold and improves the disability of patients in 3 months after the start of treatment.

Lower Formalin-Induced Pain Responses in Painless Diabetic Neuropathy Rat Correlate with the Reduced Spinal Cord NR2B Subunit of N-Methyl-D-Aspartate Receptor Activation

Diabetic neuropathy (DN) is a late complication of diabetic mellitus and may rise into painful and painless variants. Limited studies have looked at nociceptive mechanisms of painless DN variant. The study aimed to determine phosphorylation and total NR2B subunit of N-methyl-D-aspartate receptor in the spinal cord of painless DN rat during early phase following formalin injection. Thirty-six Sprague-Dawley male rats were randomly assigned into three groups: control, painful, and painless DN (n = 12). The rats were developed into the early phase of DN for 2 weeks following diabetic induction. Two weeks later, the rats were injected with 5% formalin solution and flinching and licking responses were recorded for 60 min. The rats were sacrificed 3 days later, and the spinal cord enlargement region was collected. Immunohistochemistry and Western blot procedures were conducted to determine the phosphorylated and total NR2B subunit expressions. The results showed reduced flinching and licking responses in painless DN rats compared to control and painful DN groups, followed by a significant reduction in phosphorylated and total NR2B expression at both ipsilateral and contralateral regions of the spinal cord. In conclusion, reduced pain behavior responses in painless DN rats following formalin injection is possibly contributed by the reduced expression of phosphorylated and total NR2B subunit in the spinal cord.

The Modulatory Effect of Motor Cortex Astrocytes on Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a common complication of diabetes characterized by persistent pain. Emerging evidence links astrocytes to mechanical nociceptive processing, and the motor cortex (MCx) is a cerebral cortex region that is known to play a key role in pain regulation. However, the association between MCx astrocytes and DNP pathogenesis remains largely unexplored. Here, we studied this association using designer receptors exclusively activated by designer drugs to specifically manipulate MCx astrocytes. We proved that the selective inhibition of MCx astrocytes reduced DNP in streptozocin (STZ)-induced DNP models and discovered a potential mechanism by which astrocytes release cytokines, including TNF-α and IL-1β, to increase neuronal activation in the MCx, thereby regulating pain. Together, these results demonstrate a pivotal role for MCx astrocytes in DNP pathogenesis and provide new insight into DNP treatment strategies.

Suppression of TRPV1/TRPM8/P2Y Nociceptors by Withametelin via Downregulating MAPK Signaling in Mouse Model of Vincristine-Induced Neuropathic Pain

Vincristine (VCR) is a widely used chemotherapy drug that induced peripheral painful neuropathy. Yet, it still lacks an ideal therapeutic strategy. The transient receptor potential (TRP) channels, purinergic receptor (P2Y), and mitogen-activated protein kinase (MAPK) signaling play a crucial role in the pathogenesis of neuropathic pain. Withametelin (WMT), a potential Phytosteroid isolated from datura innoxa, exhibits remarkable neuroprotective properties. The present investigation was designed to explore the effect of withametelin on VCR-induced neuropathic pain and its underlying molecular mechanism. Initially, the neuroprotective potential of WMT was confirmed against hydrogen peroxide (H₂O₂)-induced PC12 cells. To develop potential candidates for neuropathic pain treatment, a VCR-induced neuropathic pain model was established. Vincristine (75 μg/kg) was administered intraperitoneally (i.p.) for 10 consecutive days (day 1-10) for the induction of neuropathic pain. Gabapentin (GBP) (60 mg/kg, i.p.) and withametelin (0.1 and 1 mg/kg i.p.) treatments were given after the completion of VCR injection on the 11th day up to 21 days. The results revealed that WMT significantly reduced VCR-induced pain hypersensitivity, including mechanical allodynia, cold allodynia, and thermal hyperalgesia. It reversed the VCR-induced histopathological changes in the brain, spinal cord, and sciatic nerve. It inhibited VCR-induced changes in the biochemical composition of the myelin sheath of the sciatic nerve. It markedly downregulated the expression levels of TRPV1 (transient receptor potential vanilloid 1); TRPM8 (Transient receptor potential melastatin 8); and P2Y nociceptors and MAPKs signaling, including ERK (Extracellular Signal-Regulated Kinase), JNK (c-Jun N-terminal kinase), and p-38 in the spinal cord. It suppressed apoptosis by regulating Bax (Bcl2-associated X-protein), Bcl-2 (B-cell-lymphoma-2), and Caspase-3 expression. It considerably attenuated inflammatory cytokines, oxidative stress, and genotoxicity. This study suggests that WMT treatment suppressed vincristine-induced neuropathic pain by targeting the TRPV1/TRPM8/P2Y nociceptors and MAPK signaling.

Minocycline alleviates nociceptive response through modulating the expression of NR2B subunit of NMDA receptor in spinal cord of rat model of painful diabetic neuropathy

Background

It has been reported that neuropathic pain can be overcome by targeting the NR2B subunit of N-methyl-D-aspartate receptors (NR2B). This study aimed to investigate the effects of minocycline on phosphorylated and total expression of NR2B in the spinal cord of rats with diabetic neuropathic pain.

Methods

A total of 32 Sprague-Dawley male rats were randomly assigned into four groups (n = 8); control healthy, control diabetic (PDN), and PDN rats that received 80 µg or 160 µg intrathecal minocycline respectively. The rats were induced to develop diabetes and allowed to develop into the early phase of PDN for two weeks. Hot-plate and formalin tests were conducted. Intrathecal treatment of minocycline or normal saline was conducted for 7 days. The rats were sacrificed to obtain the lumbar enlargement region of the spinal cord (L4-L5) for immunohistochemistry and western blot analyses to determine the expression of phosphorylated (pNR2B) and total NR2B (NR2B).

Results

PDN rats showed enhanced flinching (phase 1: p < 0.001, early phase 2: p < 0.001, and late phase 2: p < 0.05) and licking responses (phase 1: p < 0.001 and early phase 2: p < 0.05). PDN rats were also associated with higher spinal expressions of pNR2B and NR2B (p < 0.001) but no significant effect on thermal hyperalgesia. Minocycline inhibited formalin-induced flinching and licking responses (phase 1: p < 0.001, early phase 2: p < 0.001, and late phase 2: p < 0.05) in PDN rats with lowered spinal expressions of pNR2B (p < 0.01) and NR2B (p < 0.001) in a dose-dependent manner.

Conclusion

Minocycline alleviates nociceptive responses in PDN rats, possibly via suppression of NR2B activation. Therefore, minocycline could be one of the potential therapeutic antinociceptive drugs for the management of neuropathic pain.

Tranilast attenuates neuropathic pain during type-2 diabetes by inhibiting hypoxia-induced pro-inflammatory cytokines in Zucker diabetic fatty rat model

BACKGROUND

The modulatory effect of tranilast on neuropathic pain in type-2 diabetes (T2DM) remains unclear.

METHODS

We monitored interleukin (IL)-1β, nuclear factor-κB (NF-κB) and tumour necrosis factor-α (TNF-α) levels during the progression of T2DM induced neuropathic pain in rats, and assessed the impact of tranilast treatment of increasing concentrations (0, 200 and 400 mg/kg/day via oral gavage in 1% NaHCO₃ delivered as 100 mg/kg twice a day) on the levels of cytokine production, as well as on the thermal hyperalgesia and mechanical allodynia.

RESULTS

The rats developed hyperglycaemia accompanied with elevated levels of NF-κB, IL-1β and TNF-α in the rostral ventromedial medulla at the age of 16 weeks. Tranilast administration dose dependently alleviated thermal hyperalgesia as well as mechanical allodynia, which was associated with its ability in inhibiting hypoxia-induced levels of NF-κB, IL-1β and TNF-α.

CONCLUSION

Tranilast plays crucial roles in modulating T2DM-related neuropathic pain, likely through inhibiting hypoxia.

Excessive walking exercise precipitates diabetic neuropathic foot pain: hind paw suspension treadmill exercise experiment in a rat model

The harmful effects of excessive mechanical loading on diabetic neuropathy and the reason diabetic neuropathic symptoms are common in feet are unclear. In this study, the hind paw suspension treadmill exercise model was used in rats to investigate whether mechanical loading applied to the front paws precipitates neuropathic pain, especially in diabetic conditions. Thirty-two rats were divided into six groups according to the presence of diabetes (DM) and the intensity of mechanical loading applied to the front paws: DM-Hi (high-intensity); DM-Lo (low-intensity); DM-No (non-mechanical loading); Sham-Hi; Sham-Lo; and Sham-No. DM was induced by streptozotocin injection. For high-intensity or low-intensity mechanical loading, treadmill walking exercise was conducted with or without hind paw suspension, respectively. The mechanical withdrawal threshold of the front paw decreased significantly after 8 weeks only in the DM mechanical loading groups (DM-Hi and DM-Lo), and high-intensity loading more significantly decreased the front-paw withdrawal threshold than low-intensity loading. In the DM-Hi group only, macrophage migration inhibitory factor (MIF) increased significantly, and intra-epidermal nerve fibers (IENF) in the front paws decreased significantly. In diabetic conditions, mechanical overloading such as excessive walking is likely to precipitate mechanical allodynia and damage IENF¸ which could explain why diabetic neuropathic symptoms are common in feet. This finding might be related to up-regulation of intracellular signaling cascades such as MIF, rather than inflammatory processes.

Purinergic signaling as a basis of acupuncture-induced analgesia

This review summarizes experimental evidence indicating that purinergic mechanisms are causally involved in acupuncture (AP)-induced analgesia. Electroacupuncture (EAP) and manual AP release at pain-relevant acupoints ATP which may activate purinergic P2X receptors (Rs) especially of the P2X3 type situated at local sensory nerve endings (peripheral terminals of dorsal root ganglion [DRG] neurons); the central processes of these neurons are thought to inhibit via collaterals of ascending dorsal horn spinal cord neurons, pain-relevant pathways projecting to higher centers of the brain. In addition, during AP/EAP non-neuronal P2X4 and/or P2X7Rs localized at microglial cells of the CNS become activated at the spinal or supraspinal levels. In consequence, these microglia secrete bioactive compounds such as growth factors, cytokines, chemokines, reactive oxygen, and nitrogen species, which modulate the ascending neuronal pathways conducting painful stimuli. Alternatively, ATP released at acupoints by AP/EAP may be enzymatically degraded to adenosine, stimulating in loco presynaptic A1Rs exerting an inhibitory influence on the primary afferent fibers (the above mentioned pain-sensing peripheral terminals of DRG neurons) which thereby fail to conduct action potentials to the spinal cord dorsal horn. The net effect of the stimulation of P2X3, P2X4, P2X7, and A1Rs by the AP/EAP-induced release of ATP/adenosine at certain acupoints will be analgesia.

P2Y receptors in neuropathic pain

This review summarizes and evaluates the relationship between neuropathic pain and P2Y receptors from inception to 2019. Purinergic receptors have been well studied in recent years using various molecular biological methods. The main research objective of this review is to determine the association of P2Y1, P2Y2, P2Y6, P2Y12 and P2Y13 receptors with neuropathic pain. This review includes the most comprehensive subtypes of P2Y that related to neuropathic pain and the current therapeutic method of neuropathic pain. G protein-coupled P2Y receptors are located on neurons, astrocytes, oligodendrocytes and microglial cells and regulate neurotransmission. Nerve injury is the prime reason for abnormal regulation of P2Y receptor mRNA expression, subsequently, inducing neuropathic pain. Neuropathic pain is a type of chronic pain that is divided into peripheral, central and mixed. Numerous studies demonstrated a positive correlation between the expression level of P2Y receptors and neuropathic pain generation. Also, several reports showed that P2Y short hairpin RNA (shRNA) and P2Y antagonist can be used as an analgesic to relieve neuropathic pain via decreasing P2Y receptor expression level and neural cell activation. However, the transformation process from basic experiments to clinical applications is a long process. Current deficiencies and future research directions are discussed at the end of this review.

Intrathecal TRPM8 blocking attenuates cold hyperalgesia via PKC and NF-κB signaling in the dorsal root ganglion of rats with neuropathic pain

Background: TRPM8 channel plays central roles in the sensitization of nociceptive transduction and is thought as one of the potential targets for the treatment of neuropathic pain. However, the specific molecular mechanisms are still less clear.

Methods: Sciatic chronic constriction injury (CCI) rats were intrathecally administered with AMTB (TRPM8-selective antagonist) or PDTC (nuclear factor-kappa B (NF-κB) inhibitor). Cold-, thermal- and mechanical-pain thresholds were examined in CCI and sham-operated rats before and after intrathecal administration of AMTB or PDTC. Protein expression levels of TRPM8 and NF-κB p65, p-PKC/PKC value and p-PKA/PKA value in the CCI ipsilateral L4-6 dorsal root ganglions (DRGs) were analyzed. In addition, the co-expression of TRPM8 and NF-κB was evaluated in DRG.

Results: Intrathecal injection of AMTB decreased the cold hypersensitivity and aggravated the thermal-hyperalgesia in the next 2 weeks after CCI surgery. The protein expression of TRPM8 and NF-κB p65 in the ipsilateral DRGs significantly increased after CCI surgery, which can be reversed by intrathecal administration of AMTB. The PKC, PKA, p-PKC/PKC and p-PKA/PKA values showed significantly increase after CCI surgery, while intrathecal AMTB administration offset the expression increase of PKC, p-PKC and p-PKC/PKC but PKA or p-PKA/PKA in the DRG. NF-κB inhibitor not only efficiently increased the cold-, thermal-pain threshold of CCI rats, but also enhanced AMTB’s anti-cold pain effect although exerted no anti-thermal hyperalgesia effect compared with TRPM8 blockade group. Immunofluorescence results showed co-expression of TRPM8 and NF-κB in DRG neurons.

Conclusion: TRPM8 channels in DRGs participate in the pathogenesis of cold and thermal hyperalgesia (not mechanical allodynia) in rats with neuropathic pain, which could be regulated by PKC (not PKA) and NF-κB signaling. TRPM8 channel, PKC and NF-κB are potential targets for cold hyperalgesia treatment in neuropathic pain patients.

The role of adenosine and P2Y receptors expressed by multiple cell types in pain transmission

The role of extracellular nucleotides and nucleosides as signaling molecules in cell-to-cell communication has now been clearly established. This is particularly true in the central and peripheral nervous system, where purines and pyrimidines are involved in both physiological and pathological interactions between neurons and surrounding glial cells. It can be thus foreseen that the purinergic system could represent a new potential target for the development of effective analgesics, also through the normalization of neuronal functions and the inhibition of glial cell activation. Research in the last 15 years has progressively confirmed this hypothesis, but no purinergic-based analgesics have reach the market so far; in the present review we have collected the more recent discoveries on the role of G protein-coupled P2Y nucleotide and of adenosine receptors expressed by both neurons and glial cells under painful conditions, and we have highlighted some of the challenges that must be faced to translate basic and preclinical studies to clinics.

Neurons and Microglia; A Sickly-Sweet Duo in Diabetic Pain Neuropathy

Diabetes is a common condition characterized by persistent hyperglycemia. High blood sugar primarily affects cells that have a limited capacity to regulate their glucose intake. These cells include capillary endothelial cells in the retina, mesangial cells in the renal glomerulus, Schwann cells, and neurons of the peripheral and central nervous systems. As a result, hyperglycemia leads to largely intractable complications such as retinopathy, nephropathy, hypertension, and neuropathy. Diabetic pain neuropathy is a complex and multifactorial disease that has been associated with poor glycemic control, longer diabetes duration, hypertension, advanced age, smoking status, hypoinsulinemia, and dyslipidemia. While many of the driving factors involved in diabetic pain are still being investigated, they can be broadly classified as either neuron -intrinsic or -extrinsic. In neurons, hyperglycemia impairs the polyol pathway, leading to an overproduction of reactive oxygen species and reactive nitrogen species, an enhanced formation of advanced glycation end products, and a disruption in Na⁺/K⁺ ATPase pump function. In terms of the extrinsic pathway, hyperglycemia leads to the generation of both overactive microglia and microangiopathy. The former incites a feed-forward inflammatory loop that hypersensitizes nociceptor neurons, as observed at the onset of diabetic pain neuropathy. The latter reduces neurons' access to oxygen, glucose and nutrients, prompting reductions in nociceptor terminal expression and losses in sensation, as observed in the later stages of diabetic pain neuropathy. Overall, microglia can be seen as potent and long-lasting amplifiers of nociceptor neuron activity, and may therefore constitute a potential therapeutic target in the treatment of diabetic pain neuropathy.