Synthesis of 1,4- and 1,4,4-substituted piperidines for the inhibition of neuronal T-type Ca2+ channels and mitigation of neuropathic pain in mice

BackgroundThe noradrenergic neurons of locus coeruleus (LC) project to the spinal dorsal horn (SDH), and release norepinephrine (NE) to inhibit pain transmission. However, its effect on pathological pain and the cellular mechanism in the SDH remains unclear. This study aimed to explore the analgesic effects and the anti-neuroinflammation mechanism of LC-spinal cord noradrenergic pathway (LC:SC) in neuropathic pain (NP) mice with sciatic chronic constriction injury.MethodsThe Designer Receptors Exclusively Activated by Designer Drugs (DREADD) was used to selectively activate LC:SC. Noradrenergic neuron-specific retro–adeno-associated virus was injected to the spinal cord. Pain threshold, LC and wide dynamic range (WDR) neuron firing, neuroinflammation (microglia and astrocyte activation, cytokine expression), and α2AR expression in SDH were evaluated.ResultsActivation of LC:SC with DREADD increased the mechanical and thermal nociceptive thresholds and reduced the WDR neuron firing. LC:SC activation (daily, 7 days) downregulated TNF-α and IL-1β expression, upregulated IL-4 and IL-10 expression in SDH, and inhibited microglia and astrocytes activation in NP mice. Immunofluorescence double staining confirmed that LC:SC activation decreased the expression of cytokines in microglia of the SDH. In addition, the effects of LC:SC activation could be reversed by intrathecal injection of yohimbine. Immunofluorescence of SDH showed that NE receptor α2B-AR was highly expressed in microglia in CCI mice.ConclusionThese findings indicate that selective activation of LC:SC alleviates NP in mice by increasing the release of NE and reducing neuroinflammation of astrocytes and microglia in SDH.

Neuropathic pain is a chronic pain caused by direct damage to the peripheral or central nervous system, characterized by hyperalgesia, allodynia, and spontaneous pain. Hydrogen sulfide (H2S) therapy has been applied for neuropathic pain treatment, although the underlying mechanisms remain unknown. In this study, we sought to ascertain whether H2S therapy could alleviate neuropathic pain in a model of chronic constriction injury (CCI) and, if so, the potential mechanism. A CCI model was established in mice through a spinal nerve ligation method. Intrathecal injection of NaHS was used to treat CCI model mice. The thermal paw withdrawal latency (TPWL) and mechanical paw withdrawal threshold (MPWT) were used for pain threshold evaluation in mice. A series of experiments including immunofluorescence, enzyme-linked immunosorbent assay, electrophysiological test, mitochondrial DNA (mtDNA) quantification, measurement of ATP content, demethylase activity, and western blot were performed to investigate the specific mechanism of H2S treatment in neuropathic pain. Mice with CCI exposure exhibited a decrease in MPWT and TPWL, an increase in IL-1β and TNF-α expressions, elevated eEPSP amplitude, an upregulation of mtDNA, and a reduction in ATP production, whereas H2S treatment significantly reversed these changes. Furthermore, CCI exposure induced a remarkable increase in vGlut2- and c-fos-positive as well as vGlut2- and Nrf2-positive cells, an increase in Nrf2 located in the nucleus, and an upregulation of H3K4 methylation, and H2S treatment further enhanced these changes. In addition, ML385, a selective Nrf2 inhibitor, reversed the neuroprotective effects of H2S. H2S treatment mitigates CCI-induced neuropathic pain in mice. This protective mechanism is possibly linked to the activation of the Nrf2 signaling pathway in vGlut2-positive cells.

Objective To evaluate the role of the beta3 subunit of the voltage-gated sodium channel (Scn3b) in neuropathic pain in mice.Methods The target gene Scn3b was embedded in the vector pBROAD-mcs and pBROAD3-mcs-Scn3b plasmid was then obtained.The primary mice were bred.The primary mice mated with C57/B6 mice and the transgenic mice were then generated.DNA,RT-PCR and Western blot experiments were performed to confirm the mice in which Scn3b was over-expressed.The mice with Scn3b over-expression multiplied rapidly to carry out the follow-up experiment.Ten transgenic mice (Scn3b group) and 10 control mice in the same litter (Con group) of both sexes,aged 2 months,weighing 25-30 g,were randomly chosen to establish the model of neuropathic pain.The mechanical pain threshold was measured before operation and on 3,5,7 and 14 days after operation.Results There was no significant difference in the mechanical pain threshold at each time point between the two groups (P ＞ 0.05).Conclusion Scn3b is not involved in the development and maintenance of neuropathic pain in mice. Key words: Sodium channels ; Neuralgia

Background/Objectives: Fructose-1,6-bisphosphate (FBP) is an intermediate product of the glycolytic pathway with analgesic effect in acute inflammatory pain model via the production of adenosine. However, whether FBP is active in neuropathic pain is unknown. Therefore, we reason that it would be suitable to investigate the analgesic effect and mechanism of action of FBP in a model of chronic constriction injury (CCI) of sciatic nerve-induced neuropathic pain in mice. Methods: After CCI induction, mice received FBP, adenosine, A1 and/or A2A receptor antagonists, and/or inhibitors of the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG)/ATP sensitive K channels (KATP) signaling pathway. Results: FBP (up to 85%) and adenosine (up to 84%) inhibited the mechanical hyperalgesia (electronic aesthesiometer) induced by CCI with similar profiles. FBP analgesia was dependent on adenosine because adenosine A1 and A2A receptors antagonists diminished FPB activity (100% and 79%, respectively). FBP analgesia was also dependent on activating the NO/cGMP/PKG/KATP signaling pathway. Furthermore, FBP treatment increased the production of NO in cultured dorsal root ganglia (DRG) neurons (100% increase), whereas neuronal nitric oxide synthase (nNOS) inhibition decreased (up to 70%) the analgesic effect of FBP. We also observed that FBP reduced the calcium levels of transient receptor potential ankyrin 1 (TRPA1)+ DRG neurons (85%) and paw-flinching triggered by TRPA1 activation (38%). Conclusions: FBP reduced neuropathic pain by reducing DRG neuron activation. The mechanisms involved the activation of adenosine A1 and A2A receptors to trigger the analgesic NO/cGMP/PKG/KATP signaling pathway and reducing TRPA1+ DRG neuron activity.

Oxaliplatin (L-OHP) is a platinum-based anticancer agent used to treat various types of cancer. It frequently causes acute and chronic peripheral neuropathies, such as cold allodynia and mechanical hyperalgesia. Ninjin'yoeito (NYT) is a formula used in traditional Japanese Kampo medicine to improve recovery from diseases and other medical disorders. We previously reported that treatment with a boiling-water extract of NYT prevented L-OHP-induced damage to neurite-like outgrowths from differentiated PC12 cells. The objectives of the present study were to evaluate the in vivo effects of NYT on L-OHP-induced neuropathic pain in mice and identify the active ingredients in NYT. Treatment with NYT extract significantly ameliorated both cold allodynia and mechanical hyperalgesia induced by L-OHP. While L-OHP treatment suppressed neurite outgrowths from primary dorsal root ganglion cells in vitro, NYT extract blocked this suppression in a concentration-dependent manner. Among the herbal components of NYT, the extract of ginseng (Panax ginseng roots) showed a protective effect against neurite damage induced by L-OHP, and one of its active ingredients was identified as ginsenoside Rg3. Ginseng extract partially relieved L-OHP-induced neuropathic pain in mice. Our results suggest that NYT could be an attractive agent for treating L-OHP-induced neuropathic pain, and that the active ingredient of NYT may be ginseng.

The present study was designed to investigate the effect of ramipril (angiotensin-converting enzyme inhibitor) on the chronic constriction injury of sciatic nerve induced neuropathic pain in mice. The neuropathic pain was induced by four loose ligations of the right sciatic nerve in mice. The battery of behavioral tests, i.e. plantar, pin prick, tail flick, tail pinch, rota rod tests, were performed to assess the degree of thermal and mechanical hyperalgesia in ipsilateral paw and tail, and motor in-coordination activity respectively. In addition, the biochemical tests, i.e. total protein, thiobarbituric acid reactive substances and reduced glutathione, were also performed in sciatic nerve tissue samples. The administration of ramipril (2 and 4 mg/kg, p.o.) significantly attenuated chronic constriction injury-induced rise in peripheral as well as central pain sensitivity (thermal and mechanical) along with impairment of motor in-coordination activity. Further, it also produces ameliorative effects on chronic constriction injury-induced rise in thiobarbituric acid reactive substances and decrease in glutathione levels when compared with a normal control group. It may be concluded that angiotensin-converting enzyme inhibitor may be a potential new target for the management of neuropathic pain.

GLP-1R activation ameliorated novel-object recognition memory dysfunction via regulating hippocampal AMPK/NF-κB pathway in neuropathic pain mice

Spared nerve injury (SNI) is an animal model that mimics the cardinal symptoms of peripheral nerve injury for studying the molecular and cellular mechanism of neuropathic pain in mice and rats. Currently, there are two types of SNI model, one to cut and ligate the common peroneal and the tibial nerves with intact sural nerve, which is defined as SNIs in this study, and another to cut and ligate the common peroneal and the sural nerves with intact tibial nerve, which is defined as SNIt in this study. Because the sural nerve is purely sensory whereas the tibial nerve contains both motor and sensory fibers, the SNIt model has much less motor deficit than the SNIs model. In the traditional SNIt mouse model, the common peroneal and the sural nerves are cut and ligated separately. Here a modified SNIt surgery method is described to damage both common peroneal and sural nerves with only one ligation and one cut with a shorter procedure time, which is easier to perform and reduces the potential risk of stretching the sciatic or tibial nerves, and produces similar mechanical hypersensitivity as the traditional SNIt model.

Oxaliplatin, a chemotherapy drug used to treat colorectal cancer, induces specific sensory neurotoxicity signs that are aggravated by cold and mechanical stimuli. Here we examined the preventive effects of Bee Venom (BV) derived phospholipase A2 (bvPLA2) on oxaliplatin-induced neuropathic pain in mice and its immunological mechanism. The cold and mechanical allodynia signs were evaluated by acetone and von Frey hair test on the hind paw, respectively. The most significant allodynia signs were observed at three days after an injection of oxaliplatin (6 mg/kg, i.p.) and then decreased gradually to a normal level on days 7–9. The oxaliplatin injection also induced infiltration of macrophages and upregulated levels of the pro-inflammatory cytokine interleukin (IL)-1β in the lumbar dorsal root ganglia (DRG). Daily treatment with bvPLA2 (0.2 mg/kg, i.p.) for five consecutive days prior to the oxaliplatin injection markedly inhibited the development of cold and mechanical allodynia, and suppressed infiltration of macrophages and the increase of IL-1β level in the DRG. Such preventive effects of bvPLA2 were completely blocked by depleting regulatory T cells (Tregs) with CD25 antibody pre-treatments. These results suggest that bvPLA2 may prevent oxaliplatin-induced neuropathic pain by suppressing immune responses in the DRG by Tregs.

Objective5-HOB is a novel tissue selective, 5-hydroxybenzothiazolone-derived β2 adrenoceptor agonist with minimized cardiovascular effects while retaining efficacy on skeletal muscle in preclinical experiments unlike conventional β2 adrenoceptor agonists, however its effect on the nervous system has not been evaluated yet. Therefore, 5-HOB was evaluated in a mouse model of neuropathic pain.Results5-HOB alleviated neuropathic allodynia in a dose dependent manner and reversed the changes in hind paw withdrawal thresholds to the sham control levels. The dose attenuating neuropathic allodynia was slightly lower than the dose inducing skeletal muscle hypertrophy. In conclusion, as reported with known β2 adrenoceptor agonists, 5-HOB was also effective in attenuating neuropathic pain in mice in addition to its effect on skeletal muscle.

The analgesic effect of venlafaxine (VLX), which is a selective serotonin and noradrenaline reuptake inhibitor (SNRI), has been observed on oxaliplatin-induced neuropathic pain in mice. Significant allodynia was shown after oxaliplatin treatment (6 mg/kg, i.p.); acetone and von Frey hair tests were used to assess cold and mechanical allodynia, respectively. Intraperitoneal administration of VLX at 40 and 60 mg/kg, but not 10 mg/kg, significantly alleviated these allodynia. Noradrenaline depletion by pretreatment of N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 50 mg/kg, i.p.) blocked the relieving effect of VLX (40 mg/kg, i.p.) on cold and mechanical allodynia. However, serotonin depletion by three consecutive pretreatments of para-chlorophenylalanine (PCPA, 150 mg/kg/day, i.p.) only blocked the effect of VLX on mechanical allodynia. In cold allodynia, the α2-adrenergic antagonist idazoxan (10 μg, i.t.), but not the α1-adrenergic antagonist prazosin (10 μg, i.t.), abolished VLX-induced analgesia. Furthermore, idazoxan and 5-HT3 receptor antagonist bemesetron (MDL-72222, 15 μg, i.t.), but not prazosin or mixed 5-HT1, 2 receptor antagonist methysergide (10 μg, i.t.), abolished VLX-induced analgesia in mechanical allodynia. In conclusion, 40 mg/kg of VLX treatment has a potent relieving effect against oxaliplatin-induced neuropathic pain, and α2-adrenergic receptor, and both α2-adrenergic and 5-HT3 receptors are involved in this effect of VLX on cold and mechanical allodynia, respectively.

Oxaliplatin, a chemotherapeutic drug for colorectal cancer, induces severe peripheral neuropathy. Bee venom acupuncture (BVA) has been used to attenuate pain, and its effect is known to be mediated by spinal noradrenergic and serotonergic receptors. Morphine is a well-known opioid used to treat different types of pain. Here, we investigated whether treatment with a combination of these two agents has an additive effect on oxaliplatin-induced neuropathic pain in mice. To assess cold and mechanical allodynia, acetone and von Frey filament tests were used, respectively. Significant allodynia signs were observed three days after an oxaliplatin injection (6 mg/kg, i.p.). BVA (0.25, 1, and 2.5 mg/kg, s.c., ST36) or morphine (0.5, 2, and 5 mg/kg, i.p.) alone showed dose-dependent anti-allodynic effects. The combination of BVA and morphine at intermediate doses showed a greater and longer effect than either BVA or morphine alone at the highest dose. Intrathecal pretreatment with the opioidergic (naloxone, 20 μg) or 5-HT3 (MDL-72222, 15 μg) receptor antagonist, but not with α2-adrenergic (idazoxan, 10 μg) receptor antagonist, blocked this additive effect. Therefore, we suggest that the combination effect of BVA and morphine is mediated by spinal opioidergic and 5-HT3 receptors and this combination has a robust and enduring analgesic action against oxaliplatin-induced neuropathic pain.

Anti-allodynic property of flavonoid myricitrin in models of persistent inflammatory and neuropathic pain in mice

Trigeminal neuralgia (TN) is a debilitating neuropathic facial pain disorder in which current treatments often provide incomplete or poorly tolerated relief. Microglia-driven neuroinflammation in the trigeminal system, particularly Toll-like receptor 4 (TLR4)/myeloid differentiation primary response 88 (MyD88) signaling, is increasingly recognized as a key driver of neuropathic pain, and network pharmacology suggests that Liquiritin, a major licorice flavonoid with anti-inflammatory actions, may target this pathway. We aimed to determine whether Liquiritin alleviates infraorbital nerve injury (IONI)-induced TN-like neuropathic pain by suppressing microglial M1-like polarization via the TLR4/MyD88 pathway, and to characterize its effects on head-withdrawal thresholds, conditioned place preference, inflammatory and pain mediators and TLR4/MyD88 signaling in vivo and in LPS-stimulated BV2 microglia. Adult female ICR mice (8-10weeks) underwent infraorbital nerve injury (IONI) or sham surgery and were randomly assigned to Sham + vehicle, IONI + vehicle, IONI + Liquiritin (200mg/kg, oral) or IONI + Pregabalin (10mg/kg) groups (n=10 per group) treated once daily for 18days. Mechanical allodynia (head-withdrawal thresholds to von Frey stimulation) and conditioned place preference were assessed, trigeminal tissues were analyzed by Western blotting, immunofluorescence and flow cytometry for microglial markers, inflammatory cytokines, pain mediators and TLR4/MyD88, and TLR4 antagonist/agonist administration, LPS-stimulated BV2 microglia and network pharmacology plus molecular docking were used to interrogate Liquiritin's TLR4/MyD88-dependent actions. In IONI mice, Liquiritin significantly attenuated mechanical allodynia and increased conditioned place preference compared with IONI + vehicle, yielding head-withdrawal threshold and conditioned place preference improvements. Network pharmacology identified 92 Liquiritin-related components, 194 candidate targets, and 41 neuropathic-pain-related overlapping genes enriched in Toll-like receptor signaling, and molecular docking showed favorable binding to IL-1β (-11.2kcal/mol), TNF-α (-8.35kcal/mol) and TLR4 (-8.76kcal/mol). Blocking TLR4/MyD88 signaling with LRU alleviated IONI-induced pain behaviors and reduced trigeminal IL-1β, TNF-α, Iba1, CD32, CGRP and TRPV1 expression, whereas TLR4 agonist TEA partially reversed Liquiritin-induced behavioral and molecular changes, supporting pathway involvement. In trigeminal tissues, Liquiritin decreased TLR4 and MyD88 expression and suppressed microglial M1-like markers together with IL-1β, TNF-α, TRPV1 and CGRP; in LPS-stimulated BV2 microglia, Liquiritin (100μM) and LRU (10μg/mL) reduced the production of iNOS, Iba1, CD32 and IL-1β/TNF-α, and serum alanine aminotransferase and aspartate aminotransferase activities were not significantly altered at 200mg/kg, while serum creatinine increased. Liquiritin alleviated IONI-induced TN-like neuropathic pain in mice, concomitant with reduced microglial M1-like activation, decreased IL-1β/TNF-α and TRPV1/CGRP levels, and down-regulation of TLR4/MyD88 signaling in trigeminal tissues. These in vivo, in vitro and in silico data support Liquiritin as a promising neuroinflammation-modulating candidate targeting TLR4/MyD88 in trigeminal neuropathic pain, warranting further pharmacokinetic, long-term safety and translational studies.

Triple-negative breast cancer (TNBC) is an exceptionally aggressive breast cancer subtype associated with neuropathic pain. This study explores the effects of 5'-nucleotidase domain-containing protein 2 (NT5DC2) on the progression of TNBC and neuropathic pain. Microarray analysis was conducted to identify differentially expressed genes in TNBC and the pathways involved. Gain- and loss-of-function assays of NT5DC2 were performed in TNBC cells, followed by detection of the extracellular acidification rate, adenosine triphosphate (ATP) levels, lactic acid production, glucose uptake, proliferation, migration, and invasion in TNBC cells. Macrophages were co-cultured with TNBC cells to examine the release of polarization-related factors and cytokines. A xenograft tumor model was established for in vivo validation. In addition, a mouse model of neuropathic pain was established through subepineural injection of TNBC cells, followed by measurement of the sciatic functional index and behavioral analysis to assess neuropathic pain. NT5DC2 was upregulated in TNBC and was positively correlated with epidermal growth factor receptor (EGFR). NT5DC2 interacted with EGFR to promote downstream signal transduction in TNBC cells. NT5DC2 knockdown diminished proliferation, migration, invasion, the extracellular acidification rate, ATP levels, lactic acid production, and glucose uptake in TNBC cells. Co-culture with NT5DC2-knockdown TNBC cells alleviated the M2 polarization of macrophages. Furthermore, NT5DC2 knockdown reduced tumor growth and neuropathic pain in mice. Importantly, activation of the EGFR pathway counteracted the effects of NT5DC2 knockdown. NT5DC2 knockdown protected against TNBC progression and neuropathic pain by inactivating the EGFR pathway.