Anti-allodynic effect of mangiferin in neuropathic rats: Involvement of nitric oxide-cyclic GMP-ATP sensitive K+ channels pathway and serotoninergic system

Anti-allodynic and anti-hyperalgesic activity of (±)-licarin A in neuropathic rats via NO-cyclic-GMP-ATP-sensitive K+ channel pathway

The study aimed to examine the effect of intraperitoneal and intrathecal (±)-licarin A in neuropathic pain induced by L5 and L6 spinal nerve ligation (SNL) in male Wistar rats and the possible involvement of the NO-cGMP-ATP-sensitive K+ channel pathway. Neuropathic pain signs (allodynia and hyperalgesia) were evaluated on postoperative Day 14 using von Frey filaments. Single intraperitoneal (0.01, 0.1, 1, and 10 mg/kg) and intrathecal (0.01, 0.1, 1, and 10 µg/rat) administration of (±)-licarin A improved allodynia and hyperalgesia. The (±)-licarin A-induced anti-allodynic and anti-hyperalgesic activity was prevented by the intrathecal injection of  l-NAME (100 µg/rat; nonselective nitric oxide synthase inhibitor), ODQ (10 µg/rat; guanylate cyclase inhibitor), and glibenclamide (50 µg/rat; adenosine triphosphate (ATP)-sensitive K+ channel blocker). The data suggest that (±)-licarin A exerts its anti-allodynic and anti-hyperalgesic activity by activating the NO-cGMP-ATP-sensitive K+ channel pathway.

Differential Contribution of 5-HT4, 5-HT5, and 5-HT6 Receptors to Acute Pruriceptive Processing Induced by Chloroquine and Histamine in Mice

The involvement of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing in the central nervous system (CNS) has been reported using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the roles of 5-HT receptor family in acute pruriceptive processing have not been fully elucidated in the CNS. In the present study, scratching behavior induced by chloroquine (CQ) was ameliorated by milnacipran or mirtazapine, and these effects were reversed by SB207266, a 5-HT4 antagonist, or SB258585, a 5-HT6 antagonist, but not by SB258585, a 5-HT5 antagonist. Moreover, CQ-induced scratches were mitigated by intrathecal injection of 5-HT4 agonists, such as BIMU8 and ML10302, and the 5-HT6 agonist, WAY208466. Conversely, histamine-induced scratches were not affected by the 5-HT4 agonists or a 5-HT6 agonist. Similarly, the amelioration of histamine-induced scratches by these antidepressants was not reversed by the 5-HT4, 5-HT5, or 5-HT6 receptor antagonist. Therefore, 5-HT is involved in the amelioration of CQ-induced scratches by milnacipran and mirtazapine, and 5-HT4, 5-HT5, and 5-HT6 receptors play differential roles in acute pruriceptive processing after administration of CQ or histamine.

Structure-Based Design of a Chemical Probe Set for the 5-HT5A Serotonin Receptor

The 5-HT_5A receptor (5-HT_5AR), for which no selective agonists and a few antagonists exist, remains the least understood serotonin receptor. A single commercial antagonist, SB-699551, has been widely used to investigate the 5-HT_5AR function in neurological disorders, including pain, but this molecule has substantial liabilities as a chemical probe. Accordingly, we sought to develop an internally controlled probe set. Docking over 6 million molecules against a 5-HT_5AR homology model identified 5 mid-μM ligands, one of which was optimized to UCSF678, a 42 nM arrestin-biased partial agonist at the 5-HT_5AR with a more restricted off-target profile and decreased assay liabilities versus SB-699551. Site-directed mutagenesis supported the docked pose of UCSF678. Surprisingly, analogs of UCSF678 that lost the 5-HT_5AR activity revealed that 5-HT_5AR engagement is nonessential for alleviating pain, contrary to studies with less-selective ligands. UCSF678 and analogs constitute a selective probe set with which to study the function of the 5-HT_5AR.

Atorvastatin prevents the development of diabetic neuropathic nociception by possible involvement of nitrergic system

AIMS

Diabetic neuropathy has been identified as a common complication caused by diabetes. However, its pathophysiological mechanisms are not fully understood yet. Statins, also known as HMG-CoA reductase inhibitors, alleviate the production of cholesterol. Despite this cholesterol-reducing effect of statins, several reports have demonstrated their beneficial properties in neuropathic pain. In this study, we used streptozotocin (STZ)-induced diabetic model to investigate the possible role of nitric oxide (NO) in the antineuropathic-like effect of atorvastatin.

METHODS

Diabetes was induced by a single injection of STZ. Male rats orally received different doses of atorvastatin for 21 days. To access the neuropathy process, the thermal threshold of rats was assessed using hot plate and tail-flick tests. Moreover, sciatic motor nerve conduction velocity (MNCV) studies were performed. To assess the role of nitric oxide, N(G)-nitro-L-arginine methyl ester (L-NAME), aminoguanidine (AG), and 7-nitroindazole (7NI) were intraperitoneally administered along with some specific doses of atorvastatin.

KEY FINDINGS

Atorvastatin significantly reduced the hyperalgesia in diabetic rats. L-NAME pretreatment with atorvastatin showed the antihyperalgesic effect, suggesting the possible involvement of the NO pathway in atorvastatin protective action. Furthermore, co-administration of atorvastatin with AG and 7NI resulted in a significant increase in pain threshold in diabetic rats.

SIGNIFICANCE

Our results reveal that the atorvastatin protective effect on diabetic neuropathy is mediated at least in a part via the nitric oxide system.

Sulfonylurea Receptor 1 in Central Nervous System Injury: An Updated Review

Sulfonylurea receptor 1 (SUR1) is a member of the adenosine triphosphate (ATP)-binding cassette (ABC) protein superfamily, encoded by Abcc8, and is recognized as a key mediator of central nervous system (CNS) cellular swelling via the transient receptor potential melastatin 4 (TRPM4) channel. Discovered approximately 20 years ago, this channel is normally absent in the CNS but is transcriptionally upregulated after CNS injury. A comprehensive review on the pathophysiology and role of SUR1 in the CNS was published in 2012. Since then, the breadth and depth of understanding of the involvement of this channel in secondary injury has undergone exponential growth: SUR1-TRPM4 inhibition has been shown to decrease cerebral edema and hemorrhage progression in multiple preclinical models as well as in early clinical studies across a range of CNS diseases including ischemic stroke, traumatic brain injury, cardiac arrest, subarachnoid hemorrhage, spinal cord injury, intracerebral hemorrhage, multiple sclerosis, encephalitis, neuromalignancies, pain, liver failure, status epilepticus, retinopathies and HIV-associated neurocognitive disorder. Given these substantial developments, combined with the timeliness of ongoing clinical trials of SUR1 inhibition, now, another decade later, we review advances pertaining to SUR1-TRPM4 pathobiology in this spectrum of CNS disease-providing an overview of the journey from patch-clamp experiments to phase III trials.

In Vivo Dissection of Two Intracellular Pathways Involved in the Spinal Oxytocin-Induced Antinociception in the Rat

Behavioral and electrophysiological data show that at the spinal level, oxytocin inhibits pain transmission by activation of oxytocin receptors (OTRs). Canonically, OTRs are coupled to Gq proteins, which induce a rise of intracellular Ca²⁺ by activating the phospholipase C (PLC). However, in vitro data showed that OTRs cause a plethora of intracellular events, some related to the activation of Gi proteins. Using a behavioral approach, we analyzed the main in vivo intracellular pathway elicited by spinal oxytocin during a peripheral inflammatory/persistent nociceptive stimulus. Intrathecal oxytocin reduces early (number of flinches) and late (mechanical allodynia) formalin-induced nociception, an effect abolished by the OTR antagonist (L-368,899). Furthermore, the antinociception observed during the early phase (acute inflammatory) was also reverted by U-73122 (PLC inhibitor) but not by pertussis toxin (Gα_i/o protein inhibitor) or gallein (G_βγ subunit inhibitor). In contrast, the late oxytocin-induced behavioral analgesia was blocked by pertussis and gallein but not by U-73122. Since oxytocin's effects during the early phase were also antagonized by Nω-nitro-l-arginine methyl ester, ODQ, or glibenclamide (inhibitors of nitric oxide synthase [NOS], soluble guanylyl cyclase [GC], and K⁺_ATP channels, respectively), the role of two differential pathways elicited by oxytocin is supported. Hence, we showed in in vivo experiments that oxytocin recruits two differential spinal intracellular pathways mediated by Gq (PLC/NOS/GC/K⁺_ATP) or Gi proteins during a peripheral nociceptive stimulus.

SUR1, newly expressed in astrocytes, mediates neuropathic pain in a mouse model of peripheral nerve injury

BACKGROUND

Neuropathic pain following peripheral nerve injury (PNI) is linked to neuroinflammation in the spinal cord marked by astrocyte activation and upregulation of interleukin 6 (IL-6), chemokine (C-C motif) ligand 2 (CCL2) and chemokine (C-X-C motif) ligand 1 (CXCL1), with inhibition of each individually being beneficial in pain models.

METHODS

Wild type (WT) mice and mice with global or pGfap-cre- or pGFAP-cre/ERT2-driven Abcc8/SUR1 deletion or global Trpm4 deletion underwent unilateral sciatic nerve cuffing. WT mice received prophylactic (starting on post-operative day [pod]-0) or therapeutic (starting on pod-21) administration of the SUR1 antagonist, glibenclamide (10 µg IP) daily. We measured mechanical and thermal sensitivity using von Frey filaments and an automated Hargreaves method. Spinal cord tissues were evaluated for SUR1-TRPM4, IL-6, CCL2 and CXCL1.

RESULTS

Sciatic nerve cuffing in WT mice resulted in pain behaviors (mechanical allodynia, thermal hyperalgesia) and newly upregulated SUR1-TRPM4 in dorsal horn astrocytes. Global and pGfap-cre-driven Abcc8 deletion and global Trpm4 deletion prevented development of pain behaviors. In mice with Abcc8 deletion regulated by pGFAP-cre/ERT2, after pain behaviors were established, delayed silencing of Abcc8 by tamoxifen resulted in gradual improvement over the next 14 days. After PNI, leakage of the blood-spinal barrier allowed entry of glibenclamide into the affected dorsal horn. Daily repeated administration of glibenclamide, both prophylactically and after allodynia was established, prevented or reduced allodynia. The salutary effects of glibenclamide on pain behaviors correlated with reduced expression of IL-6, CCL2 and CXCL1 by dorsal horn astrocytes.

CONCLUSION

SUR1-TRPM4 may represent a novel non-addicting target for neuropathic pain.

Synergistic antiallodynic and antihyperalgesic interaction between L-DOPA and celecoxib in parkinsonian rats is mediated by NO-cGMP-ATP-sensitive K+ channel

Pain is a usual and troublesome non-motor symptom of Parkinson's disease, with a prevalence of 29-82%. Therefore, it's vital to find pharmacological treatments for managing PD-associated pain symptoms, to improve patients' quality of life. For this reason, we tested the possible synergy between L-DOPA and celecoxib in decreasing allodynia and hyperalgesia induced by unilateral lesioning with 6-OHDA into the SNpc in rats. We also tested whether the antiallodynic and antihyperalgesic effect induced by combination of L-DOPA and celecoxib is mediated by the NO-cGMP-ATP-sensitive K⁺ channel pathway. Tactile allodynia and mechanical hyperalgesia were evaluated using von Frey filament. Isobolographic analyses were employed to define the nature of the drug interaction using a fixed dose ratio (0.5: 0.5). We found that acute and sub-acute (10-day) treatment with a single dose of L-DOPA (3-25 mg/kg, i. p.) or celecoxib (2.5-20 mg/kg, i. p.) induced a dose-dependent antiallodynic and antihyperalgesic effect in parkinsonian rats. Isobolographic analysis revealed that the ED₅₀ values obtained by L-DOPA + celecoxib combination was significantly less than calculated additive values, indicating that co-administration of L-DOPA with celecoxib produces synergistic interactions in its antiallodynic and antihyperalgesic effect in animals with nigrostriatal lesions. Moreover, the antiallodynic and antihyperalgesic effects induced by L-DOPA + celecoxib combination were blocked by intrathecal pre-treatment with L-NAME, ODQ, and glibenclamide. Taken together, the data suggest that L-DOPA + celecoxib combination produces an antiallodynic and antihyperalgesic synergistic interaction at the systemic level, and these effects are mediated, at the central level, through activation of the NO-cGMP-ATP-sensitive K⁺ channel pathway.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

Anti-allodynic effect induced by curcumin in neuropathic rat is mediated through the NO-cyclic-GMP-ATP sensitive K+ channels pathway

BACKGROUND

Recent studies pointed up that curcumin produces an anti-nociceptive effect in inflammatory and neuropathic pain. However, the possible mechanisms of action that underline the anti-allodynic effect induced by curcumin are not yet established. The purpose of this study was to determine the possible anti-allodynic effect of curcumin in rats with L5-L6 spinal nerve ligation (SNL). Furthermore, we study the possible participation of the NO-cyclic GMP-ATP-sensitive K+ channels pathway in the anti-allodynic effect induced by curcumin.

METHODS

Tactile allodynia was measured using von Frey filaments by the up-down method in female Wistar rats subjected to SNL model of neuropathic pain.

RESULTS

Intrathecal and oral administration of curcumin prevented, in a dose-dependent fashion, SNL-induced tactile allodynia. The anti-allodynic effect induced by curcumin was prevented by the intrathecal administration of L-NAME (100 μg/rat, a non-selective nitric oxide synthase inhibitor), ODQ (10 μg/rat, an inhibitor of guanylate-cyclase), and glibenclamide (50 μg/rat, channel blocker of ATP-sensitive K+ channels).

CONCLUSIONS

These data suggest that the anti-allodynic effect induced by curcumin is mediated, at least in part, by the NO-cyclic GMP-ATP-sensitive K+ channels pathway in the SNL model of neuropathic pain in rats.

Role of Melatonin in the Regulation of Pain

Melatonin is a pleiotropic hormone synthesized and secreted mainly by the pineal gland in vertebrates. Melatonin is an endogenous regulator of circadian and seasonal rhythms. Melatonin is involved in many physiological and pathophysiological processes demonstrating antioxidant, antineoplastic, anti-inflammatory, and immunomodulatory properties. Accumulating evidence has revealed that melatonin plays an important role in pain modulation through multiple mechanisms. In this review, we examine recent evidence for melatonin on pain regulation in various animal models and patients with pain syndromes, and the potential cellular mechanisms.

Mangiferin: Possible uses in the prevention and treatment of mixed osteoarthritic pain

Osteoarthritis (OA) pain has been proposed to be a mixed pain state, because in some patients, central nervous system factors are superimposed upon the more traditional peripheral factors. In addition, a considerable amount of preclinical and clinical evidence has shown that, accompanying the central neuroplasticity changes and partially driven by a peripheral nociceptive input, a real neuropathic component occurs that are particularly linked to disease severity and progression. Hence, innovative strategies targeting neuroprotection and particularly neuroinflammation to prevent and treat OA pain could be introduced. Mangiferin (MG) is a glucosylxanthone that is broadly distributed in higher plants, such as Mangifera indica L. Previous studies have documented its analgesic, anti-inflammatory, antioxidant, neuroprotective, and immunomodulatory properties. In this paper, we propose its potential utility as a multitargeted compound for mixed OA pain, even in the context of multimodal pharmacotherapy. This hypothesis is supported by three main aspects: the cumulus of preclinical evidence around this xanthone, some preliminary clinical results using formulations containing MG in clinical musculoskeletal or neuropathic pain, and by speculations regarding its possible mechanism of action according to recent advances in OA pain knowledge.

Anti-allodynic Effect of Mangiferin in Rats With Chronic Post-ischemia Pain: A Model of Complex Regional Pain Syndrome Type I

The present study reproduces chronic post-ischemia pain (CPIP), a model of complex regional pain syndrome type I (CRPS-I), in rats to examine the possible transient and long-term anti-allodynic effect of mangiferin (MG); as well as its potential beneficial interactions with some standard analgesic drugs and sympathetic-mediated vasoconstriction and vasodilator agents during the earlier stage of the pathology. A single dose of MG (50 and 100 mg/kg, p.o.) decreased mechanical allodynia 72 h post-ischemia-reperfusion (I/R). MG 100 mg/kg, i.p. (pre- vs. post-drug) increased von Frey thresholds in a yohimbine and naloxone-sensitive manner. Sub-effective doses of morphine, amitriptyline, prazosin, clonidine and a NO donor, SIN-1, in the presence of MG were found to be significantly anti-allodynic. A long-term anti-allodynic effect at 7 and 13 days post-I/R after repeated oral doses of MG (50 and 100 mg/kg) was also observed. Further, MG decreased spinal and muscle interleukin-1β concentration and restored muscle redox status. These results indicate that MG has a transient and long-term anti-allodynic effect in CPIP rats that appears to be at least partially attributable to the opioid and α2 adrenergic receptors. Additionally, its anti-inflammatory and antioxidant mechanisms could also be implicated in this effect. The association of MG with sub-effective doses of these drugs enhances the anti-allodynic effect; however, an isobolographic analysis should be performed to define a functional interaction between them. These findings suggest the possible clinical use of MG in the treatment of CRPS-I in both early sympathetically maintained pain and long-term sympathetically independent pain.

Antiallodynic effect induced by [6]-gingerol in neuropathic rats is mediated by activation of the serotoninergic system and the nitric oxide-cyclic guanosine monophosphate-adenosine triphosphate-sensitive K+ channel pathway

The present study evaluated the possible antiallodynic effect induced by [6]-gingerol in rats with L5-L6 spinal nerve ligation (SNL). Moreover, we determined the possible mechanism underlying the antiallodynic effect induced by [6]-gingerol in neuropathic rats. The animals underwent L5-L6 SNL for the purpose of developing tactile allodynia. Tactile allodynia was measured with von Frey filaments. Intrathecal administration of [6]-gingerol reversed SNL-induced tactile allodynia. The [6]-gingerol-induced antiallodynic effect was prevented by the intrathecal administration of methiothepin (30 μg per rat; nonselective 5-hydroxytryptamine [5-HT] antagonist), WAY-100635 (6 μg per rat; selective 5-HT_1A receptor antagonist), SB-224289 (5 μg per rat; selective 5-HT_1B receptor antagonist), BRL-15572 (4 μg per rat; selective 5-HT_1D receptor antagonist), and SB-659551 (6 μg per rat; selective 5-HT_5A receptor antagonist), but naloxone (50 μg per rat; nonselective opioid receptor antagonist) did not prevent the [6]-gingerol-induced antiallodynic effect. Moreover, intrathecal administration of Nω-nitro-l-arginine methyl ester (100 μg per rat; nonselective nitric oxide [NO] synthase inhibitor), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 μg per rat; inhibitor of guanylate cyclase), and glibenclamide (50 μg per rat; channel blocker of adenosine triphosphate [ATP]-sensitive K⁺ channels) prevented the [6]-gingerol-induced antiallodynic effect. These data suggest that the antiallodynic effect induced by [6]-gingerol is mediated by the serotoninergic system involving the activation of 5-HT_1A/1B/1D/5A receptors, as well as the NO-cyclic guanosine monophosphate-ATP-sensitive K⁺ channel pathway but not by the opioidergic system.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Toluene exposure enhances acute and chronic formalin-induced nociception in rats: Participation of 5-HT3 receptors

The purpose of this study was to evaluate the effect of acute toluene exposure on formalin (0.5% and 1%)-induced acute and long-lasting nociceptive hypersensitivity in rats. In addition, we sought to investigate the role of peripheral 5-HT₃ receptors in the pronociceptive effect of toluene. Toluene exposure (6000ppm) for 30min enhanced 0.5% or 1% formalin-induced acute nociception and long-lasting secondary allodynia and hyperalgesia. In contrast, exposition to toluene for 30min in rats previously injected (six days before) with 1% formalin did not affect long-lasting hypersensitivy. Local peripheral pre-treatment with alosetron (5-HT₃ receptor antagonist, 10-100 nmol) reduced the pronociceptive effect of toluene in acute nociception and long-lasting secondary allodynia and hyperalgesia. Alosetron (100nmol) was also able to reduce the nociceptive effects of 1% formalin in absence of toluene. Moreover, local peripheral injection of m-CPBG (5-HT₃ receptor agonist, 300 nmol) enhanced 0.5% formalin-induced acute and long-lasting nociception in air- and toluene-exposed rats. Alosetron (10nmol) blocked the pronociceptive effects of m-CPBG (300nmol) on 0.5% formalin-induced acute and long-lasting hypersensitivity in rats exposed to toluene. Alosetron (at 10nmol) did not modify formalin-induced nociceptive behaviors. Finally, local peripheral pre-treatment with methiothepin (non-selective 5-HT receptor antagonist, 1.5nmol), did not affect the pronociceptive effect of toluene on 1% formalin-induced acute and long-lasting hypersensitivity. Our data demonstrate that acute exposure to toluene has pronociceptive effects in formalin-induced acute nociception and long-lasting hypersensitivity. Our data suggest that this pronociceptive effect depend on activation of peripheral 5-HT₃, but not methiothepin-sensitive 5-HT, receptors.

Synergistic Interaction of a Gabapentin- Mangiferin Combination in Formalin-Induced Secondary Mechanical Allodynia and Hyperalgesia in Rats Is Mediated by Activation of NO-Cyclic GMP-ATP-Sensitive K+ Channel Pathway

Preclinical Research Gabapentin is an anticonvulsant used to treat neuropathic pain. Mangiferin is an antioxidant that has antinociceptive and antiallodynic effects in inflammatory and neuropathic pain models. The purpose of this study was to determine the interaction between mangiferin and gabapentin in the development and maintenance of formalin-induced secondary allodynia and hyperalgesia in rats. Gabapentin, mangiferin, or their fixed-dose ratio combination were administrated peripherally. Isobolographic analyses was used to define the nature of the interaction of antiallodynic and/or antihyperalgesic effects of the two compounds. Theoretical ED₅₀ values for the combination were 74.31 µg/paw and 95.20 µg/paw for pre- and post-treatment, respectively. These values were higher than the experimental ED₅₀ values, 29.45 µg/paw and 37.73 µg/paw respectively, indicating a synergistic interaction in formalin-induced secondary allodynia and hyperalgesia. The antiallodynic and antihyperalgesic effect induced by the gabapentin/mangiferin combination was blocked by administration of L-NAME, the soluble guanylyl cyclase inhibitor, ODQ and glibenclamide. These data suggest that the gabapentin- mangiferin combination produces a synergistic interaction at the peripheral level. Moreover, the antiallodynic and hyperalgesic effect induced by the combination is mediated via the activation of an NO-cyclic GMP-ATP-sensitive K⁺ channel pathway. Drug Dev Res 78 : 390-402, 2017. © 2017 Wiley Periodicals, Inc.