Amitriptyline does not block the action of ATP at human P2X4 receptor

Amitriptyline effectiveness in burning mouth syndrome: An in-depth case series analysis

OBJECTIVES

To assess the effectiveness of amitriptyline (AMT), and to identify the determinants of the treatment's effectiveness in patients diagnosed with burning mouth syndrome (BMS).

BACKGROUND

Treatment of BMS is challenging and no established treatment protocol is available. AMT may be an important treatment option, cout not all patients benefit from this drug. Studies assessing factors related to treatment response are valuable in improving decision-making.

MATERIALS AND METHODS

This case series study examined the medical records of all patients diagnosed with BMS at an oral medicine unit in a university hospital from 2008 to 2022. The patients were divided into responders to AMT and non-responders to AMT. Data on demographic information, comorbidities, medications, types of symptoms and oral subsites affected were collected. Descriptive and bivariate analyses were conducted to assess the association between the independent variables and the outcome, using the Chi-squared test (P < .05).

RESULTS

Three hundred and fourty-nine patients reported a burning mouth sensation, 50 of them (14.3%) being diagnosed with primary BMS. Of these, 35 were treated with AMT, and 26 (74.2%) responded significantly to AMT. All males responded to AMT, whereas only 67.9% of females responded. The mean dose of AMT among responders was 29.8 ± 12.3 mg, with most patients achieving a response with 25 mg (61.5% of patients), followed by 50 mg (23%). The concomitant use of an anticonvulsant resulted in non-response.

CONCLUSIONS

AMT may be effective in BMS management for most patients.

The P2X4 Receptor: Cellular and Molecular Characteristics of a Promising Neuroinflammatory Target

The adenosine 5′-triphosphate-gated P2X4 receptor channel is a promising target in neuroinflammatory disorders, but the ability to effectively target these receptors in models of neuroinflammation has presented a constant challenge. As such, the exact role of P2X4 receptors and their cell signalling mechanisms in human physiology and pathophysiology still requires further elucidation. To this end, research into the molecular mechanisms of P2X4 receptor activation, modulation, and inhibition has continued to gain momentum in an attempt to further describe the role of P2X4 receptors in neuroinflammation and other disease settings. Here we provide an overview of the current understanding of the P2X4 receptor, including its expression and function in cells involved in neuroinflammatory signalling. We discuss the pharmacology of P2X4 receptors and provide an overview of P2X4-targeting molecules, including agonists, positive allosteric modulators, and antagonists. Finally, we discuss the use of P2X4 receptor modulators and antagonists in models of neuroinflammatory cell signalling and disease.

Purinergic receptors modulators: An emerging pharmacological tool for disease management

Purinergic signaling is mediated through extracellular nucleotides (adenosine 5'-triphosphate, uridine-5'-triphosphate, adenosine diphosphate, uridine-5'-diphosphate, and adenosine) that serve as signaling molecules. In the early 1990s, purines and pyrimidine receptors were cloned and characterized drawing the attention of scientists toward this aspect of cellular signaling. This signaling pathway is comprised of four subtypes of adenosine receptors (P1), eight subtypes of G-coupled protein receptors (P2YRs), and seven subtypes of ligand-gated ionotropic receptors (P2XRs). In current studies, the pathophysiology and therapeutic potentials of these receptors have been focused on. Various ligands, modulating the functions of purinergic receptors, are in current clinical practices for the treatment of various neurodegenerative disorders and cardiovascular diseases. Moreover, several purinergic receptors ligands are in advanced phases of clinical trials as a remedy for depression, epilepsy, autism, osteoporosis, atherosclerosis, myocardial infarction, diabetes, irritable bowel syndrome, and cancers. In the present study, agonists and antagonists of purinergic receptors have been summarized that may serve as pharmacological tools for drug design and development.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

A new central post-stroke pain rat model: autologous blood injected thalamic hemorrhage involved increased expression of P2X4 receptor

Stroke is the leading cause of disability and death in the world. Central post-stroke pain (CPSP), a central neuropathic pain syndrome occurring after cerebral stroke, is a serious problem. But on account of the lack of reliable animal models, the mechanisms underlying CPSP remains poorly understood. To better understand of the pathophysiological basis of CPSP, we developed and characterized a new rat model of CPSP. This model is based on a hemorrhagic stroke lesion with intra-thalamic autologous blood (ITAB) injection in the ventral posterolateral nucleus of the thalamus. Behavioral analysis demonstrated that the animals displayed a significant decrease in mechanical allodynia threshold. We found a significant increase in P2 × 4 receptor expression in microglia in thalamic peri-lesion tissues post-hemorrhage. The mechanical allodynia in rats with CPSP were reversed by blocking P2 × 4 receptors. A significant alleviation of mechanical allodynia was achieved following the administration of adrenergic antidepressants and antiepileptics. Meanwhile, we found a significant decrease in P2 × 4 receptor expression after treatment with these drugs. Taken together, our results suggest that targeting P2 × 4 receptor may be effective in the treatment of CPSP.

Role of the neuroendocrine antimicrobial peptide catestatin in innate immunity and pain

Catestatin (CST) is a neuroendocrine peptide which is derived from the chromogranin A. It has been demonstrated that CST can affect a wide range of processes, such as innate immunity, inflammatory and autoimmune reactions, and several homeostatic regulations. Furthermore, CST is positive against several kinds of bacterial strains at micromolecular range, which shows its antimicrobial activity. Recently, the role of CST in acute and chronic pain has attracted much attention. In this review, we discussed the latest research findings of CST and its role in innate immunity and pain.

P2X4 Receptor Function in the Nervous System and Current Breakthroughs in Pharmacology

Adenosine 5′-triphosphate is a well-known extracellular signaling molecule and neurotransmitter known to activate purinergic P2X receptors. Information has been elucidated about the structure and gating of P2X channels following the determination of the crystal structure of P2X4 (zebrafish), however, there is still much to discover regarding the role of this receptor in the central nervous system (CNS). In this review we provide an overview of what is known about P2X4 expression in the CNS and discuss evidence for pathophysiological roles in neuroinflammation and neuropathic pain. Recent advances in the development of pharmacological tools including selective antagonists (5-BDBD, PSB-12062, BX430) and positive modulators (ivermectin, avermectins, divalent cations) of P2X4 will be discussed.

Blockade of P2X4 Receptors Inhibits Neuropathic Pain-Related Behavior by Preventing MMP-9 Activation and, Consequently, Pronociceptive Interleukin Release in a Rat Model

Neuropathic pain is still an extremely important problem in today's medicine because opioids, which are commonly used to reduce pain, have limited efficacy in this type of pathology. Therefore, complementary therapy is needed. Our experiments were performed in rats to evaluate the contribution of the purinergic system, especially P2X4 receptor (P2X4R), in the modulation of glia activation and, consequently, the levels of nociceptive interleukins after chronic constriction injury (CCI) of the right sciatic nerve, a rat model of neuropathic pain. Moreover, we studied how intrathecal (ith.) injection of a P2X4R antagonist Tricarbonyldichlororuthenium (II) dimer (CORM-2) modulates nociceptive transmission and opioid effectiveness in the CCI model. Our results demonstrate that repeated ith. administration of CORM-2 once daily (20 μg/5 μl, 16 and 1 h before CCI and then daily) for eight consecutive days significantly reduced pain-related behavior and activation of both spinal microglia and/or astroglia induced by CCI. Moreover, even a single administration of CORM-2 on day 7 after CCI attenuated mechanical and thermal hypersensitivity as efficiently as morphine and buprenorphine. In addition, using Western blot, we have shown that repeated ith. administration of CORM-2 lowers the CCI-elevated level of MMP-9 and pronociceptive interleukins (IL-1β, IL-18, IL-6) in the dorsal L4-L6 spinal cord and/or DRG. Furthermore, in parallel, CORM-2 upregulates spinal IL-1Ra; however, it does not influence other antinociceptive factors, IL-10 and IL-18BP. Additionally, based on our biochemical results, we hypothesize that p38MAPK, ERK1/2 and PI3K/Akt but not the NLRP3/Caspase-1 pathway are partly involved in the CORM-2 analgesic effects in rat neuropathic pain. Our data provide new evidence that P2X4R may indeed play a significant role in neuropathic pain development by modulating neuroimmune interactions in the spinal cord and DRG, suggesting that its blockade may have potential therapeutic utility.

Duloxetine Inhibits Microglial P2X4 Receptor Function and Alleviates Neuropathic Pain after Peripheral Nerve Injury

P2X4 receptors (P2X4R) are a family of ATP-gated non-selective cation channels. We previously demonstrated that activation of P2X4R in spinal microglia is crucial for neuropathic pain, a highly debilitating chronic pain condition, suggesting that P2X4R is a potential therapeutic target for treating neuropathic pain. Thus, the identification of a compound that has a potent inhibitory effect on P2X4R is an important clinical challenge. In the present study, we screened a chemical library of clinically approved drugs and show for the first time that duloxetine, a serotonin and noradrenaline reuptake inhibitor, has an inhibitory effect on rodent and human P2X4R. In primary cultured microglial cells, duloxetine also inhibited P2X4R-, but not P2X7R-, mediated responses. Moreover, intrathecal administration of duloxetine in a model of neuropathic pain produced a reversal of nerve injury-induced mechanical allodynia, a cardinal symptom of neuropathic pain. In rats that were pretreated with a serotonin-depleting agent and a noradrenaline neurotoxin, the antiallodynic effect of duloxetine was reduced, but still remained. Based on these results, we suggest that, in addition to duloxetine’s primary inhibitory action on serotonin and noradrenaline transporters, an inhibitory effect on P2X4R may be involved at least in part in an antiallodynic effect of intrathecal duloxetine in a model of neuropathic pain.

Effects of antidepressants on P2X7 receptors

Antidepressants including paroxetine, fluoxetine and desipramine are commonly used for treating depression. P2×7 receptors are member of the P2X family. Recent studies indicate that these receptors may constitute a novel potential target for the treatment of depression. In the present study, we examined the action of these antidepressants on cloned rat P2×7 receptors that were stably expressed in human embryonic kidney (HEK) 293 cells by using the whole-cell patch-clamp technique, and found that paroxetine at a dose of 10µM could significantly reduce the inward currents evoked by the P2×7 receptors agonist BzATP by pre-incubation for 6-12 but not by acute application (10µM) or pre-incubation for 2-6h at a dose of 1µM, 3µM or 10µM paroxetine. Neither fluoxetine nor desipramine had significant effects on currents evoked by BzATP either applied acutely or by pre-incubation at various concentrations. These results suggest that the sensitivity of rat P2×7 receptors to antidepressants is different, which may represent an unknown mechanism by which these drugs exert their therapeutic effects and side effects.

Medicinal chemistry of adenosine, P2Y and P2X receptors

Pharmacological tool compounds are now available to define action at the adenosine (ARs), P2Y and P2X receptors. We present a selection of the most commonly used agents to study purines in the nervous system. Some of these compounds, including A1 and A3 AR agonists, P2Y1R and P2Y12R antagonists, and P2X3, P2X4 and P2X7 antagonists, are potentially of clinical use in treatment of disorders of the nervous system, such as chronic pain, neurodegeneration and brain injury. Agonists of the A2AAR and P2Y2R are already used clinically, P2Y12R antagonists are widely used antithrombotics and an antagonist of the A2AAR is approved in Japan for treating Parkinson's disease. The selectivity defined for some of the previously introduced compounds has been revised with updated pharmacological characterization, for example, various AR agonists and antagonists were deemed A1AR or A3AR selective based on human data, but species differences indicated a reduction in selectivity ratios in other species. Also, many of the P2R ligands still lack bioavailability due to charged groups or hydrolytic (either enzymatic or chemical) instability. X-ray crystallographic structures of AR and P2YRs have shifted the mode of ligand discovery to structure-based approaches rather than previous empirical approaches. The X-ray structures can be utilized either for in silico screening of chemically diverse libraries for the discovery of novel ligands or for enhancement of the properties of known ligands by chemical modification. Although X-ray structures of the zebrafish P2X4R have been reported, there is scant structural information about ligand recognition in these trimeric ion channels. In summary, there are definitive, selective agonists and antagonists for all of the ARs and some of the P2YRs; while the pharmacochemistry of P2XRs is still in nascent stages. The therapeutic potential of selectively modulating these receptors is continuing to gain interest in such fields as cancer, inflammation, pain, diabetes, ischemic protection and many other conditions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Identification and characterization of a selective allosteric antagonist of human P2X4 receptor channels

P2X4 is an ATP-gated nonselective cation channel highly permeable to calcium. There is increasing evidence that this homomeric purinoceptor, which is expressed in several neuronal and immune cell types, is involved in chronic pain and inflammation. The current paucity of unambiguous pharmacological tools available to interrogate or modulate P2X4 function led us to pursue the search for selective antagonists. In the high-throughput screen of a compound library, we identified the phenylurea BX430 (1-(2,6-dibromo-4-isopropyl-phenyl)-3-(3-pyridyl)urea, molecular weight = 413), with antagonist properties on human P2X4-mediated calcium uptake. Patch-clamp electrophysiology confirmed direct inhibition of P2X4 currents by extracellular BX430, with submicromolar potency (IC50 = 0.54 µM). BX430 is highly selective, having virtually no functional impact on all other P2X subtypes, namely, P2X1-P2X3, P2X5, and P2X7, at 10-100 times its IC50. Unexpected species differences were noticed, as BX430 is a potent antagonist of zebrafish P2X4 but has no effect on rat and mouse P2X4 orthologs. The concentration-response curve for ATP on human P2X4 in the presence of BX430 shows an insurmountable blockade, indicating a noncompetitive allosteric mechanism of action. Using a fluorescent dye uptake assay, we observed that BX430 also effectively suppresses ATP-evoked and ivermectin-potentiated membrane permeabilization induced by P2X4 pore dilation. Finally, in single-cell calcium imaging, we validated its selective inhibitory effects on native P2X4 channels at the surface of human THP-1 cells that were differentiated into macrophages. In summary, this ligand provides a novel molecular probe to assess the specific role of P2X4 in inflammatory and neuropathic conditions, where ATP signaling has been shown to be dysfunctional.

Pharmacological insights into the role of P2X4 receptors in behavioural regulation: lessons from ivermectin

Purinergic ionotropic P2X receptors are a family of cation-permeable channels that bind extracellular adenosine 5'-triphosphate. In particular, convergent lines of evidence have recently highlighted P2X(4) receptors as a potentially critical target in the regulation of multiple nervous and behavioural functions, including pain, neuroendocrine regulation and hippocampal plasticity. Nevertheless, the role of the P2X(4) receptor in behavioural organization remains poorly investigated. To study the effects of P2X(4) activation, we tested the acute effects of its potent positive allosteric modulator ivermectin (IVM, 2.5-10 mg/kg i.p.) on a broad set of paradigms capturing complementary aspects of perceptual, emotional and cognitive regulation in mice. In a novel open field, IVM did not induce significant changes in locomotor activity, but increased the time spent in the peripheral zone. In contrast, IVM produced anxiolytic-like effects in the elevated plus maze and marble burying tasks, as well as depression-like behaviours in the tail-suspension and forced swim tests. The agent induced no significant behavioural changes in the conditioned place preference test and in the novel object recognition task. Finally, the drug induced a dose-dependent decrease in sensorimotor gating, as assessed by pre-pulse inhibition (PPI) of the acoustic startle reflex. In P2X(4) knockout mice, the effects of IVM in the open field and elevated plus maze were similar to those observed in wild type mice; conversely, the drug significantly increased startle amplitude and failed to reduce PPI. Taken together, these results suggest that P2X(4) receptors may play a role in the regulation of sensorimotor gating.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

The carbon monoxide donor, CORM-2, is an antagonist of ATP-gated, human P2X4 receptors

Carbon monoxide (CO) is produced endogenously by heme oxygenase (HO) enzymes. HO-1 is highly expressed in many inflammatory disease states, where it is broadly protective. The protective effects of HO-1 expression can be largely mimicked by the exogenous application of CO and CO-releasing molecules (CORMs). Despite a dearth of pharmacological tools for their study, molecular methodologies have identified P2X4 receptors as a potential anti-nociceptive drug target. P2X4 receptors are up-regulated in animal models of inflammatory pain, and their knock-down reduces pain behaviours. In these same animal models, HO-1 expression is anti-nociceptive, and we therefore investigated whether P2X4 was a target for CO and tricarbonyldichlororuthenium (II) dimer (CORM-2). Using conventional whole-cell and perforated-patch recordings of heterologously expressed human P2X4 receptors, we demonstrate that CORM-2, but not CO gas, is an inhibitor of these channels. We also investigated the role of soluble guanylate cyclase and mitochondria-derived reactive oxygen species using pharmacological inhibitors but found that they were largely unable to affect the ability of CORM-2 to inhibit P2X4 currents. A control breakdown product of CORM-2 was also without effect on P2X4. These results suggest that P2X4 receptors are not a molecular target of endogenous CO production and are, therefore, unlikely to be mediating the anti-nociceptive effects of HO-1 expression in inflammatory pain models. However, these results show that CORM-2 is an effective antagonist at human P2X4 receptors and represents a useful pharmacological tool for the study of these receptors given the current dearth of antagonists.