Identification of the Na+/H+ exchanger 1 in dorsal root ganglion and spinal cord: Its possible role in inflammatory nociception

Employing the sustained-release properties of poly(lactic-co-glycolic acid) nanoparticles to reveal a novel mechanism of sodium-hydrogen exchanger 1 in neuropathic pain

Neuropathic pain is caused by injury or disease of the somatosensory system, and its course is usually chronic. Several studies have been dedicated to investigating neuropathic pain-related targets; however, little attention has been paid to the persistent alterations that these targets, some of which may be crucial to the pathophysiology of neuropathic pain. The present study aimed to identify potential targets that may play a crucial role in neuropathic pain and validate their long-term impact. Through bioinformatics analysis of RNA sequencing results, we identified Slc9a1 and validated the reduced expression of sodium-hydrogen exchanger 1 (NHE1), the protein that Slc9a1 encodes, in the spinal nerve ligation (SNL) model. Colocalization analysis revealed that NHE1 is primarily co-localized with vesicular glutamate transporter 2-positive neurons. In vitro experiments confirmed that poly(lactic-co-glycolic acid) nanoparticles loaded with siRNA successfully inhibited NHE1 in SH-SY5Y cells, lowered intracellular pH, and increased intracellular calcium concentrations. In vivo experiments showed that sustained suppression of spinal NHE1 expression by siRNA-loaded nanoparticles resulted in delayed hyperalgesia in naïve and SNL model rats, whereas amiloride-induced transient suppression of NHE1 expression yielded no significant changes in pain sensitivity. We identified Slc9a1, which encodes NHE1, as a key gene in neuropathic pain. Utilizing the sustained release properties of nanoparticles enabled us to elucidate the chronic role of decreased NHE1 expression, establishing its significance in the mechanisms of neuropathic pain.

Acid-sensing ion channel blocker diminazene facilitates proton-induced excitation of afferent nerves in a similar manner that Na+/H+ exchanger blockers do

Tissue acidification causes sustained activation of primary nociceptors, which causes pain. In mammals, acid-sensing ion channels (ASICs) are the primary acid sensors; however, Na⁺/H⁺ exchangers (NHEs) and TRPV1 receptors also contribute to tissue acidification sensing. ASICs, NHEs, and TRPV1 receptors are found to be expressed in nociceptive nerve fibers. ASIC inhibitors reduce peripheral acid-induced hyperalgesia and suppress inflammatory pain. Also, it was shown that pharmacological inhibition of NHE1 promotes nociceptive behavior in acute pain models, whereas inhibition of TRPV1 receptors gives relief. The murine skin-nerve preparation was used in this study to assess the activation of native polymodal nociceptors by mild acidification (pH 6.1). We have found that diminazene, a well-known antagonist of ASICs did not suppress pH-induced activation of CMH-fibers at concentrations as high as 25 μM. Moreover, at 100 μM, it induces the potentiation of the fibers' response to acidic pH. At the same time, this concentration virtually completely inhibited ASIC currents in mouse dorsal root ganglia (DRG) neurons (IC₅₀ = 17.0 ± 4.5 μM). Non-selective ASICs and NHEs inhibitor EIPA (5-(N-ethyl-N-isopropyl)amiloride) at 10 μM, as well as selective NHE1 inhibitor zoniporide at 0.5 μM induced qualitatively the same effects as 100 μM of diminazene. Our results indicate that excitation of afferent nerve terminals induced by mild acidification occurs mainly due to the NHE1, rather than acid-sensing ion channels. At high concentrations, diminazene acts as a weak blocker of the NHE. It lacks chemical similarity with amiloride, EIPA, and zoniporide, so it may represent a novel structural motif for the development of NHE antagonists. However, the effect of diminazene on the acid-induced excitation of primary nociceptors remains enigmatic and requires additional investigations.

Inhibition of NHE1 transport activity and gene transcription in DRG neurons in oxaliplatin-induced painful peripheral neurotoxicity

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN), one of the major dose-limiting side effects of colorectal cancer treatment, is characterized by both acute and chronic syndromes. Acute exposure to low dose OHP on dorsal root ganglion (DRG) neurons is able to induce an increase in intracellular calcium and proton concentration, thus influencing ion channels activity and neuronal excitability. The Na⁺/H⁺ exchanger isoform-1 (NHE1) is a plasma membrane protein that plays a pivotal role in intracellular pH (pH_i) homeostasis in many cell types, including nociceptors. Here we show that OHP has early effects on NHE1 activity in cultured mouse DRG neurons: the mean rate of pH_i recovery was strongly reduced compared to vehicle-treated controls, reaching levels similar to those obtained in the presence of cariporide (Car), a specific NHE1 antagonist. The effect of OHP on NHE1 activity was sensitive to FK506, a specific calcineurin (CaN) inhibitor. Lastly, molecular analyses revealed transcriptional downregulation of NHE1 both in vitro, in mouse primary DRG neurons, and in vivo, in an OIPN rat model. Altogether, these data suggest that OHP-induced intracellular acidification of DRG neurons largely depends on CaN-mediated NHE1 inhibition, revealing new mechanisms that OHP could exert to alter neuronal excitability, and providing novel druggable targets.

Blocking NHE Channels Reduces the Ability of In Vitro Capacitated Mammalian Sperm to Respond to Progesterone Stimulus

Few data exist about the presence and physiological role of Na+/H+ exchangers (NHEs) in the plasma membrane of mammalian sperm. In addition, the involvement of these channels in the ability of sperm to undergo capacitation and acrosomal reaction has not been investigated in any mammalian species. In the present study, we addressed whether these channels are implicated in these two sperm events using the pig as a model. We also confirmed the presence of NHE1 channels in the plasma membrane of ejaculated sperm by immunofluorescence and immunoblotting. The function of NHE channels during in vitro capacitation was analyzed by incubating sperm samples in capacitating medium for 300 min in the absence or presence of a specific blocker (DMA; 5-(N,N-dimethyl)-amiloride) at different concentrations (1, 5, and 10 µM); acrosome exocytosis was triggered by adding progesterone after 240 min of incubation. Sperm motility and kinematics, integrity of plasma and acrosome membranes, membrane lipid disorder, intracellular calcium and reactive oxygen species (ROS) levels, and mitochondrial membrane potential (MMP) were evaluated after 0, 60, 120, 180, 240, 250, 270, and 300 min of incubation. NHE1 localized in the connecting and terminal pieces of the flagellum and in the equatorial region of the sperm head and was found to have a molecular weight of 75 kDa. During the first 240 min of incubation, i.e., before the addition of progesterone, blocked and control samples did not differ significantly in any of the parameters analyzed. However, from 250 min of incubation, samples treated with DMA showed significant alterations in total motility and the amplitude of lateral head displacement (ALH), acrosomal integrity, membrane lipid disorder, and MMP. In conclusion, while NHE channels are not involved in the sperm ability to undergo capacitation, they could be essential for triggering acrosome exocytosis and hypermotility after progesterone stimulus.

Interaction of NHE1 and TRPA1 Activity in DRG Neurons Isolated from Adult Rats and its Role in Inflammatory Nociception

Transient receptor potential ankyrin 1 (TRPA1) channel is expressed in a subset of nociceptive neurons. This channel integrates several nociceptive signals. Particularly, it is modulated by intracellular pH (pH_i). Na⁺/H⁺ exchanger 1 (NHE1) contributes to the maintenance of pH_i in nociceptors. However, it is currently unknown whether the interaction between TRPA1 and NHE1 contributes to the nociceptive processing. Thus, the purpose of this study was to assess the functional interaction between NHE1 and TRPA1 in small dorsal root ganglion (DRG) neurons from primary culture obtained from adult rats. Moreover, we also evaluated their possible interaction in acute and inflammatory pain. Zoniporide (selective NHE1 inhibitor) reduced pH_i and increased intracellular calcium in a concentration-dependent fashion in DRG neurons. Zoniporide and allyl isothiocyanate (AITC, TRPA1 agonist) increased calcium transients in the same DRG neuron, whereas that A-967079 (TRPA1 antagonist) prevented the effect of zoniporide in DRG neurons. Repeated AITC induced TRPA1 desensitization and this effect was prevented by zoniporide. Both NHE1 and TRPA1 were localized at the membrane surface of DRG neurons in culture. Local peripheral zoniporide enhanced AITC-induced pronociception and this effect was prevented by A-967079. Likewise, zoniporide potentiated Complete Freund's Adjuvant (CFA)-induced hypersensitivity, effect which was prevented by A-967079 in vivo. CFA paw injection increased TRPA1 and decresed NHE1 protein expression in DRG. These results suggest a functional interaction between NHE1 and TRPA1 in DRG neurons in vitro. Moreover, data suggest that this interaction participates in acute and inflamatory pain conditions in vivo.

Functional NHE1 expression is critical to blood brain barrier integrity and sumatriptan blood to brain uptake

Disruption of blood-brain barrier integrity and dramatic failure of brain ion homeostasis including fluctuations of pH occurs during cortical spreading depression (CSD) events associated with several neurological disorders, including migraine with aura, traumatic brain injury and stroke. NHE1 is the primary regulator of pH in the central nervous system. The goal of the current study was to investigate the role of sodium-hydrogen exchanger type 1 (NHE1) in blood brain barrier (BBB) integrity during CSD events and the contributions of this antiporter on xenobiotic uptake. Using immortalized cell lines, pharmacologic inhibition and genetic knockdown of NHE1 mitigated the paracellular uptake of radiolabeled sucrose implicating functional NHE1 in BBB maintenance. In contrast, loss of functional NHE1 in endothelial cells facilitated uptake of the anti-migraine therapeutic, sumatriptan. In female rats, cortical KCl but not aCSF selectively reduced total expression of NHE1 in cortex and PAG but increased expression in trigeminal ganglia; no changes were seen in trigeminal nucleus caudalis. Thus, in vitro observations may have a significance in vivo to increase brain sumatriptan levels. Pharmacological inhibition of NHE1 prior to cortical manipulations enhanced the efficacy of sumatriptan at early time-points but induced facial sensitivity alone. Overall, our results suggest that dysregulation of NHE1 contributes to breaches in BBB integrity, drug penetrance, and the behavioral sensitivity to the antimigraine agent, sumatriptan.

Suppressing the Na+/H+ exchanger 1: a new sight to treat depression

Na⁺/H⁺ exchanger 1 (NHE1), an important regulator of intracellular pH (pHi) and extracellular pH (pHe), plays a crucial role in various physiological and pathological processes. However, the role of NHE1 in depression has not yet been reported. This study was designed to investigate the role of NHE1 in the animal model of depression and explore the underlying mechanisms. Our results showed that inhibition of rho-associated kinase 2 (ROCK2) by fasudil (Fas) or baicalin (BA) significantly alleviated chronic unpredictable mild stress (CUMS) paradigm-induced depression-related behaviours in mice, as shown by decreased sucrose consumption in sucrose preference test (SPT), reduced locomotor activity in the open field test (OFT), and increased immobility time in the tail suspension test (TST) and forced swimming test (FST). Furthermore, ROCK2 inhibition inhibited the activation of NHE1, calpain1, and reduced neuronal apoptosis in the CUMS animal model of depression. Next, we used the lipopolysaccharide (LPS)-challenged animal model of depression to induce NHE1 activation. Our results revealed that mice subjected to 1 μl LPS (10 mg/ml) injection intracerebroventricularly (i.c.v.) showed depressive-like behaviours and NHE1 activation. Amiloride (Ami), an NHE1 inhibitor, significantly reversed the decrease in sucrose consumption and reduction in immobility time in the TST and FST induced by LPS challenge. Furthermore, Ami decreased the expression of ROCK2, NHE1, calpain1, and caspase-3 and increased the Bcl-1/Bax ratio in the hippocampus of LPS-challenged mice. Ami treatment also led to antidepressive effects in the CUMS-induced animal model of depression. Thus ROCK2 inhibition could be proposed as a neuroprotective strategy against neuronal apoptosis, and NHE1 might be a potential therapeutic target in depression.

Bactridine 2 effect in DRG neurons. Identification of NHE as a second target

Bactridine 2 (Bact-2) is an antibacterial toxin from Tityus discrepans venom which modifies isoforms 1.2, 1.4 and 1.6 voltage-dependent sodium (Na_v) channels. Bactridine-induced Na⁺ outflow in Yersinia enterocolitica was blocked by amiloride, suggesting that Bact-2 effect was mediated by an amiloride sensitive sodium channel. In this study we show that Bact-2 increases also an outward rectifying current in rat dorsal root ganglia (DRG) sensory neurons; therefore, the nature of the outward rectifying currents was characterized and then the effect of Bact-2 on these currents was studied. These currents are enhanced by amiloride, are decreased by Na⁺ when an outward pH gradient is present and its reversal potential coincides with that of a Cl^-/H⁺ exchanger, suggesting that rectifying currents are produced by the electrogenic Cl^-/H⁺ exchanger modulated by the Na⁺/H⁺ antiporter. Bact-2 also leads to an increase of the outward currents in a similar way to the produced by the inhibition of the Na⁺/H⁺ exchanger. Additionally, the subsequent application of Bact-2 after blocking the Na⁺/H⁺ exchanger does not produce any further effect, suggesting that Bact-2 modifies the outward current by modulating the activity of the Na⁺/H⁺ exchanger. The effect of Bact-2 on pH_i regulation was determined using the pH indicator BCECF. The results show that the Na⁺/H⁺ exchanger is blocked by amiloride and Na⁺ free solutions and is modulated by Bact-2 in a similar way as cariporide. This study validates that besides Na_v channels, Bact-2 modulates the activity of the Na⁺/H⁺ exchanger.

Antinociceptive effects of amiloride and benzamil in neuropathic pain model rats

Amiloride and benzamil showed antinocicepitve effects in several pain models through the inhibition of acid sensing ion channels (ASICs). However, their role in neuropathic pain has not been investigated. In this study, we investigated the effect of the intrathecal amiloride and benzamil in neuropathic pain model, and also examined the role of ASICs on modulation of neuropathic pain. Neuropathic pain was induced by L4-5 spinal nerve ligation in male Sprague-Dawley rats weighing 100-120 g, and intrathecal catheterization was performed for drug administration. The effects of amiloride and benzamil were measured by the paw-withdrawal threshold to a mechanical stimulus using the up and down method. The expression of ASICs in the spinal cord dorsal horn was also analyzed by RT-PCR. Intrathecal amiloride and benzamil significantly increased the paw withdrawal threshold in spinal nerve-ligated rats (87%±12% and 76%±14%, P=0.007 and 0.012 vs vehicle, respectively). Spinal nerve ligation increased the expression of ASIC3 in the spinal cord dorsal horn (P=0.01), and this increase was inhibited by both amiloride and benzamil (P<0.001 in both). In conclusion, intrathecal amiloride and benzamil display antinociceptive effects in the rat spinal nerve ligation model suggesting they may present an alternative pharmacological tool in the management of neuropathic pain at the spinal level.

Role of NHE1 in Nociception

Intracellular pH is a fundamental parameter to cell function that requires tight homeostasis. In the absence of any regulation, excessive acidification of the cytosol would have the tendency to produce cellular damage. Mammalian Na(+)/H(+) exchangers (NHEs) are electroneutral Na(+)-dependent proteins that exchange extracellular Na(+) for intracellular H(+). To date, there are 9 identified NHE isoforms where NHE1 is the most ubiquitous member, known as the housekeeping exchanger. NHE1 seems to have a protective role in the ischemia-reperfusion injury and other inflammatory diseases. In nociception, NHE1 is found in neurons along nociceptive pathways, and its pharmacological inhibition increases nociceptive behavior in acute pain models at peripheral and central levels. Electrophysiological studies also show that NHE modulates electrical activity of primary nociceptive terminals. However, its role in neuropathic pain still remains controversial. In humans, NHE1 may be responsible for inflammatory bowel diseases since its expression is reduced in Crohn's disease and ulcerative colitis. The purpose of this work is to provide a review of the evidence about participation of NHE1 in the nociceptive processing.

Screening of antinociceptive components in Corydalis yanhusuo W.T. Wang by comprehensive two-dimensional liquid chromatography/tandem mass spectrometry

Formalin-induced pain models were used in rats to evaluate the antinociceptive effect of the total alkaloids of Corydalis yanhusuo (TAC). The results indicated that formalin-evoked spontaneous nociceptive responses (licking behavior) could be inhibited significantly by giving (intragingival) TAC at a single dose of 150 mg/kg. Subsequently, an online comprehensive two-dimensional biochromatography method with a silica-bonded human serum albumin (HSA) column in the first dimension and a monolithic ODS column in the second was developed. The absorbed bioactive components were screened by comparing and contrasting the components detected in the plasma and striatum with those in TAC. More than 100 compounds were separated and detected in the TAC, among which 13 compounds were identified. About 40 compounds (seven compounds identified) were absorbed into the plasma with appropriate concentrations and about 20 compounds (four compounds identified) passed through the blood-brain barrier into the striatum. Of interest, four compounds (protopine, glaucine, tetrahydropalmatine, and corydaline) which were reported to possess profound antinociceptive effects exhibited high concentrations in the striatum. This may result from their synergistic effects in regulating the formalin-induced nociception. The results indicated that the comprehensive two-dimensional biochromatography method developed is capable of screening the bioactive components in Corydalis yanhusuo and providing valuable information for understanding the mechanisms by which Corydalis yanhusuo alleviates nociception.