Antinociceptive effectiveness of the inhibition of NCX reverse-mode action in rodent neuropathic pain model

Selective inhibitor of sodium-calcium exchanger, SEA0400, affects NMDA receptor currents and abolishes their calcium-dependent block by tricyclic antidepressants

The open-channel block of N-methyl-D-aspartate receptors (NMDARs) and their calcium-dependent desensitization (CDD) represent conventional mechanisms of glutamatergic synapse regulation. In neurotrauma, neurodegeneration, and neuropathic pain the clinical benefits of cure with memantine, ketamine, Mg2+, and some tricyclic antidepressants are often attributed to NMDAR open-channel block, while possible involvement of NMDAR CDD in the therapy is not well established. Here the effects of selective high-affinity sodium-calcium exchanger (NCX) isoform 1 inhibitor, SEA0400, on NMDA-activated whole-cell currents and their block by amitriptyline, desipramine and clomipramine recorded by patch-clamp technique in cortical neurons of primary culture were studied. We demonstrated that in the presence of extracellular Ca2+, 50 nM SEA0400 caused a reversible decrease of the steady-state amplitude of NMDAR currents, whereas loading neurons with BAPTA or the removal of extracellular Ca2+ abolished the effect. The decrease did not exceed 30% of the amplitude and did not depend on membrane voltage. The external Mg2+ block and 50 nM SEA0400 inhibition of currents were additive, suggesting their independent modes of action. In the presence of Ca2+ SEA0400 speeded up the decay of NMDAR currents to the steady state determined by CDD. The measured IC50 value of 27 nM for SEA0400-induced inhibition coincides with that for NCX1. Presumably, SEA0400 effects are induced by an enhancement of NMDAR CDD through the inhibition of Ca2+ extrusion by NCX1. SEA0400, in addition, at nanomolar concentrations could interfere with Ca2+-dependent effect of tricyclic antidepressants. In the presence of 50 nM SEA0400, the IC50s for NMDAR inhibition by amitriptyline and desipramine increased by about 20 folds, as the Ca2+-dependent NMDAR inhibition disappeared. This observation highlights NCX1 involvement in amitriptyline and desipramine effects on NMDARs and unmasks competitive relationships between SEA0400 and these antidepressants. Neither amitriptyline nor desipramine could affect NCX3. The open-channel block of NMDARs by these substances was not affected by SEA0400. In agreement, SEA0400 did not change the IC50 for clomipramine, which acts as a pure NMDAR open-channel blocker. Thus, NCX seems to represent a promising molecular target to treat neurological disorders, because of the ability to modulate NMDARs by decreasing the open probability through the enhancement of their CDD.

Ion Channel and Transporter Involvement in Chemotherapy-Induced Peripheral Neurotoxicity

The peripheral nervous system can encounter alterations due to exposure to some of the most commonly used anticancer drugs (platinum drugs, taxanes, vinca alkaloids, proteasome inhibitors, thalidomide), the so-called chemotherapy-induced peripheral neurotoxicity (CIPN). CIPN can be long-lasting or even permanent, and it is detrimental for the quality of life of cancer survivors, being associated with persistent disturbances such as sensory loss and neuropathic pain at limb extremities due to a mostly sensory axonal polyneuropathy/neuronopathy. In the state of the art, there is no efficacious preventive/curative treatment for this condition. Among the reasons for this unmet clinical and scientific need, there is an uncomplete knowledge of the pathogenetic mechanisms. Ion channels and transporters are pivotal elements in both the central and peripheral nervous system, and there is a growing body of literature suggesting that they might play a role in CIPN development. In this review, we first describe the biophysical properties of these targets and then report existing data for the involvement of ion channels and transporters in CIPN, thus paving the way for new approaches/druggable targets to cure and/or prevent CIPN.

Therapeutic potential of orally applied KB-R7943 in streptozotocin-induced neuropathy in rats

Both peripheral neuropathy and depression can be viewed as neurodegeneration's consequences of diabetes, at least in part coexisting with or resulting from sodium-calcium dysbalance. This study aims to assess the therapeutic potential of the orally applied reverse-mode inhibitor of the sodium-calcium exchanger (NCX) KB-R7943 in the streptozotocin (STZ) diabetes model in rats. A pilot pharmacokinetic (PK) study with high-performance liquid chromatography with high-resolution tandem mass spectrometric detection revealed higher drug exposure (AUC), lower volume of distribution (Vd) and clearance (Cl), and faster decline of the plasma concentration (ƛ) in rats with diabetes vs. controls. Brain and heart accumulation and urinary excretion of the unmetabolized KB-R7943 at least 24 h were also demonstrated in all rats. However, heart and hippocampus KB-R7943 penetration (AUCtissue/AUCplasma) was higher in controls vs. diabetic rats. The development of thermal, mechanical, and chemical-induced allodynia was assessed with the Cold plate test (CPT), Randall-Stiletto (R-S) test, and 0.5% formalin test (FT). Amitriptyline 10 mg/kg, KB-R7943 5 mg/kg, or 10 mg/kg p.o once daily was applied from the 28th to the 49th day. The body weight, coat status, CPT, R-S, and FT were evaluated on days (-5), 0, and 42. On day 41, a forced swim test and 24-h spontaneous physical activities were assessed. The chronic treatment effects were calculated as % of the maximum. A dose-depended amelioration of neuropathic and depression-like effects was demonstrated. The oral application of KB-R7943 for potentially treating neurodegenerative consequences of diabetes merits further studies. The brain, heart, and kidneys are essential contributors to the PKs of this drug, and their safety involvement needs to be further characterized.

Inhibition of Acid-Sensing Ion Channels by KB-R7943, a Reverse Na+/Ca2+ Exchanger Inhibitor

KB-R7943, an isothiourea derivative, is widely used as a pharmacological inhibitor of reverse sodium–calcium exchanger (NCX). It has been shown to have neuroprotective and analgesic effects in animal models; however, the detailed molecular mechanisms remain elusive. In the current study, we investigated whether KB-R7943 modulates acid-sensing ion channels (ASICs), a group of proton-gated cation channels implicated in the pathophysiology of various neurological disorders, using the whole-cell patch clamp techniques. Our data show that KB-R7943 irreversibly inhibits homomeric ASIC1a channels heterologously expressed in Chinese Hamster Ovary (CHO) cells in a use- and concentration-dependent manner. It also reversibly inhibits homomeric ASIC2a and ASIC3 channels in CHO cells. Both the transient and sustained current components of ASIC3 are inhibited. Furthermore, KB-R7943 inhibits ASICs in primary cultured peripheral and central neurons. It inhibits the ASIC-like currents in mouse dorsal root ganglion (DRG) neurons and the ASIC1a-like currents in mouse cortical neurons. The inhibition of the ASIC1a-like current is use-dependent and unrelated to its effect on NCX since neither of the other two well-characterized NCX inhibitors, including SEA0400 and SN-6, shows an effect on ASIC. Our data also suggest that the isothiourea group, which is lacking in other structurally related analogs that do not affect ASIC1a-like current, may serve as a critical functional group. In summary, we characterize KB-R7943 as a new ASIC inhibitor. It provides a novel pharmacological tool for the investigation of the functions of ASICs and could serve as a lead compound for developing small-molecule drugs for treating ASIC-related disorders.

Calcium-Dependent Interplay of Lithium and Tricyclic Antidepressants, Amitriptyline and Desipramine, on N-methyl-D-aspartate Receptors

The facilitated activity of N-methyl-D-aspartate receptors (NMDARs) in the central and peripheral nervous systems promotes neuropathic pain. Amitriptyline (ATL) and desipramine (DES) are tricyclic antidepressants (TCAs) whose anti-NMDAR properties contribute to their analgetic effects. At therapeutic concentrations <1 µM, these medicines inhibit NMDARs by enhancing their calcium-dependent desensitization (CDD). Li+, which suppresses the sodium−calcium exchanger (NCX) and enhances NMDAR CDD, also exhibits analgesia. Here, the effects of different [Li+]s on TCA inhibition of currents through native NMDARs in rat cortical neurons recorded by the patch-clamp technique were investigated. We demonstrated that the therapeutic [Li+]s of 0.5−1 mM cause an increase in ATL and DES IC50s of ~10 folds and ~4 folds, respectively, for the Ca2+-dependent NMDAR inhibition. The Ca2+-resistant component of NMDAR inhibition by TCAs, the open-channel block, was not affected by Li+. In agreement, clomipramine providing exclusively the NMDAR open-channel block is not sensitive to Li+. This Ca2+-dependent interplay between Li+, ATL, and DES could be determined by their competition for the same molecular target. Thus, submillimolar [Li+]s may weaken ATL and DES effects during combined therapy. The data suggest that Li+, ATL, and DES can enhance NMDAR CDD through NCX inhibition. This ability implies a drug−drug or ion−drug interaction when these medicines are used together therapeutically.

Tricyclic Antidepressant Structure-Related Alterations in Calcium-Dependent Inhibition and Open-Channel Block of NMDA Receptors

N-methyl-D-aspartate receptors (NMDARs) are an essential target for the analgetic action of tricyclic antidepressants (TCAs). Their therapeutic blood concentrations achieve 0.5–1.5 μM, which, however, are insufficient to cause in vitro the open-channel block known as the only effect of TCAs on NMDARs. Whereas structures of amitriptyline (ATL), desipramine (DES), and clomipramine (CLO) are rather similar these compounds manifest different therapeutic profiles and side effects. To study structure-activity relationships of DES and CLO on NMDARs, we measured IC₅₀s as a function of extracellular calcium ([Ca²⁺]) and membrane voltage (V_m) of NMDAR currents recorded in cortical neurons. Here two components of TCA action on NMDARs are described, which could be characterized as the Ca²⁺-dependent inhibition and the open-channel block. DES demonstrated a profound Ca²⁺-dependent inhibition of NMDARs, while the CLO effect was weak. DES IC₅₀ exhibited an e-fold change with a [Ca²⁺] shift of 0.59 mM, which is consistent with ATL. The Ca²⁺ dependence of NMDAR inhibition by DES disappeared in BAPTA loaded neurons, suggesting that Ca²⁺ acts from the inside. Since CLO differs from DES and ATL by the presence of Cl-atom in the structure, most likely, this is the atom which is responsible for the loss of pronounced [Ca²⁺] dependence. As for the NMDAR open-channel block, both DES and CLO were about 5-folds more potent than ATL due to their slow rates of dissociation either from open and closed states. DES demonstrated stronger V_m-dependence than CLO, suggesting a deeper location of the DES binding site within the ion pore. Because DES and CLO differ from ATL by the nitrogen-containing tricycle, presumably this moiety of the molecules determines their high-affinity binding with the NMDAR channel, while the aliphatic chain mono-methyl amino-group of DES allows a deep permeation in the channel. Thus, different structure-activity relationships of the Ca²⁺-dependent inhibition and V_m-dependent open-channel block of NMDARs by DES and CLO suggest that these processes are independent and most likely may represent an action on different molecular targets. The proposed model of TCA action on NMDARs predicts well the experimental values of IC₅₀s at physiological [Ca²⁺] and within a wide range of V_ms.

Neuropathic Pain in Multiple Sclerosis and Its Animal Models: Focus on Mechanisms, Knowledge Gaps and Future Directions

Multiple sclerosis (MS) is a multifaceted, complex and chronic neurological disease that leads to motor, sensory and cognitive deficits. MS symptoms are unpredictable and exceedingly variable. Pain is a frequent symptom of MS and manifests as nociceptive or neuropathic pain, even at early disease stages. Neuropathic pain is one of the most debilitating symptoms that reduces quality of life and interferes with daily activities, particularly because conventional pharmacotherapies do not adequately alleviate neuropathic pain. Despite advances, the mechanisms underlying neuropathic pain in MS remain elusive. The majority of the studies investigating the pathophysiology of MS-associated neuropathic pain have been performed in animal models that replicate some of the clinical and neuropathological features of MS. Experimental autoimmune encephalomyelitis (EAE) is one of the best-characterized and most commonly used animal models of MS. As in the case of individuals with MS, rodents affected by EAE manifest increased sensitivity to pain which can be assessed by well-established assays. Investigations on EAE provided valuable insights into the pathophysiology of neuropathic pain. Nevertheless, additional investigations are warranted to better understand the events that lead to the onset and maintenance of neuropathic pain in order to identify targets that can facilitate the development of more effective therapeutic interventions. The goal of the present review is to provide an overview of several mechanisms implicated in neuropathic pain in EAE by summarizing published reports. We discuss current knowledge gaps and future research directions, especially based on information obtained by use of other animal models of neuropathic pain such as nerve injury.