Antinociceptive effects of MSVIII-19, a functional antagonist of the GluK1 kainate receptor

Mechanisms underlying the cell-matrixed nerve grafts repairing peripheral nerve defects

Decellularized extracellular matrix (dECM), with its distinct biological properties, has gained significant attention as a natural biomaterial. Leveraging its potentials, we successfully developed a three-dimensional matrix-based oriented nerve graft by encapsulating a fibrous scaffold with multilayered conformationally intact and biologically active human bone marrow mesenchymal stem cell-derived decellularized extracellular matrix (hBMSC-dECM). Convincingly, the hBMSC-dECM group exhibited comparable functional recoveries to the autograft group by postoperative week 12. In the comprehensive analysis, the molecular regulations in the hBMSC-dECM group were more intricate and nuanced compared to the autograft group. Nevertheless, both groups displayed similar molecular regulatory processes in terms of vascularization and extracellular matrix. Notably, the hBMSC-dECM group demonstrated sustained high levels of regulation in axon and myelin regeneration at week 12, while the immunomodulation returned to the normal levels after peaking at week 2. Collectively, our findings illustrated the satisfactory construction of a cell-matrixed nerve graft that established a microenvironment conducive to nerve regeneration, and elucidated the distinct molecular regulation patterns and characteristics associated with different repair modes.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Ca2+-Permeable AMPA Receptors Contribute to Changed Dorsal Horn Neuronal Firing and Inflammatory Pain

The dorsal horn (DH) neurons of the spinal cord play a critical role in nociceptive input integration and processing in the central nervous system. Engaged neuronal classes and cell-specific excitability shape nociceptive computation within the DH. The DH hyperexcitability (central sensitisation) has been considered a fundamental mechanism in mediating nociceptive hypersensitivity, with the proven role of Ca²⁺-permeable AMPA receptors (AMPARs). However, whether and how the DH hyperexcitability relates to changes in action potential (AP) parameters in DH neurons and if Ca²⁺-permeable AMPARs contribute to these changes remain unknown. We examined the cell-class heterogeneity of APs generated by DH neurons in inflammatory pain conditions to address these. Inflammatory-induced peripheral hypersensitivity increased DH neuronal excitability. We found changes in the AP threshold and amplitude but not kinetics (spike waveform) in DH neurons generating sustained or initial bursts of firing patterns. In contrast, there were no changes in AP parameters in the DH neurons displaying a single spike firing pattern. Genetic knockdown of the molecular mechanism responsible for the upregulation of Ca²⁺-permeable AMPARs allowed the recovery of cell-specific AP changes in peripheral inflammation. Selective inhibition of Ca²⁺-permeable AMPARs in the spinal cord alleviated nociceptive hypersensitivity, both thermal and mechanical modalities, in animals with peripheral inflammation. Thus, Ca²⁺-permeable AMPARs contribute to shaping APs in DH neurons and nociceptive hypersensitivity. This may represent a neuropathological mechanism in the DH circuits, leading to aberrant signal transfer to other nociceptive pathways.

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

The Antiseizure Drug Perampanel Is a Subunit-Selective Negative Allosteric Modulator of Kainate Receptors

Perampanel (PMP) is a third-generation antiseizure drug reported to be a potent and selective noncompetitive negative allosteric modulator of one subfamily of ionotropic glutamate receptor (iGluR), the α-amino-3-hydroxy-S-methylisoxazole-4-propionic acid receptors (AMPARs). However, the recent structural resolution of AMPARs in complex with PMP revealed that its binding pocket is formed from residues that are largely conserved in two members of another family of iGluRs, the GluK4 and GluK5 kainate receptor (KAR) subunits. We show here that PMP inhibits both recombinant and neuronal KARs, contrary to the previous reports, and that the negative allosteric modulator (NAM) activity requires GluK5 subunits to be channel constituents. PMP inhibited heteromeric GluK1/GluK5 and GluK2/GluK5 KARs at IC50 values comparable to that for AMPA receptors but was much less potent on homomeric GluK1 or GluK2 KARs. The auxiliary subunits Neto1 or Neto2 also made GluK2-containing KARs more sensitive to inhibition. Finally, PMP inhibited mouse neuronal KARs containing GluK5 subunits and Neto proteins in nociceptive dorsal root ganglia neurons and hippocampal mossy fiber-CA3 pyramidal neuron synapses. These data suggest that clinical actions of PMP could arise from differential inhibition of AMPAR or KAR signaling and that more selective drugs might maintain antiseizure efficacy while reducing adverse effects.SIGNIFICANCE STATEMENT PMP is a regulatory approved antiseizure drug used for refractory partial-onset and generalized tonic-clonic seizures that acts as a selective negative allosteric modulator of AMPARs. Here, we demonstrate that PMP inhibits KARs, a second family of ionotropic glutamate receptors, in addition to AMPARs. NAM activity on KARs required GluK5 subunits or Neto auxiliary subunits as channel constituents. KAR inhibition, therefore, could contribute to PMP antiseizure action or the adverse effects that are significant with this drug. Drug discovery aimed at more selective allosteric modulators that discriminate between AMPARs and KARs could yield next-generation drugs with improved therapeutic profiles for treatment of epilepsy.

Pain-related toxins in scorpion and spider venoms: a face to face with ion channels

Pain is a common symptom induced during envenomation by spiders and scorpions. Toxins isolated from their venom have become essential tools for studying the functioning and physiopathological role of ion channels, as they modulate their activity. In particular, toxins that induce pain relief effects can serve as a molecular basis for the development of future analgesics in humans. This review provides a summary of the different scorpion and spider toxins that directly interact with pain-related ion channels, with inhibitory or stimulatory effects. Some of these toxins were shown to affect pain modalities in different animal models providing information on the role played by these channels in the pain process. The close interaction of certain gating-modifier toxins with membrane phospholipids close to ion channels is examined along with molecular approaches to improve selectivity, affinity or bioavailability in vivo for therapeutic purposes.

Kainate receptors: from synaptic activity to disease

Kainate receptors (KARs) are glutamate receptors that participate in the postsynaptic transmission of information and in the control of neuronal excitability, as well as presynaptically modulating the release of the neurotransmitters GABA and glutamate. These modulatory effects, general follow a biphasic pattern, with low KA concentrations provoking an increase in GABA and glutamate release, and higher concentrations mediating a decrease in the release of these neurotransmitters. In addition, KARs are involved in different forms of long- and short-term plasticity. Importantly, altered activity of these receptors has been implicated in different central nervous system diseases and disturbances. Here, we describe the pre- and postsynaptic actions of KARs, and the possible role of these receptors in disease, a field that has seen significant progress in recent years.

The emerging role of kainate receptor functional dysregulation in pain

Pain is a serious clinical challenge, and is associated with a significant reduction in quality of life and high financial costs for affected patients. Research efforts have been made to explore the etiological basis of pain to guide the future treatment of patients suffering from pain conditions. Findings from studies using KA (kainate) receptor agonist, antagonists and receptor knockout mice suggested that KA receptor dysregulation and dysfunction may govern both peripheral and central sensitization in the context of pain. Additional evidence showed that KA receptor dysfunction may disrupt the finely-tuned process of glutamic acid transmission, thereby contributing to the onset of a range of pathological contexts. In the present review, we summarized major findings in recent studies which examined the roles of KA receptor dysregulation in nociceptive transmission and in pain. This timely overview of current knowledge will help to provide a framework for future developing novel therapeutic strategies to manage pain.

Tissue Levels of Flurbiprofen in the Rat Plantar Heel after Short-Duration Topical Iontophoresis Are Sufficient to Induce Pharmacodynamic Responses to Local Pain Stimuli

The objective of this study was to investigate the topical iontophoresis of flurbiprofen (FBF) as a means to enhance its local bioavailability and thereby provide an improved and targeted treatment of plantar heel pain. Initial in vitro experiments using porcine ear skin investigated iontophoretic transport of FBF under different conditions. Local FBF biodistribution in the rat paw in vivo was compared after topical or oral administration. Efficacy of pain management was investigated using a plantar incisional model by evaluating pharmacodynamic responses to local pain stimuli. The results demonstrated that iontophoresis of FBF significantly increased cutaneous deposition and transdermal permeation of FBF as compared to passive delivery—it also enabled drug input to be controlled by modulation of current density and drug concentration (r2 > 0.99). Topical iontophoresis of FBF in vivo enabled higher drug levels in skin and muscle in rat plantar aspect and superior pharmacodynamic responses to local pain stimuli, in comparison to oral and passive delivery. In conclusion, short-duration topical iontophoresis of FBF may better help to relieve plantar heel pain than oral or passive administration, which should be of clinical interest.

Effective management of acute postoperative pain using intravenous emulsions of novel ketorolac prodrugs: in vitro and in vivo evaluations

The aim was to prepare intravenous fat emulsions (IFEs) of ketorolac (KTL) ester prodrugs and to investigate the pharmacokinetics and pharmacodynamics of these formulations. Three prodrugs of KTL (KTL-IS, KTL-AX and KTL-BT) were synthesized as a means to increase the lipid solubility of KTL. All KTL prodrugs with higher Log P values presented increased tendency to partition into a blank IFE using extemporaneous addition method - the encapsulation efficiency of KTL-IS IFE and KTL-BT IFE was more than 97%. The particle sizes and zeta potentials of these two formulations were comparable to that of the blank IFE. PK studies in rabbits showed significant larger AUC0-8h (646.969 ± 154.326 mg/L•h-1 for KTL-IS IFE and 559.426 ± 103.057 mg/L•h-1 for KTL-BT IFE) than that of ketorolac tromethamine (KTL-T) injectable (286.968 ± 63.045 mg/L•h-1) and approximately 2-fold increases in the elimination t1/2 over KTL-T. In a rat postoperative pain model, the paw withdrawal thresholds and the paw withdrawal latency after I.V. KTL prodrug IFEs were significantly higher than that after I.V. KTL-T at 3~4 h. Effective controlling of acute postoperative pain in a longer duration can be achieved by using non-addictive ketorolac derivatives intraveneous emulsions.

Neto2 Assembles with Kainate Receptors in DRG Neurons during Development and Modulates Neurite Outgrowth in Adult Sensory Neurons

Peripheral sensory neurons in the dorsal root ganglia (DRG) are the initial transducers of sensory stimuli, including painful stimuli, from the periphery to central sensory and pain-processing centers. Small- to medium-diameter non-peptidergic neurons in the neonatal DRG express functional kainate receptors (KARs), one of three subfamilies of ionotropic glutamate receptors, as well as the putative KAR auxiliary subunit Neuropilin- and tolloid-like 2 (Neto2). Neto2 alters recombinant KAR function markedly but has yet to be confirmed as an auxiliary subunit that assembles with and alters the function of endogenous KARs. KARs in neonatal DRG require the GluK1 subunit as a necessary constituent, but it is unclear to what extent other KAR subunits contribute to the function and proposed roles of KARs in sensory ganglia, which include promotion of neurite outgrowth and modulation of glutamate release at the DRG-dorsal horn synapse. In addition, KARs containing the GluK1 subunit are implicated in modes of persistent but not acute pain signaling. We show here that the Neto2 protein is highly expressed in neonatal DRG and modifies KAR gating in DRG neurons in a developmentally regulated fashion in mice. Although normally at very low levels in adult DRG neurons, Neto2 protein expression can be upregulated via MEK/ERK signaling and after sciatic nerve crush and Neto2^-/- neurons from adult mice have stunted neurite outgrowth. These data confirm that Neto2 is a bona fide KAR auxiliary subunit that is an important constituent of KARs early in sensory neuron development and suggest that Neto2 assembly is critical to KAR modulation of DRG neuron process outgrowth.SIGNIFICANCE STATEMENT Pain-transducing peripheral sensory neurons of the dorsal root ganglia (DRG) express kainate receptors (KARs), a subfamily of glutamate receptors that modulate neurite outgrowth and regulate glutamate release at the DRG-dorsal horn synapse. The putative KAR auxiliary subunit Neuropilin- and tolloid-like 2 (Neto2) is also expressed in DRG. We show here that it is a developmentally downregulated but dynamic component of KARs in these neurons, that it contributes to regulated neurite regrowth in adult neurons, and that it is increased in adult mice after nerve injury. Our data confirm Neto2 as a KAR auxiliary subunit and expand our knowledge of the molecular composition of KARs in nociceptive neurons, a key piece in understanding the mechanistic contribution of KAR signaling to pain-processing circuits.

Role of principal ionotropic and metabotropic receptors in visceral pain

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. It also leads to a significant economic burden due to workdays lost and reduced productivity. Further, long-term use of non-specific medications is also associated with side effects affecting the quality of life. Despite years of extensive research and the availability of several therapeutic options, management of patients with chronic visceral pain is often inadequate, resulting in frustration for both patients and physicians. This is, most likely, because the mechanisms associated with chronic visceral pain are different from those of acute pain. Accumulating evidence from years of research implicates several receptors and ion channels in the induction and maintenance of central and peripheral sensitization during chronic pain states. Understanding the specific role of these receptors will facilitate to capitalize on their unique properties to augment the therapeutic efficacy while at the same time minimizing unwanted side effects. The aim of this review is to provide a concise review of the recent literature that reports on the role of principal ionotropic receptors and metabotropic receptors in the modulation visceral pain. We also include an overview of the possibility of these receptors as potential new targets for the treatment of chronic visceral pain conditions.

Recent progress in neuroactive marine natural products

This review covers the isolation, chemical structure, biological activity, structure activity relationships including synthesis of chemical probes, and pharmacological characterization of neuroactive marine natural products; 302 references are cited.

Potent analgesic effects of a store-operated calcium channel inhibitor

Chronic pain often accompanies immune responses and immune cells are known to be involved in chronic pain. Store-operated calcium (SOC) channels are calcium-selective cation channels and play an important role in the immune system. YM-58483, a potent SOC channel inhibitor, has been shown to inhibit cytokine production from immune cells and attenuate antigen-induced hypersensitivity reactions. Here, we report that YM-58483 has analgesic actions in chronic pain and produces antinociceptive effects in acute pain and prevents the development of chronic pain in mice. Oral administration of 10mg/kg or 30 mg/kg YM-58483 dramatically attenuated complete Freund adjuvant (CFA)-induced thermal hyperalgesia and prevented the development of thermal and mechanical hypersensitivity in a dose-dependent manner. Analgesic effects were observed when YM-58483 was administered systemically, intrathecally and intraplantarly. YM-58483 decreased spared nerve injury (SNI)-induced thermal and mechanical hypersensitivity and prevented the development of SNI-induced pain hypersensitivity. Pretreatment with YM-58483 strongly reduced both the first and second phases of formalin-induced spontaneous nocifensive behavior in a dose-dependent manner. YM-58483 produced antinociception in acute pain induced by heat or chemical or mechanical stimuli at a dose of 30 mg/kg. YM-58483 diminished CFA-induced paw edema, and reduced production of TNF-α, IL-1β and PGE2 in the CFA-injected paw. In vitro, SOC entry in nociceptors was more robust than in nonnociceptors, and the inhibition of SOC entry by YM-58483 in nociceptors was much greater than in nonnociceptors. Our findings indicate that YM-58483 is a potent analgesic and suggest that SOC channel inhibitors may represent a novel class of therapeutics for pain.

Kainate receptor signaling in pain pathways

Receptors and channels that underlie nociceptive signaling constitute potential sites of intervention for treatment of chronic pain states. The kainate receptor family of glutamate-gated ion channels represents one such candidate set of molecules. They have a prominent role in modulation of excitatory signaling between sensory and spinal cord neurons. Kainate receptors are also expressed throughout central pain neuraxis, where their functional contributions to neural integration are less clearly defined. Pharmacological inhibition or genetic ablation of kainate receptor activity reduces pain behaviors in a number of animal models of chronic pain, and small clinical trials have been conducted using several orthosteric antagonists. This review will cover kainate receptor function and participation in pain signaling as well as the pharmacological studies supporting further consideration as potential targets for therapeutic development.

Association study of GRIK1 gene polymorphisms in schizophrenia: case-control and family-based studies

Glutamatergic function is one of the major hypotheses for schizophrenia. Within the glutamate system, the glutamate receptor ionotropic kainate-1 (GRIK1) gene is thought to be particularly involved in schizophrenia because of the reported reduction of GRIK1 in the dorsolateral prefrontal cortex of patients.

OBJECTIVE

We examined single-nucleotide polymorphisms (SNPs) in the GRIK1 gene for possible association with schizophrenia.

METHODS

We analyzed eight SNPs across the GRIK1 gene in 202 case-control pairs and 108 small nuclear families.

RESULTS

For the case-control study, we found nominal significant associations in the analysis of rs469472 (p = 0.028) and its haplotypes. In the family-based study, nominal significant association was also observed for rs469472 (p = 0.046), as well as rs455892 (p = 0.024). The marker rs469472 was associated with schizophrenia when we combined the case-control and family samples (p = 0.027). The association findings did not survive correction for multiple testing.

CONCLUSIONS

Because we observed similar association findings with marker rs469472 in two independent samples, further analyses in larger samples are warranted.

Psychophysics of a nociceptive test in the mouse: ambient temperature as a key factor for variation

BACKGROUND

The mouse is increasingly used in biomedical research, notably in behavioral neurosciences for the development of tests or models of pain. Our goal was to provide the scientific community with an outstanding tool that allows the determination of psychophysical descriptors of a nociceptive reaction, which are inaccessible with conventional methods: namely the true threshold, true latency, conduction velocity of the peripheral fibers that trigger the response and latency of the central decision-making process.

METHODOLOGY/PRINCIPAL FINDINGS

Basically, the procedures involved heating of the tail with a CO(2) laser, recording of tail temperature with an infrared camera and stopping the heating when the animal reacted. The method is based mainly on the measurement of three observable variables, namely the initial temperature, the heating rate and the temperature reached at the actual moment of the reaction following random variations in noxious radiant heat. The initial temperature of the tail, which itself depends on the ambient temperature, very markedly influenced the behavioral threshold, the behavioral latency and the conduction velocity of the peripheral fibers but not the latency of the central decision-making.

CONCLUSIONS/SIGNIFICANCE

We have validated a psychophysical approach to nociceptive reactions for the mouse, which has already been described for rats and Humans. It enables the determination of four variables, which contribute to the overall latency of the response. The usefulness of such an approach was demonstrated by providing new fundamental findings regarding the influence of ambient temperature on nociceptive processes. We conclude by challenging the validity of using as "pain index" the reaction time of a behavioral response to an increasing heat stimulus and emphasize the need for a very careful control of the ambient temperature, as a prevailing environmental source of variation, during any behavioral testing of mice.

Binding and selectivity of the marine toxin neodysiherbaine A and its synthetic analogues to GluK1 and GluK2 kainate receptors

Dysiherbaine (DH) and neodysiherbaine A (NDH) selectively bind and activate two kainate-type ionotropic glutamate receptors, GluK1 and GluK2. The ligand-binding domains of human GluK1 and GluK2 were crystallized as bound forms with a series of DH analogues including DH, NDH, 8-deoxy-NDH, 9-deoxy-NDH and 8,9-dideoxy-NDH (MSVIII-19), isolated from natural sources or prepared by total synthesis. Since the DH analogues exhibit a wide range of binding affinities and agonist efficacies, it follows that the detailed analysis of crystal structure would provide us with a significant opportunity to elucidate structural factors responsible for selective binding and some aspects of gating efficacy. We found that differences in three amino acids (Thr503, Ser706 and Ser726 in GluK1 and Ala487, Asn690 and Thr710 in GluK2) in the ligand-binding pocket generate differences in the binding modes of NDH to GluK1 and GluK2. Furthermore, deletion of the C(9) hydroxy group in NDH alters the ligand conformation such that it is no longer suited for binding to the GluK1 ligand-binding pocket. In GluK2, NDH pushes and rotates the side chain of Asn690 (substituted for Ser706 in GluK1) and disrupts an interdomain hydrogen bond with Glu409. The present data support the idea that receptor selectivities of DH analogues resulted from the differences in the binding modes of the ligands in GluK1/GluK2 and the steric repulsion of Asn690 in GluK2. All ligands, regardless of agonist efficacy, induced full domain closure. Consequently, ligand efficacy and domain closure did not directly coincide with DH analogues and the kainate receptors.