In vitro and antinociceptive profile of HON0001, an orally active NMDA receptor NR2B subunit antagonist

Exploring the overlapping binding sites of ifenprodil and EVT-101 in GluN2B-containing NMDA receptors using novel chicken embryo forebrain cultures and molecular modeling

N-methyl-d-aspartate receptors (NMDAR) are widely expressed in the brain. GluN2B subunit-containing NMDARs has recently attracted significant attention as potential pharmacological targets, with emphasis on the functional properties of allosteric antagonists. We used primary cultures from chicken embryo forebrain (E10), expressing native GluN2B-containing NMDA receptors as a novel model system. Comparing the inhibition of calcium influx by well-known GluN2B subunit-specific allosteric antagonists, the following rank order of potency was found: EVT-101 (EC 50 22 ± 8 nmol/L) > Ro 25-6981 (EC 50 60 ± 30 nmol/L) > ifenprodil (EC 50 100 ± 40 nmol/L) > eliprodil (EC 50 1300 ± 700 nmol/L), similar to previous observations in rat cortical cultures and cell lines overexpressing chimeric receptors. The less explored Ro 04-5595 had an EC 50 of 186 ± 32 nmol/L. Venturing to explain the differences in potency, binding properties were further studied by in silico docking and molecular dynamics simulations using x-ray crystal structures of GluN1/GluN2B amino terminal domain. We found that Ro 04-5595 was predicted to bind the recently discovered EVT-101 binding site, not the ifenprodil-binding site. The EVT-101 binding pocket appears to accommodate more structurally different ligands than the ifenprodil-binding site, and contains residues essential in ligand interactions necessary for calcium influx inhibition. For the ifenprodil site, the less effective antagonist (eliprodil) fails to interact with key residues, while in the EVT-101 pocket, difference in potency might be explained by differences in ligand-receptor interaction patterns.

Preventive treatment with dizocilpine attenuates oedema in a carrageenan model of inflammation: the interaction of glutamatergic and nitrergic signaling

Dizocilpine is a highly selective and potent non-competitive antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor. It is well known that dizocilpine has different neuroprotective effects in animal models of pain, epilepsy and oedema during trauma. The search for alternative antiinflammatory drugs is ongoing. We investigated the anti-oedematous effects of dizocilpine and the probable mechanism of action in a rat model that mimics local and persistent inflammation without tissue injury or damage. Male Wistar rats were injected with 100 μL of 0.5% carrageenan to the plantar surface of the hind paw. Anti-oedematous activity was assessed in the carrageenan-induced paw inflammatory oedema test with a plethysmometer. To assess possible mechanisms of dizocilpine action, we examined the effects of the selective inhibitor of neuronal [N-ω-propyl-L-arginine hydrochloride (L-NPA)] and inducible [S-methylisothiourea (SMT)] nitric oxide synthase (NOS). Dizocilpine after systemic (0.0005, 0.005 and 0.02 mg/kg, subcutaneous (s.c.)), but not after local peripheral administration, reduced the paw inflammatory oedema. The effect is not dose dependent, and the highest decrease by about 47% at the time of maximally developed oedema was achieved with 0.005 mg/kg. Intraperitoneally (i.p.) administered L-NPA (0.5, 1 and 2 mg/kg) or SMT (0.005, 0.01 and 0.015 mg/kg) before dizocilpine abolished or reduced the anti-oedematous effect of dizocilpine by about 70-85%. An acute single dose of dizocilpine administered before inducing oedema systemically reduced the development of inflammatory oedema. The mechanism of the anti-oedematous effect includes, at least partially, an increase in nitric oxide (NO) production.

A Novel Binding Mode Reveals Two Distinct Classes of NMDA Receptor GluN2B-selective Antagonists

N-methyl-d-aspartate receptors (NMDARs) are glutamate-gated ion channels that play key roles in brain physiology and pathology. Because numerous pathologic conditions involve NMDAR overactivation, subunit-selective antagonists hold strong therapeutic potential, although clinical successes remain limited. Among the most promising NMDAR-targeting drugs are allosteric inhibitors of GluN2B-containing receptors. Since the discovery of ifenprodil, a range of GluN2B-selective compounds with strikingly different structural motifs have been identified. This molecular diversity raises the possibility of distinct binding sites, although supporting data are lacking. Using X-ray crystallography, we show that EVT-101, a GluN2B antagonist structurally unrelated to the classic phenylethanolamine pharmacophore, binds at the same GluN1/GluN2B dimer interface as ifenprodil but adopts a remarkably different binding mode involving a distinct subcavity and receptor interactions. Mutagenesis experiments demonstrate that this novel binding site is physiologically relevant. Moreover, in silico docking unveils that GluN2B-selective antagonists broadly divide into two distinct classes according to binding pose. These data widen the allosteric and pharmacological landscape of NMDARs and offer a renewed structural framework for designing next-generation GluN2B antagonists with therapeutic value for brain disorders.

The antinociceptive effects of magnesium sulfate and MK-801 in visceral inflammatory pain model: The role of NO/cGMP/K(+)ATP pathway

CONTEXT

Magnesium and MK-801 (dizocilpine), antagonists of N-methyl-d-aspartate receptors, are involved in the processing of pain.

OBJECTIVE

This study determines whether magnesium sulfate (MS) and MK-801 affects visceral inflammatory pain and determines a possible mechanism of action.

MATERIALS AND METHODS

Analgesic activity was assessed using the acetic acid-induced writhing test in rats. MS (1-45 mg/kg) or MK-801 (0.005-0.03 mg/kg) was administrated subcutaneously (s.c.). To assess possible mechanisms of action, we examined the effects of l-NAME (10 mg/kg, intraperitoneal), methylene blue (0.5 mg/kg, s.c.), and glibenclamide (3 mg/kg, s.c.) on the effect of MS or MK-801.

RESULTS

MS and MK-801 showed biphasic and linear dose-response pattern, respectively. MS reduces the number of writhing on the dose of 1, 5, and 15 mg/kg by 60, 50, and 78%, respectively, while it has no effects on the doses of 30 and 45 mg/kg. MK-801 (0.005-0.03 mg/kg) showed decrease in the number of writhing by 33-79%. The mean effective doses of MS and MK-801 were 6.6 (first phase) and 0.009 mg/kg, respectively. Both drugs did not impair the rotarod performance. l-NAME, methylene blue, and glybenclamide reduced the effect of MK-801 by 100, 43, and 64%, respectively, but not the effect of MS.

CONCLUSIONS

The results suggest that MS and MK-801 may be useful analgesics in the management of visceral inflammatory pain, at doses that do not induce motor impairment. The modulation of NO/cGMP/K+ATP pathway plays an important role in the antinociceptive mechanism of MK-801, but does not contribute to the antinociceptive effect of MS.

NMDA receptor subunits have different roles in NMDA-induced neurotoxicity in the retina

Background

Loss of retinal ganglion cells (RGCs) is a hallmark of various retinal diseases including glaucoma, retinal ischemia, and diabetic retinopathy. N-methyl-D-aspartate (NMDA)-type glutamate receptor (NMDAR)-mediated excitotoxicity is thought to be an important contributor to RGC death in these diseases. Native NMDARs are heterotetramers that consist of GluN1 and GluN2 subunits, and GluN2 subunits (GluN2A–D) are major determinants of the pharmacological and biophysical properties of NMDARs. All NMDAR subunits are expressed in RGCs in the retina. However, the relative contribution of the different GluN2 subunits to RGC death by excitotoxicity remains unclear.

Results

GluN2B- and GluN2D-deficiency protected RGCs from NMDA-induced excitotoxic retinal cell death. Pharmacological inhibition of the GluN2B subunit attenuated RGC loss in glutamate aspartate transporter deficient mice.

Conclusions

Our data suggest that GluN2B- and GluN2D-containing NMDARs play a critical role in NMDA-induced excitotoxic retinal cell death and RGC degeneration in glutamate aspartate transporter deficient mice. Inhibition of GluN2B and GluN2D activity is a potential therapeutic strategy for the treatment of several retinal diseases.

Synthesis and structure activity relationship of tetrahydroisoquinoline-based potentiators of GluN2C and GluN2D containing N-methyl-D-aspartate receptors

We describe here the synthesis and evaluation of a series of tetrahydroisoquinolines that show subunit-selective potentiation of NMDA receptors containing the GluN2C or GluN2D subunits. Bischler-Napieralski conditions were employed in the key step for the conversion of acyclic amides to the corresponding tetrahydroisoquinoline-containing analogs. Compounds were evaluated using both two-electrode voltage clamp recordings from Xenopus laevis oocytes and imaging of mammalian BHK cells loaded with Ca(2+)-sensitive dyes. The most potent analogues had EC50 values of 300 nM and showed over 2-fold potentiation of the response to maximally effective concentrations of glutamate and glycine but had no effect on responses from NMDA receptors containing the GluN2A or GluN2B subunits AMPA, kainate, and GABA or glycine receptors or a variety of other potential targets. These compounds represent a potent class of small molecule subunit-selective potentiators of NMDA receptors.

A Randomized, placebo-controlled, crossover pilot trial of the oral selective NR2B antagonist MK-0657 in patients with treatment-resistant major depressive disorder

Converging lines of evidence suggest that the glutamatergic system may play an increasingly important role in the development of novel therapeutics for major depressive disorder (MDD), particularly agents associated with rapid antidepressant effects. Diverse glutamatergic modulators targeting N-methyl-D-aspartate receptors have shown efficacy in MDD, but their associated psychotomimetic effects presently preclude their use in larger samples. This small, randomized, double-blind, placebo-controlled, crossover pilot study evaluated the potential antidepressant efficacy and tolerability of an oral formulation of the selective N-methyl-D-aspartate NR2B antagonist MK-0657 in patients with treatment-resistant MDD (TRD). The TRD subjects underwent a 1-week drug-free period and were subsequently randomized to receive either MK-0657 monotherapy (4-8 mg/d) or placebo for 12 days. Because of recruitment challenges and the discontinuation of the compound's development by the manufacturer, only 5 of the planned 21 patients completed both periods of the crossover administration of MK-0657 and placebo. Significant antidepressant effects were observed as early as day 5 in patients receiving MK-0657 compared with those receiving placebo, as assessed by the Hamilton Depression Rating Scale and Beck Depression Inventory; however, no improvement was noted when symptoms were assessed with the Montgomery-Asberg Depression Rating Scale, the primary efficacy measure. No serious or dissociative adverse effects were observed in patients receiving this oral formulation of MK-0657. Despite the small sample size, this pilot study suggests that an oral formulation of the NR2B antagonist MK-0657 may have antidepressant properties in TRD patients. Further studies with larger sample sizes are necessary to confirm these preliminary findings.

Novel glutamatergic agents for major depressive disorder and bipolar disorder

Mood disorders such as major depressive disorder (MDD) and bipolar disorder (BPD) are common, chronic, recurrent mental illnesses that affect the lives and functioning of millions of individuals worldwide. Growing evidence suggests that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in the pathophysiology of mood disorders as well as the efficacy of glutamatergic agents as novel therapeutics.

Glutamatergic modulators: the future of treating mood disorders?

Mood disorders such as bipolar disorder and major depressive disorder are common, chronic, and recurrent conditions affecting millions of individuals worldwide. Existing antidepressants and mood stabilizers used to treat these disorders are insufficient for many. Patients continue to have low remission rates, delayed onset of action, residual subsyndromal symptoms, and relapses. New therapeutic agents able to exert faster and sustained antidepressant or mood-stabilizing effects are urgently needed to treat these disorders. In this context, the glutamatergic system has been implicated in the pathophysiology of mood disorders in unique clinical and neurobiological ways. In addition to evidence confirming the role of the glutamatergic modulators riluzole and ketamine as proof-of-concept agents in this system, trials with diverse glutamatergic modulators are under way. Overall, this system holds considerable promise for developing the next generation of novel therapeutics for the treatment of bipolar disorder and major depressive disorder.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Targeting glutamatergic signaling for the development of novel therapeutics for mood disorders

There have been no recent advances in drug development for mood disorders in terms of identifying drug targets that are mechanistically distinct from existing ones. As a result, existing antidepressants are based on decades-old notions of which targets are relevant to the mechanisms of antidepressant action. Low rates of remission, a delay of onset of therapeutic effects, continual residual depressive symptoms, relapses, and poor quality of life are unfortunately common in patients with mood disorders. Offering alternative options is requisite in order to reduce the individual and societal burden of these diseases. The glutamatergic system is a promising area of research in mood disorders, and likely to offer new possibilities in therapeutics. There is increasing evidence that mood disorders are associated with impairments in neuroplasticity and cellular resilience, and alterations of the glutamatergic system are known to play a major role in cellular plasticity and resilience. Existing antidepressants and mood stabilizers have prominent effects on the glutamate system, and modulating glutamatergic ionotropic or metabotropic receptors results in antidepressant-like properties in animal models. Several glutamatergic modulators targeting various glutamate components are currently being studied in the treatment of mood disorders, including release inhibitors of glutamate, N-methyl-D-aspartate (NMDA) antagonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) throughput enhancers, and glutamate transporter enhancers. This paper reviews the currently available knowledge regarding the role of the glutamatergic system in the etiopathogenesis of mood disorders and putative glutamate modulators.

Allosteric modulators of NR2B-containing NMDA receptors: molecular mechanisms and therapeutic potential

N-methyl-D-aspartate receptors (NMDARs) are ion channels gated by glutamate, the major excitatory neurotransmitter in the mammalian central nervous system (CNS). They are widespread in the CNS and are involved in numerous physiological and pathological processes including synaptic plasticity, chronic pain and psychosis. Aberrant NMDAR activity also plays an important role in the neuronal loss associated with ischaemic insults and major degenerative disorders including Parkinson's and Alzheimer's disease. Agents that target and alter NMDAR function may, thus, have therapeutic benefit. Interestingly, NMDARs are endowed with multiple extracellular regulatory sites that recognize ions or small molecule ligands, some of which are likely to regulate receptor function in vivo. These allosteric sites, which differ from agonist-binding and channel-permeation sites, provide means to modulate, either positively or negatively, NMDAR activity. The present review focuses on allosteric modulation of NMDARs containing the NR2B subunit. Indeed, the NR2B subunit confers a particularly rich pharmacology with distinct recognition sites for exogenous and endogenous allosteric ligands. Moreover, NR2B-containing receptors, compared with other NMDAR subtypes, appear to contribute preferentially to pathological processes linked to overexcitation of glutamatergic pathways. The actions of extracellular H+, Mg2+, Zn2+, of polyamines and neurosteroids, and of the synthetic compounds ifenprodil and derivatives ('prodils') are presented. Particular emphasis is put upon the structural determinants and molecular mechanisms that underlie the effects exerted by these agents. A better understanding of how NR2B-containing NMDARs (and NMDARs in general) operate and how they can be modulated should help define new strategies to counteract the deleterious effects of dysregulated NMDAR activity.

Targeting the glutamatergic system to develop novel, improved therapeutics for mood disorders

Mood disorders are common, chronic, recurrent mental illnesses that affect the lives of millions of individuals worldwide. To date, the monoaminergic systems (serotonergic, noradrenergic and dopaminergic) in the brain have received the greatest attention in neurobiological studies of mood disorders, and most therapeutics target these systems. However, there is growing evidence that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in mood-disorder pathophysiology as well as the efficacy of glutamatergic agents in mood disorders. We also discuss exciting new prospects for the development of improved therapeutics for these devastating disorders.

The evidence for hippocampal long-term potentiation as a basis of memory for simple tasks

Long-term potentiation (LTP) is the enhancement of postsynaptic responses for hours, days or weeks following the brief repetitive afferent stimulation of presynaptic afferents. It has been proposed many times over the last 30 years to be the basis of long-term memory. Several recent findings finally supported this hypothesis: a) memory formation of one-trial avoidance learning depends on a series of molecular steps in the CA1 region of the hippocampus almost identical to those of LTP in the same region; b)hippocampal LTP in this region accompanies memory formation of that task and of another similar task. However, CA1 LTP and the accompanying memory processes can be dissociated, and in addition plastic events in several other brain regions(amygdala, entorhinal cortex, parietal cortex) are also necessary for memory formation of the one-trial task, and perhaps of many others.