Receptor Glycine Site Agonist Research Articles

In vitro ADME characterization of a very potent 3-acylamino-2-aminopropionic acid-derived GluN2C-NMDA receptor agonist and its ester prodrugs.

The GluN2C subunit exists predominantly, but not exclusively in NMDA receptors within the cerebellum. Antagonists such as UBP1700 and positive allosteric modulators including PYD-106 and 3-acylamino-2-aminopropionic acid derivatives such as UA3-10 ((R)-2-amino-3-{[5-(2-bromophenyl)thiophen-2-yl]carboxamido}propionic acid) represent promising tool compounds to investigate the role of GluN2C-containing NMDA receptors in the signal transduction in the brain. However, due to its high polarity the bioavailability and CNS penetration of the amino acid UA3-10 are expected to be rather low. Herein, three ester prodrugs 12a-c of the NMDA receptor glycine site agonist UA3-10 were prepared and pharmacokinetically characterized. The esters 12a-c showed higher lipophilicity (higher logD 7.4 values) than the acid UA3-10 but almost the same binding at human serum albumin. The acid UA3-10 was rather stable upon incubation with mouse liver microsomes and NADPH, but the esters 12a-c were fast hydrolyzed to afford the acid UA3-10. Incubation with pig liver esterase and mouse serum led to rapid hydrolysis of the esters 12a-c. The isopropyl ester 12c showed a promising logD 7.4 value of 3.57 and the highest stability in the presence of pig liver esterase and mouse serum. These results demonstrate that ester prodrugs of UA3-10 can potentially afford improved bioavailability and CNS penetration.

Discovery of (R)-2-amino-3-triazolpropanoic acid derivatives as NMDA receptor glycine site agonists with GluN2 subunit-specific activity.

N-Methyl-d-aspartate (NMDA) receptors play critical roles in central nervous system function and are involved in variety of brain disorders. We previously developed a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid glycine site agonists with pronounced variation in activity among NMDA receptor GluN1/2A-D subtypes. Here, a series of (R)-2-amino-3-triazolpropanoic acid analogues with a novel chemical scaffold is designed and their pharmacological properties are evaluated at NMDA receptor subtypes. We found that the triazole can function as a bioisostere for amide to produce glycine site agonists with variation in activity among NMDA receptor subtypes. Compounds 13g and 13i are full and partial agonists, respectively, at GluN1/2C and GluN1/2D with 3- to 7-fold preference in agonist potency for GluN1/2C-D over GluN1/2A-B subtypes. The agonist binding mode of these triazole analogues and the mechanisms by which the triazole ring can serve as a bioisostere for amide were further explored using molecular dynamics simulations. Thus, the novel (R)-2-amino-3-triazolpropanoic acid derivatives reveal insights to agonist binding at the GluN1 subunit of NMDA receptors and provide new opportunities for the design of glycine site agonists.

Derivatives of (R)-3-(5-Furanyl)carboxamido-2-aminopropanoic Acid as Potent NMDA Receptor Glycine Site Agonists with GluN2 Subunit-Specific Activity.

NMDA receptors mediate glutamatergic neurotransmission and are therapeutic targets due to their involvement in a variety of psychiatric and neurological disorders. Here, we describe the design and synthesis of a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid analogues 8a-s as agonists at the glycine (Gly) binding site in the GluN1 subunit, but not GluN3 subunits, of NMDA receptors. These novel analogues display highly variable potencies and agonist efficacies among the NMDA receptor subtypes (GluN1/2A-D) in a manner dependent on the GluN2 subunit. Notably, compound 8p is identified as a potent partial agonist at GluN1/2C (EC50 = 0.074 μM) with an agonist efficacy of 28% relative to activation by Gly and virtually no agonist activity at GluN1/2A, GluN1/2B, and GluN1/2D. Thus, these novel agonists can modulate the activity of specific NMDA receptor subtypes by replacing the full endogenous agonists Gly or d-serine (d-Ser), thereby providing new opportunities in the development of novel therapeutic agents.

Improved synthetic route for the GluN2-specific NMDA receptor glycine site agonist AICP

(R)-2-Amino-3-(4-(2-ethylphenyl)-1H-indole-2-carboxamido)propanoic acid (AICP) is a N-methyl-d-aspartate (NMDA) receptor glycine site agonist with unprecedented high potency in the low nanomolar range, and a GluN2 subunit-dependent pharmacological profile in terms of potency and agonist efficacy (Jessen et al., 2017 [1]). Here, we report a scalable, practical and cost-efficient synthetic route for AICP, which is an improvement compared to the previously reported route. This improved synthetic route includes a versatile diphenylmethylester (DPM) protection for the amino acid moiety, which can be widely used in the synthesis of other amino acid ligands. Further functional evaluation of AICP at the different ionotropic glutamate receptor (iGluR) classes demonstrates that high affinity binding of AICP to the orthosteric binding site is selective for NMDA receptors over AMPA and kainate receptors. Furthermore, high affinity binding of AICP is not observed at GluN3A, GluN3B, and GluD2 subunits, which also bind glycine and d-serine. Thus, the new approach described here enables scalable synthesis of AICP for the use as a pharmacological tool compound to study the involvement of neuronal NMDA receptor subtypes in normal brain function and disease.

Modulating reconsolidation and extinction to regulate drug reward memory.

Drug addiction is an aberrant memory that shares the same memory processes as other memories. Brief exposure to drug-associated cues could result in reconsolidation, a hypothetical process during which original memory could be updated. In contrast, longer exposure times to drug-associated cues could trigger extinction, a process that decreases the conditioned responding. In this review, we discuss the pharmacological and non-pharmacological manipulations on the reconsolidation and extinction that could be used to interfere with drug reward memories. Pharmacological agents such as β-adrenergic receptor antagonist propranolol can interfere with reconsolidation to disrupt drug reward memory. Pharmacological agents such as the NMDA receptor glycine site agonists d-cycloserine and d-serine can facilitate extinction and then attenuate the expression of drug reward memory. Besides pharmacological interventions, drug-free behavioral approaches by utilizing the reconsolidation and extinction, such as 'post-retrieval extinction' and 'UCS-retrieval extinction', are also effective to erase or inhibit the recall of drug reward memory. Taken together, pharmacological modulation and non-pharmacological modulation of reconsolidation and extinction are promising approaches to regulate drug reward memory and prevent relapse.

D-Cycloserine reduces context specificity of sexual extinction learning

Backgroundd-Cycloserine (DCS) enhances extinction processes in animals. Although classical conditioning is hypothesized to play a pivotal role in the aetiology of appetitive motivation problems, no research has been conducted on the effect of DCS on the reduction of context specificity of extinction in human appetitive learning, while facilitation hereof is relevant in the context of treatment of problematic reward-seeking behaviors. MethodsFemale participants were presented with two conditioned stimuli (CSs) that either predicted (CS+) or did not predict (CS−) a potential sexual reward (unconditioned stimulus (US); genital vibrostimulation). Conditioning took place in context A and extinction in context B. Subjects received DCS (125mg) or placebo directly after the experiment on day 1 in a randomized, double-blind, between-subject fashion (Placebo n=31; DCS n=31). Subsequent testing for CS-evoked conditioned responses (CRs) in both the conditioning (A) and the extinction context (B) took place 24h later on day 2. Drug effects on consolidation were then assessed by comparing the recall of sexual extinction memories between the DCS and the placebo groups. ResultsPost learning administration of DCS facilitates sexual extinction memory consolidation and affects extinction’s fundamental context specificity, evidenced by reduced conditioned genital and subjective sexual responses, relative to placebo, for presentations of the reward predicting cue 24h later outside the extinction context. ConclusionsDCS makes appetitive extinction memories context-independent and prevents the return of conditioned response. NMDA receptor glycine site agonists may be potential pharmacotherapies for the prevention of relapse of appetitive motivation disorders with a learned component.

Identity of endogenous NMDAR glycine site agonist in amygdala is determined by synaptic activity level

Mechanisms of NMDA receptor-dependent synaptic plasticity contribute to the acquisition and retention of conditioned fear memory. However, synaptic rules which may determine the extent of NMDA receptor activation in the amygdala, a key structure implicated in fear learning, remain unknown. Here we show that the identity of the NMDAR glycine site agonist at synapses in the lateral nucleus of the amygdala may depend on the level of synaptic activation. Tonic activation of NMDARs at synapses in the amygdala under low-activity conditions is supported by ambient D-serine, whereas glycine may be released from astrocytes in response to afferent impulses. The release of glycine may decode the increases in afferent activity levels into enhanced NMDAR-mediated synaptic events, serving an essential function in the induction of NMDAR-dependent long-term potentiation in fear conditioning pathways.

Pharmacological Characterization of Glycine-Activated Currents in HEK 293 Cells Expressing N-Methyl-D-aspartate NR1 and NR3 Subunits

N-Methyl-D-aspartate (NMDA) receptors are important targets for drugs of abuse such as ethanol, toluene, and ketamine. Ligand-gated ion channels assembled from the NR1 and NR3 subunits have functional and pharmacological properties that are distinct from those of conventional NMDA receptors containing NR2 subunits. In the present study we used voltage-clamp electrophysiology to characterize excitatory glycine-activated receptors assembled from NR1, NR3A, and NR3B subunits expressed in human embryonic kidney (HEK) 293 cells. These glycine-activated receptors were not stimulated by glutamate or kainic acid and were resistant to magnesium block. A wide variety of NMDA receptor antagonists including d-2-amino-5-phosphonovaleric acid, ifenprodil, memantine, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) or acamprosate did not inhibit glycine-activated NR1/NR3A/NR3B receptors. Likewise, these receptors were not affected by antagonists of inhibitory glycine receptors or glycine transporters. The NMDA receptor glycine site agonist, d-serine, partially activated NR1/NR3A/NR3B receptors, whereas the antagonist, 5,7-dichloro-kynurenic acid, inhibited receptor currents. Conversely, the antagonist, 7-chlorokynurenic acid, and the partial agonist, R-(+)-3-amino-1-hydroxy-2-pyrrolidinone (HA-966), potentiated glycine-stimulated currents of these receptors. NR1/NR3A/NR3B receptor currents were inhibited by 10 to 21% by ethanol and toluene but were relatively insensitive to ketamine. Ethanol inhibition was enhanced in receptors expressing the NR1(L819A) mutant, whereas those containing NR1(F639A) or NR1(M813A) showed no change relative to the wild-type NR1. The results of this study indicate that coexpression of NR1, NR3A, and NR3B subunits in HEK 293 cells results in glycineactivated receptors with novel functional and pharmacological properties.

Vocalization responses after intrathecal administration of ionotropic glutamate receptor agonists in rats.

Inotropic glutamate receptors in the spinal cord (N-methyl-D-aspartic acid [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], and kainate receptors) seem to play a key role in acute pain transmission and the neuronal plasticity in chronic pain states. Vocalization responses produced by activation of these receptors on the pain pathways can be quantified semiautomatically and thus could be used as a research tool. We studied vocalization responses induced by intrathecal administration of various agonists acting at the glutamate receptors in normal rats and in the presence of peripheral inflammation and a chronic constriction injury model of neuropathic pain. The nonselective endogenous agonist, glutamate, and the NMDA receptor glycine site agonist D-serine did not produce vocalization, whereas selective agonists acting at AMPA, NMDA, and kainate receptors produced dose-related vocalization responses. The vocalization response evoked by the administration of AMPA was significantly increased in the neuropathic pain model. In conclusion, spinal administration of ionotropic glutamate receptor agonists produce short-lasting, dose-related vocalization responses that can be used as a basic research and screening tool for analgesic studies. However, peripheral inflammation or nerve injury did not substantially alter vocalization responses overall, possibly indicating that the vocalization test is not a good tool for studying the role of excitatory amino acids in these pathological pain conditions. Vocalization responses evoked by spinal administration of ionotropic glutamate receptor agonists can be used for experimental analgesic studies. However, pathological pain models did not substantially alter vocalization responses, possibly indicating that this test is not suitable for studying the role of spinal excitatory amino acids in central sensitization.

Presence of the N‐Methyl‐D‐Aspartate‐Associated Glycine Receptor Agonist, D‐Seine, in Human Temporal Cortex: Comparison of Normal, Parkinson, and Alzheimer Tissues

The amino acid D-serine (D-Ser), previously recognized as a pharmacological tool for potentiating neuronal activity mediated by the N-methyl-D-aspartate (NMDA) receptor complex, in vitro and in vivo, has been observed in several brain regions of the rat and mouse, most prominently in cortex. In addition to reconfirming the presence and distribution of D-Ser in rat brain, we have observed, for the first time, endogenous, free D-Ser in temporal cortex of normal human brains at a level of 2.18 +/- 0.12 nmol/mg of protein, representing 15 +/- 2% of the free L-Ser pool. The D- and L-Ser specific content and the D/L-Ser ratio obtained from temporal cortex of Parkinson and Alzheimer brains did not differ significantly from those of controls. However, at the levels observed here, and considering its specificity and affinity for the NMDA-associated glycine receptor, endogenous D-Ser is a plausible NMDA receptor glycine site agonist.