Differential profiles of binding of a radiolabeled agonist and antagonist at a glycine recognition domain on the N-methyl-D-aspartate receptor ionophore complex in rat brain

Does kynurenic acid act on nicotinic receptors? An assessment of the evidence

As a major metabolite of kynurenine in the oxidative metabolism of tryptophan, kynurenic acid is of considerable biological and clinical importance as an endogenous antagonist of glutamate in the central nervous system. It is most active as an antagonist at receptors sensitive to N-methyl-D-aspartate (NMDA) which regulate neuronal excitability and plasticity, brain development and behaviour. It is also thought to play a causative role in hypo-glutamatergic conditions such as schizophrenia, and a protective role in several neurodegenerative disorders, notably Huntington's disease. An additional hypothesis, that kynurenic acid could block nicotinic receptors for acetylcholine in the central nervous system has been proposed as an alternative mechanism of action of kynurenate. However, the evidence for this alternative mechanism is highly controversial, partly because at least eight earlier studies concluded that kynurenic acid blocked NMDA receptors but not nicotinic receptors and five subsequent, independent studies designed to repeat the results have failed to do so. Many studies considered to support the alternative 'nicotinic' hypothesis have been based on the use of analogs of kynurenate such as 7-chloro-kynurenic acid, or putatively nicotinic modulators such as galantamine, but a detailed analysis of the pharmacology of these compounds suggests that the results have often been misinterpreted, especially since the pharmacology of galantamine itself has been disputed. This review examines the evidence in detail, with the conclusion that there is no confirmed, reliable evidence for an antagonist activity of kynurenic acid at nicotinic receptors. Therefore, since there is overwhelming evidence for kynurenate acting at ionotropic glutamate receptors, especially NMDAR glutamate and glycine sites, with some activity at GPR35 sites and Aryl Hydrocarbon Receptors, results with kynurenic acid should be interpreted only in terms of these confirmed sites of action.

Different effects of lectins on the ligand binding of the NMDA receptors and sigma sites in rat brain hippocampus synaptic membranes

The effects of the lectins concanavalin A, WGA, ricin, abrin, and the mistletoe lectins from Viscum album MLI, MLII, and MLIII on the binding of ligands of the NMDA and sigma receptors in rat hippocampus synaptic plasma membranes were investigated. Binding of [3H]MK-801, [3H]glutamate, [3H]5,7-DCKA, and [3H]glycine to the membranes was decreased by 40-60% after addition of galactose-specific lectins (mistletoe lectins MLI, MLII, ricin, abrin) at concentrations of 0.01 mg/ml, but was not affected by the glucose- and mannose-specific lectin Con A, an acetylglucosamine-specific lectin WGA, or an acetylgalactosamine-specific lectin MLIII. The binding of [3H]SKF 10047 was decreased only in the presence of MLIII and did not change after addition of the other lectins. It is suggested that lectin-sensitive ligand binding sites of sigma- and NMDA receptors are located separately, and that the carbohydrate side chains of the sigma receptor do not participate in the modulation of the NMDA-receptor.

Receptor binding characteristics of the novel NMDA receptor glycine site antagonist [3H]GV150526A in rat cerebral cortical membranes

Binding of the glycine site antagonist 3-[2-(Phenylamino-carbonyl)ethenyl]-4,6-dichloro-indole-2-carboxylic acid sodium salt ([3H]GV150526A) was characterised in rat cerebral cortical membranes. Saturation experiments indicated the existence of a high affinity binding site, with a pK(d) value of 9.08 (K(d)=0. 8 nM) and a B(max) of 3.4 pmol/mg of protein. A strong linear correlation was observed between the displacement potencies for [3H]GV150526A and [3H]glycine of 13 glycine site ligands (r=0.991). The association kinetics of [3H]GV150526A binding was monophasic, with a k(on) value of 0.047 (nM)(-1) min(-1). Dissociation was induced by the addition of an excess of glycine, GV150526A, or 5,7-dichlorokynurenic acid (DCKA), another glycine antagonist. With GV150526A and DCKA, the dissociation curves presented similar k(off) values (0.068 and 0.069 min(-1), respectively), as expected from ligands binding to the same site. Conversely, a significantly lower k(off) value (0.027 min(-1)) was found with glycine. Although these data may suggest that glycine agonists and antagonists bind to discrete sites with an allosteric linkage (rather than interacting competitively), the reason for this difference remains to be elucidated. It is concluded that [3H]GV150526A can be considered a new valuable tool to further investigate the properties of the glycine site of the NMDA receptor.

Some interactions of L-DOPA and its related compounds with glutamate receptors

L-DOPA is probably a transmitter and/or modulator in the central nervous system (1). L-DOPA methyl ester (DOPA ME) is a competitive L-DOPA antagonist. However, it remains to be clarified whether there exist L-DOPAergic receptors. In Xenopus laevis oocytes injected with rat brain poly(A)+ RNA, L-DOPA induced small inward currents with ED50 of 2.2 mM at a holding potential of -70 mV. The currents were abolished by kynurenic acid or CNQX. Similar L-DOPA-currents were seen in oocytes co-injected with AMPA receptors, GluRs1,2,3 and 4. In brain membrane preparations, L-DOPA inhibited specific binding of [3H]-AMPA with IC50 of 260 microM. This inhibition was not modified by 200 microM ascorbic acid, an antioxidant. L-DOPA did not inhibit binding of [3H]-ligands of MK-801, kainate, DCKA and CGP39653. DOPA ME and L-DOPA cyclohexyl ester, a novel, potent and competitive antagonist (2), inhibited specific binding of [3H]-MK-801 with respective IC50 of 1 and 0.68 mM, but elicited no effect on that of the other [3H]-ligands. With low affinities, L-DOPA acts on AMPA receptors, while competitive antagonists act on NMDA ion channel domain. L-DOPAergic agonist and antagonist may not interact on ionotropic glutamate receptors. DOPA ME-sensitive L-DOPA recognition sites (1) seem to differ from glutamate receptors.

[3H]CGP 61594, the first photoaffinity ligand for the glycine site of NMDA receptors

Activation of NMDA receptors requires the presence of glycine as a coagonist which binds to a site that is allosterically linked to the glutamate binding site. To identify the protein constituents of the glycine binding site in situ the photoaffinity label [3H]CGP 61594 was synthesized. In reversible binding assays using crude rat brain membranes, [3H]CGP 61594 labeled with high affinity (K(D) = 23 nM) the glycine site of the NMDA receptor. This was evident from the Scatchard analysis, the displacing potencies of various glycine site ligands and the allosteric modulation of [3H]CGP 61594 binding by ligands of the glutamate and polyamine sites. Electrophysiological experiments in a neocortical slice preparation identified CGP 61594 as a glycine antagonist. Upon UV-irradiation, a protein band of 115 kDa was specifically photolabeled by [3H]CGP 61594 in brain membrane preparations. The photolabeled protein was identified as the NR1 subunit of the NMDA receptor by NR1 subunit-specific immunoaffinity chromatography. Thus, [3H]CGP 61594 is the first photoaffinity label for the glycine site of NMDA receptors. It will serve as a tool for the identification of structural elements that are involved in the formation of the glycine binding domain of NMDA receptors in situ and will thereby complement the mutational analysis of recombinant receptors.

Differentiation of glycine antagonist sites of N-methyl-D-aspartate receptor subtypes. Preferential interaction of CGP 61594 with NR1/2B receptors

The binding site for the co-agonist glycine on N-methyl-D-aspartate (NMDA) receptors has been mapped to the NR1 subunit whereas binding of the principal agonist glutamate is mediated by the NR2 subunits. Using the novel glycine site antagonist and photoaffinity label CGP 61594, distinct contributions of the NR2 subunit variants to the glycine antagonist binding domains of NMDA receptor subtypes are demonstrated. High affinity sites for CGP 61594 were exclusively displayed by NR1/2B receptors, as shown by their co-distribution with the NR2B subunit, by subunit-selective immunoprecipitation and by functional analysis of NR1/2B receptors expressed in Xenopus oocytes (inhibitory potency, IC50 = 45 +/- 11 nM). Other NMDA receptor subtypes are clearly distinguished by reduced inhibitory potencies for CGP 61594, being low for NR1/2A and NR1/2D receptors (IC50 = 430 +/- 105 nM and 340 +/- 61 nM, respectively) and intermediate for NR1/2C receptors (IC50 = 164 +/- 27 nM). Glycine antagonist sites with low and intermediate affinity for [3H]CGP 61594 were detected also in situ by radioligand binding in brain areas predominantly expressing the NR2A and NR2C subunits, respectively. Thus, [3H]CGP 61594 is the first antagonist radioligand that reliably distinguishes the glycine site of NMDA receptor subtypes. [3H]CGP 61594 is a promising tool to identify the NR2 subunit domains that contribute to differential glycine antagonist sites of NMDA receptor subtypes.

[3H]5,7-dichlorokynurenic acid recognizes two binding sites in rat cerebral cortex membranes

Binding of [3H]5,7-dichlorokynurenic acid ([3H]DCKA), a competitive antagonist of the strychnine-insensitive glycine site of the N-methyl-D-aspartate (NMDA) receptor channel complex, was characterized in synaptic plasma membranes from rat cerebral cortex. Non linear curve fitting of [3H]DCKA saturation and homologous displacement isotherms indicated the existence of two binding sites: a specific, saturable, high affinity site, with a pKD value of 7.24 (KD = 57.5 nmol/l) and a maximum binding value (Bmax) of 6.9 pmol/mg of protein and a second site, with micromolar affinity. The pharmacological profile of both binding components was determined by studying the effect on [3H]DCKA and [3H]glycine binding of a series of compounds known to interact with different excitatory and inhibitory amino acid receptors. These studies confirmed the identity of the high affinity site of [3H]DCKA binding with the strychnine-insensitive glycine site of the NMDA receptor channel complex. 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloroindole-2-carb oxylic acid sodium salt (GV 150526A), a new, high affinity, selective glycine site antagonist (1), was the most potent inhibitor of this component of binding (pKi = 8.24, Ki = 5.6 nmol/l). The low affinity component of [3H]DCKA binding was insensitive to the agonists glycine and D-serine and the partial agonist (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA 966), though recognised by glycine site antagonists. The precise nature of this second, low affinity [3H]DCKA binding site remains to be elucidated.

Characterisation of the binding of [3H]MDL 105,519, a radiolabelled antagonist for the N-methyl-D-aspartate-associated glycine site, to pig cortical brain membranes

Binding of [3H]MDL 105,519 to glycine sites on N-methyl-D-aspartate (NMDA) receptors of pig cortical brain membranes was evaluated. [3H]MDL 105,519 labelled a homogeneous population of binding sites with a Kd-value of 3.73 +/- 0.43 nM and a Bmax-value of 3030 +/- 330 fmol/mg protein. Clear correlations between the affinity (Ki) to [3H]MDL 105,519 labelled sites and the potency (EC50) to enhance or inhibit non-equilibrium [3H]MK-801 binding in the nominal absence of glycine were shown for a variety of glycine site agonists, partial agonists and antagonists. The ratio of Ki to EC50 was >1 for agonists and partial agonists and <1 for antagonists. Various cations as well as glutamate and polyamine site ligands were shown to be able to influence [3H]MDL 105,519 binding to pig cortical brain membranes substantially.

Overexpression of a functional NMDA receptor subunit (NMDAR1) in baculovirus-infected Trichoplusia ni insect cells

For overexpression of the N-methyl-D-aspartate (NMDA) receptor subunit 1b (NMDAR1b), its corresponding cDNA was extended by codons for six histidine residues at the 3'-end, cloned into a baculovirus transfer vector and integrated into the viral genome. Infection of Trichoplusia ni insect cells (High FiveTM cells) with recombinant baculovirus resulted in the production of 126- and 105-kDa NR 1b proteins in the cell membrane fraction. Enzymatic deglycosylation with PNGase F as well as infection of the insect cells in the presence of tunicamycin revealed that the two proteins represented the N-glycosylated and non-glycosylated forms of NMDAR1b, respectively. The recombinant NR1b protein was also identified with immunocytochemical methods employing a monoclonal antibody which recognized the six histidine residues. The affinity of this histidine tag to nickel ions was used for the purification of the NR1b protein. The glycine binding site of the subunit was successfully identified and analyzed with the specific antagonist 5,7-[3-3H]dichlorokynurenate (DCKA). The observed binding characteristics were similar to those obtained for native NMDA receptors. Whereas in electrophysiological measurements a functional NMDA receptor channel could not be found in infected insect cells, its expression was demonstrated in the Xenopus oocyte system after injection of the NMDAR1b cDNA construct.

Simultaneous determination of binding of a variety of radioligands related to ionotropic excitatory amino acid receptors in fetal and neonatal rat brains

Expression of ionotropic excitatory amino acid receptors was assessed by membrane binding assays using a variety of radioligands in fetal and neonatal rat brains. In fetal rat brain, receptors sensitive to N-methyl-D-aspartate (NMDA) exhibited delayed onset of expression during the last 7 days before birth as compared with those insensitive to NMDA. In addition, developmental increases in agonist-preferring sites preceded those in antagonist-preferring sites within the first 7 postnatal days in particular brain structures with respect to each domain on the NMDA receptor complex. Growth of animals led to drastic increments of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801) binding to the NMDA channel in telencephalic regions until 21 to 28 days after birth, with concomitant desensitization to inhibition by protons of [3H]MK-801 binding in cortical membranes. By contrast, three different agonists were invariably effective in more potently potentiating [3H]MK-801 binding in cortical membranes of 14- and 28-day-old rats than in those of 5-day-old rats. These results suggest that the NMDA-sensitive subclass may play more critical roles in mechanisms underlying postnatal development of rat telencephalon than do the NMDA-insensitive subclasses.

On the true affinity of glycine for its binding site at the NMDA receptor complex

To determine the exact potency of inhibitors acting at the glycine site of the NMDA receptor complex using [3H]glycine as a radioligand, the true equilibrium constant (KD) of the radioligand has to be known. To achieve this goal, (1) the contamination of water by glycine was studied, (2) the true affinity of glycine for the NMDA receptor was estimated by different methods, and (3) the inhibition constant of a number of antagonists was determined. HPLC analysis of the deionized water used for the preparation of the assay buffers and solutions resulted in glycine concentrations below 6 nmol/L in four out of seven samples analyzed. The observable equilibrium constant of [3H]glycine binding (Kobs) was found to vary, in the presence of 10-mumol glutamate per liter, between 36 and 163 nmol/L (mean +/- SD: 69 +/- 21 nmol/L, 140 determinations). Based on the observation that the polyamine spermine increased the potency of glycine, but not that of glycine antagonists, an algorithm was developed to calculate KD from Kobs- and IC50-values obtained with and without 100-mumol spermine per liter. Using four different antagonists, KD-values between 24 and 57 nmol/L were obtained. Computer analysis of glycine-stimulated [3H]MK-801 binding resulted in EC50-values between 22 and 35 nmol/L. Based on these results, a true affinity constant for [3H]glycine of 40 nmol/L was assumed and used for the determination of Ki-values of 15 different inhibitors of [3H]glycine binding.

Search for novel ligands selective at a polyamine recognition domain on the N-methyl-D-aspartate receptor complex using membrane binding techniques

Among over 60 polyamine derivatives tested, only N-(3-aminopropyl)octanediamine and bis-(3-aminopropyl)nonanediamine (TE393) markedly inhibited [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding at equilibrium in the presence of added spermidine (SPD) in "non-washed" rat brain synaptic membranes, without affecting that in the absence of added SPD. Although TE393 significantly potentiated [3H]MK-801 binding before equilibrium in the presence of L-glutamic acid (Glu) alone or both Glu and glycine (Gly) added in "Triton-treated" membranes, the putative polyamine antagonists 1,10-decanediamine (DA10) and arcaine invariably inhibited binding irrespective of the addition of agonists. In the absence of added SPD, in addition, TE393 markedly enhanced abilities of both Glu and Gly to potentiate [3H]MK-801 binding before equilibrium. However, TE393 induced a rightward shift of the concentration-response curve of SPD for [3H]MK-801 binding before equilibrium. Moreover, TE393 was effective in potentiating binding of an antagonist but not an agonist radioligand to the NMDA domain and in inhibiting binding of an antagonist but not an agonist radioligand to the Gly domain. The potentiation of NMDA antagonist binding by TE393 occurred in a manner sensitive to prevention by arcaine but not by DA10. These results suggest that TE393 may be a novel ligand at the polyamine domain with an ability to interact with both the NMDA and Gly recognition domains in antagonist-preferring forms.

Expression and characterization of the zeta 1 subunit of the N-methyl-D-aspartate (NMDA) receptor channel in a baculovirus system

Using a baculovirus expression vector system, the zeta 1 subunit of the mouse N-methyl-D-aspartate (NMDA) receptor channel was expressed in Spodoptera frugiperda insect cells. The peptide corresponding to the C-terminus of the zeta 1 subunit was synthesized by using the multiple antigen peptide (MAP) system, and an antibody to the synthetic peptide was produced. Immunoblotting using the newly developed antibody revealed the major 122-kDa and the minor 104-kDa protein bands. The effect of tunicamycin on the immunoblots and [35S]methionine/[35S]cysteine metabolic radiolabeling suggested that the two bands corresponded to glycosylated and non-N-glycosylated forms, respectively. Membranes prepared from insect cells infected with the recombinant virus had the binding activity of antagonist ligand 5,7-[3-3H]dichlorokynurenate (DCKA) of a glycine recognition domain of the receptor. Both immunofluorescence labeling and the [3H]DCKA binding assays also showed a greater level of expression (Bmax = 51 pmol/mg protein) in the insect cells. The ligand binding characteristics of the receptors expressed in insect cells suggested that the single zeta 1 subunit protein has glycine antagonist binding properties comparable to those of the native NMDA receptor channels. The lack of DCKA-binding activity of the non-N-glycosylated NMDA receptor expressed in the presence of tunicamycin suggested that N-linked oligosaccharide is essentially required for expression of a functional receptor in insect cells. This is the first report describing the importance of N-glycosylation for the acquisition of ligand binding to NMDA receptor channel subunit protein.

Alterations in the N-methyl-D-aspartate (NMDA) receptor complex in the frontal cortex of suicide victims

Chronic antidepressant treatment results in adaptation of the NMDA receptor complex in the rodent cortex. This adaptation consists of a reduction in the potency of glycine to displace [3H]5,7-dichlorokynurenic acid from strychnine-insensitive glycine receptors and a reduction in high affinity, glycine-displaceable [3H]CGP-39653 binding to glutamate receptors. We hypothesized that dysfunction of NMDA receptors might occur in frontal cortices from human suicide victims. We now report that the proportion of high affinity, glycine displaceable [3H]CGP-39653 binding to glutamate receptors is reduced from 45 +/- 5% in controls to 27 +/- 6% in age- and post-mortem interval-matched suicide victims. In contrast, neither the potency nor the maximum efficacy of glycine to inhibit [3H]CGP-39653 binding is altered in the frontal cortex of suicide victims compared to controls. Moreover, neither the potency of glycine to inhibit [3H]5,7-dichlorokynurenic acid binding to the strychnine-insensitive glycine receptor nor the specific binding of [3H]5,7-dichlorokynurenic acid binding differed in suicide victims compared to controls. Likewise, neither basal nor glycine- or glutamate enhanced non-equilibrium binding of [3H]dizocilpine was altered in the frontal cortex of suicide victims compared to controls. These data represent the first demonstration supporting the hypothesis that glutamatergic dysfunction is involved in psychopathology underlying suicide and, potentially in human major depression.

Potentiation by calcium ions of [3H]MK-801 binding to an ion channel associated with the N-methyl-D-aspartate receptor complex in rat brain

In vitro addition of Ca2+ ions was effective in almost doubling binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) before equilibrium to an ion channel associated with an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors. The addition of inhibitors for phospholipase (PLase) A2 markedly reduced binding in the absence of added Ca2+ ions, while Ca2+ ions restored the reduction to the level found in the absence of the inhibitors. Pretreatment with PLases A2 and C but not D was effective in potentiating [3H]MK-801 binding in a biphasic manner at a concentration range of 5 mU/ml-10 U/ml. Moreover, PLases A2 and C at low concentrations not only suppressed the abilities of 3 different agonists to potentiate [3H]MK-801 binding before equilibrium, but also reduced that of Ca2+ ions. These results suggest that Ca2+ ions may potentiate [3H]MK-801 binding to the NMDA channel highly permeable to Ca2+ ions through a mechanism common to that underlying potentiation by exogenous PLase A2 and/or C.