A ligand binding study of the interactions of guanine nucleotides with non-NMDA receptors

Change of intracellular calcium level causes acute neurotoxicity by antisense oligonucleotides via CSF route

Antisense oligonucleotides (ASOs) are promising therapeutics for intractable central nervous system (CNS) diseases. For this clinical application, neurotoxicity is one of the critical limitations. Therefore, an evaluation of this neurotoxicity from a behavioral perspective is important to reveal symptomatic dysfunction of the CNS and elucidate the underlying molecular mechanism. We here exploited a behavioral analysis method to categorize and quantify the acute neurotoxicity of mice administered with toxic ASOs via intracerebroventricular injection. The toxic ASOs were found to reduce consciousness and locomotor function in mice in a dose-dependent manner. Mechanistically, we analyzed the effects of modulators against receptors or channels, which regulate calcium influx of neurons, on the ASO neurotoxicity. Modulators promoting calcium influx mitigated, whereas those hindering calcium influx increased, in vivo neurotoxicity of ASOs in mice. In an in vitro assay to evaluate intracellular free calcium levels using rat primary cortical neurons, toxic ASOs reduced the calcium levels. The findings of this study demonstrated the behavioral characteristics of ASO-induced neurotoxicity and revealed that changes in intracellular free calcium levels are a part of the mechanism underlying the neurotoxic effects of ASO.

Guanosine Prevents against Glutamatergic Excitotoxicity in C. elegans

Glutamatergic neurotransmission is present in most mammalian excitatory synapses and plays a key role in central nervous system homeostasis. When over-activated, it can induce excitotoxicity, which is present in several neuropathologies. The nucleoside guanosine (GUO) is a guanine-based purine known to have neuroprotective effects by modulating glutamatergic system during glutamate excitotoxicity in mammals. However, GUO action in Caenorhabditis elegans, as well as on C. elegans glutamatergic excitotoxicity model, is not known. The GUO effects on behavioral parameters in Wild Type (WT) and knockouts worms for glutamate transporters (GLT-3, GLT-1), glutamate vesicular transporter (EAT-4), and NMDA and non-NMDA receptors were used to evaluate the GUO modulatory effects. The GUO tested concentrations did not alter the animals' development, but GUO reduced pharyngeal pumps in WT animals in a dose-dependent manner. The same effect was observed in pharyngeal pumps, when the animals were treated with 4 mM of GUO in glr-1, nmr-1 and eat-4, but not in glt-3 and glt-3;glt-1 knockouts. The double mutant glt-3; glt-1 for GluTs had decreased body bends and an increased number of reversions. This effect was reverted after treatment with GUO. Furthermore, GUO did not alter the sensory response in worms with altered glutamatergic signaling. Thus, GUO seems to modulate the worm's glutamatergic system in situations of exacerbated glutamatergic signaling, which are represented by knockout strains to glutamate transporters. However, in WT animals, GUO appears to reinforce glutamatergic signaling in specific neurons. Our findings indicate that C. elegans strains are useful models to study new compounds that could be used in glutamate-associated neurodegenerative diseases.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Effect of uridine of presynaptic NMDA and kainate receptor of rat brain cortex

It was demonstrated that uridine affects presynaptic NMDA and kainite receptors of rat brain cortex. Uridine considerably inhibited (45)Ca2+ uptake into synaptoneurosomes (IC50 = 7.1 x 10(-12) M) under conditions NMDA stimulation and increased it under conditions AMPA stimulation (157.8%).

Factors affecting guanine nucleotide binding to rat AMPA receptors

Glutamate receptors are competitively inhibited by guanine nucleotides. Insight into the physiological function of this inhibition would be greatly advanced if nucleotide binding could be eliminated through mutations without altering other aspects of receptor function, or if compounds were discovered that selectively prevent nucleotide binding. It was previously reported that a lysine in the chick kainate binding protein (cKBP) is specifically involved in guanine nucleotide binding. In the present study we mutated the equivalent lysine in the rat AMPA receptor subunit GluR1 flip to alanine (K445A) and assessed changes in nucleotide affinity from the displacement of [(3)H]fluorowillardiine. As in the cKBP, the affinity for nucleotides was greatly reduced while the binding affinity for agonists remained unchanged. The reduction in affinity was largest for GTP (factor of 5.8) and GDP (4.4) and minor for GMP and guanosine. This suggests that K445 is involved in stabilizing the second phosphate of the nucleotide. Given that bulkier analogs like GDP-fucose are also accommodated at this site, it seems likely that nucleotides bind in such a way that their phosphates project out of the cleft. In excised-patch recordings using short pulses of glutamate, the K445A mutation increased the EC(50) for the peak response 1.8-fold and accelerated desensitization and deactivation. This indicates that the effects of this mutation are not as specific as previously suggested. Efforts to selectively eliminate inhibition by nucleotides may therefore depend on mapping out further the docking site. In a first attempt using point mutations we ruled out several amino acids around the cleft as being involved in nucleotide binding. Also, the AMPA receptor modulator PPNDS which competitively inhibits nucleotide binding to purinergic receptors did not affect nucleotide inhibition, suggesting that there are major differences in the topography between purinergic and glutamate receptors. Thus new approaches, including crystallography, may be called for to identify residues uniquely involved in nucleotide binding.

Stereoselective synthesis and preliminary evaluation of new d-3-heteroarylcarbonylalanines as ligands of the NMDA receptor

New N-heteroarylcarbonylalanines of the D-series were stereoselectively prepared from enoates derived from D-mannitol. These compounds were active in binding and functional assays of the NMDA sub-type of glutamate receptors. A pyridine derivative inhibited MK801 binding, protected neurons from excitotoxic damage and blocked NMDA-induced currents in neurons. A thiophene derivative positively modulated the NMDA receptor, possibly through the allosteric glycine site.

Guanine derivatives modulate L-glutamate uptake into rat brain synaptic vesicles

Glutamate uptake into synaptic vesicles is driven by a proton electrochemical gradient generated by a vacuolar H(+)-ATPase and stimulated by physiological concentrations of chloride. This uptake plays an important role in glutamatergic transmission. We show here that vesicular glutamate uptake is selectively inhibited by guanine derivatives, in a time- and concentration-dependent manner. Guanosine, GMP, GDP, guanosine-5'-O-2-thiodiphosphate, GTP, or 5'-guanylylimidodiphosphate (GppNHp) inhibited glutamate uptake in 1.5 and 3 min incubations, however, when incubating for 10 min, only GTP or GppNHp displayed such inhibition. By increasing ATP concentrations, the inhibitory effect of GTP was no longer observed, but GppNHp still inhibited glutamate uptake. In the absence of ATP, vesicular ATPase can hydrolyze GTP in order to drive glutamate uptake. However, 5mM GppNHp inhibited ATP hydrolysis by synaptic vesicle preparations. GTP or GppNHp decreased the proton electrochemical gradient, whereas the other guanine derivatives did not. Glutamate saturation curves were assayed in order to evaluate the specificity of inhibition of the vesicular glutamate carrier by the guanine derivatives. The maximum velocity of the initial rate of glutamate uptake was decreased by all guanine derivatives. These results indicate that, although GppNHp can inhibit ATPase activity, guanine derivatives are more likely to be acting through interaction with vesicular glutamate carrier.

ATP inhibits NMDA receptors after heterologous expression and in cultured hippocampal neurons and attenuates NMDA-mediated neurotoxicity

We investigated the potential of ATP to inhibit heterologously expressed NMDA receptor subunit combinations, NMDA-induced currents in cultured hippocampal cells, and NMDA-induced neurotoxicity. The effect of ATP on diheteromeric NR1a/NR2A-D NMDA receptor (NR) combinations expressed in Xenopus laevis oocytes was studied by voltage-clamp recording. ATP strongly inhibited NMDA-induced inward currents only at the NR1a/NR2B receptor combination. At NMDA concentrations corresponding to the EC50 value (20 microm), ATP revealed an IC50 value of 135 microm. Mutation studies suggest that ATP exerts its inhibition via the glutamate-binding pocket of the NR2B subunit. Inosine 5'-triphosphate (ITP), GTP, and AMP also inhibited the recombinant NR1a/NR2B receptor, whereas UTP and CTP, ADP, or adenosine had no or only a small effect. Correspondingly, ATP inhibited NMDA-induced but not kainate-induced currents at cultured hippocampal neurons. An abundant expression of the NR2B subunit in the cultured neurons was verified by immunocytochemistry and blockade of NMDA-induced currents by the NR2B-selective antagonist ifenprodil. In addition we studied the role of ATP in NMDA-mediated neurotoxicity using cultured rat hippocampal cells. ATP exhibited a dose-dependent rescue effect when coapplied with the excitotoxicant NMDA, in contrast to ADP, AMP, and adenosine. The effect of ATP was mimicked by GTP and ITP but not by UTP and CTP. ATP had no effect on kainate-elicited neurotoxicity. Our results suggest that ATP can act as an inhibitor of NMDA receptors depending on receptor subunit composition and that it can attenuate NMDA-mediated neurotoxicity that is mediated neither by ATP nor by adenosine receptors.

Characterisation of an uridine-specific binding site in rat cerebrocortical homogenates

Parameters of [3H]uridine binding to synaptic membranes isolated from rat brain cortex (K(D)=71+/-4 nM, B(max)=1.37+/-0.13 pmol/mg protein) were obtained. Pyrimidine and purine analogues displayed different rank order of potency in displacement of specifically bound [3H]uridine (uridine>5-F-uridine>5-Br-uridine approximately adenosine>>5-ethyl-uridine approximately suramin>theophylline) and in the inhibition of [14C]uridine uptake (adenosine>uridine>5-Br-uridine approximately 5-F-uridine approximately 5-ethyl-uridine) into purified cerebrocortical synaptosomes. Furthermore, the effective ligand concentration for the inhibition of [14C]uridine uptake was about two order of magnitude higher than that for the displacement of specifically bound [3H]uridine. Adenosine evoked the transmembrane Na(+) ion influx, whereas uridine the transmembrane Ca(2+) ion influx much more effectively. Also, uridine was shown to increase free intracellular Ca(2+) ion levels in hippocampal slices by measuring Calcium-Green fluorescence. Uridine analogues were found to be ineffective in displacing radioligands that were bound to various glutamate and adenosine-recognition and modulatory-binding sites, however, increased [35S]GTPgammaS binding to membranes isolated from the rat cerebral cortex. These findings provide evidence for a rather specific, G-protein-coupled site of excitatory action for uridine in the brain.

Guanine based purines inhibit [(3)H]glutamate and [(3)H]AMPA binding at postsynaptic densities from cerebral cortex of rats

Extracellular guanine-based purines (GBPs) have been implicated in neuroprotective effects against glutamate toxicity by modulating the glutamatergic system through mechanisms without the involvement of G proteins. Accordingly, GBPs have been shown to inhibit the binding of glutamate and its analogs in different brain membrane preparations. However, brain membrane preparations used for these studies are comprised of both post- and pre-neuronal and glial synaptic components. In this study we investigated the ability of GBPs to displaced glutamate and AMPA binding at postsynaptic densities (PSDs). PSDs are markedly prominent in glutamatergic synapses and retains the native apposition of membrane components and post synaptic receptors. The PSD fraction was prepared from cerebral cortex of Wistar rats and it was characterized as PSDs by electron microscopy and by an enrichment of PSD-95, a protein marker of PSDs (90% of immunodetection). Moreover, we detected an enrichment of glutamate receptors subunits that including NR1 subunit of NMDA receptors and GluR1 subunit of AMPA receptors. GppNp (poor hydrolyzable GTP analog) and GMP displaced 40 and 36% of glutamate binding, respectively, and guanosine only 23%. AMPA binding was not affected by guanosine and was inhibited 21 and 25% by GppNp and GMP, respectively. Hence, this study demonstrates that guanine based purines inhibited glutamate and AMPA binding at postsynaptic membrane preparations, contributing for a better understanding of the mechanisms by which GBPs antagonize glutamatergic neurotoxicicity, e.g. the possible involvement of glutamatergic postsynaptic receptors in their neuroprotective roles.

Study of adenosine A2 receptors in membrane preparations from optic tectum of chicks

Binding properties of the subtypes of adenosine A2 receptors in membrane preparations and the effects of adenosine receptor ligands on cAMP accumulation in slices from the optic tectum of neonatal chicks have been investigated. [3H]2-[4-(2-p-carboxyethyl)phenylamino]-5'-N-ethylcarboxaminoadenosin e (CGS 21680), a selective ligand for adenosine A2a receptors, did not bind to optic tectal membranes, as observed with rat striatal membranes. CGS 21680 also did not induce cyclic AMP accumulation in optic tectum slices. However, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloro-adenosine or adenosine induced a 2.5- to 3-fold increase on cyclic AMP accumulation in this preparation. [3H]NECA binds to fresh non-washed-membranes obtained from optic tectum of chicks, displaying one population of binding sites, which can be displaced by NECA, 8-phenyltheophylline, 2-chloro-adenosine, but is not affected by CGS 21680. The estimated K(D) value was 400.90 +/- 80.50 nM and the Bmax was estimated to be 2.51 +/- 0.54 pmol/mg protein. Guanine nucleotides, which modulate G-proteins activity intracellularly, are also involved in the inhibition of glutamate responses by acting extracellularly. Moreover, we have previously reported that guanine nucleotides potentiate, while glutamate inhibits, adenosine-induced cyclic AMP accumulation in slices from optic tectum of chicks. However, the guanine nucleotides, GMP or GppNHp and the metabotropic glutamate receptors agonist, 1S,3R-ACPD did not alter the [3H]NECA binding observed in fresh non-washed-membranes. Therefore, the adenosine A2 receptor found in the optic tectum must be the adenosine A2b receptor which is available only in fresh membrane preparations, and its not modulated by guanine nucleotides or glutamate analogs.

Chick kainate binding protein lacks GTPase activity

Chick kainate binding protein was solubilized from cerebellar membranes and purified (x19) by use of two chromatographic steps. Measurements of [3H]kainate binding and GTPase activity in the different fractions reveal a consistent decrease of GTPase activity as the purification proceeds so that no GTPase is detectable after the final purification step. This fact, in the context of the differential involvement in nucleotide recognition of some critical amino acid residues in the p-loop motif of GTPases and in the guanine nucleotide-binding sequence of ionotropic glutamate receptors, together with significant discrepancies concerning the activity of individual nucleotides, suggests that both guanine nucleotide-recognizing sequences are unlikely to be alternative expressions of the same functional domain.

Guanine nucleotides, including GMP, antagonize kainate responses in Xenopus oocytes injected with chick cerebellar membranes

Injection of chick cerebellar membranes, rich in kainate binding sites, into Xenopus oocytes resulted in the structural integration of chick membrane patches into the oocyte plasma membrane that could be easily identified by specific immunofluorescent staining. Application of kainate to the oocyte perfusion medium, under voltage-clamp conditions, induced dose-dependent (EC50 = 87+/-14 microM) inward currents, confirming the functional incorporation to the oocyte of kainate-driven channels. Responses to kainate were consistently nondesensitizing and strongly potentiated by cyclothiazide, suggesting the selective involvement of alpha-amino-3-hydroxy-5-methyl-4isoxazolepropionate (AMPA)-preferring receptors. Binding experiments with (S)-[3H]AMPA confirmed the presence in the chick membrane preparation of low-affinity AMPA receptors (K(D) = 278 nM) amounting to <2% of the total population of kainate binding sites. A tenfold concentration of guanine nucleotides, with different degrees of phosphorylation, blocked the responses to 100 microM kainate by approximately 90%. In the case of GMP, additional concentration-inhibition studies yielded an IC50 of 180+/-11 microM. Our results illustrate the apparent failure of kainate-binding proteins to form functional channels, even when maintaining their own native membrane environment, and confirm the antagonistic behavior of guanine nucleotides, including GMP, toward glutamate receptors, in agreement with previous results of ligand-binding experiments and, more interestingly, with the marked neuroprotective effects of some guanine nucleotides in different excitotoxicity experimental paradigms.

Effects of guanine nucleotides on adenosine and glutamate modulation of cAMP levels in optic tectum slices from chicks

Glutamate and adenosine both modulate adenylyl cyclase activity through interaction of their specific receptors with stimulatory or inhibitory G-proteins. Guanine nucleotides (GN), which modulate G-protein activity intracellularly, are also involved in the inhibition of glutamate responses, acting from the outside of the cells. We had previously reported that glutamate inhibits adenosine-induced cyclic AMP (cAMP) accumulation in slices obtained from the optic tectum of chicks. In the present study we investigated the interaction of GN with these two neurotransmitters and found that GN inhibit the inhibitory effect of glutamate on adenosine-induced cAMP accumulation and potentiate adenosine-induced cAMP accumulation. These effects were observed with 5'-guanylylimidodiphosphate (GppNHp) or GMP, but not with guanosine (the nucleoside). Besides, these interactions of GN occur via a metabotropic glutamate receptor (mGluR) sensitive to (1 S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3R-ACPD) but not to L-2-amino-4-phosphonobutyrate (L-AP4). These effects were partially modulated by a mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine ((RS)M-CPG), and by an adenosine receptor antagonist, 8-phenyltheophylline. GN only potentiated the adenosine response when adenosine was acting through its receptor positively linked to adenylyl cyclase. Therefore, the data show that guanine nucleotides not only inhibit glutamate-induced responses, but also stimulate adenosine-induced responses, a fact that may contribute to the understanding of the physiological functions of guanine nucleotides.

Pharmacological characterization, anatomical distribution and sex differences of the non-NMDA excitatory amino acid receptors in the quail brain as identified by CNQX binding

The distribution of non-N-methyl-D-aspartate binding sites was studied in coronal and sagittal sections through the brain of adult Japanese quail by quantitative autoradiography, using tritiated 6-cyano-7-nitroquinoxaline-2,3-dione as a radioligand. Saturation binding experiments were, in addition, carried out in areas showing high levels of binding (cerebellar molecular layer, nucleus anterior medialis and nucleus infundibularis) and demonstrated that the binding of tritiated ligand was specific and saturable. Competition studies with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid and kainic acid indicated that kainic acid strongly inhibited ligand binding in all brain areas. alpha-Amino-3-hydroxy-methyl-4-isoxazole propionic acid was only a weak inhibitor in the hypothalamic nuclei whereas in the cerebellar molecular layer both high and low affinity inhibitions were detected. The highest binding levels of tritiated ligand were observed in the molecular layer of the cerebellum. Very high levels of binding were detected in various preoptic/hypothalamic sites including the nucleus suprachiasmaticus pars medialis, nucleus anterior medialis hypothalami, nucleus infundibularis, nucleus mammillaris medialis, nucleus posteromediale hypothalami and nucleus hypothalami ventromedialis. High levels of binding were also detected in the bulbus olfactorius, bed nucleus commissuralis anterior, bed nucleus commissuralis pallii, nucleus accumbens, bed nucleus striae terminalis and nucleus interpeduncularis. In the preoptic area/hypothalamus, high levels of binding were clearly present in all areas that contain gonadotropin releasing hormone cells or fibers. In the pons and mesencephalon, moderate levels of binding were associated with catecholaminergic areas such as the area ventralis tegmentalis (area ventralis of Tsai) and the locus coeruleus. Saturation analysis demonstrated the presence of a higher number of binding sites in females than in males in the cerebellar molecular layer, nucleus infundibularis and nucleus anterior medialis. This latter difference was confirmed in the one point assays that also identified higher levels of specific binding in the nucleus suprachiasmaticus pars medialis of males as compared with females. These anatomical data suggest a possible implication of non-N-methyl-D-aspartate receptors in the synthesis and/or release of both gonadotropin releasing hormone and catecholaminergic neurotransmitters that should now be tested by pharmacological experiments.

Guanine nucleotides protect against kainate toxicity in an ex vivo chick retinal preparation

Ex vivo preparations of chick neural retina have been successfully used in the assessment of excitotoxicity and in the evaluation of the protective effects of glutamate antagonists. Using a variation of this approach, and measuring the acute and delayed toxic effects of kainate (KA) in terms of lactate dehydrogenase release, we have shown that guanine nucleotides behave as effective neuroprotecting agents. The anti-excitotoxic potency of guanine nucleotides (in the case of GMP and GDPbetaS it is about 100 times lower than that of DNQX, a powerful kainate antagonist) correlates well with their ability to displace KA from retinal KA receptors.

The macro- and microarchitectures of the ligand-binding domain of glutamate receptors

Over the last decade, a large body of information regarding the amino acid sequences and tertiary structures of many proteins has accumulated. Subtle similarities in sequence patterns identified between glutamate receptors and bacterial periplasmic substrate-binding proteins have suggested that structural kinship exists between these protein families. Many of the bacterial periplasmic binding proteins but none of the glutamate receptors have been crystallized so far. The following article reviews how the resemblance between these two protein families led to computer-assisted structural models of crucial elements involved in ligand binding by various glutamate receptors. A plausible dynamic model of the molecular mechanism of activation and desensitization of glutamate-receptor channels is also discussed.

Guanine nucleotides inhibit cAMP accumulation induced by metabotropic glutamate receptor activation

Metabotropic glutamate receptors (mGluRs) have been shown to modulate adenylate cyclase activity via G-proteins. In the present study we report similar results to the previously observed in the literature, showing that glutamate and the metabotropic agonists, 1S,3R-ACPD or quisqualate induced cAMP accumulation in hippocampal slices of young rats. Moreover, guanine nucleotides GTP, GDP or GMP, inhibited the glutamate-induced cAMP accumulation. By measuring LDH activity in the buffer surrounding the slices, we showed that the integrity of the slices was maintained, indicating that the effect of guanine nucleotides was extracellular. GMP, GDPbeta-S or Gpp(NH)p abolished quisqualate-induced cAMP accumulation. GDPbeta-S or Gpp(NH)p but not GMP inhibited 1S,3R-ACPD-induced cAMP accumulation. The response evoked by glutamate was also abolished by the mGluR antagonists: L-AP3 abolished glutamate-induced cAMP accumulation in a dose-dependent manner and MCPG was effective only at the 2 mM dose. DNQX was ineffective. We are reporting here, an inhibition induced by guanine nucleotides, via an extracellular site (s), similar to the observed with classical glutamate antagonists on a cellular response evoked by mGluR agonists.

GMP protects against quinolinic acid-induced loss of NADPH-diaphorase-positive cells in the rat striatum

When injected into the rat striatum, quinolinic acid causes dose-dependent widespread cell death. All cell types, including the NADPH-diaphorase-positive neurons appear to be sensitive to the toxin. The latter cells are destroyed by quinolinic acid injections of 180 nmol per striatum, this effect being blocked by the concomitant administration of 5 mg/kg of the non-competitive N-methyl-D-aspartate antagonist MK-801. We report that guanosine-5'-monophosphate (GMP), at a dose of 360 nmol, is equally effective in protecting the diaphorase-positive cells against quinolinate toxicity.

Specific binding of [3H]GppNHp to extracellular membrane receptors in chick cerebellum: possible involvement of kainic acid receptors

Guanine nucleotides (GNs), including GMP, displace [3H]kainic acid binding to chick cerebellar lysed and vesiculated membranes. Saturation studies of [3H]GppNHp binding, under conditions that prevent the occupation of the nucleotide binding sites in G-proteins, demonstrate the existence of extracellular membrane receptors specific for guanine nucleotides. Affinity-labeling of a vesicle preparation with [alpha-32P]GTP gives one single labeled band, upon electrophoresis, with an apparent molecular mass of 50 kDa. Additional experiments with partially purified kainate receptors suggest that the GN extracellular sites may overlap, at least partially, the kainic acid binding sites, being then responsible for the displacement of [3H]kainic acid by GNs. The physiological significance of these findings remains unclear.

Effect of guanine nucleotides on [3H]glutamate binding and on adenylate cyclase activity in rat brain membranes

GMP-PNP, a non-hydrolyzable analog of GTP binds tightly to G-protein in the presence of Mg2+, so that the binding is stable even after exhaustive washings. This property was exploited to prepare membrane samples of rat brain where G-protein GTP-binding sites were saturated with GMP-PNP. Experiments carried out with these membranes showed that GTP, GMP-PNP, GDP-S and GMP (1 mM) inhibit the sodium-independent [3H]glutamate binding by 30-40% [F(4,40) = 5.9; p < .001], whereas only GMP-PNP activates adenylate cyclase activity [F(6,42) = 3.56; p < .01]. The inhibition of sodium-independent [3H]glutamate binding occurred in the absence of Mg2+. These findings suggest that guanine nucleotides may inhibit glutamate binding and activate adenylate cyclase through distinct mechanisms by acting on different sites.

Identification of the amino acid subsets accounting for the ligand binding specificity of a glutamate receptor

In a situation so far unique among neurotransmitter receptors, glutamate receptors share amino acid sequence similarities with the bacterial periplasmic binding proteins (PBPs). On the basis of the primary structure similarity of two bacterial periplasmic proteins (lysine/arginine/ornithine- and phosphate-binding proteins) with the chick cerebellar kainate-binding protein (KBP), a member of the ionotropic glutamate receptor family, we have generated a three-dimensional model structure of the KBP extracellular domain. By an interplay between homology modeling and site-directed mutagenesis, we have investigated the kainate binding properties of 55 different mutants (corresponding to 43 positions) and studied the interactions of some of these mutants with various glutamatergic ligands. As a result, we present here the subsets of amino acids accounting for the binding free energies and specificities of KBP for kainate, glutamate, and CNQX and propose a three-dimensional model, at the microarchitectural level, of the glutamatergic binding domain.

Characterisation of the interaction between guanyl nucleotides and AMPA receptors in rat brain

Guanyl nucleotides inhibit the binding of the AMPA receptor agonists [3H]fluorowillardiine and [3H]AMPA and the competitive antagonist [3H]CNQX to rat brain cerebrocortical membranes. The rank order of inhibition for each of the radioligands tested was GTP > GDP > GMP. The nucleotides CTP and ATP showed no effect. GTP inhibition was unaffected by the presence or absence of NaCl and MgCl2. Pre-treatment of the membranes with GTP, and its removal before addition of radioligand, did not inhibit binding. Quantitative autoradiography demonstrated that GTP inhibition occurred throughout the brain. These results are consistent with guanyl nucleotides acting at an extracellular site present on all AMPA receptor subunits, occupation of which inhibits agonist and antagonist binding.

Interaction of guanine nucleotides with the kainate binding protein from chick cerebellum

The interaction of guanine nucleotides with the chick cerebellar kainate binding protein (chKBP) was studied using binding assays, immunoblotting, and in vitro phosphorylation experiments. Guanosine 5'-triphosphate (GTP) was found to reduce [3H]kainic acid (KA) binding in a concentration-dependent manner. Similarly, an inhibition of [3H]GTP binding by KA was observed. No G protein co-purified with chKBP. chKBP phosphorylation by the cAMP-dependent protein kinase (PKA) was prevented both by KA and by GTP. Neither KA nor GTP blocked each other's effect in chKBP phosphorylation. The present findings suggest that chKBP harbours two agonistic binding sites, one in the micromolar range, detected by binding techniques and one in the millimolar range detected by phosphorylation assays. Guanine nucleotides interact with both sites.

Differential effects of guanine nucleotides on kainic acid binding and on adenylate cyclase activity in chick optic tectum

In G protein-coupled receptors, neurotransmitter-induced binding of GTP to G proteins triggers the activation of effector systems while simultaneously decreasing the affinity of the transmitter for its specific binding site within the receptor-G protein complex. In the present study we show that, in the chick optic tectum, guanine nucleotides inhibit the binding of the glutamate analog, kainate, and activate adenylate cyclase by different mechanisms and acting on different sites. GMP-PNP, a non-hydrolyzable analog of GTP, binds tightly to G proteins so that the binding is stable even after exhaustive washing. By use of this property, we have prepared membrane samples in which G protein GTP-binding sites are pre-saturated with GMP-PNP. Experiments carried out with these membranes show that GMP-PNP, GDP-S and GMP inhibit the binding of [3H]kainate by interacting with site(s) unrelated to G proteins, whereas GMP-PNP activates adenylate cyclase activity by binding to G proteins.

Kainate-binding proteins: phylogeny, structures and possible functions

Recent advances have demonstrated that the family of [3H]kainate-binding proteins and kainate receptors comprise a number of related polypeptides. In all the cases so far investigated, the kainate-binding proteins from non-mammalian vertebrates have M(r) values in the range of 40-50 kDa whereas mammalian kainate receptors and kainate-binding proteins have M(r) values in the order of 100 kDa. There have not, as yet, been any reports of 40-50 kDa kainate-binding proteins in mammalian CNS and, despite the cloning of increasing numbers of cDNAs encoding new kainate-binding proteins, the relationships between these two general groups of polypeptides remain unclear. Nonetheless, there is now a wealth of phylogenetic, structural and molecular biological data available about these proteins. In this review, Jeremy Henley outlines the properties and structures of kainate-binding proteins and offers some possibilities as to the roles of these often hugely abundant proteins.