Quisqualic acid induced sensitization and the active uptake of L-quisqualic acid by hippocampal slices

The lathyrus toxin, β-N-oxalyl-l-α,β-diaminopropionic acid (ODAP), and homocysteic acid sensitize CA1 pyramidal neurons to cystine and l-2-amino-6-phosphonohexanoic acid

A brief exposure of hippocampal slices to L-quisqualic acid (QUIS) sensitizes CA1 pyramidal neurons 30- to 250-fold to depolarization by certain excitatory amino acids analogues, e.g., L-2-amino-6-phosphonohexanoic acid (L-AP6), and by the endogenous compound, L-cystine. This phenomenon has been termed QUIS sensitization. A mechanism similar to that previously described for QUIS neurotoxicity has been proposed to describe QUIS sensitization. Specifically, QUIS has been shown to be sequestered into GABAergic interneurons by the System x(c)(-) and subsequently released by heteroexchange with cystine or L-AP6, resulting in activation of non-NMDA receptors. We now report two additional neurotoxins, the Lathyrus excitotoxin, beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (ODAP), and the endogenous compound, L-homocysteic acid (HCA), sensitize CA1 hippocampal neurons >50-fold to L-AP6 and >10-fold to cystine in a manner similar to QUIS. While the cystine- or L-AP6-mediated depolarization can be inhibited by the non-NMDA receptor antagonist CNQX in ODAP- or QUIS-sensitized slices, the NMDA antagonist D-AP5 inhibits depolarization by cystine or L-AP6 in HCA-sensitized slices. Thus, HCA is the first identified NMDA agonist that induces phosphonate or cystine sensitization. Like QUIS sensitization, the sensitization evoked by either ODAP or HCA can be reversed by a subsequent exposure to 2 mM alpha-aminoadipic acid. Finally, we have demonstrated that there is a correlation between the potency of inducers for triggering phosphonate or cystine sensitivity and their affinities for System x(c)(-) and either the non-NMDA or NMDA receptor. Thus, the results of this study support our previous model of QUIS sensitization and have important implications for the mechanisms of neurotoxicity, neurolathyrism and hyperhomocystinemia.

Analogues of homoibotenic acid show potent and selective activity following sensitisation by quisqualic acid

Quisqualic acid induces sensitisation of neurones to depolarisation by analogues of 2-amino-4-phosphonobutyric acid (AP4), phenylglycine, and homoibotenic acid (HIBO). Thus, after administration of quisqualate these analogues become active at concentrations at which they are otherwise inactive. The mechanisms behind quisqualate-induced sensitisation are poorly understood and have not previously been quantified properly. In this study, we have tested the activity of a number of 4-alkyl- and 4-aryl-substituted analogues of HIBO as regards quisqualate-sensitisation, and present a method for quantifying the sensitisation induced by quisqualate at cortical neurones. These analogues are generally more potent and selective than (S)-AP4 or its homologue (S)-AP5 following quisqualate-sensitisation. Furthermore, we found a statistically significant correlation between the ligands' ability to inhibit CaCl(2)-dependent (S)-[(3)H]glutamate uptake into rat cortical synaptosomes, and their potency following quisqualate-induced depolarisation. This demonstrates the involvement of a transport system in the mechanism underlying the quisqualate-effect.

L-Quisqualic acid transport into hippocampal neurons by a cystine-sensitive carrier is required for the induction of quisqualate sensitization

A brief exposure of hippocampal slices to L-quisqualic acid sensitizes CA1 pyramidal neurons 30-250-fold to depolarization by two classes of excitatory amino acid analogues: (1) those whose depolarizing effects are rapidly terminated following washout, e.g. L-2-amino-4-phosphonobutanoic acid (L-AP4) and L-2-amino-6-phosphonohexanoic acid (L-AP6) and (2) those whose depolarizing effects persist following washout, e.g. L-aspartate-beta-hydroxamate (L-AbetaH). This process has been termed quisqualate sensitization. In this study we directly examine the role of amino acid transport systems in the induction of quisqualate sensitization. We report that L-quisqualate is a low-affinity substrate (K(M)=0.54 mM) for a high capacity (V(max)=0.9 nmol (mg protein)(-1) min(-1)) Na(+)-dependent transport system(s) and a high-affinity substrate (K(M)=0.033 mM) for a low-capacity (V(max)=0.051 nmol (mg protein)(-1) min(-1)) transporter with properties similar to the cystine/glutamate exchange carrier, System x(c-). We present evidence that suggests that System x(c-) participates in quisqualate sensitization. First, simultaneous application of L-quisqualate and inhibitors of System x(c-), but not inhibitors of Na(+)-dependent glutamate transporters, prevents the subsequent sensitization of hippocampal neurons to phosphonates or L-AbetaH. Second, L-quisqualic acid only sensitizes hippocampal neurons to other substrates of System x(c-), including cystine. Third, immunocytochemical analysis of L-quisqualate uptake demonstrates that only inhibitors of System x(c-) inhibit the highly concentrative uptake of L-quisqualate into a widely dispersed group of GABAergic hippocampal interneurons. We conclude that quisqualate sensitization is a direct consequence of the unique interaction of various excitatory amino acids, namely L-quisqualate, cystine, and phosphonates, with the exchange carrier, System x(c-). Therefore, the results of this study have important implications for the mechanism by which L-quisqualate, and other substrates of this transporter which are also excitatory amino acid agonists (such as glutamate and beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, beta-L-ODAP) may trigger neurotoxicity.

Some metabotropic glutamate receptor ligands reduce kynurenate synthesis in rats by intracellular inhibition of kynurenine aminotransferase II

Some metabotropic glutamate receptor (mGluR) ligands, such as quisqualate, L-(+)-2-amino-4-phosphonobutyric acid (L-AP4), 4-carboxy-3-hydroxyphenylglycine (4C3HPG), and L-serine-O:-phosphate (L-SOP), reduced the formation of the endogenous excitatory amino acid receptor antagonist kynurenate in brain and liver slices. The use of novel, subtype-selective mGluR agonists and antagonists excluded a role for any known mGluR subtype in this effect. The reduction of kynurenate formation was no longer observed when slices were incubated with the active mGluR ligands in the absence of extracellular Na(+). trans-Pyrrolidine-2,4-dicarboxylate (trans-PDC), a broad-spectrum ligand of Na(+)-dependent glutamate transporters, was also able to reduce kynurenate formation. Quisqualate, 4C3HPG, L-AP4, and L-SOP did not further reduce kynurenate formation in the presence of trans-PDC, suggesting that the two classes of drugs may share the same mechanism of action. Hence, we hypothesized that the active mGluR ligands are transported inside the cell and act intracellularly to reduce kynurenate synthesis. We examined this possibility by assessing the direct effect of mGluR ligands on the activity of kynurenine aminotransferases (KATs) I and II, the enzymes that transaminate kynurenine to kynurenate. In brain tissue homogenates, KAT II (but not KAT I) activity was inhibited by quisqualate, 4C3HPG, L-AP4, L-SOP, and trans-PDC. Drugs that were unable to reduce kynurenate formation in tissue slices were inactive. We conclude that some mGluR ligands act intracellularly, inhibiting KAT II activity and therefore reducing kynurenate formation. This effect should be taken into consideration when novel mGluR ligands are developed for the treatment of neurological and psychiatric diseases.

Cyclobutane quisqualic acid analogues as selective mGluR5a metabotropic glutamic acid receptor ligands

The conformationally constrained cyclobutane analogues of quisqualic acid (Z)- and (E)-1-amino-3-[2'-(3',5'-dioxo-1',2', 4'-oxadiazolidinyl)]cyclobutane-1-carboxylic acid, compounds 2 and 3, respectively, were synthesized. Both 2 and 3 stimulated phosphoinositide (PI) hydrolysis in the hippocampus with EC50 values of 18 +/- 6 and 53 +/- 19 microM, respectively. Neither analogue stimulated PI hydrolysis in the cerebellum. The effects of 2 and 3 were also examined in BHK cells which expressed either mGluR1a or mGluR5a receptors. Compounds 2 and 3 stimulated PI hydrolysis in cells expressing mGluR5a but not in those cells expressing mGluR1a. The EC50 value for 2 was 11 +/- 4 microM, while that for 3 was 49 +/- 25 microM. Both 2 and 3 did not show any significant effect on cells expressing the mGluR2 and mGluR4a receptors. In addition, neither compound blocked [3H]glutamic acid uptake into synaptosomal membranes, and neither compound was able to produce the QUIS effect as does quisqualic acid. This pharmacological profile indicates that 2 and 3 are selective ligands for the mGluR5a metabotropic glutamic acid receptor.

The L-AP4 receptor

1. The L-2-amino-4-phosphonobutyric acid (L-AP4) receptor was originally discovered by the ability of L-AP4 to depress synaptic transmission in hippocampal glutamatergic pathways and in the retina. 2. The molecular identity of the L-AP4 receptor is not yet resolved; however, with the molecular cloning of subtypes of metabotropic glutamate receptors (mGluRs), high affinity targets for L-AP4 have been identified. 3. As the information on the pharmacology of the mGluRs and the electrophysiological and biochemical studies on L-AP4 receptor physiology becomes elaborated it seems evident that the L-AP4 receptor is not a single molecular target but may involve multiple receptor subtypes.

Quisqualate-preferring metabotropic glutamate receptor activates Na(+)-Ca2+ exchange in rat basolateral amygdala neurones

1. Inward currents evoked by metabotropic glutamate receptor (mGlu) agonists quisqualate and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) were characterized in the basolateral nucleus of the amygdala. Currents were recorded with whole-cell patch electrodes in the presence of D-2-amino-5-phosphonovaleric acid (D-APV, 50 microM), 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX, 30 microM) and tetrodotoxin (TTX, 1 microM). 2. When recording with K+ electrodes, quisqualate (10-50 microM) produced an inward current which was not associated with a significant change in membrane slope conductance (Gm) and was insensitive to Ba2+ (0.2 mM) and Cs+ (2 mM). The 1S,3R-ACPD (50-200 microM)-induced inward current was associated with a decreased Gm and reversed polarity around -95 mV. However, in Ba2+ and Cs+, the 1S,3R-ACPD inward current amplitude was enhanced and was not accompanied by a change in Gm, a response similar to that evoked by quisqualate. 3. Glutamate (1 mM) and the group I mGlu specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 microM) also evoked currents not associated with a change in Gm. 4. When recorded with Cs+ electrodes in external Ba2+ and Cs+ solution, quisqualate activated an inward current more potently than 1S,3R-ACPD, suggesting that this current is preferentially activated by quisqualate. The mGlu agonist-induced inward current was not accompanied by a Gm change under these conditions. 5. Substitution of extracellular Na+ with Li+ (117 or 50 mM) or with 100 mM choline reduced the quisqualate- and 1S,3R-ACPD-induced inward currents, results consistent with mediation by Na(+)-Ca2+ exchange. 6. The quisqualate- and 1S,3R-ACPD-induced inward currents were reduced in Ca(2+)-free EGTA (1 mM) solution and prevented by including the Ca2+ chelating agent BAPTA (10 mM) in the recording electrode. In low-Ca2+ (100 microM)- and Cd2+ (200 microM)-containing solution to block voltage-gated Ca2+ currents, the quisqualate-induced current was not altered, but the 1S,3R-ACPD inward current was blocked. These data suggest that the quisqualate- and 1S,3R-ACPD-induced currents are mediated through a rise in intracellular Ca2+ and require extracellular Ca2+, but that the 1S,3R-ACPD current may depend on Ca2+ influx via voltage-gated Ca2+ channels. 7. The quisqualate current with no Gm change was inhibited by including the Na(+)-Ca2+ exchange inhibitory peptide (XIP; 10 microM) in the K+ recording electrode. XIP did not prevent the outward current evoked by baclofen (10 microM) or the 1S,3R-ACPD-induced inward current associated with decreased conductance. 8. These data are consistent with the hypothesis that quisqualate and 1S,3R-ACPD in Ba2+ and Cs+ solution activate a Na(+)-Ca2+ exchange current not associated with a conductance change. The quisqualate exchange current mediated through a group I mGlu may result from mobilization of Ca2+ from intracellular stores. The 1S,3R-ACPD exchange current requires extracellular Ca2+ passing through voltage-gated Ca2+ channels and may be mediated through a different receptor.

Phenylglycines can evoke quisqualate-primed depolarizations in rat cingulate cortex: an effect associated with [3H]DL-AP4 uptake

Depolarization could be evoked in slices of rat cingulate cortex by the normally non-excitatory compound L-2-amino-4-phosphonobutyrate (L-AP4) if the slices had been sensitized by exposure to quisqualate. The magnitude of the response to L-AP4 was dependent on the concentrations of both L-AP4 and quisqualate and was inhibited by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor antagonism. A series of phenylglycine analogues were capable of evoking similar dose-dependent depolarizations in the rat cingulate cortex following quisqualate sensitization, the most potent being (S)-4-carboxy-3-hydroxyphenylglycine. If the superfusate collected during application of (S)-4-carboxy-3-hydroxyphenylglycine to a quisqualate-sensitized slice was administered to a slice not previously exposed to quisqualate, a small depolarization was obtained. All the compounds shown to be capable of evoking the quisqualate-sensitized response showed affinity for the L-AP4 uptake site whilst having no affinity at ionotropic glutamate receptors and different profiles of activity at metabotropic glutamate receptors. None of the compounds was active at the metabotropic glutamate 4a receptor. There was a statistically significant correlation between a compound's effectiveness in inhibiting [3H]DL-AP4 uptake into rat cortical synaptosomes and its potency in evoking quisqualate-sensitized depolarization. It is concluded that this response may be the result of hetero-exchange between L-AP4 ligands and quisqualate.

Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of L-alpha-aminoadipate (L-alpha AA) is thought to involve its entry into glia as a substrate for glutamate transporters or, alternatively, its ability to inhibit glial glutamate transport. To clarify the properties of L-alpha AA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of L-[3H] alpha AA uptake in rat cortical astrocytes. We observed two components of saturable L-alpha AA uptake, one Na(+)-dependent and the other Na(+)-independent. These components exhibited the characteristics of system X-AG, the widespread family of Na(+)-cotransporters of aspartate and glutamate, and system x-c, a Cl(-)-dependent glutamate/cystine exchanger, respectively. The K(m) value of Na(+)-dependent L-alpha AA uptake was 629 +/- 42 microM, and Vmax was 62 +/- 4 nmol.min-1.mg-1 protein, which was more than twice the capacity of Na(+)-dependent glutamate uptake. The kinetic parameters of Na(+)-dependent L-alpha AA uptake (K(m) of 20 +/- 2 microM, Vmax of 1.7 +/- 0.4 nmol.min-1.mg-1 protein did not differ from the values for Na(+)-independent glutamate uptake, indicating that L-alpha AA and glutamate are equally good substrates for system x-c.

Effects of quisqualic acid analogs on metabotropic glutamate receptors coupled to phosphoinositide hydrolysis in rat hippocampus

L-Glutamic acid (L-Glu) and L-aspartic acid (L-Asp) activate several receptor subtypes, including metabotropic Glu receptors coupled to phosphoinositide (PI) hydrolysis. Quisqualic acid (Quis) is the most potent agonist of these receptors. There is evidence that activation of these receptors may cause a long lasting sensitization of neurons to depolarization, a phenomenon called the Quis effect. The purpose of the current studies was to use Quis analogs and the recently identified metabotropic receptor antagonist, (+)-alpha-methyl-4-carboxy-phenylglycine((+)-MCPG), to define the structural properties required for interaction with the metabotropic receptors coupled to PI hydrolysis and to determine if the Quis effect is mediated by these receptors. The effects of Quis analogs on PI hydrolysis were studied in the absence or presence of the metabotropic receptor-specific agonist 1SR,3RS-1-amino-1,3-cyclopentanedicarboxylic acid (1SR,3RS-ACPD) in neonatal rat hippocampus. Some of the compounds that induce the Quis effect also stimulate PI hydrolysis, including Quis itself and 9 (homoquisqualic acid). Not all of the Quis analogs that stimulate PI hydrolysis, however, induce the Quis effect, including 7A (EC50 = 750 +/- 150 microM) and (RS)-4-bromohomoibotenic acid (BrHI) (EC50 = 130 +/- 40 microM). Although (+)-MCPG blocked PI hydrolysis stimulated by Quis (IC50 = 370 +/- 70 microM), it had no effect on the induction of the Quis effect. Other Quis analogs did not stimulate PI hydrolysis but rather blocked the effects of 1SR,3RS-ACPD. The IC50 values were 240 +/- 70 microM for 2, 250 +/- 90 microM for 3, and 640 +/- 200 microM for 4. Data for inhibition by 2 and 3 were consistent with non-competitive mechanisms of action. These studies provide new information about the structural features of Quis required for interaction with metabotropic receptors coupled to PI hydrolysis and provide evidence that the Quis effect is not mediated by (+)-MCPG sensitive subtypes of these receptors.

Novel agonists for metabotropic glutamate receptors: trans- and cis-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine (trans- and cis-MCG-I)

New derivatives of 2-(carboxycyclopropyl)glycine (CCG), (2S,1'S,2'R,3'S)- and (2S,1'S,2'R,3'R)-2-(2-carboxy-3-methoxymethylcyclopropyl) glycine (trans- and cis-MCG-I), effectively inhibited forskolin-stimulated cyclic AMP formation in a concentration dependent manner in cultured spinal neurones of rats. They effectively depressed monosynaptic excitation in the spinal reflex of newborn rats with IC50 values of 0.3 and 3 microM, respectively, which was sensitive to (+)-MCPG. They did not cause any depolarization even when the concentration was increased up to 0.3 mM. However, after treatment with quisqualate, cis-MCG-I caused a depolarization of motoneurones in the newborn rat spinal cord in a concentration dependent manner with a threshold concentration of 1 microM (quisqualate effect). The depolarizing activity developed after quisqualate treatment gradually decreased but lasted for more than 2 hr. The depolarization induced by cis-MCG-I seemed pharmacologically similar to that of phosphonate-containing analogues of glutamate such as L-AP4 or L-AP6 under the "quisqualate effect". These novel CCG derivatives would be expected to provide useful probes for elucidating the physiological function of mGluRs.

Pharmacology of selective and non-selective metabotropic glutamate receptor agonists at L-AP4 receptors in retinal ON bipolar cells

Retinal ON bipolar cells possess metabotropic glutamate receptors (mGluRs) which are sensitive to L-2-amino-4-phosphonobutyric acid (L-AP4). Recent studies suggest there are multiple subtypes of L-AP4 receptors. In order to provide a more complete description of the pharmacology of the retinal L-AP4 receptor, we examined the actions of a number of compounds which are active at L-AP4 receptors and other mGluRs. Four groups of compounds were studied: (1) AP4 analogues (e.g. L-AP5, L-SOP, cyclobutylene AP5, and N-Me-AP4), (2) non-selective mGluR agonists (ibotenate and quisqualate), (3) selective mGluR agonists (L-CCG-I), and (4) agonists proposed to be selective for specific mGluR subtypes (DCG-IV and t-ADA). Concentration-response curves were obtained using the b-wave of the electroretinogram (ERG) as an assay for L-AP4 receptor activation. Whole cell voltage clamp recordings from ON bipolar cells in the retinal slice preparation of the mudpuppy were used to determine whether the compounds acted as L-AP4 receptor agonists. All compounds were L-AP4 receptor agonists, except t-ADA which was ineffective. The results reveal pharmacological differences between L-AP4 receptors in mudpuppy ON bipolar cells and those in other systems, consistent with the proposal that there are multiple L-AP4 receptor subtypes. For example, retinal L-AP4 receptors are more potently activated by L-AP5 than L-SOP, whereas L-SOP has been shown to be more potent than L-AP5 in L-AP4 receptors in the lateral perforant path (LPP) of the rat hippocampus. L-SOP is also relatively more potent at the cloned L-AP4 receptors mGluR4, 6, and 7 than in mudpuppy ON bipolar cells in situ. The different potencies of these compounds in retina and LPP is ascribed to both steric and charge factors. The results with DCG-IV and t-ADA are consistent with the proposal that these are subtype-selective agonists, but DCG-IV is likely to be selective only at very low concentrations (< or = 1 microM).

Immunocytochemical evidence that quisqualate is selectively internalized into a subset of hippocampal neurons

Quisqualic acid (QUIS) has been shown to interact with several glutamate receptor subtypes and uptake sites. We have previously demonstrated that a brief exposure of hippocampal cells to QUIS sensitizes them to depolarization by the alpha-amino-omega-phosphonate analogues of glutamate, AP4, AP5, and AP6. This QUIS-induced sensitization is accompanied by the active uptake of QUIS into hippocampal slices. In order to localize the sites of QUIS uptake into rat hippocampal slices, a polyclonal antibody against QUIS was raised in rabbits. Utilizing immunocytochemical techniques, we have identified immunoreactive axons and dendrites after brief exposure times to QUIS, and perikarya after longer exposure times to QUIS. The intensity of the QUIS immunoreactivity increased as the exposure time to QUIS increased. QUIS immunoreactivity was primarily found in stratum oriens and stratum radiatum, of regions CA1, CA2, and CA3 of the hippocampus as well as in the hilus and molecular layer of the dentate gyrus. The distribution and morphology of QUIS immunoreactive cells appeared to be similar to those of GABAergic interneurons. Glial fibrillary acidic protein (GFAP) did not co-localize with the QUIS-internalizing cells suggesting that they are not glia. Ultrastructural analysis revealed QUIS immunoreactive profiles within the stratum radiatum. Immunostained profiles at both the light and EM levels appeared, in many cases, to be swollen and showed signs of degeneration. Such changes were only evident in tissue exposed to QUIS. These data demonstrate that QUIS is taken up by a select group of neurons in the rat hippocampus.

Utilization of the resolved L-isomer of 2-amino-6-phosphonohexanoic acid (L-AP6) as a selective agonist for a quisqualate-sensitized site in hippocampal CA1 pyramidal neurons

Brief exposure of rat hippocampal slices to quisqualic acid (QUIS) sensitizes neurons to depolarization by the alpha-amino-omega-phosphonate excitatory amino acid (EAA) analogues AP4, AP5 and AP6. These phosphonates interact with a novel QUIS-sensitized site. Whereas L-AP4 and D-AP5 cross-react with other EAA receptors, DL-AP6 has been shown to be relatively selective for the QUIS-sensitized site. This specificity of DL-AP6, in conjunction with the apparent preference of this site for L-isomers, suggested that the hitherto unavailable L-isomer of AP6 would be a potent and specific agonist. We report the resolution of the D- and L-enantiomers of AP6 by fractional crystallization of the L-lysine salt of DL-AP6. We also report the pharmacological responses of kainate/AMPA, NMDA, lateral perforant path L-AP4 receptors and the CA1 QUIS-sensitized site to D- and L-AP6, and compare these responses to the D- and L-isomers of AP3, AP4, AP5 and AP7. The D-isomers of AP4, AP5 and AP6 were 5-, 3- and 14-fold less potent for the QUIS-sensitized site than their respective L-isomers. While L-AP4 and L-AP5 cross-reacted with NMDA and L-AP4 receptors, L-AP6 was found to be highly potent and specific for the QUIS-sensitized site (IC50 = 40 microM). Its IC50 values for kainate/AMPA, NMDA and L-AP4 receptors were > 10, 3 and 0.8 mM, respectively. As with AP4 and AP5, sensitization to L-AP6 was reversed by L-alpha-aminoadipate.