Excitatory amino acid agonist-antagonist interactions at 2-amino-4-phosphonobutyric acid-sensitive quisqualate receptors coupled to phosphoinositide hydrolysis in slices of rat hippocampus

Natural Products with Heteroatom-Rich Ring Systems

This review focuses on all known natural products that contain a "heteroatom-rich" ring system, specifically a five-, six- or seven-membered ring that contains three or more heteroatoms. The isolation and biological activity of these natural products is discussed, along with the biosynthetic processes that Nature employs to assemble these rare heterocyclic frameworks.

Differential regulation of metabotropic glutamate receptor 5-mediated phosphoinositide hydrolysis and extracellular signal-regulated kinase responses by protein kinase C in cultured astrocytes

The metabotropic glutamate receptor 5 (mGluR5) exhibits a rapid loss of receptor responsiveness to prolonged or repeated agonist exposure. This receptor desensitization has been seen in a variety of native and recombinant systems, and is thought to result from receptor-mediated, protein kinase C (PKC)-dependent phosphorylation of the receptor, uncoupling it from the G protein in a negative feedback regulation. We have investigated the rapid PKC-mediated desensitization of mGluR5 in cortical cultured astrocytes by measuring downstream signals from activation of mGluR5. These include activation of phosphoinositide (PI) hydrolysis, intracellular calcium transients, and extracellular signal-regulated kinase 2 (ERK2) phosphorylation. We present evidence that PKC plays an important role in rapid desensitization of PI hydrolysis and calcium signaling, but not in ERK2 phosphorylation. This differential regulation of mGluR5-mediated responses suggests divergent signaling and regulatory pathways which may be important mechanisms for dynamic integration of signal cascades.

Modulation of VIP-Stimulated cAMP Formation by Excitatory Amino Acids in Mouse Cerebral Cortex

We have investigated the modulatory action of excitatory amino acids (EAA) on vasoactive intestinal polypeptide (VIP)-stimulated cAMP formation in mouse cerebral cortical slices. Glutamate and aspartate potentiate in a concentration-dependent manner the effect of VIP. In order to characterize the type of receptor involved, we have used three prototypical EAA receptor agonists, that is, kainate, N-methyl-d-aspartate (NMDA) and quisqualate. Kainate mimicked the effect of glutamate, NMDA was inactive and quisqualate displayed an inhibitory action. Furthermore, ibotenate also potentiated the effect of VIP on cAMP formation, while l-homocysteate exhibited an inhibitory action. Ibotenate was 4-fold more potent and 2.5 times more effective than glutamate. However, the effects of kainate and ibotenate were not additive, suggesting the activation of a common receptor. Thus, based on this metabotropic action, EAA can be categorized into the following classes: (i) those that potentiate the effect of VIP, such as glutamate, aspartate, kainate and ibotenate; (ii) those that inhibit the effect of VIP, such as l-homocysteate and quisqualate; and (iii) those that are ineffective, such as NMDA and d-homocysteate. The effects of glutamate or ibotenate on VIP-stimulated cAMP formation were completely inhibited by l-phosphoserine and only partially by kynurenate. In a low chloride medium, or in the presence of 8-(N,N-diethylamino) octyl-3,4,5-trimethoxybenzoate-hydrochloride (TMB-8), an inhibitor of calcium release from internal stores, EAA did not potentiate the effect of VIP, thus stressing the importance of these ions for the transduction of the glutamatergic signal. Our results indicate the existence of marked interactions between EAA and VIP on cAMP formation; the pharmacology of these interactions is, however, clearly distinct from the classical pharmacology of EAA which is mainly based on electrophysiological and binding studies.

Reducing conditions differentially affect the functional and structural properties of group-I and -II metabotropic glutamate receptors

We have examined the influence of reducing conditions on the activity of group-I or -II metabotropic glutamate receptors. In cultured cerebellar granule cells or in hippocampal slices, the reducing agent dithiothreitol (DTT) inhibited the stimulation of polyphosphoinositide (PPI) hydrolysis elicited by group-I mGlu receptor agonists without affecting responses to norepinephrine or carbamylcholine. Similarly, DTT reduced the increase in intracellular free Ca(2+) induced by glutamate in HEK-293 cells expressing mGlu5 receptors. In adult hippocampal slices, the selective group-II mGlu receptor agonist, (2S,1'R,2'R,3'R)-2-(2, 3-dicarboxycyclopropyl)glycine (DCG-IV) had no effect per se on PPI hydrolysis, but potentiated the response to quisqualate. Although DTT substantially attenuated the action of quisqualate, it did not affect the potentiation by DCG-IV, suggesting that group-II mGlu receptors are resistant to extracellular reduction. Accordingly, DTT did not affect the inhibition of forskolin-stimulated cAMP formation induced by maximally effective concentrations of group-II mGlu receptor agonists in hippocampal slices or in CHO cells expressing mGlu2 receptors. At structural level, DTT differentially affected the aggregation state of mGlu1a, -2/3 or -5 receptors. In immunoblots performed under non-reducing conditions, mGlu1a, -2/3 or -5 antibodies labeled exclusively a high-molecular weight band, corresponding to receptor dimers. Under reducing conditions, mGlu1a or -5 receptors were detected as monomers, whereas a large proportion of mGlu2/3 receptors was still present in a dimeric form. We conclude that reducing conditions differentially influence the aggregation state of group-I and -II mGlu receptors and suggest that dimerization affects the functional activity of native mGlu receptors.

Pharmacological agents acting at subtypes of metabotropic glutamate receptors

Metabotropic (G-protein-coupled) glutamate (mGlu) receptors have now emerged as a recognized, but still relatively new area of excitatory amino acid research. Current understanding of the roles and involvement of mGlu receptor subtypes in physiological/pathophysiological functions of the central nervous system has been recently propelled by the emergence of various structurally novel, potent, and mGlu receptor selective pharmacological agents. This article reviews the evolution of pharmacological agents that have been reported to target mGlu receptors, with a focus on the known receptor subtype selectivities of current agents.

Mediation and modulation by eicosanoids of responses of spinal dorsal horn neurons to glutamate and substance P receptor agonists: results with indomethacin in the rat in vivo

In view of the widespread use of non-steroidal anti-inflammatory drugs for treatment of inflammatory pain, we determined the effects of the non-steroidal anti-inflammatory drug, indomethacin, on dorsal horn neurons in the rat spinal cord in vivo. At 2.0-12.0 mg/kg (i.v.), indomethacin depressed the responses of spinal dorsal horn neurons to the effects of iontophoretic application of substance P, N-methyl-D-aspartate, quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. As indomethacin inhibits cyclo-oxygenase, these are the first data linking prostanoids and possibly arachidonic acid and other eicosanoids to the effects of substance P and glutamate in the spinal dorsal horn. As responses to iontophoretic application can be assumed to have been postsynaptic and as indomethacin had an effect generalized to all excitatory responses, we suggest a postsynaptic site for cyclo-oxygenase. We also suggest that elements in the cyclo-oxygenase signal transduction pathway may thus mediate at least some of the effects of substance P and glutamate receptor activation. Activation of the cyclo-oxygenase pathway in CNS neurons is Ca2- dependent, and activation of both N-methyl-D-aspartate and substance P receptors increases intracellular Ca2+. This led to the expectation that indomethacin would have a greater effect on responses to N-methyl-D-aspartate than to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, but the reverse was observed. Thus, in addition to a mediator role, we hypothesize that an element(s) of the cyclo-oxygenase pathway may regulate the efficacy of excitation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors and perhaps other membrane-bound receptors. The cyclo-oxygenase signal transduction pathway thus appears to play at least two major roles in regulation of sensory processing in the spinal cord. Therefore, non-steroidal anti-inflammatory drugs, via cyclo-oxygenase inhibition, may have multiple actions in control of spinal sensory mechanisms.

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.

Metabotropic and ionotropic glutamate receptors regulate calcium channel currents in salamander retinal ganglion cells

1. Glutamate suppressed high-voltage-activated barium currents (IBa, HVA) in tiger salamander retinal ganglion cells. Both ionotropic (iGluR) and metabotropic (mGluR) receptors contributed to this calcium channel inhibition. 2. Trans-ACPD (1-aminocyclopentane-trans-1S,3R-dicarboxylic acid), a broad-spectrum metabotropic glutamate receptor agonist, suppressed a dihydropyridine-sensitive barium current. Kainate, an ionotropic glutamate receptor agonist, reduced an omega-conotoxin GVIA-sensitive current. 3. The relative effectiveness of selective agonists indicated that the predominant metabotropic receptor was the L-2-amino-4-phosphonobutyrate (L-AP4)-sensitive, group III receptor. This receptor reversed the action of forskolin, but this was not responsible for calcium channel suppression. l-AP4 raised internal calcium concentration. Antagonists of phospholipase C, inositol trisphosphate (IP3) receptors and ryanodine receptors inhibited the action of metabotropic agonists, indicating that group III receptor transduction was linked to this pathway. 4. The action of kainate was partially suppressed by BAPTA, by calmodulin antagonists and by blockers of calmodulin-dependent phosphatase. Suppression by kainate of the calcium channel current was more rapid when calcium was the charge carrier, instead of barium. The results indicate that calcium influx through kainate-sensitive glutamate receptors can activate calmodulin, which stimulates phosphatases that may directly suppress voltage-sensitive calcium channels. 5. Thus, ionotropic and metabotropic glutamate receptors inhibit distinct calcium channels. They could act synergistically, since both increase internal calcium. These pathways provide negative feedback that can reduce calcium influx when ganglion cells are depolarized.

Evidence for functional metabotropic glutamate receptors in the dorsal cochlear nucleus

The parallel fibers (PFs) of the dorsal cochlear nucleus (DCN) molecular layer use glutamate as a neurotransmitter. Although metabotropic glutamate receptors (mGluRs) have been identified on cells postsynaptic to the PFs, little is known about the effects of mGluR activation in PF synaptic transmission in the DCN. To investigate these effects, PF-evoked field potentials were recorded from the DCN in guinea pig brain stem slice preparations. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated components of the field response were reversibly depressed by bathing the slice in the mGluR agonists (+/-)-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]. A similar depression was produced by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine, but not by the mGluR2/3 agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine or by the mGluR4/6/7/8 agonist L(+)-2-amino-4-phosphonobutyric acid. In addition to the AMPA component, an N-methyl-D-aspartate (NMDA) receptor-dependent component of the field potentials could be identified when the slices were bathed in a low magnesium solution. Under these conditions, the ACPD-induced depression of the AMPA component did not completely recover, whereas the depression of the NMDA component usually recovered and potentiated in some slices. Intracellular recordings of PF-evoked responses were obtained to ascertain which neuronal populations were affected by mGluR activation. Activation of mGluRs produced a reversible depression of PF-evoked responses in cartwheel cells that was not accompanied by any changes in paired-pulse facilitation. The PF-evoked responses recorded from pyramidal cells were unaffected by mGluR activation. Both cell types exhibited a reversible depolarization during (1S,3R)-ACPD application. Subsequent experiments explored the involvement of protein kinases in mediating the effects of mGluRs. The protein kinase C (PKC) activator phorbol-12,13-diacetate partially inhibited the mGluR-mediated depression of the field response; however, the PKC inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide or the protein kinase A inhibitor N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide had little effect on the actions of (1S,3R)-ACPD. These results demonstrate that functional mGluRs are present at PF synapses and are capable of modulating PF synaptic transmission in the DCN.

Metabotropic glutamate receptor subtypes mediating slow inward tail current (IADP) induction and inhibition of synaptic transmission in olfactory cortical neurones

1. The pharmacological features of the pre- and postsynaptic metabotropic glutamate receptors (mGluRs) present in the guinea-pig olfactory cortex, were examined in brain slices in vitro by use of a conventional intracellular current clamp/voltage clamp recording technique. 2. Bath-application of trans-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) (50 microM) produced a sustained membrane depolarization, increase in cell excitability and induction of a post-stimulus inward (after depolarizing) tail current (IADP) (measured under 'hybrid' voltage clamp) similar to those evoked by the muscarinic receptor agonist oxotremorine-M (OXO-M, 2 microM). 3. L-Glutamate (0.25 1 mM. in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM-DL-amino-5-phosphono valeric acid (DL-APV)) or the broad spectrum mGluR agonists 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), 1S,3S-ACPD (50 microM), ibotenate (Ibo; 25 microM. in the presence of 100 microM DL-APV), the selective mGluR I agonists (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG, 10 microM), (S)-3-hydroxyphenylglycine ((S)-3HPG, 50 microM), or quisqualate (10 microM, in the presence of 20 microM CNQX), but not the mGluR II agonist 2S,1'S,2'S-2-(2'-carboxycyclopropyl)-glycine (L-CCG1,1 microM) or mGluR III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 1 mM), were all effective in producing membrane depolarization and inducing a post-stimulus IADP. Unexpectedly, the proposed mGluR II-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG-IV, 10 microM, in the presence of 100 microM DL-APV) was also active. 4. The excitatory effects induced by 10 microM 1S,3R-ACPD were reversibly antagonized by the mGluR I/II antagonist (1)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG, 0.5 1 mM), as well as the selective mGluR I antagonists (S)-4-carboxyphenylglycine ((S)-4CPG) and (S)-4-carboxy-3-hydroxyphenyl glycine ((S)-4C3HPG) (both at 1 mM), but not the nonselective mGluR antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP3, 1 mM) or the selective mGluR III antagonist (S)-alpha-methyl-L-AP4 (MAP4, 1 mM). 5. The excitatory postsynaptic potentials (e.p.s.ps), induced by single focal stimulation of cortical excitatory fibre tracts, were markedly reduced by 1S,3R-ACPD or L-AP4 (both at 10 microM), and by the selective mGluR II agonists (mGluR 1 antagonists) (S)-4CPG or (S)-4C3HPG (both at 1 mM) but not (S)-3,5-DHPG or (S)-3HPG (both at 100 microM). 6. The inhibitory effects of 1S-3R-ACPD, but not L-AP4, were reversibly blocked by (+)-MCPG (1 mM), whereas those produced by L-AP4, but not 1S,3R-ACPD, were blocked by the selective mGluR III antagonist MAP4 (1 mM). 7. It is concluded that a group I mGluR is most likely involved in mediating excitatory postsynaptic effects, whereas two distinct mGluRs (e.g. group II and III) might serve as presynaptic inhibitory autoreceptors in the guinea-pig olfactory cortex.

Activation of group III metabotropic glutamate receptors is neuroprotective in cortical cultures

(RS)-alpha-Methyl-4-phosphonophenylglycine (MPPG) and (S)-alpha-methyl-3-carboxyphenylalanine (M3CPA), two novel preferential antagonists of group III metabotropic glutamate (mGlu) receptors, antagonized the neuroprotective activity of L-2-amino-4-phosphono-butanoate (L-AP4) or L-serine-O-phosphate in mice cultured cortical cells exposed to a toxic pulse of N-methyl-D-aspartate. In contrast, MPPG did not influence the neuroprotective activity of the selective group II mGlu receptor agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxy-cyclopropyl) glycine (DCG-IV). These results indicate that activation of group III mGu receptors exerts neuroprotective activity against excitotoxic neuronal death. At least one of the two major group III mGlu receptor subtypes, i.e. mGlu4 receptor, is expressed by cultured cortical neurons, as shown by immunocytochemical analysis with specific polyclonal antibodies.

Effects of excitatory amino acids on phosphoinositide metabolism in frog retina

The effects of excitatory amino acid receptor agonists on the hydrolysis of phosphoinositides were examined using frog retinal membranes prelabeled in vitro with either 32PO4 or [3H]inositol. Glutamate stimulated release of [3H]inositol phosphates (IPs) from the retinas and altered the 32P-labeling pattern of phosphatidylinositol (PI) cycle intermediates. This indicates that glutamate affects not only the hydrolysis of phosphoinositides but possibly other steps involved in the PI cycle. Among glutamate analogs, kainate (KA), quisqualate (QA), and, to a lesser extent, N-methyl-D-aspartate (NMDA) mimicked the glutamate effect, whereas L-2-amino-4-phosphonobutyrate (L-AP4) was not effective in causing either the accumulation of [3H]IPs or the alteration of the 32P-labeling pattern of PI cycle intermediates. Among QA specific agonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), but not ibotenate (IBO) or trans-1-aminocyclopentane-1,3-dicarboxylate (ACPD) was active in stimulating IPs formation. KA effect on IPs formation may be due to indirect (polysynaptic) activation of receptor(s) other than L-AP4, IBO, or ACPD specific QA receptors. To avoid activating polysynaptic pathways, retinal synaptoneurosomes prelabeled with [3H]inositol were used to examine the hydrolysis of phosphoinositides. As in whole retinas, KA, carbachol (CARB), and NMDA stimulated the release of IPs while L-AP4 had minimal effect. Glycine (GLY) had no effect. Our results show CARB and KA to be the most effective in stimulating the production of IPs. Their effects were exerted directly through separate receptors and not through polysynaptic pathways. ACPD and IBO were the least effective in eliciting the release of IPs. Our studies provide evidence that ionotropic and not metabotropic glutamate receptors are involved in PI metabolism in the retina.

Comparing the role of metabotropic glutamate receptors in long-term potentiation and in learning and memory

1. Neuronal plasticity has been suggested to be the physical substrate for changes underlying the expression of memory. One model which has attracted wide attention as a possible candidate of such neuronal plasticity is long-term potentiation (LTP), mainly investigated in the hippocampus of rodents. Moreover, various processes with different time constants may underlie LTP, and these phases show striking correspondence to different phases of memory. 2. Pharmacological evidence strongly implicates that the neurotransmitter glutamate plays a major role in LTP. Although the involvement of ionotropic glutamate receptors has been proven, the role of the newly discovered metabotropic glutamate receptors is still uncertain. 3. Metabotropic glutamate receptors (mGluRs) comprise a whole family with currently eight members grouped into three classes according to their amino acid sequence identity and pharmacological profile. They are G-protein coupled, either positively linked to phospholipase C (class I) or negatively linked to adenylate cyclase (class II and III), and among other effects are known to induce phosphorylation of ionotropic glutamate receptors as well as modulate the excitability of neurons. Finally, they are heterogeneously distributed throughout the brain. 4. In hippocampal slice preparations, mGluRs have been shown to be involved in the induction of LTP in CA1 and dentate gyrus by some investigators, but others have failed to reproduce such experiments, leaving the question: what are the appropriate conditions for mGluR-mediated LTP? 5. In vivo, metabotropic receptor antagonists have been shown to block, and agonists to facilitate, induction and maintenance of LTP, mainly at perforant path/dentate granule cell synapses. As demonstrated in behavioral investigations, mGluRs apparently play an important part in hippocampus-dependent learning paradigms. As in LTP, antagonists block memory formation; in contrast to LTP, agonists also prevent memory formation. In memory recall metabotropic receptors seem to play no role. 6. Based on current information the authors develop models for a role of mGluRs in both LTP and memory formation. Activation of metabotropic receptors plays a particular modulatory role when high frequency stimulation is weak. Strong tetanization may bypass mGluRs by stimulating other systems leading to, at least phenomenologically, similar LTP, Behaviorally, mGluRs possibly set the signal to noise ratio of the hippocampal circuit.

Differential responsiveness of metabotropic glutamate receptors coupled to phosphoinositide hydrolysis to agonists in various brain areas of the adult rat

The effects of metabotropic glutamate receptor (mGluR) agonists on inositol phosphates (IP) accumulation were investigated in slices of the cerebral cortex, hippocampus, striatum and cerebellum of adult Sprague-Dawley rats. EC(50) values for 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) did not differ significantly between various brain areas (range 10(-5) M), quisqualate was the most potent in all the brain areas (range 10(-7) - 10(-6) M), except the cerebellum (10(-5) M), ibotenate was the most potent in the striatum (range 10(-6) M) and the least potent in the cerebral cortex and hippocampus (range 10(-4) M). The efficacy in the four brain areas showed the following trend of ranking order for ACPD and quisqualate: hippocampus > striatum > cerebral cortex > cerebellum, and for ibotenate: hippocampus > cerebral cortex > striatum > cerebellum, although the observed differences reached the level of statistical significance only in the case of ACPD (hippocampus and striatum vs cerebellum) and ibotenate (hippocampus vs cerebellum). Co-incubation of the agonists at maximally effective concentrations in any pairwise combination resulted in no substantial additivity of IP accumulation. D,L-1-amino-3-phosphonopropionic acid (AP3) and D,L-2-amino-4-phosphonobutyric acid (AP4) at 0.5 mM concentration antagonized ACPD-induced IP accumulation by about 70 and 45 percent, respectively, without differences between brain areas. On the other hand, the antagonistic effects of L-serine-o-phosphate (SOP) at 1 mM concentration were the highest in the hippocampus (75 percent) and the lowest in the cerebellum (25 percent). The comparative data indicate considerable regional receptor heterogeneity, in terms of different ratios of response to the agonists (but not antagonists, except SOP). There is a robust responsiveness of mGluRs not only in the hippocampus and cerebral cortex, but also in the striatum which exhibits the highest affinity to both quisqualate and ibotenate.

Differential effects of learning and recall of a spatial task on phosphoinositide hydrolysis induced by the metabotropic glutamate receptor agonist 1S,3R-ACPD (1S,3R-1-amino-cyclopentane-1,3-discarboxylic acid) in the hippocampus and the prefrontal cortex of rats

Phosphoinositide (PI) hydrolysis, stimulated by 1S,3R-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist of metabotropic glutamate receptors (mGluRs), was measured in hippocampal and prefrontal cortical slices obtained from rats which had been trained for 8 days in a Morris water maze and had learned an allocentric spatial task. Brain slices were pre-labeled with myo-3H-inositol and 1S,3R-ACPD (100 microM) stimulation was assessed by measuring the accumulation of [3H]inositol phosphates ([3H]IPs) in the presence of Li+. Measurements conducted 24 h following the last training session revealed no differences in 1S,3R-ACPD-stimulated formation of [3H]IPs, either in the hippocampus or in the prefrontal cortex. However, a diminished response to mGluRs stimulation was detected in the hippocampus of animals re-trained after an 11-day interval. The decrease was not evident in the prefrontal cortex. These data indicate a differential involvement of the hippocampus and the prefrontal cortex in the processing of spatial information and correspond to the functional differences attributed to these areas.

N-methyl-D-aspartic acid receptor agonists: resolution, absolute stereochemistry, and pharmacology of the enantiomers of 2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid

(R,S)-2-Amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid [(R,S)-AMAA, 4] is a potent and selective agonist at the N-methyl-D-aspartic acid (NMDA) subtype of excitatory amino acid receptors. Using the Ugi "four-component condensation" method, the two diastereomers (2R)- and (2S)-2-[3-(benzyloxy)-5-methyl-4-isoxazolyl]N-tert-butyl-2- [N-[(S)-1-phenylethyl]benzamido]-acetamide (16 and 17, respectively) were synthesized and separated chromatographically. The absolute stereochemistry of 16 was confirmed by an X-ray analysis. Deprotection of these intermediates did, however, provide (R)- (8) and (S)- (9) AMAA, respectively, in extensively racemized forms. N-BOC-protected (R,S)-AMAA (21) was successfully resolved via diastereomeric salt formation using cinchonidine. The stereochemical purity and stability of 8 and 9 obtained via this resolution were determined using chiral HPLC. (R)-AMAA (8) showed peak affinity for [3H]AMPA receptor sites (IC50 = 72 +/- 13 microM) and was shown to be a more potent inhibitor of [3H]CPP binding (IC50 = 3.7 +/- 1.5 microM) than (S)-AMAA (9) (IC50 = 61 +/- 6.4 microM). Neither enantiomer of AMAA affected [3H]kainic acid receptor binding significantly. In electrophysiological studies using rat brain tissue, 8 (EC50 = 7.3 +/- 0.3 microM) was 1 order of magnitude more potent than 9 (EC50 = 75 +/- 9 microM) as an NMDA receptor agonist.

The novel metabotropic glutamate receptor agonist 2R,4R-APDC potentiates stimulation of phosphoinositide hydrolysis in the rat hippocampus by 3,5-dihydroxyphenylglycine: evidence for a synergistic interaction between group 1 and group 2 receptors

The mGlu receptor subtypes and second messenger pathways that mediate 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) responses in brain tissues are not fully understood. 1S,3R-ACPD differs from 3,5-dihydroxyphenylglycine (DHPG) or quisqualate in that 1S,3R-ACPD also activates group 2 mGlu receptors (mGlu2 and mGlu3) that are negatively linked to cAMP formation. To investigate the contribution of group 2 mGlu receptor activity of 1S,3R-ACPD to the phosphoinositide response in the rat hippocampus, we examined the effects of the novel group 2 mGlu receptor agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC). 2R,4R-APDC did not activate or inhibit group 1 mGlu receptors (human mGlu1 alpha and mGlu5a) or group 3 mGlu receptors (human mGlu4 and mGlu7), but potently decreased forskolin-stimulated cAMP formation in human mGlu2- and mGlu3-expressing cells. In slices of the adult rat hippocampus 2R,4R-APDC had no effect on basal phosphoinositide hydrolysis; however, it was found to greatly enhance phosphoinositide hydrolysis to DHPG or quisqualate. In the neonatal rat hippocampus, 2R,4R-APDC enhanced the potency of DHPG, while not affecting the maximal response to group 1 mGlu receptor agonists. Thus, the phosphoinositide response in the rat hippocampus to 1S,3R-ACPD is mediated by a synergistic interaction between group 1 and group 2 mGlu receptors.

Neuroprotection and selective vulnerability of neurons within the nucleus basalis magnocellularis

Neurons within the nucleus basalis may die due to their selective vulnerability to endogenous excitatory amino acid neurotransmitters, nitric oxide and free radicals. The factors influencing the selective vulnerability of neurons within the nucleus basalis depend upon many different factors related to the presence of these agents and the neuron's ability to defend itself against the consequences of exposure. Many different mechanisms have been investigated to provide neuroprotection for neurons within the nucleus basalis and throughout the central nervous system. This review summarizes the results of studies that have investigated our current capability to either attenuate the neurotoxicity of endogenous excitatory amino acids, such as glutamate, or to provide effective neuroprotection during circumstances of neurotoxin exposure.

Excitatory amino acid-, except 1S,3R-ACPD, induced transient high stimulation of phosphoinositide metabolism during hippocampal neuron development

Rat hippocampal neurons in culture extended their neurites until day 5 in vitro (DIV). Then, the mean neuritic length slightly decreased. Excitatory amino acid (EAA)-elicited inositol phosphate (IP) formation increased from 0.5 to 2 DIV, reached a plateau between 2 and 4-5 DIV, and then gradually decreased until 10 DIV. This decrease was likely not due to neuronal death. This developmental pattern was observed for N-methyl-D-aspartate, kainate, glutamate, ibotenate and quisqualate (QA). Interestingly, the 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD) response slightly increased during neuronal culture development. At 3 DIV, the ionotropic antagonists 6,7-dinitro-quinoxalin-2,3-dion and D-2-amino-5-phosphonopentanoate efficiently blocked N-methyl-D-aspartate and kainate-elicited IP formation, and partially inhibited glutamate and ibotenate responses. QA and 1S,3R-ACPD responses were not affected, suggesting a metabotropic action for these two compounds. Furthermore, QA and 1S,3R-ACPD potencies significantly increased between 3 and 10 DIV. The transient high activity periods induced by EAA, except for 1S,3R-ACPD, are not observed for norepinephrine, carbachol and potassium chloride responses. Taken together, these data suggest that: (i) QA and 1S,3R-ACPD can act on two different glutamate metabotropic receptors subtypes during development; and (ii) the EAA-induced transient peaks of IP stimulation, which are specific with respect to other neuroactive substances profiles, could be involved in the development of hippocampal neurons. Indeed, these transient high activities take place when the neuritic length regularly increases in vitro.

Excitation of sympathetic preganglionic neurons via metabotropic excitatory amino acid receptors

The role of excitatory amino acid metabotropic receptors in the regulation of excitability of sympathetic preganglionic neurons was investigated. This study used both conventional intracellular and whole-cell patch clamp techniques to record from sympathetic preganglionic neurons in transverse spinal cord slices of the rat (9-21 days old). The metabotropic receptor agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (10-200 microM, superfused for 2-60 s) and quisqualate (1-50 microM, superfused for 2-60 s) induced concentration-dependent depolarizing responses which did not desensitize. These responses were unaffected by the glutamate ionotropic receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-50 microM), 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10 microM), dizocilpine (MK-801, 10-40 microM), 3-[(R)-2-carboxy-piperazin-4-yl]-propyl-1-phosphonic acid (D-CPP, 10-50 microM) and DL-2-amino-5-phosphonovaleric acid (DL-AP5, 20-100 microM). Depolarizing responses to 1S,3R-ACPD and quisqualate were unaffected by L-2-amino-3-phosphonopropionic acid (L-AP3, 30 microM-1mM) and L-2-amino-4-phosphonobutanoic acid (L-AP4, 100 microM-1 mM)). The responses to 1S,3R-ACPD and quisqualate were reduced by including the G-protein blocker GDP-beta-S (400 microM) in the patch pipette solution by 77 +/- 2% (mean +/- S.E) of control (n = 3), suggesting that these agonists activate a G-protein-coupled receptor. Metabotropic receptor-mediated responses were maintained in the presence of tetrodotoxin (500 nM), progressively reduced with increased membrane hyperpolarization to around -95 mV and associated with either an increase of 16.5 +/- 2.8% (data from four neurons) in the majority of neurons (n = 22 of 34) or no measurable change (n = 12) in neuronal input resistance. These data suggest that the agonists exert a direct action on 1S,3R-ACPD and quisqualate had several effects on sympathetic preganglionic neuron membrane properties including: inhibition of a slow apamin-insensitive component of the afterhyperpolarization; a reduction in spike frequency adaptation leading to increases in firing frequency from 6.4 +/- 2.8 Hz in control experiments up to 14.7 +/- 3.0 Hz (n = 6 neurons) in the presence of a metabotropic receptor agonist: a broadening of the action potential by 37.5 +/- 6.4% (n = 6 neurons) of control. These observations suggest that the metabotropic receptor-mediated depolarization is due, at least in part, to the reduction of potassium conductances involved in the spike afterhyperpolarisation potential.(ABSTRACT TRUNCATED AT 400 WORDS)

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced burst firing is mediated by a native pertussis toxin-sensitive metabotropic receptor at rat dorsolateral septal nucleus neurons

We have reported previously that a selective metabotropic glutamate receptor agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), caused two primary postsynaptic membrane changes, namely, a slow membrane depolarization, and burst firing in rat dorsolateral septal nucleus neurons. In addition, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid also potentiates a slow after depolarization in rat dorsolateral septal nucleus neurons. We now report that, among all the postsynaptic membrane changes induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, only the burst firing was selectively blocked by pertussis toxin pretreatment. Thus, aminocyclopentane-1,3-dicarboxylic acid induced burst firing was mediated by a metabotropic receptor coupled to a pertussis toxin-sensitive GTP-binding protein, while the other induced cellular responses may be mediated by metabotropic glutamate receptors insensitive to pertussis toxin. We further characterized this receptor pharmacologically. This metabotropic receptor is activated by several metabotropic glutamate receptor agonists, but is insensitive to L-glutamate or L-aspartate. On the basis of its agonist activity profile, particularly the ineffectiveness of glutamate as an agonist, we have tentatively assigned the name aminocyclopentane-1,3-dicarboxylic acid metabotropic receptor, to this native, pertussis toxin-sensitive metabotropic receptor in the dorsolateral septal nucleus. Furthermore, this receptor is coupled to protein kinase C, probably via a phospholipase C independent pathway.

Breakdown of membrane phospholipids in Alzheimer disease. Involvement of excitatory amino acid receptors

Membrane phospholipids are not only essential membrane constituents but also determine many membrane functions and integrity. Normal receptor function, signal transduction, and transport of essential substrates depend strongly on normal membrane phospholipid metabolism. Studies of plasma membrane phospholipid composition have indicated that ethanolamine glycerophospholipids decrease, whereas serine glycerophospholipids increase significantly, in Alzheimer disease (AD). The release of arachidonate from the sn-2 position of glycerophospholipids is catalyzed by phospholipases and lipases. These enzymes are coupled to EAA receptors. Overstimulation of these receptors may be involved in abnormal calcium homeostasis, degradation of membrane phospholipids, and the accumulation of free fatty acids, prostaglandins, and lipid peroxides. Accumulation of the mentioned metabolites, as well as abnormalities in signal transduction owing to stimulation of lipases and phospholipases, may be involved in the pathogenesis of the neurodegeneration in AD.

Purification and characterization of a low molecular weight endogenous glutamate binding inhibitor (LGBI) in porcine brain

One of the endogenous substances which modulate glutamate receptor binding was isolated and highly purified from porcine brain. The purification involved extraction of brain tissue with doubled distilled water, followed by gel filtration, anion exchange, cation exchange, and several steps of C18 reverse-phase high performance liquid chromatography (HPLC). A low molecular weight glutamate binding inhibitor (LGBI) was purified to apparent homogeneity as judged from the elution profile of an HPLC column, in which a symmetrical peak was obtained when the eluate was monitored at 220 nm. The LGBI appears to be a small molecule (< 2 kD) that is heat- and acid/base-stable. The highly purified LGBI has no effect on GABAA and benzodiazepine receptor binding. The LGBI is not L-glutamate, L-aspartate or other negatively charged endogenous substances, since they are clearly separated from the LGBI in anion exchange chromatography. The inhibitory effect of the LGBI on [3H]L-glutamate binding is reversible, and it only changes the Bmax while the Kd remains the same. Since the membrane preparations used for [3H]L-glutamate binding assays for the detection of LGBI activity were enriched with quisqualate (QA)-sensitive subtypes, it was suggested that the LGBI could be a modulator of the QA receptor. Some amino acids which produce significant inhibition of glutamate binding activity were also compared with the LGBI, and they all showed no resemblance to the LGBI. The chemical structure of the LGBI remains to be determined.

Involvement of metabotropic and ionotropic glutamate receptors in inositol polyphosphate formation in carp retinal slices

The contribution of ionotropic and metabotropic glutamate receptors to inositol polyphosphate accumulation in carp retinal slices was investigated using myo-[2-3H]inositol prelabelling. In the presence of the glutamate agonists quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and trans-(+/-)-1-amino-1,3-cyclopentane-dicarboxylic acid (t-ACPD), formation of [3H]inositol phosphate was significantly increased in a dose-dependent manner, with EC50 values of 350 nM, 1.5 microM and 10 microM respectively. The complete AMPA-induced response and a large component of the quisqualate-induced response were inhibited in a competitive manner when the ionotropic antagonist 6-cyano-7-nitroquinoxalin- 2,3-dione (CNQX) was present. Furthermore, the remaining level of quisqualate-induced [3H]inositol phosphate formation closely matched that produced by ACPD alone, and coincubation of AMPA and ACPD showed additive effects, suggesting that the quisqualate-induced response resulted from coactivation of metabotropic and ionotropic glutamate receptors. The ionotropic component was partially reduced in the presence of cobalt, suggesting indirect effects resulting from synaptic interactions. We could exclude indirect effects through depolarization-induced release of other neurotransmitters. Only serotonin (EC50 1 microM) and carbachol (at a concentration of 1 mM) stimulated [3H]inositol phosphate formation, but their antagonists did not affect the quisqualate response and coactivation with quisqualate and serotonin or carbachol resulted in additive effects. The ionotropic component was completely suppressed when Ca2+ was omitted from the medium and cobalt was present. This makes it likely that the ionotropic component resulted from Ca2+ entry through AMPA-gated channels and subsequent Ca(2+)-dependent activation of phospholipase C.

(2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine positively modulates metabotropic glutamate receptors coupled to polyphosphoinositide hydrolysis in rat hippocampal slices

In rat hippocampal slices, the novel metabotropic glutamate receptor (mGluR) ligand, (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) enhanced the stimulation of polyphosphoinositide (PPI) hydrolysis elicited by quisqualate or by submaximal concentrations of ibotenate or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The enhancing effect of DCG-IV was (i) specific for mGluR agonists, (ii) restricted to hippocampal slice preparation, (iii) reversible, and (iv) not subject to homologous desensitization, in addition, DCG-IV did not interact with L-2-amino-4-phosphonobutanoate (AP4), a noncompetitive antagonist of mGluRs coupled to PPI hydrolysis in brain slices [32]. The action of DCG-IV on quisqualate-stimulated PPI hydrolysis was insensitive to antagonists of ionotropic glutamate receptors and did not appear to be a consequence of a reduction in the intracellular levels of cAMP [14]. When the stimulation of PPI hydrolysis was measured as a function of the incubation time, DCG-IV potentiated quisqualate-stimulated PPI hydrolysis after 60 min of incubation, when quisqualate had already reached its maximal effect. Knowing that activation of protein kinase C (PKC) limits the extent of mGluR agonist-stimulated PPI hydrolysis over time, we have studied the enhancing effect of DCG-IV in the presence of the PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA). As expected [9], TPA reduced quisqualate-stimulated PPI hydrolysis in control slices, but was inactive in slices incubated in the presence of DCG-IV. Taken collectively, these results suggest that DCG-IV positively modulates the activity of mGluRs coupled to PPI hydrolysis through a mechanism, which involves PKC-mediated phosphorylation processes.

Purification and Characterization of a High-Molecular-Weight Endogenous Glutamate-Binding Inhibitor in Porcine Brain

A high-molecular-weight glutamate-binding inhibitor (HGBI) from porcine brain extract was purified to homogeneity. The results of this purification process show that glutamate receptor activity can be regulated by a high-molecular-weight protein, which inhibits [(3)H]L-glutamate binding to excitatory amino acid (EAA) receptors. The purified HGBI appears to be a protein with a molecular weight of approximately 70 kD. The purified HGBI is negatively charged, suggesting that it may contain acidic amino acids, and most likely, L-glutamate- and L-aspartate-enriched regions, responsible for its surface charge as well as for its binding to glutamate receptors. Inhibition of [(3)H]L-glutamate binding by the purified HGBI is reversible, and appears to change the binding kinetics. This endogenous ligand for glutamate receptors has unique characteristics separating it from all the other ligands found so far in the EAA receptor system. This HGBI represents a new class of modulator for the EAA receptor, thus further investigation of the function and structure of the HGBI should provide new understanding of the mechanisms of EAA-mediated neutrotransmission. Copyright 1994 S. Karger AG, Basel

Dendritic Ca2+ accumulations and metabotropic glutamate receptor activation associated with an N-methyl-D-aspartate receptor-independent long-term potentiation in hippocampal CA1 neurons

Bathing hippocampal slices in the potassium channel blocker tetraethylammonium (TEA), while stimulating the Schaffer collaterals at a low frequency, induces Ca(2+)-dependent, N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation of synaptic transmission (LTPk) in CA1 neurons. We have combined ratio imaging of fura-2 and mag-fura-5 in hippocampal CA1 neurons with intracellular and field recordings to evaluate postsynaptic Ca2+ changes that occur in the induction of LTPk. Test stimuli were applied at 0.05 Hz to stratum radiatum in the presence of the NMDA receptor antagonists D,L-2-amino-5-phosphonovaleric acid (100 microM) or MK-801 (10 microM). During TEA exposure (15-25 mM; 10 min), cells fired prolonged action potentials both spontaneously and in response to test stimuli resulting in transient, micromolar Ca2+ accumulations in both somata and dendrites. The initial EPSP slope, measured 60 min after TEA wash-out, was potentiated to approximately 200% of control. The Ca2+ channel blocker nimodipine (10 microM) greatly reduced Ca2+ transients in both magnitude and duration and prevented LTPk induction. Pretreatment of slices with compounds that block metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis, L-2-amino-3-phosphonopropionic acid (L-AP3, 50-200 microM) or L-aspartate-beta-hydroxamate (50-100 microM), as well as protein kinase C (PKC) inhibitors (sphingosine, 20 microM; RO-31-8220, 0.2 microM; or calphostin C, 2 microM) also blocked LTPk. Ca2+ transients were unaffected by L-AP3 or RO-31-8220. These findings suggest that Ca2+ influx through voltage-gated channels and co-activation of PKC by mGluRs are both necessary for induction of LTPk. Activation of mGluRs must also occur in NMDA receptor-dependent induction paradigms, but is possibly of lesser importance owing to the much greater gating of Ca2+ directly into the dendritic spines.

Investigations of neurotoxicity and neuroprotection within the nucleus basalis of the rat

The present study investigated the specific ways by which cytotoxicity due to glutamate receptor stimulation could be attenuated by the administration of agonists and antagonists of the ionotropic and metabotropic glutamate receptors within the nucleus basalis magnocellularis (NBM) of rats as measured by cortical choline acetyltransferase activity. The results of these studies suggest that (1) the cytotoxicity of ibotenate to NBM cholinergic cells is not dependent upon stimulation of metabotropic glutamate receptors, but results from activation of N-methyl-D-aspartate (NMDA) receptors, (2) the cytotoxicity of quisqualate to cholinergic cells within the NBM is not dependent upon stimulation of NMDA or metabotropic receptors, and (3) the cytotoxicity of NMDA was prevented by administration (i.p.) of the un-competitive NMDA antagonist memantine (30 mg/kg), resulting in plasma levels of 2.5 micrograms/ml, a concentration known to block efficiently NMDA receptors in vitro. Finally, performance of a food-motivated, delayed-alternation task on a T-maze was impaired by injections of NMDA into the NBM, but was prevented by co-administration of NMDA with memantine.

Inhibition of endogenous dopamine release in amphibian retina by L-2-amino-4-phosphonobutyric acid (L-AP4) and trans-2-aminocyclopentane-1,3-dicarboxylate (ACPD)

The metabotropic glutamate receptor agonists 2-amino-4-phosphonobutyric acid (AP4) and trans-2-aminocyclopentane-1,3-dicarboxylate (ACPD) blocked light-stimulated dopamine release from Xenopus laevis retina. ACPD suppressed release in darkness but AP4 did not. AP4 blocked release stimulated in darkness by picrotoxin, a GABA-A receptor antagonist. The data suggest that regulation of dopamine release in Xenopus retina involves subpopulations of metabotropic glutamate receptors.

Actions of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in retinal ON bipolar cells indicate that it is an agonist at L-AP4 receptors

Metabotropic glutamate receptors (mGluRs) include receptors sensitive to L-2-amino-4-phosphonobutyrate (L-AP4) and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). To determine whether 1S,3R-ACPD is an agonist at retinal L-AP4 receptors, whole cell voltage clamp recordings were obtained from mudpuppy ON bipolar cells in a superfused retinal slice and L-AP4 and 1S,3R-ACPD were bath applied. Both compounds evoked similar outward currents which reversed near 0 mV and were accompanied by an increased input resistance. Responses to both agonists washed out in parallel suggesting they act through the same second messenger pathway(s). Inhibitors of cGMP-PDE activity suppressed responses to both L-AP4 and 1SR,3RS-ACPD, suggesting that both compounds activate cGMP-PDE. Responses to 1S,3R-ACPD were occluded by prior activation of L-AP4 receptors, but not blocked by the non-AP4, mGluR antagonists, L-aminophosphonopropionic acid (L-AP3) or 4-carboxy-3-hydroxyphenylglycine (4C3H-PG). These results indicate that 1S,3R-ACPD is an agonist at L-AP4 receptors. 1S,3S-ACPD and 4C3H-PG evoked outward currents similar to L-AP4 suggesting they may also be L-AP4 receptor agonists. Using the b-wave of the ERG as an assay for ON bipolar cell responses, concentration/response curves were obtained for ACPD enantiomers. The rank-order potency of ACPD enantiomers at L-AP4 receptors in ON bipolar cells is similar to their rank-order potency at non-AP4, mGluRs in brain which suggests that the receptors possess similar binding sites and may be members of a common receptor family.

Role of Ca2+ in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated polyphosphoinositide turnover in primary neuronal cultures

Excitatory amino acid (EAA) receptors and EAA-mediated stimulation of polyphosphoinositide (poly-PI) turnover were studied in cultured neurons at different days in vitro (DIV). Six main observations have emerged from these studies: (a) Neurons increased their sensitivity to EAAs as a function of time in culture, indicated by increasing EAA-mediated poly-PI turnover. (b) Extracellular Ca2+ concentration played an important role in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated poly-PI turnover in cells at 4 DIV, whereas poly-PI turnover mediated by L-glutamate and trans-1-amino-cyclopentane-1,3-dicarboxylic acid was not Ca(2+)-dependent. (c) A marked stimulation of poly-PI turnover by AMPA was seen in the cultured neurons at 4 DIV, but not at 17 DIV, suggesting that a distinct EAA receptor sensitive to AMPA is transiently expressed. (d) The Ca2+ ionophore A23187 increased poly-PI turnover in cultured neurons, suggesting that Ca2+ entry is involved in stimulating poly-PI turnover. (e) Stimulation of poly-PI turnover by carbachol was greater in neurons at 17 DIV as compared with 4 DIV, and appeared to be Ca(2+)-dependent across DIV. (f) 6-Cyano-7-nitroquinoxaline-2,3-dione, an antagonist for non-N-methyl-D-aspartate ionotropic EAA receptors, inhibited 100% and 35% of AMPA- and quisqualate-induced poly-PI turnover, respectively, suggesting an involvement of ionotropic AMPA/quisqualate receptors in stimulating poly-PI turnover.

Kainate-induced status epilepticus leads to a delayed increase in various specific glutamate metabotropic receptor responses in the hippocampus

Neuronal loss and gliosis were detected in the rat hippocampus soon after unilateral intra-amygdala injection of kainate (KA) (2.5 nmol) while solid mossy fiber sprouting could be seen only fourteen days after this injection. Using this experimental model, we examined the metabotropic glutamate receptor (mGluR)-induced inositol phosphate (IP) formation in hippocampal synaptoneurosomes and slices. In synaptoneurosomes prepared from ipsilateral hippocampi fourteen days following injection, there were no significant changes in mGluR- and carbachol(CARB)-stimulated IPs syntheses when sham-operated and KA-injected animals were compared. In the corresponding hippocampal slices, significant increases of the mGluR responses mediated by ibotenate (IBO) and aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD) were noted after KA application. The net stimulation values respectively expressed in a pair-wise fashion for buffer-injected control and KA-treated animals were IBO: 1,947 +/- 457 and 10,553 +/- 1,242; t-ACPD: 1,557 +/- 662 and 9,449 +/- 2,251 dpm/mg protein respectively. Significantly augmented mGluR responses in hippocampal slices were also measured at 7, 42 and 92 days after KA injection. There were, however, no significant increases in CARB-stimulated phosphoinositide hydrolysis in the hippocampal slices at all time-intervals after KA administration. These findings show that there are differences between the mGluR responses in hippocampal synaptoneurosome and slice preparations, suggesting the presence of two distinct populations of mGluR in each of these two models. The large specific increases in certain mGluR activities after KA-induced status epilepticus in hippocampal slices could represent one of the molecular mechanisms which underlie the profound morphological changes, in particular gliosis or mossy fiber sprouting, which follow the KA-induced status epilepticus.

Modulatory effects of NMDA on phosphoinositide responses evoked by the metabotropic glutamate receptor agonist 1S,3R-ACPD in neonatal rat cerebral cortex

1. The effect of NMDA-receptor stimulation on phosphoinositide signalling in response to the metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) has been examined in neonatal rat cerebral cortex slices. 2. Total [3H]-inositol phosphate ([3H]-InsPx) accumulation, in the presence of 5 mM LiCl, in [3H]-inositol pre-labelled slices was concentration-dependently increased by 1S,3R-ACPD (EC50 16.6 microM) and, at a maximally effective concentration, 1S,3R-ACPD (300 microM) increased [3H]-InsPx accumulation by 12.8 fold over basal values. 3. [3H]-InsPx accumulation stimulated by 1S,1R-ACPD was enhanced by low concentrations of NMDA (3-30 microM), but not by higher concentrations (> 30 microM). [3H]-InsPx accumulations stimulated by 1S,3R-ACPD in the absence or presence of 10 microM NMDA were linear with time, at least over the 15 min period examined; however, in the presence of 100 microM NMDA the initial enhancement of 1S,3R-ACPD-stimulated phosphoinositide hydrolysis progressively decreased with time. 4. In the presence of a maximal enhancing concentration of NMDA (10 microM), the response to 1S,3R-ACPD (300 microM) was increased 1.9 fold and the EC50 for agonist-stimulated [3H]-InsPx accumulation decreased about 4 fold. The enhanced response to the metabotropic agonist was concentration-dependently inhibited by competitive and uncompetitive antagonists of NMDA-receptor activation. 5. 1S,3R-ACPD also stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation with an initial peak response (5-6 fold over basal) at 15 s decaying to a smaller (2 fold), but persistent elevated accumulation (1-10 min). 6. Co-addition of 10 or 100 MicroM NMDA enhanced the initial peak Ins(1,4,5)P3 response to 1S,3RACPD.However, the enhancing effect was only maintained over 10 min in the presence of 1O Micro MNMDA, whilst in contrast, 100 MicroM NMDA ceased to cause a significant enhancement of the metabotropic response by 5 min and completely suppressed lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation at 10 min.7. Both basal and 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulations were reduced when slices were incubated in nominally Ca2"-free medium. Under these conditions only a concentration-dependent enhancement of the response was observed (EC50 for NMDA facilitation of lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation of 32 MicroM).8. These experiments have revealed that at low concentrations, NMDA can dramatically potentiate1S,3R-ACPD-stimulated phosphoinositide hydrolysis, probably by a Ca2"-dependent facilitation of agonist-stimulated phosphoinositide-specific phospholipase C activity. Higher concentrations of NMDA result in time-dependent inhibition of the metabotropic agonist-stimulated response. We believe the former effect could be fundamental in glutamate receptor 'cross-talk', whereas the latter may reflect a Ca2+-dependent neurotoxic effect of NMDA on the neonatal cerebral cortex slices.

An ibotenate-selective metabotropic glutamate receptor mediates protein phosphorylation in cultured hippocampal pyramidal neurons

Previous results showed that within 30 s after glutamate stimulation of cultured rat hippocampal pyramidal neurons there occurred an elevation of Ca2+ and diacylglycerol, and the phosphorylation of three acidic protein kinase C substrates, i.e., an 87-kDa protein known as myristoylated alanine-rich C kinase substrate and a 120- and a 48-kDa protein. In addition, it was suggested that a metabotropic-type glutamate receptor might be responsible for the phosphorylation observed. This work examines the ability of metabotropic and inotropic glutamate receptor agonists to quickly activate phospholipases in 1.26 mM versus 50 nM extracellular Ca2+ by measuring the generation of inositol phosphates. NMDA, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid did not stimulate the generation of inositol phosphates in the presence of normal or low extracellular Ca2+ in pyramidal neurons. Kainate stimulated the production of inositol phosphates in the presence of 1.26 mM extracellular Ca2+ but not in 50 nM extracellular Ca2+. Other than glutamate, only ibotenate was able to stimulate the generation of inositol phosphatases in both normal and low extracellular Ca2+. The maximal response to ibotenate was approximately equal to that of glutamate, when pyramidal neurons were stimulated in 50 nM extracellular Ca2+. The generation of inositol phosphates by glutamate and ibotenate could be partially blocked (50-60% reduction) by pretreatment of neurons with pertussis toxin (250 ng/ml), suggesting that a GTP-binding protein might be involved. In addition, ibotenate stimulated the immediate phosphorylation of the same three protein kinase C substrates as glutamate. The NMDA receptor blocker MK-801 had no effect on this phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabotropic glutamate receptor (MGR): pharmacology and subcellular location

A pharmacological characterization of the metabotropic glutamate receptor (MGR) was performed in striatal neurons. Among the excitatory amino acid receptor antagonists tested, only D, L-2-amino-3-phosphonopropionate (D, L-AP3) inhibited QA-induced inositol phosphate (InsP) formation in a competitive manner (mean pKi = 4.45 +/- 0.43, n = 4). However, this drug was a partial agonist of MGR since it stimulated the inositol-phosphate formation. We found that D, L-AP3 also inhibited NMDA-induced calcium increase, in a competitive manner (mean pIC50 = 4.34 +/- 0.22, n = 8, and mean pKi = 3.7 +/- 0.11 n = 5). 1 mM of the ionotropic agonists alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate (KA) or domoate (DO) (100 microM or higher) induced a significant InsP formation in striatal neurons. The InsP responses induced by all these agonists were totally blocked by the phorbol ester phorbol-12,13-dibutyrate (PdBu), but not by atropine or prazosin. Agonist-induced increases of intracellular calcium concentrations ([Ca2+]i) were insensitive to PdBu, suggesting that all these substances were able to stimulate the MGR in striatal neurons. Trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) evoked dose-dependent inositol phosphate formations with an EC50 of 29 microM but had no significant effect on NMDA or AMPA receptors, as measured by the patch clamp technique. In the presence of 30 microM of AMPA, trans-ACPD induced a significant release of arachidonic acid (AA) in striatal neurons. No important AA release was observed by any of these agonists alone. 56 mM K+ did not mimic AMPA in this associative ionotropic/metabotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Sulphur-containing excitatory amino acid-stimulated inositol phosphate formation in primary cultures of cerebellar granule cells is mediated predominantly by N-methyl-D-aspartate receptors

The stimulatory effect of excitatory sulphur-containing amino acids on inositol phosphate formation was investigated in primary cultures of cerebellar granule cells. L-Cysteine sulphinate (CSA), L-cysteate (CA), L-homocysteine sulphinate (HCSA), L-homocysteate (HCA) and S-sulpho-L-cysteine (SSC) dose-dependently stimulated the formation of [3H]inositol phosphates exhibiting EC50 values in the range 60-200 microM and maximal effects of six- to 17-fold that of basal [3H]inositol phosphate levels. Endogenous L-glutamate spontaneously released into the extracellular medium or following exposure of cells to HCSA, HCA or SSC did not contribute significantly to formation of [3H]inositol phosphates, whereas 10% of the total [3H]inositol phosphates accumulated following exposure to CSA and CA was due to released L-glutamate. The selective N-methyl-D-aspartate receptor antagonist, D,L-2-amino-5-phosphonopentanoic acid (APV, 500 microM) attenuated by 20% (HCSA) to between 80 and 100% (CSA, CA, SSC, HCA) the formation of [3H]inositol phosphates induced by 1 mM sulphur-containing amino acids. When, however, HCSA was used at 100 microM (a concentration near to its EC50 for phosphoinositide hydrolysis), APV inhibited induced responses by 70%. Sulphur-containing amino acid-stimulated [3H]inositol phosphate formation was unaffected by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). Inhibition of sulphur-containing amino acid-stimulated [3H]inositol phosphate formation by co-administration of APV and CNQX was similar to that obtained in the presence of APV alone. CSA-, CA-, SSC- and HCA-stimulated [3H]inositol phosphate formation was markedly reduced by removal of Ca2+ from the extracellular medium whereas that stimulated by HCSA was less affected. A similar inhibitory profile was observed when the levels of sulphur-containing amino acid-induced increases in intracellular free calcium ([Ca2+]i) were measured in the presence of 500 microM APV; 1 mM HCSA-induced responses being inhibited by only 30% whereas responses to the remaining sulphur-containing amino acid (also at 1 mM) were inhibited by > 45%. When the sulphur-containing amino acids were used at concentrations approximating their EC50 values for phosphoinositide hydrolysis, APV inhibited the induced increases in [Ca2+]i by 70-100%. HCA and SSC co-administered with the less efficacious but selective metabotropic glutamate receptor agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) at maximally effective concentrations (1 mM) of each agonist stimulated [3H]inositol phosphate formation in an additive manner.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of trans-ACPD, a metabotropic excitatory amino acid receptor agonist, on the responses of primate spinothalamic tract neurons

The responses of primate spinothalamic tract (STT) neurons to innocuous and noxious mechanical stimuli applied to the skin can be enhanced for more than an hour following prolonged noxious stimulation. This increased responsiveness is thought to reflect sensitization of dorsal horn neurons and may help account for secondary hyperalgesia and mechanical allodynia. The proposal that central sensitization is due to the activation of second messenger system was tested in this study by examining the effect of trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid), an agonist of metabotropic excitatory amino acid (EAA) receptors, introduced into the dorsal horn by microdialysis. A low dose of trans-ACPD resulted in an increase in the responses of STT cells to an innocuous mechanical stimulus (BRUSH), but no increase in the responses to noxious mechanical and thermal stimuli or in the excitation produced by iontophoretically applied EAAs. A high dose of trans-ACPD caused a transient increase in background activity, but no change in the responsiveness of spinothalamic cells to any of the test stimuli. It is concluded that low doses of trans-ACPD can selectively enhance transmission through interneuronal pathways mediating tactile inputs to spinothalamic cells.

Ontogenesis of quisqualate-associated phosphoinositide metabolism in various regions of the rat nervous system

The effect of postnatal age on phosphoinositide metabolism per se and on quisqualate-stimulated phosphoinositide metabolism was characterized in synaptoneurosomes prepared from nine different regions of the rat nervous system, namely the brainstem, cerebellum, cerebral cortex, colliculi, hippocampus, hypothalamus, olfactory bulb, spinal cord and striatum. In the hippocampus, striatum, cerebellum, cerebral cortex, brainstem, colliculus and spinal cord, the basal levels of inositol phosphate (inositol-1-phosphate+inositol-4,5-bisphosphate) formation were maximal two days after birth and declined steeply to steady-state levels from the age of 10 postnatal days. Similarly, in the olfactory bulb, basal inositol phosphate synthesis did not significantly change when measured during the period from postnatal day 10 to 42. The extent of [3H]-inositol labelling of phosphoinositides as a function of age presented similar profiles when measured in hippocampal, striatal, cerebellar and cerebral cortical synaptoneurosomes, i.e. maximal at perinatal ages and minimal at adult ages. In the hypothalamus, [3H]-inositol labelling of phosphoinositides showed an increase from postnatal day 12 to higher levels from postnatal days 14 to 18 subsequently followed by a dramatic increase from postnatal day 21 to 42. A similar developmental trend was also obtained for basal inositol phosphate synthesis. On the whole, four types of developmental profiles for quisqualate-stimulated inositol phosphate formation (expressed as the percentage of the basal level and as the difference between stimulated and basal levels of radioactive inositol phosphates) were obtained depending on the nervous system region studied. In the early, prenatally developed nervous system regions, namely the brainstem and the spinal cord, no postnatal stimulation peaks of quisqualate-induced inositol phosphate formation were recorded. This was also the case for the colliculi when the stimulation of IP formation was expressed as the difference in basal and stimulated levels of inositol phosphates. Secondly, in the olfactory bulb a region known to possess a continuous capacity for developmental plasticity both structurally and functionally during the first three weeks of postnatal development, a simultaneous sustained high level of quisqualate stimulation of phosphoinositide metabolism (fluctuating around 200% of the basal level) during the early postnatal period was evident. Thirdly, in regions of the central nervous system like the cerebellum, cerebral cortex, hippocampus and the striatum known to undergo intense developmental activity during the first two postnatal weeks, peaks of quisqualate-stimulated phosphoinositide metabolism were initially detected around the first week after birth in each of these brain areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Antagonism of presynaptically mediated depressant responses and cyclic AMP-coupled metabotropic glutamate receptors

The depression of monosynaptic excitation of neonatal rat motoneurones by (1S,3S)- and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) and by L-2-amino-4-phosphonobutyrate (L-AP4), which is probably presynaptically mediated, is antagonized by (+/-)- and (+)-alpha-methyl-4-carboxyphenylglycine (MCPG). The same phenylglycine derivatives also antagonize the depression of forskolin-stimulated cyclic AMP synthesis effected by the two ACPD stereoisomers and by L-AP4. These results support previous suggestions that presynaptic depression is mediated by metabotropic glutamate receptors negatively coupled to adenylyl cyclase activity. MCPG is the first antagonist to be reported for these receptors.

Excitotoxicity and neurological disorders: involvement of membrane phospholipids

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin-induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids, and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes, such as protein kinase C and calcium-dependent proteases, may also contribute to the neuronal injury. Excitotoxin-induced alterations in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. These models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs.

Excitatory amino acid receptors: a gallery of new targets for pharmacological intervention

The excitatory amino acids (EAAs) L-glutamate and L-aspartate are the most abundant amino acids in brain and play a number of roles in maintaining neuronal function. Among these are their use as protein constituents, as key intermediates in ammonia metabolism, and as precursors for other neurotransmitters. Given the widespread distribution of EAA-containing neurons, these transmitters are likely to be involved in virtually all central nervous system functions, with abnormalities in neurotransmission contributing to the symptoms of a host of neurological and psychiatric disorders. Because of the importance of EAAs in maintaining the functional integrity of the central nervous system, efforts are underway to design agents capable of regulating the activity of these transmitters for therapeutic gain. Inasmuch as potential side effects preclude a generalized modification of this system, strategies must be found to alter EAA neurotransmission in selected brain regions. In this regard, pharmacological data suggest several functionally distinct EAA receptors, a finding confirmed by cloning studies which hint at an even larger family of sites. Moreover, it appears that some excitatory amino acid receptor complexes are composed of interacting sites which orchestrate receptor function, and there is evidence that EAA receptors may influence the activity of one another. Thus, there appear to be numerous sites that can be targeted to selectively modify excitatory amino acid neurotransmission in brain. Besides the agonist recognition site for each receptor subtype, other targets include regulatory subunits, ion channels and components of receptor-coupled second messenger systems.

Cyclic GMP and regulation of cyclic nucleotide hydrolysis

Several of the different PDE isozyme families have the ability in vitro to hydrolyze cGMP. In particular they include the CaM-dependent PDEs, the cGMP-stimulated PDEs, and the cGMP binding, cGMP-specific PDEs. Existing evidence suggests or demonstrates that in different cell types, each of these can be important determinants for the control of cGMP steady-state levels. Each of these enzymes is differentially expressed and regulated; moreover, the amount of the enzyme expressed and the mode of regulation determine to a large extent the rate of rise, maximal level, rate of fall, and duration of the cGMP signal in the cell. In addition to enzymes that function to degrade cGMP at least two also are regulated by cGMP both in vitro and in the intact cell. The cGMP-stimulated PDE has the ability to decrease cAMP levels in response to cGMP and the cGMP-inhibited PDE can increase cAMP levels in response to cGMP. We are just beginning to define how many different isozymes of PDE exist in mammalian tissues, where they are located, and how they are regulated. Selective inhibitors to each are being developed and studies designed to define structural features that determine the mechanisms of action and regulation of the PDEs have been initiated. It is expected that in the next few years more PDEs will be discovered and the functions of the new an existing ones with be more clearly defined.

Metabotropic glutamate receptor modulation of cAMP accumulation in the neonatal rat hippocampus

The pharmacology and cellular mechanism by which metabotropic glutamate receptor (mGluR) activation modulates cAMP formation was studied in cross-chopped hippocampal slices from neonatal (7 day old) rats. The selective mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), and other non-selective mGluR agonists produced concentration-related stimulation of basal cAMP formation in this tissue. The relative agonist potency order was 1S,3R-ACPD = quisqualate > ibotenate >> 1R,3S-ACPD. 1S,3R-ACPD stimulated cAMP accumulation was antagonized in a stereoselective manner by L-2-amino-3-phosphonopropionate (L-AP3), but not by higher chain homologues such as L-2-amino-4-phosphonobutyrate (L-AP4) and 2-amino-5-phosphonopentanoate (AP5). 1S,3R-ACPD-enhanced cAMP formation was greatly inhibited by incubation with adenosine deaminase. In the adult rat hippocampus, 1S,3R-ACPD did not appreciably increase basal cAMP, but inhibited forskolin-stimulated cAMP formation, and this effect was observed with or without adenosine deaminase. In the presence of the adenosine receptor antagonist and cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), 1S,3R-ACPD did not enhance cAMP formation in the neonatal hippocampus, but inhibited forskolin-stimulated cAMP (like in the adult tissue). These results demonstrate that mGluRs that increase cAMP in the neonatal hippocampus have a unique pharmacology when compared to mGluRs that decrease cAMP accumulation and increase phosphoinositide hydrolysis. 1S,3R-ACPD stimulation of cAMP in the neonatal rat hippocampal slice involves potentiation of responses to endogenous adenosine. Negatively coupled cAMP linked mGluRs are also present in the neonatal tissue, but are masked by the predominance of the positively coupled mGluR cAMP response.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation

The effects of (1S,3R)-ACPD, a selective metabotropic glutamate receptor agonist, on NMDA-induced 45Ca2+ accumulation and delayed neuronal cell death were determined using primary cerebrocortical cultures. Exposure to (1S,3R)-ACPD alone, although causing small increases in 45Ca2+ accumulation, was not neurotoxic. The presence of (1S,3R)-ACPD during exposure to NMDA attenuated the resulting sustained accumulation of 45Ca2+ and delayed neuronal cell death. Reductions in sustained Ca2+ accumulation were associated both with Ca2+ efflux, in the absence of cell death, and inhibition of delayed intracellular Ca2+ accumulation. The protective effects of (1S,3R)-ACPD on NMDA-induced cell death were inhibited by pretreatment of cultures with pertussis toxin. These results suggest that activation of metabotropic glutamate receptors may stimulate intracellular processes capable of limiting sustained elevations in intracellular calcium and the resulting excitotoxic neuronal damage.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Selective excitotoxic pathology in the rat hippocampus

The pattern of cell loss and neuronal degeneration resulting from multiple microinjections of N-methyl-D-aspartate (NMDA), ibotenate (IBO), quisqualate (QUIS), and kainate (KA) into hippocampus was studied, together with the protection provided by the NMDA antagonist 3-(+/-)-2-carboxypiperazin-4-yl-propyl-1-phosphonate (CPP). Histological evaluation was carried out after 7 days of survival. NMDA and IBO resulted in an extensive loss of all cells in the hippocampus including dentate gyrus, hilar cells, and CA3-CA1 pyramidal cells, but there was an absence of damage to areas and structures outside hippocampus. After QUIS and KA injections the hippocampal damage was limited to hilar cells in the dentate gyrus, CA3 pyramidal cells, and partial loss of CA1 cells; there was extensive extrahippocampal damage including entorhinal cortex, amygdala, layers III, V, and VI of ventral neocortex, olfactory areas, and various thalamic nuclei. CPP provided almost complete protection from the effects of intrahippocampal injections of NMDA and IBO, but did not affect the hippocampal cell loss found after QUIS and KA (with the exception of minor protection of some CA1 cells). CPP protected most extrahippocampal sites from the damage resulting from QUIS and KA, indicating that such excitotoxic cell death is indirect and involves NMDA receptor activation by an endogenous agent. The use of multiple microinjections as opposed to single injections allows a clearer interpretation of selective excitotoxic vulnerability.

Taurine release evoked by NMDA receptor activation is largely dependent on calcium mobilization from intracellular stores

It is known that the activation of N-methyl-D-aspartate (NMDA) receptors leads to an increase in extracellular taurine concentration in different brain regions. The mechanism that mediates this effect is not totally understood. In this study, rat hippocampal slices were used to determine the dependence of NMDA-induced taurine release on extracellular calcium and/or on calcium mobilization from intracellular stores. NMDA was administered through a microdialysis probe inserted into the slice, at the level of CA1 stratum radiatum, which was also used to collect amino acids from the extracellular space. Field potentials evoked by stimulation of the Schaffer collaterals and recorded in the stratum pyramidale of CA1 were used as a control of NMDA receptor activation. NMDA induced a marked increase in extracellular taurine levels and a decrease in field potential amplitude, and both effects were suppressed in the presence of MK-801, a blocker of the NMDA receptor-linked channel. Dantrolene, an inhibitor of calcium release from intracellular stores, partially inhibited the extracellular taurine increase, while 2-nitro-4-carboxyphenyl-N,N-diphenyl carbamate (NCDC), an inhibitor of phosphatidylinositol-specific phospholipase C activation, had no effect. Removal of extracellular calcium diminished, but did not abolish, the extracellular taurine increase caused by NMDA. The remaining taurine response was totally suppressed by dantrolene, and also by NCDC. These results demonstrate that the release of taurine induced by NMDA receptor activation is triggered by the increase in cytoplasmic calcium concentration. We suggest that, under physiological conditions, calcium influx provides the signal for NMDA-induced taurine release, which is amplified by calcium-dependent calcium mobilization from intracellular stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR decreases cAMP) in brain slices. Among the putative mGluR agonists, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, L-2-amino-3-phosphonopropionate (AP3), quisqualate, L-glutamate and beta-N-methylamino-L-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR decreases cAMP subtype. Both L-AP4 and L-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their D-isomers, indicating that interaction of these drugs with the mGluR decreases cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR decreases cAMPS, as estimated by using 1S,3R-ACPD as an agonist, did not correlate with the steady-state levels of mGluR2 mRNA. Thus, 1S,3R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1S,3R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR decreases cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the calcium and chloride [3H]glutamate binding site in crude synaptic membranes from human brain tissue

1. The specific binding properties of [3H]glutamate to crude synaptic membranes (CSM) from postmortem human brain were studied. 2. Equilibrium binding analysis of [3H]glutamate binding to CSM from human brain cortex revealed a KD = 110 +/- 12 nM and a Bmax = 27 +/- 4 pmol/mg protein). 3. Calcium increased the number of binding sites, Bmax = 44 +/- 6 pmol/mg protein, without a significant change in the affinity constant, KD = 95 +/- 10 nM. 4. The dissociation constant of the [3H]glutamate bound to human CSM was 4.0 +/- 0.4 min-1 (n = 3). 5. The relative potencies of glutamate analogs and 2-amino-4-phosphonobutyric acid (APB) to compete for the glutamate binding sites, in human CSM, were glutamate > quisqualate = ibotenic acid > APB >> alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionate acid. 6. The glutamate specific binding in CSM from postmortem human brain was particularly rich in the gyrus hippocampus, nucleus accumbens, thalamus and frontal cortex. 7. This glutamate binding protein is related, probably, to a presynaptic neurosecretory pathway.

Dose- and age-dependent effects of prenatal ethanol exposure on hippocampal metabotropic-glutamate receptor-stimulated phosphoinositide hydrolysis

Prenatal ethanol exposure reduces the density of the N-methyl-D-aspartate (NMDA) receptor agonist binding sites and decreases the capacity to elicit long-term potentiation (LTP) in hippocampal formation of 45-day-old rat offspring. We hypothesized that prenatal ethanol exposure would reduce metabotropic-glutamate receptor (mGluR)-activated phosphoinositide hydrolysis also. Sprague-Dawley rat dams were fed a liquid diet containing either 3.35% (v/v) ethanol or 5.0% ethanol throughout gestation. Control groups were pair-fed either isocalorically matched 0% ethanol liquid diets or lab chow ad libitum. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) stimulated inositol-1-phosphate (IP1) accumulation via activation of the mGluR in offspring whose mothers consumed the 3.35% ethanol liquid diet was not different compared with the control groups. Furthermore, trans-ACPD stimulated IP1 accumulation in 10- to 13-day-old offspring of the 5.0% ethanol diet group was not different compared with the control groups. However, trans-ACPD stimulated IP1 accumulation was reduced significantly in 56- to 82-day-old offspring of dams fed the 5.0% ethanol liquid diet compared with the control groups. In contrast, bethanechol stimulated IP1 accumulation, mediated via activation of muscarinic cholinergic receptors, was not affected by maternal consumption of either ethanol liquid diet. These results suggest both dose- and age-dependent effects of prenatal ethanol exposure on hippocampal responsiveness to trans-ACPD-activated phosphoinositide hydrolysis. Furthermore, the ability of the 3.35% ethanol diet to alter hippocampal NMDA receptors without altering the mGluR response suggests a differential sensitivity to the effects of ethanol exposure in utero among hippocampal glutamate receptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)