Glutamate receptors and phosphoinositide metabolism: stimulation via quisqualate receptors is inhibited by N-methyl-D-aspartate receptor activation

Regional changes in CNS and retinal glycerophospholipid profiles with age: a molecular blueprint

We present here a quantitative molecular blueprint of the three major glycerophospholipid (GPL) classes, phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE), in retina and six regions of the brain in C57Bl6 mice at 2, 10, and 26 months of age. We found an age-related increase in molecular species containing saturated and monoenoic FAs and an overall decrease in the longer-chain PUFA molecular species across brain regions, with loss of DHA-containing molecular species as the most consistent and dramatic finding. Although we found very-long-chain PUFAs (VLC-PUFAs) (C28) in PC in the retina, no detectable levels were found in any brain region at any of the ages examined. All brain regions (except hippocampus and retina) showed a significant increase with age in PE plasmalogens. All three retina GPLs had di-PUFA molecular species (predominantly 44:12), which were most abundant in PS (∼30%). In contrast, low levels of di-PUFA GPL (1-2%) were found in all regions of the brain. This study provides a regional and age-related assessment of the brain's lipidome with a level of detail, inclusion, and quantification that has not heretofore been published.

Nicotine attenuates the effect of HIV-1 proteins on the neural circuits of working and contextual memories

BACKGROUND

Human immunodeficiency virus (HIV)-1-associated neurocognitive disorders (HAND) are characterized by synaptic damage and neuronal loss in the brain. Excessive glutamatergic transmission and loss of cholinergic neurons are the major indicators of HAND. Nicotine acts as a cholinergic channel modulator, and its cognitive-enhancing effect in neurodegenerative and cognitive disorders has been documented. However, it is unclear whether nicotine has any positive effect on memory and synaptic plasticity formation in HAND.

METHODS

We investigated the effects of nicotine on synaptic plasticity and hippocampus-prefrontal cortex (PFC)-amygdala-dependent memory formation in the HIV-1 transgenic (Tg) and F344 control rats.

RESULTS

Chronic nicotine treatment (0.4 mg/kg nicotine, base, subcutaneously) significantly attenuated the cognitive deficits in the HIV-1Tg rats in both the spatial and contextual fear memories but impaired the contextual learning memory in the F344 rats. To determine the role of nicotine in the synaptic dysfunction caused by HIV-1 proteins, we analyzed the expression of key representative genes related to synaptic plasticity in the hippocampus, PFC, and amygdala of the HIV-1Tg and F344 rats using a custom-designed qRT-PCR array. The HIV-1 proteins significantly altered the glutamate receptor-mediated intracellular calcium cascade and its downstream signaling cascade in a brain region-specific manner. Further, chronic nicotine treatment reversed the effect of HIV-1 proteins on the expression of genes involved in synaptic plasticity in the three brain regions. The effects of nicotine differed significantly in the HIV-1Tg and F344 rats.

CONCLUSIONS

Our findings indicate that nicotine can attenuate the effect of HIV viral proteins on cognitive function and produce a brain region- and strain-specific effect on the intracellular signaling cascades involved in synaptic plasticity and memory formation.

The role of group I metabotropic glutamate receptors in neuronal excitotoxicity in Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease, ischemia, and Alzheimer's disease (AD) are major causes of death. Recently, metabotropic glutamate receptors (mGluRs), a group of seven-transmembrane-domain proteins that couple to G-proteins, have become of interest for studies of pathogenesis. Group I mGluRs control the levels of second messengers such as inositol 1,4,5-triphosphate (IP3), Ca2+ ions and cAMP. They elicit the release of arachidonic acid via intracellular Ca2+ mobilization from intracellular stores such as mitochondria and endoplasmic reticulum. This facilitates the release of glutamate and could trigger the formation of neurofibrillary tangles, a pathological hallmark of AD. mGluRs regulate neuronal injury and survival, possibly through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrially mediated programmed cell death. They may also play a role in glutamate-induced neuronal death by facilitating Ca(II) mobilization. Hence, mGluRs have become a target for neuroprotective drug development. They represent a pharmacological path to a relatively subtle amelioration of neurotoxicity because they serve a modulatory rather than a direct role in excitatory glutamatergic transmission.

Physiological roles and therapeutic potential of metabotropic glutamate receptors

Discovery of mGlu receptors has dramatically influenced our understanding of glutamatergic neurotransmission in the central nervous system. This receptor family provides a mechanism by which activation by glutamate can regulate a number of important neuronal and glial functions that are not typically modulated by ligand-gated ion channels. This includes modulation of neuronal excitability, synaptic transmission, and various metabolic functions. Because of the ubiquitous distribution of glutamatergic synapses, discovery of the mGlu receptors immediately raised the likelihood that mGlu receptors would participate in most, if not all, major functions of the CNS. In addition, the wide diversity and heterogeneous distribution of mGlu receptor subtypes could provide an opportunity for development of pharmacological agents that selectively target specific CNS systems to achieve a therapeutic effect. Over the past decade, an increasing number of agonists and antagonists selective for specific mGlu receptor subtypes have been developed. Use of these pharmacological tools along with genetic approaches has led to major advances in our understanding of the roles of mGlu receptors in regulating CNS systems and animal behavior. These studies suggest that drugs active at mGlu receptors may be useful in treatment of a wide variety of neurological and psychiatric disorders.

Glutamate Receptors of a Quisqualate-Kainate Subtype are Involved in the Presynaptic Regulation of Dopamine Release in the Cat Caudate Nucleus in vivo

Experiments were conducted with halothane-anesthetized cats implanted with a push-pull cannula in the caudate nucleus in order to estimate the effects of glutamate (GLU) agonists on the release of 3H-dopamine continuously synthesized from 3H-tyrosine. In the presence of tetrodotoxin (TTX), glutamate (10-8 M, 10-4 M) and kainate (KAI) (10-5 M) stimulated the release of 3H-dopamine while quisqualate (10-5 M) and N-methyl-D-aspartate (NMDA) (10-5 M) were without effect. The stimulatory effect of kainate (10-5 M) on 3H-dopamine release did not seem to be mediated by glutamate released from corticostriatal fibers, as not only kainate, but also quisqualate (QUI) and N-methyl-D-aspartate enhanced the efflux of glutamate through a tetrodotoxin-resistant process. Riluzole (10-5 M), gamma-D-glutamyl-glycine (GDGG) (10-5 M) and glutamine-diethyl-ester (10-5 M) prevented the stimulatory effect of kainate (10-5 M) while 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) (10-5 M), kynurenate (10-5 M) and 2-amino-5-phosphonovalerate (APV) (10-5 M) were without effect. In the presence of concanavalin A (CONA) (10-7 M), a lectin which is known to prevent the quisqualate-evoked desensitization of glutamate receptors, quisqualate (10-5 M) stimulated the release of 3H-dopamine. In addition, in the absence of concanavalin A, quisqualate (10-5 M) blocked the stimulatory effects of kainate (10-5 M) or glutamate (10-4 M) on 3H-dopamine release. These results suggest the involvement of receptors of the quisqualate/kainate subtype in the direct glutamate-induced presynaptic facilitation of dopamine release. In contrast to what was observed in the presence of tetrodotoxin, in the absence of the neurotoxin, high concentrations of glutamate (10-4 M) and kainate (10-5 M) reduced rather than stimulated the release of 3H-dopamine. A weak inhibitory effect was also observed with quisqualate (10-5 M) while N-methyl-D-aspartate (10-5 M) was without effect. In the light of previous studies, these latter observations suggest that glutamate can also exert an indirect inhibitory presynaptic influence on the release of dopamine from nerve terminals of the nigrostriatal dopaminergic neurons by acting on receptors of the quisqualate/kainate subtype located on striatal GABAergic neurons.

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.

Glutamate regulates IP3-type and CICR stores in the avian cochlear nucleus

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Norepinephrine promotes long-term potentiation in the adult rat hippocampus in vitro

We previously found a reduction in the ability of a single 100 Hz x 1 sec tetanus to induce long-term potentiation (LTP) in the CA1 region of hippocampal slices prepared from adult animals. To determine whether this reduction in LTP generation results from changes in neuromodulator function, we examined the ability of several neuromodulators to promote LTP in slices prepared from mature rats. Although acetylcholine, N-methyl-D-aspartate, and an agonist at metabotropic glutamate receptors failed to promote LTP, administration of norepinephrine allowed robust LTP. The effects of norepinephrine were mimicked by an alpha1-adrenergic agonist and were blocked by an alpha1-receptor antagonist. Beta-adrenergic agonists and antagonists were ineffective. These results suggest that norepinephrine acting via alpha1-adrenoceptors may be an important cofactor in promoting lasting synaptic plasticity in the adult central nervous system and that changes in adrenergic function may contribute to maturation- or aging-associated changes in memory function.

Activation of NMDA receptors reverses desensitization of mGluR5 in native and recombinant systems

The metabotropic glutamate receptor, mGluR5, has a critical role in induction of NMDA-receptor-dependent forms of synaptic plasticity and excitotoxicity. This is likely mediated by a reciprocal positive-feedback interaction between these two glutamate receptor subtypes in which activation of mGluR5 potentiates NMDA receptor currents and NMDA receptor activation potentiates mGluR5-mediated responses. We have investigated the mechanism by which NMDA receptor activation modulates mGluR5 function and find evidence that this response is mediated by activation of a protein phosphatase and a resultant dephosphorylation of protein kinase C phosphorylation sites on mGluR5. This form of neuromodulation may be important in a number of normal and pathological processes that involve activation of the NMDA receptor.

Modulation of LTP induction by NMDA receptor activation and nitric oxide release

In the CA1 hippocampal region, the induction of long-term potentiation (LTP) requires activation of N-methyl-D-aspartate receptors (NMDARs). However, untimely NMDAR activation either immediately prior to or following tetanic stimulation inhibits LTP generation. This NMDAR-mediated LTP inhibition is overcome by inhibitors of nitric oxide synthase (NOS) and hemoglobin, suggesting the involvement of NO. Additionally, NO inhibitors can promote the ability of weak tetanic stimuli to produce LTP under basal conditions in hippocampal slices. Recent experiments indicate that untimely NMDAR activation contributes to the failure of LTP induction during periods of low glucose exposure and hypoxia. Following hypoxia there is also a delayed form of LTP inhibition that is reversed by NMDAR antagonists and NO inhibitors. These results suggest that there are physiological and pathological conditions during which NMDAR activation and NO release modulate the induction of synaptic plasticity.

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.

Inhibition by NMDA of carbachol-stimulated inositol tetrakisphosphate accumulation in rat brain cortical slices

The present studies examined the effect of NMDA on carbachol-stimulated accumulation of inositol polyphosphates, with emphasis on the accumulation of inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4), at short time periods in rat brain cortical slices. There was a stimulatory effect of NMDA on accumulation of labeled inositol mono-, bis- and trisphosphates but not on labeled inositol tetrakisphosphates. In the presence of carbachol Ins 1,3,4,5-P4 accumulation was preferentially inhibited by NMDA at early time periods (within 30 seconds after NMDA addition). Subsequently, total phosphoinositide breakdown was inhibited by NMDA. NMDA did not stimulate accumulation of total Ins 1,3,4,5-P4 but immediately inhibited carbachol stimulated accumulation of Ins 1,3,4,5-P4. The inhibitory effect of NMDA (1 mM) was not mimicked by increasing K+ in the medium from 10 to 30 mM. However 30 mM K+ reversed the inhibitory effect of 1 mM NMDA on carbachol-stimulated Ins 1,3,4,5-P4. Parallel experiments with veratridine (a sodium channel activator) suggest that the early inhibitory effects of NMDA on Ins 1,3,4,5-P4 accumulation are not due to decreases in ATP availability or elevations in intracellular Na+. These data indicate that NMDA increases inositol mono-, bis- and trisphosphate accumulation while blocking muscarinic cholinergic stimulated accumulation of Ins 1,3,4,5-P4.

Effects of nitric oxide availability on responses of spinal wide dynamic range neurons to excitatory amino acids

The role of nitric oxide (NO) in responses of spinal dorsal horn neurons to excitatory amino acids and to cutaneous mechanical stimuli was examined. Extracellular recordings were made from wide dynamic range neurons excited with iontophoretically applied excitatory amino acid agonists, N-methyl-D-aspartate (NMDA) and (R,S)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or kainic acid. Nitric oxide availability was decreased by iontrophoretic application of NO synthase inhibitors, N omega-nitro-L-arginine methyl ester (L-NAME) or L-N5-(1-iminoethyl)ornithine (L-NIO), or elevated by the NO donating compound, S-nitroso-N-penicillamine (SNAP). When cells were excited with successive application of NMDA and non-NMDA excitatory amino acid receptor agonists, application of NO synthase inhibitors led to a decrease in responses to NMDA in 60% of neurons. In more than a third of the cells tested, inhibition of NO synthase caused reciprocal changes in responses to glutamate receptor agonists: NMDA-evoked responses were significantly decreased whereas responses to the non-NMDA receptor agonists (AMPA or kainic acid) were increased. Application of the NO donating compound, S-nitroso-N-penicillamine, revealed an opposite tendency, increasing responses to NMDA in more than half of the neurons tested. In approximately 40% of the cells, reciprocal changes in responses to excitatory amino acid receptor agonists of NMDA versus non-NMDA types were observed after application of S-nitroso-N-penicillamine, such that the increase in NMDA responses was accompanied by decreases in the responses to kainic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

AMPA receptor activation regulates the glutamate metabotropic receptor stimulated phosphatidylinositol turnover in human cerebral cortex slices

The effect of excitatory amino acids (EAA) on phosphatidylinositol (PI) turnover in human cerebral cortical slices was investigated. Trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) increased inositol phosphate (IP) formation in the 1-1000 microM range. Quisqualic acid (QA) was maximally effective at 10-100 microM, showing an inverse correlation between concentration and effect in the 100-1000 microM range. The glutamate metabotropic receptor antagonist 2-amino-3-phosphonopropionic acid (AP3), the ionotropic non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the NMDA channel blocker dizolcipine (MK-801) failed to prevent the PI response to ACPD (1000 microM). However, CNQX (100 microM) modified the concentration-response curve of QA reducing the effect of QA 10 microM by approx. 50% and enhancing that of QA 1000 microM by 2-fold. In addition, CNQX (100 microM) together with MK-801 (100 microM) unmasked the ability of L-glutamate (L-GLU) 3000 microM to stimulate PI turnover. The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on the EAA-induced PI turnover was also studied. AMPA (0.1-1 microM) potentiated the response to submaximal (30 microM) ACPD and (1 microM) QA concentrations. However, higher AMPA concentrations (10 microM) failed to synergize with ACPD 30 microM and, in addition, inhibited the PI turnover maximally stimulated by QA 10 microM. These results further support the presence of the glutamate metabotropic receptor in the human neocortex. In addition, they show the occurrence of a concentration-related dual interaction between AMPA and glutamate metabotropic receptor activation in the IP formation in this brain area.

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.

Actions of phenylglycine analogs at subtypes of the metabotropic glutamate receptor family

The functional effects of phenylglycine analogs on metabotropic glutamate receptor (mGluR) subtypes mGluR1 alpha, mGluR2 and mGluR4 were examined. (S)-4-Carboxyphenylglycine (IC50 = 65 +/- 5 microM), (R,S)-alpha-methyl-4-carboxyphenylglycine (IC50 = 155 +/- 38 microM) and (S)-3-carboxy-4-hydroxyphenylglycine (IC50 = 290 +/- 47 microM) competitively antagonized glutamate-stimulated phosphoinositide hydrolysis in baby hamster kidney (BHK) cells stably expressing mGluR1 alpha. (S)-4-Carboxyphenylglycine and (R,S)-alpha-methyl-4-carboxyphenylglycine competitively antagonized glutamate-induced inhibition of forskolin-stimulated cAMP-formation in BHK cells stably expressing mGluR2 with IC50 values of 577 +/- 74 microM and 340 +/- 59 microM, respectively. (R,S)-4-carboxy-3-hydroxyphenylglycine, (R)-3-hydroxyphenylglycine and (S)-3-carboxy-4-hydroxyphenylglycine were agonists at mGluR2 with EC50 values of 48 +/- 5 microM, 451 +/- 93 and 97 +/- 12 microM, respectively. In parallel experiments, no activities of these phenylglycine analogs at mGluR4 were observed. The present report demonstrates that phenylglycine analogs possess differential functional activities at subtypes of the mGluR family.

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.

Glutamate-stimulated phosphatidylinositol metabolism in the avian cochlear nucleus

This study examined the ability of the excitatory amino acid glutamate and its analogs to stimulate phosphatidylinositol metabolism in isolated cochlear nucleus tissue from young chicks. In the presence of lithium chloride, glutamate and (+/-)-1-aminocyclopentyl-trans-1,3-dicarboxylate (ACPD) stimulated the formation of inositol phosphates to levels significantly above unstimulated control levels. Unexpectedly, quisqualate did not stimulate inositol phosphates formation. The N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV), the ionotropic kainate/quisqualate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the putative metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionate (AP3) had no effect on the glutamate stimulated formation of inositol phosphates. We conclude that a metabotropic glutamate receptor is present on cochlear nucleus neurons of posthatch chicks and is able to stimulate formation of inositol phosphates.

Excitatory amino acids (EAAs) stimulate phosphatidylinositol turnover in adult rat striatal slices: interaction between NMDA and EAA metabotropic receptors

The effect of excitatory amino acids (EAAs) on phosphatidylinositol (PI) turnover in adult rat striatal slices was investigated. Quisqualic acid (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainic acid (KA), ibotenic acid (IBO) and N-methyl-D-aspartic acid (NMDA) maximally increased inositol phosphate (IP) formation at 10 microM while trans-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was maximally effective at 100 microM. The NMDA channel blocker dizolcipine (MK-801) counteracted the effect of NMDA 10 microM and IBO 10 microM while it potentiated that of IBO 100 microM and IBO 1000 microM. Conversely, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) prevented the effect of AMPA and KA and reduced that of QA (all at 10 microM). Lowering extracellular Ca2+ concentrations ([Ca2+]0) differentially affected the PI response to EAAs. The ACPD 30 microM effect was unchanged at low [Ca2+]0 (but abolished when EGTA 2 mM was added), while that of ACPD 100 microM was halved in 0.1 mM and almost abolished in a nominally free Ca2+ medium. NMDA 10 microM and AMPA 10 microM were ineffective at low [Ca2+]0 while NMDA 100 microM, ineffective in a 1.2 mM Ca2+ medium, strongly stimulated IP formation in 0.1 mM Ca2+ but not in a nominally free Ca2+ medium. The effect of NMDA on EAA metabotropic receptor agonist stimulated PI turnover was also studied. NMDA 10 microM potentiated the effect of ACPD 30 microM. This positive cooperation persisted at low [Ca2+]0 but not in the presence of EGTA. Conversely, NMDA 100 microM prevented the effect of ACPD 100 microM. This negative interference was reversed when Ca2+ was omitted from the medium. This study shows that in the adult rat striatum both EAA metabotropic and ionotropic receptor activation increases IP formation. A positive and negative interaction between NMDA and metabotropic receptor activation was also found to regulate PI turnover. The role of [Ca2+]0 in subserving the PI response to EAAs was made evident.

Metabotropic glutamate receptors trigger postsynaptic protein synthesis

K+ depolarization or addition of glutamate to a synaptoneurosome preparation triggers a rapid increase in size of polyribosomal aggregates isolated by centrifugation of lysate through 1 M sucrose. The profile of response to the glutamate analogues quisqualate, ibotenate, and 1-aminocyclopentane-1,3-dicarboxylate corresponds to that of metabotropic receptors. Glutamate stimulation is mimicked by the diacylglycerol analogue 1-oleoyl-2-acetylglycerol and by the protein kinase C activator phorbol dibutyrate. The phospholipase blockers 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate and quinacrine reduce the late phase of the response. The protein kinase C inhibitor calphostin C suppresses the response to 1-aminocyclopentane-1,3-dicarboxylate. These data indicate that glutamatergic synapses upregulate postsynaptic protein synthesis via metabotropic glutamate receptors coupled to the phosphatidylinositol second-messenger system. This mechanism could underlie the reported involvement of metabotropic glutamate receptors in long-term potentiation and other forms of neural plasticity.

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

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)

Excitatory amino acid receptor-stimulated phosphoinositide turnover in primary cerebrocortical cultures

1. Characterization of excitatory amino acid-induced accumulation of [3H]-phosphoinositides was carried out in primary cerebrocortical cultures isolated from foetal rats. 2. All of the excitatory amino acid receptor agonists examined caused concentration-dependent enhancement of phosphoinositide (PI) formation. The most potent excitatory amino acid receptor agonists were quisqualate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), ibotenate and glutamate with mean EC50 values of 0.9 +/- 0.4 microM, 15 +/- 5 microM, 15 +/- 3 microM and 41 +/- 8 microM respectively. 3. The selective ionotropic receptor antagonists kynurenic acid (1 mM), 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX, 10 microM) and (+/-)-4-(3-phosphonopropyl)-2 piperazinecarboxylic acid (CPP, 100 microM), failed to block responses to quisqualate, (1S,3R)-ACPD or glutamate. D,L-2-Amino-3-phosphonopropionate (D,L-AP3) did not block 1S,3R-ACPD or quisqualate-induced PI turnover, but had an additive effect with quisqualate or (1S,3R)-ACPD. 4. Exposure of cultures to agonists in the absence of added extracellular calcium reduced the maximal quisqualate response by approximately 45%, revealing a two-component concentration-response curve. Concentration-response curves to ibotenate and glutamate became flattened by omission of extracellular calcium, whereas (1S,3R)-ACPD-stimulated PI turnover was unaffected. 5. Pretreatment of cultures with pertussis toxin markedly inhibited PI responses evoked by (1S,3R)-ACPD. 6. These results suggest that excitatory amino acid-stimulated PI turnover in cerebrocortical cultures is independent of ionotropic receptor activation and is mediated via specific G-protein-linked metabotropic receptors. The partial dependence of the responses to quisqualate, ibotenate and glutamate on the presence of extracellular calcium suggests that the effects of these agonists may be mediated by more than one receptor subtype.

Characterization of the metabotropic glutamate receptor in mouse cerebellar granule cells: lack of effect of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline (NBQX)

The ability of excitatory amino acids to stimulate phosphoinositide hydrolysis in mouse cerebellar granule cells was characterized. Quisqualic acid (EC50 = 2 microM), ibotenic acid (EC50 = 15 microM), kainic acid (EC50 = 30 microM), glutamate (EC50 = 51 microM) and (1S,3R)-1-amino-cyclo-pentane-1,3-dicarboxylic acid (t-ACPD) (EC50 = 175 microM) dose-dependently stimulated phosphoinositide hydrolysis. The stimulation of phosphoinositide hydrolysis was dose-dependently blocked by 2-amino-3-phosphonopropionic acid (L-AP3) and pertussis toxin, but was unaffected by other excitatory amino acid agonists or antagonists. These data suggest that the pharmacology of excitatory amino acid-stimulated phosphoinositide hydrolysis in the mouse cerebellar granule cells is mediated through the G protein coupled metabotropic glutamate receptor. The overall pharmacology of the metabotropic receptor present in mouse cerebellar granule cells differs from that of previously reported tissue preparations such as rat cerebellar granule cells. In addition, the effect of the alpha-amino-3-hydroxyl-5-methyl-1-isoxazole-4-propionic acid (AMPA) receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX), on excitatory amino acid-stimulated phosphoinositide hydrolysis was also examined. NBQX was without effect on either basal phosphoinositide hydrolysis or excitatory amino acid-stimulated phosphoinositide hydrolysis, suggesting that the neuroprotective effect of NBQX is not mediated through the metabotropic glutamate receptor.

(2s,1'R,2'R,3'R)-2-(2,3-Dicarboxycyclopropyl) glycine enhances quisqualate-stimulated inositol phospholipid hydrolysis in hippocampal slices

In adult rat hippocampal slices, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) did not affect the basal hydrolysis of polyphosphoinositides but dramatically enhanced (EC50 value = 30 nM) the stimulation of [3H]inositol monophosphate (InsP) formation by quisqualate, without substantially affecting the stimulation produced by maximal concentrations of 1S,3R-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) or carbamylcholine. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) was virtually devoid of activity on [3H]InsP formation, either in the absence or presence of DCG-IV. These results suggest that DCG-IV acts, directly or indirectly, as a positive modulator of metabotropic glutamate receptors in the rat hippocampus.

Glutamate metabotropic receptor activation in neonatal rat cerebral cortex by sulphur-containing excitatory amino acids

The sulphur-containing acidic amino acids (SAAs) display neuroexcitatory actions similar to those of L-glutamate and are widely regarded as bona fide transmitter candidates. In this study, L-cysteine sulphinic acid, L-cysteic acid, DL-homocysteic acid and L-homocysteine sulphinic acid were investigated for their ability to stimulate phosphatidylinositol hydrolysis in rat pup cerebrocortical slices and compared with L-glutamate and the selective agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD). Each of the SAAs possessed agonist activity at metabotropic glutamate receptors (mGluRs) and, although they exhibited lower efficacy than L-glutamate, they were more potent (EC50s of 401-487 microM compared with 791 microM for L-glutamate). These data are consistent with the possibility that SAAs may have a physiological role as endogenous activators of metabotropic (and presumably ionotropic) excitatory amino acid receptors.

Co-activation of metabotropic glutamate and N-methyl-D-aspartate receptors is involved in mechanisms of long-term potentiation maintenance in rat hippocampal CA1 neurons

Slices of hippocampal area CA1 in the rat were employed to test the hypothesis that the activation of metabotropic glutamate receptors during tetanization is necessary for the late maintenance of long-term potentiation. If the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionate was present during tetanization, post-tetanic and early long-term potentiation of the population spike as well as field excitatory postsynaptic potential developed almost normally. However, 100 min after tetanization, long-term potentiation of the field excitatory postsynaptic potential decreased in an irreversible manner. The same concentration of D-2-amino-3-phosphonopropionate was ineffective. If L-2-amino-3-phosphonopropionate was applied 120 min after tetanization, it did not influence long-term potentiation. The presence of the metabotropic glutamate receptor agonist trans-D,L-1-aminocyclopentane-1,3-dicarboxylic acid during tetanization weakly enhanced the slope of field excitatory postsynaptic potential long-term potentiation. The influence of L-2-amino-3-phosphonopropionate and D,L-1-aminocyclopentane-1,3-dicarboxylic acid on ionotropic glutamate receptors was studied using whole-cell voltage-clamp and pressure application techniques. No effect of L-2-amino-3-phosphonopropionate on either early or late components of excitatory postsynaptic currents could be detected at the concentration used to block long-term potentiation. It is therefore unlikely that the effect of L-2-amino-3-phosphonopropionate on long-term potentiation is due to an interaction with N-methyl-D-aspartate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors. However, bath-applied 1S,3R-D,L-1-aminocyclopentane-1,3-dicarboxylic acid facilitated the N-methyl-D-aspartate-induced depolarization in response to N-methyl-D-aspartate pressure application in a reversible manner. These data suggest that besides the involvement of N-methyl-D-aspartate receptors the activation of a 2-amino-3-phosphonopropionate-sensitive metabotropic glutamate receptors during or immediately after tetanization is necessary for subsequent mechanisms responsible for the maintenance of long-term potentiation. A link between metabotropic glutamate receptors and protein kinase C activation during long-term potentiation is discussed considering the similar time course of long-term potentiation blockade after application of L-2-amino-3-phosphonopropionate and protein kinase C inhibitors.

(R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors mediate a calcium-dependent inhibition of the metabotropic glutamate receptor-stimulated formation of inositol 1,4,5-trisphosphate

L-glutamate (3-1,000 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 10-1,000 microM), a selective agonist for the metabotropic glutamate receptor, stimulated the formation of inositol 1,4,5-trisphosphate in a concentration-dependent manner. L-Glutamate was half as efficacious as 1S,3R-ACPD. N-methyl-D-aspartate (NMDA; 1 nM to 1 mM) did not significantly influence the response to a maximally effective concentration of 1S,3R-ACPD (100 microM). On the other hand, coapplication of (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 1-300 nM) produced a concentration- and time-dependent inhibition of the 1S,3R-ACPD effect, with a maximal inhibition (97%) at 100 nM. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the AMPA receptor, blocked the inhibitory effect of AMPA. Reduced extracellular calcium concentration, as well as 10 microM nimodipine, an L-type calcium channel antagonist, inhibited the AMPA influence on the 1S,3R-ACPD response. W-7, a calcium/calmodulin antagonist, prevented the inhibition by AMPA, whereas H-7, an inhibitor of protein kinase C, had no effect. These data suggest that activation of AMPA receptors has an inhibitory influence on inositol 1,4,5-trisphosphate formation mediated by stimulation of the metabotropic glutamate receptor. The mechanism of action involves calcium influx through L-type type calcium channels and possible activation of calcium/calmodulin-dependent enzymes.

Norepinephrine reverses N-methyl-D-aspartate-mediated inhibition of long-term potentiation in rat hippocampal slices

Application of 1 microM N-methyl-D-aspartate (NMDA) either immediately before or following tetanic stimulation inhibits the induction of CA1 hippocampal long-term potentiation (LTP). We have examined the effect of trans-amino-1,3-cyclopentanedicarboxylic acid (ACPD), norepinephrine and acetylcholine on this NMDA-mediated LTP inhibition using extracellular recordings from in vitro rat hippocampal slices. When NMDA is accompanied by 100 microM ACPD or 10 microM norepinephrine, the block of LTP is overcome. The norepinephrine effect is mimicked by phenylephrine and methoxamine and is blocked by phentolamine and prazosin suggesting the involvement of alpha 1-adrenergic receptors.

Effects of N-methyl-D-aspartate on quisqualate-stimulated phosphoinositide hydrolysis in slices of kitten striate cortex

Stimulation of phosphoinositide (PI) hydrolysis by excitatory amino acids (EAAs) was studied in coronal slices of kitten visual cortex. Coincubation with N-methyl-D-aspartate (NMDA) markedly reduced the stimulation by quisqualate, however, this inhibition developed with a latency of > 10 min and occurred even when the NMDA exposure preceded, but did not overlap with, incubation in quisqualate. This time-course of NMDA inhibition of EAA-stimulated PI turnover places new constraints on its possible mechanism of inhibition.

N-methyl-D-aspartate-mediated injury enhances quisqualic acid-stimulated phosphoinositide turnover in perinatal rats

Previous work in our laboratory demonstrated that ischemic-hypoxic brain injury in postnatal day 7 rats causes a substantial increase in phosphoinositide (PPI) turnover stimulated by the glutamate analogue quisqualic acid (QUIS) in the hippocampus and striatum. To examine this phenomenon in more detail, we performed similar experiments after producing injury by unilateral intracerebral injections of the glutamate analogue N-methyl-D-aspartate (NMDA). The 7-day-old rodent brain is hypersensitive to NMDA neurotoxicity and NMDA injection causes histopathology that closely resembles that produced by ischemia-hypoxia. NMDA, 17 nmol in 0.5 microliter, was injected into the right posterior striatum of 7-day-old rat pups and they were killed 3 days later. Hippocampal or striatal tissue slices were prepared from ipsilateral and contralateral hemispheres from vehicle-injected control and from noninjected control rat pups. Slices were then incubated with myo-[3H]inositol plus glutamate agonists or antagonists in the presence of lithium ions and [3H]inositol monophosphate ([3H]IP1) accumulation was measured. The glutamate agonists, QUIS, L-glutamic acid, and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, stimulated greater [3H]IP1 release in tissue ipsilateral to the NMDA injection compared with that in the contralateral side and in control pups. The glutamate antagonists, D,L-2-amino-7-phosphonoheptanoic acid, 3-[(+)-2-carboxypiperazin-4-yl]-propyl-1-phosphoric acid, kynurenic acid, and 6,7-dinitroquinoxaline-2,3-dione did not inhibit QUIS-stimulated [3H]IP1 release. The enhanced PPI turnover in the lesioned tissue was specific to glutamate receptors because carbachol (CARB) failed to elicit preferential enhanced stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate Inhibits Adenylate Cyclase Activity in Dispersed Rat Hippocampal Cells Directly via an N-Methyl-d-Aspartate-Like Metabotropic Receptor

Three major subtypes of glutamate receptors that are coupled to cation channels--N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors--are known as ionotropic receptors in the mammalian CNS. Recently, an additional subtype that is coupled to GTP binding proteins and stimulates (or inhibits) metabolism of phosphoinositides has been proposed as a metabotropic receptor. Incubation of dispersed hippocampal cells from adult rats with glutamate or NMDA decreased forskolin-stimulated cyclic AMP (cAMP) accumulation; half-maximal effects were obtained with 5.6 +/- 2.2 and 6.4 +/- 2.3 microM, respectively. Kainate and quisqualate were less potent. The effect of glutamate was antagonized by 2,3-diaminopropionate and 2-amino-5-phosphonovalerate, NMDA/glutamate receptor antagonists, but not by 0.5 microM Joro spider toxin, a specific blocker of the AMPA receptor. The inhibitory effect of glutamate on cAMP formation was not blocked by 2 microM tetrodotoxin or by the absence of Ca2+. In hippocampal membranes, glutamate, similar to carbachol, inhibited adenylate cyclase activity in a GTP-dependent manner. These findings suggest that the glutamate inhibition of adenylate cyclase is direct and is not due to a result of the release of other neurotransmitters. The effect of glutamate on cAMP accumulation was observed in an assay medium containing 0.7 mM MgCl2, which is known to inhibit both ionotropic NMDA receptor/channels in the hippocampus and metabotropic NMDA receptors in the cerebellum. The inhibitory effect of glutamate was abolished by pertussis toxin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Magnesium-dependent inhibition of agonist-stimulated phosphoinositide breakdown in rat cortical slices by excitatory amino acids

The excitatory amino acid agonists kainate, N-methyl-D-aspartate (NMDA), and quisqualate inhibited ligand-stimulated phosphoinositide hydrolysis in rat cortical slices. The NMDA channel blocker MK-801 antagonized the inhibition by NMDA but had no effect on the inhibition due to kainate or quisqualate. The antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the effects of quisqualate and kainate but not the effect of NMDA. These data indicate that activation of the NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate types of ionotropic receptors has the same effect. In membranes prepared from cortical slices, there was no inhibition of carbachol-stimulated phosphoinositidase C activity by excitatory amino acids, suggesting that excitatory amino acids indirectly affect carbachol-stimulated phosphoinositide hydrolysis. The inhibition by excitatory amino acids of carbachol-stimulated phosphoinositide breakdown was dependent on extracellular Mg2+ and was abolished by procedures that increase intracellular Ca2+. Veratridine inhibition of carbachol-stimulated phosphoinositide hydrolysis was reversed by ouabain but not by other procedures that increase intracellular Ca2+. In contrast to excitatory amino acids, veratridine potentiated carbachol-stimulated phosphoinositide breakdown in the presence of 10 mM extracellular Mg2+. These data suggest that excitatory amino acids inhibit carbachol-stimulated phosphoinositide breakdown in rat cortex by lowering intracellular Ca2+ through a mechanism dependent on extracellular Mg2+.

Inhibition of the phospholipase C-linked metabotropic glutamate receptor by 2-amino-3-phosphonopropionate is dependent on extracellular calcium

D,L-2-Amino-3-phosphonopropionate (AP-3), a proposed metabotropic receptor antagonist, produced a concentration-dependent increase in the formation of inositol 1,4,5-trisphosphate in rat hippocampal slices. The response was maximal at 1 mM and completely due to the L-isomer. D,L-AP-3 was half as efficacious as (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist of this receptor. The response produced by maximally effective concentrations of L-AP-3 and 1S,3R-ACPD together for 5 min was not significantly different from that produced by 1S,3R-ACPD alone. However, pretreatment for 40 min with either 1 mM L-AP-3 or D,L-AP-3 completely inhibited the response to 1S,3R-ACPD. This inhibition was long-lasting (wash-resistant) and was reversed by reduction of the extracellular Ca2+ concentration. Also, pretreatment for 40 min with 1S,3R-ACPD reduced, but did not completely block, the response to readdition of 1S,3R-ACPD. L-AP-3 (1 mM) also produced a stereoselective 2.3-fold increase in the efflux of glutamate from the hippocampal slices. These data suggest that incubation of hippocampal slices with AP-3 induces a time-dependent desensitization of the metabotropic response by a mechanism that is dependent on extracellular Ca2+. The possible roles of receptor occupancy and inhibition of glutamate uptake by AP-3 are also discussed.

Induction of NMDA receptor-independent long-term potentiation (LTP) in visual cortex of adult rats

The aim of this study was to examine: (1) whether long-term potentiation (LTP) can be induced in slices from adult rat visual cortex under conditions where inhibition is not antagonized, and (2) the role of N-methyl-D-aspartate (NMDA) receptors in its induction. The field potential elicited in layer III in response to stimulation of the subcortical white matter consisted of a component with peak latency 5-8 ms (N1) and, in most slices, a second component with peak latency 13-19 ms (N2). N1 was generated via both kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and NMDA receptor activation as revealed by bath application of 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D,L-2-amino-5-phosphonovalerate (APV). N2 was insensitive to APV in most of the slices and was probably polysynaptic since it did not follow stimulation at 0.5 Hz. Tetanic stimulation of the white matter in normal medium induced LTP of N1; in some slices N2 also potentiated. Tetanic stimulation in the presence of APV also induced LTP of N1 and sometimes N2. LTP of N1 induced in APV was of a larger magnitude, and was expressed more quickly than LTP induced in normal medium. It appears that the known reduction of NMDA receptor activity in adult neocortex is accompanied by the development of other mechanisms that maintain synaptic plasticity; these mechanisms seem to operate more efficiently in absence of NMDA receptor activation.

L-glutamate and acetylcholine mobilise Ca2+ from the same intracellular pool in cerebellar granule cells using transduction mechanisms with different Ca2+ sensitivities

Ca2+ mobilisation induced by L-glutamate (Glu) and acetylcholine (ACh) has been studied in cultured cerebellar granule cells using digital fluorescence microscopy. The ability of Glu-receptor activation to mobilise Ca2+ was decreased when [Ca2+]o was lowered to 10 microM (from 1.8 mM). It was enhanced when [Ca2+]i was raised using 25 mM external K+ or by N-methyl-D-aspartate (NMDA), which selectively activates a distinct Glu-receptor subtype. The enhancement was dependent on entry of external Ca2+. In contrast, the ability of ACh receptor activation to mobilise Ca2+ was not affected by these conditions. Furthermore, pretreatment with pertussis toxin inhibited Ca2+ mobilisation in response to Glu-receptor activation without affecting mobilisation in response to ACh. However, activation of both receptors mobilised Ca2+ from a common, thapsigargin-sensitive pool. We conclude that there are differences in the Ca2+ mobilization pathways for the two receptor systems in cerebellar granule cells. The Ca(2+)-sensitivity of this Ca2+ mobilizing Glu receptor may have implications for its function in neuronal synaptogenesis and plasticity.

Astrocyte Heterogeneity: Endogenous Amino Acid Levels and Release Evoked by Non-N-Methyl-D-Aspartate Receptor Agonists and by Potassium-Induced Swelling in Type-1 and Type-2 Astrocytes

The aim of the present study was to determine whether endogenous amino acids are released from type-1 and type-2 astrocytes following non-N-methyl-D-aspartate (NMDA) receptor activation and whether such release is related to cell swelling. Amino acid levels and release were measured by HPLC in secondary cultures from neonatal rat cortex, highly enriched in type-1 or type-2 astrocytes. The following observations were made. (a) The endogenous level of several amino acids (glutamate, alanine, glutamine, asparagine, taurine, serine, and threonine) was substantially higher in type-1 than in type-2 astrocytes. (b) The spontaneous release of glutamine and taurine was higher in type-1 than in type-2 astrocytes; that of other amino acids was similar. (c) Exposure of type-2 astrocyte cultures to 50 microM kainate or quisqualate doubled the release of glutamate and caused a lower, but significant increase in that of aspartate, glycine, taurine, alanine, serine (only in the case of kainate), and glutamine (only in the case of quisqualate). These effects were reversed by the antagonist CNQX. (d) Exposure of type-1 astrocyte cultures to 50-200 microM kainate or 50 microM quisqualate did not affect endogenous amino acid release, even after treating the cultures with dibutyryl cyclic AMP. (e) Exposure of type-1 or type-2 astrocyte cultures to 50 mM KCl (replacing an equimolar concentration of NaCl) enhanced the release of taurine greater than glutamate greater than aspartate. The effect was somewhat more pronounced in type-2 than in type-1 astrocytes. Veratridine (50 microM) did not cause any increase in amino acid release. (f) The release of amino acids induced by high [K+] appeared to be related to cell swelling, in both type-1 and type-2 astrocytes. Swelling and K(+)-induced release were somewhat higher in type-2 than in type-1 astrocytes. In contrast, neither kainate nor quisqualate caused any appreciable increase in cell volume. It is concluded that non-NMDA receptor agonists stimulate the release of several endogenous amino acids (some of which are neuroactive) from type-2 but not from type-1 astrocytes. The effect does not seem to be related to cell swelling, which causes a different release profile in both type-1 and type-2 astrocytes. The absence of kainate- and quisqualate-evoked release in type-1 astrocytes suggests that the density of non-NMDA receptors in this cell type is very low.

Biochemical responses mediated by N-methyl-D-aspartate receptors in rat cortical slices are differentially sensitive to magnesium

The effects of magnesium on the inhibition of phosphoinositide (PI) hydrolysis and the stimulation of [3H]norepinephrine release by N-methyl-D-aspartate (NMDA) in rat cortical slices were investigated. Removal of the magnesium from the buffer resulted in a small reduction of the inhibitory effect of 100 microM NMDA (34% inhibition in the absence of magnesium, compared with 51% for the control) when slices were coincubated with NMDA and carbachol. Addition of 10 mM Mg2+ also allowed the inhibitory effect of 100 microM NMDA on carbachol-stimulated PI hydrolysis to be expressed (44% inhibition) under these conditions. Concentration-effect curve analysis for the NMDA-induced inhibition of carbachol-stimulated PI hydrolysis indicated that the IC50 for NMDA was decreased from 14.9 microM for the control to 4.2 microM in the absence of magnesium. The absence of magnesium also had small effects on the concentration-effect curve for (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate reversal of the inhibitory effects of NMDA on carbachol-stimulated PI hydrolysis. The absence of magnesium also shifted slightly downward and flattened the NMDA concentration-effect curve if the cortical slices were pretreated with NMDA in the presence or absence of magnesium followed by removal of the NMDA and subsequent stimulation with carbachol. In contrast, cortical slices that had been prepared and treated similarly to the slices used in the PI experiments were very sensitive to the inhibitory effects of magnesium when using the NMDA stimulation of [3H]norepinephrine release assay in the presence or absence of carbachol.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of excitatory amino acid-stimulated phosphoinositide hydrolysis in rat hippocampus by l-aspartate-β-hydroxamate

The effect of a series of glutamate uptake inhibitors was tested on ibotenate-stimulated phosphoinositide hydrolysis. The pharmacological profile of the inhibitory effect of these compounds on the ibotenate response was quite different from that on glutamate uptake. Aspartate-beta-hydroxamate was the most potent compound with the L-isomer (IC50 11 +/- 2 microM) being considerably more potent than the D-isomer (IC50 104 +/- 12 microM). The effect of the L-aspartate-beta-hydroxamate was found to be specific for ibotenate and quisqualate-stimulated phosphoinositide hydrolysis; this compound did not affect hydrolysis stimulated by carbachol, K+ or sodium fluoride. The inhibition of the ibotenate response was found to involve a non-competitive and irreversible mechanism.

Quisqualate-stimulated phosphatidylinositol breakdown in rat cerebellar membranes

The effect of quisqualate, an excitatory amino acid agonist, on the breakdown of exogenously added phosphatidylinositol was investigated in a membrane preparation from the cerebellum of young rats. Quisqualate stimulated phospholipase C activity in a dose-dependent manner in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). Half-maximal activation of the quisqualate response required 0.15 microM GTP gamma S and was optimal at a free Ca2+ concentration of 300 nM. Phosphoinositide breakdown was also stimulated by quisqualate using either exogenous phosphatidylinositides 4,5-bisphosphate or endogenous labeled phosphoinositides as the substrate for phospholipase C in cerebellar membranes. In the presence of guanine nucleotides, other excitatory amino acid agonists, such as L-glutamate, trans-D,L-1-aminocyclopentyl-1,3-dicarboxylic acid, and ibotenate, but not N-methyl-D-aspartate, stimulated phosphatidylinositol breakdown. However, quisqualate displayed the highest response among these excitatory amino acid agonists. These data indicate that there is a direct activation of phosphoinositide-specific phospholipase C by excitatory amino acids through a process dependent on the presence of guanine nucleotides.

Activation of the metabotropic glutamate receptor attenuates N-methyl-D-aspartate neurotoxicity in cortical cultures

Excitatory amino acid receptor-mediated neurotoxicity (excitotoxicity) has been proposed to contribute to neuronal loss in a wide variety of neurodegenerative conditions. Although considerable evidence has accumulated implicating N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in the processes of excitotoxicity, relatively little research has focused on the ability of other neurotransmitter systems to influence excitotoxic neuronal injury. In the present study, we examined the effects of trans-1-aminocyclopentyl-1,3-dicarboylic acid (ACPD), a selective agonist for the metabotropic glutamate, or ACPD, receptor, and carbachol, an agonist at the acetylcholine receptor, on neuronal degeneration produced by brief exposure to NMDA in murine cortical cultures. Since excitotoxic neuronal injury is probably caused by increases in intracellular Ca2+ concentrations, the two transmitter agonists were of particular interest as both have been shown to mobilize intracellular calcium stores. Contrary to what might be expected, ACPD and, to a lesser degree, carbachol attenuated NMDA neurotoxicity. The neuroprotective effect of ACPD, but not of carbachol, was dependent upon the developmental state of cultures; in older cultures (greater than or equal to 18 days in vitro), the protective effect decreased. The neuroprotection by ACPD may be, in part, mediated by protein kinases, since protection is partially reversed by the protein kinase antagonists H-7 and HA-1004. These data suggest that concomitant activation of the ACPD receptor may serve as a protective mechanism against neurotoxicity that could be produced by brief intense NMDA receptor activation during normal or abnormal brain function.

GABAB receptor stimulation by baclofen and taurine enhances excitatory amino acid induced phosphatidylinositol turnover in neonatal rat cerebellum

Excitatory amino acid stimulation of phosphatidylinositol (PI) hydrolysis has been associated with development of the CNS. Normally minimally ineffective in stimulating PI hydrolysis in the neonatal rat cerebellum, N-methyl-D-aspartate (NMDA) increased levels of PI hydrolysis 82.3 +/- 5.5% above basal values in the presence of 1 microM baclofen, a gamma-aminobutyric acidB (GABAB) receptor agonist. This effect was observed at day 7 but not in adult cerebellum. The effect of baclofen could be mimicked by low dose GABA and taurine, actions which were blocked by prior application of a specific GABAB antagonist. Therefore, the ability of NMDA to stimulate PI hydrolysis in neonatal cerebellar tissue may be regulated by the degree of GABAB receptor stimulation.

A metabotropic glutamate receptor agonist does not mediate neuronal degeneration in cortical culture

In light of the evidence that calcium plays a critical role in excitotoxic neuronal death, it has been speculated that the metabotropic glutamate receptor may also contribute to excitotoxic damage through the mobilization of Ca2+ from intracellular stores. In the present study we examined this possibility by studying the neurotoxicity of trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD), a selective agonist of the metabotropic glutamate receptor. Exposure of cortical neurons to 100 microM trans-ACPD substantially increased phosphoinositide hydrolysis and intraneuronal free calcium in the presence of CPP and CNQX. Despite the presence of functional metabotropic receptors on cultured neurons, however, exposure of cultures to as high as 1 mM trans-ACPD for 24 h failed to produce any morphological or chemical signs of neuronal damage. Furthermore, trans-ACPD did not potentiate submaximal neurotoxicity produced by other non-N-methyl-D-aspartate (NMDA) agonists, kainate and D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA).

Comparison of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD)- and 1R,3S-ACPD-stimulated brain phosphoinositide hydrolysis

(1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and (1R,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1R,3S-ACPD) were characterized for potency, efficacy, and selectivity at metabotropic excitatory amino acid receptors. 1S,3R-ACPD stimulated [3H]phosphoinositide hydrolysis in slices of the neonatal and adult rat hippocampus with full efficacy and twice the potency relative to what has been shown for (+/-)-trans-ACPD. 1S,3R-ACPD was up to 30 times more potent in activating metabotropic excitatory amino acid receptors, compared to its affinity for [3H]CGS19755 binding to NMDA receptors. In contrast, 1R,3S-ACPD was much less potent, efficacious, and selective than 1S,3R-ACPD. Although 1S,3R-ACPD is not specific, it is a most selective and efficacious agonist at metabotropic excitatory amino acid receptors.