Pharmacological and functional characteristics of metabotropic excitatory amino acid receptors

N-methyl-D-aspartate receptor-independent long-term potentiation in area CA1 of rat hippocampus: input-specific induction and preclusion in a non-tetanized pathway

We previously reported that an N-methyl-D-aspartate receptor-independent component of long-term potentiation with an apparent delayed onset can be induced in area CA1 of the hippocampus. Here we show that some but not all of this delay in onset can be accounted for by a transient heterosynaptic depression. We also show that N-methyl-D-aspartate receptor-independent long-term potentiation is induced only in the input pathway tetanized, and not in a second pathway. However, prior induction of N-methyl-D-aspartate receptor-independent long-term potentiation in one pathway precludes later induction in an independent pathway. Calcium entry through dihydropyridine-sensitive Ca2+ channels may be a critical step for induction of N-methyl-D-aspartate receptor-independent long-term potentiation in area CA1 [Grover L. M. and Teyler T.J. (1990) Nature 347, 477-479]. Since the distribution [Westenbroek R. E. et al. (1990) Nature 347, 281-284] of dihydropyridine-sensitive Ca2+ channels in CA1 neuron dendrites does not suggest a basis for input-specific induction of long-term potentiation, an additional process may confer the specificity we observed. Tetanic stimulation of afferents into area CA1 can elicit several processes: a transient heterosynaptic depression, and a transient homosynaptic potentiation, as well as N-methyl-D-aspartate receptor-dependent and -independent long-term potentiation.

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.

Effects of central administration of kynurenic acid on spontaneous locomotor activity in the kindled rat: a multivariate approach using the automated Digiscan monitoring system

Changes in spontaneous motor activity in kindled hooded rats were measured following intracerebroventricular administration of three doses of kynurenic acid (65, 39, and 6.5 micrograms, dissolved in 3.3 microliters isotonic saline). Behavior was measured in the automated Digiscan system on every third day during 13 days of drug administration to assess initial behavioral impairment and the development of tolerance. Activity data were collected beginning 5 min after drug administration for six consecutive 5-min samples. The results revealed a suppressive effect of central administration of kynurenic acid on the pattern of spontaneous locomotor activity and showed the development of behavioral tolerance. Initially, the degree of suppression was dose related, but as tolerance developed group differences were minimized. Most measures returned to predrug levels by day 13 except vertical movement, which remained suppressed in the 65-micrograms group throughout testing. This measure may have been more sensitive to the subtle and long-lasting motor impairments resulting from kynurenic acid.

Alternative splicing generates metabotropic glutamate receptors inducing different patterns of calcium release in Xenopus oocytes

A splice variant of the metabotropic glutamate receptor (mGluR) 1a, named mGluR1c, was isolated. Compared to mGluR1a, the predicted mGluR1c protein is 302 amino acids shorter at its C-terminal end. Despite this difference, mGluR1c activates phospholipase C in Xenopus oocytes with a pharmacological profile identical to that of mGluR1a. However, in contrast to the large fast transient responses induced by mGluR1a, mGluR1c receptors elicit a small more slowly generated long-lasting oscillatory current, suggesting that these two receptors do not generate the same pattern of Ca2+ release in Xenopus oocytes. In situ hybridization data show that mGluR1c mRNA is expressed at a lower level than the other splice variants of mGluR1. Some differences in the regional distribution of these transcripts were observed in the cerebellum, the olfactory bulb, and the striatum.

Multiple metabotropic glutamate receptors regulate hippocampal function

Selective activation of metabotropic glutamate receptors with trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) stimulates phosphoinositide hydrolysis and elicits three major physiological responses in area CA1 of the hippocampus. These include direct excitation of pyramidal cells, blockade of synaptic inhibition, and decreased transmission at the Schaffer collateral-CA1 pyramidal cell synapse. Physiological effects of trans-ACPD are thought to be mediated by activation of phosphoinositide hydrolysis. However, it is now clear that multiple metabotropic glutamate receptor subtypes exist, some of which are not coupled to phosphoinositide hydrolysis. Thus, we performed a series of studies aimed at determining whether the physiological effects of trans-ACPD in the hippocampus are mediated by activation of the predominant phosphoinositide hydrolysis-linked glutamate-receptor. We report that L-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of trans-ACPD-stimulated phosphoinositide hydrolysis, does not inhibit the physiological effects of trans-ACPD in area CA1 at concentrations that maximally inhibit trans-ACPD-stimulated phosphoinositide hydrolysis in this region. Furthermore, 1S,3S-ACPD activates the phosphoinositide hydrolysis-linked glutamate receptor but does not reduce evoked field excitatory postsynaptic potentials (EPSPs) in area CA1. However, we report that the physiological effects of 1R,3S- and 1S,3R-ACPD are consistent with the hypothesis that these effects are mediated by activation of a metabotropic glutamate receptor. Thus, our data are consistent with the hypothesis that the physiological effects of trans-ACPD in area CA1 of the hippocampus are mediated by metabotropic glutamate receptors that are distinct from the AP3-sensitive phosphoinositide hydrolysis-linked glutamate receptor.

Multiple mechanisms for inhibition of excitatory amino acid receptors coupled to phosphoinositide hydrolysis

Excitatory amino acid (EAA) analogues activate receptors that are coupled to the increased hydrolysis of phosphoinositides (PIs). In these studies, hippocampal slices were prepared from neonatal rats (6-11 days old) to characterize the effects of EAA analogues on these receptors. The concentrations of ibotenate and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) required to evoke half-maximal stimulation (EC50 values) were 28 and 51 microM, respectively. Although the data for stimulation of PI hydrolysis by ibotenate and trans-ACPD were best fit to theoretical curves that had Hill slopes of 1, data for stimulation of PI hydrolysis by quisqualate were best fit to two sites. The EC50 values were 0.43 and 44 microM. The high-affinity sites were 70% of the total. A number of EAA analogues were tested for inhibition of PI metabolism. One of these, L-aspartate-beta-hydroxamate (L-A beta HA), was identified as a novel inhibitor of this response. L-A beta HA was equipotent as an inhibitor of PI metabolism stimulated by ibotenate, quisqualate, and trans-ACPD. The data for this inhibition were best fit to two sites. Between 32 and 48% of the total sites had high affinity with IC50 values in the range of 1.2-6.3 microM. The low-affinity sites had IC50 values between 610 and 2,700 microM. DL-2-Amino-3-phosphonopropionate (DL-AP3) was also equipotent as an inhibitor of PI hydrolysis stimulated by ibotenate, quisqualate, and trans-ACPD (IC50 values were 480-850 microM). In contrast to the data for L-A beta HA, the data for DL-AP3 were best fit to a single site. Both of these inhibitors reduced the maximal response caused by the agonists, consistent with noncompetitive mechanisms of action. Several experiments were designed to examine potential mechanisms for these noncompetitive effects. These studies suggest that either L-A beta HA and DL-AP3 bind to a site on the receptor and irreversibly block activation of the receptor, or that these inhibitors act via a distinct site that specifically regulates EAA receptors coupled to PI hydrolysis.

Molecular diversity of glutamate receptors and implications for brain function

The glutamate receptors mediate excitatory neurotransmission in the brain and are important in memory acquisition, learning, and some neurodegenerative disorders. This receptor family is classified in three groups: the N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate, and metabotropic receptors. Recent molecular studies have shown that many receptor subtypes exist in all three groups of the receptors and exhibit heterogeneity in function and expression patterns. This article reviews the molecular and functional diversity of the glutamate receptors and discusses their implications for integrative brain function.

Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylyl cyclase and involvement of a pertussis toxin-sensitive G protein

We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.

Neuroprotective synergism of 2-amino-3-phosphonoproprionate (d,l-AP3) and MK-801 against ibotenate induced brain injury

The neuroprotective characteristics of the functional antagonist of metabotropic stimulated phosphoinositide hydrolysis, 2-amino-3-phosphonoproprionate (D,L-AP3), were examined alone and in combination with the non-competitive N-methyl-D-aspartate (NMDA) antagonist, MK-801, against ibotenate induced brain injury. Postnatal day (PND) 7 rats received unilateral stereotaxic intrastriatal injections of 10 nmol ibotenate and treated with either D,L-AP3 (600 nmol i.c.), MK-801 (1 mg/kg i.p.) or both. The severity of brain injury was assessed on PND 12 by comparison of the weights of injected and contralateral cerebral hemispheres. Ibotenate induced injury was partially reduced by treatment with MK-801 (34.0 +/- 4.4% protection, P < 0.05 vs. PBS treated, independent t-test) but not D,L-AP3. However, combined treatment with both MK-801 and D,L-AP3 produced marked synergistic neuroprotection (83.5 +/- 7.6% protection, P < 0.001 vs. PBS treated, independent t-test). The data suggest that metabotropic stimulated phosphoinositide hydrolysis contributes to excitotoxic neuronal injury in the presence of concurrent ionotropic receptor activation.

Trans-ACPD-induced phosphoinositide hydrolysis and modulation of hippocampal pyramidal cell excitability do not undergo parallel developmental regulation

The selective metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD), stimulates phosphoinositide hydrolysis and elicits a number of electrophysiological responses in the hippocampus. If these effects are mediated by the same receptor subtype, they should undergo parallel developmental regulation. Therefore, we compared the phosphoinositide hydrolysis response and the electrophysiological responses to trans-ACPD at two different developmental stages. Trans-ACPD-stimulated phosphoinositide hydrolysis was significantly greater in hippocampal slices from immature (6-11-day-old) rats than from adults. In contrast, trans-ACPD elicited decreases in spike frequency adaptation and in the amplitude of the slow afterhyperpolarization in roughly equal percentages of immature and adult CA1 pyramidal cells. Similar results were obtained using the putative endogenous agonist, glutamate. These data support the hypothesis that certain electrophysiological effects of trans-ACPD are mediated by a metabotropic glutamate receptor that is distinct from the phosphoinositide hydrolysis-linked glutamate receptor.

Agonist analysis of 2-(carboxycyclopropyl)glycine isomers for cloned metabotropic glutamate receptor subtypes expressed in Chinese hamster ovary cells

1. 2-(Carboxycyclopropyl)glycines (CCGs) are conformationally restricted glutamate analogues and consist of eight isomers including L- and D-forms. The agonist potencies and selectivities of these compounds for metabotropic glutamate receptors (mGluRs) were studied by examining their effects on the signal transduction of representative mGluR1, mGluR2 and mGluR4 subtypes in Chinese hamster ovary cells expressing the individual cloned receptors. 2. Two extended isomers of L-CCG, L-CCG-I and L-CCG-II, effectively stimulated phosphatidylinositol hydrolysis in mGluR1-expressing cells. The rank order of potencies of these compounds was L-glutamate > L-CCG-I > L-CCG-II. 3. L-CCG-I and L-CCG-II were effective in inhibiting the forskolin-stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation in mGluR2-expressing cells. Particularly, L-CCG-I was a potent agonist for mGluR2 with an EC50 value of 3 x 10(-7) M, which was more than an order of potency greater than that of L-glutamate. 4. L-CCG-I evoked an inhibition of the forskolin-stimulated cyclic AMP production characteristic of mGluR4 with a potency comparable to L-glutamate. 5. In contrast to the above compounds, the other CCG isomers showed no appreciable effects on the signal transduction involved in the three mGluR subtypes. 6. This investigation demonstrates not only the importance of a particular isomeric structure of CCGs in the interaction with the mGluRs but also a clear receptor subtype specificity for the CCG-receptor interaction, and indicates that the CCG isomers would serve as useful agonists for investigation of functions of the mGluR family.

Region-specific age effects on AMPA sensitivity: electrophysiological evidence for loss of synaptic contacts in hippocampal field CA1

The effects of aging on the responsiveness of hippocampal neurons to iontophoretic application of L-glutamate and AMPA were studied in vitro. There were no effects of age on neuronal responses to L-glutamate; however, CA1 pyramidal cells of old rats, but not granule cells in the fascia dentata, showed both a smaller reduction in extracellularly-recorded synaptic responses following application of AMPA (presumably mediated by depolarization), and smaller extracellular "DC" fields (measured by subtracting the DC potentials at the dendrite and soma following AMPA application in the dendrites). To examine the cellular bases of this age-related alteration in AMPA sensitivity, two additional electrophysiological approaches were used: (1) measurement of the amplitude ratios of extracellular EPSP and fiber potential components of the Schaffer collateral-CA1 response; (2) measurement of intracellularly recorded unitary EPSPs and quantal analysis of their fluctuations. The interpretations that would be placed on four hypothetical possible outcomes of such experiments are outlined and assessed in relation to the experimental data. The pattern of results obtained in the present experiments supports the following conclusions: In old rats, individual Schaffer collateral synapses do not appear to have altered AMPA receptor properties, as neither the mean size of the unitary synaptic response nor the apparent quantal size differs between age groups; however, the data do support the conclusion that there are fewer synapses per Schaffer collateral branch in old versus young CA1 pyramidal cells.

1S,3R-ACPD-sensitive (metabotropic) [3H]glutamate receptor binding in membranes

Metabotropic glutamate receptors are selectively activated by 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). [3H]Glutamate binding sites in rat brain membranes were characterized in the presence of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) to block binding to ionotropic glutamate receptors. 1S,3R-ACPD displaced a single population of [3H]glutamate binding sites and was mimicked by other metabotropic glutamate agonists with a potency order of L-glutamate > 1S,3R-ACPD > ibotenate > 1R,3S-ACPD. Quisqualate interacted at two populations of binding sites. 1S,3R-ACPD-sensitive [3H]glutamate binding was saturable (Bmax = 2.50 +/- 0.27 pmol/mg protein), reversible, and had high-affinity (KD = 187 +/- 60 nM). 1S,3R-ACPD-sensitive [3H]glutamate binding likely represents labeling of metabotropic glutamate receptors in rat brain membranes.

L-aspartate-beta-hydroxamate exhibits mixed agonist/antagonist activity at the glutamate metabotropic receptor in rat neonatal cerebrocortial slices

L-aspartate-beta-hydroxamate, a glutamate uptake inhibitor, was investigated for activity at a glutamate metabotropic receptor (mGluR) in neonatal rat cerebral cortical slices. Stimulation of phosphatidylinositol hydrolysis by 100 microM (1S,3R)-ACPD was inhibited only very weakly, to a maximal extent of 28%, L-aspartate-beta-hydroxamate did however exhibit agonist activity (EC50 = 760 microM) and, although much less potent than (1S,3R)-ACPD (EC50 = 20 microM), its efficacy was approximately 70% of the latter. These results indicate that, at least in this preparation, offspartate-beta-hydroxamate is of little value as an antagonist at the mGluR receptor.

Modulation of AMPA and NMDA responses in rat spinal dorsal horn neurons by trans-1-aminocyclopentane-1,3-dicarboxylic acid

In freshly isolated spinal dorsal horn (DH) neurons (laminae I-IV) of the young rat the effects of 25-100 microM of (+/-)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD), 1S,3R-ACPD and 1R,3S-ACPD, a metabotropic glutamate receptor (mGluR) agonist, on inward currents induced by glutamate (Glu), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA) and kainate were studied under whole-cell voltage-clamp conditions. When the cells were clamped to -60 mV, the racemic mixture and both stereo isomers of trans-ACPD increase the responses elicited by Glu, AMPA, and NMDA, but little those of kainate. In addition, quisqualate (10-50 microM), in the presence of CNQX (5-20 microM) or NBQX (5 microM), potentiated NMDA-induced currents. The enhancing effect lasted 10-75 min, depending upon both dose and length of application. In a smaller proportion of dorsal horn neurons, the enhancing effect was preceded by a transient depression of the responses to Glu, AMPA, and NMDA. 2-Amino-3-phosphonopropionic acid (L-AP3), a putative antagonist of mGluR exerted little effect on responses to AMPA itself, but reduced or prevented the enhancing effect of 1S,3R-ACPD. It is concluded that activation of a metabotropic glutamate receptor by trans-ACPD, and its two enantiomers, may mediate the enhancement of AMPA and NMDA responses in acutely isolated rat spinal dorsal horn neurons. These results are consistent with the possibility that the activation of metabotropic glutamate receptor may contribute to the regulation of the strength of excitatory amino-mediated primary afferent neurotransmission, including nociception.

Actions of a metabotropic glutamate receptor agonist in immature and adult rat cerebellum

The electrophysiological actions of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on Purkinje and granule cells were studied in immature and adult cerebellar slices. ACPD elicited a depolarising response when applied to Purkinje cells (EC50 approximately 20 microM). Granule cells hyperpolarised when exposed to low (3-10 microM) concentrations of ACPD; higher concentrations produced a depolarisation (EC50 approximately 40 microM) that was rapidly curtailed by a hyperpolarisation. The hyperpolarisation was abolished when Ca2+ was removed. In Purkinje cells, the amplitude of the depolarisation was greater in adult slices compared to those in immature slices. The responses were not blocked by ionotropic glutamate receptor antagonists or (L)-2-amino-3-phosphonopropionate (AP3).

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.

Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.

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)

Developmental changes in the modulation of cyclic AMP formation by the metabotropic glutamate receptor agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid in brain slices

Metabotropic glutamate receptors (mGluRs) have been recently described as a family of guanine nucleotide-binding regulatory protein-coupled receptors with multiple signal transduction pathways. At least one of these receptors appears to be negatively coupled to adenylyl cyclase when stably expressed in transfected cells. We have studied how activation of native mGluRs modulates cyclic AMP (cAMP) formation in brain slices prepared from rats at different ages. 1S,3R-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,1R-ACPD), a selective agonist of mGluRs, slightly increased basal cAMP formation but reduced forskolin-stimulated cAMP formation in adult hippocampal slices, in agreement with previous results. The action of 1S,3R-ACPD on basal cAMP formation was not reproduced by the ionotropic receptor agonists N-methyl-D-aspartate, kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and was antagonised by L-2-amino-3-phosphonopropionate (L-AP-3). L-AP-3, however, did not prevent but rather mimicked the inhibitory action of 1S,3R-ACPD on forskolin-stimulated cAMP formation. In hippocampal slices from 1-, 8-, or 15-day-old rats, 1S,3R-ACPD increased basal cAMP formation but failed to reduce the action of forskolin. A similar development pattern of modulation was observed in hypothalamic slices with the difference that 1S,3R-ACPD did not stimulate basal cAMP formation in the hypothalamus of adult animals. These results suggest that inhibition of forskolin-stimulated cAMP formation by 1S,3R-ACPD is mediated by a specific mGluR subtype that is preferentially expressed in the adult.

A metabotropic l-Glutamate receptor agonist: Pharmacological difference between rat central neurones and crayfish neuromuscular junctions

1. 2S,3S,4S-2-(carboxycyclopropyl)glycine (L-CCG-I), a conformationally restricted glutamate analogue, is a potent metabotropic L-glutamate receptor agonist in the mammalian central nervous system. 2. Depolarizing actions of L-CCG-I and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) in the newborn rat spinal motoneurone are temperature-sensitive, and are not depressed by 3-[(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP) and/or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. L-CCG-I and trans-ACPD induced oscillatory responses in Xenopus oocytes injected with rat brain mRNA. Oocytes with oscillatory responses to L-CCG-I and trans-ACPD showed reversal potential of about -20 mV, which was very close to the equilibrium potential of chloride ions. 4. In rat hippocampal synaptoneurosomes, L-CCG-I stimulated phosphoinositide hydrolysis in a concentration dependent manner. L-CCG-I was less potent than quisqualate but more potent than trans-ACPD. 5. At low concentrations, L-CCG-I did not cause any depolarization of newborn rat spinal motoneurones, but reduced substantially amplitudes of monosynaptic reflexes. 6. At the crayfish neuromuscular junction L-CCG-I, acting presynaptically, reduced the amplitude of excitatory junctional potentials. This action was prevented by application of picrotoxin but not pertussis toxin. The actions of trans-ACPD differ from those of either L-CCG-I or ibotenate at the crayfish neuromuscular junction. 7. L-CCG-I has a potential to provide further useful information on metabotropic L-glutamate receptor function.

Macrophage-induced cytotoxicity of N-methyl-D-aspartate receptor positive neurons involves excitatory amino acids rather than reactive oxygen intermediates and cytokines

The co-localization of activated macrophages and damaged neurons observed in brain injury and degenerative brain diseases may hint to macrophage-induced neuronal cytotoxicity. Recently, macrophages have been found to secrete neurotoxic molecules such as radical oxygen intermediates and glutamate, the latter interacting with N-methyl-D-aspartate (NMDA) receptors. As shown in the present study, brain macrophages termed microglial cells co-cultured with differentiated cerebellar neurons excert potent neurotoxic effects. Neurotoxicity is unlikely to be due to cytokines since tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, IL-6 and interferon (IFN)-alpha/IFN-beta/IFN-gamma had no such effects. In contrast, when treating neurons with H2O2 or oxygen radical-generating systems cytotoxicity was induced. Furthermore, microglia were found to produce O2- and H2O2 when triggered with phorbol 12-myristate 13-acetate. However, in co-cultures of neurons and microglia, oxygen-radical scavengers catalase and superoxide dismutase, failed to protect neurons from microglia-induced killing. Moreover, when using undifferentiated neurons which are susceptible to H2O2 but not to NMDA receptor-dependent killing, microglia did not destroy the neurons. Thus, the amount of reactive oxygen intermediates produced by microglia in co-culture do not reach the critical concentrations required for neurotoxicity. As dibenzocyclohepteneimide, an antagonist to NMDA receptors neutralized neurotoxicity in microglia-neuronal co-cultures, excitatory amino acids released by microglia are suggested to compose the major determinant of neurotoxicity.

Cellular localization of a metabotropic glutamate receptor in rat brain

In rat brain, the cellular localization of a phosphoinositide-linked metabotropic glutamate receptor (mGluR1 alpha) was demonstrated using antibodies that recognize the C-terminus of the receptor. mGluR1 alpha, a 142 kd protein, is enriched within the olfactory bulb, stratum oriens of CA1 and polymorph layer of dentate gyrus in hippocampus, globus pallidus, thalamus, substantia nigra, superior colliculus, and cerebellum. Lower levels of mGluR1 alpha are present within neocortex, striatum, amygdala, hypothalamus, and medulla. Dendrites, spines, and neuronal cell bodies contain mGluR1 alpha. mGluR1 alpha is not detectable in presynaptic terminals. mGluR1 alpha and ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits show differential distributions, but in Purkinje cells, mGluR1 alpha and specific AMPA receptor subunits colocalize. The postsynaptic distribution of mGluR1 alpha is consistent with postulated physiological roles of this subtype of glutamate receptor.

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.

Distribution of the mRNA for a metabotropic glutamate receptor (mGluR1) in the central nervous system: an in situ hybridization study in adult and developing rat

Distribution of the mRNA for a metabotropic glutamate receptor (mGluR1), which is linked to phosphoinositide (PI) hydrolysis, was investigated in adult and developing rat central nervous system (CNS) by in situ hybridization. Transcripts of mGluR1 were specifically localized to neurons and widely distributed throughout the adult rat brain. Most intensely labeled neurons were Purkinje cells of the cerebellum, mitral and tufted cells of the olfactory bulb, and neurons in the hippocampus, lateral septum, thalamus, globus pallidus, entopeduncular nucleus, ventral pallidum, magnocellular preoptic nucleus, substantia nigra, and dorsal cochlear nucleus. Moderately labeled neurons were seen in high density in the dentate gyrus, striatum, islands of Calleja, superficial layers of the retrosplenial, cingulate and entorhinal cortices, mammillary nuclei, red nucleus, and superior colliculus. In the developing rat brain, the level of mGluR1 expression gradually increased during early postnatal days in accordance with the maturation of neuronal elements. These results show prominent expression of mGluR1 in the major targets of putative glutamatergic pathways and unique distribution pattern of mGluR1 distinct from those reported for ionotropic subtypes of glutamate receptors, suggesting specific roles of mGluR1 in the glutamatergic system.

Activation of metabotropic glutamate receptors in the hippocampus increases cyclic AMP accumulation

The selective metabotropic glutamate receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) stimulates phosphoinositide hydrolysis and elicits several physiological responses in rat hippocampal slices. However, recent studies suggest that the physiological effects of trans-ACPD in the hippocampus are mediated by activation of a receptor that is distinct from the phosphoinositide hydrolysis-linked receptor. Previous experiments indicate that cyclic AMP mimics many of the physiological effects of trans-ACPD in hippocampal slices. Furthermore, recent cloning and biochemistry experiments indicate that multiple metabotropic glutamate receptor subtypes exist, some of which are coupled to yet unidentified effector systems. Thus, we performed a series of experiments to test the hypothesis that ACPD increases cyclic AMP levels in hippocampal slices. We report that 1S,3R- and 1S,3S-ACPD (but not 1R,3S-ACPD) induce a concentration-dependent increase in cyclic AMP accumulation in hippocampal slices. This effect was blocked by the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonoproprionic acid but not by selective antagonists of ionotropic glutamate receptors. Furthermore, our results suggest that 1S,3R-ACPD-stimulated increases in cyclic AMP accumulation are not secondary to increases in cell firing or to activation of phosphoinositide hydrolysis.

A novel kainate receptor in the insect nervous system

The pharmacology of excitatory amino acid receptors on an identified neurone in the insect central nervous system was investigated using single electrode recording techniques. Application of kainate, domoate and quisqualate elicited large, slow depolarizations with a rise time of approximately 4 min and recovery time of 23 min. Concentration-response curves were constructed giving an order of potency, domoate greater than quisqualate greater than kainate, and fitted curves demonstrated quisqualate to be a possible partial agonist, compared to domoate and kainate. Agonist responses were insensitive to the antagonists CNQX and picrotoxin and the vertebrate receptor-subtype selective agonists AMPA and trans-ACPD did not elicit any response, suggesting a novel type of excitatory amino acid receptor present on neurones in the insect central nervous system.

Evidence for metabotropic excitatory amino acid receptor heterogeneity: developmental and brain regional studies

To examine whether multiple subtypes of the excitatory amino acid (EAA) receptor coupled to phosphoinositide (PPI) hydrolysis exist, we have pharmacologically characterized the PPI response in neonatal and adult rat brain. Activation of PPI hydrolysis was determined by the accumulation of [3H]inositol monophosphate in brain slices prelabeled with [3H]inositol. In neonatal hippocampus, D,L-2-amino-3-phosphonopropionic acid (AP3; 1 mM) inhibited the cis-1-aminocyclopentane-1,3-dicarboxylic acid (IUPAC nomenclature; ACPD; 100 microM)- and quisqualate (Quis; 100 microM)-stimulated PPI hydrolysis by 73 and 66%, respectively, but had no effect in neonatal cerebellum. In adult hippocampus, AP3 stimulated PPI hydrolysis with potency and efficacy comparable to those of Quis and ACPD and completely masked the Quis concentration-response curve. In adult cerebellum, only Quis behaved as a full agonist on the PPI response. The Quis concentration-response curve was shifted rightward with a fourfold decrease in potency in the presence of ACPD (5 mM), whereas it was nearly additive with the PPI response induced by AP3 (5 mM). Thus, our data reveal significant developmental and brain regional differences in metabotropic EAA receptor responses and support the notion that this receptor is heterogeneous, in both a regionally specific and a developmentally dependent manner.

(S)-homoquisqualate: a potent agonist at the glutamate metabotropic receptor

The synthetic quisqualate analogue, (S)-homoquisqualate was examined for activity at the glutamate metabotropic receptor, in relation to its ability to stimulate phosphoinositide hydrolysis in rat pup cerebro-cortical slices. The compound produced a robust increase in hydrolysis (EC50 = 50.2 +/- 1.6 microM), which, in common with responses to quisqualate and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate ((1S,3R)-ACPD), was antagonized uncompetitively by L-2-amino-3-phosphonopropionate (L-AP3). In contrast to quisqualate which exhibits low efficacy, (S)-homoquisqualate behaves as a full agonist at the metabotropic receptor.

Metabotropic glutamate receptor activation produces extrapyramidal motor system activation that is mediated by striatal dopamine

Little is known about the in vivo function of the GTP-binding protein-coupled "metabotropic" excitatory amino acid (EAA) receptor. In vitro studies on agonist-induced brain phosphoinositide hydrolysis have shown that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid is a highly selective and efficacious metabotropic EAA agonist. We have recently reported that in vivo unilateral intrastriatal injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid induces transient extrapyramidal motor activation that manifests itself as contralateral turning. In this study, we fully characterized the onset of turning behavior following intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid injection and the possible involvement of striatal dopamine neurons in the mediation of this effect. Rats were anesthetized with the short-acting agent halothane to allow for rapid surgical recovery and thus early behavioral measurements. Intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 mumol/2 microliters) produced an incremental increase in contralateral turning starting at 1 h and plateauing 3-6 h after injection (peak effect, 39.1 +/- 6.7 rotations per 5 min). Dopamine depletion with alpha-methyl-DL-p-tyrosine (250 mg/kg i.p., 80% depletion) resulted in greater than 85% inhibition of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced contralateral turning. The dopamine antagonist haloperidol (0.3 mg/kg i.p.) produced 48% inhibition of the (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid response. In time course studies, turning behavior correlated with increases in levels of the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid. These results suggest a functional interaction between the metabotropic EAA receptor and the dopaminergic system in the striatum.

Metabotropic glutamate receptors are required for the induction of long-term potentiation

Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.

Activation of metabotropic receptors has a neuroprotective effect in a rodent model of focal ischaemia

The role of the glutamate 'metabotropic' receptor was investigated in an experimental model of focal ischaemia-induced neurodegeneration. The metabotropic agonist, trans-1-amino cyclopentane-1,3-dicarboxylic acid (t-ACPD, 20 mg/kg i.p.), was administered to mice immediately after middle cerebral artery occlusion (MCAO), which causes cerebral infarct. Seven days after MCAO, the mean infarct volume value of the t-ACPD-treated group (mean +/- S.E. = 4.57 +/- 0.73 mm3) was significantly reduced, by 34.3%, compared to the vehicle-treated group (mean +/- S.E. = 6.95 +/- 0.59 mm3, P less than 0.01). This suggests that metabotropic receptor activation in the adult brain reduces excitotoxicity.

Morphological response of endoplasmic reticulum in cerebellar Purkinje cells to calcium deprivation

Mobilisable intracellular Ca2+ stores are highly enriched in the cerebellum, particularly in Purkinje cells. We have detected, by light and electron microscopy, striking morphological changes in the presumed Ca2+ stores of Purkinje cells when slices of eight-day-old rat cerebellum were incubated in Ca(2+)-deficient media. After 30 min under these conditions, the endoplasmic reticulum became thinned and elongated. By 2 h, it was transformed into multilamellar, whorl-like inclusions with electron-dense cores. These changes were reversed on reintroduction of Ca2+. Analogous changes in other neurons were not observed. The results suggest that Ca2+ storage sites within Purkinje cells are capable of dramatic morphological change depending on the availability of Ca2+. The transformations may reflect, initially, depletion of Ca2+ from the stores and then homeostatic alterations in their capacity.

2-Amino-3-phosphonopropionate fails to block postsynaptic effects of metabotropic glutamate receptors in rat hippocampal neurones

Metabotropic glutamate receptors (mGluRs) operate via the phosphoinositide second messenger cascade and have various modulatory effects on central neurones. 2-Amino-3-phosphonopropionate (AP3) has been proposed as a selective antagonist of mGluR and used to explore the physiological functions of mGluR. We have compared the effects of mGluR agonists in the presence and absence of AP3 using intracellular recording from CA1 pyramidal neurones in rat hippocampal slices. Two mM D,L-AP3 or 1 mM L-AP3 did not cause any detectable change in the effects of mGluR agonists trans-1-amino-cyclopentyl-1,3-dicarboxylate (t-ACPD) or quisqualate. These agonists still induced depolarization, inhibition of the slow after-hyperpolarization and slowing of the spike repolarization. The results argue against AP3 being an antagonist of postsynaptic mGluRs.

Activation of hippocampal metabotropic excitatory amino acid receptors leads to seizures and neuronal damage

A role for ionotropic (NMDA, AMPA, and kainate) excitatory amino acid (EAA) receptors in seizure and seizure-related brain damage is well documented. To study the possible role of metabotropic (G-protein linked) EAA receptors in this regard, a highly selective metabotropic EAA agonist was injected into the hippocampus of halothane-anesthetized rats. This resulted in delayed-onset seizures and selective hippocampal neuronal damage that was indirectly mediated by NMDA receptors. This provides direct evidence for a novel role of metabotropic EAA receptors in the etiology of seizures and neuronal damage.

Activation of quisqualate metabotropic receptors reduces glutamate and GABA-mediated synaptic potentials in the rat striatum

The role of quisqualate (QUIS) metabotropic receptors in the synaptic transmission in the striatum was investigated using the cortico-striatal slice preparation. Low concentrations (1-30 microM) of trans-1-amino-cyclopentyl-1,3- dicarboxylic acid (t-ACPD), a selective agonist of QUIS metabotropic receptors, decreased glutamate-mediated synaptic potentials (EPSPs) evoked in the striatum by the stimulation of cortico-striatal fibers. This agonist decreased also GABA-mediated depolarizing synaptic potentials evoked by intrastriatal stimulation in the presence of 6-cyano-7-nitro-quinoxaline-2,3- dione (CNQX); this effect was less potent than the action of t-ACPD on glutamate-mediated potentials. Low concentrations of t-ACPD did not affect the intrinsic membrane properties of striatal neurons and their postsynaptic responses to exogenous glutamate and GABA. Higher concentrations (50-100 microM) to t-ACPD caused membrane depolarizations and inward currents in several neurons. Our data suggest that low concentrations of t-ACPD selectively reduce synaptic transmission while higher concentrations of this agonist may cause a direct excitatory action on striatal neurons.

Glutamate selectively increases the high-threshold Ca2+ channel current in sensory and hippocampal neurons

Previous studies resulted in conflicting conclusions that glutamate application either decreases or increases the activity of Ca2+ channels in hippocampal neurons. We studied whole-cell Ca2+ currents (ICa) in chick dorsal root ganglion neurons and rat hippocampal cells. For both cell types glutamate (1-30 microM) increased high-threshold Ca2+ current. It was independent of the charge carriers, Ca2+ or Ba2+. Low-threshold Ca2+ channel current and the fast sodium current were not changed with glutamate application. The effect developed within 1-2 min and then further facilitated after washout of the agonist. A second application of glutamate produced no additional increase in ICa. No changes in the time-course of whole-cell currents were observed, suggesting that glutamate recruits 'sleepy' Ca2+ channels. Whatever its mechanism, overlasting increase of ICa by glutamate may be important in neuronal plasticity.

Trans-ACPD inhibits cAMP formation via a pertussis toxin-sensitive G-protein

In primary cultured striatal neurons we found that (+-)-trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) could inhibit forskolin-induced cAMP formation in a dose-dependent manner (EC50 156 +/- 38 microM, n = 5, maximal inhibition 37.8 +/- 1.2, n = 37). The trans-ACPD-induced inhibition was totally abolished in neurons preincubated with Bordetella pertussis toxin (1 microgram/ml), demonstrating the involvement of a G-protein. This is the first report in intact neurons of a glutamate metabotropic receptor negatively coupled to cAMP formation.

Signal transduction and pharmacological characteristics of a metabotropic glutamate receptor, mGluR1, in transfected CHO cells

The signal transduction and pharmacological properties of a metabotropic glutamate receptor, mGluR1, were studied in CHO cells permanently expressing the cloned receptor. mGluR1 stimulated phosphatidylinositol (PI) hydrolysis in the potency rank order of quisqualate greater than L-glutamate greater than or equal to ibotenate greater than L-homocysteine sulfinate greater than or equal to trans-ACPD. This receptor also evoked the stimulation of cAMP formation and arachidonic acid release with comparable agonist potencies. DL-AP3 and L-AP4, the effective antagonists reported for glutamate-stimulated PI hydrolysis in brain slices, showed no appreciable effects on mGluR1, suggesting the existence of an additional subtype of this receptor family. Pertussis toxin and phorbol ester produced distinct effects on the three transduction cascades, implying that mGluR1 independently links to the multiple transduction pathways probably through different G proteins.

Metabotropic receptors and 'slow' excitatory actions of glutamate agonists in the hippocampus

The actions of glutamate in the CNS can be divided into ionotropic and metabotropic effects. The ionotropic receptors participate in synaptic transmission by directly opening nonselective cation channels. Recently, a so-called 'metabotropic effect' of glutamate has been described and is attributed to a novel metabotropic glutamate receptor. This effect consists of increased hydrolysis of membrane phosphoinositides, production of the second messengers diacylglycerol and inositol 1,4,5-trisphosphate, and mobilization of intracellular Ca2+. Activation of metabotropic glutamate receptors blocks the slow Ca(2+)-dependent K+ conductance and increases the membrane excitability of neurones. In addition, metabotropic agonists block the excitatory synaptic transmission supported by the ionotropic glutamate receptor, and may therefore play a critical role in synaptic plasticity. However, intracellular mechanisms linking metabotropic glutamate receptors with ionic channels remain unclear. This article discusses recent findings concerning metabotropic agonist effects on membrane currents and synaptic transmission, the pharmacology of the agonists and the roles played by G proteins and second messengers in mediating their effects.

Examination of Parameters Influencing [3H]MK-801 Binding in Postmortem Human Cortex

[3H]MK-801 binding was used as an index of the glutamate receptor N-methyl-D-aspartate-subtype channel to examine the influence of gender, age, mode of death (agonal status), interval between death and autopsy (postmortem delay), and time in storage at -70 degrees C in well washed homogenate preparations from postmortem human frontal cortex. Basal binding and the modulatory effects of glutamate, glycine, spermidine, and zinc were examined with respect to these variables. Basal binding was sensitive to agonal status, being higher in sudden death cases. The effect of added glutamate and glycine was sensitive to age, with a trend toward lower binding with increasing age. The effect of added spermidine alone was sensitive to storage time at -70 degrees C, the binding being higher with longer storage time. The effect of added zinc was also sensitive to postmortem delay, with zinc causing a greater reduction in binding with shorter postmortem delays. Thus, with the exception of gender, all variables examined influenced [3H]MK-801 binding, highlighting the attention that should be given to these factors in postmortem studies in normal and diseased human subjects.

Inhibition of cyclic AMP formation by a selective metabotropic glutamate receptor agonist

It is well documented that the effects of excitatory amino acid (EAA) agonists on phosphoinositide hydrolysis involve a GTP-binding protein-linked or "metabotropic" receptor mechanism. The mechanisms by which EAAs alter cyclic AMP levels in brain slices, however, are not yet clear. In this study, the selective metabotropic EAA agonist trans-(+-)-1-aminocyclopentane-1,3-dicarboxylic acid and its isomers were examined for effects on basal and forskolin-stimulated cyclic AMP formation in slices of the rat hippocampus. Trans-(+-)-1-Aminocyclopentane-1,3-dicarboxylic acid had little effect on basal cyclic AMP but inhibited forskolin-stimulated cyclic AMP formation in a biphasic manner. The 1S,3R isomer of 1-aminocyclopentane-1,3-dicarboxylic acid produced potent but only partial (approximately 50%) inhibition of forskolin-stimulated cyclic AMP formation. 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid fully inhibited forskolin-stimulated cyclic AMP but with lower potency than the 1S,3R isomer. These results show that in addition to the formation of phosphoinositide-derived second messengers, the cellular consequences of selectively activating hippocampal metabotropic EAA receptors include an alteration of cellular cyclic AMP levels.

Burst firing of rat septal neurons induced by 1S,3R-ACPD requires influx of extracellular calcium

We have demonstrated that burst firing in rat dorsolateral septal nucleus neurons is a specific response following activation of metabotropic glutamate receptors. Now we report that the burst firing induced by 1S,3R-ACPD (1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid), a selective agonist, is blocked by inorganic calcium channel blockers. Our data suggest that influx of external calcium is required for this metabotropic glutamate response. Intracellular second messenger pathways coupled to the metabotropic glutamate receptor may be more complex than releasing calcium from IP3-sensitive internal stores as is currently hypothesized.

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.

Long-lasting enhancement of metabotropic excitatory amino acid receptor-mediated polyphosphoinositide hydrolysis in the amygdala/pyriform cortex of deep prepiriform cortical kindled rats

We have previously demonstrated that ibotenate (IBO)-stimulated polyphosphoinositide (PPI) hydrolysis is increased for a long period in the amygdala/pyriform cortex (AM/PC) of amygdala (AM)- and hippocampal (HIPP)-kindled rats. This finding indicates that enhanced function of the PPI-coupled excitatory amino acid (EAA) receptor may be associated with the long-lasting seizure susceptibility of kindling. The present study further examined PPI hydrolysis induced by trans-ACPD, a selective agonist of the metabotropic EAA receptor, as well as by IBO in brain slices of rats kindled from the deep prepiriform cortex (DPC). IBO-stimulated accumulation of [3H]inositol monophosphate ([3H]InsP) was significantly increased in the AM/PC by 162 (P less than 0.0001), 130 (P less than 0.005) and 81% (P less than 0.03) at 24 h, 7 days and 28 days, respectively, after the last kindled seizure, whereas it was increased significantly only at 24 h after the last seizure in the HIPP and did not change at any time in the limbic forebrain (LFB). The IBO-stimulated accumulation of [3H]InsP was significantly increased by 55% (P less than 0.01) in the AM/PC of partially kindled rats reaching an average stage of 3.7, but not in the AM/PC of those remaining at stage 1, 7 days after the last kindled seizure. Trans-ACPD-stimulated PPI hydrolysis was significantly increased in the AM/PC of DPC-kindled rats by 65 (P less than 0.05) and 45% (P less than 0.005) at 7 and 28 days, respectively, after the last kindled seizure. Cis-ACPD-stimulated PPI hydrolysis was also significantly increased in the AM/PC of DPC-kindled rats by 45 (P less than 0.03) and 30% (P less than 0.04) at 7 and 28 days, respectively, after the last seizure. There was no increase in trans-ACPD- or cis-ACPD-stimulated PPI hydrolysis in the HIPP or LFB. These results further confirm our previous studies showing that the metabotropic EAA receptor-stimulated PPI hydrolysis exhibited a long-lasting increase in the AM/PC irrespective of the primary stimulation site for kindling.