Pharmacological and functional characteristics of metabotropic excitatory amino acid receptors

Reduction of phosphoinositide hydrolysis by L-amino-3-phosphonopropionate may be caused by the inhibition of synthesis of phosphatidylinositols

L-Amino-3-phosphonopropionate (L-AP3), a putative antagonist of metabotropic glutamate receptors, inhibited the formation of [3H]inositol phosphates induced by (1S,3R)1-aminocyclopentane-1,3-dicarboxylic acid in rat hippocampal slices. The inhibition was accompanied by a decrease in the levels of [3H]phosphatidylinositols ([3H]PIs). Preincubation of slices with L-AP3 inhibited the incorporation of [3H]myo-inositol into PI fractions. The effects of L-AP3 was in contrast with those of a typical receptor antagonist, atropine; atropine inhibited carbachol-induced phosphoinositide hydrolysis, but the levels of [3H]PIs were not affected. These findings suggest that the inhibition of phosphoinositide hydrolysis by L-AP3 is not due to the receptor antagonism but may be caused by the inhibition of synthesis of PIs.

NMDA glutamatergic receptors, labelled with [3H]MK-801, in brain samples from drug-free depressed suicides

Glutamate receptors of the NMDA-subtype were quantitated by binding of [3H]dizocilpine maleate (MK-801) in nine brain regions from 22 suicide victims (20-60 yr), with a firm retrospective diagnosis of depression, who had not recently received antidepressant drugs, and 20 age- and sex-matched controls. [3H]MK-801-binding did not differ between suicides and controls in any region studied. Suicides who died violently did not differ from non-violent suicides and controls. A significative negative correlation was found between age and NMDA receptor-binding in the frontal cortex of suicide victims, but not in controls. This preliminary study provides little evidence for an important role of NMDA-binding sites in the pathophysiology of depression.

A comparison of two alternatively spliced forms of a metabotropic glutamate receptor coupled to phosphoinositide turnover

A comparison of the pharmacological and physiological properties of the metabotropic glutamate 1 alpha and 1 beta receptors (mGluR1 alpha and mGluR1 beta) expressed in baby hamster kidney (BHK 570) cells was performed. The mGluR1 beta receptor is an alternatively spliced form of mGluR1 alpha with a modified carboxy terminus. Immunoblots of membranes from the two cell lines probed with receptor-specific antipeptide antibodies showed that mGluR1 alpha migrated with an M(r) = 154,000, whereas mGluR1 beta migrated with an M(r) = 96,000. Immunofluorescence imaging of receptors expressed in BHK 570 cells revealed that the mGluR1 alpha receptor was localized to patches along the plasmalemma and on intracellular membranes surrounding the nucleus, whereas mGluR1 beta was distributed diffusely throughout the cell. Agonist activation of the mGluR1 alpha and the mGluR1 beta receptors stimulated phosphoinositide hydrolysis. At both receptors, glutamate, quisqualate, and ibotenate were full agonists, whereas trans-(+)-1-aminocyclopentane-1,3-dicarboxylate appeared to act as a partial agonist. The stimulation of phosphoinositide hydrolysis by mGluR1 alpha showed pertussis toxin-sensitive and insensitive components, whereas the mGluR1 beta response displayed only the toxin-insensitive component. The mGluR1 alpha and mGluR1 beta receptors also increased intracellular calcium levels by inducing release from intracellular stores. These results indicate that the different carboxy terminal sequences of the two receptors directly influences G protein coupling and subcellular deposition of the receptor polypeptides and suggest that the two receptors may subserve different roles in the nervous system.

Kainate toxicity in energy-compromised rat hippocampal slices: differences between oxygen and glucose deprivation

The effects of kainate (KA) on the recovery of neuronal function in rat hippocampal slices after hypoxia or glucose deprivation (GD) were investigated and compared to those of (R,S)-alpha-amino-3-hydroxy-5-methyl-4- isoxazoleproprionate (AMPA). KA and AMPA were found to be more toxic than either N-methyl-D-aspartate (NMDA), quinolinate, or glutamate, both under normal conditions and under states of energy deprivation. Doses as low as 1 microM KA or AMPA were sufficient to significantly reduce the recovery rate of neuronal function in slices after a standardized period of hypoxia or GD. The enhancement of hypoxic neuronal damage by both agonists could be partially blocked by the antagonist kynurenate, by the NMDA competitive antagonist AP5, and by elevating [Mg2+] in or by omitting Ca2+ from the perfusion medium. The AMPA antagonist glutamic acid diethyl ester was ineffective in preventing the enhanced hypoxic neuronal damage by either KA or AMPA. The antagonist of the glycine modulatory site on the NMDA receptor, 7-chlorokynurenate, did not block the KA toxicity but was able to block the toxicity of AMPA. 2,3-Dihydroxyquinoxaline completely blocked the KA- and AMPA-enhanced hypoxic neuronal damage. The KA-enhanced, GD-induced neuronal damage was prevented by Ca2+ depletion and partially antagonized by kynurenate but not by AP5 or elevated [Mg2+]. The results of the present study indicate that the KA receptor is involved in the mechanism of neuronal damage induced by hypoxia and GD, probably allowing Ca2+ influx and subsequent intracellular Ca2+ overload.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic receptor stimulation coupled to weak tetanus leads to long-term potentiation and a rapid elevation of cytosolic protein kinase C activity

We have previously shown that short-term potentiation (STP) inducing weak tetanus induces long-term potentiation (LTP) when it is coupled with activation of metabotropic glutamate (mGlu) receptors by trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD) in rat CA1 slices. In the present study, we examined if this conversion of STP to LTP involves activation of protein kinase C (PKC). Two minutes but not 30 min after coupling, there was a significant increase in the activator-dependent PKC activity in the cytosolic fraction. STP induction or t-ACPD application did not change PKC activity. There was no activity increase in the membrane fraction. STP was also induced by a co-application of gamma-amino-3-hydroxy-5-methyllisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA). Coupling this STP with t-ACPD, however, did not result in an LTP or PKC activity increase, indicating a requirement for synaptic activity. A rapid and transient (< 5 min) increase in cytosolic PKC activity was also seen after the induction of LTP by stronger tetanic stimulation. No LTP tested in the present study was accompanied by activator-independent, persistent increases in PKC activity. STP induction depends on NMDA receptor activation, and the activation of mGlu receptors results in the production of intracellular second messengers. Our results therefore indicate that these separate components may add and bring about PKC activation and LTP.

Antagonists of excitatory amino acids and endogenous opioid peptides in the treatment of experimental central nervous system injury

Trauma to the central nervous system can lead to primary injuries occurring at the time of impact as well as secondary or delayed injury processes that can result from cellular hypoxia, oligemia/ischemia, edema and swelling, and intracranial hypertension that are manifested over a period of hours to weeks after the initial event. Although the mechanisms underlying delayed tissue injury are poorly understood, they appear to be associated with endogenous neurochemical changes resulting from traumatic nervous system injury. These neurochemical changes may include excessive neurotransmitter release, deregulation of ion homeostasis, and the synthesis, release, or activation of various "autodestructive" neurochemical factors. Experimental studies over the past decade indicate that these alterations mediate important components of the neurochemical cascade leading to central nervous system injury. Furthermore, pharmacologic manipulations of these neurochemical changes have been reported to attenuate secondary central nervous system damage, ameliorate neuronal death, and promote functional recovery after central nervous system injury. This article focuses on the role of excitatory amino acid neurotransmitters, endogenous opioid peptides, and magnesium in the pathophysiology of central nervous system injury and on the therapeutic manipulation of these systems to improve functional outcome after central nervous system injury.

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.

Functional evidence for a L-AP3-sensitive metabotropic receptor different from glutamate metabotropic receptor mGluR1

The efficacy of mGluR agonists quisqualate and 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in stimulating the inositol phosphate (IP) formation in primary cultures of cerebellar granule neurons correlated with mGluR1 mRNA expression and was affected by the medium KCl content. L-2-Amino-3-phosphonopropionic acid (L-AP3) mimicked the stimulatory action of mGluR agonists. Maximal stimulatory doses of mGluR agonist 1S,3R-ACPD and L-AP3 were additive, suggesting the action of L-AP3 on a receptor different from mGluR1. Indeed, in embryonic kidney 293 cells transfected with mGluR1 cDNA quisqualate and 1S,3R-ACPD but not L-AP3 stimulated the IP formation.

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.

4-Carboxy-3-hydroxyphenylglycine, an antagonist at type I metabotropic glutamate receptors

The pharmacology of (DL)-4-carboxy-3-hydroxyphenylglycine was examined in baby hamster kidney cells which stably express the type I metabotropic glutamate receptor (mGluR1 alpha). DL-4-Carboxy-3-hydroxyphenylglycine dose-dependently inhibited glutamate-stimulated phosphoinositide hydrolysis in a competitive manner with a KB of 29 microM as estimated from Schild-plot analysis. The present report is the first demonstration of a potent and competitive antagonist of the mGluR1 alpha subtype.

A postsynaptic G-protein in hippocampal long-term potentiation

The effects of guanosine triphosphate (GTP)-binding protein (G-protein) blockade on hippocampal LTP at stratum radiatum-CA1 synapses was studied. Bath application of 20 mM lithium chloride (LiCl) inhibited long-term potentiation (LTP) of extracellularly-recorded excitatory postsynaptic potentials (EPSPs). Inclusion of 100 mM LiCl in intracellular recording electrodes was shown to block postsynaptic G-proteins by bath-application of baclofen, an agonist at the G-protein linked gamma-aminobutyric acid (GABAB) receptor. Under normal conditions, GABAB receptor activation causes a hyperpolarization postsynaptically, and a decrease in neurotransmitter release presynaptically. With LiCl in the recording electrodes, the postsynaptically-mediated hyperpolarization was blocked, while the presynaptically-mediated depression of EPSPs was unaffected. With postsynaptic G-proteins blocked in this manner, LTP at these synapses was inhibited. These studies provide evidence for the involvement of a postsynaptic G-protein in LTP of stratum radiatum-CA1 synapses.

Glutamate receptor agonists enhance the expression of BDNF mRNA in cultured cerebellar granule cells

The influence of glutamate and its analogues on the expression of BDNF mRNA was studied in cultured cerebellar granule cells. Four-hour exposure of the neurons to the glutamate receptor agonists, quisqualate, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA), increased levels of BDNF mRNA. Glutamate in combination with antagonists of the ionotropic glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), D-2-amino-5-phosphonovalerate (AP-5) and/or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine hydrogen maleate (MK-801), also increased levels of BDNF mRNA. However, the addition of glutamate itself to the cultures produced severe neuronal death and failed to increase the mRNA level. The onset of the increase in BDNF mRNA by kainate and NMDA lagged behind that by quisqualate. These results indicate that the non-ionotropic glutamate receptor might be involved in the induction of BDNF mRNA. Quisqualate is known to be a potent agonist of both the AMPA/kainate receptor and the metabotropic glutamate receptor. The specific antagonists of the AMPA/kainate receptor, CNQX and 6,7-dinitroquinoxaline-2,3-dione (DNQX) failed to block the increase of BDNF mRNA by quisqualate. Moreover, the desensitization of the metabotropic glutamate receptor by phorbol ester abolished the increase of BDNF mRNA by quisqualate. These results suggest that stimulation of the metabotropic glutamate receptor may be the most predominant component to increase BDNF mRNA in cerebellar granule cell culture.

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.

Pharmacological characterization of metabotropic glutamate receptors in cultured cerebellar granule cells

A detailed pharmacological characterization of metabotropic glutamate receptors (mGluR) was performed in primary cultures of cerebellar granule cells at 6 days in vitro (DIV). The rank order of agonists induced polyphosphoinositide (PPI) hydrolysis (after correcting for the ionotropic component in the response) was as follows: in terms of efficiency, Glu > quisqualate (quis) = ibotenate (ibo) > (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) > beta-methyl-amino-L-alanine (BMAA) and in terms of potency, quis > ACPD > Glu > ibo = BMAA. Ionotropic excitatory amino acid (EAA) receptor agonists, such as alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) were relatively inactive (in the presence of Mg2+). Quis and ACPD-induced PPI hydrolysis was unaffected by ionotropic Glu receptor antagonists, but was inhibited, in part by L-2-amino-3-phosphonopropionate (AP3). In contrast, Glu-or ibo- induced PPI hydrolysis was reduced, in part, by both AP3 and NMDA receptor antagonists. Characteristic interactions involving different transmitter receptors were noted. PPI hydrolysis evoked by quis and 1S,3R-ACPD was not additive. In contrast, PPI hydrolysis stimulated by quis/ACPD and carbamylcholine was additive (indicating different receptors/transduction pathways). In the presence of Mg2+, the metabotropic response to quis/AMPA and NMDA was synergistic (this being consistent with AMPA receptor-induced depolarization activating NMDA receptor). On the other hand, in Mg(2+)-free buffer the effects of quis and NMDA, at concentrations causing maximal PPI hydrolysis, were additive (indicating that PPI hydrolysis was effected by two different mechanisms).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptors in the ventrolateral medulla of rats

We investigated the hypothesis that stimulation of metabotropic excitatory amino acid receptors in the ventrolateral medulla evokes cardiovascular responses. Thus, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic excitatory amino acid receptors, was microinjected into the rostral or caudal ventrolateral medulla of halothane-anesthetized Sprague-Dawley rats. Microinjections of (1S,3R)-ACPD (100 pmol-1 nmol) into the rostral ventrolateral medulla produced dose-dependent increases in mean arterial pressure (+20 +/- 4 mm Hg by 100 pmol and +35 +/- 2 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and integrated splanchnic sympathetic nerve activity (+17 +/- 3% and +46 +/- 4%, respectively, p < 0.01), whereas (1S,3+)-ACPD microinjected into the caudal ventrolateral medulla decreased mean arterial pressure (-28 +/- 2 mm Hg by 100 pmol and -48 +/- 6 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and splanchnic sympathetic nerve activity (-24 +/- 4% and -49 +/- 5%, p < 0.01). The blockade of ionotropic excitatory amino acid receptors by the combined injection of 2-amino-7-phosphonoheptanoic acid (200 pmol) and 6,7-dinitroquinoxaline-2,3-dione (200 pmol), which effectively blocked the responses elicited by either N-methyl-D-aspartate (20 pmol) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol), failed to affect the responses evoked by either (1S,3R)-ACPD (100 pmol) or L-glutamate (2 nmol) microinjected in the rostral and caudal ventrolateral medulla. These results suggest that metabotropic receptors are present and mediate cardiovascular responses evoked by L-glutamate injections into the rostral and caudal ventrolateral medulla.

Distribution of the messenger RNA for a metabotropic glutamate receptor, mGluR2, in the central nervous system of the rat

Distribution of the messenger RNA for a metabotropic glutamate receptor, mGluR2, which is coupled to the inhibitory cyclic AMP cascade, was investigated in the central nervous system of the adult rat by in situ hybridization. Transcripts of mGluR2 were specifically localized to neuronal cells of the brain. Although the hybridization signals were widely distributed in the brain, the most prominent expression of mGluR2 messenger RNA was seen in Golgi cells of the cerebellum. Marked expression of mGluR2 messenger RNA was further observed in the mitral cells of the accessory olfactory bulb, neurons in the external part of the anterior olfactory nucleus, and pyramidal neurons in the entorhinal and parasubicular cortical regions. The granule cells of the accessory olfactory bulb, and many pyramidal and non-pyramidal neurons in the neocortical, cingulate, retrosplenial and subicular cortices, were moderately labeled. All of the granule cells in the dentate gyrus were also labeled moderately, whereas no significant hybridization signals were detected in Ammon's horn. In the basal forebrain regions, moderately labeled neurons were distributed in the triangular septal nucleus, in the lateral, basolateral and basomedial amygdaloid nuclei, and in the medial mammillary nucleus. Weakly labeled neurons were sparsely scattered in the striatum, globus pallidus, ventral pallidum and claustrum. The subthalamic nucleus was also labeled weakly. No significant labeling was found in the entopeduncular nucleus and substantia nigra. In the thalamus, moderately labeled neurons were distributed in the anterodorsal, anteromedial, ventromedial, intralaminar and midline nuclei; the ventrolateral part of the anteroventral nucleus and the rostral pole of the ventrolateral nucleus also contained moderately labeled neurons. No significant labeling was found in the thalamic reticular, submedius, ventroposterior, lateral geniculate and medial geniculate nuclei. In the lower brainstem, labeling was generally weak. No significant hybridization signals were found in the spinal cord. Some neurons in the inner part of the inner nuclear layer of the retina and some retinal ganglion cells were labeled moderately. The pattern of distribution of mGluR2 messenger RNA revealed in the present study indicates specific roles of mGluR2 in the glutamatergic system in the brain.

Nuclear magnetic resonance characterization of secondary mechanisms following traumatic brain injury

Much of the injury that occurs following a traumatic insult to the central nervous system is the result of physiological and biochemical processes initiated by the primary traumatic event. These processes occur over a period of hours to days following the insult, and although a number of factors have been identified as being associated with this secondary injury process, their role and interrelationship with one another is unclear. Nuclear magnetic resonance spectroscopy has been utilized to characterize many of these secondary factors and their relationship to eventual neurological outcome. In particular, the role of high energy phosphates, pH, lactic acid, excitatory amino acids, and magnesium has been investigated, along with pharmacotherapies directed toward altering the status of these factors following traumatic injury. This review critically examines the role that each of these factors may play in the secondary injury process, and proposes a scheme which theoretically accounts for the interrelationships among the various factors.

Cerebral protection by AMPA- and NMDA-receptor antagonists administered after severe insulin-induced hypoglycemia

Excitatory amino acids are implicated in the development of neuronal cell damage following periods of reversible cerebral ischemia or insulin-induced hypoglycemic coma. To explore the importance of glutamate receptor activation in the posthypoglycemic phase, we exposed rats to 20 min of insulin-induced severe hypoglycemia. The rats were treated immediately after the hypoglycemic insult with four regimes of glutamate receptor antagonists: (1) the AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propriate)-receptor antagonist NBQX [2.3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F) quinoxaline] given as a bolus dose of 30 mg.kg-1 i.p., followed by an i.v. infusion of 225 micrograms.kg-1.min-1 for 6 h; (2) the non-competitive NMDA-receptor antagonist, dizocilpine (MK-801) 1 mg.kg-1 given i.v.; (3) a combined NBQX treatment, (a bolus dose of 10 mg.kg-1 i.p., followed by an i.v. infusion of 225 micrograms.kg-1.min-1 for 6 h), with dizocilpine 0.33 mg.kg-1 given twice i.p. at 0 and 15 min after recovery and (4) the competitive NMDA-receptor blocker CGP 40,116 [D-(E)-2-amino-4-methyl-5-phosphono-3- pentenoic acid] 10 mg.kg-1 given i.p. In the striatum, all glutamate receptor blockers significantly decreased neuronal damage by approximately 30%. An approximately 50% decrease in neuronal damage was demonstrated in neocortex and hippocampus following the combined treatment with NBQX and dizocilpine, while protection was variable following the treatment with a single glutamate-receptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Interplay between expressed non-NMDA receptors and endogenous calcium-activated chloride currents in Xenopus laevis oocytes

Current-voltage (I-V) relationships of glutamate receptors activated by the non-NMDA receptor agonist, kainate (KA), were determined in Xenopus laevis oocytes injected with either calf or chick brain mRNA. In most injected oocytes (n = 44; 84%), I-V plots to 80 microM KA exhibited inward rectification; however, in some oocytes (n = 7; 16%), the KA-evoked membrane current showed only slight outward rectification. A comparison between the resting membrane properties of injected oocytes with the KA-evoked currents revealed that weak outward rectification was often observed in oocytes possessing predominant voltage-activated calcium-dependent chloride currents (ICl(Ca)). In these oocytes, replacement of extracellular calcium (2 mM Ca2+) with magnesium ions (Mg2+), or the addition of the chloride channel blocker, anthracene-9-carboxylic acid (500 microM to 1 mM A9C), reduced ICl(Ca) and also altered the nature of the KA I-V plot revealing inward rectification. It is proposed that the responses mediated by expressed non-NMDA receptors may be influenced by the activation of endogenous calcium-dependent membrane currents in Xenopus laevis oocytes.

Glutamate metabotropic receptor mediated depression of synaptic inputs to lamprey reticulospinal neurones

The transmission of vestibular inputs to reticulospinal (RS) neurones of the posterior rhombencephalic nucleus (PRRN) has been shown to be depressed by the bath application of N-methyl-D-aspartate (NMDA). The aim of this study was to investigate the pharmacological mechanism involved using patch clamp recordings of reticulospinal neurones. It is demonstrated that the chemical component of vestibular inputs to the PRRN is mediated by glutamatergic synapses utilising alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors on the PRRN neurones. Monosynaptic excitatory postsynaptic currents (EPSCs) from octavomotorius relay cells to RS neurones are markedly depressed by the application of NMDA, a depression which was insensitive to competitive and non-competitive NMDA receptor antagonists. The effect of NMDA was eliminated by inactivation of G proteins. A similar depressive effect was observed following application of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) to the superfusate. It is concluded that NMDA acts at a metabotropic receptor located most likely presynaptically to reticulospinal neurones on terminals of octavomotorius relay cells.

Pharmacology of metabotropic glutamate receptor inhibition of cyclic AMP formation in the adult rat hippocampus

Phosphoinositide-linked metabotropic glutamate receptors have been well characterized in a variety of CNS tissues. In this study the pharmacology of metabotropic glutamate receptors negatively coupled to cAMP formation was investigated in cross-chopped slices of the adult rat hippocampus. Excitatory amino acid agonists and antagonists were examined for effects on forskolin (30 microM)-simulated cAMP formation. The selective metabotropic glutamate agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and various nonselective metabotropic/ionotropic agonists were found to inhibit forskolin-stimulated cAMP formation. Inhibition of cAMP formation was observed using 1S,3R-ACPD (57% at 100 microM), quisqualate (92% at 500 microM), ibotenate (44% at 500 microM), L-glutamate (41% at 1000 microM), and L-aspartate (59% at 1000 microM). Inhibition of forskolin-stimulated cAMP formation induced by these agonists was observed even in the presence of the ionotropic antagonists MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione. Up to 500 microM of the ionotropic agonists N-methyl-D-aspartate, AMPA, and kainate did not inhibit forskolin-stimulated cAMP formation. L-2-amino-3-phosphonopropionate (500 microM) greatly inhibited the stimulatory effect of 1S,3R-ACPD on phosphoinositide hydrolysis, even in the presence of forskolin. However when measuring cAMP formation, L-2-amino-3-phosphonoproprionate (500 microM) mimicked the effect of 1S,3R-ACPD, producing 64% inhibition of forskolin-stimulated cAMP. These studies show that in the adult rat hippocampus metabotropic glutamate receptors that are negatively linked to cAMP formation have a pharmacology that is distinct from ionotropic glutamate receptors and phosphoinositide-linked metabotropic glutamate receptors.

Biochemical and immunocytochemical characterization of antipeptide antibodies to a cloned GluR1 glutamate receptor subunit: cellular and subcellular distribution in the rat forebrain

Antibodies were made to synthetic peptides corresponding to residues 253-367, 757-771 and 877-889 of the published amino acid sequence of the rat brain glutamate receptor GluR1 subunit [Hollmann et al. (1989) Nature 342, 643-648]. The peptides were synthesized both as multiple copies on a branching lysyl matrix (multiple antigenic peptides) and conventional linear peptides using solid-phase synthesis. Rabbits were immunized with these peptides either without conjugation (multiple antigenic peptides) or following coupling to ovalbumin with glutaraldehyde (monomeric peptides). The antibodies from immune sera were then purified by affinity chromatography using reactigel coupled monomeric peptides. All the rabbits produced good antipeptide responses, and were characterized by immunoprecipitation of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and kainate binding activity and by their staining patterns on immunoblots. Antibody to peptide 253-267 specifically immunoprecipitated 12 +/- 3, 50 +/- 3 and 44 +/- 4% of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding activity from cortex, hippocampus and cerebellum, respectively. Under identical conditions, antibody against the 877-889 peptide removed 23 +/- 4, 9 +/- 4 and 15 +/- 9% of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites from these areas. On immunoblots of rat brain membrane samples separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, antibodies labelled a 105,000 mol. wt immunoreactive band. GluR1 was immunoaffinity-purified using subunit-specific antibodies against both N-terminal (253-267) and C-terminal (877-889) residues, covalently attached to protein A-agarose. Analysis of the purified product from each column showed a major immunoreactive band, recognized by both sera at 105,000 mol. wt and silver staining identified the same major protein. After exhaustive immunoprecipitation of solubilized membrane samples with antibody against the C-terminal of the subunit, a subpopulation of GluR1 was labelled with antibodies specific for the N-terminal part of the receptor. These observations suggest that the GluR1 subunit consists of at least two isoforms possessing a common N-terminal region but a distinct C-terminus. Immunocytochemistry, using immunoperoxidase staining, was performed for the GluR1 subunit in rat forebrain with antisera raised against the N-terminal (253-267) and the C-terminal parts (877-889) of the molecule. Both antisera gave a similar distribution of immunoreactivity at the light-microscopic level. Immunoreactivity for the GluR1 subunit was selectively distributed throughout the rat forebrain. The hippocampus, septum, amygdala and olfactory bulb exhibited the strongest immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Early postnatal undernutrition impairs protein kinase C-dependent phosphorylation in rat brain synaptosomes

Undernutrition in rat pups was established by restricting feeding time daily. Protein kinase C-dependent phosphorylation in vitro was studied by incubating the mitochondrial-synaptosomal membrane fractions from adult, 18-day-old control and undernourished rats with gamma-[32P]ATP in presence of Ca2+ and phosphatidylserine. In adult and 18-day-old control rats, an increased phosphorylation of three major proteins (49, 53, 84 kDa) were detected in presence of calcium and phosphatidylserine. However, in 18-day-old undernourished rats, calcium/phosphatidylserine activated phosphorylation was found to be significantly impaired with only a slightly increased labelling detected in the 49 kDa protein.

Metabotropic glutamate receptors in cultured cerebellar granule cells: developmental profile

Excitatory amino acid (EAA)-induced polyphosphoinositide (PPI) hydrolysis was studied during the development in culture of cerebellar granule cells. The developmental pattern was similar using metabotropic glutamate (Glu) receptor (mGluR) agonists, including L-Glu, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid: The stimulation of [3H]inositol monophosphate ([3H]-InsP) formation was low at 2 days in vitro (DIV), but the response increased steeply, reaching a peak at 4 DIV, followed by a progressive decline. In contrast, carbamylcholine-induced PPI hydrolysis exhibited a plateau after a pronounced increase during the first week in vitro. At 6 DIV, but not at 4 DIV, when the activity peaked, PPI hydrolysis elicited by Glu was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, indicating that in cultured granule cells, NMDA receptors contribute to [3H]-InsP formation and that this component of the response develops relatively late. Accordingly, NMDA-induced [3H]-InsP formation, estimated under Mg(2+)-free conditions, increased markedly from very low values at 2 DIV to a plateau at 8-10 DIV. The developmental pattern of EAA-induced PPI hydrolysis was paralleled by changes in the level of an mRNA for a specific mGluR subtype (mGluR1 mRNA). RNA blot analysis performed with the pmGR1 cDNA probe revealed that the hybridization signal in RNA extracts from cultures at 1 DIV was very weak, but mGluR mRNA levels increased dramatically between 1 and 3 DIV, followed by a progressive decrease, so that by 15 DIV the mRNA levels were only approximately 10% of the values at 3 DIV.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediation of thalamic sensory responses in vivo by ACPD-activated excitatory amino acid receptors

The existence of the so-called metabotropic excitatory amino acid receptor has been known for some years. Various functions have been suggested for this receptor, but the lack of selective antagonists for (IS, 3R)-aminocyclopentane dicarboxylic acid (ACPD) has precluded the direct demonstration of a functional role for this receptor in synaptic processes. We describe here a specific antagonism of the excitatory responses of thalamic neurons to ACPD by two novel antagonists, and a parallel antagonism by these compounds of sensory synaptic responses to noxious stimuli of the same neurons. This provides the first direct pharmacological evidence for a functional role of ACPD-sensitive receptors in central neurotransmission, and indicates that these receptors may play an important part in central sensory processes.

Actions of agonists of metabotropic glutamate receptors on synaptic transmission and transmitter release in the olfactory cortex

1. The effects of agonists of on the evoked N-wave complex in slices of mouse have been studied: most experiments were carried out using slices perfused with Mg(2+)-free solution to which 10 microM of either 6,7-dinitroquinoxaline-2,3-dione or 6-cyano-7-nitroquinoxaline-2,3-dione was applied. 2. Following agonist washout, a slowly developing, long lasting potentiation of the complex occurred which was confined to the mediated component of the potential. The relative agonist potencies were 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 5-250 microM) = quisqualate (5-50 microM) > 1RS,3RS-cis-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 25-1000 microM) > L-glutamate (0.25-2.5 mM); NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and L-aspartate were inactive. 3. Potentiation of the NMDA receptor-mediated component by 1S,3R-ACPD (0.1 mM) was non-competitively antagonised by S-(+)- but not R-(-)-2-amino-3-phosphonopropionate (AP3, 0.125 mM), equally by D-(-) and L-(+)-2-amino-4-phosphonobutyrate (0.25 mM) and also by the protein kinase C inhibitors sphingosine, (25 microM), sangivamycin (25 microM) and 5-(isoquinolinylsulphonyl)-3-methylpiperazine (50 microM). 4. In a series of input-output experiments, 1S,3R-ACPD (0.1 mM) reversibly reduced the latency to peak of the NMDA receptor-mediated component at submaximal stimulus intensities, an effect blocked by S-(+)-AP3 (0.125 mM). On agonist washout, there was an increase in the area of the receptor-mediated component over all stimulus intensities, an effect blocked by the inhibitors of protein kinase C and by S-(+)-AP3 (0.125mM). 4-beta-Phorbol-12,13-diacetate (2.5 muM) also potentiated the component, an action inhibited by protein kinase C inhibitors but not by S-(+)-AP3. IS,3R-ACPD (0.1mM) had no significant effect on postsynaptic responses evoked by NMDA, AMPA and kainate, but significantly reversed a partial antagonism of NMDA responses produced by 7-chlorokynurenate (2.5 muM). The K+evoked release of glycine was selectively and significantly increased in the presence 0.1mM 1S,3R-ACPD(antagonized by 0.125 mM S-(+)-AP#) whereas following agonist washout, release of glycine fell to control levels but there was a significant increase in release of aspartate(antagonized by 25 muM sangivamycin and 0.125 muM S-(+)-AP3). It is concluded that mediate (i) a reduction in the latency of the mediated component of potentials by a mechanism that is independent of protein kinase C but which may depend on increased glycine release release and (ii) a long lasting increase in the total area of the potential by increasing transmitter (possibly aspartate) release by a mechanism that is protein kinase C-dependent.

Endogenous adenosine regulates the apparent efficacy of 1-aminocyclopentyl-1S,3R-dicarboxylate inhibition of forskolin-stimulated cyclic AMP accumulation in rat cerebral cortical slices

In rat cerebral cortical slices, the 1-aminocyclopentyl-1S,3R-dicarboxylate (1S,3R-ACPD) isomer of the selective metabotropic excitatory amino acid agonist ACPD inhibited forskolin-stimulated cyclic AMP (cAMP) accumulation in a concentration-dependent manner with a maximal inhibition of 51 +/- 3% and a half-maximally effective concentration of 8.8 +/- 3.4 microM. Similarly, 1R,3S-ACPD inhibited the forskolin response in a concentration-dependent manner, but with an inhibition of 80 +/- 5% at 3 mM. In addition to inhibiting forskolin-stimulated cAMP levels, 1S,3R-ACPD, but not 1R,3S-ACPD, enhanced the cAMP response to A2b adenosine receptor activation. In the presence of 1.2 U/ml of adenosine deaminase (included to reduce the contribution of endogenous adenosine), the efficacy of 1S,3R-ACPD was increased (88 +/- 3% inhibition), but the potency was unchanged. The adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine also increased the inhibitory effect of 100 microM 1S,3R-ACPD, from 57 +/- 1 to 78 +/- 5%. These results indicate that endogenous adenosine plays an important role in regulating the apparent efficacy of 1S,3R-ACPD inhibition of forskolin-stimulated cAMP accumulation in rat cerebral cortical slices and that previous studies in rat hippocampus and hypothalamus in the absence of added adenosine deaminase may have underestimated the efficacy of this compound.

Characterization of metabotropic glutamate receptors coupled to a pertussis toxin sensitive G-protein in bovine brain coated vesicles

Glutamate metabotropic receptors (mGluRs) in bovine brain coated vesicles have been characterized by pharmacological and kinetic binding experiments. Saturation experiments revealed a single binding site with a Kd = 607.9 +/- 78.5 nM and a Bmax = 6.45 +/- 0.88 pmol/mg protein. The specific binding of L-[3H]glutamate to mGluRs is regulated by guanine nucleotides. Guanosine-5'-triphosphate (GTP; 100 microM) shifts the agonist competition curves to the right, increasing the IC50 values. Pertussis toxin treatment produces a pharmacological binding profile for quisqualate similar to that obtained in the presence of 100 microM GTP. These results indicate the presence of metabotropic glutamate receptors in coated vesicles and its coupling to a pertussis toxin sensitive G-protein.

Inhibitory glutamate response on cyclic AMP formation in cultured astrocytes

We examined the effects of glutamate receptor agonists on cyclic AMP (cAMP) formation in cultured astrocytes. L-Glutamate reduced the cAMP formation induced by either isoproterenol (IC50 7 microM) or forskolin without affecting the basal level. Glutamate agonists reduced the cAMP formation in astrocytes with the following rank order of potency: L-glutamate > trans-(+/-)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) = quisqualate. Pretreatment of astrocytes with pertussis toxin resulted in a partial reduction of the glutamate response and a complete attenuation of the t-ACPD response. These results suggest that astrocytes have another type of metabotropic glutamate receptor which inhibits adenylate cyclase through pertussis toxin-sensitive G-proteins.

Glutamate and quisqualate regulate expression of metabotropic glutamate receptor mRNA in cultured cerebellar granule cells

Metabotropic glutamate receptor (type 1; mGluR1) is expressed predominantly in the hippocampus and the cerebellum. Using cultured cerebellar granule cells, we investigated the regulation of the mGluR1 mRNA expression. Levels of mGluR1 mRNA were decreased to less than half by high potassium stimulation and by glutamate and quisqualate. Although these glutamate receptor agonists tested are also known to cause neuronal cell death in culture, the effect of cell death cannot explain the observed reduction in mGluR1 mRNA because of the following reasons: (a) antagonists of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors inhibited cell death, but not the reduction of the level of mGluR1 mRNA; (b) mGluR1 mRNA returned to its initial level 48 h after the agonist application; and (c) the mRNA level of one of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptors (GluR1) was not altered by these conditions. Therefore, we conclude that the glutamate or quisqualate stimulation can specifically inhibit the expression of mGluR1 mRNA. The dose response of quisqualate for the reduction in mGluR1 mRNA is consistent with that for inositol phosphate formation stimulated through the cloned mGluR1. The mRNA reduction did not require extracellular calcium. Desensitization of mGluR1 with phorbol ester abolished the mRNA reduction. These results suggest that the reduction in mGluR1 mRNA is mediated by the activation of the metabotropic receptor itself.

Neuronal degeneration and spinal cavitation following intraspinal injections of quisqualic acid in the rat

Microinjections of quisqualic acid were made in the spinal cord to evaluate the excitotoxic effects of this excitatory amino acid agonist on spinal neurons in the rat. Animals were divided into four groups based on post injection survival times of 7-49 days. Injections ranging from 0.3 to 2.0 microL of 8.3, 83, and 125 mM quisqualic acid or normal saline were made in the lower thoracic and upper lumbar spinal cord. At all survival times evaluated unilateral injections of quisqualic acid produced unilateral or bilateral cell death and a prominent inflammatory reaction. In 23/25 animals spinal cavities were also observed. Spinal cord segments at or near quisqualate injection sites contained darkly stained, hypertrophied neuronal profiles, and increased staining for glial fibrillary acidic factor. Immunostaining for glial fibrillary acidic factor was especially intense in areas of neuronal degeneration and in border areas of spinal cavities. The results of this study suggest that the intraspinal injection of quisqualic acid may be an effective method to study the mechanisms of excitatory amino acid neurotoxicity, and the pathogenesis of spinal cavitation following neuronal injury.

Novel synaptic potentials in cerebellar Purkinje cells: probable mediation by metabotropic glutamate receptors

Glutamate receptors of both the ionotropic (ion channel-linked) and metabotropic (enzyme-linked) categories are abundantly expressed by Purkinje cells in the cerebellum but the functional significance of the latter receptors is unknown. We have tested the possibility that they are activated by the parallel fibre input by recording from Purkinje cells within a biplanar cerebellar slice preparation using the grease-gap technique. Under conditions where ionotropic (NMDA and non-NMDA) glutamate and GABA receptors were blocked pharmacologically, electrical stimulation of parallel fibres gave rise to two very slow potentials. The first peaked about 400 msec from the start of stimulation and was depolarising. It was not evident with single stimuli but reached maximum amplitude after 6 shocks delivered at 50 Hz. The wave was abolished when the slices were perfused with Ca(2+)-free solution or with drugs that inhibit synaptic transmission, but it was resistant to blockade of GABAB receptors, acetylcholine receptors and adrenergic receptors. Next came a slow hyperpolarising potential that peaked about 30 sec after stimulation and which was also Ca(2+)-dependent. The sequence of potentials was replicated by perfusion of an exogenous agonist acting selectively on metabotropic glutamate receptors. We conclude that parallel fibre-to-Purkinje cell synaptic transmission involves not only fast signals generated through ionotropic non-NMDA receptors but also much slower potentials that are likely to be mediated by metabotropic glutamate receptors. These potentials are likely to be significant both for shorter-term (seconds to minutes) Purkinje cell excitability as well as for the induction of longer-term synaptic plasticity.

Metabotropic glutamate receptors in brain function and pathology

Metabotropic glutamate receptors (mGluRs) are a novel family of recently cloned G protein-coupled receptors. These receptors are heterogeneous and coupled to multiple second messenger systems that include increases in phosphoinositide hydrolysis, activation of phospholipase D, decreases in cAMP formation, increases in cAMP formation, and changes in ion channel function. Using the selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (1s,3R-ACPD), considerable progress has been made towards understanding the role of this glutamate receptor class in the central nervous system. This article reviews the molecular aspects and pharmacology of mGluRs, and recent studies elucidating their role in brain function and pathology.

Subtypes of sodium-dependent high-affinity L-[3H]glutamate transport activity: pharmacologic specificity and regulation by sodium and potassium

Some data suggest that the sodium-dependent, high-affinity L-glutamate (Glu) transport sites in forebrain are different from those in cerebellum. In the present study, sodium-dependent transport of L-[3H]Glu was characterized in cerebellum and cortex. In both cerebellar and cortical tissue, activity was enriched in synaptosomes. Approximately 100 excitatory amino acid analogues were tested as potential inhibitors of transport activity. Many of the compounds tested inhibited transport activity by < 65% at 1 mM and were not studied further. One group of compounds exhibited inhibition conforming to theoretical curves with Hill coefficients of 1 and were < 10-fold selective as inhibitors of transport activity. These included three of the putative endogenous substrates for transport: L-Glu, L-aspartate, and L-cysteate. Four of the compounds exhibited inhibition conforming to theoretical curves with Hill coefficients of 1 and were > 10-fold selective as inhibitors. These included beta-N-oxalyl-L-alpha,beta-diaminopropionate, alpha-methyl-DL-glutamate, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine, and (2S,1'S,2'S,3'S)-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine. Data obtained with a few of the inhibitors were consistent with two sites in one or both of the brain regions. (2S,1'R,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG-II) was identified as the most potent (IC50 = 5.5 microM) and selective (60-100-fold) inhibitor of transport activity in cerebellum. One of the potential endogenous substrates, L-homocysteate, was also a selective inhibitor of cerebellar transport activity. The data for inhibition of transport activity in cortex by both L-CCG-II and L-homocysteate were best fit to two sites. Kainate was equipotent as an inhibitor of transport activity, and in both brain regions the data for inhibition were best fit to two sites. The possibility that there are four subtypes of excitatory amino acid transport is discussed. Altering sodium and potassium levels affects cerebellar and cortical transport activity differently, suggesting that the differences extend to other recognition sites on these transporters.

Effects of age on L-glutamate-induced depolarization in three hippocampal subfields

The effects of aging on the translation of L-glutamate-induced depolarization into hippocampal neuronal firing frequency were studied in vitro. L-glutamate was iontophoretically-applied to the somatic region of extracellularly recorded single units. In none of the three principal hippocampal subfields (fascia dentata, CA3, and CA1) were there any effects of age on neuronal sensitivity to L-glutamate. Because there are pronounced, region-specific age effects on AMPA sensitivity (3), these results are in agreement with the conclusions of other investigators that the depolarization caused by exogenously applied L-glutamate probably exerts its effects through nonsynaptic mechanisms. These mechanisms, however, which lead to powerful depolarization and action potentials in hippocampal cells, are unaffected by age.

Blockade of NMDA receptor-mediated mobilization of intracellular Ca2+ prevents neurotoxicity

NMDA receptor activation leads to elevated Ca2+ in cultured rat cortical and retinal ganglion cell neurons. If excessive, this Ca2+ response is associated with delayed neurotoxicity. We used dantrolene and ionomycin to test if the Ca2+ response to NMDA was due to mobilization of intracellular Ca2+ stores rather than merely to Ca2+ influx. In the presence of EGTA, ionomycin resulted in release and subsequent depletion of intracellular Ca2+ stores. Henceforth, despite normal extracellular Ca2+, NMDA elicited only about half of its former Ca2+ response. Similarly, when dantrolene was used to block Ca2+ release from intracellular stores, we observed > 50% smaller NMDA-evoked Ca2+ responses. These results quite surprisingly indicate that at least half the Ca2+ response to NMDA is due to release of intracellular Ca2+, a process triggered by influx of extracellular Ca2+. Dantrolene also protected neurons from NMDA receptor-mediated neurotoxicity. Release of intracellular Ca2+ may therefore be a necessary step in the cascade leading to neuronal damage induced by excessive NMDA receptor stimulation and may be amenable to pharmacological intervention.

Differential roles of NMDA and non-NMDA receptor activation in induction and maintenance of thermal hyperalgesia in rats with painful peripheral mononeuropathy

Central activation of excitatory amino acid receptors has been implicated in neuropathic pain following nerve injury. In a rat model of painful peripheral mononeuropathy, we compared the effects of non-competitive NMDA receptor antagonists (MK 801 and HA966) and a non-NMDA receptor antagonist (CNQX) on induction and maintenance of thermal hyperalgesia induced by chronic constrictive injury (CCI) of the rat common sciatic nerve. Thermal hyperalgesia to radiant heat was assessed by using a foot-withdrawal test and NMDA/non-NMDA receptor antagonists were administered intrathecally onto the lumbar spinal cord before and after nerve injury. Four daily single treatments with 20 nmol HA966 or CNQX beginning 15 min prior to nerve ligation (pre-injury treatment), reliably reduced thermal hyperalgesia in CCI rats on days 3, 5, 7 and 10 after nerve ligation. Thermal hyperalgesia was also reduced in CCI rats receiving a single post-injury treatment with HA966 (20 or 80 nmol) or MK 801 (5 or 20 nmol) on day 3 after nerve ligation when thermal hyperalgesia was well developed. In contrast, a single post-injury CNQX (20 or 80 nmol) treatment failed to reduce thermal hyperalgesia or to potentiate effects of HA966 or MK 801 (5 or 20 nmol) on thermal hyperalgesia in CCI rats. Moreover, multiple post-injury CNQX treatments utilizing the same dose regime as employed for the pre-injury treatment attenuated thermal hyperalgesia but only when the treatment began 1 or 24 h (but not 72 h) after nerve ligation.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippocampal slices

Metabotropic excitatory amino acid (EAA) receptors are coupled to effector systems through G proteins. Because various G protein-coupled receptors stimulate the hydrolysis of phosphatidylcholine by phospholipase D (PLD), we examined the possibility that metabotropic EAA receptors exist that are coupled to the activation of PLD. We found that the selective metabotropic glutamate receptor (mGluR) agonists 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (ACPD) and 1S,3S-ACPD, but not the inactive isomer, 1R,3S-ACPD, induce a concentration-dependent increase in PLD activity in hippocampal slices. Selective ionotropic glutamate receptor (iGluR) antagonists did not block 1S,3R-ACPD-induced PLD stimulation. Furthermore, although selective iGluR agonists did not activate this response, the nonselective mGluR-iGluR agonists, ibotenate and quisqualate, caused significant increases in PLD activity (all in the presence of iGluR antagonists). L-2-Amino-3-phosphonopropionic acid, which blocks the mGluR that is coupled to phosphoinositide hydrolysis in various brain regions, activates PLD to the same extent as the active isomers of ACPD. These data suggest that metabotropic EAA receptors exist in hippocampus that are coupled to PLD activation and are pharmacologically distinct from phosphoinositide hydrolysis-coupled mGluRs.

Organization and expression of the gene encoding chick kainate binding protein, a member of the glutamate receptor family

The gene encoding chick cerebellar Bergmann glia-specific kainate binding protein (chKBP), has been isolated, characterized and expressed in heterologous systems. The structural gene spans 11.2 kb and contains 11 exons and 10 introns. Several of the exons encode specific receptor domains, including each of the predicted transmembrane regions. Exon/intron boundaries flanking the second, putative channel-forming transmembrane domain are conserved between chKBP and other glutamate/kainate receptor subunits. The putative promoter region 5' to the first exon displays high GC content and TATA, CAAT and AP1 consensus sequences. Transcription of the chKBP gene is evident prior to full cerebellar cortical maturation. Transcripts are abundant in cells consistent with Bergmann glia, as revealed by in situ hybridization. Transfection of 293 kidney cell cultures with chKBP cDNA or chKBP gene expression constructs confers CNQX-sensitive kainate binding with the pharmacological specificity displayed by both chKBP and kainate receptors. However, expression of the same constructs in Xenopus oocytes fails to yield detectable agonist-activated currents.

The expression of presynaptic t-ACPD receptor in rat cerebellum

The expression of a receptor subtype for one type of excitatory amino acid agonist, t-ACPD, was examined in developing Purkinje cells of cerebellar slices. The t-ACPD-induced responses were compared with those induced by QA in current response, single cell Ca2+ imaging and changes in the miniature currents in the same preparation. It was found that t-ACPD induced a single component of inward current, and an increase in the frequency of miniature currents associated with the presence of external Ca2+, but was ineffective at mobilizing intracellular Ca2+ even in the presence of external Ca2+. The present study suggests the expression of at least two types of metabotropic receptors in the Purkinje cell region, one of which, expressed in the Purkinje cell dendrites, is highly sensitive to QA, but relatively insensitive to t-ACPD, and the other of which is a t-ACPD-sensitive receptor expressed on the presynaptic terminals of the neurons making synapses onto Purkinje cells.

Modulation of transient type K channel cloned from rat heart

We have cloned a transient type K channel from rat heart (RH10) and coexpressed a metabotropic glutamate receptor (mGluR5) to study the functional modulation of RH10 coupled to the phosphatidylinositol (PI) hydrolysis. Stimulation of mGluR5 suppressed peak amplitude of RH10 current and affected voltage dependence of activation and inactivation of the channel.

Cyclic adenosine 3'5'-monophosphate potentiates excitatory amino acid and synaptic responses of rat spinal dorsal horn neurons

Intracellular recordings were made from rat dorsal horn neurons in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). In the presence of TTX, bath application of the membrane permeable analogue of cyclic AMP, 8-Br cyclic AMP (25-100 microM) caused a small depolarization of the resting membrane potential accompanied by a variable change in membrane input resistance. In addition, 8-Br cyclic AMP caused a long-lasting increase in the spontaneous synaptic activity and the amplitude of presumed monosynaptic excitatory postsynaptic potentials evoked in the substantia gelatinosa neurons by orthodromic stimulation of a lumbar dorsal root. When the fast voltage-sensitive Na conductance was blocked by TTX, 8-Br cyclic AMP enhanced in a reversible manner, the depolarizing responses of a proportion of dorsal horn neurons to N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and kainic acid (KA). The effects of 8-Br cyclic AMP on the resting membrane potential and the NMDA response of dorsal horn neurons were mimicked by reducing phosphodiesterase activity with bath application of 3-isobutyl-1-methylxanthine, but not by cyclic AMP applied extracellularly. Moreover, we have found that intracellular application of a protein inhibitor of cyclic AMP-dependent protein kinase (PKI) into dorsal horn neurons prevents the 8-Br cyclic AMP-induced potentiation of the NMDA response of these cells. These results suggest that in the rat spinal dorsal horn the activation of the adenylate cyclase-cyclic AMP-dependent protein kinase system may be involved in the enhancement of the sensitivity of postsynaptic excitatory amino acid (NMDA, AMPA, KA) receptors and modulation of primary afferent neurotransmission, including nociception.