Enteral supplements of a carbon monoxide donor CORM-A1 protect against cerebrovascular dysfunction caused by neonatal seizures.

Cerebral blood flow dysregulation caused by oxidative stress contributes to adverse neurologic outcome of seizures. A carbon monoxide (CO) donor CORM-A1 has antioxidant and cytoprotective properties. We investigated whether enteral supplements of CORM-A1 can improve cerebrovascular outcome of bicuculline-induced seizures in newborn piglets. CORM-A1 (2 mg/kg) was given to piglets via an oral gastric tube 10 minutes before or 20 minutes after seizure onset. Enteral CORM-A1 elevated CO in periarachnoid cerebrospinal fluid and produced a dilation of pial arterioles. Postictal cerebral vascular responses to endothelium-, astrocyte-, and vascular smooth muscle-dependent vasodilators were tested 48 hours after seizures by intravital microscopy. The postictal responses of pial arterioles to bradykinin, glutamate, the AMPA receptor agonist quisqualic acid, ADP, and heme were greatly reduced, suggesting that seizures cause injury to endothelial and astrocyte components of the neurovascular unit. In contrast, in the two groups of piglets receiving enteral CORM-A1, the postictal cerebral vascular responsiveness to these dilators was improved. Overall, enteral supplements of CORM-A1 before or during seizures offer a novel effective therapeutic option to deliver cytoprotective mediator CO to the brain, reduce injury to endothelial and astrocyte components of cerebral blood flow regulation and to improve the cerebrovascular outcome of neonatal seizures.

Anxiolytic properties of Valeriana officinalis in the zebrafish: a possible role for metabotropic glutamate receptors.

Valerian extract is used in complementary and alternative medicine for its anxiolytic and sedative properties. Our previous research demonstrated valerian interactions with glutamate receptors. The purpose of this study was to determine if valerian anxiolytic properties are mediated by metabotropic glutamate receptors (mGluR) such as mGluR (1/5) (mGluR I) and mGluR (2/3) (mGluR II). Adult wild-type zebrafish (Danio rerio) prefer the black compartment and avoid the white compartment in the dark/light preference task. Zebrafish exposed to 1 mg/mL of valerian extract or 0.00117 mg/mL valerenic acid increased their residence time in the white side by 84.61 ± 6.55 % and 58.30 ± 8.97 %, respectively. LAP3 (mGluR I antagonist) and EGLU (mGluR II antagonist) significantly inhibited the effects of valerian and valerenic acid. These results demonstrated that valerian and valerenic acid have anxiolytic properties in the zebrafish. Moreover, the selective interaction of valerian with mGluR I and II represent an alternative explanation for the anxiolytic properties of this plant and support the role of mGluR in anxiety.

Group I, II, and III mGluR compounds affect rhythm generation in the gastric circuit of the crustacean stomatogastric ganglion.

We have studied the effects of group I, II, and III metabotropic glutamate receptor (mGluR) agonists on rhythm generation by the gastric circuit of the stomatogastric ganglion (STG) of the Caribbean spiny lobster Panulirus argus. All mGluR agonists and some antagonists we tested in this study had clear and distinct effects on gastric rhythm generation when superfused over combined oscillating or blocked silent STG preparations. A consistent difference between group I agonists and group II and III agonists was that group I agonists acted excitatory. The group I-specific agonists L-quisqualic acid and (S)-3,5-dihydroxyphenylglycine, as well as the nonspecific agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid accelerated ongoing rhythms and could induce gastric rhythms in silent preparations. The group II agonist (2S,1'S, 2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) and the group III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) slowed down or completely blocked ongoing gastric rhythms and were without detectable effect on silent preparations. The action of L-CCG-I was blocked partially by the group-II-specific antagonist, (RS)-1-amino-5-phosphonoindan-1-carboxylic acid [(RS)APICA], and the group-III-specific antagonist (RS)-alpha-methyl-4-phosphonophenylglycine completely blocked the action of L-AP4. Besides its antagonistic action, the group-II-specific antagonist (RS)APICA had a remarkably strong apparent inverse agonist action when applied alone on oscillating preparations. The action of all drugs was dose dependent and reversible, although recovery was not always complete. In our experiments, the effects of none of the mGluR-specific agonists were antagonized or amplified by the N-methyl-D-aspartate (NMDA)-receptor-specific antagonist D(-)-2-amino-5-phosphonopentanoic acid, excluding the contamination of responses to mGluR agonists by nonspecific cross-reactivity with NMDA receptors. Picrotoxin did not prevent the inhibitory action of L-CCG-I and L-AP4. We conclude that mGluRs, probably similar to those belonging to groups I, II, and III described in mammals, may play a role as modulators of gastric circuit rhythm generation in vivo.

NGF treatment potentiates c-fos expression in the rat nucleus basalis upon excitotoxic lesion with quisqualic acid.

The induction of the c-fos gene in the rat brain by NGF was studied in a model of acute cholinergic hypofunction, i.e., the lesion of the nucleus basalis magnocellularis (NBM) with quisqualic acid. Choline acetyltransferase and Fos immunoreactivity (IR) in the NBM were analyzed at different times after the excitotoxic lesion. NGF treatment induced a potentiation of Fos expression 4 and 24 h after lesion. The possibility is discussed that c-fos induction is one of the early mechanisms of the neuroprotective action of NGF.

Activation of group I mGluRs increases spontaneous IPSC frequency in rat frontal cortex.

The effect of metabotropic glutamate receptor (mGluR) activation on inhibitory synaptic transmission was examined by using whole cell patch-clamp recordings. Spontaneous (s) and miniature (m) inhibitory postsynaptic currents (IPSCs) were recorded from visually identified layer II/III pyramidal neurons in rat neocortex in vitro. Excitatory postsynaptic currents (EPSCs) were blocked by using bath application of 20 microM D(-)2-amino-5-phosphonovaleric acid and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione. In the presence of 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (30-100 microM), Lp4-quisqualate (5 microM), and the group I selective mGluR agonist (S)-3,5-dihydroxyphenylglycine (100 microM), the frequency of sIPSCs was increased. Decay kinetics of sIPSCs were unaffected. No enhancement of mIPSCs was observed. Bath application of group II (2S,3S,4S-alpha-carboxycyclopropyl-glycine; 5 microM) and group III selective mGluR agonists (L-2-amino-4-phosphonobutyric acid; 100 microM) had no detectable effects on the frequency or amplitude of sIPSCs. These findings indicate that activation of group I mGluRs (mGluR1 and/or mGluR5) enhances gamma-aminobutyric acid-mediated synaptic inhibition in layer II/III pyramidal neurons in neocortex. The lack of effect on mIPSCs suggests a presynaptic action via excitation of inhibitory interneurons.

The effect of excitatory aminoacids on GABA release from mediobasal hypothalamus of female rats.

The purpose of the present study was to examine the in vitro effect of L-glutamate and its agonists on basal and potassium-evoked GABA release from incubated mediobasal hypothalamus (MBH) of intact, ovariectomized (OVX) and OVX-estrogenized female rats. L-glutamate (100 microM) decreased evoked GABA release from MBH of intact female rats in diestrus. NMDA and quisqualate (10 and 100 microM) modified neither basal nor evoked hypothalamic GABA release of intact rats. However, kainate (10 and 100 microM) decreased hypothalamic basal and evoked GABA release of intact rats. Kainate induced no changes in basal or in evoked GABA release from hypothalami of OVX rats, but decreased GABA release in chronically estrogenized rats. DNQX (6,7-dinitroquinoxaline-2,3-dione), a non-NMDA receptor antagonist, failed to affect GABA release but blocked the inhibitory effect of kainate. The kainate effect was not Mg2+-sensitive and was not inhibited by D-AP5 (D(-)-2-amino-5-phosphonopentanoic acid), an NMDA-specific receptor antagonist. Kainate induced no changes in nitric oxide synthase activity in MBH of either intact or estrogenized rats. These data indicate that kainate decreases GABA release from MBH of female rats through a non-NMDA receptor subtype, and provide evidence to support the view that kainate-mediated decrease of the hypothalamic GABAergic tone is affected by estrogens.

Immunocytochemical characterization of quisqualic acid- and N-methyl-D-aspartate-induced excitotoxicity in the retina of chicks.

A single, large dose of N-methyl-D-aspartate (NMDA) or quisqualic acid (QA) injected into the chick eye has been shown previously to destroy many retinal amacrine cells and to induce excessive ocular growth accompanied by myopia. The purpose of this study was to identify distinct populations of retinal cells, particularly those believed to be involved in regulating ocular growth, that are sensitive to NMDA or QA. Two pmol of NMDA or 0.2 micromol of QA were injected unilaterally into eyes of 7-day-old chicks, and retinas were prepared for observation 1, 3, or 7 days later. Retinal neurons were identified by using immunocytochemistry, and cells containing fragmented DNA were identified by 3'-nick-end labelling in frozen sections. NMDA and QA destroyed many amacrine cells, including those immunoreactive for vasoactive intestinal polypeptide, Met-enkephalin, and choline acetyltransferase, but they had little effect upon tyrosine hydroxylase-immunoreactive cells. Other cells affected by both QA and NMDA included those immunoreactive for glutamic acid decarboxylase, gamma-aminobutyric acid, parvalbumin, serotonin, and aminohydroxy methylisoxazole propionic acid (AMPA) receptor subunits GluR1 and GluR2/3. Cells largely unaffected by QA or NMDA included bipolar cells immunoreactive for protein kinase C (alpha and beta isoforms) and amacrine cells immunoreactive for glucagon. DNA fragmentation was detected maximally in many amacrine cells and in some bipolar cells 1 day after exposure to QA or NMDA. We propose that excitotoxicity caused by QA and NMDA induces apoptosis in specific populations of amacrine cells and that these actions are responsible for the ocular growth-specific effects of QA and NMDA reported elsewhere.

Excitotoxic lesions of histaminergic neurons by excitatory amino acid agonists in the rat brain.

Excitotoxic lesions of histaminergic (HA) neurons in the rat hypothalamus were observed by the injection of ibotenate and N-methyl-D-aspartate (NMDA) while kainate, quisqualate and quinolinate had no significant effects on HA neurons. An NMDA selective antagonist, 2-amino-5-phosphonopentanoic acid, was able to block the toxic effects of both NMDA and ibotenate almost completely, but kynurenate was a weaker inhibitor of ibotenate or NMDA-induced neurotoxicity. These results suggest that there exists an ibotenate-sensitive and quinolinate-insensitive NMDA receptor subtype on HA neurons.

G-protein activation by metabotropic glutamate receptors reduces spike frequency adaptation in neocortical neurons.

Intracellular recordings were obtained from neocortical brain slices of adult rats maintained in vitro. The effect of metabotropic glutamate receptor activation on spike frequency adaptation in regular spiking layer II and III neurons was determined. Putative metabotropic glutamate receptor agonists and antagonists, as well as inhibitors of intracellular signaling systems, were tested. Activation of metabotropic glutamate receptors by bath applied (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD; 50-200 microM) reduced the first interspike interval and increased action potential frequency at all current intensities. This effect was not blocked by ionotropic glutamate receptor antagonists. Under these recording conditions, quisqualate (1-10 microM) similarly reduced spike frequency adaptation. Neither 1R,3S-ACPD, L-2-carboxycyclopropylglycine-I nor the putative presynaptic metabotropic glutamate receptor agonist, L-2-amino-4-phosphonobutyrate, mimicked the effects of 1S,3R-ACPD or quisqualate. Bath application of the putative metabotropic glutamate receptor antagonist, alpha-methyl-4-carboxyphenylglycine, competitively antagonized the excitatory actions of 1S,3R-ACPD. Another putative antagonist, L-2-amino-3-phosphonopropionate, failed to antagonize the reduction in spike frequency adaptation. Intracellular injection of guanosine-5'-O-(2-thiodiphosphate), a non-hydrolysable analog of GTP, inhibited the postsynaptic metabotropic glutamate receptor-mediated effects. However, the depression of synaptic transmission by 1S,3R-ACPD was not antagonized by this compound. The decrease in spike frequency adaptation by 1S,3R-ACPD was not prevented by prior exposure to the non-specific protein kinase inhibitors H-7 or H-8 (10 microM), the protein kinase A inhibitor H-89 (0.25 microM) or the protein kinase C inhibitor staurosporine (0.10 microM). These data suggest that the metabotropic glutamate receptor-mediated reduction in spike adaptation requires the activation of specific G-protein-coupled metabotropic glutamate receptor subtypes located on postsynaptic sites. The increase in neuronal excitability observed in the adult neocortex may be mediated either by an unidentified G-protein-coupled second messenger or via a membrane-delimited G-protein action.

Immortalized hypothalamic neurons express metabotropic glutamate receptors positively coupled to cyclic AMP formation.

We have characterized the expression pattern and pharmacological profile of activation of metabotropic glutamate receptors (mGluRs) in immortalized, gonadotropin releasing hormone (GnRH)-secreting GT1-7 cells, which represent a homogeneous cellular population of hypothalamic origin. These cells are known to respond to the mGluR agonist (1S,3R)-cyclopentanedicarboxylic acid (1S,3R-ACPD) with increased GnRH release. To establish which specific mGluR subtypes are expressed by GT1-7 cells, we used polyclonal antibodies raised against non-conserved regions of the carboxy-terminal domains of individual subtypes. The selectivity of these antibodies was tested in HEK 293 cells transiently transfected with each mGluR subtype. GT1-7 cells stained positively for the subtypes mGluR1a, -1b and -5 (belonging to group I mGluR2/3 (group II) and mGluR7 (group III). Agonists of group I mGluRs, including 1S,3R-ACPD, activated phosphoinositide hydrolysis in GT1-7 cells. This effect, however, was manifested only when cell density was low, and it disappeared when cells reached confluence. Stimulation of phosphoinositide hydrolysis could not therefore have been related to hormone secretion because 1S,3R-ACPD effectively released GnRH in confluent cultures. We then focused on group II and III mGluRs, which in transfected cells are negatively linked to adenylate cyclase activity. Unexpectedly, however, agonist which preferentially activate group II and III mGluRs increased both basal and forskolin-stimulated cAMP accumulation in GT1-7 cells. Stimulation of cAMP accumulation by mGluR agonists was not prevented by enzymatic depletion of endogenous adenosine, but was obliterated when cells were incubated with agonists of receptors positively coupled to adenylate cyclase, such as beta-adrenergic and prostaglandin E2 receptors. These results suggest that GT1-7 cells express a novel mGluR subtype positively coupled to adenylate cyclase, which shares the same transduction pathway of other classical receptors coupled with a Gs-type of GTP-binding protein.

Role of glutamate in regulating hypothalamic proglucagon-derived peptide secretion in vitro.

We have previously demonstrated expression of the proglucagon gene and synthesis and secretion of the proglucagon-derived peptides (PGDPs) in the fetal rat hypothalamus. The excitatory amino acid glutamate has been found to be a key regulator of hypothalamic neuroendocrine hormone secretion. Therefore, the effects of glutamate on hypothalamic PGDP secretion and synthesis were examined in the present study, using the hypothalamic culture model. Glutamate (10 microM) significantly stimulated PGDP secretion (P < 0.01), but had no effect on the total PGDP content of the cultures over 24 hr of incubation. Similarly, the metabotropic/ionotropic glutamate receptor agonist, quisqualic acid (10 microM) stimulated PGDP secretion only ( P < 0.05). In contrast, the ionotropic glutamate receptor agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 10 microM) and antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), had no effects on either secretion or content of the hypothalamic PGDPs. These findings suggest that excitatory amino acids, and glutamate in particular, regulate secretion but not synthesis of the PGDPs in the hypothalamus.

Behavioral and biochemical consequences of combined lesions of the medial septum/diagonal band and nucleus basalis in the rat when ibotenic acid, quisqualic acid, and AMPA are used.

Combined lesions in the medial septum/diagonal band and nucleus basalis magnocellularis (NBM) in rats were produced using three excitotoxins, ibotenate (Ibo), quisqualate (Quis), and AMPA. Reductions in choline acetyltransferase (ChAT) activity differed in the cortical regions for the three toxins (AMPA > Quis > Ibo), but were fairly similar in the hippocampus. ChAT activities were not reduced in the globus pallidus, but AMPA reduced ChAT in the amygdala. Lesions with all three toxins produced similar decrements in hippocampal and posterior cortical serotonin levels. A small reduction in posterior cortical norepinephrine was detected for Quis and Ibo lesions. Spatial memory impairments were found for all three toxin groups compared with controls in acquisition, platform reversal, and a spatial probe in the water maze. The learning deficit was greatest with the Quis lesion and equivalent for the Ibo and AMPA lesions. There was no deficit in single trial passive avoidance retention for the Ibo and AMPA groups. The AMPA group was slower than controls on both training and retention trials to enter the dark compartment. This group also showed a tendency to hypoactivity as measured in an open-field test. Excitotoxic infusions into medial septum/diagonal band and NBM produced spatial mnemonic deficits which do not parallel reductions in overall ChAT activity and do not resemble the profile of behavioral changes previously reported for NBM lesions alone using these toxins.

Distribution and ontogeny of 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid-sensitive and quisqualate-insensitive [3H]glutamate binding sites in the rat brain.

Displacement of [3H]glutamate by 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid and quisqualate (in the presence of saturating concentrations of ionotropic glutamate receptor agonists) was used to characterize optimal ionic conditions, distribution, and the ontogeny of glutamate receptor binding sites in rat brain. Using rat forebrain membranes or receptor autoradiography, optimal 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid-sensitive [3H]glutamate binding was found in the presence of 100 mM bromide ions and in the absence of calcium ions. Under these conditions, [3H]glutamate binding was relatively quisqualate insensitive. In regions of the neonatal (11-day-old) and adult rat brain, this [3H]glutamate binding was highest in forebrain (striatum, cerebral cortex, and hippocampus) and hypothalamus/mid-brain but was lower in the cerebellum, olfactory bulb, and pons/medulla regions. 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid-sensitive and quisqualate-insensitive [3H]glutamate binding was present in the rat forebrain at 1 day of age and gradually increased more than twofold by day 50 (adult). Thus, in the presence of bromide ions and in the absence of calcium ions, [3H]glutamate labels a subpopulation of metabotropic glutamate receptors that are sensitive to 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid but insensitive to quisqualate. Expression of [3H]glutamate binding under these conditions was both regionally and developmentally regulated in rat brain, suggesting that [3H]glutamate is labeling a distinct population of metabotropic glutamate receptors.

Mechanisms underlying developmental changes in the expression of metabotropic glutamate receptors in cultured cerebellar granule cells: homologous desensitization and interactive effects involving N-methyl-D-aspartate receptors.

Glutamate receptors coupled to polyphosphoinositide (PPI) hydrolysis (metabotropic glutamate receptors, mGluR), are highly efficient during the early stages of postnatal life and are thought to be involved in developmental plasticity. The dramatic decrease with age in mGluR activity suggests the existence of mechanisms that down-regulate this receptor after a certain stage of neuronal maturation. In cultured cerebellar granule neurons grown under conditions that promote the survival and maturation of cells (serum-containing medium with 25 mM K+), enzymatic depletion of extracellular glutamate prevented the age-dependent decrease in mGluR agonist-stimulated PPI hydrolysis that normally occurs after 4 days of maturation in vitro, suggesting that mGluR activity declines as a result of developmental changes affecting homologous desensitization. This was borne out by the observation that glutamate at low concentrations (1-10 microM) readily desensitized mGluR at 7 days but not at 4 days in culture. Furthermore, the critical period during which the high sensitivity to agonist-induced desensitization of mGluR developed coincided with the period when phorbol ester-activated protein kinase C acquired the ability to suppress mGluR activity. The developmental pattern of mGluR agonist-induced PPI hydrolysis was similar in granule cells grown under "trophic" and "nontrophic" conditions (in cultures in 25 mM K+ and in a medium containing "low" K+, in this study, 10 mM, respectively). However, the developmental decline in the response to mGluR stimulation after 4 days in vitro was not prevented in cells grown in 10 mM K+ by the removal of extracellular glutamate; rather, it could be counteracted by treatment with N-methyl-D-aspartate (NMDA) (EC50, approximately 4 microM), which blocked the development of mGluR desensitization. The effect was NMDA receptor mediated and required DNA transcription and protein synthesis. However, NMDA exerted a different effect in cells grown in 25 mM K+, inducing a substantial decrease rather than an increase in mGluR activity. The effect of growth conditions was also examined on mGluR mRNA levels, which were not always correlated with mGluR activity. In general, either increases in the medium K+ concentrations or NMDA supplementation of the cultures resulted in a decrease in mGluR mRNA levels. It is noteworthy that NMDA could also restore mGluR activity after the metabotropic response had reached its peak. This implies that NMDA receptor activation may be involved in the increase in mGluR activity in adult life under conditions that elicit plastic changes in the nervous system.

Molecular properties of the glutamate receptor mediating synaptic excitation in rat hypothalamic neurons.

AMPA-type glutamate receptors (GluRs) mediate synaptic excitation in networks of cultured rat hypothalamic neurons [18, 25]. Under voltage clamp the agonists quisqualate and AMPA induce current responses which consist of a maintained and/or transient component depending on the concentrations applied. The current-voltage relationship for both components is linear. The biphasic response patterns are due to receptor desensitization which is fast and does not require intracellular second messengers for its activation. Several GluR-subtype-encoding transcripts were found in these neurons using polymerase chain reaction (PCR) methods. While mRNAs encoding the GluR2 and 3 flip forms are expressed early, mRNAs encoding the GluR1, 2 and 3 flop forms and the GluR4 flip form appear only in cultures older than 3 weeks. By comparison to recombinant receptors, the properties of the native receptor can be accommodated by a heteromeric receptor containing GluR2 as one of the subunits.

Molecular cloning, functional expression, and pharmacological characterization of an N-methyl-D-aspartate receptor subunit from human brain.

A cDNA encoding a full-length N-methyl-D-aspartate (NMDA) receptor subunit 1, hNR1, was isolated from a human brain cDNA library. The hNR1 cDNA encodes an open reading frame of approximately 2.7 kb that shares high homology with the rat brain NMDA receptor subunit 1 and the mouse zeta 1 subunit. The hNR1 sequence, however, diverges from the rodent and murine homologs near the C terminus, suggesting that they represent alternatively spliced messages of the same gene. Oocytes injected with cRNA synthesized from the hNR1 cDNA express glutamate and NMDA-activated currents in the presence of glycine. Currents are blocked by the NMDA-receptor-specific antagonists 2-amino-5-phosphovaleric acid and 7-chlorokynurenate, and the open channel blockers MK-801 and phencyclidine, by Mg2+ ions in a voltage-dependent manner, and by Zn2+. Expressed hNR1 homomeric receptor channels exhibit the high Ca2+ permeability characteristic of neuronal NMDA receptors. Therefore, the cDNA clone hNR1 codes for a human brain NMDA receptor subunit cognate to the rodent and murine brain NR1 subunits.

Inhibition of high voltage-activated calcium currents by L-glutamate receptor-mediated calcium influx.

The modulation of high voltage-activated (HVA) Ca2+ currents by L-glutamate and its agonists was investigated in cultured rat hypothalamic neurons. L-Glutamate and agonists selective for NMDA or non-NMDA receptors reversibly inhibited HVA Ca2+ currents. The putative presynaptic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid and the selective metabotropic agonist trans-ACPD were ineffective. Inhibition was dependent on the presence of extracellular Ca2+ and blocked by internal perfusion of the cells with BAPTA. The calmodulin antagonists trifluoperazine and calmidazolium completely prevented the inhibition. Increases in the intracellular Ca2+ concentration due to Ca2+ influx through non-NMDA receptor channels were visualized using fura-2. These results indicate that not only NMDA but also non-NMDA receptor channels in these neurons are permeable for Ca2+ and that Ca2+ influx through these channels activates a calmodulin-dependent mechanism, which leads to HVA Ca2+ current inhibition.

N-methyl-D-aspartate induces regular firing patterns in the cat lateral habenula in vivo.

The present study was undertaken to elucidate the action of excitatory amino acids in the dorsal diencephalic pathway. Single neurons in the lateral habenula of halothane-anesthetized cats were recorded extracellularly, and excitatory amino acid receptor agonists and antagonists were applied by iontophoresis. Most neurons in the lateral habenula were spontaneously active. This spontaneous firing could be inhibited by kynurenic acid, a broad spectrum antagonist of excitatory amino acid receptors, but not by the selective N-methyl-D-aspartate receptor antagonist 2-amino-7-phosphono-heptanoic acid. Iontophoretic application of alpha-amino-3-hydroxy-5-methyl-5-isoxazolepropionate, quisqualate and kainate mostly elicited a non-burst, regular firing pattern which was sensitive to kynurenic acid. Surprisingly, 116 (96%) out of 121 neurons in the lateral habenula responded to iontophoretic application of N-methyl-D-aspartate with a regular non-burst firing pattern, in contrast to previously published observations from other brain regions where N-methyl-D-aspartate predominantly elicited phasic firing patterns. When cells were recorded with electrode assemblies where one iontophoretic barrel contained MgCl2 or MgSO4, only 10 (43%) out of 23 cells responded with regular firing upon application of N-methyl-D-aspartate, while 13 (57%) now displayed a phasic firing pattern. In these cells iontophoretically applied alpha-amino-3-hydroxy-5-methyl-5-isoxazolepropionate or quisqualate still evoked only regular firing. In a few cases, an initially regular N-methyl-D-aspartate-induced firing pattern could be changed to a phasic pattern following active ejection of Mg2+ ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential activation and survival of basal forebrain neurons following infusions of excitatory amino acids: studies with the immediate early gene c-fos.

These experiments investigated, by studying patterns of c-fos expression, the distribution of neurons activated or destroyed by the infusion into the basal forebrain of various excitatory amino acids at toxic and subtoxic doses. The results of experiment 1 showed that N-methyl-D-aspartic acid (NMDA), quisqualic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) differentially increased the expression of c-fos in magnocellular cholinergic nucleus basalis, dorsal and ventral pallidal neurons. AMPA was the most, and NMDA the least, effective in inducing FOS in nucleus basalis magnocellularis (nbM) neurons, with quisqualic acid having an intermediate effect, whereas the reverse was true in terms of the induction of FOS in pallidal neurons. In experiment 2, it was demonstrated that, in animals with ibotenic acid-induced lesions of the basal forebrain that were targetted on the nbM, virtually no pallidal neurons could be visualized that expressed FOS following AMPA-induced excitation of the dorsal and ventral striatum. By contrast, in animals with AMPA-induced lesions of the nbM, excitation of the striatum was followed by the expression of FOS in many dorsal and ventral pallidal neurons. Thus, infusions of AMPA into the basal forebrain appears preferentially to activate or destroy, depending on the concentration infused, cholinergic nbM neurons, whereas ibotenic acid or NMDA preferentially destroys or activates neurons of the dorsal and ventral pallidum. These results provide novel and complementary information regarding the organization of the basal forebrain and allow a clearer understanding of the different behavioural consequences of NMDA agonist-induced and non-NMDA agonist-induced excito-toxic lesions of this area.