Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex

Immunocytochemical characterization of AMPA-selective glutamate receptor subunits: laminar and compartmental distribution in macaque striate cortex

Subunit proteins that comprise functional AMPA receptors were localized by immunocytochemical methods in the adult macaque primary visual cortex (V1). GluR1, GluR2/3/4c, and GluR4 immunoreactivity consisted of rich plexuses of punctate profiles scattered throughout the neuropil, in radial arrays, and outlining the membrane of somata and proximal dendrites. Cytoplasmic immunoreactivity was limited. GluR2/3/4c immunostaining was more prominent along the somata surface and exhibited greater levels of cytoplasmic immunoreactivity than GluR1 and GluR4 immunostaining. The density of AMPA subunit immunoreactive elements also varied across layers and compartments of macaque V1. Immunoreactivity for GluR1, GluR2/3/4c, and GluR4 was densest in three bands that corresponded to layers IVA, IVC, and VI. Immunostaining for each subunit was also unevenly distributed within many of the layers. In layers II-III, patches of intense immunostaining coincided with cytochrome oxidase (CO)-rich blobs. In layer IVA, intense subunit staining formed a conspicuous honeycomb pattern. In layer IVC, subunit staining formed a radial lattice. GluR2/3/4c subunit immunostaining was also preferentially distributed within the CO-rich blobs of layers V-VI. These findings demonstrate that AMPA subunit immunoreactivity is densely concentrated in layers and compartments receiving direct geniculocortical innervation. This distribution, which differs from that of excitatory synapses, suggests that the density of AMPA receptors is unevenly distributed at synaptic and possibly extrasynaptic sites within macaque visual circuits.

Postnatal changes in the laminar and subcellular distribution of NMDA-R1 subunits in the cat visual cortex as revealed by immuno-electron microscopy

Although it is recognized that nearly all synapses in the cerebral cortex form postnatally, little is known about the emergence of molecules necessary to render these synapses functional. This study visualized the emergence of synaptically localized NMDA receptors by immuno-electron microscopic labeling of the receptor's obligatory subunit, NMDA-R1, in the developing cat visual cortex. Prior to eye-opening (postnatal day 2-10), NMDA-R1 immunoreactivity is already present within dendritic and growth cones, even though these profiles are devoid of synaptic specializations. This indicates that synthesis and incorporation of NMDA-R1 into plasma membranes are independent of form vision. During the next 2-3 weeks, i.e., preceding the onset of the critical period for ocular dominance plasticity (around the fourth week), NMDA-R1 immunoreactivity changes from a diffuse distribution within dendrites to a more discrete aggregation over postsynaptic densities of axo-spinous junctions. Such clustering of NMDA-R1 at synapses may be a prerequisite for stabilization and strengthening of synapses activated by visual stimulation during the critical period. Furthermore, only during the first several weeks, intensely NMDA-R1-immunoreactive neurons are present in the infragranular layers and the white matter. Enrichment of NMDA-R1 in the deep-layer neurons may reflect the neurons' supportive role in the development of cortical circuitry, serving as transient synaptic targets for geniculate and cortico-cortical afferents while these afferents 'wait' in the infragranular for their ultimate, life-long target neurons to become receptive in the upper layers.

Excitatory amino acids and monoamines in parahippocampal gyrus and frontal cortical pole of adults with Down syndrome

Aspartate (ASP), glutamate (GLU), noradrenaline (NA), dopamine (DA) and its acidic metabolites DOPAC and HVA, serotonin (5-HT) and its metabolite 5-HIAA were simultaneously investigated in post-mortem tissue samples from right parahippocampal gyrus (temporal cortex) and frontal cortical pole (frontal cortex) of adults with Down syndrome (DS), and of neurologically healthy controls by use of high performance liquid chromatography (HPLC). In parahippocampal gyrus, ASP, GLU, NA, DOPAC and 5-HT levels were significantly decreased in patients with DS, compared to levels found in control subjects (approximately 50%). No significant changes were observed in frontal pole. ASP and GLU levels were significantly lower in parahippocampal gyrus than in frontal pole of DS, a regional distribution that could not be observed in control subjects. In conclusion, the results of this study suggest that the temporal cortex would be more affected than the frontal cortex in adult patients with DS, a finding in line with reports showing a marked hypometabolism and extensive cell loss in temporal cortex of DS, and with those showing that parahippocampal gyrus abnormality may correlate with the extent of mental retardation affecting this type of patients.

Glutamate and aspartate immunoreactivity in the reciprocal projections between the anterior thalamic nuclei and the retrosplenial granular cortex in the rat

We have used retrograde and anterograde labelling with wheat germ agglutinin-horseradish peroxidase and immunohistochemistry with antibodies against glutamate and aspartate to examine the reciprocal connections between the anterior thalamic nuclei and the retrosplenial granular cortex in the rat, and to characterize those projection neurones that contain glutamate and/or aspartate. Injections into superficial layers of the retrosplenial granular cortex resulted in retrogradely labelled cell bodies in the anterodorsal, anteroventral, and to a lesser extent the anteromedial subnuclei. Approximately 70% of these cell bodies were also immunolabelled for glutamate or aspartate. Injections confined to deep layers (V-VI) resulted in the presence, in anterior thalamic neuropil, of anterogradely labelled fibre and terminal-like structures, many of which appeared to be immunolabelled for glutamate or aspartate. Injections into the anterior thalamic nuclei resulted in retrogradely labelled pyramidal cells in layers V-VI of the retrosplenial granular cortex. Most (90-95%) of these cells were immunolabelled for glutamate or aspartate. Thus, approximately 70% of thalamocortical and 90-95% of corticothalamic projection neurones in these circuits may use glutamate and/or aspartate as neurotransmitters.

Immunohistochemical studies of localization and co-localization of glutamate, aspartate and GABA in the anterior thalamic nuclei, retrosplenial granular cortex, thalamic reticular nucleus and mammillary nuclei of the rat

Localization and possible co-localization of glutamate, aspartate and GABA immunoreactivities was examined in the anterior thalamic nuclei, retrosplenial granular cortex, thalamic reticular nucleus and mammillary nuclei of the rat by double antigen immunohistochemistry using diaminobenzidine and benzidine dihydrochloride in one series and double immunofluorescence labelling with rhodamine and fluorescein in a second series of animals. In three of these regions, retrosplenial granular cortex, anterior thalamic nuclei, and mammillary nuclei, glutamate immunoreactivity was co-localized with aspartate immunoreactivity in a majority of the projection neurons (pyramidal neurons, predominantly in layers V and VI in retrosplenial granular cortex; rounded polygonal multipolar neurons throughout the rostrocaudal extent of the anterior thalamic and mammillary nuclei). None of the cells showing glutamate and/or aspartate immunoreactivity in these regions also displayed GABA immunoreactivity, which was present in non-pyramidal cells in the retrosplenial granular cortex (chiefly in layers I-III) and in small numbers of cells within the anterior thalamic nuclei. In the thalamic reticular nucleus, in contrast, most neurons were immunoreactive for GABA and in the majority of these neurons glutamate (and/or aspartate) immunoreactivity was co-localized with GABA.

Anatomical evidence for glutamate and/or aspartate as neurotransmitters in the geniculo-, claustro-, and cortico-cortical pathways to the cat striate cortex

Data obtained by using various experimental approaches suggest that in the mammalian brain, most neurons within the visual system projecting to the striate cortex employ excitatory amino acids as transmitters. In order to investigate further the neurotransmitter phenotype of the ipsilateral afferents to area 17 of the cat, we have injected D-[3H]-aspartate, a retrograde tracer which selectively reveals putative glutamatergic and/or aspartatergic pathways, into this area. Retrogradely labelled neurons were observed in the dorsal lateral geniculate nucleus, visual claustrum, cortical areas 18, 19, 21a, and in both posteromedial and posterolateral parts of the suprasylvian areas but not in other known thalamic afferents such as the lateral posterior-pulvinar complex and the intralaminar nuclei. The distribution and localization of the labelled cells in all these regions were similar to that observed by using the non-selective tracer horseradish peroxidase conjugated to wheat germ agglutinin, though the number of cells was higher with the latter. Our findings provide additional evidence for the presence of excitatory amino acids as neurotransmitters in the major afferents to the cat striate cortex.

Expression of N-methyl-D-aspartate glutamate receptor subunits in the prefrontal cortex of the rat

The laminar distribution and cellular levels of expression of mRNAs encoding N-methyl-D-aspartate receptor subunits (NMDAR1, NMDAR2A-D and the alternatively spliced isoforms of NMDAR1) were examined in prefrontal cortex of rat by in situ hybridization using film and emulsion autoradiography. Film autoradiograms demonstrated a distinctive laminar distribution of hybridization signals for each of the probes recognizing NMDAR1, NMDAR2A, and NMDAR2B messenger RNA; hybridization with probes for NMDAR2C and NMDAR2D resulted in scattered signals without laminar organization. Grain counting disclosed that neurons in layer V displayed the highest and neurons in layer IV the lowest absolute number of grains for all probes examined. Correction for cell size demonstrated statistically significant differences in cellular labelling density of up to 50% across neurons in different cortical layers. The cellular density profiles across cortical laminae differed between probes. Hybridization with a probe recognizing all isoforms of NMDAR1 resulted in significantly lower densities of cellular labelling in neurons of layer IV than of layers II/III, V and VI. Cellular labelling densities following hybridization with probes recognizing alternatively spliced segments of NMDAR1 were examined. Densities were low in neurons of the upper cortical layers II/III and IV using probes for the messenger RNA encoding the amino terminal insert, NMDAR11XX and the second carboxy terminal deletion, NMDAR1XX1; hybridization with a probe for the messenger RNA encoding the first carboxy terminal deletion, NMDAR1X1X, resulted in low cellular signal densities in neurons of layers IV and VIb. NMDAR2A messenger RNA expression was of relatively uniform intensity in neurons of layers II-V but significantly lower in neurons of the inner part of layer VI. NMDAR2B expression was most dense in layer II neurons. These data indicate that neurons in different cortical laminae express distinct N-methyl-D-aspartate receptor subunit messenger RNA phenotypes. In addition, the observed differences in density of N-methyl-D-aspartate receptor subunit messenger RNA expression suggest that cortical laminae differ in the relative contribution of N-methyl-D-aspartate receptors to their excitatory responses.

Metabolic and neurochemical plasticity of gamma-aminobutyric acid-immunoreactive neurons in the adult macaque striate cortex following monocular impulse blockade: quantitative electron microscopic analysis

The purpose of the present study was to examine the effects of retinal impulse blockade on gamma-aminobutyric acid (GABA)-immunoreactive (GABA-IR) neurons in cytochrome oxidase (CO)-rich puffs of the adult monkey striate cortex. Specifically, we wished to know if changes occurred in their CO activity, GABA immunoreactivity, and synaptic organization. A double-labeling technique, which combined CO histochemistry and postembedding GABA immunocytochemistry on the same ultrathin sections, was used to reveal simultaneously the distribution of the two markers. We quantitatively compared changes in GABA-IR neurons of deprived puffs (DPs) with respect to non-deprived puffs (NPs) 2 weeks after monocular tetrodotoxin treatment. We found that the proportion of darkly CO reactive mitochondria in GABA-IR neurons of DPs drastically decreased to about half of those in NPs. There was a greater reduction of CO levels in GABA-IR axon terminals than in their cell bodies and dendrites. In contrast, most non-GABA-IR neurons displayed no significant change in their CO levels. Morphologically, GABA-IR neurons and axon terminals in DPs showed a significant shrinkage in their mean size. GABA immunoreactivity, as indicated by the density of immunogold particles in GABA-IR neurons, declined in DPs, and a greater decrease was also found in axon terminals than in cell bodies or dendrites. Moreover, the numerical density of GABA-IR axon terminals and synapses in DPs was significantly reduced without changes in that of asymmetric and symmetric synapses. Thus, the present results support the following conclusions: 1) Oxidative metabolism and neurotransmitter expression in GABA-IR neurons are tightly regulated by neuronal activity in adult monkey striate cortex; 2) GABA-IR neurons are much more vulnerable to functional deprivation than non-GABA-IR ones, suggesting that these inhibitory neurons have stringent requirement for sustained excitatory input to maintain their heightened oxidative capacity; and 3) intracortical inhibition mediated by GABA transmission following afferent deprivation may be decreased in deprived puffs, because the oxidative capacity and transmitter level in GABAergic neurons, especially in their axon terminals, are dramatically reduced.

Differential glutamatergic innervation in cytochrome oxidase-rich and -poor regions of the macaque striate cortex: quantitative EM analysis of neurons and neuropil

One of the hallmarks of the primate striate cortex is the presence of cytochrome oxidase (CO)-rich puffs and CO-poor interpuffs in its supragranular layers. However, the neurochemical basis for their differences in metabolic activity and physiological properties is not well understood. The goals of the present study were to determine whether CO levels in postsynaptic neuronal compartments were correlated with the proportion of excitatory glutamate-immunoreactive (Glu-IR) synapses they received and if Glu-IR terminals and synapses in puffs differed from those in interpuffs. By combining CO histochemistry and postembedding Glu immunocytochemistry on the same ultrathin sections, the simultaneous distribution of the two markers in individual neuronal profiles was quantitatively analyzed. As a comparison, adjacent sections were identically processed for the double labeling of CO and GABA, an inhibitory neurotransmitter. In both puffs and interpuffs, most axon terminals forming asymmetric synapses (84%)--but not symmetric ones, which were GABA-IR--were intensely immunoreactive for Glu. GABA-IR neurons received mainly Glu-IR synapses on their cell bodies, and they had three times as many mitochondria darkly reactive for CO than Glu-rich neurons, which received only GABA-IR axosomatic synapses. In puffs, GABA-IR neurons received a significantly higher ratio of Glu-IR to GABA-IR axosomatic synapses and contained about twice as many darkly CO-reactive mitochondria than those in interpuffs. There were significantly more Glu-IR synapses and a higher ratio of Glu- to GABA-IR synapses in the neuropil of puffs than of interpuffs. Moreover, Glu-IR axon terminals in puffs contained approximately three times more darkly CO-reactive mitochondria than those in interpuffs, suggesting that the former may be synaptically more active. Thus, the present results are consistent with our hypothesis that the levels of oxidative metabolism in postsynaptic neurons and neuropil are positively correlated with the proportion of excitatory synapses they receive. Our findings also suggest that excitatory synaptic activity may be more prominent in puffs than in interpuffs, and that the neurochemical and synaptic differences may constitute one of the bases for physiological and functional diversities between the two regions.

GluR1-immunopositive interneurons in rat neocortex

Recent in vitro studies suggest that inhibitory interneurons in cortex may express the GluR1 glutamate receptor subunit in the absence of GluR2, leading to calcium-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) channels. We performed a study of rat somatic sensory cortex to confirm and extend these observations, using quantitative immunocytochemistry for multiple antigens. A morphologically distinct subpopulation of nonpyramidal neurons in neocortex was intensely immunoreactive for GluR1. Electron microscopic analysis of these cells revealed somatic staining for GluR1, mainly in the rough endoplasmic reticulum. Dendritic staining was concentrated at the synaptic active zone and in the adjacent subsynaptic cytoplasm. Double immunostaining revealed that the large majority of intensely GluR1-positive cells contained gamma-aminobutyric acid or its synthetic enzyme, glutamic acid decarboxylase, but little or no GluR2. Thus, AMPA receptors on a subpopulation of inhibitory interneurons in cortex are likely to be calcium permeable. This calcium permeability is likely to influence functional properties of these neurons; it may underlie the high levels of calcium-binding proteins they contain; and may render them liable to excitotoxic injury

Localization of AMPA-selective glutamate receptor subunits in the adult cat visual cortex

We have studied the presence and distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptor subunits (GluR1, 2, 3, and 4) in the adult cat visual cortical areas 17, 18, 19, and the lateral suprasylvian areas (LSA). Reverse transcription-polymerase chain reaction (RT-PCR) amplification indicated that the genes encoding GluR1, 2, 3, and 4 are expressed in these areas and Western blot analysis revealed that the size of the corresponding peptides is similar to those described in the rat brain. In situ hybridization (ISH) using digoxigenin-labeled riboprobes showed that mRNAs coding for GluR1 and GluR3 were located in cells in all layers of the areas examined and also in the underlying white matter. GluR1 mRNA was relatively abundant throughout layers II-VI while GluR3 mRNA revealed a more laminated pattern of expression, preferentially labeling cells in layers II, III, V, and VI. The distribution of AMPA-selective receptor subunit peptides was studied by immunohistochemistry using subunit specific antibodies and found to be consistent with ISH results. In addition, we observed that most of the cells strongly labeled by the anti-GluR1 antibody were non-pyramidal neurons and that intense GluR2/3 immunoreactivity was seen preferentially in pyramidal neurons. Interestingly, double-labeling experiments indicated that neurons expressing gamma-aminobutyric acid (GABA) as well as the GluR1 subunit were particularly abundant in deeper layers. The GluR4 peptide was predominantly found in a relatively low number of layer III and layer V neurons with either pyramidal or non-pyramidal morphology. Finally, the distribution of neurons expressing the various receptor subunits was similar in all the visual cortical areas studied. These findings indicate a high expression of GluR1-3 subunits in the cat visual cortex and that GluR1 and GluR2/3 subunits are particularly abundant in non-pyramidal and pyramidal neurons, respectively. In addition, the results described here provide a reference for future studies dealing with the effect of visual deprivation on the expression of this receptor type.

Differential effect of whisker trimming on excitatory and inhibitory transmission in primary somatosensory cortex of the adult rat in vivo

The effects of sensory deprivation on excitatory and inhibitory activity in the primary somatosensory cortex were studied in the adult rat. Excitatory and inhibitory transmission generated by whisker stimulation, and neuronal responsiveness to iontophoretically applied excitatory amino acids were recorded. Whisker input deprivation, through whisker trimming for a median of 24 days, resulted in a significant decrease in excitatory transmission to surround whisker stimulation. In contrast, the response magnitude to principal whisker stimulation remained unchanged. However, the response latencies to principal whisker and surround whisker stimulation were significantly reduced, which led to altered temporal response distributions in deprived cells. Neurons deprived of sensory input were significantly less responsive to glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate and kainate. Following deprivation, no change was observed in cortical inhibitory transmission measured 30-200 ms post-stimulus. These results show that excitatory transmission (including excitatory amino acid receptor function) is altered by adult whisker deprivation.

Lactic acid and amino acid fluctuations measured using microdialysis reflect physiological derangements in head injury

We examined the extracellular neurochemical milieu in 34 head injured patients using microdialysis while simultaneously monitoring intracranial pressure, cerebral perfusion pressure, and jugular venous oxygen saturation. Derangements of anaerobic metabolism reflected by increased lactate and lactate/pyruvate ratios, and release of amino acids were seen at the same time as physiological deterioration in the majority of instances. Clinical microdialysis may provide insights into the neurochemistry of head injury, and such information may lead to new methods of monitoring and treating head injured patients.

Augmentation of prefrontal cortical monoaminergic activity inhibits dopamine release in the caudate nucleus: an in vivo neurochemical assessment in the rhesus monkey

Prefrontal cortical modulation of caudate nucleus dopamine release was investigated in the rhesus monkey using the in vivo microdialysis technique. Reliable and stable basal caudate nucleus dopamine levels were quickly attained within hours following insertion of the dialysis probes. High-potassium (60 mM) or tetrodotoxin (10 microM) infusions significantly altered caudate nucleus dopamine levels in the dialysate indicating that measured dopamine levels reflected impulse-dependent release from the presynaptic pool. Pharmacological augmentation of monoaminergic transmission in the sulcus principalis region of the prefrontal cortex resulted in significant alterations in caudate nucleus dopamine levels. Increase of monoaminergic activity by infusion of either D-amphetamine (100 microM) or cocaine hydrochloride (100 microM) resulted in a gradual and prolonged decrease in caudate nucleus dopamine levels. Similar decreases were noticed in caudate nucleus dopamine metabolite levels. The present results indicate that in non-human primates modulation of dorsolateral prefrontal cortical monoaminergic transmission results in alterations in dopamine levels in subcortical structures. This observation may have clinical implications for therapeutic management of certain neuropsychiatric disorders, particularly schizophrenia.

A light and electron microscopic study of calbindin D-28k immunoreactive double bouquet cells in the human temporal cortex

Correlative light and electron microscopic methods were used to examine the morphology, distribution and synaptic connections of double bouquet cells immunoreactive for the calcium-binding protein calbindin D-28k in the human temporal neocortex. Double bouquet cells form symmetric synapses with small dendritic shafts and dendritic spines. The distribution and proportion of synapses found in the present work are very similar to those found in previous studies on the synaptic connectivity of double bouquet cells in the monkey cerebral cortex. Thus, double bouquet cells are probably involved in similar synaptic circuits in monkeys and humans.

Backpropagation of action potentials generated at ectopic axonal loci: hypothesis that axon terminals integrate local environmental signals

This review deals with the fascinating complexity of presynaptic axon terminals that are characterized by a high degree of functional distinctiveness. In vertebrate and invertebrate neurons, all-or-none APs can take off not only from the axon hillock, but also from ectopic axonal loci including terminals. Invertebrate neurons display EAPs, for instance alternating with somatic APs, during survival functions. In vertebrate, EAPs have been recorded in the peripheral and central nervous systems in time relationship with physiological or pathological neuronal activities. In motor or sensory axon, EAP generation may be the cause of motor dysfunctioning or sensory perceptions and pain respectively. Locomotion is associated with rhythmic depolarizations of the presynaptic axonal membrane of primary afferents, which are ridden by robust EAP bursts. In central axons lying within an epileptic tissue EAP discharges, coinciding with paroxysmal ECoG waves, get longer as somatic discharges get shorter during seizure progression. Once invaded by an orthodromic burst, an ectopic axonal locus can display an EAP after discharge. Such loci can also fire during hyperpolarization or the postinhibitory excitatory period of the parent somata, but not during their tonic excitation. Neurons are thus endowed with electrophysiological intrinsic properties making possible the alternate discharges of somatic APs and EAPs. In invertebrate and vertebrate neurons, ectopic axonal loci fire while the parent somata stop firing, further suggesting that axon terminal networks are unique and individual functional entities. The functional importance of EAPs in the nervous systems is, however, not yet well understood. Ectopically generated axonal APs propagate backwards and forwards along the axon, thus acting as a retrograde and anterograde signal. In invertebrate neurons, somatically and ectopically generated APs cannot have the same effect on the postsynaptic membrane. As suggested by studies related to the dorsal root reflex, EAPs may not only be implied in the presynaptic modulation of transmitter release but also contribute significantly during their backpropagation to a powerful control (collision process) of incoming volleys. From experimental data related to epileptiform activities it is proposed that EAPs, once orthodromically conducted, might potentiate synapses, initiate, spread or maintain epileptic cellular processes. For instance, paroxysmal discharges of EAPs would exert, like a booster-driver, a powerful synchronizing synaptic drive upon a large number of excitatory and inhibitory postsynaptic neurons. We have proposed that, once backpropagated, EAPs are likewise capable of initiating (and anticipating) threshold and low-threshold somatodendritic depolarizations. Interestingly, an antidromic EAP can modulate the excitability of the parent soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical study of two phosphoinositide-linked metabotropic glutamate receptors (mGluR1 alpha and mGluR5) in the cat visual cortex before, during, and after the peak of the critical period for eye-specific connections

The distribution of two phosphoinositide-linked metabotropic glutamate receptors (mGluR1 alpha and mGluR5) was studied immunohistochemically in area 17 before, during and after the peak of use-dependent modification of eye-specific connections. In the adult, mGluR1 alpha immunoreactivity is high in all layers except layer IV, where mGluR5 immunoreactivity is concentrated. This difference in distribution indicates different functions for these two receptor subtypes. The laminar pattern of mGluR1 alpha immunoreactivity is similar in all three ages, but the overall labeling intensity decreases after the peak (6 weeks of age) of the critical period. The laminar pattern of mGluR5 immunoreactivity changes with age. It is expressed in most layers at 2 days of age and is found mainly in layer IV in the adult. This laminar distribution and developmental pattern match the distribution and the development of the geniculocortical terminals. The change in mGluR1 alpha labeling intensity and mGluR5 laminar distribution over time is consistent with both of these mGluRs being involved in sensory-dependent plasticity for eye-specific connections in the visual cortex.

Immunocytochemical localization of non-NMDA ionotropic excitatory amino acid receptor subunits in human neocortex

The distribution of immunocytochemically localized subunits that comprise ionotropic non-NMDA excitatory amino acid receptors was examined in human frontal, parietal and temporal association neocortex. AMPA/kainate receptor subunits were identified using a monoclonal antibody (3A11) that recognizes an epitope common to GluR2 and GluR4 [GluR2(4)], as well as polyclonal antisera that recognize GluR2 and GluR3 (GluR2/3). Kainate receptor subunits were identified using a monoclonal antibody (4F5) that recognizes an epitope common to GluR5/6/7. For all three antibodies used, labeling was observed in a large number of neurons throughout the human association neocortex with the highest immunoreactivity present in pyramidal-like neurons, a cellular pattern largely similar to that observed in the monkey neocortex. These data demonstrate the cellular localization patterns for some non-NMDA receptor subunits in human neocortex, details upon which further studies on the roles of these subunits in human neurological diseases can be based.

Inhibitory effects of antidepressants on NMDA-induced currents in Xenopus oocytes injected with rat brain RNA

Although it has been reported that desipramine affects ion-channel activity of NMDA receptor/ion-channel complexes, the binding sites remain unclear. To identify the binding site, influences of desipramine on NMDA-induced current were examined in Xenopus oocytes injected with rat brain RNA and compared with those of blockers, MK-801, Zn2+ and Mg2+. Application of 100 microM desipramine irreversibly inhibited NMDA-induced inward current as well as 1 microM MK-801. Mg2+ and Zn2+ showed a reversible inhibition. Pretreatment with Mg2+ or Zn2+ abolished the irreversible inhibition of desipramine. In contrast, the irreversible inhibition of desipramine was still observed after application of Mg2+ and Zn2+. These results suggest that Mg2+/Zn2+ and desipramine bind on different sites from each other and affect the cation permeability via different mechanisms. Regarding inhibitory effects of other antidepressant drugs, imipramine and setiptiline were found to markedly inhibit NMDA current, while maprotiline, amitriptyline and lofepramine slightly inhibited the current. Mianserin, a potent antagonist of 5-HT1c receptors, however, had no influence.

Cellular localization and laminar distribution of AMPA glutamate receptor subunits mRNAs and proteins in the rat cerebral cortex

The cellular and laminar distributions of the alpha-amino-3-hydroxy-5- methyl-4-isoxazole propionate (AMPA) receptor subunits GluR1-4 have been investigated in the cerebral cortex of adult rats by in situ hybridization with 35S-labeled cRNA probes and by immunocytochemistry with subunit-specific antibodies. In sections incubated with the GluR1-4 antisense probes, specific hybridization signal was observed in many but not all cortical cells. Experiments with in situ hybridization and antibodies to glial fibrillary acidic protein (GFAP) showed that percentages of GFAP-immunoreactive cells labeled by the GluR1-4 probes were 20%, 9.4%, 8.2%, and 57.3%, respectively. A semiquantitative evaluation revealed that about 56% of cortical neurons contained the GluR1 subunit, 80% the GluR2, 63% the GluR3, and 44% the GluR4. The number of grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and labeled neurons were divided into four groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), and D (> 30 grains). The number of neurons belonging to each of these groups was then evaluated for their occurrence in each cortical layer. Immunocytochemistry with subunit-specific antibodies showed that 1) GluR1-immunoreactive neurons were mostly layers V and VI nonpyramidal neurons; 2) GluR2/3-immunoreactive neurons were more numerous in layers II-III and V-VI, and most of them were pyramidal; and 3) GluR4-positive cells were the least numerous, and they were either neurons (pyramidal and nonpyramidal) or astrocytes. These observations indicate that cortical neurons exhibit a remarkable degree of heterogeneity with regard to both the composition and the number of AMPA receptors and suggest that this diversity might be correlated with the functional attributes of neurons receiving glutamatergic afferents and with the physiological features of corticifugal neurons.

Cellular and synaptic localization of NMDA and non-NMDA receptor subunits in neocortex: organizational features related to cortical circuitry, function and disease

Excitatory amino acid (EAA) receptors are an important component of neocortical circuitry as a result of their role as the principal mediators of excitatory synaptic activity, as well as their involvement in use-dependent modifications of synaptic efficacy, excitoxicity and cell death. The diversity in the effects generated by EAA-receptor activation can be attributed to multiple receptor subtypes, each of which is composed of multimeric assemblies of functionally distinct receptor subunits. The use of subunit-specific antibodies and molecular probes now makes it feasible to localize individual receptor subunits anatomically with a high level of cellular and synaptic resolution. Initial studies of the distribution of immunocytochemically localized EAA-receptor subunits suggest that particular subunit combinations exhibit a differential cellular, laminar and regional distribution in the neocortex. While such patterns might indicate that the functional heterogeneity of EAA-receptor-linked circuits, and the cell types in which they operate, are based partly on differential subunit parcellation, a definitive integration of these anatomical details into current schemes of cortical circuitry and organization awaits many further studies. Ideally, such studies should link a high level of molecular precision regarding subunit localization with synaptic details of identified connections and neurochemical features of neocortical cells.

Synaptic input-induced increase in intraneuronal Ca2+ in the medial vestibular nucleus of young rats

In the medial vestibular nucleus (MVN), an input-dependent influx of Ca2+ into neurons through N-methyl-D-aspartate (NMDA) receptor-linked channels and/or voltage-dependent Ca2+ channels is suggested as underlying certain mechanisms of plasticity of the vestibular system. To see whether there is an increase in intracellular Ca2+ induced by afferent synaptic inputs to MVN neurons, we measured changes in [Ca2+]i with microfluorometry using a Ca2+ indicator, rhod-2, following electrical stimulation of ipsilateral vestibular afferents and commissural fibers in slice preparations of the brainstem of young rats (4-7 days postnatal). Single shock stimulation of ipsilateral afferents or commissural fibers induced an increase in fluorescence intensity lasting for several seconds. An application of 2-amino-5-phosphonovaleric acid (APV), an antagonist of NMDA receptors, almost completely blocked this stimulus-induced rise in fluorescence intensity. Nifedipine, an L-type Ca2+ channel blocker, also reduced the stimulus-induced rise in fluorescence intensity to 44-51% of the control value. These results suggest that synaptic inputs from the afferent and commissural pathways induce an influx of Ca2+ into MVN neurons due, at least in part, to the activation of NMDA receptors and the subsequent operation of L-type Ca2+ channels in young rats.

NMDA receptors in layers II and III of rat cerebral cortex

Glutamate receptors are found in all layers of the cerebral cortex, but NMDA receptors are concentrated in layers II and III in the adult. We investigated the location of these receptors, and their contribution to the responses of cells in layers V and VI, by iontophoresing NMDA at various distances from the cell body along the apical dendrite of the cells, first in artificial CSF, then in TTX to abolish action potentials. Comparison of responses at various distances along the apical dendrite showed that the response generally increases as distance from the cell body decreases. Comparison of responses in layers II and III, before and after TTX, showed that TTX reduced the response considerably. We conclude first that NMDA receptors in layers II and III are located primarily on cells in layers II and III, rather than on the apical dendrites of cells in layers V and VI, and second that the contribution of NMDA receptors to the response of cells in layers V and VI comes primarily from receptors close to the cell body.

N-methyl-D-aspartate receptors contribute to afferent synaptic transmission in the medial vestibular nucleus of young rats

In the medial vestibular nucleus (MVN), the non-N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors has been reported as operating at synapses between ipsilateral vestibular afferents and neurons. In the present study, we addressed the question of whether or not NMDA receptors contribute to afferent synaptic transmission in the MVN and if so, to what degree. Using nystatin-perforated or conventional whole-cell patch clamp methods in brainstem slices of young rats (postnatal day 4-6), we found that NMDA receptors contribute to a substantial extent to afferent synaptic transmission in the MVN of young rats.

Distinct laminar differences in the distribution of excitatory amino acid receptors in adult ferret primary visual cortex

In order to explore the relative contributions of the different ionotropic excitatory amino acid receptor subtypes to signalling in primary visual cortex, we have mapped their distributions in area 17 of adult ferret cerebral cortex by quantitative in vitro autoradiography. D,L-alpha-amino-3-hydroxy-5-methoxy-4-isoxazole propionate (AMPA) and kainate receptors, gating fast, Na(+)-permeable channels, were localized with [3H]dizocilpine maleate ([3H]MK-801). All three radioligands bound to single sites, with KDs of 414 nM [3H]AMPA and [3H]kainate, respectively. Slower-acting N-methyl-D-aspartate receptors, which gate the influx of Ca2+ as well as Na+, were localized with ([3H]AMPA), 78 nM ([3H]kainate) and 16 nM ([3H]MK-801), and each receptor subtype displayed a different laminar distribution pattern within area 17. AMPA receptors were concentrated in superficial layers, with intermediate densities in deep layers and lowest levels in layer IV. Kainate receptor levels were high in layers V and VI and low in all other layers. N-methyl-D-aspartate receptors were more homogeneously distributed than AMPA or kainate receptors, but were expressed at highest levels in layers I and IV and lowest levels in layers V and VI. The binding site densities found in the layers containing most receptors were Bmax = 2812 fmol/mg for [3H]AMPA, Bmax = 626 fmol/mg for [3H]MK-801 maleate and Bmax = 278 fmol/mg for [3H]kainate. Thus, while AMPA receptors were predominant and kainate receptors least abundant in all cortical layers, a complementary relative distribution of excitatory amino acid receptors was apparent, with AMPA receptor density highest in superficial layers, kainate receptor density highest in inferior layers and N-methyl-D-aspartate receptor density highest in the middle granular layer, as well as in layer I. The results indicate that although AMPA receptors are principally involved in excitatory signalling in adult ferret primary visual cortex, kainate receptors in layers V and VI and N-methyl-D-aspartate receptors in layers I and IV may have particularly important roles in mediating synaptic transmission.

Neuropeptide Y immunoreactive axons in the corpus callosum of the cat during postnatal development

Many immunocytochemical studies have identified different types of neurotransmitters localized in the corpus callosum (CC) axons in the adult mammal. Few studies have looked at the development of different neurochemically identified CC systems. Previous studies on the development of cat CC axons have indicated that a large number of transitory CC axons project to the cortex during early postnatal development. The present study focuses on the development of one neurochemically identified group of CC axons in the cat, labeled with an antibody against neuropeptide Y (NPY), to determine if this group participates in transitory CC axonal growth. Cats at specified ages from birth to adulthood were studied with a routine method of immunocytochemistry for antiserum to NPY. NPY-immunoreactive (ir) CC axons were detected at all stages examined, from newborn to adult; the peak density occurred during postnatal weeks (PNW) 3-4. During PNW 1-2, the density of NPY-ir CC axons increased gradually; some NPY-ir axons at this age had growth cones located within the CC bundle between the cerebral hemispheres. The density of the NPY-ir CC axons decreased gradually during PNW 5-7, and from PNW 8 to maturity only a few NPY-ir CC axons were observed. These results indicate that at least two types of NPY-ir CC axons (i.e., transitory and permanent) exist during development, and that most of these axons are eliminated or only express NPY-ir for a short period during development. The results also indicate that neurochemical subsets of CC axons participate in the extensive transitory growth observed by means of the membrane tracer DiI but they may follow unique developmental timetables.

Effects of kainic acid, quisqualic acid, and their antagonist, pCB-PzDA, on rat electrocorticograms and monoamine metabolite levels in rat striatum

The action of kainic acid (KA), quisqualic acid (QA), and 1-(4-chlorobenzoyl)-piperazine-2,3-dicarboxylic acid (pCB-PzDA) was investigated in the central nervous system of male Sprague Dawley rats. Intracerebroventricularly injected KA and QA (100 nmol) induced spike discharges, and pCB-PzDA (100 nmol) suppressed electrocorticograms for one hour. pCB-PzDA enhanced the KA-induced spike discharges and inhibited those induced by QA. 2,3-Di-hydroxyphenylacetic acid(DOPAC) and homovanillic acid (HVA) levels were increased transiently by 10 nmol and continuously by 100 nmol of KA. KA dose-dependently increased 5-hydroxyindoleacetic acid (5-HIAA) levels 2 hours after administration. While 10 nmol of QA slightly increased the HVA level, 100 nmol of QA significantly increased DOPAC, HVA, and 5-HIAA levels. DOPAC and HVA levels were increased by 100 nmol of pCB-PzDA, although this agent inhibited KA-induced increases in DOPAC, HVA, and 5-HIAA levels. On the other hand, while pCB-PzDA first inhibited QA-induced increases in DOPAC, HVA and 5-HIAA levels for one hour, DOPAC and HVA levels thereafter increased additively. These findings suggest that pCB-PzDA may act not only as a NMDA antagonist, but that it may also act directly on dopaminergic neurons.

Modulation of cellular excitability in neocortex: muscarinic receptor and second messenger-mediated actions of acetylcholine

Muscarinic-type acetylcholine (ACh) receptor are involved in a variety of cortical functions. ACh "activates" neocortex; simultaneously modifying spontaneous subthreshold activity, intrinsic neuronal oscillations and spike discharge modes, and responsiveness to fast (putative glutamatergic) synaptic inputs. However, beyond the general involvement of muscarinic receptors, a mechanistic understanding of integrated cholinergic actions, and interactions with non-cholinergic transmission, is lacking. We have addressed this problem using intracellular recordings from the in vitro auditory neocortex. First, we investigated cholinergic modification of responses to the excitatory amino acid glutamate. ACh, or the muscarinic agonist methacholine, produced a lasting enhancement of glutamate-mediated membrane depolarizations. Muscarinic receptors of the M1 and/or M3 subtype, rather than M2 or nicotinic receptors, mediated this enhancement. Subsequently, we investigated whether second messenger systems contribute to observed muscarinic actions. Activation of protein kinase C with phorbol 12,13-dibutyrate (4 beta-PDBu), enhanced neuronal responses to glutamate. The effect of 4 beta-PDBu was attenuated by the kinase antagonist H7. Finally, we attempted to identify postsynaptic actions of endogenous ACh. Tetanic stimulation of cholinergic afferents elicited voltage-dependent effects, including reduced spike frequency adaptation and reduced slow afterhyperpolarization (sAHP) elicited by transmembrane depolarizing stimuli. These effects were mimicked by methacholine, enhanced by eserine, and antagonized by muscarinic receptor antagonists. These data suggest that cholinergic modulation in neocortex likely involves the integrated actions of diverse mechanisms, primarily gated by muscarinic receptors, and at least partly involving second messenger systems.

Columnar activity regulates astrocytic beta-adrenergic receptor-like immunoreactivity in V1 of adult monkeys

Recent results indicate that astrocytic beta-adrenergic receptors (beta AR) participate in noradrenergic modulation of synaptic activity. In this study, we sought to examine whether neural activity can, in turn, regulate astrocytic beta AR. To address this question, an antiserum that recognizes beta-adrenergic receptors (beta AR) specifically in astrocytes was used to assess the distribution of the receptors across ocular dominance columns in V1 of two monocular and four visually intact adult monkeys. Cytochrome oxidase histochemistry (CO) was used to identify the position of the cortical laminae and of the ocular dominance columns receiving visual inputs from the intact and enucleated eyes. This stain revealed the expected pattern within V1 of monocular monkeys--i.e. darker and lighter bands of equal widths (ca. 500 microns) spanning laminae 4-6, each associated with larger and smaller blobs, respectively, in lamina 2/3. Alignment of CO sections with adjacent sections stained for astrocytic beta AR by the immunoperoxidase method revealed intense beta AR-like immunoreactivity (beta AR-li) in the superficial laminae, a slightly weaker staining in the infragranular laminae and weakest staining in lamina 4C. Within lamina 4C, a prominent striped pattern was evident. The darker bands of the stripe closely matched widths and positions of the lighter CO columns associated with the enucleated eye. On the other hand, immunocytochemical staining for the astrocytic intermediate filament protein, GFAP, within V1 of monocular monkeys revealed no inter-columnar difference in the density of astrocytic cell bodies or processes. Nissl stain also revealed no overt inter-columnar differences in cell density.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid receptors participate in synaptic transmission of visual responses in the superficial layers of the cat superior colliculus

The contribution of NMDA and non-NMDA receptors to visual synaptic transmission in the superficial layers of the superior colliculus of the cat has been studied using extracellular recording and iontophoretic techniques. Neuronal responses to natural visual stimulation and the ejection of NMDA and AMPA were observed in the absence and presence of the antagonists CNQX, CPP and AP5. CNQX routinely reduced the responses to visual stimulation at ejection currents which selectively blocked the responses to AMPA but not those to NMDA. Agonist selective ejection currents of CPP and AP5 also reduced visual responses of most SC neurons, but there was a substantial majority whose visual responses were resistant to these antagonists. Neurons with CPP/AP5 resistant visual responses were more commonly found 750-1000 microns from the dorsal surface of the SC. The data indicate that, while non-NMDA receptors are heavily involved in visual synaptic transmission in the superficial SC, the involvement of NMDA receptors varies with recording depth.

Localization of non-N-methyl-D-aspartate glutamate receptors in normal and Alzheimer hippocampal formation

The hippocampi and adjacent temporal cortices of 24 human brains were examined with antibodies to the GluR1, GluR2/3, and GluR4 subunits of the D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-preferring glutamate receptor. GluR1 immunoreactivity was most dense in the dentate gyrus, with lower densities in other hippocampal and cortical regions. GluR2/3 immunoreactivity was the most intense of the three antibodies, with high levels throughout most hippocampal subfields, where it was localized to cell bodies, proximal axons, and dendrites. GluR4 immunoreactivity was very sparse in all regions. In Alzheimer's disease brains, the general pattern of staining was similar to that seen in control brains. GluR1 and GluR4 immunoreactivity was seen in some but not all neuritic plaques. All three antibodies recognized some neurons undergoing neurofibrillary degeneration.

Glutamate in terminals of thalamocortical fibers in rat somatic sensory cortex

Thalamocortical terminals in layer IV of the somatic sensory cortex (SI) were identified by anterograde transport from ventrobasal thalamus. They were large, contained loosely packed round clear vesicles and abundant mitochondria, and made asymmetric contacts mainly with dendritic spines. Post-embedding immunocytochemistry revealed these terminals to be enriched in glutamate. Levels of glutamate in thalamocortical terminals, and in dendritic spines postsynaptic to them, were significantly higher than in nearby dendrites, astroglia, or in GABAergic terminals (identified by double immunostaining). These results support glutamate as neurotransmitter in thalamocortical fibers, and suggest that dendritic spines may take up glutamate released by these terminals.

NMDA, AMPA, and benzodiazepine binding site changes in Alzheimer's disease visual cortex

Quantitative receptor autoradiography was used to measure the laminar distribution of [3H]glycine and [3H]glutamate binding to the N-methyl-D-aspartate (NMDA) receptor complex, [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) binding to the AMPA receptor, and [3H]flunitrazepam binding to the benzodiazepine (BDZ) receptor in three areas of visual cortex in control and Alzheimer's disease (AD) postmortem human brains (primary or striate visual cortex, visual association cortex, and higher-order visual association cortex, corresponding to Brodmann Areas 17, 18, and 21, respectively). In Area 17, binding to the NMDA, AMPA, and BDZ receptors was not significantly altered in the AD brains (except in layer VI for [3H]glycine and layer III for [3H]AMPA, where binding was reduced in the AD brains). Ligand binding to the two EAA receptors in Area 18 was, however, significantly reduced in the AD brains (layers I through III for [3H]glycine and layers III through VI for [3H]AMPA). In Area 21, binding to both the NMDA and BDZ receptors but not to the AMPA receptor, was significantly reduced in almost all laminae of the AD brains (layers I through VI for [3H]glycine and layers I through V for [3H]flunitrazepam). This hierarchical pattern of laminar binding loss with increasing complexity of association visual cortices is consistent with the increasing numbers of neurofibrillary tangles found in those areas, implicating NMDA and BDZ receptor bearing cells in AD neuropathology. AMPA receptor losses do not parallel the pathology, suggesting that AMPA receptors are not directly correlated with the pathology.

Nitric oxide increases interstitial excitatory amino acid release in the rat dorsomedial medulla oblongata

In vivo microdialysis was employed to measure release of the neuroactive amino acids L-glutamate (Glu) and L-aspartate (Asp) in the dorsomedial medulla oblongata of the anaesthetised rat. Basal levels of endogenous extracellular Glu and Asp were increased over 5-fold and 3-fold, respectively, following perfusion with a depolarising stimulus of KCl. Intracerebral administration of the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine (SNAP, 30 microM) caused a 5-fold increase in extracellular Asp and a 2-fold increase in extracellular Glu, which was blocked by Methylene blue. These data suggest that NO, acting through guanylate cyclase, can affect excitatory amino acid neurotransmission in the dorsomedial medulla oblongata.

Intermediary metabolism disturbance in AD/SDAT and its relation to molecular events

1. Early-onset dementia of Alzheimer type (EODAT; AD) and late-onset dementia of Alzheimer type (LODAT; SDAT) are heterogenous in origin. 2. A common superordinate pathobiochemical principle in the etiopathogenesis of both types of dementia is neuronal energy failure with subsequent abnormalities in cellular Ca2+ homeostasis and glucose-related amino acid metabolism. 3. These metabolic abnormalities are assumed to occur first at axodendritic terminals of the acetylcholinergic-glutamatergic circuit and to cause morphological damage at synaptic sites. 4. Metabolic stress and structural damage at synaptic sites may induce enhanced formation of APP and its cleavage product amyloid. 5. Energy-metabolism related abnormalities along with functional and structural changes at synaptic sites of the acetylcholinergic-glutamatergic circuit may precede the formation of amyloid in DAT brain.

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

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

Fast and slow components of unitary EPSCs on stellate cells elicited by focal stimulation in slices of rat visual cortex

1. Voltage and current recordings were made from visually identified non-pyramidal neurones in slices of layer IV of rat primary visual cortex using the whole-cell configuration of the patch clamp technique. These neurones are characterized by a high input resistance (0.5-2 G omega) and a non-adaptive behaviour of action potential frequency following depolarizing current injection, which suggests that they are stellate cells. 2. Excitatory postsynaptic currents (EPSCs) were recorded from these neurones during focal stimulation of neighbouring cells by a second patch pipette, the tip of which was placed on the soma of the stimulated cell. The response amplitude as a function of stimulus strength showed a sharp increase at a critical stimulus strength suggesting that stimulus-evoked currents represent unitary EPSCs. 3. In most cases the latencies of stimulus-evoked EPSCs were unimodally distributed with means in the range of 2.1-3.6 ms. In some experiments two peaks were seen in the distribution of latencies. The EPSC rise times, measured as the time from 20 to 80% peak amplitude, fell into a distribution ranging from 0.1 to 0.8 ms with a peak at 0.2 ms. The EPSC decay time course at -70 mV membrane potential was fitted by a single exponential with a time constant of 2.39 +/- 0.99 ms (mean +/- S.D.). The rise and decay times were independent of EPSC peak amplitudes. 4. The peak amplitude of successive unitary EPSCs, elicited by a constant stimulus, fluctuated at random. At a holding potential of -70 mV the peak amplitudes varied between 5 and 90 pA. In two out of ten cells the histogram of peak amplitudes could be well fitted by the sum of several equidistant Gaussians with a peak distance of around 10 pA. This suggests that the quantal conductance change underlying the peak current fluctuations is of the order of 100 pS. 5. At membrane potentials more positive than -70 mV the decay of stimulus-evoked EPSCs showed two components with very different time courses. In standard extracellular solution the current-voltage (I-V) relation for the fast component was almost linear whereas the slow component showed a J-shaped I-V relation with a region of negative slope conductance between -30 and -70 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo modulation of excitatory amino acid receptors: microdialysis studies on N-methyl-d-aspartate-evoked striatal dopamine release and effects of antagonists

Striatal dopamine (DA) release was measured following intrastriatal (i.s.) administration of N-methyl-D-aspartate (NMDA) to unanesthetized, freely-moving rats. One hour after insertion of a removable microdialysis probe and perfusion with normal Ringer's solution, a modified Ringer's solution containing 100 mM potassium (high-K+ Ringer's) was used to standardize the preparation. DA release following i.s. administration of NMDA (12.5 mM in normal Ringer's) was dose-dependent. When NMDA (12.5 mM) was administered in high-K+ Ringer's, DA release was greatly potentiated. Administration of the competitive NMDA receptor antagonist aminophosphonovalerate (APV) in normal Ringer's prior to treatment with NMDA in high-K+ Ringer's resulted in a significant reduction of DA release compared to control animals. In contrast, administration of APV priot to treatment with NMDA in normal Ringer's resulted in a significantly increased release of DA compared to controls. Administration of the non-competitive NMDA antagonist, dextromethorphan (DXT) prior to treatment with NMDA in normal Ringer's or NMDA in high-K+ Ringer's caused significant reductions of DA release compared to controls. Intrastriatal DXT also caused dose-dependent inhibition of high-K+ Ringer's-induced DA release. Similarly, administration of the non-specific calcium channel blocker, cadmium, prior to treatment with NMDA resulted in a significant decrease when compared to control values. Results of this study indicate that dose-dependent NMDA-induced striatal DA release is greatly potentiated by potassium suggesting that under physiological conditions in vivo, striatal NMDA receptors are mostly inactivated.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor subtypes involved in callosally-induced postsynaptic potentials in rat frontal agranular cortex in vitro

A slice preparation of rat frontal agranular cortex preserving commissural inputs has been used for intracellular recording from layer V pyramidal cells, in order to characterize the synaptic potentials induced by stimulation of the corpus callosum and to reveal the subtypes of amino acid receptors involved. Stimulation of the corpus callosum induced EPSPs followed by early IPSPs with a peak latency of 30 +/- 2 ms and late IPSPs with a peak latency of 185 +/- 18 ms. Reversal potentials for early and late IPSPs were -75 +/- 5 mV (early) and -96 +/- 5 mV (late). Late IPSPs were more dependent on extracellular K+ concentration. The early IPSPs were blocked by GABAA antagonists, bicuculline and picrotoxin, whereas the late IPSPs were reduced by the GABAB antagonist, phaclofen. CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), an antagonist of non-NMDA (N-methyl-D-aspartate) receptors, suppressed both EPSPs and late IPSPs at 5 microM. Early IPSPs remained at this concentration but were suppressed by 20 microM CNQX. In Mg(2+)-free solution, EPSPs were larger and more prolonged than in control solution. These enhanced EPSPs persisted after 5 to 20 microM CNQX, but were reduced in amplitude, and their onset was delayed by 3.6 +/- 0.8 ms. The remaining EPSPs were suppressed by 50 microM APV (DL-2-amino-5-phosphono-valeric acid), an antagonist of NMDA receptors. In Mg(2+)-free solution containing 5 to 20 microM CNQX, the late IPSPs were not diminished. The remaining late IPSPs were suppressed by APV or by phaclofen.(ABSTRACT TRUNCATED AT 250 WORDS)