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

Long-Term Consumption of Purified Water Altered Amino Acid, Fatty Acid and Energy Metabolism in Livers of Rats

The consumption of low-mineral water has been increasing worldwide. Drinking low-mineral water is associated with cardiovascular disease, osteopenia, and certain neurodegenerative diseases. However, the specific mechanism remains unclear. The liver metabolic alterations in rats induced by drinking purified water for 3 months were investigated with a metabolomics-based strategy. Compared with the tap water group, 74 metabolites were significantly changed in the purified water group (6 increased and 68 decreased), including 29 amino acids, 11 carbohydrates, 10 fatty acids, 7 short chain fatty acids (SCFAs), and 17 other biomolecules. Eight metabolic pathways were significantly changed, namely aminoacyl-tRNA biosynthesis; nitrogen metabolism; alanine, aspartate and glutamate metabolism; arginine and proline metabolism; histidine metabolism; biosynthesis of unsaturated fatty acids; butanoate metabolism; and glycine, serine and threonine metabolism. These changes suggested that consumption of purified water induced negative nitrogen balance, reduced expression of some polyunsaturated fatty acids and SCFAs, and disturbed energy metabolism in rats. These metabolic disturbances may contribute to low-mineral-water-associated health risks. The health risk of consuming low-mineral water requires attention.

β-Catenin in the Adult Visual Cortex Regulates NMDA-Receptor Function and Visual Responses

The formation, plasticity and maintenance of synaptic connections is regulated by molecular and electrical signals. β-Catenin is an important protein in these events and regulates cadherin-mediated cell adhesion and the recruitment of pre- and postsynaptic proteins in an activity-dependent fashion. Mutations in the β-catenin gene can cause cognitive disability and autism, with life-long consequences. Understanding its synaptic function may thus be relevant for the treatment of these disorders. So far, β-catenin's function has been studied predominantly in cell culture and during development but knowledge on its function in adulthood is limited. Here, we show that ablating β-catenin in excitatory neurons of the adult visual cortex does not cause the same synaptic deficits previously observed during development. Instead, it reduces NMDA-receptor currents and impairs visual processing. We conclude that β-catenin remains important for adult cortical function but through different mechanisms than during development.

Control of the superior colliculus by the lateral prefrontal cortex

Several decades of patient, functional imaging and neurophysiological studies have supported a model in which the lateral prefrontal cortex (PFC) acts to suppress unwanted saccades by inhibiting activity in the oculomotor system. However, recent results from combined PFC deactivation and neural recordings of the superior colliculus in monkeys demonstrate that the primary influence of the PFC on the oculomotor system is excitatory, and stands in direct contradiction to the inhibitory model of PFC function. Although erroneous saccades towards a visual stimulus are commonly labelled reflexive in patients with PFC damage or dysfunction, the latencies of most of these saccades are outside of the range of express saccades, which are triggered directly by the visual stimulus. Deactivation and pharmacological manipulation studies in monkeys suggest that response errors following PFC damage or dysfunction are not the result of a failure in response suppression but can best be understood in the context of a failure to maintain and implement the proper task set.

AMPA receptor expression is increased post-mortem samples of the anterior cingulate from subjects with major depressive disorder

BACKGROUND

Glutamate is thought to be involved in the pathophysiology of major depressive disorder and bipolar disorder; however, the molecular changes underlying abnormal glutamatergic signalling remain poorly understood. Whilst previous studies have suggested that the NMDA receptor may be involved in the pathophysiology of mood disorders, it is unclear whether the non-NMDA receptors are also involved. Therefore, we sought to examine whether the expression of the non-NMDA, ionotropic glutamate receptors, AMPA receptor and kainate receptor, is altered in mood disorders.

METHODS

We used [3H]AMPA and [3H]kainate to measure the levels of AMPA and kainate receptor, respectively, in the anterior cingulate (BA 24) and dorsolateral prefrontal cortex (BA 46) from post-mortem CNS in 10 subjects with major depressive disorder, 10 subjects with bipolar disorder and 10 control subjects.

RESULTS

A 20.7% to 27.7% increase in [3H]AMPA binding density was seen in BA 24 (p<0.05) but not BA 46 (p>0.05) in major depressive disorder compared to control levels. [3H]AMPA binding density was not changed in bipolar disorder in either BA 24 or BA 46 (p>0.05) compared to controls. [3H]Kainate binding was not changed in either BA 24 or BA 46 in either disorder compared to controls (p>0.05).

LIMITATIONS

Small sample sizes (n=10) were used in this study. The subjects were not drug naïve.

CONCLUSIONS

Our data suggests increased in AMPA receptor levels in the anterior cingulate are involved in the pathophysiology of major depressive disorder. This data has relevance for the development of new anti-depressant drugs targeted towards the AMPA receptors.

Modulation of NMDA and AMPA-mediated synaptic transmission by CB1 receptors in frontal cortical pyramidal cells

Although the endogenous cannabinoid system modulates a variety of physiological and pharmacological processes, the specific role of cannabinoid CB1 receptors in the modulation of glutamatergic neurotransmission and neural plasticity is not well understood. Using whole-cell patch clamp recording techniques, evoked or spontaneous excitatory postsynaptic currents (eEPSCs or sEPSCs) were recorded from visualized, layer II/III pyramidal cells in frontal cortical slices from rat brain. Bath application of the CB1 receptor agonist, WIN 55212-2 (WIN), reduced the amplitude of NMDA receptor-mediated EPSCs in a concentration-dependent manner. When co-applied with the specific CB1 antagonists, AM251 or AM281, WIN did not suppress NMDA receptor-mediated EPSCs. WIN also reduced the amplitude of evoked AMPA receptor-mediated EPSCs, an effect that was also reversed by AM251. Both the frequency and amplitude of spontaneous AMPA receptor-mediated EPSCs were significantly reduced by WIN. In contrast, WIN reduced the frequency, but not the amplitude of miniature EPSCs, suggesting that the suppression of glutamatergic activity by CB1 receptors in the frontal neocortex is mediated by a presynaptic mechanism. Taken together, these data indicate a critical role for endocannabinoid signaling in the regulation of excitatory synaptic transmission in frontal neocortex, and suggest a possible neuronal mechanism whereby THC regulates cortical function.

Cholinergic terminals in the cat visual cortex: Ultrastructural basis for interaction with glutamate-immunoreactive neurons and other cells

Acetylcholine (ACh) is one of the transmitters utilized by extrathalamic afferents to modulate stimulus-driven neurotransmission and experience-dependent plasticity in the visual cortex. Since these processes also depend on the activation of glutamatergic receptors, cholinergic terminals may exert their effects via direct modulation of excitatory neurotransmission. The objective of this study was to determine whether the ultrastructural relationships between cholinergic terminals, glutamate-immunoreactive neurons, and other unlabeled cells support this idea. Sections from aldehyde-fixed visual cortex (area 17) of adult cats were immunolabled for the following molecules: (1) choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme; (2) L-glutamate; or (3) ChAT simultaneously with L-glutamate by combining electron-microscopic immunogold and immunoperoxidase techniques. None of the cortical terminals were dually labeled, suggesting that (1) the labeling procedure was free of chemical or immunological cross reactions; and (2) glutamate immunoreactivity probably reflects the transmitter, and not metabolic, pool of L-glutamate. Comparisons between cholinergic and noncholinergic axons revealed that (1) ChAT-immunoreactive axons formed fewer identifiable synaptic contacts within single ultrathin sections (P < 0.01 using chi-square test); and (2) more of the cholinergic axons occurred directly opposed to other terminals (P < 0.0015 by chi-square test), including 21% of which resided directly across asymmetric, axo-spinous junctions. Dual labeling showed that a third of the synaptic targets for cholinergic terminals contained detectable levels of glutamate immunoreactivity. Some of the axo-spinous junctions juxtaposed to cholinergic axons also exhibited glutamate immunoreactivity presynaptically. These observations provide ultrastructural evidence for direct, cholinergic modulation of glutamatergic pyramidal neurons within the mammalian neocortex. Prevalence of juxtapositions between cholinergic terminals and axo-spinous synapses supports the following ideas: (1) ACh may modulate the release of noncholinergic transmitters, including Glu; (2) Glu may modulate ACh release; and (3) these processes may be concurrent with cholinergic modulation of glutamatergic synapses at postsynaptic sites.

A neurochemical signature of visual recovery after extrastriate cortical damage in the adult cat

In adult cats, damage to the extrastriate visual cortex on the banks of the lateral suprasylvian (LS) sulcus causes severe deficits in motion perception that can recover as a result of intensive direction discrimination training. The fact that recovery is restricted to trained visual field locations suggests that the neural circuitry of early visual cortical areas, with their tighter retinotopy, may play an important role in attaining perceptual improvements after damage to higher level visual cortex. The present study tests this hypothesis by comparing the manner in which excitatory and inhibitory components of the supragranular circuitry in an early visual cortical area (area 18) are affected by LS lesions and postlesion training. First, the proportion of LS-projecting pyramidal cells as well as calbindin- and parvalbumin-positive interneurons expressing each of the four AMPA receptor subunits was estimated in layers II and III of area 18 in intact animals. The degree to which LS lesions and visual retraining altered these expression patterns was then assessed. Both LS-projecting pyramidal cells and inhibitory interneurons exhibited long-term, differential reductions in the expression of glutamate receptor (GluR)1, -2, -2/3, and -4 following LS lesions. Intensive visual training post lesion restored normal AMPAR subunit expression in all three cell-types examined. Furthermore, for LS-projecting and calbindin-positive neurons, this restoration occurred only in portions of the ipsi-lesional area 18 representing trained visual field locations. This supports our hypothesis that stimulation of early visual cortical areas-in this case, area 18-by training is an important factor in restoring visual perception after permanent damage to LS cortex.

From benefit to damage. Glutamate and advanced glycation end products in Alzheimer brain

The glutamatergic system is the most widespread neurotransmitter system in the mammalian brain. It is connected to the acetylcholinergic neurotransmitter system to form the glutamatergic/aspartatergic-acetylcholinergic circuit, which is the morphobiochemical basis of learning, memory and cognition assisted by the glutamatergic N-methyl-D-aspartate receptor, which mediates long-term potentiation as the fundamental molecular mechanisms of these mental capacities. Glutamate and acetylcholine as ligands of the two neurotransmitter systems are products of the neuronal glucose metabolism as holds true also for advanced glycation end products (AGEs), which are markers of damaged and/or aged proteins. During normal aging, both the neurotransmitters glutamate and acetylcholine undergo strong functional variations. Their synthesis was found to be reduced as a common feature. In contrast, basal release of acetylcholine and receptor number decrease, whereas basal release of glutamate and receptor number increase. AGEs increase during aging obviously preferentially in glutamatergic pyramidal neurons in cerebral cortical layers prone to neurodegeneration. In sporadic Alzheimer disease (SAD), glutamate concentration was shown to fall since it may serve as a substitute for lacking glucose in the beginning of the disease. In contrast, glutamate receptor density was found to be much less involved indicating an excessive activation of the glutamatergic neurotransmitter system particularly via the NMDA receptor, mediating endogenous excitotoxicity. The morphological hallmarks of SAD neuritic plaques and neurofibrillary tangles have been demonstrated to crosslink with AGEs causing an increased rate of free radical production. First data from animal studies and investigations on human beings may indicate that the NMDA receptor antagonist memantine may have beneficial effects on the course of SAD and its clinical symptoms.

Effects of Agonists and Antagonists of NMDA and ACh Receptors on Plasticity of Bat Auditory System Elicited by Fear Conditioning

In big brown bats, tone-specific plastic changes [best frequency (BF) shifts] of cortical and collicular neurons can be evoked by auditory fear conditioning, repetitive acoustic stimuli or cortical electric stimulation. It has been shown that acetylcholine (ACh) plays an important role in evoking large long-term cortical BF shifts. However, the role of N-methyl-d-aspartate (NMDA) receptors in evoking BF shifts has not yet been studied. We found 1) NMDA applied to the auditory cortex (AC) or inferior colliculus (IC) augmented the auditory responses, as ACh did, whereas 2-amino-5-phosphovalerate (APV), an antagonist of NMDA receptors, reduced the auditory responses, as atropine did; 2) although any of these four drugs did not evoke BF shifts, they influenced the development of the long-term cortical and short-term collicular BF shifts elicited by conditioning; 3) like ACh, NMDA augmented the cortical and collicular BF shifts regardless of whether it was applied to the AC or IC; 4) endogenous ACh of the AC and IC is necessary to produce the long-term cortical and short-term collicular BF shifts; 5) blockade of collicular NMDA receptors by APV abolished the development of the collicular BF shift and made the cortical BF shift small and short-term; 6) blockade of cortical NMDA receptors by APV reduced the cortical and collicular BF shifts and made the cortical BF shift short-term; and 7) conditioning with NMDA + atropine applied to the AC evoked the small, short-term cortical BF shift, whereas conditioning with APV + ACh applied to the AC evoked the small, but long-term cortical BF shift.

Development of reorganization of the auditory cortex caused by fear conditioning: effect of atropine

Reorganization of the frequency map in the central auditory system is based on shifts in the best frequencies (BFs; hereafter, BF shifts), together with the frequency-response curves, of auditory neurons. In the big brown bat, conditioning with acoustic stimulation followed by electric leg-stimulation causes BF shifts of collicular and cortical neurons. The collicular BF shift develops quickly and is short term, whereas the cortical BF shift develops slowly and is long term. The acetycholine level in the auditory cortex must be high during conditioning to develop these BF shifts. We studied the effect of atropine (an antagonist of muscarinic acetylcholine receptors) applied to the auditory cortex on the development of the long-term cortical BF shift in the awake bat caused by a 30-min conditioning session. We found 1) the cortical BF shift starts to develop approximately 15 min after the onset of the conditioning, gradually increases over 60 min, and reaches a plateau, 2) the cortical BF shift changes from short to long term approximately 45 min after the onset of the conditioning, 3) the cortical BF shift can plateau at different frequencies between the BF of a given neuron in the control condition and the frequency of the conditioning tone, 4) the maximum BF shift is determined approximately 70 min after the onset of the conditioning, and 5) acetylcholine plays an important role in the development of the cortical BF shift. Its role ends approximately 180 min after the onset of the conditioning.

Distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor subunits (GluR2/3) along the ventral visual pathway in the monkey

By using immunohistochemical methods, we examined the distribution of cells expressing subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptors (GluR2/3) in the cortical areas of the occipitotemporal pathway in monkeys. GluR2/3-immunoreactive (-ir) cells were primarily pyramidal cells; this category, however, also included large stellate cells in layer IVB of the striate cortex (V1) and fusiform cells in layer VI of all the areas examined. GluR2/3 immunoreactivity differed among the areas in laminar distribution and intensity. In V1, GluR2/3-ir cells were identified mainly in layers II, III, IVB, and VI. The prestriate areas V2 and V4 and the inferior temporal areas TEO and TE contained GluR2/3-ir cells in layers II, III, and VI. In the TE, GluR2/3-ir cells were also abundant in layer V. In area 36 of the perirhinal cortex, neurons in layers II, III, V, and VI were labeled in a similar manner to the TE labeling, but with greater staining intensity and numbers, especially in layer V. Thus, GluR2/3 immunoreactivity increased rostrally along the pathway. Within V1 and V2, cells strongly stained for GluR2/3 formed clusters that colocalized with cytochrome oxidase (CO)-rich regions. These distinct laminar and regional distribution patterns of GluR2/3 expression may contribute to the specific physiological properties of neurons within various visual areas and compartments.

Retinal lesions affect extracellular glutamate levels in sensory-deprived and remote non-deprived regions of cat area 17 as revealed by in vivo microdialysis

This study aimed at gaining insight into the role of the excitatory neurotransmitter glutamate in topographic map reorganization in the sensory systems of adult mammals after restricted deafferentations. Hereto, in vivo microdialysis was used to sample extracellular glutamate from sensory-deprived and non-deprived visual cortex of adult awake cats 18 to 53 days after the induction of restricted binocular retinal lesions, and in topographically corresponding cortical regions of control animals. A microbore HPLC-ED method was applied for the analysis of the microdialysates. In normal subjects, the visual cortex subserving central and peripheral vision showed similar extracellular fluid glutamate concentrations. In contrast, in animals with homonymous central retinal lesions, the extracellular glutamate concentration was significantly lower in central, sensory-deprived cortex compared to peripheral, non-deprived cortex. Compared to control regions in normal subjects, glutamate decreased in the extracellular fluid of deprived cortex but increased significantly in remote non-deprived visual cortex. These results not only suggest an activity-dependent regulation of the glutamate levels in visual cortex but also imply a role for perilesional cortical regions in topographic map reorganization following sensory deafferentation.

Distribution of the AMPA2 glutamate receptor subunit in adult cat visual cortex

In this study, we revealed the distribution of the AMPA2 glutamate receptor subunit (AMPA2) in the visual cortical areas 17 and 18 of the adult cat by means of different techniques. In situ hybridization, with a cat-specific radioactively labeled oligonucleotide probe, showed that AMPA2 mRNA was expressed mainly in cortical layers II/III and V/VI with a lower expression in layer IV and practically no signal in layer I. Immunocytochemistry, using a polyclonal AMPA2 subunit-specific antibody, showed immunoreactivity almost exclusively in the somata and dendrites of pyramidal neurons in cortical layers II/III and V/VI. Only a very faint signal was detected in layer IV. Neurons with little or no AMPA2 have AMPA receptors that are highly permeable to calcium. By determining the location of AMPA2, this study therefore provides a clear examination of the distribution of Ca(2+)-impermeable AMPA receptors over the supra- and infragranular layers of cat visual cortex. The functional implication of the absence of AMPA2 in cortical layer IV and thus the presence of Ca(2+)-permeable AMPA receptors in this layer, is still speculative and has yet to be elucidated.

AMPA GluR2 subunit is differentially distributed on GABAergic neurons and pyramidal cells in the macaque monkey visual cortex

The cellular and synaptic distribution of the AMPA receptor subunit GluR2 was analyzed in the monkey primary visual cortex (area V1), by immunocytochemistry and postembedding immunogold methods. GluR2 immunoreactivity was widely distributed in all of the layers of area V1. A quantitative double labeling analysis in layers II and III revealed that the vast majority of GABAergic interneurons in this area also contained GluR2. Postembedding immunogold analysis revealed that GluR2 immunoreactivity was present at asymmetric synapses on both GABAergic interneurons and pyramidal cells. A quantitative study indicated that the number of GluR2 immunogold particles at asymmetric synapses on pyramidal cells was significantly higher than that on GABAergic interneurons. These results from the primate neocortex are in agreement with and extend our previous studies on the rat hippocampus and amygdala. In view of the dominant role of the GluR2 subunit in regulating calcium flux through AMPA receptors, the differential synaptic distribution of GluR2 on different neuronal types might provide a mechanism for cell-specific response properties to glutamate as well as clues to selective neuronal vulnerability and cell death mediated by calcium-dependent excitotoxic mechanisms.

Effects of acetylcholine and atropine on plasticity of central auditory neurons caused by conditioning in bats

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.

Transplanted neuroblasts differentiate appropriately into projection neurons with correct neurotransmitter and receptor phenotype in neocortex undergoing targeted projection neuron degeneration

Reconstruction of complex neocortical and other CNS circuitry may be possible via transplantation of appropriate neural precursors, guided by cellular and molecular controls. Although cellular repopulation and complex circuitry repair may make possible new avenues of treatment for degenerative, developmental, or acquired CNS diseases, functional integration may depend critically on specificity of neuronal synaptic integration and appropriate neurotransmitter/receptor phenotype. The current study investigated neurotransmitter and receptor phenotypes of newly incorporated neurons after transplantation in regions of targeted neuronal degeneration of cortical callosal projection neurons (CPNs). Donor neuroblasts were compared to the population of normal endogenous CPNs in their expression of appropriate neurotransmitters (glutamate, aspartate, and GABA) and receptors (kainate-R, AMPA-R, NMDA-R. and GABA-R), and the time course over which this phenotype developed after transplantation. Transplanted immature neuroblasts from embryonic day 17 (E17) primary somatosensory (S1) cortex migrated to cortical layers undergoing degeneration, differentiated to a mature CPN phenotype, and received synaptic input from other neurons. In addition, 23.1 +/- 13.6% of the donor-derived neurons extended appropriate long-distance callosal projections to the contralateral S1 cortex. The percentage of donor-derived neurons expressing appropriate neurotransmitters and receptors showed a steady increase with time, reaching numbers equivalent to adult endogenous CPNs by 4-16 weeks after transplantation. These results suggest that previously demonstrated changes in gene expression induced by synchronous apoptotic degeneration of adult CPNs create a cellular and molecular environment that is both permissive and instructive for the specific and appropriate maturation of transplanted neuroblasts. These experiments demonstrate, for the first time, that newly repopulating neurons can undergo directed differentiation with high fidelity of their neurotransmitter and receptor phenotype, toward reconstruction of complex CNS circuitry.

Analysis of extracellular gamma-aminobutyric acid, glutamate and aspartate in cat visual cortex by in vivo microdialysis and capillary electrophoresis-laser induced fluorescence detection

To investigate the influence of a partial sensory deprivation on the extracellular concentration of amino acid neurotransmitters in cat visual cortex, a capillary electrophoresis method was developed for the quantification of gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) in in vivo microdialysis samples of cat brain. Microdialysis samples from different regions of area 17 were obtained every 15-min using CMA 12 2-mm probes perfused with synthetic cerebrospinal fluid and derivatized using fluorescein isothiocyanate (FITC). Laser-induced fluorescence (LIF) detection was employed. Good selectivity was obtained with a borate buffer (20 mM, pH 10.25). The whole procedure, including the washing step takes only 15 min. The conditions for derivatization and separation were optimized. The parameters for validation such as linearity, precision and detection limit are also reported. The results are consistent with those of HPLC but, as the sample volumes needed are only 1--5 nl, a much better time resolution can be obtained.

Cooperative changes in GABA, glutamate and activity levels: the missing link in cortical plasticity

Different intracortical mechanisms have been reported to contribute to the substantial topographic reorganization of the mammalian primary visual cortex in response to matching lesions in the two retinas: an immediate expansion of receptive fields followed by a gradual shift of excitability into the deprived area and finally axonal sprouting of laterally projecting neurons months after the lesion. To gain insight into the molecular mechanisms of this adult plasticity, we used immunocytochemical and bioanalytical methods to measure the glutamate and GABA neurotransmitter levels in the visual cortex of adult cats with binocular central retinal lesions. Two to four weeks after the lesions, glutamate immunoreactivity was decreased in sensory-deprived cortex as confirmed by HPLC analysis of the glutamate concentration. Within three months normal glutamate immunoreactivity was restored. In addition, the edge of the unresponsive cortex was characterized by markedly increased glutamate immunoreactivity 2-12 weeks postlesion. This glutamate immunoreactivity peak moved into the deprived area over time. These glutamate changes corresponded to decreased spontaneous and visually driven activity in unresponsive cortex and to strikingly increased neuronal activity at the border of this cortical zone. Furthermore, the previously reported decrease in glutamic acid decarboxylase immunoreactivity was found to reflect decreased GABA levels in sensory-deprived cortex. Increased glutamate concentrations and neuronal activity, and decreased GABA concentrations, may be related to changes in synaptic efficiency and could represent a mechanism underlying the retinotopic reorganization that occurs well after the immediate receptive field expansion but long before the late axonal sprouting.

Neuropeptide Y- and somatostatin-immunoreactive axons in the corpus callosum during postnatal development of the rat

Corpus callosum (CC) projections in adult mammals were generally thought to be excitatory and to use excitatory amino acids as their transmitters. Little information has been available about the electrical properties and neurochemical status of developing CC connections. The present study investigated the chemical status of rat CC axons during postnatal development by using antibodies to neuropeptide Y (NPY) and to somatostatin (SOM). Both NPY-immunoreactive (ir) and SOM-ir axons were found in the CC of the rat from newborn through adult; however, the number of SOM-ir CC axons is less than that of NPY-ir CC axons at corresponding ages. The density of both NPY-ir and SOM-ir CC axons initially increased, then peaked, and finally decreased to the mature level. In the adult, only a few NPY-ir and SOM-ir CC axons were found in the CC. These results indicate that many NPY-ir and SOM-ir CC axons are transitory during early postnatal development. The results also suggest that the functions of CC connections in adult mammals may be different from that of developing ones. The present results as well as the previous results demonstrate that both developing and mature CC connections are chemically heterogeneous.

GABA, 5-HT and amino acids in the rotifers Brachionus plicatilis and Brachionus rotundiformis

gamma-Aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) have been shown to increase the reproduction of the Brachionus plicatilis (NH3L strain). In the present study, the endogenous presence of GABA and 5-HT in the rotifers B. plicatilis (NH3L and Kamiura strains) and Brachionus rotundiformis (Langkawi strain) were confirmed by dot blot immunoassay and high-performance liquid chromatography (HPLC). HPLC showed that GABA and 5-HT concentrations in the three rotifer strains range from 71 to 188 pmol/mg and from 12 to 64 pmol/mg, respectively. A total of 33 amino acids were also detected in B. plicatilis and B. rotundiformis, with glutamic acid, serine, glycine, taurine, threonine, alanine, arginine, proline, valine and isoleucine in high concentrations relative to other amino acids.

Cerebral cortical muscarinic cholinergic and N-methyl-D-aspartate receptors mediate increase in cortical blood flow elicited by chemical stimulation of periaqueductal gray matter

The periaqueductal gray matter is implicated in the central processing of defensive reactions. We found previously that, when stimulated by N-methyl-D-aspartate, the caudal third of its lateral subdivision elicited an increase in blood flow over widespread cerebral neocortical areas and that a major proportion of the flow increase was inhibited by topical cortical application of scopolamine, an antagonist of muscarinic receptors. The present study was undertaken to elucidate the roles of cortical nicotinic and excitatory amino acid receptors in the mediation of the flow increase in 66 anaesthetized, cervically cordotomized, artificially ventilated rats with open cranial windows. We found that the flow increase (laser-Doppler flowmetry) was resistant to antagonists of non-N-methyl-D-aspartate receptors and of nicotinic receptors. The response was, however, attenuated to a substantial extent by topical and intravenous N-methyl-D-aspartate receptor antagonists, MK-801 and topical D(-)-2-amino-5-phosphonopentanoic acid. Combined topical application of the latter antagonist with scopolamine attenuated the flow increase to a further extent beyond that achieved with either of the antagonists alone. Topical applications of acetylcholine and N-methyl-D-aspartate individually increased the cortical blood flow. A modest synergism was observed between the actions of these two agonists.Overall, we suggest that in the face of stimuli which provoke defensive reactions, the periaqueductal gray matter may elicit an increase in cortical blood flow by utilizing the cortical acetylcholine-muscarinic receptor system and the cortical excitatory amino acid-N-methyl-D-aspartate receptor system. The vasomotor actions of these two transmitter-receptor systems may operate independently of each other as well as in harmony with each other.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Effects of blocking non-N-methyl-D-aspartate receptors on visual responses of neurons in the cat visual cortex

To elucidate the function of non-N-methyl-D-aspartate types of glutamate receptors in the primary visual cortex of the adult cat, we studied the effects of the iontophoretically applied glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and D-amino-5-phosphonovalerate. Antagonists were applied with ejecting currents that selectively blocked non-N-methyl-D-aspartate receptors. Among 93 cells in which stable recordings were obtained, 6-cyano-7-nitroquinoxaline-2,3-dione reduced the visual response in all cells. The average response magnitude during 6-cyano-7-nitroquinoxaline-2,3-dione administration was reduced to 11.7% of the control (average ejecting current: 41.2 nA). The effect of 6-cyano-7-nitroquinoxaline-2,3-dione was obvious throughout all cortical layers. The effect of D-amino-5-phosphonovalerate on the visual response was tested in 14 cells and it was also effective in blocking the visual response: the average response magnitude during D-amino-5-phosphonovalerate administration was 45.0% of the control (average ejecting current: 41.4 nA). The effect of 6-cyano-7-nitroquinoxaline-2,3-dione on the response was compared in individual cells at both high and low firing rates in order to determine whether a differential effect exists on the level of firing activity of cells due to secondary inactivation of voltage-dependent N-methyl-D-aspartate receptors. However, no indication of response dependency on firing rate was seen with 6-cyano-7-nitroquinoxaline-2,3-dione. We suggest that excitatory transmission at the geniculocortical and corticocortical synapses seems to be strongly dependent on non-N-methyl-D-aspartate receptors throughout the primary visual cortex of the adult cat, and that both non-N-methyl-D-aspartate and N-methyl-D-aspartate type glutamate receptors function additively.

The synaptic pharmacology underlying sensory processing in the superior colliculus

The superior colliculus (SC) is one of the most ancient regions of the vertebrate central sensory system. In this hub afferents from several sensory pathways converge, and an extensive range of neural circuits enable primary sensory processing, multi-sensory integration and the generation of motor commands for orientation behaviours. The SC has a laminar structure and is usually considered in two parts; the superficial visual layers and the deep multi-modal/motor layers. Neurones in the superficial layers integrate visual information from the retina, cortex and other sources, while the deep layers draw together data from many cortical and sub-cortical sensory areas, including the superficial layers, to generate motor commands. Functional studies in anaesthetized subjects and in slice preparations have used pharmacological tools to probe some of the SC's interacting circuits. The studies reviewed here reveal important roles for ionotropic glutamate receptors in the mediation of sensory inputs to the SC and in transmission between the superficial and deep layers. N-methyl-D-aspartate receptors appear to have special responsibility for the temporal matching of retinal and cortical activity in the superficial layers and for the integration of multiple sensory data-streams in the deep layers. Sensory responses are shaped by intrinsic inhibitory mechanisms mediated by GABA(A) and GABA(B) receptors and influenced by nicotinic acetylcholine receptors. These sensory and motor-command activities of SC neurones are modulated by levels of arousal through extrinsic connections containing GABA, serotonin and other transmitters. It is possible to naturally stimulate many of the SC's sensory and non-sensory inputs either independently or simultaneously and this brain area is an ideal location in which to study: (a) interactions between inputs from the same sensory system; (b) the integration of inputs from several sensory systems; and (c) the influence of non-sensory systems on sensory processing.

Immunogold localization of AMPA and NMDA receptors in somatic sensory cortex of albino rat

We performed an electron microscopic study of S-1 cortex by using postembedding immunogold histochemistry to examine the subcellular distribution of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (assessed with an antibody recognizing the glutamate receptor 2 and 3 [GluR2 and GluR3] subunits) and to compare this distribution with that of N-methyl-D-aspartate (NMDA) receptors (assessed with an antibody for the NR1 subunit). Both receptors were concentrated at active zones of asymmetric synapses, often directly apposed to presynaptic dense bodies. GluR2/3 showed a bias for long active zones, whereas short active zones expressed GluR2/3 at substantially lower levels; in contrast, labeling for NR1 was independent of synaptic size. Particle counts suggested that synaptic labeling was Poisson distributed and implied that the majority of synapses express both receptors. Quantitative analysis indicates that approximately one-half of synapses express high levels of GluR2/3 and that the remainder express GluR2/3 at a much lower level. Approximately three-fourths of synapses express NR1 at a uniform level; the remainder, which may lack NR1 completely, include synapses with especially large active zones. The present results suggest that the smallest active zones may play a special role in synaptic plasticity.

Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

We have carried out an ultrastructural study to determine the characteristics and distribution of glutamate-containing constituents of the anterodorsal (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a polyclonal antibody to glutamate and a postembedding immunogold detection method in animals in which the neurons of AD/AV projecting to the cortex had been retrogradely labelled and the terminals of corticothalamic afferents anterogradely labelled by injection of cholera toxin-horseradish peroxidase (HRP) into the retrosplenial granular cortex. The heaviest immunogold labelling was over axon terminals 0.42 to 2.2 microm in diameter containing round synaptic vesicles and establishing Gray type 1 (asymmetric) synaptic contact (type 1 terminals) on HRP-labelled or non-labelled dendrites. Mean gold particle densities over such terminals were 3-4 times higher than the densities over the dendrites to which they were presynaptic and 5-6 times higher than over terminals establishing Gray type 2 (symmetric) synaptic contacts (type 2 terminals). Gold particle densities over neuronal cell bodies and dendrites and over a subpopulation of myelinated axons were intermediate between the densities over type 1 and type 2 terminals. In adjacent serial sections immunoreacted for gamma aminobutyric acid, type 2 terminals were heavily immunolabelled whereas type 1 terminals and other profiles with moderate gold particle densities after glutamate immunoreaction displayed very low labelling. A subpopulation of small type 1 axon terminals (up to 1 microm diameter) contained HRP reaction product identifying them as cortical in origin; they contacted small dendritic profiles (most <1 microm diameter) many of which also contained HRP reaction product. We conclude that terminals of the corticothalamic projection from retrosplenial granular cortex to AD/AV are glutamatergic and innervate predominantly distal dendrites of thalamocortical projection neurons.

Role of muscarinic receptors, G-proteins, and intracellular messengers in muscarinic modulation of NMDA receptor-mediated synaptic transmission

Previously, we reported that activation of muscarinic receptors modulates N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission in auditory neocortex [Aramakis et al. (1997a) Exp Brain Res 113:484-496]. Here, we describe the muscarinic subtypes responsible for these modulatory effects, and a role for G-proteins and intracellular messengers. The muscarinic agonist oxotremorine-M (oxo-M), at 25-100 microM, produced a long-lasting enhancement of NMDA-induced membrane depolarizations. We examined the postsynaptic G-protein dependence of the modulatory effects of oxo-M with the use of the G-protein activator GTP gamma S and the nonhydrolyzable GDP analog GDP beta S. Intracellular infusion of GTP gamma S mimicked the facilitating actions of oxo-M. After obtaining the whole-cell recording configuration, there was a gradual, time-dependent increase of the NMDA receptor-mediated slow-EPSP, and of iontophoretic NMDA-induced membrane depolarizations. In contrast, intracellular infusion of either GDP beta S or the IP3 receptor antagonist heparin prevented oxo-M mediated enhancement of NMDA depolarizations. The muscarinic receptor involved in enhancement of NMDA iontophoretic responses is likely the M1 receptor, because the increase was prevented by pirenzepine, but not the M2 antagonists methoctramine or AF-DX 116. Oxo-M also reduced the amplitude of the pharmacologically isolated slow-EPSP, and this effect was blocked by M2 antagonists. Thus, muscarinic-mediated enhancement of NMDA responses involves activation of M1 receptors, leading to the engagement of a postsynaptic G-protein and subsequent IP3 receptor activity.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for alpha-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methyl-D-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or alpha type II Ca2+/calmodulin-dependent protein kinase (CaMKII-alpha), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

Irreversible impairment of inhibitory neurons and nitric oxide release in the neocortex produced by low temperature and hypoxia in vitro

Brain ischemia causes irreversible hyperexcitability, which may be attributed to irreversible impairment of inhibitory neurons. However, the conditions required for selective and irreversible impairment of inhibitory interneurons in vitro are unknown. In this study, we found that a combination of low temperature and hypoxia produced hyperexcitability in the neocortex. Neocortical tissue blocks isolated from rats were exposed to low temperature (1-3 degrees C) for 45 min and subsequently to room temperature (21-23 degrees C) for 60 min in the non-oxygenated medium. In experimental slices prepared from the processed blocks, hyperexcitability, similar to that elicited by an antagonist of GABA(A) receptors, was observed. Exposure of the neocortical tissue blocks to low temperature alone or room temperature alone did not elicit hyperexcitability. The excitability of pyramidal neurons, excitatory synaptic transmission and inhibitory effects of an agonist of GABA(A) receptors were normal in experimental slices. However, excitation of pyramidal neurons was inhibited after local stimulation of inhibitory neurons in control slices, but not in experimental slices. Nitric oxide (NO) release from cortical interneurons was also markedly reduced in experimental slices. These results indicate that irreversible impairment of neocortical inhibitory neurons was produced by low temperature combined with hypoxia produced in vitro.

Temperature dependence and independence of effects of pentobarbital on visual evoked potentials of rats

Visual cortex flash evoked potentials (FEPs), pattern reversal evoked potentials (PREPs), and body temperature were measured in hooded rats following IP injections on separate days of saline, and of 15 and 30 mg pentobarbital/kg body weight. Two experiments were performed, differentiated by standard (23 degrees C) and warm (31 degrees C) room temperatures. The 30 mg/kg dose produced hypothermia of 2.6 degrees C in the standard environment, but not in the warm environment. Early components of FEPs were generally increased in amplitude by the 15 mg/kg dose, and decreased by the 30 mg/kg dose at 23 degrees C. At 31 degrees C, the 30 mg/kg dose did not decrease early component amplitude, suggesting that hypothermia can potentiate some effects of pentobarbital. Amplitudes of late FEP components were depressed at both ambient temperatures. The main PREP components N1P1 and P1N3 were increased in amplitude by the 15 mg/kg dose, but returned to near baseline levels at 30 mg/kg, at both temperatures. PREP component N2P2 was reduced in amplitude by the 30 mg/kg dose only at 23 degrees C. Treatment with 30 mg/kg pentobarbital increased FEP and PREP latencies at both ambient temperatures, but the magnitudes of the increases at 31 degrees C were typically less than half those observed at 23 degrees C. These results indicate that hypothermia contributes to some pentobarbital-induced changes in both FEPs and PREPs, but that pentobarbital also produces effects independent of hypothermia.

Spatiotemporal patterning of glutamate receptors in developing ferret striate cortex

We have studied glutamate receptor levels during very early phases of cortical formation by using quantitative in vitro autoradiography to map the expression of NMDA, AMPA and kainate receptors in the developing primary visual cortex of the ferret. NMDA and non-NMDA receptors exhibit very different developmental profiles in primary visual cortex. NMDA receptor density is low at birth and increases throughout the first 2 postnatal months, rising between threefold (layers II/III) and ninefold (layer VI). In contrast, AMPA receptors are abundant at birth and their density remains constant for the first postnatal month, before rising by a maximum of 1.7-fold (layer I) at around the time of eye-opening (postnatal day 32). Kainate receptors are also present in high levels at birth and their expression levels rise in the early postnatal period by between 1. 5-fold (layer I) and threefold (layers V/VI) to a peak just after eye-opening. The proportion of the total ionotropic glutamate receptor binding contributed by NMDA receptors thus rises from 5% at birth to a maximum of 22% at 2 months of age, while the AMPA receptor contribution falls from 87% to 72% over the same period. Below cortex, all three glutamate receptor subtypes are expressed in the subplate region for the first 3 postnatal weeks. These developmental patterns, combined with the fact that AMPA receptors are densely expressed in the proliferative zones underlying presumptive area 17, indicate that non-NMDA receptor expression levels in primary visual cortex are mostly specified much earlier than those of NMDA receptors.

Selective suppression of forward and recurrent "rapid" inhibition by local application of picrotoxin in area CA1 of rat hippocampal slices

Living slices of rat hippocampus were used to study the possibility of suppressing forward and recurrent "rapid" inhibition in area CA1 by local application of picrotoxin, an antagonist of ionotropic gamma-aminobutyric acid (GABAA) receptors. Application of picrotoxin to the apical dendrites increased the duration of focal potentials recorded in the radial layer (143.0 +/- 7.5%, n = 5; here and subsequently, results are presented as mean +/- error of the mean and n is the number of experiments) but had no effect on the population peak in the pyramidal layer (103.0 +/- 19.6%, n = 5). This is evidence for the existence of suppression of direct, but not of recurrent inhibition. Application of picrotoxin to the cell body layer, on the other hand, significantly increased the population peak (654.5 +/- 245.1%, n = 4) and provoked convulsive activity in neurons, demonstrating suppression of recurrent inhibition. Local application of picrotoxin was further used to study the question of how completely antagonists of glutamate ionotropic receptors sensitive to alpha-amino-3-hydroxy-S-methyl-4-isoxazole propionic acid (AMPA) suppress inhibition in solutions with low magnesium contents. This question is important for interpreting experimental data obtained from measurements of the components of excitatory postsynaptic potentials (EPSP), which depend on activation of ionotropic glutamate receptors sensitive to N-methyl-d-aspartic acid (NMDA). A number of studies have suggested that even at low concentrations, AMPA receptor antagonists suppress forward inhibition to such an extent that it has no significant effect on measurements of the NMDA component of EPSP. Our data do not contradict this suggestion.

The NMDA receptor antagonist CPP impairs conditioned taste aversion and insular cortex long-term potentiation in vivo

It has been proposed that long-term potentiation (LTP) a form of activity-dependent modification of synaptic efficacy, may be a synaptic mechanism for certain types of learning. Recent studies on the insular cortex (IC) a region of the temporal cortex implicated in the acquisition and storage of conditioned taste aversion (CTA), have demonstrated that tetanic stimulation of the basolateral nucleus of the amygdala (Bla) induce an N-methyl-d-aspartate (NMDA) dependent LTP in the IC of adult rats in vivo. Here we present experimental data showing that intracortical administration of the NMDA receptor competitive antagonist CPP (-3(-2 carboxipiperazin-4-yl)-propyl-1-phosphonic acid) disrupts the acquisition of conditioned taste aversion, as well as, the IC-LTP induction in vivo. These findings are of particular interest since they provide support for the view that the neural mechanisms underlying NMDA dependent neocortical LTP, constitute a possible mechanism for the learning related functions performed by the IC.

Involvement of NMDA and non-NMDA receptors in the neuronal responses of the primary motor cortex to input from the supplementary motor area and somatosensory cortex: studies of task-performing monkeys

The involvement of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in mediating the excitatory responses of neurons in the primary motor cortex (MI) to electrical stimulation of the supplementary motor area (SMA) and the somatosensory cortex (SI) was examined in monkeys performing a trained motor task. During the task, a total of 109 MI neurons were identified and classified as movement related (91), motor set related (7), or mixed (11). Subsequently, the influence of receptor antagonists on the stimulus-evoked and task-related activities of these neurons was examined. The selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (APV) and the selective non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were applied iontophoretically through multibarreled micropipettes. One barrel was used for extracellular unit recording. The excitatory response evoked by SI stimulation was suppressed by CNQX in the vast majority (83%) of the motor task related neurons, and only 10% were suppressed by APV. On the other hand, the response evoked by SMA stimulation was suppressed by APV in 56% of the neurons and by CNQX in 54%. APV and CNQX had parallel effects on the stimulus-evoked responses and the task-related neuronal activity. These results indicate that NMDA and non-NMDA receptors are both involved in mediating the excitatory responses of MI neurons to input from the SMA and SI. On the other hand, the data suggest a greater contribution of non-NMDA receptors in response to SI input and greater involvement of NMDA receptors in mediating the response to SMA input, especially among set-related MI neurons.

Role of NMDA receptors in the propagation of excitation in rat visual cortex as studied by optical imaging

To examine the role of the N-methyl-D-aspartate (NMDA) type of glutamate receptors in the propagation of information in visual cortex, optical imaging with high spatial and temporal resolution of neuronal activity was used in cortical slices of rats. Single-shock stimulation of the white matter elicited a vertical propagation of excitation toward the cortical surface simultaneously with a horizontal spread of excitation in lower layers. The horizontal spread in upper layers occurred subsequent to the vertical spread reaching these layers. The results from perfusion of Ca2+-free medium and application of an antagonist of non-NMDA receptors indicated that this intracortical propagation of signals is due mostly, if not exclusively, to the postsynaptic excitation of cortical neurons. Blockade of NMDA receptors attenuated the rising and peak phases of the upper horizontal spread, but did not affect those of the lower horizontal or vertical propagation of excitation. Perfusion with Mg2+-free solution enhanced the upper horizontal spread, but in most cases did not significantly change the spread of excitation in the other pathways. These results indicate that NMDA receptors are involved in the flow of information in the upper layers of visual cortex, and further suggest that this propagation of activity is mediated mainly by horizontal connections intrinsic to the upper layers.

In vivo microdialysis study of (+/-)-kavain on veratridine-induced glutamate release

This is the first microdialysis study to address the effects of (+/-)-kavain on veratridine-induced glutamate release in freely moving rats. (+/-)-Kavain (100 mg/kg, p.o.) significantly reduced veratridine-induced glutamate release compared with that of vehicle-treated controls. Maximum extracellular glutamate levels were obtained 20-40 min after veratridine stimulation (500 microM, added to the perfusate). In the control group the increase was 301% and in the (+/-)-kavain group the increase was significantly reduced to 219% (the basal value was 100%). These results demonstrate that (+/-)-kavain reduces veratridine-induced glutamate release in vivo, which confirms previous in vitro data.

Neurochemical organization of the macaque striate cortex: correlation of cytochrome oxidase with Na+K+ATPase, NADPH-diaphorase, nitric oxide synthase, and N-methyl-D-aspartate receptor subunit 1

Previously, we found that cytochrome oxidase-rich zones in the supragranular layers of the macaque striate cortex had more asymmetric, glutamate-immunoreactive synapses than the surrounding, cytochrome oxidase-poor regions. A major glutamate receptor family is N-methyl-D-aspartate, which is implicated in the stimulation of nitric oxide synthase and in the production of nitric oxide, a gaseous intra- and inter-cellular messenger. To determine if energy-generating and energy-utilizing enzymes bore any spatial relationship with neurochemicals associated with glutamatergic neurotransmission in the monkey visual cortex, serial cortical sections were processed histochemically for cytochrome oxidase and NADPH-diaphorase, and immunohistochemically for sodium/potassium-ATPase, nitric oxide synthase, and N-methyl-D-aspartate receptor subunit 1 protein, respectively. The general patterns were similar among the five neurochemicals, with layers 4C, 6 and supragranular puffs being labelled, although the intensity of labelling differed among them. Monocular impulse blockade with tetrodotoxin for two to four weeks induced a down-regulation of all five neurochemicals not only in deprived layer 4C ocular dominance columns, but also in deprived rows of puffs. Thus, the regulation of all five neurochemicals in the mature visual cortex is activity-dependent. Combined cytochrome oxidase histochemistry and nitric oxide synthase immunohistochemistry in the same sections revealed that double-labelled cells were primarily medium-sized non-pyramidals in various cortical layers. Likewise, those that were double-labelled by N-methyl-D-aspartate receptor subunit 1 immunohistochemistry and nitric oxide synthase immunogold silver staining in the same sections were of the medium-sized non-pyramidal neurons. At the ultrastructural level, combined cytochrome oxidase cytochemistry and postembedding immunogold labelling for nitric oxide synthase showed that immunogold particles for nitric oxide synthase were more heavily concentrated in cytochrome oxidase-rich type C cells. These medium-sized non-pyramidal cells were previously found to be gamma aminobutyric acid-immunoreactive and received both gamma aminobutyric acid- and glutamate-immunoreactive axosomatic synapses. Thus, our results are consistent with an enrichment of excitatory synaptic interactions in metabolically active regions of the primate visual cortex that involves glutamate-related neurochemicals, such as N-methyl-D-aspartate receptors and nitric oxide synthase. These interactions impose a higher energy demand under normal conditions and are down-regulated by retinal impulse blockade.

Simultaneous determination of total and extracellular concentrations of the amino acid neurotransmitters in cat visual cortex by microbore liquid chromatography and electrochemical detection

To investigate the influence of a partial sensory deprivation on the total and extracellular concentration of the amino acid neurotransmitters in cat visual cortex, two microbore HPLC methods were developed for the simultaneous determination of aspartate, glutamate, glycine, taurine and gamma-aminobutyric acid in cat brain extracts or microdialysis samples. For the determination of the total neurotransmitter concentrations in the visual cortex, the brains were quickly frozen and 200-microns cryostat sections were made. From these sections tissue samples of 2 x 2 mm2 containing the six cortical layers were dissected out of the central and peripheral parts of area 17. After homogenisation and centrifugation, the supernatants were used for quantitative amino acid analysis using an o-phthalaldehyde-tert.-butylthiol pre-column derivatisation HPLC gradient elution method on a microbore column (100 x 1 mm I.D.; C8) and single electrochemical detection. Microdialysis samples from area 17 were obtained every 15 min using 2-mm probes perfused with synthetic cerebrospinal fluid at a flow-rate of 1 microliter/min. After o-phthalaldehyde-tert.-butylthiol derivatisation they were analysed on a microbore column by isocratic elution and dual electrochemical detection. The instrumentation and the different separation parameters were optimised and standard curve, recovery, analytical precision and detection limits for each neurotransmitter were determined.

In vivo long-term potentiation in the insular cortex: NMDA receptor dependence

It has been demonstrated that the insular cortex (IC) plays an important role in the acquisition and storage of different aversive motivated learning tasks like conditioned taste aversion, spatial maze and inhibitory avoidance. It is of particular interest to investigate whether activity-dependent modification of synaptic efficacy, a presumptive mechanism for learning and memory, is present in this cortical region. Here, we address this issue by examining the induction of synaptic plasticity, long-term potentiation (LTP) in in vivo preparations. The results showed that high frequency stimulation of the basolateral amygdaloid nucleus (Bla) induced LTP in the IC. The LTP induced by tetanus was blocked by application of the N-methyl-D-aspartate (NMDA) receptor antagonists CPP and MK-801, indicating that NMDA receptors were responsible for its induction. These results suggest that in vivo tetanus induced LTP of the Bla-IC projection is a possible mechanism for the memory-related functions performed by the IC.

Cocaine and amphetamine preferentially stimulate glutamate release in the limbic system: studies on the involvement of dopamine

The effects of cocaine and d-amphetamine on extracellular glutamate and aspartate levels in the nucleus accumbens, prefrontal cortex, and striatum were studied by in vivo microdialysis in awake, freely moving rats. In the nucleus accumbens, glutamate levels were stimulated by cocaine (15-30 mg/kg, i.p.), GBR 12909 (15 mg/kg, i.p.), and d-amphetamine (2 mg/kg, i.p.), while aspartate levels were not affected. The increase in nucleus accumbens glutamate levels following cocaine (30 mg/kg) was calcium-dependent and was blocked by pretreatment with dopamine antagonists; haloperidol (0.2 mg/kg, i.p.), SCH 23390 (0.02 mg/kg, i.p.), and raclopride (1 mg/kg, i.p.), as well as local 6-OHDA lesions of the nucleus accumbens. In the prefrontal cortex, glutamate levels were stimulated by both cocaine (15-30 mg/kg, i.p.) and d-amphetamine (2 mg/kg, i.p.), while aspartate levels were moderately stimulated by d-amphetamine only. The increase in prefrontal cortex glutamate levels following cocaine (30 mg/kg) was calcium-dependent and was blocked by pretreatment with SCH 23390 (0.02 mg/kg, i.p.), but not haloperidol (0.2 mg/kg, i.p.) or raclopride (1 mg/kg, i.p.). In the striatum, glutamate and aspartate levels were not affected by either cocaine (15-30 mg/kg, i.p.) or d-amphetamine (2 mg/kg, i.p.). These findings demonstrate that stimulants enhance glutamate release in limbic brain structures, nucleus accumbens, and prefrontal cortex, but not extrapyamidal brain structures, striatum. Furthermore, the increase in glutamate release in the nucleus accumbens may be mediated by dopamine.

Intracerebral microdialysis study of glutamate reuptake in awake, behaving rats

The central nervous system has high-affinity uptake systems for the clearance of amino acid transmitters. These systems are found in both neurons and astrocytes. Previous studies have shown that the uptake of amino acid transmitters by astrocytes in culture can be modulated by adrenergic agents. The objectives of this study were to develop a methodology that evaluates the brain's reuptake capacity for glutamate in awake, behaving animals and to determine whether glutamate reuptake is under alpha-adrenergic regulation in the intact central nervous system. Male Sprague-Dawley rats weighing 250-450 g were used in this study. The extraction fraction of L-[3H]glutamate with [14C]mannitol as a reference was measured. The cortical extraction fraction of L-[3H]glutamate corrected for [14C]mannitol (EL-glu) reaches steady state rapidly and is both stable and repeatable. EL-glu is a measure of L-glutamate reuptake and not metabolism. EL-glu is decreased in a dose-dependent manner by the addition of the glutamate reuptake blocker D,L-threo-beta-hydroxyaspartic acid or unlabeled L- glutamate. In addition, EL-glu is increased in a dose-dependent manner by the alpha1-adrenergic agonist phenylephrine, and this increase is blocked by the alpha-adrenergic antagonist phentolamine.

The somatosensory cortex of human: cytoarchitecture and regional distributions of receptor-binding sites

The aim of this study is to characterize the regional and laminar distribution patterns of various neurotransmitter binding sites in areas 3a, 3b, 1, and 2 of the human primary somatosensory cortex, and to compare these receptor-based "maps" with the cytoarchitectonic parcelation. Cryostat sections from a dorsomedial region of the postcentral gyrus close to the interhemispheric fissure and from a ventrolateral region close to the Sylvian fissure were examined. Neurotransmitter-binding sites were analyzed with quantitative in vitro receptor autoradiography. Different muscarinic-binding sites were labeled with [3H]pirenzepine and [3H]oxotremorine-M, noradrenergic-binding sites with [3H]prazosin, different serotoninergic-binding sites with [3H]5-hydroxytryptamine and [3H]ketanserine, glutamate-binding sites with l-[3H]glutamate, and GABA-binding sites with [3H]muscimol. Adjacent sections were stained with a modified Nissl method for cytoarchitectonic analysis. The binding sites either were preferentially localized in the superficial layers ([3H]5-hydroxytryptamine, [3H]prazosin, l-[3H]glutamate, [3H]muscimol, and [3H]pirenzepine) or were more homogeneously distributed with highest densities in layers III-V ([3H]oxotremorine-M and [3H]ketanserine). Changes in the distribution patterns of [3H]oxotremorine-M- and [3H]ketanserine-binding sites precisely matched the borders between areas 4/3a, 3b/1, and 1/2, as defined cytoarchitectonically. In addition, the autoradiographs showed that area 1 possibly consists of two subregions which cannot be distinguished cytoarchitectonically. The results demonstrate that the regional and laminar distribution patterns of some, but not all, transmitter-binding sites are precisely correlated with the cytoarchitectonic parcelation of the human primary somatosensory cortex. In addition, binding sites may reveal new borders not detectable in Nissl-stained sections. Finally, the human primary somatosensory cortex differs clearly from the primary motor cortex due to higher densities of l-[3H]glutamate-, [3H]muscimol-, [3H]pirenzepine-, [3H]oxotremorine-M-, and [3H]ketanserine-binding sites.

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

Muscarinic reduction of GABAergic synaptic potentials results in disinhibition of the AMPA/kainate-mediated EPSP in auditory cortex

The present study is concerned with the ability of muscarinic actions of acetylcholine (ACh) to modulate glutamate and gamma-aminobutyric acid (GABA)-mediated synaptic transmission in the in vitro rat auditory cortex. Whole-cell patch clamp recordings were obtained from layer II-III pyramidal neurons, and the fast-EPSP (AMPA/kainate), fast-IPSP (GABA(A)), and slow-IPSP (GABA(B)), were elicited following a stimulus to deep gray/white matter. Acetyl-beta-methylcholine (MCh), a muscarinic receptor agonist, applied by either superfusion or iontophoresis, produced an atropine-sensitive increase or decrease in the amplitude of the fast-EPSP. The effect of MCh could be predicted by the response of the fast-EPSP to paired-pulse stimulation (i.e. a conditioning pulse followed 300 ms later by a test pulse). The fast-EPSP was decreased in amplitude by MCh in cases where the test-EPSP was suppressed in the pre-MCh condition, and increased in amplitude when the test-EPSP was facilitated. The fast- and slow-IPSPs were always reduced by MCh. In several experiments, the strength of synaptic inhibition was systematically modified by varying stimulus intensity. When the fast-EPSP was elicited in the absence of IPSPs, it was decreased in amplitude by MCh. However, when the fast-EPSP was elicited in conjunction with large IPSPs it was increased in amplitude during MCh. Because the magnitude of the fast-EPSP is influenced by the degree of temporal overlap with IPSPs, it was hypothesized that enhancement of the fast-EPSP was the result of disinhibition produced as a consequence of muscarinic reduction of GABAergic IPSPs. This view was supported by the finding that MCh could reduce the amplitude of pharmacologically isolated GABAergic IPSPs (i.e. elicited in the absence of glutamatergic transmission). Our results suggest that ACh at muscarinic receptors can modify fast glutamatergic neurotransmission differently as a function of strength of inhibition, to suppress that produced by 'weak' inputs and enhance that produced by 'strong' inputs.