Glutamate and Aspartate as Transmitter Candidates for Systems of the Cerebral Cortex

An in silico agent-based model demonstrates Reelin function in directing lamination of neurons during cortical development

The characteristic six-layered appearance of the neocortex arises from the correct positioning of pyramidal neurons during development and alterations in this process can cause intellectual disabilities and developmental delay. Malformations in cortical development arise when neurons either fail to migrate properly from the germinal zones or fail to cease migration in the correct laminar position within the cortical plate. The Reelin signalling pathway is vital for correct neuronal positioning as loss of Reelin leads to a partially inverted cortex. The precise biological function of Reelin remains controversial and debate surrounds its role as a chemoattractant or stop signal for migrating neurons. To investigate this further we developed an in silico agent-based model of cortical layer formation. Using this model we tested four biologically plausible hypotheses for neuron motility and four biologically plausible hypotheses for the loss of neuron motility (conversion from migration). A matrix of 16 combinations of motility and conversion rules was applied against the known structure of mouse cortical layers in the wild-type cortex, the Reelin-null mutant, the Dab1-null mutant and a conditional Dab1 mutant. Using this approach, many combinations of motility and conversion mechanisms can be rejected. For example, the model does not support Reelin acting as a repelling or as a stopping signal. In contrast, the study lends very strong support to the notion that the glycoprotein Reelin acts as a chemoattractant for neurons. Furthermore, the most viable proposition for the conversion mechanism is one in which conversion is affected by a motile neuron sensing in the near vicinity neurons that have already converted. Therefore, this model helps elucidate the function of Reelin during neuronal migration and cortical development.

Bigenomic regulation of cytochrome c oxidase in neurons and the tight coupling between neuronal activity and energy metabolism

Cytochrome c oxidase is the terminal enzyme of the mitochondrial electron transport chain, without which oxidative metabolism cannot be carried to completion. It is one of only four unique, bigenomic proteins in mammalian cells. The holoenzyme is made up of three mitochondrial-encoded and ten nuclear-encoded subunits in a 1:1 stoichiometry. The ten nuclear subunit genes are located in nine different chromosomes. The coordinated regulation of such a multisubunit, multichromosomal, bigenomic enzyme poses a challenge. It is especially so for neurons, whose mitochondria are widely distributed in extensive dendritic and axonal processes, resulting in the separation of the mitochondrial from the nuclear genome by great distances. Neuronal activity dictates COX activity that reflects protein amount, which, in turn, is regulated at the transcriptional level. All 13 COX transcripts are up- and downregulated by neuronal activity. The ten nuclear COX transcripts and those for Tfam and Tfbms important for mitochondrial COX transcripts are transcribed in the same transcription factory. Bigenomic regulation of all 13 transcripts is mediated by nuclear respiratory factors 1 and 2 (NRF-1 and NRF-2). NRF-1, in addition, also regulates critical neurochemicals of glutamatergic synaptic transmission, thereby ensuring the tight coupling of energy metabolism and neuronal activity at the molecular level in neurons.

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.

Relation Between Putative Transmitter Phenotypes and Connectivity of Subplate Neurons During Cerebral Cortical Development

During development, the earliest generated neurons of the mammalian telencephalon reside in a region of the white matter, the subplate, just beneath the cortical plate. Neurons in the subplate are only transiently present in the telencephalon: shortly after birth in the cat the majority have disappeared. During their brief life, however, subplate neurons mature; they extend long-distance and local projections, and express immunoreactivity for GABA and several neuropeptides. In the present study we examined the relation between possible transmitter phenotypes of subplate neurons and their connectivity. To do so, we used a double-label technique in which immunohistochemistry for neuropeptide Y (NPY), somatostatin (SRIF) or calbindin (CaBP) was combined with retrograde tracing. Experiments were performed in neonatal cats and in ferret kits at equivalent postconceptional ages, times when subplate neurons are numerous. Subplate neurons immunoreactive for neuropeptides and CaBP could be double-labelled by an injection of retrograde tracer either into the cortical plate or the white matter, indicating that this particular subset of subplate neurons can make local circuit projections. In contrast, peptide or CaBP immunoreactive subplate neurons could never be retrogradely labelled from a tracer injection into the thalamus. Taken together, these observations indicate that subplate neurons immunoreactive for NPY, SRIF and CaBP are likely to be interneurons exclusively. On the other hand, subplate neurons with long-distance projections to the thalamus or the contralateral hemisphere could be labelled by the retrograde transport of d-[3H]aspartate, suggesting that at least some projection subplate neurons might use an excitatory amino acid as a neurotransmitter. These results indicate that there is a defined relationship between the putative transmitter phenotypes of subplate neurons and their patterns of projection. Interneurons of the subplate express peptidergic properties while projection neurons to the thalamus may use an excitatory amino acid. Thus, these basic organizational features of the transient subplate are reminiscent of those found in the adult cortical layers.

Chandelier cells and epilepsy

The main goal of this article is to review certain aspects of the circuitry of the human cerebral cortex that may be particularly relevant for the development, maintenance or spread of seizures. There are a number of different structural abnormalities that are commonly found in the cortex of epileptic patients, but these abnormalities do not appear to be intrinsically epileptogenic, since some patients displaying them are epileptic (after variable delays) whereas others are not. Therefore, cortical circuits in an affected brain may undergo a series of changes that finally cause epilepsy. In this article, it is proposed that the chandelier cell, which is considered to be the most powerful cortical GABAergic inhibitory interneuron, is probably a key component of cortical circuits in the establishment of human intractable temporal lobe epilepsy. These cells (among other types) have been found to be lost or reduced at epileptic foci in both experimental animals and epileptic patients. A hypothesis is presented by which the normal variability in the number of interneurons might explain the predisposition of some individuals to develop epilepsy more than others as a result of a lesion or other precipitating factors that lead to loss of neurons. The sources of GABAergic input on dendrites and somata of cortical pyramidal cells originate from many and diverse types of interneurons but, at the level of the axon initial segment of these cells, all synapses come from a few chandelier cells (five or less). Loss of one class of interneurons ending on soma and dendrites might have relatively little impact on the inhibitory control of the pyramidal cell. However, if chandelier cells were affected, it would have serious consequences for the inhibitory control of the pyramidal cells. Evidence suggests that the loss of chandelier cells may be non-specific and that when this occurs epilepsy may develop. Therefore, these cells might represent a key component in the aetiology of human temporal lobe epilepsy.

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.

Cellular and subcellular localization of the 2B-subunit of the NMDA receptor in the adult rat telencephalon

NMDA receptors (NR) are encoded by a family of genes including those of the NR1 and NR2A-D subunits. In situ hybridization has revealed that NR1, comprising eight splice variants, is ubiquitously expressed in the central nervous system (CNS) while the expression of NR2 isoforms is restricted to particular CNS regions. We report on the cellular and ultrastructural distribution of the NR2B polypeptide in rat telencephalon. In the telencephalon, the hippocampus represented the most intensively immunolabeled region. Here, predominantly the CA pyramidal neurons were heavily stained. Intense immunoreactivity (IR) was also detected in cortical neurons, in particular in pyramidal-like ones of layers II/III and V. On the ultrastructural level, the NR2B subunit was present not only in synaptic complexes where it usually was present in postsynaptic sites but in addition could be located at extrasynaptic sites. Furthermore, preliminary evidence indicates a presynaptic location of NR2B in some rare cases. NR2B antigen distribution is consistent with that of corresponding transcripts. Indeed, NR2B immunoreactivity coincides largely with that for NR1, indicating that both subunits are coexpressed in numerous cortical and hippocampal neurons.

Reinforcement learning by Hebbian synapses with adaptive thresholds

A central problem in learning theory is how the vertebrate brain processes reinforcing stimuli in order to master complex sensorimotor tasks. This problem belongs to the domain of supervised learning, in which errors in the response of a neural network serve as the basis for modification of synaptic connectivity in the network and thereby train it on a computational task. The model presented here shows how a reinforcing feedback can modify synapses in a neuronal network according to the principles of Hebbian learning. The reinforcing feedback steers synapses towards long-term potentiation or depression by critically influencing the rise in postsynaptic calcium, in accordance with findings on synaptic plasticity in mammalian brain. An important feature of the model is the dependence of modification thresholds on the previous history of reinforcing feedback processed by the network. The learning algorithm trained networks successfully on a task in which a population vector in the motor output was required to match a sensory stimulus vector presented shortly before. In another task, networks were trained to compute coordinate transformations by combining different visual inputs. The model continued to behave well when simplified units were replaced by single-compartment neurons equipped with several conductances and operating in continuous time. This novel form of reinforcement learning incorporates essential properties of Hebbian synaptic plasticity and thereby shows that supervised learning can be accomplished by a learning rule similar to those used in physiologically plausible models of unsupervised learning. The model can be crudely correlated to the anatomy and electrophysiology of the amygdala, prefrontal and cingulate cortex and has predictive implications for further experiments on synaptic plasticity and learning processes mediated by these areas.

Light microscopic comparison of the patterns of glutamate-like and homocysteate-like immunoreactivities in rat dorsal lateral geniculate after combined visual cortical and retinal ablations

To study the contribution of retinal and cortical afferents to the patterns of glutamate- and homocysteate-like immunoreactivities in dorsal lateral geniculate, combined retinal and cortical ablations were performed in rats. In controls, glutamate immunoreactivity was in terminal-like dots and neurons. Homocysteate immunoreactivity was in small puncta. In lesioned animals, most glutamate-immunoreactive dots disappeared. In contrast, abundant puncta resembling parts of glial cells were immunoreactive for homocysteate.

A two-process theory of schizophrenia: evidence from studies in post-mortem brain

Glutamate and GABA are the principle neurotransmitters of the cerebral cortex and are known to modulate dopaminergic function. Evidence of structural abnormalities in the cortex raises the possibility that schizophrenia involves disturbances of cortical amino-acid neurotransmission. The psychotomimetic effects of phencyclidine, a glutamate antagonist, have been taken to suggest that schizophrenia involves reduced brain glutamate function. Direct evidence for diminished glutamate function in schizophrenia is lacking. However, in polar temporal cortex and hippocampus we reported evidence of an asymmetric loss of glutamate terminals, and of reduced GABA function, which may be secondary to the loss of glutamatergic input. Glutamate cell body markers are spared in temporal lobe; the neurones which degenerate may originate in frontal cortex. A number of studies have reported increases in markers of glutamatergic cell bodies and terminals in orbital frontal cortex in schizophrenia. These findings are consistent with the presence of an abnormally abundant glutamatergic innervation, which may be the result of an arrest in the normal process of cellular and synaptic elimination which occurs during development. There is evidence that frontal abnormalities in schizophrenia are genetically determined. We suggest that glutamatergic abnormalities in anterior temporal cortex in schizophrenia are the result of the degeneration of fronto-temporal projections. Orbital frontal projections to polar temporal cortex may be prone to degeneration because they arise from an unstable frontal cortical cytoarchitecture which has not completed the normal process of post-natal remodelling. The structural abnormality of the orbital frontal region may confer vulnerability to some intrinsic or extrinsic mechanism, which brings about a progressive degeneration of projections to polar temporal lobe.

Fos induction in subtypes of cerebrocortical neurons following single picrotoxin-induced seizures

In adult rats single seizures of varying behavioural severities were caused by slow, systemic infusion of picrotoxin, an antagonist of the C1- channel at the GABAA receptor. We used a double labelling immunohistochemical method to define the subclasses of neurons that contained Fos protein following seizures. In four cortical regions (piriform, entorhinal, motor and sensory) neuronal subclasses were defined with antibodies against the calcium-binding proteins D-28K, parvalbumin and calretinin (aspiny neurons), and neurofilament protein (spiny neurons). The remaining spiny neuron population was estimated by subtraction of defined subclasses from total neuronal numbers determined from Nissl stain. After seizures, most of the calbindin D-28K immunoreactive interneurons (> 80%) and many of the unlabelled spiny neurons (60-80%) were FOs positive. Co-localisation of Fos was found in about 30% of parvalbumin, calretinin and neurofilament protein immunoreactive neurons. Paradoxically, mild seizures were associated with induction of Fos in up to 80% of cortical cells and more severe seizures with 60%, the difference being due to different levels of Fos induction in spiny neurons. These results also demonstrate that seizures induce Fos predominantly in excitatory cortical neurons.

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.

Changes in glutamate immunoreactivity in the somatic sensory cortex of adult monkeys induced by nerve cuts

Antibodies to glutamate (Glu) were used to study the effects of reduced afferent input on excitatory neurons in the somatic sensory cortex of adult monkeys. In each monkey, immunocytochemical staining was compared to thionin and cytochrome oxidase (CO) staining in adjacent sections. In the cervical spinal cord, dorsal column nuclei, ventroposterior thalamus, and primary somatic sensory cortex (SI), Glu immunoreactivity (Glu-ir) was analogous to that described in normal animals; regions with reduced or absent Glu-ir were never observed and no appreciable differences were noted between the experimental and normal side. There were also no differences in CO or thionin-stained sections from the affected hemisphere. In the insuloparietal operculum, sections in the hemisphere contralateral to the nerve cut showed that most cortical fields had a normal pattern of Glu-ir (pattern a), some exhibited a reduction of Glu-ir (pattern b), and that in the central portion of the upper bank of the central sulcus, which corresponds to the general location of the hand representation of the second somatic sensory cortex (SII), Glu-ir had virtually disappeared (pattern c). Adjacent sections processed for CO or stained with thionin showed that in the regions corresponding to those characterized by pattern c, CO was slightly decreased and that glial cells had increased in number. In the regions of SII characterized by pattern c, small intensely stained glial cells displayed Glu-ir. These findings indicate that Glu-ir is regulated by afferent activity and suggest that changes in Glu levels in neurons as well as in glial cells may trigger the biochemical processes underlying the functional and structural changes occurring during a slow phase of reorganizational plasticity in the cerebral cortex of adult monkeys.

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

The specification of neuronal fate: a common precursor for neurotransmitter subtypes in the rat cerebral cortex in vitro

Neurotransmitter choice is a crucial step in neural development. In the cerebral cortex, pyramidal neurons use the excitatory neurotransmitter glutamate, whereas non-pyramidal cells use the inhibitory neurotransmitter GABA. We are interested in how these two neuronal types are generated. We labelled precursor cells from embryonic rat cerebral cortex with a retroviral vector in dissociated cell cultures, and examined the neurotransmitter phenotype of their progeny immunohistochemically after 2 weeks in vitro. We discovered, first, that precursor cells in culture generate glutamatergic and GABAergic neurons in proportions similar to those in vivo. Second, we found that neuronal precursor cells gave rise to both GABAergic and glutamatergic neurons. These results suggest that neuronal precursor cells in the cerebral cortex have the potential to generate both neuronal subtypes. Moreover, these data are consistent with a stochastic model of neurotransmitter specification.

Getting there and being there in the cerebral cortex

The mammalian neocortex is composed of functional areas that are specified to process particular aspects of information. How is this specification achieved during development? Since cells migrate to their final positions in the developing nervous system, a central issue is the relation between cellular migration and positional information. This review combines evidence for early positional specification in the developing cortex with evidence for cellular dispersion during migration. A model is suggested whereby stable cues provide positional information and minorities of 'displaced' cells are respecified accordingly. Comparison with other parts of the CNS reveals that cellular dispersal is ubiquitous and has to be included in any mechanism relaying positional specification. Ontogenetic and phylogenetic considerations suggest that radial glial cells might provide the positional information in the developing nervous system.

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.

Differentiation of transmitter phenotypes in rat cerebral cortex

Cortical neurons differ in their neurochemical properties. Projection neurons use excitatory amino acids as transmitters, most local interneurons contain the inhibitory transmitter GABA, and specific subtypes of local circuit neurons express distinct neuropeptides. How this cellular diversity is generated during development is not known. We have been studying the transmitter differentiation of cortical neurons in different in vitro systems using immunohistochemical techniques. Transmitter phenotypes of cortical neurons were examined in slice cultures, i.e. in the absence of extrinsic cortical connections, and in dissociated cortical cell cultures, i.e. in the absence of extrinsic and intrinsic cortical connections. The expression of vasoactive intestinal polypeptide in cortical interneurons occurred normally in slice cultures prepared from neonatal rats between birth and 2 days of age, but was strongly impaired in dissociated cell cultures prepared at the same time. These results suggest that the intact cortical environment present in the slice cultures exerts crucial influences for neuropeptide differentiation. In contrast, the transmitters glutamate and GABA were expressed normally in the appropriate cell types and similar in proportions in dissociated cell cultures prepared from cortices at embryonic day 19. Only cells dissociated during S-phase failed to express glutamate and GABA in vitro. When cells were kept for 24 h after mitosis in a cortical slice preparation in vitro, however, they later expressed their appropriate transmitter phenotypes. Thus, signals from the local cortical environment that act early in the cell cycle are required for the specification of transmitter phenotypes of cortical neurons.

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.

Excitatory amino acids as neurotransmitters of cortical and cerebellar projections to the red nucleus: an immunocytochemical study in the guinea pig

We combined a retrograde labeling technique with peroxidase immunocytochemistry to verify whether cortical and cerebellar neurons projecting to the red nucleus (RN) contain high concentrations of glutamate and aspartate as possible neurotransmitters. Injections of a tracer, colloidal gold-labeled enzymatically inactive horseradish peroxidase conjugated to wheatgerm agglutinin, into the RN of adult guinea pigs produced retrograde labeling of layer V cortical neurons, with a large predominance in the ipsilateral hemisphere. Corticorubral neurons were located in the granular parietal cortex (Gr), agranular frontal cortex (Ag), agranular cingulate cortex (Cg), and retrobulbar cortex (Rb). Large numbers of retrogradely labeled neurons were concentrated in contralateral interpositus and dentate cerebellar nuclei. We found the majority of corticorubral neurons to be immunostained by antibodies raised in rabbits against glutamate or aspartate conjugated to invertebrate hemocyanin by glutaraldehyde, supporting the hypothesis that excitatory amino acids are neurotransmitters of corticorubral projections. With either antiserum, immunostaining was found in 58-72% of corticorubral neurons in Ag and Gr; higher percentages were observed in Rb (80-85%) and Cg (up to 96%). Cross-sectional area measurements indicated that the perikarya of corticorubral neurons were larger in Ag and Gr than in Rb and Cg; in each area, soma size values of immunopositive corticorubral neurons tended to be larger than those of immunonegative ones. In the cerebellar nuclei, virtually all retrogradely labeled neurons were immunostained by glutamate and aspartate antisera, suggesting that excitatory amino acids might also be considered as possible neurotransmitters for cerebellorubral projections.

A selective reduction of excitatory amino acids in cerebrospinal fluid of patients with Alzheimer type dementia compared with vascular dementia of the Binswanger type

We determined the concentrations of the putative transmitter amino acids in the cerebrospinal fluid of patients with Alzheimer type dementia (ATD) and vascular dementia of the Binswanger type (VDBT). In ATD, aspartate and glutamate concentrations were significantly and selectively reduced, while in VDBT, concentrations of aspartate, glutamate, gamma-aminobutyric acid (GABA) and many other amino acids were decreased non-selectively. In both ATD and VDBT, we found a tendency for all amino acids to increase with progression of the disease, and this reached statistical significance for some amino acids.

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Proportion of glutamate- and aspartate-immunoreactive neurons in the efferent pathways of the rat visual cortex varies according to the target

Immunohistochemistry, with antisera directed against glutamate (Glu) or aspartate (Asp), was combined with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) histochemistry to examine the distribution, morphology, and proportions of Glu- and Asp-containing neurons that give rise to corticofugal and callosal projections of the rat visual cortex. WGA-HRP injections in the dorsal lateral geniculate nucleus resulted in retrograde labelling of small and medium-sized cells throughout layer VI of the visual cortex. Of these cells, 60% were also Glu-immunoreactive and 61% Asp-positive. WGA-HRP injections in the superior colliculus labelled large and medium-sized neurons in the upper portion of layer V of the visual cortex. Of these cells, 46% were also stained for Glu and 66% for Asp. Injections in the pontine nuclei resulted in retrograde labelling of cells in the deeper part of cortical layer V. Retrogradely labelled cells, which were also immunoreactive for Glu or Asp, were large pyramidal cells. Corticopontine neurons, which were also Glu-positive, accounted for 42% of the total number of WGA-HRP labelled cells, whilst for Asp-positive neurons this percentage was 51%. Finally, after injections in the visual cortex, retrogradely labelled small and medium-sized cells were found throughout layers II-VI in the contralateral visual cortex. Of these neurons, 38% were also labelled for Glu while 49% were also Asp-immunoreactive. The present results demonstrate that substantial proportions of projection neurons in the rat visual cortex are immunoreactive for Glu or Asp, suggesting that these excitatory amino acids are the major transmitters used by the cortical efferent systems examined. Furthermore, the proportions of these immunoreactive neurons in the efferent pathways vary according to the target.

Appearance of putative amino acid neurotransmitters during differentiation of neurons in embryonic turtle cerebral cortex

Pyramidal and nonpyramidal neurons can be recognized early in the development of the cerebral cortex in both reptiles and mammals, and the neurotransmitters likely utilized by these cells, glutamate and gamma-aminobutyric acid, or GABA, have been suggested to play critical developmental roles. Information concerning the timing and topography of neurotransmitter synthesis by specific classes of cortical neurons is important for understanding developmental roles of neurotransmitters and for identifying potential zones of neurotransmitter action in the developing brain. We therefore analyzed the appearance of GABA and glutamate in the cerebral cortex of embryonic turtles using polyclonal antisera raised against GABA and glutamate. Neuronal subtypes become immunoreactive for the putative amino acid neurotransmitters GABA and glutamate early in the embryonic development of turtle cerebral cortex, with nonpyramidal cells immunoreactive for GABA and pyramidal cells immunoreactive for glutamate. The results of controls strongly suggest that the immunocytochemical staining in tissue sections by the GABA and glutamate antisera corresponds to fixed endogenous GABA and glutamate. Horizontally oriented cells in the early marginal zone (stages 15-16) that are GABA-immunoreactive (GABA-IR) resemble nonpyramidal cells in morphology and distribution. GABA-IR neurons exhibit increasingly diverse morphologies and become distributed in all cortical layers as the cortex matures. Glutamate-immunoreactive (Glu-IR) cells dominate the cellular layer throughout development and are also common in the subcellular layer at early stages, a distribution like that of pyramidal neurons and distinct from that of GABA-IR nonpyramidal cells. The early organization of embryonic turtle cortex in reptiles resembles that of embryonic mammalian cortex, and the immunocytochemical results underline several shared as well as distinguishing features. Early GABA-IR nonpyramidal cells flank the developing cortical plate, composed primarily of pyramidal cells, shown here to be Glu-IR. The earliest GABA-IR cells in turtles likely correspond to Cajal-Retzius cells, a ubiquitous and precocious cell type in vertebrate cortex. Glutamate-IR projection neurons in vertebrates may also be related. The distinctly different topographies of GABA and glutamate containing cells in reptiles and mammals indicate that even if the basic amino acid transmitter-containing cell types are conserved in higher vertebrates, the local interactions mediated by these transmitters may differ. The potential role of GABA and glutamate in nonsynaptic interactions early in cortical development is reinforced by the precocious expression of these neurotransmitters in turtles, well before they are required for synaptic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of dopamine and noradrenaline transport in synaptosomes from cerebellum, striatum and frontal cortex of normal and reeler mice

Recent evidence indicates that the cerebellum has a dopaminergic system. In order to elucidate further the dopaminergic system in the cerebellum, we investigated the transport of dopamine (DA) in synaptosomal preparations of normal and reeler mice. For comparative purposes we also studied DA transport in synaptosomal preparations from striatum and frontal cortex and compared DA transport to noradrenaline (NA) transport. [3H]-DA transport into cerebellar synaptosomes was found to be a Na(+)-dependent, two component system--a high affinity, low capacity and a low affinity, high capacity. In striatum [3H]-DA is transported by a similar high but different low affinity component. Maximal velocities of both transport components in the striatum were higher than the corresponding ones in the cerebellum. In the frontal cortex we also observed two [3H]-DA transport components with affinities significantly lower than those in cerebellum and striatum. [3H]-NA transport into synaptosomes, prepared from the three brain regions studied, showed two transport components with similar Kt and Vmax values, except for the high affinity component in striatum whose affinity is lower. In reeler mice [3H]-DA transport was different from normal only in the cerebellum where the maximal velocity for both transport components was significantly higher (2x). In contrast, no significant difference was observed in the transport of [3H]-NA. The accumulated [3H]-DA from cerebellar slices was found to be releasable by K+ stimulation, in a Ca(++)-dependent manner, and most of the released radioactivity was in the form of [3H]-DA. These results indicate that in the cerebellum there is a low-density dopaminergic system which is distinct from the corresponding noradrenergic system.

Identification of mitochondrial and non-mitochondrial glutaminase within select neurons and glia of rat forebrain by electron microscopic immunocytochemistry

Antibodies against the mitochondrial enzyme glutaminase (EC 3.5.1.2), have been used in previous immunocytochemical studies to help identify glutamate-releasing neurons among all glutamate-containing neurons. The studies were based on the idea that glutaminase is enriched within the releasable "transmitter" pools of glutamate. However, evidence is also available to suggest that the enzyme does not occur exclusively within glutamate-releasing neurons. Thus we sought to determine whether glutaminase was immunocytochemically detectable within presynaptic terminals forming asymmetric (putatively excitatory) synapses or, alternatively, occurs in association with mitochondria throughout the cell. For this purpose, we examined the cellular and subcellular distribution of glutaminase- immunoreactivity in neocortical (visual and somatosensory) areas known to contain glutamatergic perikarya. This localization was compared with the distribution in striatal (caudate-putamen and nucleus accumbens) regions recognized to contain high densities of glutamatergic terminals but fewer, if any, glutamatergic perikarya. Glutaminase-immunoreactive perikarya were numerous within the infragranular laminae of neocortex (approximately 1 per 1,000 microns 2 tissue area) but sparse within the caudate-putamen nuclei and accumbens nuclei (less than 1 per 20,000 microns 2.). In addition, heterogeneous distribution of small (less than 1 microns) punctate immunoreactive structures was notable. Relatively high densities of these punctate structures occurred within the supragranular laminae of neocortex, dorsolateral quadrant of the caudate-putamen nuclei, and surrounding certain groups of myelinated fiber bundles throughout the striatum. Electron microscopy revealed diffusely distributed peroxidase immunoreactivity in a select population of dendritic spines, glial processes, and axons. Eight percent of all synapses within the supra-granular laminae were formed by terminals labeled for glutaminase. These principally formed asymmetric junctions on spiny processes. When tissue was incubated with the antibody in the presence of a permeabilizing agent, Photo-flo, high levels of glutaminase immunoreactivity was detectable by electron microscopy within select mitochondria of neocortical (4%) and striatal (8%) perikarya and dendrites, while the diffuse distribution of immunoreactivity within axons and glia was greatly diminished. The differential ultrastructural conditions provide direct demonstration that glutaminase in brain occurs in at least two forms discriminable by their diffuse distribution within non-mitochondrial cytoplasm versus discrete localization within mitochondria. The morphological characteristics of synapses formed by axons exhibiting diffuse distributions of glutaminase immunoreactivity are consistent with the idea that glutaminase-enriched terminals mediate excitatory chemical transmission via the release of glutamate. Because glia containing glutaminase occur juxtaposed to the asymmetric junctions, the glia may utilize neuronally released glutamate for energy metabolism.

Synapses made by axons of callosal projection neurons in mouse somatosensory cortex: emphasis on intrinsic connections

This is one of a series of papers aimed at identifying the synaptic output patterns of the local and distant projections of subgroups of pyramidal neurons. The subgroups are defined by the target site to which their main axon projects. Pyramidal neurons in areas 1 and 40 of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of these neurons ("intrinsic" terminals) were identified in somatosensory areas 1 and 40, and their distribution and synaptic connectivity were examined. Also examined were the synaptic connections of "extrinsic" callosal axon terminals labeled by lesion induced degeneration consequent to the severing of callosal fibers. A post-lesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by HRP. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where they formed only asymmetrical synapses. Layers II and III contained the highest concentrations of both types of callosal axon terminal. Analyses of serial thin sections through layers II and III in both areas 1 and 40 yielded similar results: 97% of the extrinsic (277 total sample) and of the intrinsic (1215 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal contrasts with the proportion of asymmetrical, axospinous synapses that occur in the surrounding neuropil where only about 80% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of both extrinsic and intrinsic axonal pathways in the cortex (White and Keller, 1989: Cortical Circuits; Boston: Birkhauser): in all instances, axonal pathways are highly selective for the types of elements with which they synapse.

Quantitative analysis of a vulnerable subset of pyramidal neurons in Alzheimer's disease: I. Superior frontal and inferior temporal cortex

Various cytoskeletal proteins have been implicated in the cellular pathology of Alzheimer's disease. A monoclonal antibody (SMI32) that recognizes nonphosphorylated epitopes on the medium (168 kDa) and heavy (200 kDa) subunits of neurofilament proteins has been used to label and analyze a specific subpopulation of pyramidal neurons in the prefrontal and inferior temporal cortices of normal and Alzheimer's disease brains. In Alzheimer's disease, the distribution of neuropathological markers predominates in layers III and V in these association areas. In these neocortical regions, SMI32 primarily labels the perikarya and dendrites of large pyramidal neurons, predominantly located within layers III and V. In Alzheimer's disease, a dramatic loss of SMI32-immunoreactive (ir) cells was observed, affecting particularly the largest cells (i.e., cells with a cross-sectional perikaryal area larger than 350 microns 2). The staining intensity of the largest SMI32-ir neurons was significantly reduced in Alzheimer's disease cases, suggesting that an inappropriate phosphorylation of these cytoskeletal proteins may take place in the course of the pathological process. In addition, the SMI32-ir neuron loss and total neuron loss were highly correlated with neurofibrillary tangle counts, whereas such a correlation was not observed with neuritic plaque counts. These quantitative data suggest that SMI32-ir neurons represent a small subset of pyramidal cells that share certain anatomical and molecular characteristics and are highly vulnerable in Alzheimer's disease. Other studies have suggested that SMI32-ir neurons are likely to furnish long corticocortical projections. Thus, their loss would substantially diminish the effectiveness of the distributed processing capacity of the neocortex, resulting in a neocortical isolation syndrome as reflected by the clinical symptomatology observed in these patients. Such correlations between the expression of a selective cellular pathology and specific elements of cortical circuitry will increase our understanding of the molecular and cellular characteristics underlying a given neuronal subclass vulnerability in Alzheimer's disease or other neurodegenerative disorders.

Neurons of the lateral entorhinal cortex of the rhesus monkey: a Golgi, histochemical, and immunocytochemical characterization

This study identifies the neuronal types of the rhesus monkey lateral entorhinal cortex (LEC) and discusses the importance of these data in the context of the connectional patterns of the LEC and the possible role of these cells in neurodegenerative diseases. These neuronal types were characterized with the aid of Golgi impregnation techniques. These characterizations were based upon their spine densities, dendritic arrays, and, where possible, axonal arborizations. The cells could be segregated into only spinous and sparsely spinous types. The most numerous spinous types were pyramidal neurons. Other spinous types included multipolar, vertical bipolar and bitufted, and vertical tripolar neurons. The sparsely spinous neuronal types consisted of multipolar, horizontal bipolar and bitufted, and neurogliaform cells. These cells were further classified with the aid of histochemical stains and immunocytochemical markers. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry stained multipolar, bipolar, and bitufted neurons. Stain for cytochrome oxidase (CO) was found in pyramidal and nonpyramidal cell types. Immunocytochemical techniques revealed several nonpyramidal neurons that contain somatostatin (Som) or substance P (SP). This study complements previous analyses of the neuronal components described in the LEC and adds further information about the distribution of selected neurochemicals within this cortex.

Excitatory transmitter amino acid-containing neurons in the rat visual cortex: a light and electron microscopic immunocytochemical study

The distribution and morphology of neurons labelled with antisera to glutamate or aspartate were examined, at the light and electron microscope levels, in the rat visual cortex. Using widely accepted light microscopic features as well as well-established nuclear, cytoplasmic, and synaptic criteria, we noted that glutamate-immunoreactive neurons were pyramidal cells distributed in layers II-VI, with an increased concentration in layers II and III. Aspartate immunoreactivity was localized chiefly to pyramidal neurons in layers II-VI. However, approximately 10% of immunolabeled cells were nonpyramidal neurons scattered throughout the cortex. Cell-body measurements revealed that, for both groups of neurons, layer V contained the largest labelled neurons, whereas layers IV and VI contained the smallest. Furthermore, in every layer, aspartate-stained neurons were larger than glutamate-positive cells. Finally, glutamate- and aspartate-labelled axon terminals formed asymmetrical synapses, which are presumably excitatory in nature, primarily with dendritic spines. These findings, together with recent detailed studies of the projections of glutamate- and aspartate-labelled cortical neurons, may provide essential background information for studies aimed to elucidate the function(s) of excitatory amino acids in the cortex and their role in pathological conditions.

Glutamate and aspartate immunoreactivity in corticospinal neurons of rats

A combination of retrograde tracers and immunostaining was employed to test whether corticospinal tract (CST) neurons in rats may use amino acid excitatory neurotransmitters. CST neurons were identified following injections of either Diamidino Yellow (DY) or colloidal gold-labeled enzymatically inactive horseradish peroxidase conjugated to wheat germ agglutinin (WGAapoHRP-Au) in the spinal cord. As retrograde tracers, the two substances seemed to be equally effective, but WGAapoHRP-Au was better suited than DY as a tracer to use in combination with immunocytochemistry. Sections through the primary sensorimotor cortex, which contained the bulk of identified CST neurons, and the secondary somatosensory cortex were processed with antisera raised in rabbits against glutamate (Glu) or aspartate (Asp) conjugated by glutaraldehyde to hemocyanin. In rats with DY injections, about 60-75% of the CST neurons were Glu-immunopositive, with higher ratios in SI and MI than in SII. Similar results were obtained in all areas examined from the rats with injections of WGAapoHRP-Au. Only sections from rats with injections of WGAapoHRP-Au were processed for Asp immunostaining. In this material, between 65 and 75% of the CST neurons were Asp-immunopositive, with a slightly higher ratio in SI and MI than in SII. The possibility that these results might reflect limited penetration of the antiserum and/or staining of the same population of CST neurons by either antiserum was addressed in sections processed with both the Glu and Asp antisera. In sections incubated in a mixture of the two antisera, the percentage of immunostained CST neurons was higher, about 90%, than in sections processed for only one of the two antisera. Furthermore, in rats in which Glu and Asp antibodies were visualized by two distinguishable immunostainings, four populations of CST neurons were identifiable: 1) neurons only immunopositive for Glu, 2) neurons only immunopositive for Asp, 3) neurons likely to be stained by both, and 4) neurons immunonegative for both antisera. Twenty-five to 30% of CST neurons were positive for only one antiserum, and about 50% were positive for both. No preferential distribution was evident for any one of these populations of neurons. However, perikaryal cross-sectional areas were larger for the double-stained than for the single-stained CST neurons. Glutamergic and aspartergic transmission in CST neurons has been proposed in several publications in which methods other than immunocytochemistry were employed.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced glycine stimulation of [3H]MK-801 binding in Alzheimer's disease

The novel N-methyl-D-aspartate receptor channel ligand (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5, 10-imine maleate ([3H]MK-801) has been utilized to label this receptor in human brain tissue. Characteristics of [3H]MK-801 binding to well-washed membranes from 17 control subjects and 16 patients with Alzheimer's disease were determined in frontal, parietal, and temporal cerebral cortex and cerebellar cortex. In control tissue the pharmacological specificity of the binding of this substance is entirely consistent with the profile previously reported for rat brain. Binding could be stimulated by the addition of glutamic acid to the incubation medium; addition of glycine produced further enhancement which was not prevented by strychnine. The specificity of the effects of these and other amino acids on the binding was the same as in the rat. In Alzheimer's disease significantly less binding was observed in the frontal cortex under glutamate- and glycine-stimulated conditions. This appears to be associated with a reduced affinity of the site whereas the pharmacological specificity of the site remained unchanged. The effect did not appear to be due to differences in mode of death between Alzheimer's disease and control subjects and is unlikely to be related to factors for which the groups were matched. In contrast, binding was not altered in the absence of added amino acids and presence of glutamate alone. These results imply that in the cerebral cortex the agonist site and a site in the cation channel of the receptor are not selectively altered, but that their coupling to a strychnine-insensitive glycine recognition site is impaired.

Frontal cortical and left temporal glutamatergic dysfunction in schizophrenia

Glutamatergic mechanisms have been investigated in postmortem brain samples from schizophrenics and controls. D-[3H]Aspartate binding to glutamate uptake sites was used as a marker for glutamatergic neurones, and [3H]kainate binding for a subclass of postsynaptic glutamate receptors. There were highly significant increases in the binding of both ligands to membranes from orbital frontal cortex on both the left and right sides of schizophrenic brains. The changes are unlikely to be due to antemortem neuroleptic drug treatment, because no similar changes were recorded in other areas. A predicted left-sided reduction in D-[3H]aspartate binding was refuted at 5% probability, but not at 10%. Previously reported high concentrations of dopamine in left amygdala were strongly associated with low concentrations of D-[3H]aspartate binding in left polar temporal cortex in the schizophrenics. The findings are compatible with an overabundant glutamatergic innervation of orbital frontal cortex in schizophrenia. The results also suggest that schizophrenia may involve left-sided abnormalities in the relationship between temporal glutamatergic and dopaminergic projections to amygdala.

Release of neuroactive substances: homocysteic acid as an endogenous agonist of the NMDA receptor

Sulfur containing amino acids such as homocysteic acid (HCA), cysteinsulfinic acid, homocysteinsulfinic acid are released by depolarization of slices from various rat brain regions in a Ca++-dependent manner. L-HCA excites caudate neurons through their N-methyl-D-aspartic acid (NMDA) receptor and potentiates their cortically evoked excitatory postsynaptic potentials. 35S-methionine can label the releasable pool of HCA, and thus appears as a precursor of HCA. Thus HCA is a transmitter candidate which acts predominantly on the NMDA receptor.

Immunohistochemical study of glutaminase-containing neurons in the cerebral cortex and thalamus of the rat

In an attempt to identify glutamatergic neurons, the cerebral cortex and thalamus of the rat were examined immunohistochemically by using a monoclonal antibody against phosphate-activated glutaminase (PAG), a major synthetic enzyme of transmitter glutamate in the central nervous system. In both the neocortex and mesocortex, pyramidal cells in layers V and VI showed intense PAG-like immunoreactivity (PAG-LI), whereas neuronal cell bodies in layers I-IV showed weak PAG-LI. At the deep border of layer VI, neurons with horizontally elongated cell bodies showed PAG-LI. In the pyriform and entorhinal cortices, neurons with intense to moderate PAG-LI were seen in layer II as well as in the deeper layers. In the hippocampal formation, pyramidal cells in CA1, CA2, and CA3 and polymorphic cells in CA4 showed PAG-LI; PAG-LI was most intense in pyramidal cells of CA3. Fine granules with weak PAG-LI were also seen on and/or within the cell bodies of granule cells in the dentate gyrus. In the thalamus, neurons with PAG-LI were distributed in all nuclei, although regional differences were observed in the distribution pattern of neurons with PAG-LI and in the intensity of PAG-LI in individual neurons. The largest neurons in each thalamic nucleus showed intense PAG-LI; these were considered to be projection neurons. In addition to perikaryal labeling, many fine, PAG-like immunoreactive granules were distributed in the neuropil of both the cerebral cortex and thalamic nuclei. Some of these fine granules with PAG-LI in the neuropil were assumed to represent fiber terminals with PAG-LI, because the distribution pattern of the deposits in the primary somatosensory and primary visual cortices resembled that of thalamocortical fiber terminals. Glutamate is rather ubiquitous in the mammalian central nervous system, and it is still debatable whether the monoclonal antibody to PAG from brain mitochondria can distinguish transmitter-related glutaminase from the other metabolism-related ones. In the present study, however, large neurons in the thalamic nuclei, as well as pyramidal neurons in the cerebral cortex, showed PAG-LI most intensely, supporting the assumption that projection neurons of the cerebral cortex and thalamus are primarily glutamatergic.

D-[3H]aspartate retrograde labelling of callosal and association neurones of somatosensory areas I and II of cats

Experiments were carried out on cats to ascertain whether corticocortical neurones of somatosensory areas I (SI) and II (SII) could be labelled by retrograde axonal transport of D-[3H]aspartate (D-[3H]Asp). This tritiated enantiomer of the amino acid aspartate is (1) taken up selectively by axon terminals of neurones releasing aspartate and/or glutamate as excitatory neurotransmitter, (2) retrogradely transported and accumulated in perikarya, (3) not metabolized, and (4) visualized by autoradiography. A solution of D-[3H]Asp was injected in eight cats in the trunk and forelimb zones of SI (two cats) or in the forelimb zone of SII (six cats). In order to compare the labelling patterns obtained with D-[3H]Asp with those resulting after injection of a nonselective neuronal tracer, horseradish peroxidase (HRP) was delivered mixed with the radioactive tracer in seven of the eight cats. Furthermore, six additional animals received HRP injections in SI (three cats; trunk and forelimb zones) or SII (three cats; forelimb zone). D-[3H]Asp retrograde labelling of perikarya was absent from the ipsilateral thalamus of all cats injected with the radioactive tracer but a dense terminal plexus of anterogradely labelled corticothalamic fibres from SI and SII was observed, overlapping the distribution area of thalamocortical neurones retrogradely labelled with HRP from the same areas. D-[3H]Asp-labelled neurones were present in ipsilateral SII (SII-SI association neurones) in cats injected in SI. In these animals a bundle of radioactive fibres was observed in the rostral portion of the corpus callosum entering the contralateral hemisphere. There, neurones retrogradely labelled with silver grains were present in SI (SI-SI callosal neurones). Association and callosal neurones labelled from SI showed a topographical distribution similar to that of neurones retrogradely labelled with HRP. The laminar patterns of corticocortical neurones labelled with D-[3H]Asp or with HRP were also similar, with one exception. In the inner half of layer II, SII-SI association neurones and SI-SI callosal neurones labelled with the radioactive marker were much less numerous than those labelled with HRP. In cats injected in SII, D-[3H]Asp retrogradely labelled cells were present in ipsilateral SI (SI-SII association neurones). Their topographical and laminar distribution overlapped that of neurones labelled with HRP but, as in cats injected in SI, association neurones labelled with silver grains were unusually rare in the inner layer III.(ABSTRACT TRUNCATED AT 400 WORDS)

Hemidecortication selectively alters release of glutamate in perfusates collected from cerebral cortex of unrestrained rats

The effect of hemidecortication on the in vivo release of amino acids was examined in different areas of the cerebral cortex of the freely-moving rat. After one side of the cortex was lesioned by aspiration, four guide tubes for push-pull perfusion were implanted chronically on the contralateral side so as to rest above the frontal, parietal, temporal and occipital areas of the cortex. After 10-14 days elapsed, each of these regions was perfused with an artificial cerebrospinal fluid (CSF) at a rate of 25.0 microliter/min. Two types of assays were undertaken to determine the release of either newly synthesized amino acids from [14C]glucose precursor or the actual endogenous content in samples of perfusate. The separation of the [14C]amino acids was performed by thin layer chromatography, whereas endogenous amino acids were separated by HPLC with electrochemical detection and quantitated in the range of 1.0-10.0 picomoles. When compared to the control group, samples collected in the hemidecorticate rat showed no significant differences in the new synthesis of glutamate, aspartate, glutamine, glycine, and GABA from the precursor. On the other hand, the analysis of the endogenous amino acid neurotransmitters revealed that the levels of glutamic acid and glutamine declined in samples obtained from the parietal and frontal cortex, respectively. These results implicate further the potential role of glutamic acid as a neurotransmitter of interhemispheric connections in the cerebral cortex.

Glial glutamate dehydrogenase: ultrastructural localization and regional distribution in relation to the mitochondrial enzyme, cytochrome oxidase

Glutamate dehydrogenase (GDH) is primarily a mitochondrial enzyme involved in the metabolism of glutamate. We have recently shown by light microscopic immunocytochemistry that, within detergent-permeabilized brain tissue, GDH is enriched in glial cells, particularly in regions utilizing L-glutamate as a neurotransmitter. In this study, we used immunogold labeling to quantitatively establish that the form of the enzyme recognized by the presently used GDH antiserum is associated primarily with a subpopulation of mitochondria in ultrathin, plastic-embedded sections of the rat cortex and striatum. Permeabilization with detergents was omitted in these studies, so as to preserve the ultrastructure. As expected, labeled mitochondria occurred both in neurons and glia. Furthermore, light microscopic comparisons of the regional distributions of peroxidase immunoreactivity for GDH and a histochemical reaction product for a second mitochondrial enzyme, cytochrome oxidase (CO), were used to demonstrate that high levels of GDH in glia of glutamate-receptive areas do not necessarily reflect the areas' demand for elevated oxidative metabolism. While all regions showing intense labeling for glial GDH also exhibited high levels of CO activity, many additional regions showing high levels of CO activity contained no detectable immunoreactivity for glial GDH. These light-microscopic comparisons reveal that the energy requirements are not the only factors accounting for the regional heterogeneity of the enzyme. We conclude that glial mitochondria are heterogeneous with respect to their GDH content and that GDH is enriched in areas exhibiting chronically active glutamatergic transmission.