Mosaic distribution of phosphate-activated glutaminase-like immunoreactivity in the rat striatum

Glutaminases

Mammalian glutaminases catalyze the stoichiometric conversion of L-glutamine to L-glutamate and ammonium ions. In brain, glutaminase is considered the prevailing pathway for synthesis of the neurotransmitter pool of glutamate. Besides neurotransmission, the products of glutaminase reaction also fulfill crucial roles in energy and metabolic homeostasis in mammalian brain. In the last years, new functional roles for brain glutaminases are being uncovered by using functional genomic and proteomic approaches. Glutaminases may act as multifunctional proteins able to perform different tasks: the discovery of multiple transcript variants in neurons and glial cells, novel extramitochondrial localizations, and isoform-specific proteininteracting partners strongly support possible moonlighting functions for these proteins. In this chapter, we present a critical account of essential works on brain glutaminase 80 years after its discovery. We will highlight the impact of recent findings and thoughts in the context of the glutamate/glutamine brain homeostasis.

Modular control of glutamatergic neuronal identity in C. elegans by distinct homeodomain proteins

The choice of using one of many possible neurotransmitter systems is a critical step in defining the identity of an individual neuron type. We show here that the key defining feature of glutamatergic neurons, the vesicular glutamate transporter EAT-4/VGLUT, is expressed in 38 of the 118 anatomically defined neuron classes of the C. elegans nervous system. We show that distinct cis-regulatory modules drive expression of eat-4/VGLUT in distinct glutamatergic neuron classes. We identify 13 different transcription factors, 11 of them homeodomain proteins, that act in distinct combinations in 25 different glutamatergic neuron classes to initiate and maintain eat-4/VGLUT expression. We show that the adoption of a glutamatergic phenotype is linked to the adoption of other terminal identity features of a neuron, including cotransmitter phenotypes. Examination of mouse orthologs of these homeodomain proteins resulted in the identification of mouse LHX1 as a regulator of glutamatergic neurons in the brainstem.

Difference in organization of corticostriatal and thalamostriatal synapses between patch and matrix compartments of rat neostriatum

The neostriatum, which possesses a mosaic organization consisting of patch and matrix compartments, receives glutamatergic excitatory afferents from the cerebral cortex and thalamus. Differences in the synaptic organization of these striatopetal afferents between the patch and matrix compartments were examined in the rat using confocal laser scanning and electron microscopes. Thalamostriatal terminals immunopositive for vesicular glutamate transporter (VGluT) 2 were less dense in the patch than in the matrix compartment, although the density of VGluT1-immunopositive corticostriatal terminals was almost evenly distributed in both the compartments. Quantitative analysis of ultrastructural images revealed that 84% of VGluT2-positive synapses in the patch compartment were formed with dendritic spines, whereas 70% in the matrix compartment were made with dendritic shafts. By contrast, VGluT1-positive terminals display a similar preference for specific synaptic targets in both compartments: about 80% made synapses with dendritic spines. In addition, VGluT2-positive axospinous synapses in the patch compartment were larger than the VGluT1-positive axospinous synapses in both compartments. As axospinous synapses are generally found in neuronal connections showing high synaptic plasticity, the present findings suggest that the thalamostriatal connection requires higher synaptic plasticity in the patch compartment than in the matrix compartment.

Vesicular glutamate transporters in the brain

Glutamate is an excitatory amino acid that acts as a major neurotransmitter throughout the brain. Although its neurotransmitter action has been evidenced by the identification of various receptor subtypes at synapses, a cellular mechanism by which this amino acid accumulates in synaptic vesicles has long been in doubt until the discovery in recent years of specific vesicular transporters. Three kinds of transporter isoforms have so far been cloned and their transport properties and distribution in the brain have been studied extensively. In contrast with the apparently similar ability of all transporter isoforms to highly selectively transport glutamate and their presence in synaptic vesicles, their regional distribution of gene expression and immunoreactivity in the rodent or human brain are surprisingly different from one another. This indicates that the glutamatergic neuron system of mammalian brains is substantially comprised of at least three different neuron subpopulations, each of which uses a unique transport system for the vesicular storage of glutamate. Thus, we now have highly useful and reliable tools for a comprehensive understanding of the glutamatergic neuron system in the brain from a new viewpoint different from that of other components, such as receptors. The scope of the present review is to provide an overview of the history and present status of the study of vesicular glutamate transporters and to highlight some unresolved issues requiring clarification for the progress of future brain function research.

Expression of vesicular glutamate transporters, VGluT1 and VGluT2, in axon terminals of nociceptive primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns of the rat

We examined immunohistochemically whether the vesicular glutamate transporters (VGluTs), VGluT1 and VGluT2, might be expressed in synaptic terminals of nociceptive primary afferent fibers within laminae I and II of the medullary and spinal dorsal horns of the rat. VGluT1 immunoreactivity (IR) was intense in the inner part of lamina II but weak in lamina I and the outer part of lamina II. VGluT2-IR was most intense in lamina I and the outer part of lamina II. Expression of VGluTs in synaptic terminals was confirmed by dual immunofluorescence histochemistry for VGluTs and synaptophysin. Expression of VGluTs in axon terminals of primary afferent fibers terminating in laminae I and II was also confirmed immunohistochemically after unilateral dorsal rhizotomy. The dual immunofluorescence histochemistry indicated expression of VGluTs in substance P (SP)-containing axon terminals in lamina I and the outer part of lamina II. Electron microscopy confirmed the coexpression of VGluTs and SP in axon terminals within laminae I and II; VGluTs was associated with round synaptic vesicles at the asymmetric synapses. It was further observed that isolectin IB4, a marker for unmyelinated axons, often bound with VGluT2-immunopositive structures but rarely with VGluT1-immunopositive structures in lamina II. Thus, the results indicated in laminae I and II of the medullary and spinal dorsal horns that both VGluT1 and VGluT2 were expressed in axon terminals of primary afferent fibers, including SP-containing nociceptive fibers and that VGluT in unmyelinated primary afferent fibers terminating in lamina II was primarily VGluT2.

Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons

We have cloned and functionally characterized a third isoform of a vesicular glutamate transporter (VGLUT3) expressed on synaptic vesicles that identifies a distinct glutamatergic system in the brain that is partly and selectively promiscuous with cholinergic and serotoninergic transmission. Transport activity was specific for glutamate, was H(+)-dependent, was stimulated by Cl(-) ion, and was inhibited by Rose Bengal and trypan blue. Northern analysis revealed higher mRNA levels in early postnatal development than in adult brain. Restricted patterns of mRNA expression were observed in presumed interneurons in cortex and hippocampus, and projection systems were observed in the lateral and ventrolateral hypothalamic nuclei, limbic system, and brainstem. Double in situ hybridization histochemistry for vesicular acetylcholine transporter identified VGLUT3 neurons in the striatum as cholinergic interneurons, whereas VGLUT3 mRNA and protein were absent from all other cholinergic cell groups. In the brainstem VGLUT3 mRNA was concentrated in mesopontine raphé nuclei. VGLUT3 immunoreactivity was present throughout the brain in a diffuse system of thick and thin beaded varicose fibers much less abundant than, and strictly separated from, VGLUT1 or VGLUT2 synapses. Co-existence of VGLUT3 in VMAT2-positive and tyrosine hydroxylase -negative varicosities only in the cerebral cortex and hippocampus and in subsets of tryptophan hydroxylase-positive cell bodies and processes in differentiating primary raphé neurons in vitro indicates selective and target-specific expression of the glutamatergic/serotoninergic synaptic phenotype.

The essence of excitation

The amino acid glutamate is the major excitatory neurotransmitter in a range of organisms from Caenorhabditis elegans to mammals, and it mediates the information processing that underlies essentially all behavior. Recent advances in our understanding of glutamate storage and release now illuminate how this ubiquitous amino acid can function as a signalling molecule.

Intracerebroventricular administration of quinolinic acid induces a selective decrease of inositol(1,4,5)-trisphosphate receptor in rat brain

[3H]inositol(1,4,5)-trisphosphate (IP3) binding studies have shown decreased [3H]IP3 binding to brain tissue in several neurodegenerative diseases, including Alzheimer's and Huntington's diseases. In addition, previous results obtained from brains of Alzheimer patients indicated a reduction of IP3-receptor protein correlated to neuronal loss. The neurotoxic effect of the glutamate receptor agonist quinolinic acid (QUIN) was therefore examined with respect to the level of IP3-receptor immunoreactivity in rat brain. Neuronal lesions were estimated with antibodies to marker proteins for striatal medium-sized spiny neurons (dopamine- and cyclic AMP-regulated phosphoprotein, Mr 32,000; DARPP-32), synaptic vesicles (synaptophysin), mitochondria (phosphate-activated glutaminase; PAG) and glial cells (glial fibrillary acidic protein; GFAP). Injection of QUIN into rat neostriatum induced a massive loss of striatal medium-sized spiny neurons, and led to a comparable loss of IP3-receptor and PAG immunoreactivity, suggesting a neuronal localisation of both these proteins. In an effort to induce less pronounced excitotoxic damage, intracerebroventricular infusion of QUIN was performed. Following this lesion, the neostriatum showed a negligible loss of DARPP-32 immunoreactivity (-11+/-5%), but contained only 43+/-3% of IP3-receptor immunoreactivity levels compared to controls. In the hippocampus, cerebellum and entorhinal cortex, the IP3-receptor loss was less pronounced. The decrease in the level of IP3-receptor immunoreactivity appears to be selective with respect to the other proteins studied, and the IP3-receptor thus shows extreme sensitivity to QUIN neurotoxicity in the neostriatum.

The neostriatal mosaic: basis for the changing distribution of neurokinin-1 receptor immunoreactivity during development

The pattern of neurokinin-1 receptor-like immunoreactivity (NK-1Rir) was mapped in perinatal and adult mouse striatum by using a new polyclonal antiserum. NK-1Rir was detected in the differentiating regions of the ganglionic eminences on embryonic day 12.5 (E12.5). NK-1Rir structures were enriched in the striatal patch compartment between E16.5 and approximately postnatal day 3 (P3); distributed more uniformly, within portions of both the patch and matrix compartments on P7; and enriched in the matrix compartment in the adult. Analysis of the phenotype of NK-1Rir cells on P2, P7, and in the adult suggested that cholinergic cells accounted for the majority of NK-1Rir cells early postnatally, with increasing contributions from somatostatinergic cells later postnatally. In the adult, approximately half of NK-1Rir cells were cholinergic and half were somatostatinergic. The transient enrichment of NK-1R-bearing cells and processes in the patch compartment which contains cells that express substance P (SP), a putative ligand for the NK-1R, may be a consequence of compartment formation or may be functionally important for compartment development.

Long-term enhancement of dopamine release by high frequency tetanic stimulation via a N-methyl-D-aspartate-receptor-mediated pathway in rat striatum

We studied the effects of high frequency tetanic stimulation of the striatum on the KCl (20 mM)-evoked dopamine release in rat striatal slices. The KCl-evoked dopamine release was potentiated by high frequency tetanic stimulation (10-20 Hz) of the striatum including the corticostriatal fibers, and this potentiation was observed until 3 h after high frequency tetanic stimulation. Potentiation of dopamine release after high frequency tetanic stimulation was induced not only by KCl but also by glutamate in Mg(2+)-free medium, N-methyl-D-aspartate in Mg(2+)-free medium, and by DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. 2-amino-5-phosphovalerate, 3-[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonate or dibenzocycloheptaneimine, N-methyl-D-aspartate receptor inhibitors, abolished enhancement by tetanus, whereas, 6,7-dinitroquinoxaline-2,3-dione, an antagonist of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid ionotropic receptors, or L-2-amino-4-phosphonobutyrate, an antagonist of glutamate metabotropic receptors, showed no effect. Moreover, pretreatment with glutamate or N-methyl-D-aspartate in the absence of Mg2+ also facilitated dopamine release evoked by KCl concentrations. When extracellular Ca2+ was removed from the medium during pretreatment, potentiation by glutamate disappeared. We conclude that activation of N-methyl-D-aspartate receptors on dopaminergic nerve terminals in the striatum produces the long-term changes in efficacy of the response to KCl or glutamatergic agents. That is, plastical phenomena could exist at presynaptic levels between glutamatergic neurons and dopaminergic neurons in striatum.

Glutamate in some retinal neurons is derived solely from glia

Glutamate is the most abundant excitatory neurotransmitter in the vertebrate central nervous system. It is widely assumed that neurons using this transmitter derive it from several sources: (i) synthesizing it themselves from alpha-ketoglutarate or aspartate, (ii) synthesize it from glial-derived glutamine, or (iii) take up glutamate from the extracellular space. By use of immunocytochemistry we show that glutamate is abundant in the retinal ganglion and bipolar cells of the rabbit, but that immunoreactivity for glutamate in these neurons is reduced below immunocytochemical detection limits after the specific inhibition of glutamine synthesis in glial cells by D,L-methionine D,L-sulphoximine. GABA immunoreactivity in retinal amacrine cells was also reduced after inhibition of glutamine synthetase but the patterns and densities of immunoreactivity for taurine and glycine were unaffected. Therefore, this experimental paradigm does not induce generalized metabolic changes in neurons or glia. This study demonstrates that some glutamatergic neurons are dependent on the synthetic processes in glia for their neurotransmitter content.

Conceptual history of the nigrostriatal dopamine system

The history of the nigrostriatal dopamine system may provide a prime example of the two faces of scientific development. First, a given concept is replaced by another simply as a result of methodologies being improved, and second, successive technical improvements make seemingly settled controversies even more complicated and disputable. The nigrostriatal pathway, which had been unrecognizable with Nauta's silver impregnation method, became apparent by use of the more sensitive silver impregnation method of Fink-Heimer. The sensitivity of the latter method, however, was still insufficient to reveal the whole extent of another ascending dopamine system, the mesocortical dopamine system, until its existence was established through the application of glyoxylic acid fluorescent histochemistry. Electron microscopic analysis of nigrostriatal dopamine synapses in properly fixed tissue was initiated by the demonstration of dark type terminal degeneration, which was induced by either electrolytic lesions or chemical destruction with a specific toxin (6-hydroxydopamine) of the substantia nigra and medial forebrain bundle. The degenerating terminal boutons, thus produced, invariably formed postsynaptic membrane specializations of asymmetric type. However, the asymmetric nature of the synaptic morphology, although later confirmed by the combined study of chemical lesions and autoradiographic anterograde tracing, was seriously challenged with the introduction of electron microscopic immunohistochemistry. The latter method has consistently revealed that symmetric en passant synapses or axonal varicosities with no synaptic membrane specializations are the only tissue compartments immunoreactive to antibodies against dopamine and its synthetic enzyme tyrosine hydroxylase. In view of the fact that more than 95% of the nigrostriatal projection neurons are dopaminergic, it is difficult to satisfactorily interpret all the available and seemingly paradoxical fine structural data. In this context, a novel concept has emerged in the process of eliminating all the possible alternative interpretations. The concept is that single nigrostriatal neurons form two chemically distinct types of synapses, one dopaminergic symmetric en passant bouton and another non-dopaminergic (still chemically unclassified) asymmetric terminal bouton. If the concept is a valid one, it contradicts Dale's long standing principle, as defined by Eccles: at all the axonal branches of a neuron there is liberation of the same transmitter substance or substances. Furthermore, a certain population of substantia nigra pars reticulata neurons has recently been recognized to be immunoreactive to both dopamine synthetic tyrosine hydroxylase and GABA synthetic glutamate decarboxylase. These single neurons send projections to both the striatum and superior colliculus by way of axon collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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