Aspartate aminotransferase-like immunoreactivity as a marker for aspartate/glutamate in guinea pig photoreceptors

Localization of chick retinal visinin-like immunoreactivity in the rat forebrain and diencephalon

The present study is an examination, using an indirect immunofluorescence method, of the distribution of visinin, a 24,000 dalton peptide, in the rat forebrain and diencephalon. Immunoreactive structures were localized in the neuronal elements showing an uneven distribution. Immunoreactive neurons were found in the olfactory bulb, anterior olfactory nucleus, cerebral cortex, amygdaloid complex, ventral portion of the nucleus caudatus putamen, septal area, nucleus accumbens, nucleus paratenialis, nucleus rhomboideus, nucleus reuniens, nucleus paraventricularis hypothalami, nucleus supraopticus, nucleus anterior hypothalami, preoptic area, hypothalamic periventricular nucleus, nucleus mammillaris medialis, medial habenular nucleus, zona incerta, nucleus lateralis thalami, nucleus tractus optici and gyrus dentatus. Immunoreactive fibers were observed in the above areas, particularly near the labelled cells, forming fiber plexuses of varying density. In addition, dense plexuses were also seen in the globus pallidus, anteroventral nucleus of the thalamus, substantia nigra and hippocampus. In the former three structures, no labelled cells were present and in the latter, a few scattered neurons were found, indicating that these fibers originate from extrinsic sources.

Localization of elevated glutaminase immunoreactivity in small DRG neurons

Glutamate has long been considered to be a neurotransmitter candidate in vertebrate spinal sensory nerve cells. We report here the first immunohistochemical evidence in support of this hypothesis. We find that up to 30% of the moderately small dorsal root ganglion neurons in the rat contain elevated levels of glutaminase immunoreactivity. This enzyme, which mediates the synthesis of glutamate from glutamine, is not found at these high levels in large diameter neurons of the same ganglia. In contrast, another enzyme associated with glutamate metabolism, aspartate aminotransferase, is rather uniformly distributed within neurons of the sensory ganglia. These data define a subpopulation of sensory neurons which appear to contain an elevated capacity to synthesize glutamate through the glutamine cycle and suggest that glutaminase immunoreactivity may be an indicator of glutamatergic function in some nerve cells.

Immunohistochemical localization of chick retinal 24 kdalton protein (visinin) in various vertebrate retinae

Antiserum against a protein (24,000 daltons, visinin) of chick retina has been provided for immunohistochemical study on the localization of visinin in chick retinae during development, as well as in various vertebrate retinae. The photoreceptor cells were stained with anti-visinin serum from 7th day embryonic retinae and its intensity was gradually increased with embryonic age. In addition, visinin-like immunoreactivity was found in some kinds of amacrine and displaced amacrine cells from 11th-day embryonic retinae. When human, cat, frog and carp retinae which contain both rods and cones were examined, staining of cone cells was clearly observed in the photoreceptor cell layer, but not in the rods. Furthermore visinin-like immunoreactivity was barely detectable in the photoreceptor cells of bovine, rat and mouse retinae containing mostly rod cells. These results suggest that visinin is mainly located in the cone cells in various vertebrate retinae and is a good marker for the cone cells.

Immunocytochemical localization of glutaminase-like and aspartate aminotransferase-like immunoreactivities in the rat and guinea pig hippocampus

There is considerable evidence that pathways of the hippocampus use an excitatory amino acid as transmitter. We have attempted to immunocytochemically identify excitatory amino acid neurons in the hippocampus of the rat and guinea pig using antiserum to glutaminase and antiserum to aspartate aminotransferase, which have been proposed as markers for aspartergic/glutamergic neurons. Glutaminase-like immunoreactivity was seen in granule cells in the dentate gyrus and fibers and puncta associated with the mossy fiber pathway in the hilus and stratum lucidum of the hippocampus. At the ultrastructural level, glutaminase-like immunoreactivity was observed in mossy fiber terminals in the stratum lucidum. Glutaminase-like immunoreactivity was also seen in pyramidal cells in regio inferior and regio superior and in cells in layer two of the entorhinal cortex. Schaffer collateral terminals, commissural fiber terminals and perforant pathway terminals were not seen at the light microscopic level. Glutaminase-like immunoreactivity is thus found in the cell bodies of proposed excitatory amino acid neurons of hippocampal pathways, but does not appear to label all terminals. Aspartate aminotransferase-like immunoreactivity was not seen in any cells, fibers or terminals in the rat or guinea pig hippocampus.

Aspartate aminotransferase-like immunoreactivity in the guinea pig and monkey retinas

The excitatory amino acids, aspartate and glutamate, have been proposed as retinal neurotransmitters. Aspartate aminotransferase (AAT) is an enzyme which is involved in the routine metabolism of these amino acids and may be involved in the specific synthesis of glutamate and/or aspartate for use as a neurotransmitter. On the basis of the hypothesis that increased levels of aspartate aminotransferase may reflect a transmitter role for aspartate and/or glutamate, we have localized aspartate aminotransferase in the guinea pig and cynamolgus monkey retinas with light and electron microscopic immunohistochemical techniques. AAT-like immunoreactivity is localized to the cones of guinea pig retina and to monkey rods. Both species contain a subpopulation of immunoreactive amacrine cells as well as a subpopulation of immunoreactive cells in the ganglion cell layer. Immunostaining is seen in bipolar cells and terminals in the monkey but not in the guinea pig retina. We have performed quantitative analysis of the immunoreactive staining in the outer plexiform layer and described the synaptic organization of immunoreactive processes in the inner plexiform layer (IPL). Labeled amacrine processes in both species form synaptic contacts predominantly to and from bipolar terminals in the inner third of the IPL and to and from other amacrine and small unidentified processes in the outer portion of the IPL. The majority of labeled bipolar terminals in the monkey retina are seen in the inner third of the IPL where they synapse exclusively onto amacrine processes. Labeled bipolar terminals in the outer third of the IPL occasionally synapse onto ganglion processes.

Dopamine inhibits calcium-independent gamma-[3H]aminobutyric acid release induced by kainate and high K+ in the fish retina

Kainic acid (KA) at micromolar concentrations stimulated the release of gamma-[3H]aminobutyric acid [( 3H]GABA) from a particulate fraction of the carp (Cyprinus carpio) retina. The KA action was dose-dependent but Ca2+-independent. A similar response was elicited by another glutamate receptor agonist, quisqualic acid, and high K+, but not by an aspartate agonist, N-methyl-D-aspartic acid. The stimulatory action of KA on the [3H]GABA release was selectively blocked by the KA blockers gamma-D-glutamylglycine and cis-2,3-piperidine dicarboxylic acid. Dopamine (DA), which is contained in DA interplexiform cells in the carp retina, inhibited the [3H]GABA release induced by KA and high K+ in a dose-dependent manner. 5-Hydroxytryptamine and two well-known GABA antagonists, bicuculline (Bic) and picrotoxin (Pic), also mimicked the DA effect on the GABA release at a comparable concentration. This inhibitory effect of DA as well as Bic and Pic on the [3H]GABA release evoked by KA was clearly antagonized by a DA blocker, haloperidol. The action of these agents (KA, DA, GABA antagonist) belonging to three different receptor categories on the GABAergic neurons (possibly external horizontal cells; H1 cells) is discussed in relation to other electrophysiological studies on the lateral spread of S-potentials between H1 cells.

Localization of aspartate aminotransferase and cytochrome oxidase in the cat retina

Using immunohistochemical techniques, we demonstrate aspartate aminotransferase (AAT)-like immunoreactivity in cone pedicles and ganglion cells of the cat retina. An identical pattern was seen when we stained for cytochrome oxidase activity, a marker for neurons which have a high metabolic activity. Tetrodotoxin selectively blocked the cytochrome oxidase labeling of ganglion cells. AAT is a key enzyme in the metabolism of aspartate and glutamate and has been proposed as a marker for neurons which use aspartate/glutamate as a neurotransmitter. Due to the close correlation between AAT-like immunoreactivity and cytochrome oxidase activity, we suggest that, at least in the retina, AAT-like immunoreactivity in fact labels cells which have a high metabolic activity.

Immunocytochemical localizations of cytosolic and mitochondrial glutamic oxaloacetic transaminase isozymes in rat retina as markers for the glutamate-aspartate neuronal system

The localization of cytosolic (s) or mitochondrial (m) glutamic oxaloacetic transaminase (GOT) was examined in the rat retina by means of an indirect immunofluorescence method using antibodies specific for s- and m-GOT. The m-GOT-like immunoreactive structures were seen on the inner segments of the photoreceptor cells and other outer and inner plexiform layers. These structures were dot-like in appearance. Somas were not labeled. In contrast, s-GOT-like structures were found on the inner segments and inner fibers of the photoreceptor cells, numerous cell somas in the inner nuclear layer (horizontal, amacrine and bipolar cells), and ganglion cell layer (displaced amacrine cells) and inner plexiform layer. The difference in distribution between s- and m-GOT isozymes suggests that they may be useful as markers for glutamatergic and/or aspartinergic neurons.

Bovine retinal glutamine synthetase 1. Purification, characterization and immunological properties

Glutamine synthetase (GS) from bovine retina was purified to apparent homogeneity by ammonium sulfate fractionation followed by Sephacryl S-200, hydroxylapatite, and Sephadex G-150 chromatography. The purified enzyme showed a single band on polyacrylamide gel electrophoresis. Based on the purification data, retinal GS was shown to be approximately 2% of the total soluble retinal protein. By gel filtration, sedimentation velocity centrifugation, and gel electrophoresis, it was shown that the enzyme has a subunit molecular weight of 45 000 daltons and a native molecular weight of 360 000 daltons, which is consistent with an octameric structure. Throughout the various stages of purification, it was found that GS and glutamyl transferase (GT) activities were maintained at a constant ratio. Thus, the GS and GT reactions are catalyzed by the same enzyme. Immunodiffusion of antiretinal GS antibodies gave a single line of precipitation with both crude retinal and brain enzymes as well as purified enzyme preparations. Precipitation lines of retinal and brain enzymes completely fused with each other without any spur formation. The immunochemical titration of brain enzyme activity with antiretinal GS antibodies also revealed an immunological homology between retinal and brain enzymes.

Bovine retinal glutamine synthetase 2. Regulation and properties on the basis of glutamine synthetase and glutamyl transferase reactions

Glutamine, the end product formed by the glutamine synthetase (GS) reaction, inhibits retinal GS activity in the presence of Mn2+, but not in the presence of Mg2+. In the presence of Mg2+, Mn2+ itself inhibits retinal GS activity. Other compounds which inhibit retinal GS activity significantly are methionine sulfoximine, D-alanine and carbamyl phosphate. Amino acids, such as L-alanine, L-serine and glycine, do not affect the enzyme activity. These amino acids, however, significantly inhibit the enzyme activity when measured on the basis of the glutamyl transferase (GT) reaction. GS isolated from neuronal tissues is regulated differently from that previously reported by others for non-neuronal tissues. The enzyme activity, as measured by GS activity, shows three-fold higher activity with Mg2+ over Mn2+ or Co2+ and on the basis of GT activity, shows about three-fold higher activity with Mn2+ over Mg2+ or Co2+. The optimum pH for the GS reaction lies in the range of 7.2-7.8 and for the GT reaction is 6.4-7.0. Both the GS and GT activities of the enzyme show similar heat stabilities.

Proline, glutamate and glutamine metabolism in mouse brain synaptosomes

In nerve terminals, glutamate (Glu) may serve as precursor of the inhibitory neurotransmitter, GABA, and the putative excitatory transmitter, aspartate (Asp), in addition to exerting its own excitatory neurotransmitter role in brain. Glu carbon can originate from glucose through glycolysis and the Krebs cycle, from glutamine (Gln) subsequent to uptake, and from proline (Pro) and ornithine (Orn). Orn, but not Glu, is an effective precursor in nerve terminals of Pro, a putative inhibitory neurotransmitter. [3H]Arg can be converted in mouse brain nerve terminals to Orn, which in turn gives rise to Glu, Pro and GABA. In the present study, the conversion subsequent to uptake of labeled Glu, Gln and Pro to other amino acids was studied in unfractionated and subfractionated synaptosomal particles which layered, respectively, on 1.0 M, 1.2 M, 1.3 M and 1.5 M sucrose after centrifugation in a discontinuous gradient (fractions 1-4, respectively). Fraction 1 contained small synaptosomal fragments with vesicles and almost no mitochondria. Fractions 2 and 3 showed numerous normal-appearing mitochondria-containing synaptosomes, and fraction 4 contained large synaptosomes and more free mitochondria than the other fractions. Glu was readily taken up in all fractions and converted to Asp, Gln and GABA, the greatest formation of Asp from Glu occurring in fractions 2 and 3 and of Gln in fraction 4. In contrast, Gln was taken up poorly in fraction 1 and not metabolized, converted extensively to Glu and GABA in fractions 2-4, giving rise only to very small amounts of Asp in fractions 2 and 3. Although Pro was taken up to the greatest extent in fraction 2, it was by far most readily converted to Glu, Gln and GABA in fraction 1, showing only small amounts of Asp formation in fractions 1-3 and none in 4. There was no significant production of Pro from Glu or Gln or of Arg and Orn from any of the 3 precursors studied. The above results suggest that Glu, Gln and Pro may be taken up largely in different classes of synaptosomes which are distributed among the centrifugally separated fractions and which possess differing transport and metabolic characteristics. Determination of glutamate decarboxylase activity (GAD) indicated that GABA-forming nerve terminals were present in all synaptosomal fractions studied. Amino acid determinations by HPLC in the subfractionated synaptosomes showed a similar distribution for Glu, Asp and GABA contents, peaking in fraction 2, and an inverse relationship of the latter 3 with Arg contents.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localizations of cytosolic and mitochondrial glutamic oxaloacetic transaminase isozymes in rat brain

The localizations of cytosolic (s-) and mitochondrial (m-) glutamic oxaloacetic transaminase (GOT) were examined by immunocytochemical methods using specific antibodies. Staining of s-GOT-like immunoreactivity was seen in periglomerular cells of the olfactory bulb, and basket, stellate cells of the cerebellum, and second layer cells of the neocortex. On the other hand, m-GOT-like immunoreactivity was found in mitral cells and glomerular regions of the olfactory bulb and deep Golgi cells of the cerebellum. These different distributions of s- and m-GOT isozymes suggest that these isozymes are available as markers of glutamergic or aspartergic neurons.

Immunocytochemical localization of glutaminase-like immunoreactivity in the auditory nerve

The immunocytochemical localization of glutaminase, which we have proposed as a marker for excitatory amino acid neurotransmitters was determined in the guinea pig auditory nerve. Glutaminase-like immunoreactivity was seen in auditory nerve terminals in the cochlear nucleus and in the cell bodies of the auditory nerve in the cochlea. This staining was seen in type I and not type II spiral ganglion cells. Glutaminase-like immunoreactivity was also observed in granule cells in the cochlear nucleus.

Measurement of aminotransferases: Part 1. Aspartate aminotransferase

Aminotransferases are ubiquitous enzymes of mammalian cells and several are of important diagnostic use. The application of aspartate aminotransferase activity measurements in serum from individuals suffering from myocardial infarction brought about a new dimension in clinical laboratory testing in the 1950s. This review focuses on measurement techniques for aspartate aminotransferase and their application (a subsequent article will review other aminotransferases). Assay techniques measuring enzyme activity are direct spectrophotometric measurements, manometric techniques, assays using dye substances, coupled enzyme techniques, and radiometric procedures. Of these procedures, the one employing malate dehydrogenase and NADH is the most important and is covered in particular detail. The estimation of the mitochondrial isoenzyme of aspartate aminotransferase is also of clinical interest, in particular for estimating severity of disease or in specific applications (e.g., chronic alcoholism). Methods reviewed for estimation of this enzyme are electrophoresis, chromatography, differential kinetic behavior, and immunochemical separation. Determination of the enzyme protein by techniques independent of its catalytic activity are also reviewed.

A few axonal proteins distinguish ventral spinal cord neurons from dorsal root ganglion neurons

A series of proteins putatively involved in the generation of axonal diversity was identified. Neurons from ventral spinal cord and dorsal root ganglia were grown in a compartmented cell-culture system which offers separate access to cell somas and axons. The proteins synthesized in the neuronal cell somas and subsequently transported into the axons were selectively analyzed by 2-dimensional gel electrophoresis. The patterns of axonal proteins were substantially less complex than those derived from the proteins of neuronal cell bodies. The structural and functional similarity of axons from different neurons was reflected in a high degree of similarity of the gel pattern of the axonal proteins from sensory ganglia and spinal cord neurons. Each axonal type, however, had several proteins that were markedly less abundant or absent in the other. These neuron-population enriched proteins may be involved in the implementation of neuronal diversity. One of the proteins enriched in dorsal root ganglia axons had previously been found to be expressed with decreased abundance when dorsal root ganglia axons were co-cultured with ventral spinal cord cells under conditions in which synapse formation occurs (P. Sonderegger, M. C. Fishman, M. Bokoum, H. C. Bauer, and P.G. Nelson, 1983, Science [Wash. DC], 221:1294-1297). This protein may be a candidate for a role in growth cone functions, specific for neuronal subsets, such as pathfinding and selective axon fasciculation or the initiation of specific synapses. The methodology presented is thus capable of demonstrating patterns of protein synthesis that distinguish different neuronal subsets. The accessibility of these proteins for structural and functional studies may contribute to the elucidation of neuron-specific functions at the molecular level.

L-glutamic acid: a neurotransmitter candidate for cone photoreceptors in human and rat retinas

We have combined immunocytochemical localization of L-aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1; glutamic-oxaloacetic transaminase) with autoradiographic localization of high-affinity uptake sites for L-glutamate or L-aspartate to identify the neurotransmitters of mammalian photoreceptors. In both human and rat retinas, high aspartate aminotransferase immunoreactivity is found in cones but not in rods; certain putative bipolar and amacrine cells are also heavily stained. In the human retina, and perhaps also in the rat retina, cones possess a high-affinity uptake mechanism for L-glutamate but not L-aspartate, whereas rods and Müller (glial) cells take up both L-glutamate and L-aspartate. Taken together, our results indicate that (i) L-glutamate is much more likely than L-aspartate to be the transmitter for human cones, and possibly for cones of other mammalian species as well, and (ii) major differences exist between mammalian cones and rods in the transport and metabolism or utilization of L-aspartate and L-glutamate.