L-glutamate transporter derived from mRNAs of primary glial cultures: expression in Xenopus laevis oocytes

Immunocytochemical visualization of d-glutamate in the rat brain

Using highly specific antisera directed against conjugated d-amino acids, the distribution of d-glutamate-, d-tryptophan-, d-cysteine-, d-tyrosine- and d-methionine-immunoreactive structures in the rat brain was studied. Cell bodies containing d-glutamate, but not d-glutamate-immunoreactive fibers, were found. Perikarya containing this d-amino acid were only found in the mesencephalon and thalamus of the rat CNS. Thus, the highest density of cell bodies containing d-glutamate was observed in the dorsal raphe nucleus, the ventral part of the mesencephalic central gray, the superior colliculus, above the posterior commissure, and in the subparafascicular thalamic nucleus. A moderate density of immunoreactive cell bodies was observed in the dorsal part of the mesencephalic central gray, above the rostral linear nucleus of the raphe, the nucleus of Darkschewitsch, and in the medial habenular nucleus, whereas a low density was found below the medial forebrain bundle and in the posterior thalamic nuclear group. Moreover, no immunoreactive fibers or cell bodies were visualized containing d-tryptophan, d-cysteine, d-tyrosine or d-methionine in the rat brain. The distribution of d-glutamate-immunoreactive cell bodies in the rat brain suggests that this d-amino acid could be involved in several physiological mechanisms. This work reports the first visualization and the morphological characteristics of conjugated d-glutamate-immunoreactive cell bodies in the rat CNS using an indirect immunoperoxidase technique. Our results suggest that the immunoreactive neurons observed have an uptake mechanism for d-glutamate.

Direct immunocytochemical evidence for the transfer of glutamine from glial cells to neurons: use of specific antibodies directed against the d-stereoisomers of glutamate and glutamine

We have raised antibodies against D-stereoisomers of the amino acids glutamate and glutamine. These stereoisomers are not naturally occurring in mammals but can be taken up into cells by transporters that normally handle the endogenous L-amino acids. Exposure of isolated rabbit retinae to 50 microM D-glutamate resulted in a strong accumulation of D-glutamate, and hence immunoreactivity for D-glutamate in radial glial cells (Müller cells). By contrast the glutamatergic ganglion cells exhibited no immunoreactivity for D-glutamate. D-Glutamate can be converted into D-glutamine by the glial enzyme glutamine synthetase. Immunolabelling for D-glutamine revealed the presence of D-glutamine in somata of subsets of neurons including the glutamatergic ganglion cells. Labelling was also present in the inner plexiform layer, possibly indicating labelling of neuronal processes. These data indicate that after D-glutamate has been taken up into glial cells it is converted into D-glutamine. This D-glutamine is then exported from the glial cells and taken up by a subset of neurons, including the glutamatergic ganglion cells.

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

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

Pharmacological characterization of a cloned rat glutamate transporter (GluT-1)

Pharmacological properties of a cDNA clone encoding a high affinity, Na(+)-dependent L-glutamate transporter (GluT-1) were examined using Xenopus oocytes. L-[3H]glutamate transport was inhibited by the putative endogenous substrates L-aspartate (Ki = 65 microM) and L-glutamate (Ki = 70 microM). L-Homocysteate did not significantly inhibit high-affinity glutamate transport (Ki = 2.7 mM). Of the previously identified uptake inhibitors, DL-threo-beta-hydroxyaspartate (Ki = 65 microM), L-cysteine sulfinate (Ki = 80 microM), beta-glutamate (Ki = 475 microM) and L-aspartate-beta-hydroxamate (Ki = 950 microM) inhibited L-glutamate uptake. The other L-glutamate uptake blockers examined, including dihydrokainate, L-alpha-aminoadipate and SITS, weakly inhibited uptake of L-glutamate with Ki values in excess of 1 mM. These features of the inhibitor sensitivities of GluT-1 transport show that GluT-1 is less sensitive to these agents than previously characterized glutamate transporters in rat brain, suggesting that GluT-1 is a novel glutamate transporter with a unique pharmacologic profile.

Expression cloning of a rat glutamate transporter

A functional cDNA clone for the rat glutamate transporter was isolated by a cloning approach using a Xenopus oocyte expression system. The cDNA sequence predicts a protein of 543 amino acids with 6 putative transmembrane domains. The glutamate transporter shows no sequence similarity to other members of the known neurotransmitter transporter family. Expression of the cDNA clone in Xenopus oocyte indicates that glutamate transport activity is Na+- and Cl(-)-dependent and sensitive to selective glutamate transporter inhibitor. The glutamate transporter mRNA is expressed in both the brain and the peripheral tissues at different levels.