Characteristics of excitatory amino acid uptake in cultures from neurons and glia from the retina

Glutamate transporter EAAC1 is expressed on Müller cells of lower vertebrate retinas

The expression of glutamate transporter EAAC1 was investigated in carp and bullfrog retinas using Western blotting, immunofluorescence double labeling and confocal laser scanning microscopic techniques. In addition to a variety of retinal neurons, radially oriented elements spanning the whole neural retinas of carp and bullfrog were also EAAC1-immunoreactive, and EAAC1 was found to be predominantly on the cell membrane. Virtually all EAAC1-labeled radial elements were immunopositive to glial fibrillary acidic protein (GFAP), a specific marker for retinal Müller cells of carp and bullfrog, indicating that they were Müller cells. This finding suggests that EAAC1, which has been thought to be an exclusively neuronal type, may be a glial transporter as well. EAAC1 of Müller cells may play an important modulatory role in the retina by making contributions to glutamate homeostasis.

Sodium-dependent glutamate transport in Müller glial cells: regulation by phorbol esters

The regulation of the Na(+)-dependent high affinity glutamate/aspartate transporter system expressed in cultured Müller glia cells from chick retina was studied. Treatment of the cells with the Ca(2+)/diacylglycerol dependent protein kinase C (PKC) activator, phorbol 12-tetradecanoil-13-acetate (TPA) produced a decrease in [(3)H]D-aspartate uptake which was reversed by staurosporine and partially by H7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochoride], two PKC inhibitors. Long-term treatment with TPA resulted in a drastic decrease in the uptake activity, correlated with a substantial fall in the expression of the transporter protein. These findings suggest that PKC is involved in transport modulation at two different levels: phosphorylation and transporter expression in retinal Müller glial cells.

Changes in GluR4 expression induced by metabotropic receptor activation in radial glia cultures

The expression of neurotransmitter receptors in glial cells has suggested a regulatory role of these cells in synaptic function. In radial glia, glutamate receptors elicit a cascade from the membrane to the nucleus and a consequent change in gene expression. In order to gain insight into this process, we address the question of whether receptor activation leads to changes in the repertoire of AMPA/KA glutamate receptor subunits in Bergmann and Müller glial cells. Of the subunits investigated, only GluR4 was up-regulated in Bergmann glial cells both at mRNA and protein levels. In contrast, in Müller glial cells Glu treatment leads to a reduction in GluR4 mRNA and protein expression. Both effects are receptor-mediated and must probably involve group I of metabotropic glutamate receptors. Accordingly, using Northern blot analysis and RT-PCR we detected the expression of both mGluR1 and mGluR5 transcripts in the cultured cells. Our results confirm that glutamate receptors in Bergmann and Müller cells modulate gene expression and further strengthen a plausible role of glial cells in long-lasting changes in the central nervous system.

Serum affects the characteristics of excitatory amino acid-binding sites on Müller cells

The binding of [3H]L-aspartate to membranes obtained from primary cultures of chick retinal Müller cells (glia) was studied Cells seeded in low-serum-containing medium (1%) and maintained in this condition showed an increased number of binding site from 1 to 5 days in vitro (DIV), when compared with controls cultured in medium containing 10% serum; these changes were not reversed by the addition of 10% serum after 48 h in vitro. Increased binding at this age was due to the expression of a low affinity binding system, competitively inhibited by the glutamate uptake blocker L-aspartate-beta-hydroxamate, suggesting that high serum might inhibit the expression of uptake sites at precise maturation stages. Experiments showed the effect was due to a thermolabile serum component. The increase in binding sites is parallel in time to both an increase in aspartate uptake and the initiation of synaptogenesis in the whole retina. Our results suggest that the presence of serum at defined stages in retinal development, could result in the elevation of extracellular glutamate and the concomitant excitotoxic death of neuronal cells, due to a decreased glutamate uptake by glial cells.

Pharmacological characterization of inositol-1,4,5,-trisphosphate binding to membranes from retina and retinal cultures

Light and excitatory amino acids (EAA) stimulate the phosphoinositide cycle in the vertebrate retina. The regulation of Ca2+ release from intracellular stores by inositol-1,4, 5-trisphosphate (IP3) involves an interaction of this compound with specific receptors. By means of [3H]IP3-specific binding, we studied the kinetic and pharmacological properties of IP3 receptors in the chick retina as well as in primary cultures of neurons and glia from this tissue. The equilibrium time for the binding reaction was 15 min and was optimal at alkaline pH (8.3). IP3 receptor displayed high affinity (K(B) approximately 40 nM) and selectivity for D-IP3, compared to D-IP4 > L-IP3 > D-IP2 > D-IP1. These characteristics were the same in subcellular fractions from outer (P1) and thinner (P2) plexiform layers, binding sites being more abundant in P2 (2.65 pmol/mg protein). IP3 receptors were present in both neuronal and glial cultures, but were concentrated in neuronal cultures. Binding was not affected by ryanodine, or caffeine, related to calcium-induced calcium release (CICR) channels, nor by the endoplasmic reticulum Ca2+ ATPase inhibitor thapsigargin, while heparin affectively inhibited IP3 binding. GSSG and thimerosal increased the affinity of [3H]IP3 binding from IC50 approximately 80 nM to IC50 approximately 40 nM; this effect was reversed by DTT. Binding in zero Ca2+ was decreased by low concentrations of Ca2+ (350 nM). These results suggest that actions of IP3 in the retina are regulated by physiological changes in intracellular pH and Ca2+ concentrations, as well as by the oxidation state of the receptor. Additionally, the presence of IP3 receptors in Müller glia opens the possibility of IP3 participation in nonsynaptic signalling through Ca2+ waves in glial cells.

High affinity uptake of glutamate and aspartate in the developing rat retina

Uptake for glutamate and aspartate in both retina and synaptosomes was found to be saturable, temperature sensitive, sodium dependent and reduced by metabolic inhibitors. The P1 and P2 synaptosomal fractions showed high affinity systems for glutamate (3 and 9 microM) and aspartate (6 and 3 microM) respectively. Early after birth, glutamate accumulation was much higher than that of aspartate. It showed a rapid increase reaching the adult values about day 15. Aspartate uptake progressively increases with age up to about day 30. Our findings suggest that glutamate and aspartate may be transmitters at specific cell populations in the rat retina.

Excitatory amino acid receptors in primary cultures of glial cells from the retina

Binding of 3H-L-aspartate to membranes from retinal glial cells in primary culture was characterized. Binding kinetics showed a saturable, reversible binding to three populations of sites with KB = 40, 200, and 1,300 nM. The first two were present at 1 day in vitro (DIV), whereas the latter two were observed at 12 DIV. The possibility of the 40 nM site being neuronal cannot be discarded, since some neurons are present at 1 DIV. In 12 DIV cultures, the presence or absence of sodium determined two different pharmacological patterns, comparable to those described for electrogenic glutamate transport in Müller cells, and QA metabotropic receptors in astrocytes, respectively. Results suggest that, as has been shown for some receptors in nerve tissue, the properties of glial cell receptors undergo age-dependent changes. In turn, this could be related to changes in the function of neurotransmitter substances during development.