Localization and possible function of the glutamate transporter, EAAC1, in the rat retina

Abstract

We investigated the localization and possible function of EAAC1 in the rat retina. Immunocytochemical localization of EAAC1 at the light-microscopic level revealed a fine dust-like labelling pattern across the two synaptic layers. Horizontal cell and subpopulations of amacrine cell somata were labelled, as were some somata within the ganglion cell layer. Some immunoreactive puncta were observed within the cytoplasm of amacrine cells, in regions well away from synaptic sites. At the ultrastructural level, EAAC1 immunolabelled one postsynaptic element at synapses and also processes well away from the synaptic release site. Since EAAC1 was localized away from synaptic sites, we evaluated the role EAAC1 plays in GABA formation by measuring GABA concentrations via reversed-phase high-performance liquid chromatography following incubation of retinae in enzyme and glutamate uptake inhibitors. Incubation of retinae in D-threo-beta-hydroxyaspartate or D/ L-threo-beta-benzyloxyaspartate, which are known to inhibit the glutamate transporters GLAST1, GLT1, and EAAC1, caused a decrease in GABA synthesis by around 50%. Incubation in 6-diazo-5-oxo- L-norleucine, a phosphate-activated glutaminase inhibitor, decreased GABA formation by 40%. Taken together with the anatomical data, the results of this study suggest that EAAC1 plays very little role in GABA synthesis - indeed GABA formation occurs predominantly from glutamine. By virtue of its location both near and well away from synaptic release sites, EAAC1 may regulate glutamate uptake differentially.

Expression of glutamate transporters in human and rat retina and rat optic nerve

“…2. Scale bars 20 mm in a, b, d-g; 10 mm in c and Wiessner et al (2002) in so far as we also detected the EAAC1 protein in horizontal, bipolar, amacrine and ganglion cells. Additionally, we demonstrated EAAC1 (by ISH and immunocytochemistry) in photoreceptor and Müller cells.…”

“…EAAC1 (EAAT3) seems to be present in several neuronal cell types of the retina, i.e. horizontal, amacrine and ganglion cells (Rauen et al 1996;Schultz and Stell 1996;Wiessner et al 2002). To date, limited in situ hybridisation (ISH) data are available only about the expression of EAAC1 (Kanai et al 1995), GLT1v (or GLT1B;Schmitt et al 2002) and GLAST (Otori et al 1994).…”

Differential cellular and subcellular distribution of glutamate transporters in the cat retina

“…The lack of labeling in the A17 cells which are presumed to be GABAergic again argues against a role for EAAC1 in the metabolic processing of GABA. As in the rat (Wiessner et al, 2002), staining in the cat IPL was confined to a single member of the dyad pairing, indicating that the two postsynaptic elements utilize different modes of glutamate inactivation. This is consistent with a large volume of data which shows that the two postsynaptic members at such contacts express different glutamate receptors (Brandstätter et al, 1998;Cai & Pourcho, 1999;Qin & Pourcho, 1999, 2001.…”

Functional Properties of the Retinal Glutamate Transporters GLT-1c and EAAT5

“…These data indicate that EAAT1/GLAST is the major retinal glutamate transporter that controls synaptic signal transmission, whereas EAAT2/GLT-1 mostly plays a role in neuroprotection against glutamate excitotoxicity. EAAT3/EAAC1 can be found in amacrine, ganglion, and horizontal cells (9,11,12) with predominant non-synaptic localization so that an important role in glutamatergic neurotransmission seems unlikely (13). EAAT4 has been identified in retinal astrocytes (9,14) and in the retinal pigment epithelium (15).…”