Laminar and subcellular heterogeneity of GLAST and GLT‐1 immunoreactivity in the developing postnatal mouse hippocampus

Astrocytes express two sodium‐coupled transporters, glutamate–aspartate transporter (GLAST) and glutamate transporter‐1 (GLT‐1), which are essential for the maintenance of low extracellular glutamate levels. We performed a comparative analysis of the laminar and subcellular expression profile of GLAST and GLT‐1 in the developing postnatal mouse hippocampus by using immunohistochemistry and western blotting and employing high‐resolution fluorescence microscopy. Astrocytes were identified by costaining with glial fibrillary acidic protein (GFAP) or S100β. In CA1, the density of GFAP‐positive cells and GFAP expression rose during the first 2 weeks after birth, paralleled by a steady increase in GLAST immunoreactivity and protein content. Upregulation of GLT‐1 was completed only at postnatal days (P) P20–25 and was thus delayed by about 10 days. GLAST staining was highest along the stratum pyramidale and was especially prominent in astrocytes at P3–5. GLAST immunoreactivity indicated no preferential localization to a specific cellular compartment. GLT‐1 exhibited a laminar expression pattern from P10–15 on, with the highest immunoreactivity in the stratum lacunosum‐moleculare. At the cellular level, GLT‐1 immunoreactivity did not entirely cover astrocyte somata and exhibited clusters at processes. In neonatal and juvenile animals, discrete clusters of GLT‐1 were also detected at perivascular endfeet. From these results, we conclude there is a remarkable subcellular heterogeneity of GLAST and GLT‐1 expression in the developing hippocampus. The clustering of GLT‐1 at astrocyte endfeet indicates that it might serve a specialized functional role at the blood–brain barrier during formation of the hippocampal network. J. Comp. Neurol. 522:204–224, 2014. © 2013 Wiley Periodicals, Inc.