Distribution of the glutamate transporters GLAST and GLT-1 in rat circumventricular organs, meninges, and dorsal root ganglia

Abstract

The glial glutamate transporters GLAST and GLT-1 are primarily responsible for the removal of glutamate from brain extracellular fluid. This study compares the distribution of GLAST and GLT-1 expression in the circumventricular organs of the brain, in the meninges, and in the dorsal root ganglion. By using a highly sensitive nonisotopic in situ hybridization method and immunostaining, we demonstrate marked differences in the expression patterns for the two transporters. In the three sensory circumventricular organs that contain neuronal elements, i.e., the subfornical organ, the vascular organ of the lamina terminalis, and the area postrema, GLAST is strongly expressed, whereas GLT-1 is faintly expressed or absent. Both transporters are absent from the choroid plexus, and only GLAST mRNA is found in the subcommisural organ. In the pineal gland, GLAST is expressed by astrocytic cells near the pineal stalk, whereas GLT-1 is expressed by pinealocytes throughout the gland. In the pituitary gland, GLAST is likely expressed by folliculo-stellate cells in the anterior lobe, by a group of astrocyte-like cells and by marginal cells in the intermediate lobe, and by pituicytes in the posterior lobe, whereas GLT-1 is expressed only by the astrocyte-like cells in the intermediate lobe. Finally, GLAST, but not GLT-1, is expressed by specific layers of the meninges, and by satellite cells in the dorsal root ganglion. These results show that GLAST is the primary glutamate transporter in the circumventricular organs. The data provide further evidence that these two glutamate transporters fulfill markedly different functions in the nervous system.