Distribution of mRNA for the facilitated urea transporter UT3 in the rat nervous system

Recently, the cDNA encoding the rat urea transporter UT3 has been cloned from rat kidney. Here we describe the cellular localization of this transporter in the brain as detected by non-radioactive in situ hybridization. UT3 is expressed in astrocytes throughout the central nervous system as well as in Bergmann glia in the cerebellum. The expression in astrocytes was verified by double staining using the astrocytic marker GFAP. UT3 mRNA is also strongly expressed by the ependymal cells lining the cerebral ventricles and by Müller cells in the retina. Furthermore, UT3 expression was detected in subgroups of neurons in the inferior colliculus and dorsal root ganglia, as well as in cells in the anterior pituitary gland. Other types of brain cells, including oligodendrocytes, microglia, tanycytes, endothelial cells of blood vessels, and epithelial cells in the choroid plexus were devoid of UT3 mRNA. Northern blot analysis confirmed that the mRNA species in the brain and in dorsal root ganglia are identical, and that cultured astrocytes and C6 cells also express the UT3 mRNA. UT3 mRNA expression by astrocytes is markedly upregulated in quinolinic acid-induced gliosis, possibly as a result of increased urea levels during gliosis induced polyamine formation. We propose that UT3 in astrocytes represents a mechanism to control urea formed in the brain by equilibrating it throughout the astrocyte network and guiding it to blood vessels and the CSF for disposal.