Elevated expression of glucose transporter-1 in hypothalamic ependymal cells not involved in the formation of the brain-cerebrospinal fluid barrier

Glucose transporters play an essential role in the acquisition of glucose b y the brain. Elevated expression of glucose transporter-1 has been detected in endothelial cells of the blood-brain barrier and in choroid plexus cell s of the blood-cerebrospinal fluid barrier. On the other hand, there is a p aucity of information on the expression of glucose transporters in the epen dymal cells that line the walls of the cerebral ventricles. The tanycytes a re specialized ependyma[ cells localized in circumventricular organs such a s the median eminence that can be segregated into at least three types, alp ha, beta1 and beta2. The beta2 tanycytes form tight junctions and participa te in the formation of the cerebrospinal fluid-median eminence barrier. Usi ng immunocytochemistry and in situ hybridization, we analyzed the expressio n of hexose transporters in rat and mouse hypothalamic tanycytes. In both s pecies, immunocytochemical analysis revealed elevated expression of glucose transporter-1 in alpha and beta1 tanycytes. Intense anti-glucose transport er-1 staining was observed in cell processes located throughout the arcuate nucleus, in the end-feet reaching the lateral sulcus of the infundibular r egion, and in cell processes contacting the hypothalamic capillaries. On th e other hand, there was Very low expression of glucose transporter-1 in bet a2 tanycytes involved in barrier function. In contrast with the results of the cytochemical analysis, in situ hybridization revealed that tanycytes al pha, beta1, and beta2 express similar levels of glucose transporter-1 mRNA. Further analysis using anti-glial fibrillary acidic protein antibodies to identify areas rich in astrocytes revealed that astrocytes were absent from areas containing alpha and beta1 tanycytes, but were abundant in regions c ontaining the barrier-forming beta2 tanycytes. Overall, our data reveal a l ack of correlation between participation in barrier function and expression of glucose transporter-1 in hypothalamic tanycytes. Given the virtual abse nce of astrocytes in areas rich in alpha and beta1 tanycytes, we speculate whether the tanycytes might have astrocyte-like functions and participate i n the metabolic coupling between glia and neurons in the hypothalamic area. J. Cell. Biochem. 80:491-503, 2001. (C) 2001 Wiley-Liss, Inc.