Species differences in the localisation of gamma-glutamyl transpeptidase immunopositive cells at the blood-brain interface.

The expression of gamma-glutamyl transpeptidase (GGT) has been associated with the emergence of a functional blood-brain barrier. We have undertaken a precise localisation of this enzyme in the cerebral cortical vessels of different species, by immunocytochemistry using a polyclonal antibody and light and electron microscopy. GGT immunoreaction product was present on the luminal surface of endothelial cells in 1-day-old to 3-month-old rats, whereas in the mouse, monkey and human cortex, this protein was present in astrocytic endfeet around the vessels. No labelling was observed in the other cellular components of the vessel walls, such as pericytes, fibroblasts, smooth muscle cells and perivascular cells. The localisation of GGT in astrocytes in mice, monkeys and humans suggests that these cells could play a role in the detoxication of lipophilic xenobiotic substances that cross the endothelial barrier. In these species, astrocytes can be viewed as a second line of defense against xenobiotics, beyond the capillary endothelium.