Membrane-transport systems in the fenestrated capillaries of the area postrema in rat and calf.

The capillaries of the area postrema (AP) lack the morphological peculiarity of the blood-brain barrier (BBB), and the AP neurons are considered located outside the BBB. Using the immunofluorescent method, we have investigated the expression of membrane transport systems that are instrumental to the BBB function, such as caveolin-1, -2, P-glycoprotein, and glut-4, in the capillary endothelium of the rat and calf AP. The expression of these molecules was verified after fibronectin immunostaining of the microvessels. Both in the rat and calf, caveolin-1, -2, and P-glycoprotein were expressed in the AP capillaries. A quantitative analysis revealed that the proportion of the capillary profiles expressing these transport systems was very close to 100% of the fibronectin immunolabelled profiles. On the contrary, none of the AP capillaries showed glut-4 immunoreactivity. The present investigation demonstrates that the endothelial layer of the AP capillaries, in spite of the paracellular passage of polar molecules through the leaky tight junctions and fenestrations, could be an active interface which is able to control the entry of a wide range of blood-borne compounds into the brain by means of specific mechanisms, including an efflux pump.