The effect of serotonin on the blood-brain barrier to proteins.

Since increased concentration of serotonin (5-HT) has been demonstrated in areas of the brain exposed to ischemia and lesions, and since the elevation might be responsible for the enhanced permeability to proteins across cerebral vessels, studies were carried out to elucidate the effect of the amine, perfused through the cerebral ventricular system, on the transport of horseradish peroxidase (HRP) from blood to brain. The amounts of 5-HT were large (50--800 microgram per mouse). The permeability across cerebral vessels was increased, especially across arterioles. The endothelium was intact. HRP did not form a continuous line between endothelial cells, from the vessel lumen to the subendothelial basement membrane. Furthermore, channels through the endothelium, that could allow HRP to pass, were not observed. However, several vesicles, filled with HRP were observed in the cytoplasm of endothelial cells. They could be open to the vessel lumen or to the subendothelial basement membrane. Freely situated HRP-containing vesicles were also found. Based on the observations it is most reasonable to assume that the 5-HT, perfused through the cerebral ventricles increased the normally occurring vesicular transport of protein from blood to brain.