Impermeability of hagfish cerebral capillaries to radio-labelled polyethylene glycols and to microperoxidase.

The rate and extent of penetration of radiolabelled polyethylene glycols (900 and 4000 daltons) from plasma into brain was examined in the Atlantic hagfish (Myxine glutinosa), a primitive vertebrate, and, for comparison, in the rat. Tracer penetration into brain was limited to a similar extent in hagfish and in rats, indicating a basic similarity between the two species in the blood-brain barrier to polar non-electrolytes. The permeability of hagfish brain capillaries to the electron microscopic tracer microperoxidase (2000 Daltons) was also examined. Intravenously injected microperoxidase remained confined to the capillary lumen and was not found in the basal lamina or surrounding brain parenchyma. In contrast to an earlier study, these results indicate that the hagfish has a blood-brain barrier to hydrophilic molecules larger than 900 Daltons. It is concluded that a blood-brain barrier is a general characteristic of all vertebrates.