Chapter 29: Tracer uptake by circumventricular organs — a relative measure of blood supply to the brain

Abstract

The permeability difference between “tight brain regions” and “leaky brain structures” is illustrated by example, in which the tritiated adenosine analog N -ethylcarboxamidoadenosine was injected intracarotidally. The radioactivity was in this case revealed by autoradiography. The tracer amount accumulated in a given experiment by the blood–brain barrier (BBB)-protected, tight brain regions frequently shows a rather pronounced fluctuation from one animal to another. One main reason for this is that the portion of the bolus injected into the common carotid artery that distributes to the brain via the internal carotid artery varies, because of the variable amounts of the bolus diverted largely to the external carotid and pterygopalatine arteries. To be able to disregard interindividual uptake differences in the tight brain regions caused by such a differing tracer supply, it has proved useful to relate the measured accumulation values to the corresponding values obtained for the circumventricular organs. Because of the leakiness of their capillaries, the tracer uptake by these regions indicates very well the actual tracer supply to the brain.