Circumventricular organs (CVOs) were identified in 1958 as brain structures with similar specialized morphological characteristics and a common phylogenetic ancestry. Metabolism in CVOs is specialized, resembling that of both neural and endocrine systems. Cerebral tissue, under physiological conditions, utilizes only glucose as an energy source, with metabolism via the tricarboxylic acid cycle. In contrast, endocrine organs can use a variety of circulating substances as substrates for their metabolic activity. Triglycerides and free fatty acids are used as well as glucose that can be metabolized through the hexosemonophosphate shunt. Using histochemistry, glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the hexosemonophosphate shunt has been localized in high activity in CVOs and in magnocellular neurons of the hypothalamo-neurohypophyseal system. Using cytochemical techniques, in the rat brain the distribution of G6PD activity and of the Type I pathway for hydrogen utilization from NADPH in CVOs surrounding the third cerebroventricle and in the magnocellular system of neurons were examined. One hypothesis being tested was that, in the absence of a blood–brain barrier, a chemical barrier in the form of the mixed function oxidase system developed in CVOs to protect these brain regions from circulating substances that could be harmful.
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