SOME ASPECTS OF CARBOHYDRATE-METABOLISM IN THE BRAIN

A convenient physiology of the nervous system closely depends on the a vailability of glucose, the lack of which quickly results in syncope a nd death. Carbohydrate metabolism in the brain was long thought of as being specific and different from liver carbohydrate metabolism. The p resent report tries to summarize current data and advances in our know ledge about carbohydrate metabolism. Glucose is brought to the brain b y blood flowing through a special network of arteries and is quickly c atabolized by the glycolytic and tricarboxylic acid cycle pathways to synthesize energy. It is also used in the synthesis of numerous amino acids, nucleotides and NADPH. Glucose can be polymerized into glycogen in the brain. The nerve tissue is capable of synthesizing glucose-6-p hosphate in the gluconeogenic pathway since the fructose-1,6-bisphosph atase, the key enzyme believed to be absent, is actually active and ha s been purified up to electrophoretic homogeneity. Moreover, the possi bility of free glucose synthesis by astrocytes exists. Although the ex act role of glycogen in the brain is not totally clear, it is known th at the polysaccharide content generally decreases when the functioning of the brain is stimulated and increases in sedative state. This carb ohydrate can therefore serve as an indicator for the level of brain ac tivity. Through the administration of methionine sulfoximine, it is po ssible to increase the amount of glycogen in the brain massively and o btain particles similar to those found in the liver. These in vivo fin dings have been confirmed by studies based on cultured astrocytes. It has been shown with cultured astrocytes that glutamate increases glyco gen synthesis in a pathway which still remains to be elucidated. Brain carbohydrate metabolism is thus in many ways similar to liver carbohy drate metabolism. The astrocyte constitutes the main cell implicated i n this metabolism. Improvement in our knowledge about brain carbohydra te metabolism should spread the use of brain glucose metabolism in the diagnosis of certain diseases.