Interference of the nutritional condition of the rat with peripheral glucose regulation determined by CNS mechanisms

Glucose and insulin levels of the cerebrospinal fluid (CSF) were changed by an infusion of either glucose, insulin or a mixture of glucose and insulin in the third ventricle of freely moving undisturbed rats. Before, during and after infusions venous blood samples were withdrawn to determine insulin- and glucose concentrations. In rats on carbohydrate rich food neither plasma insulin nor blood glucose changed during infusion of either glucose, insulin or a mixture of both. However, in rats on carbohydrate free food an immediate decline in blood glucose of about 7 mg/dl occurred during and after glucose infusion. During insulin infusion in the third ventricle a rise in blood glucose of about 7 mg/dl occurred 15 min after the start of the infusion. During infusion of a mixture of glucose and insulin, blood glucose rose slightly and reached a value which was in between those attained when glucose and insulin were infused separately. Plasma insulin did not change following central infusions. It is argued that the central nervous system (CNS) is sensitive to changes in CSF levels of glucose and insulin and affects peripheral glucose homeostasis mainly by changing glucose output from the liver. This action of the CNS, however, is dependent on the nutritional condition of the animal.