The hypothalamus plays a pivotal role in the nervous control of glucose homeostasis. This area contains gluco-sensitive neurons. Some of them detect increases in glucose levels and regulate glucose homeostasis by stimulating insulin secretion or inhibiting food intake. It is widely accepted that astrocytes are metabolically coupled to neurons. Lactate, resulting from the metabolism of glucose by astrocytes, is transported via the monocarboxylate transporters (MCTs). In addition, gap junctions (GJ), that form networks within astrocytes, are essential to transfer glucose from the bloodstream to the active neurons. These astroglial GJ mainly consist of connexins 43 and 30 (Cxs).The aims of my thesis are twofold: first, to show that an intracarotid lactate injection toward the brain, as for glucose, triggers insulin secretion and, second, to investigate the role of astroglial Cxs.Our results demonstrate that lactate and glucose sensing are altered in 48h hyperglycemic rats (accompanied by high blood lactate level). These alterations are not due to changes in protein expression of astroglial or neuronal MCTs in the hypothalamus. We then show that Cx43 is highly expressed in astrocytic end-feet enwraping blood vessels, in medio-basal hypothalamus (MBH) where many gluco-sensitive neurons are present. The Cx30 expression is more diffuse in this structure. We also show that the protein expression of astroglial Cxs varies very rapidly due to changes in metabolic status (fasting, refeeding and hyperglycemia). To evaluate the involvement of astroglial Cx43 (the major isoform) in the hypothalamic glucose sensing, we silenced its expression in the MBH in vivo by injecting specific siRNA. A 30% diminution in protein levels (after 72h) induced a decrease in food intake without changes in weight, blood glucose and insulin levels compared to vehicle treated animals. The central response to glucose is drastically inhibited in terms of insulin secretion in siCx43 animals. Similarly, an intracarotid injection of glucose towards the brain does not reduce refeeding in siRNA treated animals.These results demonstrate for the first time in vivo, the importance of connexins and astroglial networks in hypothalamic glucose sensing mechanism. These new data reinforce the importance of the metabolic role of astrocytes in specific neuronal functions
Astrocytes and hypothalamic glucose sensing : metabolic role and involvement of astroglial connexins