Insulin receptors and signal transduction proteins in the hypothalamo-hypophyseal system: a review on morphological findings and functional implications

Receptors for insulin are widely distributed in the brain and pituitary. The current hypothesis on receptor function in these regions points to a role of insulin as a mediator in the communication of the peripheral endocrine system with the brain via various steps of the neuroendocrine axis. Recent data demonstrate that receptor-positive neurons in the brain, i.e. in the hypothalamus, and secretory cells in the anterior pituitary gland possess specific proteins that are thought to be involved in key steps of post receptor signal transduction, in particular insulin receptor substrate-1 and phosphatidylinositol 3'-kinase (PI3k). PI3k is a critical enzyme of the intracellular signaling pathway that is activated by a number of receptor tyrosine kinases, including receptors for insulin and IGF-1. This information further completes the framework indicating in vivo activity of insulin receptors in central neuroendocrine cells and their involvement in one branch of several physiological mechanisms that control body metabolism and nutritional behaviour.