Leptin and insulin action in the central nervous system

Body adiposity is known to be carefully regulated and to remain relatively stable for long periods of time in most mammalian species. This review summarizes old and recent data implicating insulin and leptin as key circulating signals to the central nervous system, particularly the ventral hypothalamus, in communicating the size and the distribution of body fat stores. This input ultimately alters food intake and energy expenditure to maintain constancy of the adipose depot The key primary neurons in the arcuate nucleus containing NPY/AgRP and POMC/CART appear be critical constituents of the CNS regulating system, and are shown to contribute to anabolic and catabolic signaling systems to complete the feedback loop. New data to indicate shared intracellular signaling from leptin and insulin is provided. The satiety system for meals, consisting of neural afferents to the hind-brain from the gastrointestinal tract, is described and its effectiveness is shown to vary with the strength of the insulin and leptin signals. This provides an efferent mechanism that plays a key role in a complex feedback system that allows intermittent meals to vary from day to day, but provides appropriate long-term adjustment to need. Recently described contributions of this system to obesity are described and potential therapeutic implications are discussed.