Food in, energy out: how does the brain monitor fat stores to maintain balance?

Leptin is a hormone produced in, and released from, adipose tissue. One function of leptin is to regulate plasma glucose levels, much like insulin. More specifically, eating increases leptin in blood, which acts in the brain to excite the fight-or-flight sympathetic nervous system. One physiologic consequence is stimulation of glucose uptake, mediated by sympathetic nerves innervating skeletal muscle. In addition, because the plasma levels of leptin increase with more fat, another major function is that leptin tells the brain how much stored fat there is: leptin is the body’s fat meter. This second function is important, because leptin (fat) levels affect essentially every system in the body (e.g. reproduction, immune system, body temperature, etc.) via actions in the brain; it is integrative. Leptin receptors are found throughout the brain, but particularly in the hypothalamus. Interestingly, one hypothalamic area, the paraventricular nucleus, expresses scarce or undetectable leptin receptors. Yet, leptin acts here to activate the sympathetic nervous system, raising the question: how can leptin can act in areas with few or no receptors? A main theme is that leptin complements insulin in its actions: whereas insulin levels rise rapidly to stimulate glucose uptake by muscle, plasma leptin levels increase and act slowly, which is appropriate for a long-term regulator of energy balance, as well as other physiological systems in the body.

Speaker

Virginia Leah Brooks

Oregon Health & Science University

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