Leptin is secreted primarily from white adipose tissue, and to a lesser extend from the bone marrow, the placenta and stomach. Leptin circulates in the blood, bound to a protein, enters the brain, binds to its receptors in the hypothalamus and acts on central neural networks.
Normally, Leptin signals the brain to inhibit feeding. If Leptin is absent, feeding is uncontrolled and relentless. The brain does not get the message to stop eating.