Hormones involved in hunger

Leptin is a protein produced by fat cells (i.e. adipocytes) that has important roles in weight regulation and metabolism. It is absent in some individuals who are severely obese at an early age. Leptin acts on hypothalamic centers that control hunger.

Leptin binds to its specific receptor - leptin receptor. Some people with obesity suffer from leptin resistance, in that their leptin receptors are defective and do not bind well to leptin. This causes exaggerated appetite. A person who feels hungry all the time is likely to eat more calories, leading to weight gain. Scientists have been working on ways to utilize the body's system to treat obesity.

The mice in the picture are genetically different - one has been programmed to be fat by affecting its hunger/eating/metabolism.

Ghrelin is a hormone produced by the stomach, and also to some extent in the intestine and hypothalamus. The molecule binds a receptor in the anterior pituitary gland that stimulates growth hormone secretion. But it also binds ghrelin receptors in the hypothalamus in an area known to be involved in the regulation of feeding and metabolism, as well as reward centers of the brain (in the hippocampus). Ghrelin causes a feeding response (i.e. hunger) and suppresses fat use in metabolism (i.e. weight retention).

Neuropeptide Y is an abundant protein in the brain that acts as a neurotransmitter. It has been pinpointed in feeding behavior, particularly to stimulate eating. The actions of leptin in hunger suppression include blocking this neuropeptide from acting in the hypothalamus. Neuropeptide Y has also been implicated in the regulation of fat metabolism - causing decreased fat use for energy and increased fat storage. Genetic mutations in NPY that lead to aberrant signaling by the hormone have been shown to result in obesity in rats.