Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including the regulation (decrease) of appetite and (increase) of metabolism. Leptin circulating  levels give the brain a reading of energy storage for the purposes of regulating appetite and metabolism. Leptin works by inhibiting the activity of neurons that contain neuropeptide Y (NPY) and agouti-related peptide (AgRP), and by increasing the activity of neurons expressing α-melanocyte-stimulating hormone (α-MSH). The NPY neurons are a key element in the regulation of appetite; small doses of NPY injected into the brains of experimental animals stimulates feeding, while selective destruction of the NPY neurons in mice causes them to become anorexic. Conversely, α-MSH is an important mediator of satiety, and differences in the gene for the receptor at which α-MSH acts in the brain are linked to obesity in humans.

It is unknown whether leptin must cross the blood-brain barrier to access receptor neurons, because the blood-brain barrier is somewhat absent in the area of the median eminence, close to where the NPY neurons of the arcuate nucleus are. If it does cross the blood-brain barrier, it is unknown whether this occurs via an active or passive process. It is generally thought that leptin might enter the brain at the choroid plexus, where there is intense expression of a form of leptin receptor molecule that might act as a transport mechanism.

Once leptin has bound to the Ob-Rb receptor, it activates the molecule stat3, which is phosphorylated and travels to the nucleus, it is presumed, to effect changes in gene expression. One of the main effects on gene expression is the down-regulation of the expression of endocannabinoids, responsible—among their many other functions—for increasing appetite. There are other intracellular pathways activated by leptin, but less is known about how they function in this system. In response to leptin, receptor neurons have been shown to remodel themselves, changing the number and types of synapses that fire onto them.

In mice, leptin is also required for male and female fertility. In mammals generally, and in humans in particular, puberty in females is linked to a critical level of body fat. When fat levels fall below this threshold (as in anorexia), the ovarian cycle stops and females stop menstruating.