Ghrelin and the vagus nerve.

Ghrelin, a gastrointestinal hormone, stimulates feeding and secretion of growth hormone (GH). Ghrelin is thought to directly affect neurons involved in feeding or GH secretion through growth hormone secretagogue receptor (GHS-R; ghrelin receptor); however, it is still unclear whether ghrelin crosses through the blood-brain barrier. Recently, several gastrointestinal hormones have been shown to transmit signals involved in feeding to the brain at least in part via the vagal afferent system. In fact, ghrelin's action on feeding or GH secretion is abolished or attenuated in rats that have undergone vagotomy or treatment with capsaicin, a specific afferent neurotoxin. GHS-R is present in the vagal afferent neurons as well as the brain and is transported to the afferent terminals. In addition, the firing rate of vagal afferent fibers significantly decreases after ghrelin administration. Taken together, these data show that the vagal afferent system is the major pathway conveying ghrelin's signals for feeding and GH secretion to the brain.