Hunger signals

The peptide ghrelin is a hunger signal that acts in both the hypothalamus and the reward system to increase food intake. In general blood concentrations of ghrelin are lower in obese and higher in anorexic individuals compared to those with normal body mass implying that ghrelin release is inversely related to energy intake. There is also an inverse relation between hours of sleep per night and blood ghrelin concentrations. The fewer hours slept the higher the ghrelin, and individuals who sleep less are more likely to be obese.

Ghrelin cannot cross the blood–brain barrier. It exerts its effects by three routes:

- As a peripheral signal  secreted from  the  epithelium of the  empty stomach it enters the  circulation (i.e., it is a gut  hormone) and  acts  on  ghrelin receptors on  the  area postrema.

- Locally in the gut it acts on terminals of vagus nerve axons that run to the NST.

- It is a transmitter of neurons in the hypothalamus. It acts presynaptically to excite orexigenic (NPY/AgRP) neurons, and on corticotrophin releasing hormone (CRH) neurons in the hypothalamus.

Neuropeptide Y (NPY) is a gut hormone and the most potent orexigenic agent known. It is also a transmitter in hypothalamic orexigenic pathways.The area postrema is a circumventricular organ located adjacent to the nucleus of the solitary tract (NST). It has chemosensory cells that respond to a variety of peptides. In response to toxins in the blood it triggers vomiting.