Background & aims: Visceral sensory information is transmitted to the brain through the afferent vagus nerve. Ghrelin, a peptide primarily produced in the stomach, stimulates both feeding and growth hormone (GH) secretion. How stomach-derived ghrelin exerts these central actions is still unknown. Here we determined the role of the gastric afferent vagal nerve in ghrelin's functions.
Methods: Food intake and GH secretion were examined after an administration of ghrelin intravenously (IV) to rats with vagotomy or perivagal application of capsaicin, a specific afferent neurotoxin. We investigated Fos expression in neuropeptide Y (NPY)-producing and growth hormone-releasing hormone (GHRH)-producing neurons by immunohistochemistry after administration IV of ghrelin to these rats. The presence of the ghrelin receptor in vagal afferent neurons was assessed by using reverse-transcription polymerase chain reaction and in situ hybridization histochemistry. A binding study on the vagus nerve by (125)I-ghrelin was performed to determine the transport of the ghrelin receptor from vagus afferent neurons to the periphery. We recorded the electric discharge of gastric vagal afferent induced by ghrelin and compared it with that by cholecystokinin (CCK), an anorectic gut peptide.
Results: Blockade of the gastric vagal afferent abolished ghrelin-induced feeding, GH secretion, and activation of NPY-producing and GHRH-producing neurons. Ghrelin receptors were synthesized in vagal afferent neurons and transported to the afferent terminals. Ghrelin suppressed firing of the vagal afferent, whereas CCK stimulated it.
Conclusions: This study indicated that the gastric vagal afferent is the major pathway conveying ghrelin's signals for starvation and GH secretion to the brain.