Abstract
Ghrelin, an endogenous appetite hormone, is secreted from the gastro-intestinal tract, and the ghrelin-induced appetite is conveyed through the afferent vagus nerves. However, it has not been elucidated how the orexigenic sensation produced by ghrelin is transmitted to the afferent vagus nerves that innervate gastro-intestinal tract. To address this issue, ghrelin-induced food intake in Wister rats was evaluated in the presence or absence of vagal denervation or the administration of a type-3 serotonin receptor antagonist ramosetoron. Cumulative food intake was significantly increased 1 hr and 2 hr after the intraperitoneally-applied ghrelin (20 μg/kg). Desensitization of C-fiber of the afferent vagus nerves by intraperitoneally-applied capsaicin completely abolished the increases in the ghrelin-induced food intake. Surgical vagotomy of the hepatic or gastric branches markedly suppressed the ghrelin-evoked feeding. Furthermore, pretreatment with ramosetoron also significantly suppressed the ghrelin-evoked feeding. Stimulatory effects of ghrelin on the appetite were transient and the overall food intake was comparable after 24 hr among all groups. These data suggest that 1) exogenous ghrelin rapidly promotes feeding, but such feeding lasts no more than 2 hr; 2) afferent C-fiber of the hepatic or gastric vagus nerves appear to convey ghrelin-induced hungry sensation to the brain; 3) type-3 serotonin receptors at the nerve endings of the C-fiber of the gastric or hepatic afferent vagus nerves may involve in transmitting the ghrelin-evoked hungry sensation.
