Distribution of ghrelin-immunoreactive neuronal networks in the human hypothalamus

Ghrelin has been discovered as the endogenous ligand of the growth hormone secretagogue receptor (GHS-R). It stimulates growth hormone secretion and also potently increases food intake. To date, ghrelin is the only known peripheral orexigenic hormone. Recent studies have demonstrated that in addition to peripheral organs, ghrelin is also synthesized in the hypothalamus. In the present study, we examined the distribution of the ghrelin-immunoreactive (IR) elements in the human hypothalamus. Ghrelin-IR fibers were widely distributed throughout the hypothalamus. Based on the thickness of fibers, major subtypes of ghrelin-IR axons were observed: thick fibers with large varicosities and very fine axons with or without small varicosities. Dense networks of ghrelin-IR axons were observed in the hypothalamic suprachiasmatic, paraventricular, supraoptic, dorsomedial, ventromedial and infundibular nuclei and in the periventricular area. Ghrelin-IR axons also appeared in the external layer of the pituitary stalk. Ghrelin-IR cell bodies were not detected. Since hypothalamic regions innervated by ghrelin-IR axons also take part in the regulation of food intake and energy balance, the centrally synthesized ghrelin may play a major role in the central regulation of energy metabolism in humans.