Evidence that endogenous SST inhibits ACTH and ghrelin expression by independent pathways

Corticosterone and total ghrelin levels are increased in somatostatin (SST) knockout mice (Sst-/-) compared with SST-intact controls (Sst+/+). Because exogenous ghrelin can increase glucocorticoids, the question arises whether elevated levels of ghrelin contribute to elevated corticosterone levels in Sst-/- mice. We report that Sst-/- mice had elevated mRNA levels for pituitary proopiomelanocortin (POMC), the precursor of adrenocorticotropic hormone (ACTH), whereas mRNA levels for hypothalamic corticotropin-releasing hormone (CRH) did not differ from Sst+/+ mice. Furthermore, SST suppressed pituitary POMC mRNA levels and ACTH release in vitro independently of CRH actions. In contrast, it has been reported that ghrelin increases glucocorticoids via a central effect on CRH secretion and that n-octanoyl ghrelin is the form of ghrelin that activates the GHS-R1a and modulates CRH neuronal activity. Consistent with elevations in total ghrelin levels, Sst-/- mice displayed an increase in stomach ghrelin mRNA levels, whereas hypothalamic and pituitary expression of ghrelin was not altered. Despite the increase in total ghrelin levels, circulating levels of n-octanoyl ghrelin were not altered in Sst-/- mice. Because glucocorticoids and ghrelin increase in response to fasting, we examined the impact of fasting on the adrenal axis and ghrelin in Sst+/+ and Sst-/- mice and found that endogenous SST does not significantly contribute to this adaptive response. We conclude that endogenous SST inhibits basal ghrelin gene expression in a tissue specific manner and independently and directly inhibits pituitary ACTH synthesis and release. Thus endogenous SST exerts an inhibitory effect on ghrelin synthesis and on the adrenal axis through independent pathways.