Nicotinamide phosphoribosyltransferase and the hypothalamic-pituitary-adrenal axis of the rat

Nicotinamide phosphoribosyltransferase (Nampt), also termed visfatin, catalyses the rate‑limiting step in the nicotinamide adenine dinucleotide (NAD) salvage pathway. In addition to its intracellular function (iNampt), extracellular Nampt (eNampt) also affects numerous intracellular signalling pathways. The current study investigated the role of Nampt in the regulation of the hypothalamic‑pituitary‑adrenal (HPA) axis in rats. At 1 h after intraperitoneal administration of eNampt (4 µg/100 g) in adult male rats, serum adrenocorticotropic hormone(ACTH) and aldosterone levels remained unchanged, while corticosterone levels were notably elevated compared with the control group, as determined by ELISA. The results of reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) demonstrated that, in the hypothalami of eNampt‑treated rats, the mRNA expression levels of Fos proto‑oncogene, which is also termed c‑Fos, were not significantly different compared with the control group; however, the mRNA expression levels of proopiomelanocortin (POMC) were markedly increased in the pituitary gland of eNampt‑treated rats compared with the control group. Furthermore, in hypothalamic explants, ELISA results demonstrated that the addition of the eNampt protein exhibited no effect on corticotropin‑releasing hormone (CRH) release into the incubation medium and prevented potassium ion‑induced CRH release. Additionally, the eNampt‑induced increase in ACTH output by pituitary gland explants was not statistically significant, compared with the control group. However, RT‑qPCR indicated that exposure of pituitary gland explants to eNampt and CRH increased the levels of POMC mRNA expression; the effect of eNampt, but not CRH, was inhibited by FK866, which is a specific Nampt inhibitor. In primary rat adrenocortical cell cultures, eNampt exhibited no effect on basal aldosterone or corticosterone secretion, while increases in aldosterone and corticosterone levels in response to ACTH were retained. To assess the potential role of iNampt in the regulation of adrenal steroidogenesis, experiments involving a specific Nampt inhibitor, FK866, were performed. Exposure of cultured cells to FK866 notably lowered basal aldosterone and corticosterone output compared with the control group, and completely eliminated the response of cultured cells to ACTH. The results of the present study indicated that the injected eNampt may have increased the corticosterone serum levels by acting at the pituitary level. In addition, iNampt may exert a tonic stimulating effect on the secretion of aldosterone and corticosterone from rat adrenocortical cells, as normal iNampt levels were required to retain the response of cultured rat adrenocortical cells to ACTH. Thus, these data suggest an important physiological role of both iNampt and eNampt in the regulation of the HPA axis activity in the rat.