Expression Analysis of Hypothalamic and Pituitary Components of the Growth Hormone Axis in Fasted and Streptozotocin-Treated Neuropeptide Y (NPY)-Intact (NPY^+/+) and NPY-Knockout (NPY^–/–) Mice

Abstract

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY^+/+) and NPY-knockout (NPY^–/–) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY^+/+ and NPY^–/– did not differ. In the NPY^+/+ mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY^–/– mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY^–/– mouse. Induction of diabetes in NPY^+/+ mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY^+/+ and NPY^–/– mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY^–/– mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/fasting) control of growth hormone axis in mice

“…Fasting has been shown to suppress HPT SST mRNA in male mice and ewes (21,29,46), consistent with the current observations showing an overall reduction in HPT SST mRNA, which reached significance in females (Fig. 3A).…”

Differential responses of the growth hormone axis in two rat models of streptozotocin-induced insulinopenic diabetes

“…NPY maybe a key inhibitor of the GH axis in that intracerebroventricular administration of NPY inhibits pulsatile GH release in rats and decreases GHRH mRNA levels in both rats and mice (Pierroz et al 1996, Raposinho et al 2000, Sainsbury & Herzog 2001, Suzuki et al 1996. The significance of endogenous NPY in regulation of hypothalamic GHRH expression in catabolic states is supported by a recent report from our laboratory demonstrating that NPY-knockout mice do not exhibit fasting-induced suppression of GHRH mRNA (Park et al 2005). In that NPY levels did not rise as dramatically LO STZ-treated rats, compared with HI STZ rats, we might speculate that these changes were not adequate to suppress GHRH expression.…”

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

“…In Sst ϩ/ϩ mice, fasting tended to suppress hypothalamic and stomach SST mRNA levels; however, these effects did not reach statistical significance. A modest inhibitory effect of fasting on hypothalamic SST expression has been previously reported in mice (35). However, in the rat, fasting has been reported to enhance SST mRNA levels in the pyloric antrum but not the acid-secreting region of the stomach (38,48), although others report a regional increase or decrease in SST mRNA following fasting, depending on the housekeeping gene used as the internal control (49).…”