Role of glucocorticoids in mediating effects of fasting and diabetes on hypothalamic gene expression

Abstract

Background: Fasting and diabetes are characterized by elevated glucocorticoids and reduced insulin, leptin, elevated hypothalamic AGRP and NPY mRNA, and reduced hypothalamic POMC mRNA. Although leptin replacement can reverse changes in hypothalamic gene expression associated with fasting and diabetes, leptin also normalizes corticosterone; therefore the extent to which the elevated corticosterone contributes to the regulation of hypothalamic gene expression in fasting and diabetes remains unclear. To address if elevated corticosterone is necessary for hypothalamic responses to fasting and diabetes, we assessed the effects of adrenalectomy on hypothalamic gene expression in 48-hour-fasted or diabetic mice. To assess if elevated corticosterone is sufficient for the hypothalamic responses to fasting and diabetes, we assessed the effect of corticosterone pellets implanted for 48 hours on hypothalamic gene expression.

Results: Fasting and streptozotocin-induced diabetes elevated plasma glucocorticoid levels and reduced serum insulin and leptin levels. Adrenalectomy prevented the rise in plasma glucocorticoids associated with fasting and diabetes, but not the associated reductions in insulin or leptin. Adrenalectomy blocked the effects of fasting and diabetes on hypothalamic AGRP, NPY, and POMC expression. Conversely, corticosterone implants induced both AGRP and POMC mRNA (with a non-significant trend toward induction of NPY mRNA), accompanied by elevated insulin and leptin (with no change in food intake or body weight).

Conclusion: These data suggest that elevated plasma corticosterone mediate some effects of fasting and diabetes on hypothalamic gene expression. Specifically, elevated plasma corticosterone is necessary for the induction of NPY mRNA with fasting and diabetes; since corticosterone implants only produced a non-significant trend in NPY mRNA, it remains uncertain if a rise in corticosterone may be sufficient to induce NPY mRNA. A rise in corticosterone is necessary to reduce hypothalamic POMC mRNA with fasting and diabetes, but not sufficient for the reduction of hypothalamic POMC mRNA. Finally, elevated plasma corticosterone is both necessary and sufficient for the induction of hypothalamic AGRP mRNA with fasting and diabetes.

Figure 1

Effects of fasting on blood hormones in intact and adrenalectomized mice. (A) Fasting induces serum corticosterone in intact mice but not in adrenalectomized mice. Fasting decreases both serum insulin (B), and serum leptin (C) to the same levels in intact and adrenalectomized mice. Data are expressed as mean ± SEM. Groups with different letters are statistically different (p < 0.05), reflecting ANOVA followed by Tukey-Kramer post hoc tests comparing every group to every other group. Thus groups with the same letter do not differ from each other at a p < 0.05 level.

Figure 2

Effects of fasting on hypothalamic gene expression in intact and fasted mice. (A) Hypothalamic AGRP is induced by fasting in intact mice however the fasting associated induction is blocked by adrenalectomy. (B) Fasting reduces hypothalamic POMC mRNA in intact mice but not in adrenalectomized mice. (C) Similar to AGRP, hypothalamic NPY mRNA is induced in intact mice but not in adrenalectomized mice. Data are expressed as mean ± SEM. Groups with different letters are statistically different (p < 0.05), reflecting ANOVA followed by Tukey-Kramer post hoc tests comparing every group to every other group. Thus groups with the same letter do not differ from each other at a p < 0.05 level.

Figure 3

Effects of adrenalectomy, streptozotocin injection and fasting on levels of blood hormones. (A) Streptozotocin injection elevates levels of serum corticosterone however adrenalectomy prevents the induction associated with streptozotocin. Both serum insulin (B), and serum leptin (C) are reduced to the same extent in all experimental conditions. Data are expressed as mean ± SEM. Groups with different letters are statistically different (p < 0.05), reflecting ANOVA followed by Tukey-Kramer post hoc tests comparing every group to every other group. Thus groups with the same letter do not differ from each other at a p < 0.05 level.

Figure 4

Effects of adrenalectomy, streptozotocin injection and fasting on levels of hypothalamic gene expression. (A) Hypothalamic AGRP mRNA is induced by streptozotocin injection, however the induction is partially blocked by adrenalectomy. (B) Hypothalamic POMC mRNA is reduced by streptozotocin injection, but adrenalectomy completely blocks the reduction associated with streptozotocin. (C) Similar to AGRP, NPY mRNA is induced by streptozotocin injection and partially blocked by adrenalectomy. Data are expressed as mean percentage ± SEM of the control group. Data are expressed as mean ± SEM. Groups with different letters are statistically different (p < 0.05), reflecting ANOVA followed by Tukey-Kramer post hoc tests comparing every group to every other group. Thus groups with the same letter do not differ from each other at a p < 0.05 level.

Figure 5

Effects of corticosterone implants on levels of blood hormones. Corticosterone implants elevates levels of serum corticosterone (A), insulin (B), and leptin (C). Statistical significance was determined by unpaired student's t-test (p < 0.05) and is denoted by *.

Figure 6

Effects of corticosterone implants on hypothalamic gene expression. Corticosterone implants induces both hypothalamic AGRP (A) and POMC (B) mRNA. (C) NPY mRNA is also elevated by corticosterone implants but the effect did not reach statistical significance. Data are expressed as a mean percentage ± SEM of placebo implant group. Statistical significance was determined by unpaired student's t-test (p < 0.05) and is denoted by *.