THE GLUCOCORTICOID ANTAGONIST MIFEPRISTONE REVEALS ABNORMAL REGULATION OF THE ADRENOCORTICAL SYSTEM IN OBESE ZUCKER RATS

The effects of the potent glucocorticoid type-II receptor (GR) antagonist, mifepristone, on corticosterone secretion and on expression of preprocorticotrophin-releasing factor (preproCRF) mRNA in the hypothalamic paraventricular nucleus (PVN) and of pro-opiomelanocortin (POMC) mRNA in the pituitary gland were investigated in lean and obese Zucker rats. Treatment with mifepristone for 4 days (10 mg/kg orally twice daily) significantly (P < 0.05) stimulated corticosterone secretion in lean but not in obese rats. In lean rats the enhanced corticosterone secretion was associated with non-significant increments in the expression of preproCRF mRNA in the PVN and of POMC mRNA in the pituitary gland, while mifepristone significantly (P < 0.05) reduced the expression of preproCRF mRNA in the PVN of obese Zucker rats. It is concluded that antagonism of GR by mifepristone results in persistent activation of the adrenocortical axis in lean Zucker rats due to blockade of feedback inhibition by circulating corticosterone. In obese animals the abnormal response to mifepristone suggests that the neuroendocrine control of the HPA axis is altered in genetically determined obesity.