Participation of the hypothalamus-hypophysis axis in the sympathetic activation of human obesity.

Previous studies have shown that hypothalamic and hypophyseal factors are involved in the acute sympathoexcitation induced by a variety of laboratory stimuli. Whether a chronic condition of sympathetic activation, such as that characterizing human obesity, is also dependent on these factors has never been investigated. In 40 normotensive obese subjects ([mean+/-SEM] age, 39.1+/-0.8 years) we measured blood pressure (Finapres), heart rate (ECG), and postganglionic muscle sympathetic nerve activity (MSNA) (microneurography). In 20 subjects measurements were repeated, according to a double-blind randomized sequence, after a midnight oral dose of dexamethasone (1 mg) (n=10) or placebo (n=10), while in the remaining subjects they were performed again after 1 week of a daily evening oral administration of 1 mg of dexamethasone (n=10) or placebo (n=10). The same protocol was performed in 16 age-matched lean normotensives. In both groups acute dexamethasone administration markedly reduced plasma cortisol (radioimmunoassay), without affecting hemodynamic and neural variables. In contrast to the acute administration, in obese subjects prolonged dexamethasone administration, although not affecting blood pressure and heart rate, significantly reduced both plasma cortisol (from 16.0+/-1.3 to 0.7+/-0.1 microg/dL; P<0.01) and MSNA (from 59.5+/-2.8 to 39.6+/-2.9 bursts per 100 heartbeats; P<0.02; -33.1+/-4.1%). This was not the case in lean subjects, in which the dexamethasone-induced reduction in plasma cortisol was associated with a slight and nonsignificant MSNA decrease. In both lean and obese subjects, placebo administration caused no change in any variable. Thus, prolonged dexamethasone administration exerts in obese subjects marked sympathoinhibitory effects that are not detectable in lean individuals. This suggests that hypothalamic and hypophyseal factors substantially contribute to the sympathoexcitation of obesity.