Adrenal steroid receptor activation in rat brain and pituitary following dexamethasone: implications for the dexamethasone suppression test.

The dexamethasone suppression test (DST) has been used extensively to evaluate feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis by adrenal steroids. Nevertheless, it remains unclear at what level of the HPA axis and through which adrenal steroid receptor subtype dexamethasone exerts its inhibitory effect. Because adrenal steroid receptor activation is an important prerequisite for dexamethasone to affect cellular function, HPA axis tissues that exhibit evidence of receptor activation following dexamethasone administration are likely site(s) of action for this synthetic hormone to inhibit HPA axis activity. Therefore, type-I and type-II adrenal steroid receptor activation was assessed in the pituitary, hypothalamus, and hippocampus of intact and adrenalectomized rats after overnight exposure to various oral doses of dexamethasone. Results with dexamethasone were compared to similar studies using corticosterone, the endogenous glucocorticoid of the rat. All dexamethasone doses led to significant type-II receptor activation in the pituitary, whereas only an exceedingly high dexamethasone dose activated type-II receptors in the hippocampus and hypothalamus. Dexamethasone had little effect on type I receptors in any tissue at any dose. In contrast, corticosterone significantly activated type-I receptors in all tissues, whereas it activated type-II receptors in the brain and not the pituitary at physiological concentrations. Because dexamethasone activated pituitary type-II receptors at blood concentrations that did not activate type-II receptors in the brain, these results suggest that the DST in humans may primarily be a measure of type-II adrenal steroid receptor feedback inhibition at the level of the pituitary.