Glucocorticoid inhibition of neurohypophysial vasopressin secretion.

Several lines of evidence have suggested that neurohypophysial vasopressin secretion is under the influence of glucocorticoid negative feedback. Studies in clinical and experimental adrenal insufficiency have suggested that the impaired water excretion accompanying that syndrome may be due to elevated vasopressin levels. Furthermore, both the impaired water excretion and elevated vasopressin levels observed in adrenal insufficiency may be normalized by glucocorticoid treatment. This topic remains controversial, with a considerable body of evidence suggesting that vasopressin is elevated during adrenal insufficiency not because of a loss of central steroid negative feedback but because of alterations in plasma volume osmolality (renal mechanisms). Vasopressin responses to a variety of stimuli (hemorrhage, hypoxia, hypertonic saline) in normal humans and animals appear to be attenuated or eliminated by pretreatment with glucocorticoids. However, the vasopressinergic system appears to be considerably less sensitive to negative feedback than the corticotropin-releasing factor-adrenocorticotropic hormone (ACTH) system. There is evidence that the locus for this inhibitory effect is both directly at the posterior pituitary and within the hypothalamus. It is unlikely that corticosteroid negative feedback closes a direct hypothalamo-neurohypophysial-adrenocortical feedback loop. Since neurohypophysial vasopressin is involved in the control of ACTH secretion, it is more likely that the modulation of neurohypophysial vasopressin by glucocorticoid is an integral part of the overall negative-feedback control of ACTH secretion. The physiological role of glucocorticoid inhibition of vasopressin secretion remains speculative.