Interaction of arginine vasopressin and corticotropin releasing factor demonstrated with an improved bioassay

Effect of exogenous glucocorticoid on osmotically stimulated antidiuretic hormone secretion and on water reabsorption in man

OBJECTIVE

Glucocorticoids exert tonic suppression of antidiuretic hormone (ADH) secretion. Hypocortisolism in secondary adrenocortical insufficiency can result in a clinical picture similar to the syndrome of inappropriate ADH secretion. On the other hand, in vitro and in vivo results provide evidence for ADH suppression in states of hypercortisolism. To test the hypothesis that ADH suppression is of relevance during glucocorticoid therapy, we investigated the influence of prednisolone on the osmotic stimulation of ADH.

DESIGN AND METHODS

Seven healthy men were subjected to water deprivation tests with the measurement of plasma ADH (pADH) and osmolality (posmol) before and after glucocorticoid treatment (5 days 30 mg prednisolone per day).

RESULTS

Before glucocorticoid treatment, the volunteers showed a normal test with an adequate increase of pADH (basal 0.54 +/- 0.2 to 1.9 +/- 0.72 pg/ml (mean +/- S.D.)) in relation to posmol(basal 283.3 +/- 8.5 to 293.7 +/- 6 mosmol/kg). After prednisolone intake, pADH was attenuated (<0.4 pg/ml) in spite of an increase of posmol from 289.3 +/- 3.6 to 297.0 +/- 5.5 mosmol/kg. However, urine osmolar concentration increased normally during water deprivation after prednisolone. Urinary cAMP excretion increased during water deprivation without glucocorticoid treatment from 3.56 +/- 0.55 to 6.07 +/- 0.76 micro mol/l, reflecting the increased pADH levels. The rise in cAMP excretion was completely blunted by prednisolone treatment.

CONCLUSIONS

We speculate that there may be an ADH-independent stimulation of the formation or function of aquaporin-2 channels by prednisolone and/or a direct osmotic stimulation of water reabsorption independent of ADH and glucocorticoid control.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Circadian variation in response to CRF-41 and AVP

The diurnal response to ovine corticotropin-releasing factor (CRF-41), arginine vasopressin (AVP), and adrenocorticotropic hormone (ACTH) was studied in rats in which the endogenous release of CRF was blocked by chlorpromazine, morphine sulfate, and pentobarbital sodium. This procedure resulted in a markedly attenuated circadian rhythm at base-line levels of plasma corticosterone and ACTH. Decreased pituitary responsiveness to CRF-41 and AVP at 0400 compared with 1600 was observed. The plasma corticosterone response 30 min after intravenous injection of ovine CRF-41 (0.1 microgram/kg) or AVP (5.0 micrograms/kg) remained nearly constant over the major portion of the 24-h light-dark cycle. However, in the early morning (0400), 2 h before lights on, there was an approximately threefold decrease in response. The time of this decrease in response coincided with the normal decline in the concentrations of plasma corticosterone and ACTH. Rats exposed to constant darkness for 10 days continued to show a significantly greater response to CRF or AVP at 1600 than at 0400. In contrast, rats exposed to constant light for 10 days failed to demonstrate a differential response to CRF or AVP at different times of the day. These results demonstrate that there is a diurnal rhythm in pituitary response to CRF and AVP.