Sensitivity of the hypothalamic-pituitary-adrenal system of pigeons (Columbia livia domestica) to suppression by dexamethasone, cortisol, and prednisolone

The sensitivity of the hypothalamic-pituitary-adrenal system in pigeons (Columba livia domestica) to glucocorticoid feedback was studied after single intravenous administration of dexamethasone (500, 100, 50, 10, 5, 1, 0.5, and 0.1 microgram/kg), cortisol (15,000, 3000, 1500, 300, 150, 30, 15, and 3 microgram/kg), and prednisolone (3500, 700, 350, 70, 35, 7, 3.5, 0.7, and 0.35 microgram/kg). Dose responses, the threshold doses, and the onset of maximum suppression were determined by measuring plasma corticosterone concentrations around the expected peak of plasma corticosterone in pigeons kept on a shifted-light regimen. At 52 hr following the highest dose of dexamethasone, and at 48 hr following the highest doses of cortisol and prednisolone, plasma corticosterone concentrations were similar to the initial values and to the control values. Following the minimum doses that resulted in suppression of the normal diurnal variation in plasma corticosterone concentration (0.5 microgram dexamethasone/kg, 15 microgram cortisol/kg, and 0.7 microgram prednisolone/kg), plasma corticosterone concentrations were similar to values of the control group and to initial values within 24 hr. The onset of suppression of plasma corticosterone concentrations was between 30 and 60 min following 1 microgram dexamethasone/kg, 7 microgram prednisolone/kg, or 30 microgram cortisol/kg. Suppression was greatest at 60 min following prednisolone and cortisol administration, and at 90 min following dexamethasone administration. It is concluded that the hypothalamic-pituitary-adrenal system of pigeons reacts to exogenous glucocorticoids by early delayed feedback, is more sensitive to suppression by glucocorticoids than that of mammals, and is suppressed for the longest time by dexamethasone. These observations indicate that some of the side effects of glucocorticoid therapy are likely to be at least as frequent and as severe in birds as in mammals