SENSITIVITY OF THE HYPOTHALAMIC-PITUITARY-ADRENAL SYSTEM OF PIGEONS (COLUMBA-LIVIA-DOMESTICA) TO SUPPRESSION BY DEXAMETHASONE, CORTISOL, AND PREDNISOLONE

The sensitivity of the hypothalamic-pituitary-adrenal system in pigeon s (Columba livia domestica) to glucocorticoid feedback was studied aft er single intravenous administration of dexamethasone (500, 100, 50, 1 0, 5, 1, 0.5, and 0.1 mug/kg), cortisol (15,000, 3000, 1500, 300, 150, 30, 15, and 3 mug/kg), and prednisolone (3500, 700, 350, 70, 35, 7, 3 .5, 0.7, and 0.35 mug/kg). Dose responses, the threshold doses, and th e onset of maximum suppression were determined by measuring plasma cor ticosterone concentrations around the expected peak of plasma corticos terone 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 concentratio ns were similar to the initial values and to the control values. Follo wing the minimum doses that resulted in suppression of the normal diur nal variation in plasma corticosterone concentration (0.5 mug dexameth asone/kg, 15 mug cortisol/kg, and 0.7 mug prednisolone/kg), plasma cor ticosterone concentrations were similar to values of the control group and to initial values within 24 hr. The onset of suppression of plasm a corticosterone concentrations was between 30 and 60 min following 1 mug dexamethasone/kg, 7 mug prednisolone/kg, or 30 mug cortisol/kg. Su ppression was greatest at 60 min following prednisolone and cortisol a dministration, and at 90 min following dexamethasone administration. I t is concluded that the hypothalamic-pituitary-adrenal system of pigeo ns reacts to exogenous glucocorticoids by early delayed feedback, is m ore sensitive to suppression by glucocorticoids than that of mammals, and is suppressed for the longest time by dexamethasone. These observa tions 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 ma mmals.