RESPONSIVENESS OF THE GLUCOCORTICOID-SUPPRESSED PITUITARY-ADRENOCORTICAL SYSTEM OF PIGEONS (COLUMBA-LIVIA DOMESTICA) TO STIMULATION WITH ARGININE-VASOPRESSIN

This paper reports on the duration of the suppressive effects of singl e high and low doses of dexamethasone, cortisol, and prednisolone on t he pituitary-adrenocortical (PA) system in pigeons. Tn addition, the e ffects of long-term daily administration of a high dose of dexamethaso ne are reported. In Expt. 1, low and high doses of dexamethasone (0.5 mu g/kg and 0.5 mg/kg), cortisol (15 mu g/kg and 1.5 mg/kg), and predn isolone (3.5 mu g/kg and 3.5 mg/kg)were administered around tile expec ted nadir (4 hr before the peak) of the diurnal plasma corticosterone concentration. The recovery of the PA system was investigated by measu ring plasma corticosterone concentrations before and 30 min after argi nine vasopressin (AVP) stimulation (10 mu g/kg) around the expected pe ak of plasma corticosterone concentrations. In Expt. 2, one high dose of each of the three glucocorticoids was administered 4 hr before the peak, followed by AVP stimulation around the expected peak of plasma c orticosterone concentrations each day for 3 days. In Expt. 3, the reco very of the PA system following long-term daily administration of 1 mg /kg dexamethasone was investigated by repeated stimulations with AVP a fter cessation of the treatment. The results of these experiments (I) confirm that the PA system of pigeons is very sensitive to the suppres sive effects of dexamethasone, cortisol, and prednisolone in a dose-de pendent manner, (II) demonstrate that after cessation of glucocorticoi d treatment the response to AVP stimulation is restored earlier than b asal corticosterone concentrations, (III) demonstrate thar after long- term glucocorticoid treatment the responsiveness of the PA system reco vers relatively fast, and (IV) demonstrate that long-term treatment wi th a high dose of dexamethasone not only results in complete suppressi on of the PA system but is also a serious risk for infection and death .