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 single high and low doses of dexamethasone, cortisol, and prednisolone on the pituitary-adrenocortical (PA) system in pigeons. In addition, the effects of long-term daily administration of a high dose of dexamethasone are reported. In Expt. 1, low and high doses of dexamethasone (0.5 microgram/kg and 0.5 mg/kg), cortisol (15 micrograms/kg and 1.5 mg/kg), and prednisolone (3.5 micrograms/kg and 3.5 mg/kg) were administered around the expected nadir (4 hr before the peak) of the diurnal plasma corticosterone concentration. The recovery of the PA system was investigated by measuring plasma corticosterone concentrations before and 30 min after arginine vasopressin (AVP) stimulation (10 micrograms/kg) around the expected peak 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 corticosterone concentrations each day for 3 days. In Expt. 3, the recovery of the PA system following long-term daily administration of 1 mg/kg dexamethasone was investigated by repeated stimulations with AVP after cessation of the treatment. The results of these experiments (I) confirm that the PA system of pigeons is very sensitive to the suppressive effects of dexamethasone, cortisol, and prednisolone in a dose-dependent manner, (II) demonstrate that after cessation of glucocorticoid treatment the response to AVP stimulation is restored earlier than basal corticosterone concentrations, (III) demonstrate that after long-term glucocorticoid treatment the responsiveness of the PA system recovers relatively fast, and (IV) demonstrate that long-term treatment with a high dose of dexamethasone not only results in complete suppression of the PA system but is also a serious risk for infection and death.