Adaptation of the adrenocortical response during repeated stress in thalamic pigeons.

Chronic vascular catheterization allowed to obtain serial blood samples before and after stress application to thalamic pigeons. Daily repetition of the same stress, at the same hour, for 5 weeks led to drastic changes in the stress-induced adrenocortical reaction. The rebounding, long-lasting rise in plasma corticosterone occurring after initial presentation of electrical footshocks was replaced by a single peak of corticosterone, lower and shorter than the initial response. Moreover, an anticipatory conditioned rise in corticosterone appeared before stress. Random distribution of stressful stimuli, thrice a day, for 5 weeks resulted in the single peak pattern of post-stress adrenocortical reaction, without any anticipatory component. When electrical footshocks were omitted after 5 weeks of daily regular presentation of stress, only the anticipatory peak in plasma corticosterone occurred. Shifting the daily lighting from the usual 06.00 a.m. to 04.00 a.m. led to a shift in the anticipatory conditioned endocrine response which appeared 2 h sooner than usual. Thalamic involvement in the process of adaptation to chronic stress is discussed.