Ontogeny of the adrenal and behavioral responses of lambs to emotional stress.

The ontogeny of adrenocortical responsiveness in lambs was studied from 24 hr of age to maturity. Throughout the neonatal period, the adrenal axis was responsive to exogenous ACTH and the stresses of both restraint and exposure to open field testing. Newborn lambs both had higher resting levels of plasma corticosteroids and responded to the test stimuli with greater increases in corticosteroids than did other age groups. From the second week of life, the responsiveness of the adrenal axis was comparable to that of mature ewes. On exposure to an open field arena, lambs vocalized less and initiated movement sooner with increasing age and experience through 35 days of age, after which there was little change. Once locomotion was initiated, there was little difference in the amount of time spent moving in the arena. Open field testing was as potent a stimulator of the adrenal axis as restraint stress or exogenous ACTH.