FUNCTIONING OF THE PORCINE PITUITARY-ADRENOCORTICAL AXIS DURING NEONATAL DEVELOPMENT

A study was conducted with neonatal boars to measure age-related chang es in functioning of the pituitary-adrenocortical axis. Pigs were rand omly assigned to control (n = 7-10/age) or treated (1-min restraint, n = 9-11/age) groups to be sampled at either 12, 19, or 26 days of age. Blood samples were taken via catheter 10 min before and 3, 10, and 20 min after restraint or at similar time intervals in controls. One day later, pigs were killed and adrenal glands obtained for ACTH receptor measurements. Basal plasma ACTH concentrations were greatest (p = 0.0 35) on day 12 when compared with later ages, but basal plasma cortisol concentrations were comparable at the three ages. Compared with contr ols, restraint elevated incremental plasma ACTH and cortisol responses at each age (p < 0.004). On day 12, maximal plasma ACTH (p = 0.0006) and incremental cortisol (p < 0.006) responses to restraint were great er than at later ages. Binding to adrenal ACTH receptors was greatest (p < 0.05) at day 13, which may help explain the apparently increased in vivo response of the adrenal gland to ACTH at this time. Restrained pigs had increased growth rates with increasing age (p = 0.016) where as growth rates for control pigs did not differ with age. At day 27, 2 4 h after the 1-min restraint, body weights of restrained pigs exceede d those of control pigs (p = 0.045). At day 20, adrenal DNA and protei n in pigs restrained 24 h previously were greater than in control pigs (p < 0.05). These data suggest age-related changes in functioning of the pituitary-adrenal axis in neonatal boars, and an absence of a peri od during neonatal life when the porcine pituitary adrenocortical axis cannot respond to a stressor. The data also indicate both rapid and l ong-term responses of the adrenal to a very modest stressor and sugges t an extreme sensitivity of neonatal pigs to environmental perturbatio ns.