Increased cortisol response to exogenous adrenocorticotropic hormone in chronically stressed pigs: influence of housing conditions.

In a longitudinal experiment, the influence of tethered housing (a condition of chronic stress) on the reactivity of the adrenal cortex to exogenous ACTH was investigated in gilts. To that end, the plasma cortisol response to synthetic ACTH (1-24; 10 micrograms/kg of BW; i.v. bolus injection via a permanent catheter) was determined before and after prolonged tethered housing. Two systems for tethered housing were used, one more restrictive than the other with regard to possibilities for visual and tactile contacts with conspecifics and visual control over the environment. The ACTH treatment induced a marked, transient plasma cortisol response in all gilts studied, irrespective of their housing conditions. Long-term tethered housing increased the ACTH-induced cortisol response. Possible effects of the experimental procedure or age-related effects could be excluded, because in control gilts, which were housed loose during the entire experimental period, the cortisol response to ACTH remained unaltered. The chronic stress-induced increase in the ACTH-induced cortisol response was considerably more pronounced and persistent in gilts that were deprived of possibilities for social contacts with conspecifics and visual control over the environment than in gilts with such possibilities. These data indicate that in tethered gilts adaptational changes occur at the level of the adrenal cortex that affect the ACTH-induced adrenocortical response. In addition, not only physical restraint but also restriction of social contact and visual control play an important role in the development of these changes.