Responses of the pituitary-adrenal system of the pig to environmental changes and drugs.

1 The reactivity of the pituitary-adrenal axis of the young pig was tested for its suitability as a sensitive index for any discomfort that might be experienced under certain conditions of intensive husbandry.2 In a thermoneutral environment, most undisturbed piglets showed only slight variations in the plasma concentrations of adrenocorticotrophic hormone (ACTH) and corticosteroids.3 Stimuli such as exposure to ambient temperatures of +40 degrees C or -5 degrees C were required to cause large rises in the plasma concentrations of ACTH and corticosteroids.4 Apparently milder stimuli, such as change of environment, slight frustration or changes in ambient temperatures between +5 degrees C and +30 degrees C only rarely caused a significant rise in plasma corticosteroids. Thus changes in plasma corticosteroid concentrations are not a sensitive index for the reaction of a piglet to its environment.5 Increases in plasma ACTH concentrations occurred faster than those of the corticosteroids, were larger when expressed as a percentage of the basal values and occurred following relatively small disturbances such as omission of the reward in an operant behaviour test when corticosteroid changes were often not detectable. Thus rises in plasma ACTH might be a useful indication that a given situation is disturbing to a pig. The reaction of plasma ACTH concentrations to chronic irritations as they might occur in intensive husbandry remains to be investigated.6 Azaperone (2 mg/kg i.m.), a drug which is used as a sedative in pigs, caused a rise of about 50% in plasma corticosteroid concentrations. It did not diminish the large steroid output seen when the animals were exposed to high and low ambient temperatures.