Reactivity of the hypophyseal-adrenocortical system to stress in rats with active and passive behavioral strategies.

Rats with active (KHA) and passive (KLA) behavioral strategies showed no strain-related differences in basal corticosterone levels or in changes in corticosterone levels after exposure to mildly stressful stimuli. Only severe immobilization stress produced significant interstrain differences in the reactivity of the hypophyseal-adrenocortical system, as evidenced by the greater increase in blood corticosterone in KHA rats 30 min after stressing. The hormonal stress response in KHA rats was prolonged, as the elevated blood corticosterone level in these animals persisted longer than in KLA rats. The data provide evidence not only that the hypophyseal-adrenocortical systems have different sensitivities to severe stresses in these strains, but also that active and passive animals have different rates of inactivation of the stress reaction.