Effects of HPA hormones on adapted lymphocyte responsiveness to repeated stress.
Acute stress-induced immune alterations can result in adapted function with prolonged exposure to the same stressor. The present study was designed to evaluate the possible role of the hypothalamic-pituitary-adrenocortical (HPA) axis on the adaptation of spleen lymphocyte responsiveness to repeated stress. For this purpose, we selected a stressful protocol (aversive auditory stimulation) that induced an initial suppression (1 day), followed by a return to control values with repeated application (4 days), of mitogen-induced lymphocyte proliferation. Because rats exposed to 4 days of noise sessions show enhanced adrenocorticotropin (ACTH) and corticosterone levels, we tested the possibility that adaptation of lymphoproliferation by repeated stress was due to a desensitization of splenic lymphocytes to stress-released HPA hormones. The results showed that corticotropin-releasing factor (10(-9) M) and corticosterone (5 x 10(-8) and 10(-7) M), as well as dexamethasone (10(-8), 5 x 10(-8), and 10(-7) M), significantly suppressed lymphoproliferation from both control and stressed rats in a similar way. ACTH (10(-9) and 5 x 10(-9) M) did not significantly influence Concanavalin-A-stimulated spleen lymphocytes. These data indicate that adaptation of lymphocyte proliferation by repeated noise stress occurs without accompanying alterations in lymphocyte responsiveness to HPA hormones.