CRIBBS, A.M.; HAUSSMANN, M.F.; VLECK, C.M.; Iowa State University, Ames; Iowa State University, Ames; Iowa State University, Ames: Effects of Aging on Glucocorticoids and Immune Responses in Zebra Finches

Age affects many physiological parameters. Corticosterone is an adrenal hormone that improves an animal�s ability to deal with stress. Chronically elevated glucocorticoids, however can compromise resistance to disease by suppressing the immune system. We tested whether the basal level of corticosterone or the extent to which it increases after 30 minutes of handling varies with age in male zebra finches, a species that lives a maximum of about five years. Basal and stress-induced levels of corticosterone were also measured 24 hours later to determine whether age affects the ability to recover from the handling stress. In addition we measured the proliferation of circulating T lymphocytes in response to the mitogen phytohaemagglutinin (PHA). We found no effect of age on either basal corticosterone level or the stress response to handling on either the first or second day of measurement. Basal levels did not vary between days, nor did the stress-induced levels (paired t-tests, P > 0.3). The local swelling in response to the PHA was lowest in the youngest individuals, highest in the one-year old birds, and then declined significantly with age (P = 0.002). PHA response was also influenced by variance in basal corticosterone (P = 0.02). Those birds with the lowest adrenocorticoid levels had the highest immune response. These data suggest that in the zebra finch, immunosenescence occurs in the cellular-mediated immune pathways. Suppression of the immune response also varies with adrenocorticoid level. However we found no effect of senescence on basal or stress-induced corticosterone or the ability to reduce corticosterone to baseline 24 hours after handling.