OWEN-ASHLEY, N. T.*; HASSELQUIST, D.; WINGFIELD, J. C.; Univ. of Washington, Seattle; Lund University, Sweden; Univ. of Washington, Seattle:
Examining the indirect effects of testosterone-induced immunosuppression in male songbirds. II.) Effect of short-term pharmacological adrenalectomy on humoral immune responses in testosterone-implanted birds

In many species of birds, implants of testosterone (T) suppress immune function in males. However, this effect could be attributed to chronically elevated levels of the stress hormone corticosterone (CORT), which rise in the blood after T implantation (via binding globulins). We attempted to eliminate the T-induced rise in CORT by temporarily deactivating adrenocortical cells in male nonbreeding Gambel�s white-crowned sparrows (Zonotrichia leucophrys gambelii) using daily mitotane injections, while measuring humoral immune responses to diphtheria-tetanus vaccination. Male white-crowned sparrows were implanted with T or control (blank) silastic capsules, and T treatment suppressed primary humoral responses to tetanus, but not to diphtheria, relative to controls. Males were then divided into 4 groups and injected daily with mitotane or peanut oil (vehicle) for 12 days: (1) T + mitotane, (2) T + oil, (3) control + mitotane, and (4) control and oil. Daily mitotane injections effectively reduced CORT in males implanted with T relative to vehicle injections. Secondary humoral responses to tetanus were higher in T-implanted birds given mitotane injections compared to those given vehicle. There was a similar trend for secondary humoral responses to diphtheria. This is the first experimental evidence to demonstrate that T-induced immunosuppression in birds is largely attributed to glucocorticosteroids.