Stress responses of testosterone and corticosterone-binding globulin in a multi-brooded species, Eurasian Tree Sparrows (Passer montanus): Does CBG function as a mediator?

In avian plasma, testosterone (T) and corticosterone (CORT) compete to bind with corticosterone-binding globulin (CBG). Elevation of CBG may function to "buffer" the tissues against high circulating levels of T and stress-induced levels of CORT. To demonstrate the effects of acute stress on CBG and T levels and their biological functions, we investigated seasonal changes of baseline and stress-induced T and CBG levels in Eurasian Tree Sparrows (Passer montanus) during different life stages using the capture-handling-restraint stress method. Our results show that (1) male sparrows had significantly higher baseline T levels and CBG capacities during the nest building, the first egg-laying, and the first nestling stages, and significantly decreased stress-induced T levels only during the nest building and the first egg-laying stages. They also expressed significantly increased stress-induced CBG capacities during the second nestling stage. (2) Females showed significantly higher baseline CBG capacities but significantly decreased stress-induced CBG capacities during the nest building stage, and females also showed significantly increased stress-induced CBG capacities during the second egg-laying and the second nestling stages. Therefore, the seasonal fluctuations of baseline CBG in both sexes and baseline T in males reflect their adaptive strategies for optimizing their physiological and behavioral states to the life history cycle. The different patterns of stress-induced CBG in females suggest CBG functions as an essential mediator in regulating stress response to unpredictable perturbations. Our results highlight the need for future studies of stress-induced CBG and T levels on a wide range of vertebrate species that vary in different life history stages to gain a full understanding of the mechanisms that underlie biological functions of CBG and T for unpredictable stressors.