Meeting Abstract
The use of silicone (Silastic) tube implants to manipulate hormone levels in free-living animals was a key breakthrough for comparative field endocrinology. In particular, this method has been a reliable and minimally invasive means to chronically elevate sex steroid hormones in birds in the field and in the lab. However, using this method to elevate glucocorticoids (e.g., corticosterone [CORT]) has proved problematic. Often plasma CORT levels were not found to be elevated in CORT-implanted, leading to a view that CORT does not easily diffuse through silicone tubing. In many cases researchers report puncturing implants or leaving one end of the implant unsealed to facilitate CORT delivery. We have tested the utility of silicone tube implants to chronically manipulate CORT in two sparrow species: Melospiza melodia and Zonotrichia albicollis. In vitro silicone implants rapidly deliver high doses of CORT, even with no punctures. In vivo implants elevate baseline plasma CORT, but only for a few days before levels return to normal. Despite not having elevated baseline CORT, implanted birds appear to have a suppressed CORT response to restraint stress and a suppressed CORT response to ACTH challenge. Our results indicate that silicone tube CORT implants deliver high pharmacological doses of CORT to birds, and that birds respond by altering negative feedback and clearance mechanisms. Silicone CORT implants thus alter the HPA axis, but not in the way most researchers intend, and in a way that could have drastic consequences on HPA axis dynamics and far reaching physiological implications. As such, the utility of silicone tube implants to manipulate CORT is limited, and the practice of puncturing such implants is ill-advised.
