The glucocorticoid hormone-driven stress response is a potent modulator of physiology and behavior across the animal kingdom. This stress response is regulated at many levels, demonstrating specificity to the stressor, the environmental context, and the physiological condition of the organism. How can an organism facilitate such fine control over what was historically presumed to be a generalized response to any noxious stressor? Over the last decade binding proteins have emerged as one of the central integrators of the endocrine regulatory axis. Corticosteroid binding globulin (CBG) is a significant regulator of the stress response, altering the potency and tissue specificity of corticosteroid action depending on environmental and physiological conditions. Nevertheless, the mechanism by which CBG alters physiological function is under dispute. This research will investigate the role of CBG in mediating the stress response in white-crowned sparrows by incorporating studies from the cellular to the ecological level. The availability of CBG in plasma, tissues, and whole birds will be altered and the physiological and behavioral responses to exogenous corticosterone will be monitored. Such an approach will evaluate the relative importance of tissue versus plasma CBG in orchestrating the organismal response to stress. This research will be extended to ecologically relevant organismal studies to determine how CBG is involved in mediating the stress response in wild populations of birds. This project incorporates undergraduate internships with integration of research and teaching as their main focus. These undergraduate biology and science education majors will be trained in field research techniques and ideas, while learning how to develop lesson plans for high school students. In mid-May, the students will travel with the PI to Lee Vining, California for the 6-week field season. During that time they will complete the field research outlined in the grant, while also giving their prepared lessons at the local high school one day each week. The entire internship will culminate with presentation of the field-collected data at the University of Montana Conference on Undergraduate Research. These experiences strengthen the education majors' knowledge of the scientific process, and forge an understanding and commitment to developing authentic research experiences for students in their future science classrooms. For the biology undergraduates, these experiences will foster positive experiences with educational settings and encourage future relationships with secondary science classrooms. This program will promote active participation in both the scientific process and learning the nature of science; it will improve the interns' potential as future teachers of science.
CAREER: corticosteroid binding globulin and the vertebrate stress response: integrating cellular, plasma and organismal studies As vertebrates become stressed, secretion of glucocorticoids increase in circulation. When people face deadlines, get in car accidents, or can’t pay bills, these hormones increase, and cause a wide variety of changes in physiology and behavior. The same hormones exist in all vertebrates, and the effects of these hormones are often similar across people, birds, reptiles and fish. Our understanding of these hormones come from measuring total amounts of hormones in the blood. Historically it has been assumed that total blood measures represent the amount of hormone reaching tissues and having effects on the brain, the liver, and other target organs. However, there are proteins in the blood that bind glucocorticoids and may limit their access to tissues. They are called corticosteroid binding globulins (CBG). When I began working on these proteins, it was unclear what role they played in limiting glucocorticoid entry into tissues. In fact, one of my early papers was primarily cited by biomedical studies trying to understand whether these proteins were important or not. Even your medical doctors disagreed on whether they should be measuring total hormone in your plasma, or just the free hormone that wasn’t bound to CBG. This grant was created to help understand whether CBG is important in regulating the activity of glucocorticoids in physiology and behavior. In the plasma, about 80-90% of hormone is bound to CBG. CBG is a large glycoprotein and cannot normally pass through capillary walls, so normally glucocorticoids bound to CBG will also not pass out of the blood into tissues. If this is true, then only 10-20% of total hormone measured in the blood is actually making it to tissues, and having effects. In this grant I set out to test whether free or total hormone better predicted physiological and behavioral effects in animals. I chose to work in birds (the white-crowned sparrow), because they are easy to study in the lab and in the field, and there is a huge amount known about glucocorticoids in this species. Our results support the ‘free hormone hypothesis’, which states that only free, unbound hormone is available to enter tissues and have effects. We demonstrated that the rate of hormone clearance from the blood (how quickly it leaves) is dependent on how much binding globulin is available to bind up hormone. The more CBG available, the less glucocorticoids will be free, and the less can exit the bloodstream to be cleared. We demonstrated this measuring clearance rates of corticosterone (the primary avian glucocorticoid) with more and less CBG available. Our results showed that clearance rates of corticosterone were higher when there was less CBG available. We also showed that field behavior is more closely related to free hormone estimations than total hormone measures. When we captured sparrows in the wild and measured the amount of CORT present in their blood, it did not relate to how soon they abandoned their territories when a storm came through. However, when we measured CBG levels and estimated free CORT from that, there was a very tight relationship between high free CORT levels and time of departure during a storm. Individuals that had more free CORT in their blood left earlier when the storm hit. Finally, we demonstrated that behavior measured in the lab is more closely related to the amount of free hormone in the blood than the amount of total hormone. We increased CORT levels experimentally in sparrows by feeding it to them in a meal worm. We already knew that this would increase activity levels, that was work I did for my PhD. However, when we altered the amount of CBG available to bind to CORT, we altered the amount of activity change. So, total hormone didn’t predict behavior change, but free hormone estimations did. There are still researchers who disagree with us on the role of CBG, but the results collected from this grant have helped move more people into believing that free estimates will better represent what is happening in the body when an animal (or a person!) gets stressed.
