Hormonal and Cellular Mechanisms of Fattening in Migratory Songbirds

In preparation for migration, birds accumulate large amounts of fat to serve as fuel during migratory flight. Corticosterone (CORT), the primary energy-regulating steroid in birds, is well known to be an important regulator of feeding and fattening but its precise role is still unclear. Elevations in CORT concurrent with increased hepatic fatty acid synthase (FAS) activity during migration suggest that CORT may play a functional role in increasing de novo synthesis of fatty acids. I tested this hypothesis through controlled captive studies of migratory songbirds in which CORT was manipulated and subsequent effects on food intake, FAS activity, body mass and fat deposition were measured. First, I compared patterns of CORT secretion and FAS activity in two contexts in which an increase in CORT would occur: the short-term response to fasting/refeeding in a non-migratory context, and during the development of migratory condition in response to increasing day length. Following a period of fasting, birds increased food intake compared to controls and birds with greater food intake had higher FAS activity. I did not, however, detect a difference in baseline CORT between control and fasted birds. During the development of migratory condition, however, the onset of long days triggered an increase in baseline CORT concurrent with increased FAS activity, independently of food intake. This increase in FAS occurred in anticipation of migratory hyperphagia and may be an important preparatory mechanism to facilitate migratory fattening. Later, I used intraperitoneal osmotic pump implants to elevate baseline CORT continuously for 13 days in birds that were photostimulated to initiate migratory condition. I discovered that birds with elevated CORT had greater food intake, insulin levels, and gained more fat than control birds with vehicle implants. These results suggest that insulin, also a potent stimulator of FAS, may interact with CORT to enhance fatty acid synthesis. My research demonstrates that CORT has distinct functional roles during short-term changes in energy demand versus predictable lifehistory demands such as migration. Changes in CORT secretion also influence other signals, such as insulin, which in turn potentiate CORT\u27s effects on fatty acid synthesis to enable birds to rapidly accumulate fat