The role of corticosterone in supporting migratory lipogenesis in the dark-eyed Junco, Junco hyemalis: a model for central and peripheral regulation.

The functional role of corticosterone (CORT) in regulating migratory hyperphagia and lipogenesis was investigated in an annual migrant, the dark-eyed junco (Junco hyemalis). Intraperitoneal injections of either dexamethasone (9 microg DXM/500 microL of 5% EtOH in saline, n=10) to inhibit an increase in baseline CORT or saline (5% EtOH, n=9) were given every 48 h for 15 d after transfer from short (10.5L:13.5D) to long (15.5L:8.5D) days. Food intake, body mass, furcular fat deposition scores, and nocturnal migratory activity were recorded for 29 d after photostimulation. Both groups showed the same increase in daily food intake over the study period (DXM=52%, control=41%). Controls began to increase baseline CORT and mass about 2 wk after photostimulation. DXM-treated birds maintained low CORT and did not increase mass or CORT until injections ceased, at which time they gained mass at the same rate shown earlier by controls. DXM-treated birds did not show greater levels of migratory activity despite experiencing an increase in energy intake during the CORT-inhibited period. Collectively, the results support the migration modulation hypothesis, illustrating how an increase in baseline CORT is needed to support the development of migratory condition. We address the apparent conflict with earlier studies on CORT and migratory food intake and propose a model in which migratory hyperphagia is supported by changes in centrally regulated responses to CORT that can occur even if CORT remains low and lipogenesis is regulated predominantly by peripheral mechanisms that require an increase in baseline CORT.

Development of the corticosterone stress response in young northern mockingbirds (Mimus polyglottos).

In birds, additional adrenocortical secretion in response to stressors often redirects an individual's ongoing activities toward immediate life-saving activities, usually by facilitating an increase in food searching and food intake needed to meet periods of increased energy demand. We asked whether young birds, who are entirely dependent on parents for food acquisition and therefore unable to manipulate their own food intake, fail to show an adult-like adrenocortical response to the acute stress of capture and handling. In 1998, plasma profiles of acute corticosterone secretion (e.g., samples taken at the time of capture and 30 min later) were compared across seven age classes of Northern Mockingbirds (Mimus polyglottos) representing various age-related stages of foraging ability and opportunity. As predicted, young birds less able, or entirely unable, to readjust their own foraging effort exhibited significantly lower stress responses compared to adults. The magnitude of the stress response (at 30 min postcapture) increased and approached that of adults as young birds approached independence. Energetic condition was not correlated with the magnitude of the stress response at any age, suggesting that variation in its expression was most likely due to age alone. We also investigated at what level within the hypothalamic-pituitary-adrenal (HPA) axis the corticosterone response may be controlled in young birds. In 1999, baseline corticosterone samples were taken in 8-day-old nestlings and were immediately followed by intrajugular injections of adrenocorticotropic hormone (ACTH) or saline. While plasma corticosterone concentrations did not change in saline-injected nestlings, ACTH-injected nestlings showed a significant increase in plasma corticosterone concentrations similar to 30-min samples taken from adults. These results indicate that, while young birds do not normally show the corticosterone response, the adrenocortical tissue has the capacity to do so, and the control appears to be within the hypothalamic-pituitary component of the HPA axis. Collectively, our results indicate that the expression of the corticosterone stress response develops in concert with a young, altricial bird's ability to utilize it as it approaches independence; the reduced corticosterone secretion may also allow young, rapidly growing birds to avoid potential deleterious exposure to elevated glucocorticosteroid concentrations.