An integrated evaluation of costs and benefits of corticosterone secretion through development
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Glucocorticoids (GCs) play critical roles during development: transient increases in GCs facilitate anticipatory physiological changes and trigger ontogenetic transitions such as promoting fetal/embryonic organ maturation and initiating birth/hatching. In contrast, chronically elevated GCs can be detrimental to growth, cognition, and survival. Thus, animals going through substantial growth may have higher corticosteroid binding globulin (CBG) levels, or enhance negative feedback/tonic inhibition on the hypothalamic-pituitary-adrenal (HPA) axis to keep GCs levels low. Here I investigated these hypotheses using altricial white-crowned sparrow nestlings. I examined 1) the ontogeny of the corticosterone (CORT) response (both total and free hormone levels), 2) changes in corticosteroid receptor levels in brain with age, and 3) effects of acute and extended elevation of CORT on behavior and growth. In response to acute stress, nestlings showed a low HPA reactivity in total CORT during the first 1/3 of the nestling period. When free CORT is considered, this hyporesponsive period was extended to 2/3 of the nestling period, suggesting CBG is one of the mechanisms to keep free CORT low. These periods coincided with rapid mass gain and acquiring thermoregulatory ability. The low reactivity was partly due to a dampened sensitivity at pituitary level or higher as all stages of nestlings responded to adrenocorticotropic hormone challenges; however it was not due to an enhanced negative feedback/tonic inhibition on hypothalamus or hippocampus. When CORT levels were artificially elevated, I only observed detrimental effects on begging behavior and growth. These series of data elucidated the ontogeny of the HPA axis in altricial nestlings regarding CORT, binding globulin, and receptor levels. In addition, I found that measured effects of exogenous CORT are primarily costly and highly age-specific.