Browsing by Subject "Glucocorticoids--Receptors"
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Item An integrated evaluation of costs and benefits of corticosterone secretion through development(2007-05) Wada, Haruka, 1976-; Jansen, Robert K., 1954-; Breuner, Creagh W.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.Item Regulation of elements of the thyroid hormone and corticosteroid systems by stress, hormone treatment, and atrazine during ontogeny of red drum (Sciaenops ocellatus)(2008-08) Applebaum, Scott Lee, 1974-; Nunez, B. ScottSurvival of teleost larvae requires growth and development which depend upon endocrine processes. In this dissertation I have examined the ontogeny of elements of the thyroid hormone (TH) and corticosteroid (CS) systems in red drum (Sciaenops ocellatus). Basal cortisol production was first detected 3 days post-hatch (DPH) and a cortisol stress response was present from 6 DPH forward. Changes in steroidogenic enzyme mRNA (CYP11B and CYP21) levels did not correlate with these events. The time necessary to reach peak cortisol levels as well as return to basal levels declined as larvae developed. A second set of studies examined ontogenetic patterns in levels of mRNAs encoding thyroid (soTR[alpha] and soTR[beta]) and corticosteroid (soGR) hormone receptors and assessed the regulation of these mRNAs by exogenous triiodothyronine (T3) and cortisol. soTR[alpha], soTR[beta] and soGR were expressed in all stages of red drum examined. soTR[alpha] levels increased during the time when surging TH levels have been reported. soTR[beta] levels did not differ significantly during development. soGR levels were strongly correlated with those of soTR[alpha]. T3 up-regulated soTR[alpha] and soTR[beta] levels in 7 DPH, but not older larvae. Cross-regulation of receptor mRNAs by exogenous treatment with T3 or cortisol was not observed. Finally, I assessed the influence of a common herbicide, atrazine, on receptor mRNA, TH levels and growth of red drum. In two experiments, red drum exposed to environmentally relevant levels of atrazine did not alter hormone receptor mRNA levels, or TH content. However, atrazine did depress growth in some instances. In conclusion, the expression patterns of hormone receptor mRNA in embryos suggests receptor proteins could be activated by maternal hormones prior to the onset of endogenous hormone production. A correlation between soTR[alpha] and soGR mRNA levels suggests coordinated function of TH and CS systems, although regulatory interactions between these systems were not evident under the conditions in this study. Patterns in soTR[alpha] and soTR[beta] mRNA levels support an important role for TH in the larval to juvenile transformation of red drum larvae. The results also support growing evidence indicating atrazine exposure effects larval growth and may impact their survival in the wild.