UBC Theses and Dissertations
Cortisol and corticosterone in the immune system and brain of developing songbirds Schmidt, Kimberly L
Glucocorticoids (GCs) are primarily produced by the adrenal glands, however GCs can also be synthesized in extra-adrenal tissues. For example, immune and brain tissues contain all the enzymes necessary to synthesize GCs in vitro suggesting local GC synthesis. However, few studies have measured endogenous GC levels in neural or immune tissues. The primary objectives of this thesis were: (1) to determine if GCs are synthesized in immune and brain tissue of developing songbirds by measuring endogenous GC levels, (2) to determine how plasma and tissue GC levels change with age and (3) to determine how plasma and tissue GC levels are affected by restraint stress. In Chapter 2, baseline corticosterone and cortisol levels were measured in plasma, immune and brain tissue of zebra finches on the day of hatch (P0), P3, and P30. Cortisol levels in immune organs were higher than cortisol levels in plasma at all ages, suggesting local cortisol synthesis. Interestingly, corticosterone was the predominant plasma GC but cortisol was the predominant GC in immune tissues. In Chapter 3.1 corticosterone and cortisol levels were measured in wild European starlings on P0 and P10 (at baseline and after restraint). At P0, neither GC increased with restraint in plasma or any of the tissues studied. At P10, restraint increased corticosterone in plasma and all tissues, and cortisol in plasma, diencephalon and thymus. In contrast to Chapter 2, local GC levels were low at both ages suggesting that there may be a difference in local GC levels between starlings and zebra finches. To test this hypothesis, in Chapter 3.2, local GC levels in P4 zebra finches and starlings were compared. Again, in zebra finches, cortisol levels in the immune system were higher than cortisol levels in plasma. In starlings, cortisol levels in the immune system were similar to cortisol levels in plasma. These data suggest a difference in immunosteroid synthesis between zebra finches and starlings. These studies suggest that circulating GC levels are not always representative of tissue GC levels. Measuring GC levels in tissues can provide important insights into how GCs are regulated in age-specific, tissue-specific, and species-specific manners.
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