BREUNER, Creagh W.: Plasma Binding Proteins As Mediators Of Corticosteroid Action In Vertebrates

In trying to understand how corticosteroids alter physiology and behavior we investigated properties of corticosteroid binding proteins (CBG) in the plasma of several vertebrate species. These comparative studies have led to a number of disparate but interesting findings. 1) The binding specificity of CBG varies considerably between species. In an amphibian, a reptile, and at least three species of birds, CBG is not specific for corticosteroids (CORT). In contrast to mammalian CBG, we found that CBG has an affinity for androgens that is similar or higher than the affinity of CBG for endogenous CORT. Therefore, ‘CBG’ is something of a misnomer for non-mammalian species. This may be especially relevant for avian studies, since adult birds appear to lack a distinct SHBG, and it is commonly assumed that androgen present in avian plasma is free. On the contrary, we estimate that physiological levels of testosterone (T) are more than 90% bound to CBG in the plasma of dark-eyed juncos in all seasons. In addition, it is possible that during a stress response, the increase in circulating CORT will displace T from the binding globulin, causing a spike in free T. 2) When considering hormone levels, total CORT levels are often considered without reference to CBG levels. We have found examples in three different species where CBG capacity drastically changes the pattern of hormones across season, morph, or sub-species. Together, these data suggest plasma “CBG” may serve diverse roles in mediating the actions of CORT in different species. The data also indicate that levels of “CBG” are subject to dynamic regulation and therefore appear to be an important component in the plasticity of CORT action in vertebrates.