Pharmacological characterization of intracellular, membrane, and plasma binding sites for corticosterone in house sparrows

The diversity and specificity of glucocorticoid effects are dependent on cell-specific receptor mechanisms. Three known corticosteroid receptors mediate tissue effects of glucocorticoids in vertebrates: two intracellular receptors that act primarily as ligand-activated transcription factors, and a membrane-associated receptor. The intracellular receptor sub-types have been well characterized in mammals, however relatively little is known about them across non-mammalian vertebrates. The membrane-associated receptors are poorly characterized in most vertebrate taxa. To explore the basis for glucocorticoid action in birds, we pharmacologically characterized the three putative corticosteroid receptors in the brain, as well as a plasma corticosterone binding globulin, in the house sparrow (Passer domesticus). We found that house sparrow brain cytosol contained two distinguishable binding sites for corticosterone. A high affinity, mineralocorticoid-like receptor had subnanomolar affinity for corticosterone (K_d ∼ 0.2 nM). However, this 'MR-like' high-affinity receptor did not bind RU28318 or canrenoic acid, two compounds that bind mammalian MR with high affinity. A lower-affinity, glucocorticoid-like receptor in brain cytosol bound corticosterone with an average K_d = 5.61 nM. This GR-like receptor showed subnanomolar affinity for RU 486. MR- and GR-like receptors were found in equal numbers in whole brain assays (average B_max = 69 and 62 fmol/mg protein, respectively). House sparrow brain membranes contain a single binding site specific for glucocorticoids, with characteristics consistent with a steroid/receptor interaction. Corticosterone affinity for this putative membrane receptor was approximately 24 nM, with apparent B_max = 177 fmol/mg protein. House sparrow plasma contained a single binding site for [³H]corticosterone. Specific binding to plasma sites was inhibited by glucocorticoids, progesterone, and testosterone. Testosterone binding to this corticosteroid binding globulin is noteworthy as sex steroid-specific binding globulins have not been identified in birds. Taken together, these data extend our ability to evaluate the comparative actions of glucocorticoids, increase our understanding of mechanisms behind the tissue specificity of glucocorticoid action, and offer insight into the evolution of glucocorticoid action in vertebrates.