BREUNER, CW; ORCHINIK, M; HAHN, TP; WINGFIELD, JC: Mechanisms for plasticity of the stress response in three subspecies of white-crowned sparrow

Among closely related taxa, species with shorter breeding seasons may incur higher fitness costs when the first breeding attempt fails. Thus, animals with short breeding seasons should be less responsive to stressors, and therefore less likely to abandon their young. We tested this idea in three subspecies of white-crowned sparrow (Zonotrichia leucophrys gambelii, Z.l. oriantha, and Z.l. pugetensis), which rear 1, 1-2, and 2-3 clutches per breeding season, respectively. The objectives of this study were two-fold. First, we determined the binding parameters for three corticosteroid binding sites in plasma and brain. In plasma, corticosteroid binding globulin (CBG) had high affinity for corticosterone (CORT), dexamethasone, progesterone, and testosterone. In brain, we found two intracellular corticosteroid receptors with ~10X difference in affinity for CORT (Kd = 0.28 and 3.3 nM). The specificity of the low-affinity receptor is similar to mammalian GR. In contrast, the high-affinity receptor had EC50=CORT>ALDO>>>canrenoic acid=RU318. The second objective compared several aspects of the stress response (CORT levels, CBG, and receptors) among the three subspecies. CBG levels were higher in oriantha than in pugetensis. As a result, we estimate stress-induced free CORT levels to be 2.5-fold lower in oriantha which has a shorter breeding season than pugetensis. In liver tissue, low-affinity glucocorticoid-like receptor levels were significantly lower in gambelii (which has the shortest breeding season), whereas there was no difference in the high-affinity corticosteroid receptors. In the brain, both receptor types were present in lower levels in gambelii.