Hypothalamic-pituitary-adrenal axis changes allow seasonal modulation of corticosterone in a bird

We examined possible mechanisms underlying seasonal stress modulation in Lapland longspurs (Calcarius lapponicus), a species that breeds and molts (the energetically costly replacement of feathers) in the Alaskan Arctic. Free-living Lapland longspurs show dramatically reduced maximal corticosterone release during molt compared with the breeding season, an effect lost in captive birds. Neither changes in corticosterone binding proteins nor the overall condition of the bird (assessed by weight and fat storage) can explain different seasonal corticosterone responses. Adrenal insensitivity also does not fully explain reduced maximal output because exogenous ACTH enhanced corticosterone release during molt. Exogenous ACTH in molting birds, however, cannot stimulate corticosterone to stress-induced levels during breeding, implying reduced adrenal capacity. Lapland longspur pituitaries appeared to respond to exogenous corticotropin-releasing factor, arginine vasotocin, and mesotocin (the avian equivalents of arginine vasopressin and oxytocin) during molt, suggesting that a mechanism upstream of the pituitary blunts corticosterone release. Taken together, these results indicate that seasonal modulation of corticosterone release in this species is controlled at multiple sites in the hypothalamic-pituitary-adrenal axis.