Noninvasive corticosterone treatment rapidly increases activity in Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii)

Recent evidence supports the hypothesis that corticosteroids influence behavioral changes associated with stressful events. Most investigations into this relationship focus on the long-term behavioral effects of corticosterone. Because many behavioral responses to environmental perturbations occur within minutes, we determined what rapid effects corticosterone may have on behavior. With this goal in mind, we devised and evaluated a method of corticosterone delivery which allowed us to examine immediate effects of corticosterone on behavior in a noninvasive manner. White-crowned sparrows (Zonotrichia leucophrys gambelii) were allowed access to mealworms (Tenebrio molitor) injected with corticosterone. Once ingested, the corticosterone moves across the digestive epithelium into the circulation. This method was evaluated using two vehicles: dimethyl sulfoxide and peanut oil. We tested the efficiency and consistency of corticosterone transfer into the circulation for both vehicles. Dimethyl sulfoxide gave a more efficient transfer of corticosterone. Injecting mealworms with corticosterone (carried in dimethyl sulfoxide) and feeding those worms to white-crowned sparrows increased circulating corticosterone to a discrete, repeatable level which peaked within 7 min and was cleared within 60 min. Using this method, we demonstrated that intermediate levels of corticosterone caused an increase in perch hopping in white-crowned sparrows within 15 min of hormone administration. An increase in perch hopping indicated elevated locomotor activity that is consistent with behavioral responses to natural perturbations. High levels of corticosterone did not induce this behavioral change. In light of the rapid effect of corticosterone on behavior, we propose that corticosterone was acting through a nongenomic mechanism.