Rapid behavioral response to corticosterone varies with photoperiod and dose

The magnitude of the adrenocortical response to stress can be modulated by a variety of environmental and physiological factors, such as daylength and body condition. To determine the consequences of this modulation for the organism, one also needs to investigate behavioral sensitivity to glucocorticoids. We examined the behavioral response of Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) to elevated glucocorticoids. Using a behavioral assay in which a rapid and transient dose of corticosterone (CORT: the avian glucocorticoid) rapidly increases perch hopping, we first investigated the photoperiodic regulation of this behavioral response. Intermediate levels of CORT ( approximately 24 ng/ml), which increased activity in sparrows exposed to a long-day (breeding) photoperiod, had no behavioral effect in sparrows exposed to a short-day (winter) photoperiod. Hence, the neural mechanisms which regulate the behavioral response to CORT appear to be less sensitive under a winter photoperiod. Using the same behavioral assay, we also measured a dose-response curve for CORT's effects on activity in sparrows exposed to the long-day photoperiod. Intermediate levels (24 and 40 ng/ml) increased activity to threefold background levels, whereas high physiological levels (65 and 97 ng/ml) had no effect. Given that the behavioral response does not increase linearly with CORT, we can no longer assume that modulation of the adrenocortical response to stress will result in linear changes in behavior.