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 o rganism, one also needs to investigate behavioral sensitivity to glucocorti coids. We examined the behavioral response of Gambel's white-crowned sparro ws (Zonotrichia leucophrys gambelii) to elevated glucocorticoids. Using a b ehavioral assay in which a rapid and transient dose of corticosterone (CORT : the avian glucocorticoid) rapidly increases perch hopping, we first inves tigated the photoperiodic regulation of this behavioral response. Intermedi ate levels of CORT (similar to 24 ng/ml), which increased activity in sparr ows 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 sparr ows exposed to the long-day photoperiod. Intermediate levels (24 and 40 ng/ ml) increased activity to threefold background levels, whereas high physiol ogical levels (65 and 97 ng/ml) had no effect. Given that the behavioral re sponse does not increase linearly with CORT, we can no longer assume that m odulation of the adrenocortical response to stress will result in linear ch anges in behavior. (C) 2000 Academic Press.