Exposure to chronic stress downregulates corticosterone responses to acute stressors

Abstract

(Sturnus vulgaris) to test whether corticosterone responses differed in birds held under normal laboratory conditions or conditions of chronic stress. Surprisingly, both basal corticosterone concentrations and corticosterone responses to acute stress were significantly reduced when birds were chronically stressed. To determine the mechanism underlying this reduced response, animals under both conditions were injected with lactated Ringer's solution (control), adrenocorticotropin (ACTH), arginine vasotocin (AVT), or dexamethasone (DEX). ACTH increased corticosterone concentrations above stress-induced levels in both cases, although maximum responses were lower in chronically stressed birds. AVT did not augment the corticosterone response under nonchronically stressed conditions, but it did under chronically stressed conditions. DEX reduced maximal corticosterone concentrations in both cases. Neither ovine nor rat corticotropin-releasing factor (CRF) altered normal stress responses. These data indicate that changes in responsiveness of the hypothalamic-pituitary-adrenal axis to ACTH and AVT serve to downregulate corticosterone responses during chronic stress. Furthermore, these data lead to the following hypothesis: ACTH output from the pituitary limits maximum corticosterone concentrations under normal conditions, but reduced AVT release from the hypothalamus regulates lower corticosterone concentrations under chronic stress conditions. environmental stress; negative feedback; hypothalamic-pituitary-adrenal axis; conservation GLUCOCORTICOIDS ARE STEROID hormones released from adrenal tissue in response to stressful stimuli. Upon perception of stress, the avian hypothalamus is activated to secrete arginine vasotocin (AVT, a congener of the mammalian arginine vasopressin) and corticotropin-releasing factor (CRF), which stimulate the pituitary to release adrenocorticotropin (ACTH), in turn, causing the release of glucocorticoids from the adrenals (7,8). This pathway constitutes the hypothalamic-pituitaryadrenal (HPA) axis, and its activation is one component of the physiological stress response. This response is believed to be aimed at maintaining or restoring homeostasis, thereby helping the animal to survive a stressful episode (45, 50). The primary avian glucocorticoid is corticosterone (CORT) (22).The presumed benefits of an acute activation of the HPA axis contrast with chronic activation. Various deleterious effects of chronic CORT elevation have been documented in many species, including suppression of reproductive function and behavior, immune system suppression, muscle wasting, growth suppression, and neuronal cell death (45, 49). Chronic stress generally produces chronic elevations in baseline CORT concentrations. However, studies of chronically stressed rats indicate that facilitation of the HPA axis takes place to maintain responsiveness to acute stressors (11,13,25), perhaps by increasing the role of vasopressin over CRF in the release of ACTH (1, 16, 17). Therefore, animals must avoid chronic stress ...

Stress and translocation: alterations in the stress physiology of translocated birds

“…Our results indicate that captivity and additional anthropogenic stressors while in captivity may exacerbate chronic stress with the potential to compromise the health of animals and their ability to cope with stressors following release. The decrease in baseline CORT concentration that we observed in chukar following their release parallels other recent data on chronic stress in wild, free-living and wild-caught captive birds (Rich & Romero 2005;Cyr & Romero 2007). The captivity-translocate chukar also experienced greater weight loss compared with the other groups.…”

“…We partitioned major components of translocation (capture, transport, captivity and release to novel area) in order to establish how each step of the translocation process affected CORT release. We measured changes in baseline and stress-induced CORT as well as changes in adrenal sensitivity and efficacy of the negative feedback system (Rich & Romero 2005) in addition to changes in chukar mass. We hypothesized that if translocation procedures cause chronic stress, then subjecting individuals to a progressive series of translocation components will result in increasingly disrupted stress physiology.…”

The effects of neonatal handling on adrenocortical responsiveness, morphological development and corticosterone binding globulin in nestling American kestrels (Falco sparverius)

“…Many studies have challenged birds with one or more pituitary and/or hypothalamic hormones (i.e., ACTH, CRF, and arginine vasotocin) to elicit downstream endocrine responses [3,55,64,54,20]. In our study, neither CRF nor ACTH challenges elicited any greater CORT release than did saline in handled or non-handled nestling kestrels.…”

Seasonal stress physiology and body condition differ among co-occurring tropical finch species

“…This could either help to promote the maintenance of breeding behavior later in the season, be a product of lowered stress due to abundant food and good condition, or alternatively, attenuation of the stress response, a symptom of chronic stress during breeding. However, based on current knowledge, chronically stressed birds tend to reduce both their CBG capacity and CORT response (Fleshner et al 1995;Rich and Romero 2005;Cyr et al 2007). Thus these breeding Gouldian finches do not fit the profile of chronically stressed birds.…”