Compared Corticosterone Levels Research Articles

Social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli, which is termed “social buffering.” We previously reported that social buffering in male rats ameliorated behavioral responses, as well as hypothalamic-pituitary-adrenal axis activation, elicited by an auditory conditioned stimulus (CS). In addition, subjects that experienced social buffering did not show stress responses when re-exposed to the CS the next day in the absence of an accompanying rat. However, the mechanisms underlying this enhancement of between-session extinction are poorly understood. In Experiment 1, we compared corticosterone levels at 0, 10, and 15 min after extinction training. Subjects that experienced social buffering had lower corticosterone levels than subjects that trained alone at the end of extinction training. However, corticosterone levels at 10 and 15 min after training were not affected by the experience of social buffering. These results suggest that a lower level of corticosterone during extinction training had an important role in the enhancement of extinction. To directly assess this, in Experiment 2, we manipulated the corticosterone level during extinction training. We found that a subcutaneous injection of corticosterone before extinction training blocked the enhancement of extinction by social buffering. These results demonstrate that the enhancement is caused by a low level of corticosterone during the training. Taken together, we suggest that social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats.

Assessing space use in meadow voles: the relationship to reproduction and the stress axis

Abstract Voles are key mammals in understanding how social interactions can affect large-scale population processes. Previous studies have shown that at high population densities, meadow voles (Microtus pennsylvanicus) have a lower proportion of breeding animals, higher average corticosterone levels, and can be limited by female territorial spacing. Based on this, we compared corticosterone levels and spatial use between breeding and nonbreeding free-ranging adult meadow voles within populations. We measured intrasexual spatial overlap to examine if breeding females minimize occupying the same areas as other females, and noninvasively assessed corticosterone levels using fecal corticosterone metabolites (FCMs). We found that female meadow voles have much lower intrasexual spatial overlap than males, even though both sexes have similar range sizes, and that females have generally higher FCM levels than males. However, breeding and nonbreeding females did not differ from one another in spatial use or in FCM levels. Conversely, reproductive classes of males differed greatly in all measures: nonbreeding males had FCM levels that were two times higher than those of breeding males, occupied a smaller range, and had lower spatial overlap, indicating they were moving less widely than breeding males. We additionally validated an enzyme immunoassay for noninvasively measuring FCMs in meadow voles. The assay was successful in detecting an increase in corticosterone stimulated by adrenocorticotropic hormone injection; however, dexamethasone did not induce negative feedback. FCMs reflect circulating corticosterone levels approximately 5 h prior. These results highlight differences in FCMs and spacing in meadow voles related to sex and reproductive status, and reflect the respective strategies males and females employ during the breeding season.

Decreased Fecal Corticosterone Levels Due to Domestication: A Comparison Between the White‐Backed Munia (Lonchura striata) and Its Domesticated Strain, the Bengalese Finch (Lonchura striata var. domestica) With a Suggestion for Complex Song Evolution

The Bengalese finch (BF; Lonchura striata var. domestica) is a domesticated strain of the white-backed munia (WBM; Lonchura striata). Environmental stresses activate the hypothalamic-pituitary-adrenal (HPA) axis and release corticosterone (CORT). We hypothesized that domesticated songbirds have reduced CORT levels because of reduced levels of environmental stresses (compared to wild conditions) and reductions in the role of CORT, which is necessary for survival in the wild. However, no study has examined the effects of domestication on songbird CORT levels. To explore the domestication effects, we compared CORT levels between domesticated BFs and their wild ancestors WBMs. We also compared CORT levels between bought and aviary-raised BFs, and between wild-caught and captive-raised WBMs to examine the influence of being raised. However, blood collection causes stress, which affects endocrine dynamics and makes continuous sampling difficult in small birds. Therefore, we used a non-invasive method to measure fecal CORT. Parallelism between diluted fecal extracts and a CORT standard, extraction efficiency, and ACTH challenge demonstrated the effectiveness of this method. This study demonstrates that BFs have lower fecal CORT than do WBMs, regardless of whether the WBMs were wild-caught or captive-raised. In addition, BFs sing more complex songs than WBMs. Considerable evidence suggests that song complexity is related to CORT levels. Previously, we found that the corticosteroid receptors were expressed in song-control areas of the BF brain. Based on these results, we hypothesize that reduced CORT levels through domestication might be one factor allowing for the development of more complex songs in BFs.

Assessment of Corticosterone Levels in American Alligators (Alligator mississippiensis) with Dermatitis

Bacterial and fungal dermatitis is a common problem in captive reared alligators in commercial operations, and stress has been suggested as a predisposing factor. We compared corticosterone levels between alligators with dermatitis (Tx) and alligators without dermatitis (Cx). There was no statistically significant difference between the baseline corticosterone concentrations (time 0) of the Tx and Cx groups (P = 0.272). At 15 min postcapture, there was a statistically significant difference between treatment and control animals (P = 0.006), with the Tx alligators having higher corticosterone concentrations compared with the Cx alligators. At 3 h (after dexamethasone administration) values for both groups decreased from the 15 min levels, showing a functional negative feedback loop of the hypothalamic–pituitary axis; there was no statistically significant difference between groups (P = 0.90) at that time. The results do not show an association between stress and dermatitis, but suggest that animals may be more prone to increases in corticosterone release once dermatitis is present.

Taking the stress out of blood collection: comparison of field blood‐sampling techniques for analysis of baseline corticosterone

Many ecological studies use stress hormones to assess the condition, health or disturbance levels of wild organisms. Common blood sampling protocols for this research involve trapping individuals and taking blood within three minutes to obtain a “baseline” for analysis of stress hormones (“conventional method”). In some situations it may be difficult to get an accurate measure of baseline values; therefore, alternative sampling techniques may be preferable. We compared corticosterone levels in samples taken via a newly developed, minimally invasive blood sampling technique with corticosterone levels in blood taken via the conventional method. We collected samples from incubating adult common terns Sterna hirundo via blood sucking bugs (Heteroptera, Triatominae) contained in “dummy eggs” (“bug method”) and compared measured corticosterone concentrations to concentrations in blood taken from the same birds using the conventional method. We found no significant differences in mean or variance of baseline corticosterone levels between samples collected via the different methods. This suggests that the bug method offers a viable alternative for hormone sampling.

Corticosterone, reproductive status, and body mass in a cooperative breeder, the Florida scrub-jay (Aphelocoma coerulescens).

In the cooperatively breeding Florida scrub-jay, nonbreeders are subordinate to the breeders with which they share a territory. Corticosterone is secreted in response to a wide range of stressors, including social stress, and suppresses reproductive and territorial behaviors in several taxa; thus, elevated baseline levels of corticosterone might be a causal mechanism of reproductive suppression. To test the hypothesis that nonbreeder Florida scrub-jays are reproductively suppressed through the actions of corticosterone, we compared corticosterone levels of nonbreeders and breeders and found no differences. However, baseline corticosterone levels only provide information about a bird's current hormonal status. Virtually all species exhibit a rapid rise in glucocorticoids in response to an acute stressor. If the adrenocortical response of nonbreeders is greater than that of breeders, this might be a mechanism whereby nonbreeders remain reproductively quiescent. We compared the responses of breeders and nonbreeders to the acute stress of being captured and held for 1 h. Both exhibited significant but equivalent increases in corticosterone titers. Because in some species heavier or fatter individuals have reduced glucocorticoid responses to stressors, we examined whether body mass was correlated with corticosterone titers. Both baseline and maximum corticosterone levels covaried with body mass, and the effect of body mass on corticosterone levels explained the increase due to capture and handling. Our data do not support the hypothesis that Florida scrub-jay nonbreeders are reproductively suppressed through the actions of corticosterone.