Corticosterone implants make stress hyporesponsive birds

Abstract

In birds, the use of corticosterone (Cort) implants is a frequent tool aimed at simulating systemic elevations of this hormone and studying effects on biological traits (e.g. physiology, morphology, behavior). This manipulation may alter adrenocortical function, potentially changing both baseline (Cort) and stress-induced (Cort) plasma Cort levels. However, implant effects on the latter trait are rarely measured, disregarding downstream consequences of potentially altered stress responses. Here, we analyzed the effects of Cort implants on both Cort and Cort in nestling and adult European white storks, In addition, we performed a review of 50 studies using Cort implants in birds during the last two decades to contextualize stork results, assess researchers' patterns of use and infer current study biases. High and low doses of Cort implants resulted in a decrease of both Cort (31-71% below controls) and Cort (63-79% below controls) in storks. Our literature review revealed that Cort generally increases (72% of experiments) whereas Cort decreases (78% of experiments) following implant treatment in birds. Our results challenge and expand the prevailing assumption that Cort implants increase circulating Cort levels because: (i) Cort levels show a quadratic association with implant dose across bird species, and decreased levels may occur at both high and low implant doses, and (ii) Cort implants also decrease Cort levels, thus producing stress-hyporesponsive phenotypes. It is time to work towards a better understanding of the effects of Cort implants on adrenocortical function, before addressing downstream links to variation in other biological traits.

Experimental Increases in Glucocorticoids Alter Function of the HPA Axis in Wild Red Squirrels without Negatively Impacting Survival and Reproduction

“…We show that elevation of plasma cortisol concentrations within the physiological range for 1-2 weeks had profound effects on measures of HPA axis reactivity and CBG concentrations. This is similar to the results of Torres-Medina et al (2018) that showed that treatment with corticosterone implants can also cause reduced corticosterone levels in response to capture and handling. However, our study took the result from Torres-Medina et al (2018) one step further as we showed how treatment with exogenous GCs suppresses CBG concentrations and we also investigated the possibility of fitness consequences of reduced HPA axis reactivity.…”

“…Although these issues were highlighted 10-15 years ago (Romero, 2004;Fusani, 2008; see also Sopinka et al 2015;Crossin et al, 2016), detailed studies about the potential complications of manipulating hormones in wild animals have not been widely performed except in birds. Torres-Medina et al (2018) reviewed the consequences of experimentally elevated GCs on baseline and stress-induced corticosterone levels from previous studies on multiple bird species that were published 2005-2015. Many but not all of these studies were conducted in free-living birds.…”

Experimental increases in glucocorticoids alter function of the HPA axis in wild red squirrels without negatively impacting survival and reproduction

“…This is similar to the results of Torres-Medina et al (2018) that showed that treatment with corticosterone implants can also cause reduced corticosterone levels in response to capture and handling. However, our study took the result from Torres-Medina et al (2018) one step further as we showed how treatment with exogenous GCs suppresses CBG concentrations and we also investigated the possibility of fitness consequences of reduced HPA axis reactivity. Despite these observed shifts in the functionality of the neuroendocrine stress axis and the sustained elevations in GCs, we found no change in body mass or offspring and adult survival.…”

“…When it is feasible, daily supplementation may be effective in maintaining sustained elevations in hormone concentrations and provide an alternative to other methods like implants that carry some disadvantages (Sopinka et al, 2015). For example, Torres-Medina et al (2018) showed that silastic implants, time release pellets, or osmotic pumps that contain corticosterone can also suppress the responsiveness of the HPA axis in birds, which could decrease overall exposure to circulating corticosterone. Our study shows that even regular provisioning of exogenous GCs rather than implants, such as through food in our study (see also Dantzer et al, 2017) or through other methods (Vitousek et al 2018), may also decrease the activity of the HPA axis and cause a reduced ability to mount an increase in circulating GCs in response to an environmental challenge.…”

Maternal glucocorticoid levels during incubation predict breeding success, but not reproductive investment, in a free-ranging bird

“…Across species, silastic implants have been successfully used to raise CORT levels for anywhere from a few days ( Astheimer et al, 2000 ; Hayward and Wingfield, 2004 ; Criscuolo et al, 2005 ; Martin et al, 2005 ; Angelier et al, 2007 ) to 3 weeks post-implantation in vivo ( Ouyang et al, 2013 ) and in vitro ( Newman et al, 2010 ). However, the use of implants to raise CORT levels has not been consistently successful ( Crossin et al, 2012 ; Ouyang et al, 2013 ; Hau and Goymann, 2015 ; Lattin et al, 2016 ; Torres-Medina et al, 2018 ). Although the implants used in our study may have failed to release CORT, this seems unlikely given that in vitro studies have shown that CORT continues to be released across the membrane over 4 weeks ( Newman et al, 2010 ).…”