Cocoa Butter Injections, but not Sealed or Perforated Silastic Implants, of Corticosterone can be used to Chronically Elevate Corticosterone in Free-Living Painted Turtles (Chrysemys picta)
Chronic stress can result in an elevation of circulating levels of glucocorticoid (GC) hormones in vertebrates, which may affect their fitness. To isolate the effect of GCs on stressed organisms, one approach consists of manipulating circulating levels of GCs. We investigated the usefulness of two corticosterone (CORT) administration methods, Silastic® implants and cocoa butter injections, in chronically elevating circulating CORT levels in Painted Turtles (Chrysemys picta). First, free-living turtles received subcutaneous Silastic implants for 2 mo. We observed no significant difference in baseline CORT levels between two doses of CORT and sham-treated turtles. Then, captive turtles received a subcutaneous Silastic implant for 28 d. We observed no effect on baseline CORT levels, hormonal stress response, or body mass, suggesting that sealed and perforated Silastic implants of CORT may not be an effective way to elevate CORT in Painted Turtles. Second, we tested injections of CORT-laden cocoa butter for the first time in an ectothermic tetrapod. Free-living turtles received an epicoelomic injection of liquid cocoa butter mixed with CORT and were recaptured in the field over 2 mo. Despite large interindividual variation, we found that this injection approach generally kept circulating CORT levels elevated for up to 3 wk. Achieved CORT concentrations were probably physiologically and ecologically relevant for the species, although concentrations possibly remained elevated longer than would be the case in wild animals. Cocoa butter injections, but not sealed or perforated Silastic implants, can be used in Painted Turtles to chronically elevate CORT. Further, this represents a promising method for other temperate ectotherms such as amphibians and reptiles.Abstract
Plasma total baseline corticosterone concentrations ([CORT]) as a function of time since treatment in free-living adult Painted Turtles (Chrysemys picta) (90 data points from 28 individuals) in Experiment 1A. Individuals were randomly assigned to one of three treatment groups: two empty Silastic implants (Sham, n = 10), one sealed CORT-filled Silastic implant and one empty implant (1 CORT, n = 9), or two sealed CORT-filled Silastic implants (2 CORT, n = 9). (A) [CORT] values are presented on a linear scale. (B) Individual profiles over time. [CORT] values are presented on a logarithmic scale to allow better visualization of each profile.
Plasma total corticosterone concentrations ([CORT]) from captive adult male Painted Turtles (C. picta) in Experiment 1B. Individuals were randomly assigned to one of four treatment groups (n = 7 for each group): no implant (Control), one empty Silastic implant (Sham), one sealed CORT-filled Silastic implant (CORT sealed), or one perforated CORT-filled Silastic implant (CORT holes). Baseline samples were collected as quickly as possible after taking the turtle out of its tank while stress-induced samples were collected following 30 min of restraint. (A) Baseline (scatter on left) and stress-induced (scatter on right) [CORT] on day 0, prior to treatment, as a function of time since first handling the animal (n = 28 pairs). (B) Baseline [CORT] as a function of time since implantation. (C) Stress-induced [CORT] as a function of time since implantation. (D) Individual profiles of baseline [CORT] over time. [CORT] values are presented on a logarithmic scale to allow better visualization of each profile.
Plasma total corticosterone concentrations ([CORT]) from free-living adult Painted Turtles (C. picta) in Experiment 2. Individuals were randomly assigned to one of three treatment groups: without injection (Control), injected with cocoa butter only (Sham), or injected with CORT-laden cocoa butter (CORT). Baseline samples were collected as quickly as possible after disturbing the turtle while stress-induced samples were collected following a 30-min restraint. (A) Baseline (scatter on left) and stress-induced (scatter on right) [CORT] on day 0, prior to treatment, as a function of time since first handling the animal (n = 22 pairs). (B) Baseline [CORT] as a function of time since treatment (88 data points from 28 individuals: 6 controls, 5 sham-treated, and 17 CORT-treated). The solid line shows the significant linear regression calculated after day 0 for the CORT-treated group. Regressions were not significant for the other groups. (C) Individual profiles of baseline [CORT] over time. [CORT] values are presented on a logarithmic scale to allow better visualization of each profile. Six baseline samples (n = 2 Control and 4 CORT) collected after 10 min (11–21 min) on day 0 were excluded from other analyses and figures but are shown (×) to allow presentation of all profiles and still demonstrate increases because of CORT treatment.
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