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)

Abstract

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 t 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 CORTand 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. When vertebrates face a stressful stimulus, they release hormones that help them cope with and survive the adverse conditions. One key aspect of the vertebrate stress response is the activation of the hypothalamic-pituitary-adrenal (HPA) axis, culminating in glucocorticoid (GC; corticosterone, CORT, in reptiles) release several minutes after initiation of a stressful stimulus (Sapolsky et al., 2000). The GCs then induce a variety