Effects of corticosterone treatment and wound healing on reproductive traits of American bullfrogs

Abstract

During reproductive season, calling anuran males display high testosterone (T) and episodically high corticosterone (CORT) plasma levels, which are positively associated with higher calling rates and immunocompetence. However, exposure to constant stress stimuli can result in chronically elevated CORT levels, possibly leading to inhibition of reproductive and immune activity. Reproduction and immune responses are energetically expensive, so when an animal is immunologically challenged, a tradeoff might be expressed, with CORT potentially mediating it. Our aim was to test how episodic and chronic CORT treatments, alongside wound healing, would affect reproduction in American bullfrog males (Lithobates catesbeianus). Forty animals were divided in four groups: Episodic CORT (daily transdermic application of CORT), placebo (daily transdermic application of sesame oil), chronic CORT (subcutaneous CORT silastic implants), and sham control (subcutaneous empty silastic implants). One week after treatments began, animals were punctured in the leg with a biopsy needle and the wound was photographed after 45 days to determine wound healing status (WS). Blood samples were collected throughout the experiment to measure CORT and T plasma levels. After animal euthanasia, testes were dissected, fixed, and analyzed histologically to determine spermatogenic activity (germinative cyst [GmC] morphometrics). As expected, the episodic CORT treatment had no effect on T plasma levels or spermatogenic activity. On the other hand, chronic CORT treatment reduced GmC morphometric traits, indicating suppression of reproduction, although T levels were not altered. In addition, animals from sham control and chronic CORT treatments with higher T levels presented higher WS, which indicates an immune‐enhancing T effect.

Contrasting effects of transdermal and implant corticosterone treatments in the American bullfrog wound healing

“…Compared to the physiological levels achieved 30 min after transdermal application, the increased CORT plasma levels in animals from group 4 (CORT implant) was (on average) 3.2-fold greater. Besides the expected chronic increase of CORT plasma levels, the CORT implant treatment also resulted in body mass loss (present study) and inhibited spermatogenesis in these males [9]. Although implants do not mimic the physiological pulsatile profile of CORT, the data indicate that the pharmacological increase of CORT levels resulted in the expected deleterious effects of chronic CORT plasma elevation, including reproductive traits [9], lower body mass and the tendency of lower immune response (present study).…”

“…Besides the expected chronic increase of CORT plasma levels, the CORT implant treatment also resulted in body mass loss (present study) and inhibited spermatogenesis in these males [9]. Although implants do not mimic the physiological pulsatile profile of CORT, the data indicate that the pharmacological increase of CORT levels resulted in the expected deleterious effects of chronic CORT plasma elevation, including reproductive traits [9], lower body mass and the tendency of lower immune response (present study). Sesame oil-treated (group 1) and empty implant individuals (group 3) maintained the same CORT plasma levels as the baseline for at least five weeks.…”

“…Even greater increase in GCs may occur when animals face unpredictable and threatening events [3][4][5][6][7]. The ubiquity of GC receptors, with expression in organs, tissues and immune cells, results in broad physiological and behavioural adjustments that help individuals cope with these events [1,8,9].…”

Testosterone immunomodulation in free-living and captive Rhinella icterica male toads

“…Studies have shown T suppresses the action of inflammatory dendritic cells and macrophages in humans [33], reduces the expression of receptors in immune cells in mice [34], decreases the phagocytosis activity of blood cells and the lymphocyte proliferation in snakes [35], and impairs antibody production and the cell-mediated response in birds [36,37]. Although immunosuppression is mostly associated with high T, it has been recently demonstrated that plasma T levels and immunity can positively covary under natural and experimental conditions in birds [38][39][40], lizards [11], fishes [41] and amphibians [42][43][44][45]. Namely, intraperitoneal T injection with 2 and 5 µg g −1 doses significantly increased plasma T levels at physiological levels, and promoted augmented complement and peroxidase activities 3 days after the T injection in gilthead seabream [41].…”