Plasma steroids and immune measures vary with restraint duration in a toad (Rhinella icterica).

Immunoenhancing effects have been widely described following acute stressors in several vertebrates, and valuable contributions have been made from studies on acute stress to understand hormonal-immune interactions. However, most studies focus on hormonal and immune responses after standardized time lapses, neglecting potential influence of duration of exposition to stressor. Herein, we investigate fluctuations of plasma hormone concentrations (corticosterone and testosterone) and immunity (neutrophil to lymphocyte ratio, phagocytosis of blood cells, and plasma bacterial killing ability) in a toad species (Rhinella icterica) in response to six different periods of exposure to restraint stress. We observed increased plasma corticosterone concentrations following restraint in all sampled times (0.5 to 48 h), with the highest values being observed during the first hour (0.5 to 1 h). Restraint-induced increases in the neutrophil to lymphocyte ratio and phagocytosis percentage were observed from the first 0.5 h, gradually increasing after that with the time of restraint. We also observed decreased testosterone plasma concentrations in response to a more prolonged restraint (24 and 48 h). No changes were observed in plasma bacterial killing ability following restraint. Together, our results demonstrate dynamic time-related hormonal and immune changes. These results point to the fact that for some species measuring hormonal and immune variables at single time points following a stressor might work better when preceded by a study of the temporal changes of the response variables to the stimuli applied. Also, time of response needs to be considered when different variables are used as proxies of stress.

Contaminated meal intake (Aeromonas hydrophila) does not elicit hormonal or immune modulation in bullfrogs (Lithobates catesbeianus).

The gastrointestinal tract (GIT) is colonized by resident microbiota but contact with foreign microbiota during feeding can impair GIT functions. During meal digestion, several vertebrates modulate the systemic immune function and immunoregulatory hormones concentration. However, in ectothermic animals, it is not known if this hormonal and immune modulation during the postprandial period is affected by the presence of pathogenic microbiota in the food. This study aimed to investigate the effects of contaminated meal ingestion on hormonal and innate immune responses in bullfrogs (Lithobates catesbeianus). Bullfrogs were divided into three treatments: fed three times with sterilized fish feed (control group), fed twice with sterilized fish feed and once with fish feed containing live bacteria (Aeromonas hydrophila, 109 UFC/mL), and fed three times with fish feed containing live bacteria. Blood and GIT tissues were collected 24 h after treatments to measure plasma and tissue corticosterone levels, NL ratio, and plasma bacterial killing ability. The ingestion of contaminated meal did not affect the hormonal and immune parameters. In conclusion, ingestion of contaminated food was not capable of intensifying the hypothalamic-pituitary-interrenal axis activation and the consequent hormonal and immune responses observed after feeding in bullfrogs. However, our results suggest that the ingestion of three contaminated meals tended to decrease stomach corticosterone levels (nonstatistically significant), possibly contributing to preventing the transmigration of the bacteria to organs outside the GIT.