Immune and hormonal modulation in the postprandial period of bullfrogs (Lithobates catesbeianus)
Mammals show immune up-regulation and increased plasma and local (gastrointestinal tract) concentrations of some immunoregulatory hormones, such as corticosterone and melatonin, after feeding. However, little is known about the endocrine and immune modulation in the postprandial period of ectothermic animals. This study investigated the effects of feeding on endocrine and immune responses in the bullfrog (Lithobates catesbeianus). Frogs were fasted for 10 days and divided into two groups: fasted and fed with fish feed (5% of body mass). Blood and gastrointestinal tract tissues (stomach and intestine) were collected at 6, 24, 48, 96, and 168 h to measure neutrophil/lymphocyte ratio, plasma bacterial killing ability, phagocytosis of blood leukocytes, plasma corticosterone and melatonin; and stomach and intestine melatonin. Feeding increased plasma corticosterone at 24 h and decreased at 168 h; and increased neutrophil/lymphocyte ratio at 6, 24, and 96 h. We also observed decreased bacterial killing ability 48 h after feeding. Stomach melatonin increased after 17-days fasting. We show that feeding activates the hypothalamic-pituitary-interrenal axis and promotes transient immunosuppression, without stimulating an inflammatory response. Increased CORT may mobilize energy to support the digestive processes and melatonin may protect the stomach during fasting. We conclude feeding modulate secretion of immunoregulatory hormones, increasing plasma CORT levels in the beginning followed by a decrease in the end of meal digestion; and systemic immune cell redistribution, increasing NL ratio during almost all meal digestion in bullfrogs. Also, fasting modulate secretion of melatonin in the stomach.