BUDDINGTON, RK; KROGDAHL, A; Mississippi State University; Norwegian College of Veterinary Medicine: Hormonal Regulation of the Gastrointestinal Tract of Fish

The gastrointestinal tract (GIT) of fish and other vertebrates is challenged with a diversity of functional demands that are associated with changes and differences in dietary inputs and environmental conditions. Many of the hormones, associated receptors, and signaling pathways that have been identified in the GIT of mammals and regulate its structural and functional characteristics have been localized to the GIT of fish. This contribution reviews how hormonal regulation plays a critical role in adaptation of GIT structure and functions throughout the life history of fish. Exemplary is the modulation of digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs by hormones produced by the GIT and the associated organs (e.g., pancreas), and the influences of hormones secreted by other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads). More is known about how the osmoregulatory functions of the GIT (secretion and absorption of electrolytes and water) are modulated by various hormones, including sex steroids, as fish migrate between environments of different salinities, and during metamorphosis. The specific osmoregulatory responses of the GIT to hormones vary among euryhaline and stenohaline fish. Cells associated with the endocrine and immune systems are present throughout the GIT and accessory organs of fish, yet there is very little known about their regulation by hormones. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.