Chapter 4 Growth Hormone Regulation in Fish

In fish, growth hormone (GH) affects many functions, including somatic growth, energy metabolism, reproduction, feeding, osmoregulation and immune functions. GH release and synthesis are controlled by neuroendocrine factors from the brain and peripheral tissues. Hypothalamic regulators influence the expression of one another, forming an interacting network in GH regulation. GH release is inhibited tonically by somatostatin with insulin‐like growth factor as a major feedback regulator; however, the actual amount of GH released reflects the balance of total inhibitory and stimulatory influences. Sex steroids and nutritional status also modulate the expression and pituitary actions of hypothalamic factors. Intrapituitary regulators, including GH, gonadotropin and inhibin/activin, provide autocrine/paracrine control over GH synthesis and secretion. At the somatotrope level, receptor expression for a multitude of neuroendocrine factors can integrate the regulatory signals from various regulators. The distinct and yet overlapping signaling cascades utilized by different regulators allow for ligand‐ and function‐specificity for GH regulation.