Chapter 1 - An overview of chemical bioregulation in vertebrates

Abstract

Bioregulation in vertebrate animals involves secretion of chemical messengers primarily by cells of the endocrine system, the immune system, and the nervous system. These bioregulators include hormones, neurocrines (neurotransmitters, neuromodulators, and neurohormones), cytocrines (autocrines, paracrines), and semiochemicals (pheromones, allelomones). Bioregulators work by binding to specific receptor molecules located on the cell membrane or with cells, where they impact a variety of intracellular processes, more or less specific to each bioregulator. Homeostasis is the process whereby bioregulators maintain relatively constant conditions in the body. Allostasis is the process whereby homeostasis can be altered to respond to changing environmental conditions. These systems are regulated in part through positive and negative feedback and can be disrupted by natural and manmade chemicals in the environment that mimic or prevent the actions of natural bioregulators. The vertebrate neuroendocrine system involves the hypothalamus of the brain that produces neurohormones and the pituitary gland that produces tropic hormones that in turn affect four major endocrine glands (thyroid, gonad, adrenal cortex, and liver), each of which produces additional hormones that affect various tissues throughout the body. Other endocrine glands and tissues are independent of the neuroendocrine system (e.g., the endocrine pancreas, parathyroid glands, adipose cells, and cells in the lining of certain digestive organs) and also produce bioregulators.