Neural Secretions and Regulation of Gut Functions

Abstract

Neural secretions of the gut are substances released from the neurons or nerve terminals of the gastrointestinal (GI) tract to regulate the functioning of the GI system as well as extragut tissues and organs. The neural secretions may be mediated and released centrally or peripherally to act on the GI tract and extragut tissues/organs. The central neural secretions that act on the gut are mediated by the brain and spinal cord, whereas peripherally mediated neural secretions are regulated and released from the intrinsic nervous system of the gut (enteric nervous system) and extrinsic nervous system. Progressive advancement of knowledge on neural secretions of the gut was dictated mainly by technological progress. This area of study (regarding neural secretions and regulation of gut) has increasingly been recognized as a new field of science called neurogastroenterology. The science of neural secretions and regulation of GI functioning began around the eighteenth to nineteenth centuries. The founding fathers of this new science were William Maddock Bayliss (1860–1924), Ernest Henry Starling (1866–1927), Georg Meissner (1829–1905), Leopold Auerbach (1828–1897), Santiago Ramón y Cajal (1852–1934), Alexander Stanislavovich Dogiel (1852–1922), John Newport Langley (1852–1925), Paul Trendelenburg (1884–1931), and Harold Hirschsprung (1830–1916) among others. The second half of the twentieth century was marked by substantial advancement in neurogastroenterology, and generally in science. Development of the broader field “gastroenteroneuro endocrinology,” which comprises neurogastroenterology and gut endocrinology, was enhanced by progress in genetics and molecular biology pioneered by James Watson (1928–), Francis Crick (1916–2004), and Maurice Hugh Frederick Wilkins (1916–2004); peptide biochemistry, and the invention of the radioimmunoassay, which enabled the measurement of minute quantities of neuropeptides. The gut produces over 50 types of neural secretions (neurotransmitters, neuromodulators), and tens of different types of neurons have been identified in the gut alone. This makes the gut one of the most important organs that can mediate neural signals at the peripheral level and in extragut tissues and organs such as the brain and adipose tissues. With extensively growing data on neurotransmitters of the gut and their functions, it is necessary to provide state-of-the-art information on neural secretions of the gut and their mechanisms of regulation. This chapter provides contemporary information on fundamental aspects of the neural secretions of the gut, gives an account of the course of discovery of these secretions (neurotransmitters) of the gut, and provides the mechanisms of neural regulation of GI functions. The clinical importance of the neurotransmitters is systematically described.