The control and regulation of such a complex system so that all the various processes occur in a coordinated and timely way are complex. The secretion of digestive juices, coordination of muscle contractions to mix and move the bolus, and activation of enzymes, as well as communication to the brain to stimulate or suppress appetite and influence feeding behavior, all must oc- cur in an appropriate and measured process. This is accomplished through a combination of hormonal and nervous control. Few studies have been done with fish in this area, therefore some of the material in this section has been extrapolated from what is known for other animals (mostly mammals).
This should be kept in mind, as future studies may find differences between the mammalian model and fish.
The overall metabolic state of the organism influences feeding and di- gestion. Many of the regulators of metabolism (insulin, insulin-like growth factor, glucagon, glucagon-like peptide, pancreastatin, somatostatin, growth hormone, and so on) also affect the digestive tract; however, these effects tend to be coordinated at the organismal level. This includes control of appetite, transport of nutrients within the body, uptake of nutrients from the blood by tissues, control of blood nutrient level, and cell and tissue growth. There is also a system of gastroenteropancreatic (GEP) hormones/
neurotransmitters (Wendelaar Bonga 1993) and specific autonomic nerves (Smith 1989) that exert control over the specific processes of the gastroin- testinal tract. It is the latter set of controls that is the subject of this section.
The understanding of the GEP endocrine and nervous system is likely to have practical application for the improvement of prepared diets for fish being raised in aquaculture, because this is the system that regulates and controls the efficiency of digestion on a single meal basis. For information on control and regulation of metabolism see Chapter 6, by Dabrowski and Guderley.
Nerves from the cranial, spinal, and enteric sections of the autonomic system innervate various parts of the gastrointestinal tract. The vagus nerve (cranial X) runs to the pharynx, esophagus, and stomach. The (spinal) celiac ganglion innervates all of the visceral organs and also connects to the vagus nerve. The splanchnic nerve (also spinal) connects to the uro- genital organs and rectum. The enteric nerves connect to the circular and longitudinal muscle layers of the gut wall and gland cells and receive inputs from extrinsic cranial and spinal nerves (Smith 1989). The roles of these nerves, in concert with GEP hormones, include the detection of fullness, control of peristalsis, vasoconstriction/dilation, control of smooth muscle (for regurgitation or relaxation of the stomach to allow distension), and secretion of digestive fluids (Smith 1989). For example, in humans, the strongest stimulation of acid and pepsin secretion is produced via the vagus nerve (Lentner 1981), although a number of GEP hormones also result in these secretions.
Endocrine cells of the GEP system are located in the pancreas (as dis- cussed in Section 7.5.6.) and in the gut (stomach, intestine, and ceca).
Within the gut, diffuse endocrine cells are of two types, open and closed (Wendelaar Bonga 1993). Both cell types are in contact with the basal lamina, but only the open type extends all the way through to the lumen. The open cell has microvilli-like processes that are likely to contain chemorecep- tors for compounds in the gut (Wendelaar Bonga 1993). This gives the open cell the opportunity to communicate to both the lumen, on one side, and capillaries adjacent to the basal lamina, on the other side (Smith 1989). Both cells secrete into the lateral and basal cell membranes (Wendelaar Bonga 1993). The gut endocrine cells can produce the same major hormones as the pancreatic islets (glucagon, glucagon-like peptide, somatostatin, pancreatic polypeptide, pancreastatin, and, in some species, insulin) as well as sev- eral others (gastrin, cholecystokinin, bombesin, enkephalin, vasointestinal peptide, tachykinins, neuropeptide Y-like peptides, neurotensin, secretin, gastric inhibitory peptide, and serotonin) (Wendelaar Bonga 1993).
Some of the known actions of the various neuropeptide and nerves of the GEP system are listed in Table 7.3. The study of digestive endocrinol- ogy in fish is lagging compared to other aspects of endocrinology such as reproduction, development, growth, and metabolism, so much of the infor- mation in Table 7.3 and this section has been based on what is known in higher vertebrates. There is a tremendous opportunity to utilize modern molecular methods to better understand the workings of the GEP system (Schmitzet al. 1996; Johnsonet al. 1997; Peyonet al. 1998, 1999; Suzukiet al.
1999). This understanding may provide the insight needed to control and improve digestion in farmed fish in the future.
Selected Hormones and Neurotransmitters of Gastroenteropancreatic (GEP) Systema
Hormone Synthesis location Activity location Function(s) References
Gastrin Released by stomach Stomach oxynticopeptic Release of HCl and Elbalet al. (1988), Cimini
(pyloric) and intestinal cells pepsinogen et al. (1989), Holmgren
endocrine cells in (1993), Wendelaar Bonga
response to food in lumen (1993), Barrenecheaet al.
(1994), Panet al. (1995), Nielsenet al. (1998) Cholecystokinin Released by anterior Gallbladder, pancreas, Stimulates contraction Ciminiet al. (1989),
(CCK) intestinal endocrine and brain of the gallbladder and Holmgren (1993), Wendelaar
cells in response to secretion of pancreatic Bonga (1993), Barrenechea
food in lumen and digestive enzymes and et al. (1994), Peng and
vagal stimulation. hormones; decreases Peter (1997), Nielsen
feeding behavior and et al. (1998) gastric emptying
Bombesin and Released by stomach Systemic Stimulates endocrine Ciminiet al. (1989), Rajjo
gastrin releasing and/or anterior cells to release gastrin/ et al. (1989), Jensen and
peptide intestinal endocrine CCK, growth hormone, Conlon (1992), Holmgren
cells and other stimulatory (1993), Wendelaar Bonga
compounds; decreases (1993), Barrenecheaet al.
feeding behavior (1994), Peng and Peter (1997), Phale (1998)
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peptide (VIP) and/or intestinal gut wall, pancreas, gut wall and pancreatic (1993), Wendelaar Bonga endocrine cells in intestinal endocrine exocrine and endocrine (1993), Barrenecheaet al.
response to gut cells, and blood secretion; increases (1994), Pleschet al. (1999)
distention vessels blood flow to gut;
activates salt secretion in rectum and inhibits gastrin secretion
Somatostatin Released by endocrine Gut and brain Inhibits gastrointestinal Abadet al. (1987), Elbal
cells in stomach and motility, rectal salt et al. (1988), Ciminiet al.
pancreas secretion, and gastrin (1989), Holmgren (1990),
and growth hormone Chan and Hale (1992),
release Barrenecheaet al. (1994),
Pleschet al. (1999) Neuropeptide Released by stomach Gut and brain Enhances ion transport Wendelaar Bonga (1993),
Y-like peptides and/or intestinal and stimulates feeding Peter (1997), Gomez-Visus
endocrine cells behavior et al. (1998)
Secretin Released by anterior Pancreas Releases sodium Ince (1983), Wendelaar
intestine endocrine bicarbonate; inhibits Bonga (1993)
cells in response to gastric acid secretion;
gastric acids may increase insulin
and other pancreatic hormone levels
aAll are peptides, and many have functions and tissue locations in addition to those listed here.
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7.11