and MK-329 also abolished the intake depression caused by CCK (Ebenezeret al., 1990). This suggests that endogenous CCK does play a role as a negative feedback inhibitor of food intake.
It is likely that the doses of exogenous CCK used have mostly been above the physiological and have caused malaise, but the recent work with low doses of exogenous CCK, with antagonistic drugs and with immunization against CCK, provides very strong evidence for a role for CCK in normal satiety.
Ruminants
In ruminants there are delays between eating and the arrival of food at the duodenal CCK-producing sites. Thus, CCK might be less important in these types of animal than in those such as the pig in which the simple stomach releases digesta into the duodenum as soon as a meal starts. However, a dose- dependent increase in plasma CCK concentrations to feeding different amounts of fat to cows has been observed 3 h post-feeding (Choi and Palmquist, 1996), giving some support to a role for CCK in controlling food intake in ruminants.
Grovum (1981) infused CCK into several blood vessels in sheep and found that there was no greater effect on food intake when infusion was into the carotid artery or portal vein than when it was given into the jugular vein. He concluded that neither the brain nor the liver were involved in the reduction of food intake in response to CCK. The main effect is probably on the digestive tract. However, the flow of digesta through the duodenum is relatively constant in free-feeding ruminants, and Furuse et al. (1991) found no significant fluctuations in plasma levels of CCK in cows either with concentrate or roughage feeding. Without such meal-related fluctuations it is difficult to see how CCK can be involved in the control of feeding.
Other gut hormones
Pentagastrin depressed food intake of sheep whether given into the jugular vein or portal vein (Anil and Forbes, 1980a), while secretin had no effect by either route. Two other gut peptides, somatostatin and bombesin, also depress food intake and the former has many properties that make it a possible satiety hormone. The effect of somatostatin administered peripherally is blocked by gastric vagotomy, but that of bombesin is not and the route whereby the latter influences the CNS is unknown.
vasopressin concentrations achieved in plasma as a result of the injections were similar to those seen during stress, and it was suggested that this hormone might be involved in stress-induced anorexia.
Hormones involved in growth and reproduction are covered in Chapters 15 and 16, respectively.
Conclusions
This chapter cannot claim to do justice to the complex subject of the roles of metabolites and hormones in the control of voluntary food intake. Although the glucostatic theory in the form originally proposed is no longer tenable, it is clear that animals can monitor the availability of energy-yielding materials and use this information to control their food intake. Because glucose is the major energy-yielding substrate in most non-ruminants, its infusion causes a reduction in food intake, especially when given into the liver. The liver is sensitive to oxidizable nutrients including glucose (propionate in the ruminant).
The effects of infusion of glucose or propionate into the liver can be blocked by local section of the vagus and/or splanchnic nerves, showing that the information to the brain is transmitted via the autonomic nervous system. A major role of the liver is to prevent the uneven flow of nutrients from the digestive tract causing undue fluctuations in the energy supply to the rest of the body, especially the CNS, so it is well placed to play a major role in the control of feeding.
Fats and proteins can also yield energy and therefore affect the energostatic control of intake in situations where they are oxidized. This utilization is influenced by metabolic hormones, and the effect of metabolites is modified according to the insulin and glucagon status of the animal. The role of insulin in the control of food intake is complex and dependent on the nutritional status of the animal as well as on other hormones and metabolites.
Most hormones affect food intake when given in large amounts, but it is often difficult to decide whether they are acting directly on the CNS or peripherally and whether they are important under normal, physiological conditions. This is true for the gut peptides such as CCK, which act at both sites and whose natural sites and rates of secretion are difficult to mimic experimentally.
5 Central Nervous System and the Special Senses
The central nervous system is clearly the integrator of most of the actions of the animal and, as such, plays a vital role in the control of voluntary food intake as well as in growth, fattening and reproduction. It is axiomatic that information from receptors in other parts of the body, including the special senses, is relayed to the brain, which integrates this with what it has previously learned about the consequences of feeding and makes decisions whether or not to seek and eat food.
The centres in the brain that are involved in feeding control were originally thought to be in the hypothalamus, a region with a volume of about 0.5 cm3in the sheep, lying just above the pituitary gland and optic chiasma at the base of the brain. These centres have been associated with the glucostatic, thermostatic and lipostatic theories of control of feeding (see Chapter 1). The ventromedial hypothalamus (VMH) is important in the control of anterior pituitary function as well as in food intake, while the lateral hypothalamic area (LHA), being a part of the medial forebrain bundle, receives information both from the visceral receptors and from higher centres of the brain.
Early studies used lesioning techniques to destroy small parts of the brain, followed by observation of changes in behaviour following recovery from the anaesthetic. Subsequently the effects of electrical stimulation were studied, while more recently effort has been concentrated on the neurochemistry of intake control using techniques such as injection of drugs that mimic or block the effects of the naturally occurring brain chemicals.
It is now accepted that feeding is organized by circuits and networks of neurones rather than by discrete ‘centres’, but the full picture is far from being complete. The involvement of the brain in the control of food intake with particular respect to poultry has been reviewed by Denbow (1985), while Forbes and Blundell (1989) have covered the subject for pigs and Baile and McLaughlin (1987) for ruminants. None of these reviews is recent and, while subsequent research is covered in this chapter, studies with farm animals
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have decreased in the last two decades. Research using laboratory animals is presumably of relevance to farm animals, but there is evidence of significant differences between species. Reference in this chapter to species other than farm animals is minimal, and the reader is referred to the book edited by Stricker and Woods (2004).