Insulin
In farm animals, the effects of insulin administration have sometimes been to depress food intake and sometimes to stimulate it. In the chicken, mammalian insulin depresses intake but avian insulin is structurally different from that of mammals and any future work will need to use chicken insulin.
Exogenous insulin has been shown to stimulate voluntary intake of pigs under some circumstances (Houpt et al., 1977). Similarly, some experiments with sheep have shown increased intake (e.g. Houpt, 1974) while others have not. Intravenous administration of 0.7 g/kg of insulin at the beginning of a meal depressed food intake during the following 30 min by 14% when given to cows deprived of food for 11 h, but not in those fed 4 h beforehand (Faverdin, 1986), so the role of insulin in short-term control of intake depends on nutritional status.
Evidence that insulin does play an important physiological role in maintaining food intake in ruminants comes from the observation that alloxan- induced diabetes causes inappetence and death in sheep (Reid et al., 1963).
This is due to the lack of insulin rather than to the general toxic effects of alloxan, because replacement therapy with insulin maintains intake at normal levels.
A collation of data from the literature on effects of intravenous insulin in ruminants (Dulphy and Faverdin, 1987) shows that, in the short term (15–30 min after injection), doses up to about 0.5 g/kg depress feeding but doses above this have less effect; higher doses (84–170 g/kg) stimulate intake from 2–4 h after injection. Infusion of glucose to maintain plasma levels prevents the insulin-induced hypophagia in sheep, but in cows a euglycaemic clamp applied for long-term infusions (4 days) during hyperinsulinaemia generally depressed intake (Faverdin, 1999). Given the many physiological responses, both direct and indirect, to changing the concentration of insulin in the blood, it is not possible to explain these diverse results without a deeper understanding of all of the factors effecting, and affected by, changes in insulin secretion.
In summary, insulin may play a role in long-term intake and weight regulation in ruminants but it does not seem likely that it plays a role in the depressed intake in dairy cows in early lactation when the insulin concentration is low.
Glucagon
Glucagon has been found to increase in blood during feeding. The liver is the focus of attention for effects of this hormone on feeding, as injections into the portal vein are more effective in depressing feeding than those into the general circulation, and hepatic vagotomy blocks the effect of glucagon given portally.
Glucagon injected into the portal vein of cockerels at doses of 5–50 g was found to depress food intake in a dose-related manner during the subsequent 90 min (Howes and Forbes, 1987b). This effect was prevented by vagotomy, although interpretation of the results was somewhat clouded by the fact that control levels of feeding at the time of day the experiments were carried out were lower in vagotomized birds than intact birds. The reason for this is probably that vagotomized chickens eat fewer meals and the infusion period happened to include less meals after vagotomy.
Of the few studies of glucagon and glucagon-like peptide 1 (GLP1) on food intake in ruminants, one shows that glucagon administered intravenously at physiological concentrations does reduce intake in sheep. More research is required in ruminants on the effect of glucagon and GLP1 to unravel their importance in intake regulation.
Corticotropin-releasing factor (CRF)
CRF is the hypothalamic releasing hormone that controls the output of pituitary corticotrophin and thus the adrenal output of corticosteroids. Centrally administered, CRF reduces food intake in cattle and it may be that it plays a role in the reduced intake around calving and during other stressful events (Ingvartsen and Andersen, 2000).
Growth hormone and somatostatin
In the sheep, spontaneous meals of a highly digestible food are preceded by peaks of growth hormone (GH) in plasma, as shown in Fig. 4.4 (Driver and Forbes, 1981). Unexpected withdrawal of food resulted in much greater peaks of GH secretion within 2–3 h and, within a few minutes of replacement of food, levels were low once again. The concentration of GH in lactating cows also tends to fall during spontaneous meals. Growth hormone secretion is a sensitive index of nutritional status, and this suggests that the pre-meal peak is an indication of the need to replenish body stores, rather than a direct cause of eating. Injection of GH to mimic spontaneous peaks has no effect on feeding behaviour (P.M. Driver and J.M. Forbes, unpublished results), so that the release of GH from the anterior pituitary (under the control of the hypothalamus) and the onset of feeding seem to be independent consequences of a relative shortfall in nutrient supply from the digestive tract.
Somatostatin is the hypothalamic factor that maintains inhibitory control on GH secretion. In a summary of 11 studies, it was found that growing cattle
immunized against somatostatin ate an average of 4.2% more food than did controls (Ingvartsen and Andersen, 2000). This effect is likely to be via the increase in growth, and thus in nutrient requirements, caused by the immunization. It may seem paradoxical that GH should both signal a shortage of nutrients and stimulate growth. However, it should be borne in mind that insulin secretion is reduced by underfeeding so that mild underfeeding will allow fat mobilization (increased GH and reduced insulin), while encouraging deposition of non-fat tissues.
Adrenaline
Eating stimulates the sympathetic and parasympathetic branches of the autonomic nervous system, causing the release of adrenaline both from the Fig. 4.4. Plasma growth hormone concentrations and meal sizes during 27-h periods in a sheep; (a) with free access to food throughout; (b) food removed from 17.30 to 03.30 h (from Driver and Forbes, 1981).
adrenal medulla and from sympathetic nerve endings in the liver, which might contribute to satiety via actions on the liver. Intramuscular injection of adrenaline at doses of 0.2–1.0 mg/bird caused hypophagia for several hours (Sykes, 1983).
This inhibition of feeding was not overcome by prior starvation for 24 h, but pre- treatment with 1.5 mg phentolamine or 0.4 mg propranalol (␣-adrenergic antagonists) completely blocked this effect. Injection of up to 2.5 mg adrenaline into the hepatic portal vein of cockerels depressed intake in a dose-related manner, but the effect of the highest dose was not attenuated by vagotomy at the level of the proventriculus (Howes and Forbes, 1987a). Phenylephrine, a pure
-blocker, given intraportally at doses of up to 3.0 mg, had no effect on food intake whereas the -blocker salbutamol gave a dose-related depression, whether or not the vagus nerves were intact. The biological significance of these results is in doubt, however, as the doses used are likely to be well above the physiological range.
Leptin
It had long been suspected that adipose tissue secretes a substance in proportion to its mass, that acts as a feedback signal to the CNS. Leptin possesses many of the characteristics necessary for this function, as adipose tissue produces this hormone increasingly as adipocyte size increases (see review by Houseknecht and Spurlock, 2003). Leptin is taken up in the CNS where it inhibits feeding via the NPY system, with pro-opiomelanocortin, melanocortin-stimulating hormone and aguti-related peptide also likely to be involved as mediators. Leptin primarily binds to the dorso- and ventromedial nuclei of the hypothalamus and to the arcuate nucleus, areas of the brain that are involved in the control of hormone secretion (including GH) and food intake.
While much of the research on leptin has been performed with laboratory animals and humans, there is evidence that it has similar functions in farm animals (Ingvartsen and Andersen, 2000). Leptin also affects energy expenditure, glucose metabolism, insulin secretion and action, the adrenal axis and hormones of the growth hormone axis, but virtually no research in these areas has been conducted with farm animals, although it has been demon- strated that growing pigs injected intra-cerebroventricularly with 10, 50 or 100 mu porcine leptin showed a dose-related depression in food intake over approximately the next 24 h (Barbet al., 1998).
Although there have been some studies on the effects of GH on plasma leptin levels in cattle, there appears to have been no work on voluntary intake.
Leptin in the plasma of sheep shows a mean concentration of 2.9 ng/ml, but with secretory episodes at 4.8 pulses/day with a mean amplitude 0.67 ng/ml and length of 1.13 h. These parameters are not affected by pattern of feeding or light (Tokuda et al., 2000b). There appears to be a seasonal influence on the effects of leptin in sheep, as ICV injection in the autumn reduces food intake by 30%, when it is already low (see Chapter 17), while the same injection in the spring has no effect (Adam and Mercer, 2004).
Administration of single injections of 200 g of mouse leptin intra- cerebroventricularly daily for 7 days in sheep reduced intake and body weight (Tokuda et al., 2000a), and continuous infusion for 3 days of recombinant human leptin at 20 g/h into the cerebroventricles of sheep decreased the food intake significantly (Henryet al., 1999).
If we can envisage leptin as generating ‘discomfort’ in the CNS, then we can integrate it into our concept of MTD, whereby its effects are added to those of other feedback signals (see Chapter 10). A small but persistent effect of leptin could, when added to the short-term signals from the digestive tract and liver, result in significant long-term reduction in intake in obese animals. There is no need to consider short-term and long-term controls of intake as being different in nature, just in the rate of change of the signals involved.
While there have been hopes that our increasing knowledge of the biology of leptin would lead to advances in the treatment of obesity in humans, it is difficult to see how knowledge of the leptin system could be made use of commercially in farm animals. If leptin is contributing significantly to the control of intake, then neutralizing leptin (e.g. with specific antibodies) would allow food intake to increase but the extra nutrients would be used for fat synthesis, which is not usually required. If, on the other hand, the animal is eating insufficient to support its metabolic needs, then adipose tissue will have been depleted and its production of leptin will be low, rendering neutralization ineffective.
Not all agree that the leptin hypothesis is valid. Speakman et al. (2002) point out that there are very great differences in the concentrations of leptin in people of similar fatness; different fat depots produce different amounts of leptin, subcutaneous fat a great deal, omental fat hardly any; in some species leptin is produced in the liver as well as in adipose tissue. Leptin production is very sensitive to immediate food intake, so leptin should be thought of as a starvation signal, i.e. a memory of energy deficit, and body mass is then considered an output variable rather than part of a closed loop, and to be susceptible to social factors, etc. Speakman’s alternative proposals to explain the maintenance of more-or-less constant body fat content are discussed in Chapter 15.
Ghrelin
This is a hormone produced by the stomach wall that stimulates feeding by acting on the hypothalamus. Plasma levels increase during fasting, and humans injected with ghrelin have reported sensations of intense hunger. In pigs, intravenous infusion of 2 g/kg/day of ghrelin for 5 days stimulated weight gain and insulin, GH and cortisol secretion without affecting food intake (Salfen et al., 2004). This reinforces the endocrine effects of ghrelin without giving any support for an effect on intake.
Ghrelin and its cDNA-encoding precursor have been identified from the chicken proventriculus, and Kaiya et al. (2007) review ghrelin structure, distribution and function in birds.
In sheep, peaks of ghrelin in plasma have been noted just before scheduled feeding times, falling back to baseline 1 h after feeding (Suginoet al., 2002). It was suggested that ghrelin was acting as a hunger hormone.
Another hormone, obestatin, was found in late 2005 to decreaseappetite.
It is encoded by the same gene as ghrelin, but the purpose of this mechanism remains unknown.
Cholecystokinin
The secretion of several gut hormones is increased during feeding and subsequently as digesta pass through the stomach and duodenum. Of these, cholecystokinin (CCK) is perhaps the most studied as a satiety signal, but others, such as bombesin and gastrin, are also likely to be involved; CCK is produced principally in the wall of the duodenum. Strong evidence that endogenous CCK is important in the limitation of feeding has been provided by autoimmunization of rats to CCK, which increases food intake and weight gain and by the conditioning of a flavour preference to CCK with very low doses; higher doses condition an aversion.
The receptors for CCK that are involved in the feeding response were initially assumed to be those in the brain, but it has been shown that there are also receptors in the stomach, as gastric vagotomy prevents the effects of exogenous CCK on intake. The major effect of peripheral injection is probably on the digestive tract, stimulated to contract and thereby activate mechanoreceptors whose information is relayed to the CNS via the vagus nerves. The effect of injection into the brain is direct, and the effect on feeding is probably independent of that on the gastrointestinal tract (see Chapter 5).
Poultry
Intravenous injection of CCK in chickens reduces food intake, depresses gizzard motility and stimulates muscular activity in the duodenum. It has been found that intravenous infusion of CCK had less effect in birds offered diluted food and it was suggested that CCK may induce gastrointestinal responses that lead to abdominal discomfort; these are reduced when the motivation to food is higher (Savory and Gentle, 1984). Savory (1987) used conditioning tests to demonstrate that bombesin or CCK-8 at 1–10 g/kg were mildly aversive, increased heart rate and induced abnormal gastrointestinal motility. Cholecystokinin is less effective when arousal is reduced with reserpine, when the birds are very hungry or when their attention is distracted, all these points being used to support the contention that CCK acts by inducing abdominal discomfort rather than normal satiety.
Covasa and Forbes (1994b) confirmed the effects of CCK in broiler chickens by intraperitoneal injection and demonstrated a learned aversion to the colour of food offered for 2 h after 14 g/kg CCK injections, compared with saline. Vagotomy at the level of the proventriculus (equivalent to sub- diaphragmatic vagotomy in the mammal) prevented the effect of CCK on feeding and the learned aversion to CCK-paired coloured food. A very low
dose of CCK (2g/kg) had no significant effect on feeding and did not elicit a preference for the colour paired with the injection.
The CCK-blocker, MK-329, given intraperitoneally at doses of ⭓90g/kg, significantly increased food intake over the next 2 h while, when given intravenously, somewhat lower doses were effective (Covasa and Forbes, 1994a). Lower doses of MK-329 did not affect food intake and failed to condition a preference or aversion for the colour of food given for 2 h after the injection. Cholecystokinin (14 mg/kg) caused a reduction in feeding, but this effect was not blocked by pre-treatment with intraperitoneal injection of MK- 329 (32, 90, 180 and 360 mg/kg), thus questioning the role of endogenous CCK in satiety in chickens. A logical next step would be to see whether MK-329 still stimulates intake when given intraperitoneally in vagotomized birds.
Pigs
Cholecystokinin is thought either to constrict the pylorus, reducing the rate of stomach emptying, or to increase the sensitivity of vagal afferent receptors.
CCK given into the jugular vein of pigs at a rate sufficient to depress food intake by 25% did not affect gastric emptying, however, suggesting that an effect on stomach motility is not necessary for CCK to exert at least some of its effect on feeding. This is confirmed by the observation that MK-329 reverses the depression of intake caused by fat infusion, but does not reverse the effect on gastric emptying. MK-329 increases intake in operant-fed pigs and also in pigs given a single meal after an overnight fast (Rayneret al., 1991). Another CCK antagonist (L-364,718) blocked the inhibition of food intake induced by jugular or abdominal aortic infusion of CCK or duodenal infusion of emulsified fat or monoglyceride, while the responses to duodenal glucose, glycerol or oleic acid were not blocked.
Feeding in the pig is depressed by infusion of CCK into the general circulation but not by that into the hepatic portal vein or gastric artery. Probably CCK is most active in the upper intestine, as it is most effective when given into the branch of the mesenteric circulation going to this part of the intestine. The site of action is concluded to be in the intestines, but it is not yet known whether this is dependent on vagal innervation. The fact that duodenal infusion of phenylalanine or tryptophan in pigs increased plasma concentrations of CCK, but did not affect feeding (Rayner and Gregory, 1985), casts further doubt on the role of CCK as a true hormone of satiety. However, pigs immunized against CCK increased their intake by 8.2% and their growth by 10.6% (Pekas, 1983), strong support for a physiological role for CCK in the control of food intake.
Baldwin et al. (1983) have suggested that exogenous CCK causes a general malaise in the pig. They trained pigs by operant conditioning to respond for food, water, sucrose solution or heat. Doses of 20 or 40 units of CCK octapeptide given intravenously transiently reduced responding for food, water or sucrose but had no effect on responses for heat. These results show that CCK affects several types of behaviour and is thus unlikely to be a specific food satiety factor. However, the CCK receptor antagonist MK-329 caused a dose-related increase in food intake during 2 h following intravenous injection,
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.