Peptidergic control of food intake in food-producing animals

Roles of brain and intestinal peptides in the control of food intake may vary among species for specific peptides depending on the degree of complexity of the gastrointestinal tract. Cholecystokinin (CCK) in the brain and intestine is the most widely studied of the peptides involved in the control of feeding. Although CCK released from the intestine may act on peripheral receptors in producing satiety in the pig, a monogastric animal, it has little effect on feeding after peripheral administration in sheep. CCK injected peripherally in chickens decreases food intake, but because of the delay in gastric emptying related to the crop and gizzard, it may be of minor importance. Possible roles for brain CCK have been suggested because CCK injected into the cerebrospinal fluid (CSF) decreases feeding in all three species. In sheep, food intake was stimulated by sequestration of endogenous CCK in CSF with specific CCK antibodies, which suggests a physiological role for brain CCK controlling food intake in this species. Opioid peptides increased feeding in sheep after i.v. and CSF injections. Only peripheral, and not CSF, injections of naloxone, a specific opiate antagonist, decreased feeding and blocked both peripheral and central opioid peptide-stimulated feeding. The balance of CCK and the opioid peptide activity in either the central nervous system or the periphery appears important in the control of feeding, but specific peptide functions and sites of action probably vary among species.