Page 426 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Digestion / 411

Neuroendocrine Control leptin is produced by adipose tissue and
of Feeding
inhibits food intake and increases energy
VetBooks.ir Feeding behavior, or appetite, is regulated expenditure through its receptors in the
brain. In sheep and cattle, leptin may also
by a complex system of central and periph- modulate growth hormone secretion from
eral signals which interact in order to the anterior pituitary. Equine metabolic
modulate the individual response to nutri- syndrome is characterized by horses that
ent ingestion. The hypothalamus is the are obese (body condition score greater
regulating center of appetite and energy than 7 out of 10), with insulin resistance,
homeostasis. Specific hypothalamic nuclei and either have laminitis or a predisposi-
serve as control centers for appetite and tion to laminitis. Measurement of plasma
integrate orexigenic (appetite stimulating) insulin or leptin can be useful in predict-
and anorexigenic (appetite inhibiting) ing laminitis in horses with metabolic
signals from other hypothalamic nuclei, syndrome. Obesity‐associated insulin
the midbrain and brainstem, and tissues resistance is thought to develop as a
outside the central nervous system. result of adipokine‐associated inflamma-
Adipose tissue is connective tissue made tion, dysregulation of glucocorticoid
up of adipocytes or fat cells, which is the metabolism, oxidative cellular damage,
result of a positive energy balance (greater and excess lipid production, resulting in
intake than expenditure). Ruminants are impaired insulin signaling within fat and
unique in their ability to produce volatile muscle cells.
fatty acids by rumen fermentation and have In the early 1970s, cholecystokinin was
a very high capacity for lipid synthesis in the first enteroendocrine hormone shown
adipocytes where excess energy can be to have anorexigenic action. Since that
readily stored and utilized. Different types time, several other anorexigenic hormones
of cytokines have been found to be secreted from the gastrointestinal tract have been
from adipose tissue (adipokines), the liver identified, but one is uniquely orexigenic.
(hepatokines), and muscle tissue (myokines) Discovered in 1999, ghrelin has been
and play an important role in growth, repro- shown to increase feed intake by stimulat-
duction, lactation, and fattening (marbling) ing the hypothalamus and brainstem.
periods in cattle and other species. Ghrelin also has a role in signaling the dep-
Peripheral signals of satiety (feeling of “full- osition of fat tissue by increasing food
ness” after ingestion of food) and energy intake and reducing fat utilization. New
balance influence appetite as well as home- research suggests that leptin and ghrelin
ostasis and basic biological functions act in opposition for signaling satiation,
including development, metabolism, immu- regulating energy balance, and therefore
nity, and inflammation. One of the major other biological functions that rely on
adipokines is leptin. First identified in 1994, energy balance (reproduction).