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Hypothalamus and Pituitary Gland (Prolactin (stimulates lactation…
Hypothalamus and Pituitary Gland
Hypothalamus
works with pituitary gland to regulate thyroid, adrenal, and reproductive glands
controls growth, milk production/ejection, and osmoregulation
controls release of pituitary hormones by neuronal and hormonal mechanisms
hormones
prolactin releasing hormone
target cell: lactotroph
prolactin inhibitory hormone
target cell: lactotroph
thyroid releasing hormone
target cell: thyrotroph
corticotropin releasing hormone
target cell: corticotroph
growth hormone releasing hormone
target cell: somatotroph
growth hormone inhibiting hormone
target cell: somatotroph
gonadotropic releasing hormone
target cell: gonadotroph
Pituitary gland
cell types
somatotroph
produces growth hormone
(regulates growth and metabolism)
thyrotroph
produces thyroid stimulating hormone
(controls secretion of thyroid hormone)
corticotroph
produces adrenocorticotropic hormone
(ACTH - stimulates secretion of glucocorticoids)
lactotroph
produces prolactin
(regulates milk production)
gonadotroph
produces luteinizing hormone and follicle stimulating hormone
(regulates reproductive development and function)
Gonadotropins
gonadotropic releasing hormone secretion is pulsatile
secreted from gonadotrophs
luteinizing hormone
glycoprotein
in females
initiates ovulation
stimulates formation of corpus luteum and maintians its steroid hormone production
in males
stimulates Leydig cells to synthesize testosterone
follicle stimulating hormone
glycoprotein
in females
stimulates growth of granulosa cells
stimulates estradiol synthesis
in males
stimulates spermatogenesis
stimulates Sertoli cell function
regulation
negative and positive feedback loops
testosterone, estrogen, and progesterone inhibit Gonadotropic Releasing Hormone in hypothalamus and inhibit LH/FSH in the pituitary gland
kisspeptin and estradiol also play a role by directly impacting the GnRH neurons thru the Kiss1 neuron
Thyroid Stimulating Hormone
similar in structure to LH and FSH
glycoprotein
secreted from thyrotrophs
stimulates thyroid hormone synthesis and secretion
promotes thyrocyte growth
increases conversion of T4 to T3
initiates iodide uptake from the blood
pathophysiology
hypothyroidism
primary: thyroid dysfunction
(98-99% of cases)
secondary: pituitary dysfunction
tertiary: hypothalamus dysfunction
note: TSH is normal in 60-80% of dogs with hypothyroid because majority of cases are due to thyroid dysfunction, not from lack of TSH production
ACTH
secreted from corticotrophs
episodic and pulsatile secretion
overrides normal feedback controls,
has elevated levels during times of stress
increased cortisol concentration
development of blood capillary network
enhances transport of cholesterol
can be used to determine source of hyperadrenocorticism
produced from fragments of Proopiomelanocortin (POMC)
lipoprotein: precursor to beta-endorphin
beta-endorphin and Met-enkephalin: opiods
melanocyte stimulating hormone (MSH)
controls melanin pigmentation in the skin, melanotrophs under inhibitory control of dopamine from hypothalamic neurons
Growth Hormone
aka Somatotropin
secreted from somatotrophs
most important hormone for normal growth
stimulates cell growth and division
increases protein synthesis, uptake of amino acids, DNA and RNA synthesis
stimulates lipolysis (catabolic)
diabetogenic: decreases glucose uptake and causes insulin resistance (increased blood glucose)
increases linear growth
increases metabolism in cartilage
pulsatile secretion
regulation
stimulation
ghrelin
peptide produced in the stomach that stimulates appetite, gastric, and intestinal emptying and inhibits POMC
growth hormone releasing hormone
stimulates cellular proliferation somatotrophs, stimulates synthesis and secretion of growth hormone (during exercise, when blood sugar is low, when amino acid levels are high, or high stress)
arginine
inhibition
somatostatin, increased glucose levels, increased free fatty acids
pathophysiology
feline acromegaly
common signs
enlarged head, abdomen, tongue, extremities, and paws
PU/PD and weight gain
due to GH secreting tumor
more common in older male cats with uncontrolled diabetes mellitus
canine acromegaly
common signs
enlarged head and abdomen, thickened limbs and skin
caused by excess GH secretion from mammary tissue or GH secreting mammary tumor
more common in old intact females
pituitary dwarfism in dogs
congenital
growth hormone deficiency
overall puppy appearance, mental retardation
Prolactin
secreted as prohormone
secreted from lactotrophs
pulsatile secretion
required for pregnancy maintenance
(dogs, rodents, skunks)
stimulates lactation
stimulates synthesis of milk components
stimulates growth of mammary gland
inhibits ovulation by inhibiting gonadotropin releasing hormone
stimulates maternal behaviors
regulation
inhibition
in mammals: under tonic constant inhibition by dopamine
(dopamine production needs to be overridden)
somatostatin
prolactin (negative feedback loop)
stimulation
gonadotropin releasing hormone, thyroid releasing hormone, stress, suckling reflex, pregnancy
pathophysiology
deficiency (not common): destruction of anterior pituitary or lactotrophs
causes failure to lactate
excess: destruction of hypothalamus, interruption of the hypophyseal tract or prolactin secreting tumors
causes galactorrhea (excessive/inappropriate milk production) and infertility