Lecture 14 Part B: Endocrine 3 Hypo and Pituitary Axis Flashcards
What is the purpose of the hypothalamus and the pituitary in the brain?
Hypothalamus is the major integration centre
-Regulates the autonomic NS
-Regulates most of the endocrine system
-Processes most sensory info
Pituitary is a small gland attached to the hypothalamus
-Anterior lobe and intermediate after considered as anterior pituitary
What are the 3 sections of pituitary?
-3 parts: posterior, intermediate, and anterior
-Posterior lobe: neurones from hypothal.
-Anterior lobe: major endocrine part (glandular tissue)
-Intermediate: major fxn in amphibians and fish (MSH)
What is the hypothalamo-pituitary axis?
-Hypothalamus composed of neuroendocrine cells
-Some project axons down the posterior pituitary lobe
-Some release factors into the pituitary stalk portal venous system to feed the anterior pituitary
-Endocrine cells from the anterior and intermediate pituitary release their hormones in a second capillary network –> enter systemic circulation
What is the role of the Intermediate Pituitary?
-Not clear the role in mammals could be a source of β-endorphins?
Produces
-MSH: melanocyte stimulating hormone (leads to increase skin pigment)
-β-LPH (β-lipotropin): degraded to β-endorphin=analgesia during stress
-All derived from the common gene POMC (pro-opiomelanocortin)
What is the role of the posterior (nervous) pituitary?
-Releases antidiuretic hormone (ADH or vasopressin) and oxytocin produced in the cell body of hypothalamic neurones
-Transported to the pituitary along axons in vesicles
-Stored in nerve endings, released when AP is fired
-After secretion, hormones diffuse into blood vessels
-Sum up 2 main neuropeptides: oxytocin and ADH (aka vasopressin)
What is Anti Diuretic Hormone and what does it act on?
aka Vasopressin
-Released by posterior pituitary
-Most important regulator of extracellular fluid
-Acts in the kidneys: regulates the density of aquaporins (water channels) in the distal tubule and collecting duct
-ADH increases reabsorption of water
-Primarily regulated by hypothalamic osmoreceptors and stretch receptors in blood vessels
What is oxytocin and what does it act on?
-Again released by posterior pituitary
-Acts like reflex has 5 components
Primarily acts on:
-Uterus smooth muscle: contraction during parturition (fetus physically stimulates uterus)
-Mammary gland: contraction increase pressure to drive milk towards excretory ducts and the teats (milk ejection reflex)
-Receptor= G-coupled receptor with activation of PLC (Ca pathway) Gq should know bc Ca
-Secretion of oxytocin regulated by several reflexes
What section of the pituitary is known as the master gland?
Anterior Pituitary
-Only endocrine and has major hormones that the animal will use
What is the role of the Anterior pituitary (master gland)?
-Endocrine part: contains 5 different cell types producing 6 different hormones
-Protein or glycoproteins with longer half-lives than their releasing hormones
*TSH (thyroid stimulating hormone)-> thyrotrope (stimulates the thyroid to release its own hormones: T3 and T4)
*LH & FSH (gonadotropins) -> gonadotrope (reproductive gonads)
*ACTH (adrenocorticotropin)-> corticotrope (Stress hormone: glucocorticoids are main one, affects cortisol in lab related it to cushings)
*GH (growth hormone)-> somatotrope (trigger of IGF1)
*PRL (prolactin) -> mammotrope (lactotrope) (target is mammary gland)
-Tropic effects = relate other endocrine glands
-Under direct control (pos and/or neg) from hypothalamus
*note whenever you see ‘trope’ means targets another gland to produce another hormone
What is the function of the GH (somatotropin)?
-Cytokine receptor type
Direct effects:
-Stimulates lipolysis and reduces lipogenesis in adipose tissue (catabolic); promotes synthesis of protein (anabolic) (uptake of AA)
indirect effects:
-By stimulating synthesis of IGF1 (somatomedin) and its binding proteins in the liver
-Stimulates chondrocyte (cartilage cells) proliferation to increase bone growth
-Stimulates satellite cells in muscle (muscle fibre growth)
-Stimulates AA uptake and protein synthesis
Adults: mostly for metabolic purpose bc growing
Kids: Stimulates growth in young animals
What is the function of the TSH (thyrotropic hormone)?
-TSH binds to its G-coupled receptor on membrane of follicular cells in thyroid gland
-Stimulates cAMP pathway which in turn stimulates the synthesis of thyroid hormones
What is the function of ACTH (adrenocorticotropic hormone)?
-ACTH receptor is a G-coupled receptor stimulating the cAMP pathway
-Stimulates the metabolism of cholesterol in adrenal cortex= more substrate for cytochrome P-450 (enzyme) -> increase release of corticosteroids
What is the function of LH?
-G-protein couple receptor (cAMP pathway)
In males: stimulates testosterone production by Lydia cells in the testis
In Females: controls sex steroid production by the ovary and is responsible for ovulation (surge)
What is the function of FSH?
-G-protein coupled receptor (cAMP pathway)
In males: stimulates secretion of inhibin by Sertoli cells (spermatozoa)
In females: Development of follicles and secretion of sex steroids
What is the function of PRL (prolactin)?
-Cytokine receptor type
-Stimulates the synthesis of milk proteins (casein, lactalbumin)
-In poultry, responsible for the initiation and maintenance of incubation behaviour (broodiness)
What is the hypothalamic control of the anterior pituitary?
-Neurohormones released in small amount (bypass general circulation)
-Hypothalamic neurons receive info from:
-Higher brain centre, emotions
-Exterior, environment and social stimuli
-Internal rhythms
-Metabolic state (temp, energy level, osmolality)
-Endogenous hormones by feedback
What is tonic inhibition? Are there species differences?
-Some hormones are under tonic inhibition (GH, MSH, PRL)
-These hormones are needed early in life as the hypothalamus matures and animals ages, secretion decreases
Species differences: In mammals PRL Is mainly under dopaminergic tonic inhibition, in birds it is mainly under VIP stimulation
TRUE OR FALSE: Each anterior pituitary hormone has a corresponding hypothalamic releasing hormone and/or a corresponding hypothalamic inhibitory factor?
TRUE
What are hypothalamic factors?
-All reactively small peptides generally fragments from proprotein of larger size
-Different releasing hormones synthesized by specific neurones
What is the hormonal cascade of signals from CNS to ultimate hormone?
-Releasing hormone precursors are made in cell bodies and transported down the axons to the nerve endings for storage
-Eletrical stimulus = released by hypothalamus -> delivered to pituitary by portal system -> release of hormone by pituitary -> release of ultimate hormone in target gland
-Amplification of signal at each step
What is pulsatility?
Many hormones from the hypothalamus and pituitary are pulsatile or episodic
-Regulated by the biological clock of hypothalamic suprachiasmatic nucleus
-May prevent the down regulation of receptors from continuous level of hormone secretion (what prof was talking about when continuously stimulated)
-Can trigger specific action depending on pulse frequency
-Basically if constant can’t respond, so pulse means have time for cells to respond
What are the 2 types of feed back control and how do they differ?
-Feedback serves to regulate the secretion of hormones
-Circulating hormones from the endocrine glands provide neg feedback both to the hypothalamus and pituitary
Short loop: pituitary hormones (ACTH, FSH, LH, TSH, GH) feedback to hypothalamus
Long loop: hormones form target glands feed back to the pituitary and hypothalamus
What are the 2 types of feed back control and how do they differ?
-Feedback serves to regulate the secretion of hormones
-Circulating hormones from the endocrine glands provide neg feedback both to the hypothalamus and pituitary
Short loop: pituitary hormones (ACTH, FSH, LH, TSH, GH) feedback to hypothalamus
Long loop: hormones form target glands feed back to the pituitary and hypothalamus
