Download Posterior pituitary gland

Posterior pituitary gland
Embryonic origin
• Infundibulum
– Base of mesencephalon
– Downward growth
• Composition
– Axons of hypothalamic neurons
• Magnicellular neurons
– Blood vessels
• Part of peripheral circulation
Cellular composition
• Pituicytes
– Unknown function
Hormone secretion
• Magnicellular neurosecretion
– Hypothalamic nuclei
• Hormone production
• Extend axons to posterior pituitary gland
• Specific nuclei produce specific hormone
– Oxytocin
– Vasopressin/ADH
• Specific localization of nuclei within the specific
region of hypothalamus
– Supraoptic
– Paraventricular
Regulation of hormone secretion
• Sensory stimuli
– Changes in blood pressure
– Suckling stimuli
– Generation of neural impulses
• Interaction between magnicellular neurons
and higher brain
– Generation of action potential
• Travels through axons to the posterior pituitary
• Action potential
– Increased Ca permeability and increased Ca
influx
• Migration and fusion of secretory granules
– Release of hormones into perivascular space
• Into the capillary subsequently
• Rate of hormone production
– Transcription level
• Increased transcription after receiving stimuli
• Transport of hormones from hypothalamic neural
cells
– Transport stops when synthesis stops
• Some storage in the posterior pituitary gland
– Asynchrony between synthesis and release of
hormones
• Paracrine/autocrine regulation of hormone
production
– Intrinsic pattern of firing by neurons
• Oxytocin – high amplitude burst followed by long pause
(pumping action of smooth muscle within the mammary
gland)
• VP – strongly and weakly active neurons alternatively fire
• Structural plasticity
– Contraction of dendrites during firing
• Efficient propagation of action potential
Posterior pituitary hormones
• Nonapeptide
– 9 amino acids
• Formation of ring via
disulfide bridge
• Highly conserved amino
acid sequences
– Pigs have lysinevasopressin instead
of argininevasopressin
• Structurally similar
– Completely different
function
Vasopressin
• Synthesis and secretion
– Two systems
• Osmotic
• Pressure-volume
• Action
– Different receptors
• V1 (blood vessels)
• V2 (renal collecting duct)
• V3 (corticotrophs)
Vasopression
• Function
– Regulation of osmolarity
• Control/conservation of water
– Simple relationship
• Regulation of Na concentrations in plasma
– Pressure-volume
– Different system (renin-angiotensin system and
aldosterone)
– Complicated
• Regulation of osmolarity
– Osmoreceptors in brain (outside of bloodbrain barrier)
• Hypothalamic neurons
• Cells in organum vasculosum of lamina terminalis
• Extracellular fluid
osmolarity
– Affected greatly by
vasopressin
• Change in plasma
osmolarity
• Change in urine
osmolarity
• Change in urine volume
• Vasopressin
– Acts on the collecting duct of the kidney
• Increased water permeability and uptake
– Increased number of aquaporin (water channel) of the
cell surface (cAMP)
• Result in production of concentrated urine and
decreased urine output (antidiuresis)
Thirst
• Replacement of water in the body
– Urine production
– Insensible water loss
• Thirst
– Defense mechanism
– Triggered by changes in osmolarity or volume
• Strongly triggered by hypovolemia
• Decrease in blood pressure
• Generally people ingest excess fluid
Vasopressin and thirst
• Water balance
– Osmolarity
• Changes are usually too small to trigger thirst
– 1 to 2 % of basal level
– Under normal condition
• Regulated by water excretion
– Vasopressin
Oxytocin
• Physiological regulation of oxytocin
secretion
– Complicated
• Difference among species
• Extrapituitary synthesis of oxytocin
– Ovaries (corpus luteum)
– Uterus in some species
– Regulated by suckling stimuli
• Classical regulatory mechanism
Function of oxytocin
• Lactation
– Critical for milk let-down
• Oxytocin receptors
– Grandular cells in the
mammary alveoli
– Myoepithelial layers in
the mammary ducts
• Contraction of
myoepithelial layer
– Secretion stimulated by
suckling
• Tactile response
• Regulated by the CNS
Function of oxytocin
• Contraction of smooth muscle around
uterus during parturition
– Uterine myometrium
• Relaxed during pregnancy
– Progesterone and relaxin (hormone from cervix)
• Become responsive to oxytocin as parturition
approaches
– Increased number of receptors
– Formation of gap junctions (synchronous contraction)
• Works in concert with prostaglandin F2a
Function of oxytocin
• Contraction of smooth muscle around
uterus during parturition
– Burst of oxytocin secretion by the posterior
pituitary gland during labor
• Pulsatile manner
• Triggered by Fergusson reflex (dilation of cervix
and vagina)
• Postpartum secretion of oxytocin
– Primed by changes in steroid hormone
concentrations during parturition
• Increase in estradiol
• Decrease in progesterone
• Affects oxytocin responsiveness
– Receptors in the mammary gland
• Other functions
– Action at the CNS level
• Maternal behavior
• Sexual arousal
– Regulation of reproductive cycle
• Ruminants