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 lysine-vasopressin instead of arginine-vasopressin

• 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 blood-

brain 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 F2

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