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 F 2 

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