Lecture 5 - Pituitary-Hypothalamic integration of hormone action Flashcards
Describe the integration of hypothalamic and pituitary function
a. Hypothalamus = major integration centre
i. Regulates autonomic nervous system
ii. Regulates more of the endocrine system
iii. Processes all sensory information and receives information from baroreceptors, osmoreceptors and other external stimuli
b. Posterior pituitary – neuroendocrine cells from hypothalamus project their axons down the pituitary lobe where they release their hormones
c. Anterior pituitary – major endocrine part
i. Pituitary stalk contains the hypothalamic pituitary portal venous system – delivers hypothalamic hormones to anterior pituitary
What is the nature of the hypothalamic releasing and release inhabiting hormones (function and structure)?
Function = control the secretion of anterior pituitary hormones
- for each type of anterior pituitary hormone there is usually a corresponding hypothalamic releasing hormone
- for some of the anterior pituitary hormones there is also a corresponding hypothalamic inhibitory factor
Hypothalamic releasing hormones - usually a larger pro-hormone, which is cleaved later –> active peptide hormone (simple peptides/proteins = 3-44 AAs)
Some hormones are under tonic inhibition by release inhibiting hormones - needed early in life but decrease as hypothalamus matures and becomes more active
What hormones are produced by posterior pituitary gland (structure, synthesis, function)?
Oxytocin and vasopressin
- oxytocin –> contraction of smooth muscles
- vasopressin aka ADH –> stimulates reabsorption of water from the distal tubular kidney to maintain blood osmolarity when blood volume/blood pressure decreases
Both polypeptides containing 9 AAs with disulphide bridge between 2 cysteines
- in vasopressin, phenylalanine and arginine replace isoleucine and leasing in oxytocin
Synthesized as pre-prohormones in cell bodies of specific neurons of hypothalamus
- the pro hormones are cleaved to active hormones during fast axonal transport from the cell body down axon in posters pituitary and before they are released into the circulation
- released in response to changes in osmotic or barometric pressure, pain, fright/stress, adrenal insufficiency, hypoxia, cardiac failure
What are the properties of the hormones produced in anterior pituitary gland?
Causes a number of trophic hormones that cause hormone release from target tissues
- proteins or glycoproteins
- longer half lives than releasing hormones produced in hypothalamus
- made in specific cells (e.g. gonadotrope) in the anterior pituitary gland
–> GH, PRL, TSH, FSH, LH, ACTH, MSH, B-LPH
How is the release of anterior pituitary hormones regulated?
Controlled by releasing factors which are produced in the hypothalamus and act on the anterior pituitary
- a feedback system can act on the anterior pituitary or hypothalamus
= short loop - trophic hormones –> hypothalamus
= long loop - ultimate hormones (secreted by endocrine tissues) –> anterior pituitary or hypothalamus
Describe experiments used to demonstrate the hypothalamic control of anterior pituitary
- mechanical barrier between hypothalamus and pituitary
- diminishes or enhances secretion of pituitary hormones - ectopic transplantation of pituitary gland
- leads to decreased/increased secretion of pituitary hormones - electrode stimulation of discrete hypothalamic sites
- leads to inhibition or release of certain pituitary hormones - antibodies against trophic hormones
- leads to enhanced or inhibited pituitary hormone secretion
Describe the function and control of pulsatile release of pituitary hormones
Many hypothalamic and pituitary hormone secretions are pulsatile or episodic
- regulated by biological clock of hypothalamic suprachiasmatic nucleus
- may prevent down regulation of receptors from continuous level of hormone secretion
- can trigger specific action depending on pulse height and frequency
- problem for experimentation - need to take measures over time due to variability
