Neurohormones Flashcards
How are neurohormones released?
→ DIrectly in the blood circulation
What is point to point communication like?
→ Fast and restricted
What are the secretory hypothalamus neurons like?
→ Slow but widespread
What are networks of interconnected neurons of the autonomic nervous system like?
→ Fast
→ widespread influence
What are diffuse modulatory systems like?
→ Slower and widespread
What are the 4 characteristics of the endocrine system?
→ Mediators travel within blood vessels
→ utilises chemical mediators
→ slow communication
→ long lasting effects
What are the 4 characteristics of the nervous system?
→ Signalling along nerve fibres
→ Transmission of electrical impulses
→ fast communication
→effects are short acting
What are neurohormones produced by?
→ specialised nerve cells called neurosecretory cells
What can neurohormones act like?
→ Neurotransmitters
→ autocrine or paracrine messengers
What are the 3 types of hormones?
→ Protein and peptide
→ amino acid derivatives
→ steroid hormones
What are the 4 features of protein and peptide hormones?
→ Vary considerably in size
→ Can be synthesised as large precursors prior to secretion
→ post translationally modified
→ can have multiple subunits synthesized independently and assembled
What are 2 features of amino acid derivative neurohormones?
→ Mostly tyrosine derived
→ Neurotransmitter that can also act as a hormon
What are 3 examples of amino acid derivative neurohormones?
→ adrenaline
→ noradrenaline
→ dopamine
Where are steroid hormones derived from?
→ Class of lipids derived from cholesterol
What are 5 examples of steroid neurohormones?
→ Cortisol
→ Aldosterone
→ testosterone
→ progesterone
→ estradiol
What is the base of the brain called?
→ Hypothalamus
What is the hypothalamus connected to?
→ pituitary
Where do magnocellular neurons project to?
→ posterior pituitary
What hormones are secreted from the posterior pituitary?
→ Vasopressin
→ oxytocin
Where are neurohormones released in the anterior pituitary?
→ Portal system
What time frame are circadian rhythms?
→ 24 hour cycle
What time frame are pulsatile rhythms?
→ less than 24 hours
What time frame are infradian rhythms?
→ longer than 24 hours
What is an example of hormones that follow the circadian cycle?
→ Cortisol
→ GH
→ PRL
What is an example of a hormone that follows pulsatile cycles?
→ Gonadotrophins
What is an example of an infradian rhythm?
→ Menstrual cycle
Where is the pituitary?
→ Sella turcica
→ in the sphenoid bone
Where are hypothalamic hormones secreted?
→ portal vein system
→ at the median eminence
Describe the pathway of gonadotrophin releasing hormone?
→ neurons release GnRH
→ transported to the anterior hypothalamus and acts on receptors
→ Induces release of FSH and LH
Describe the pathway of growth hormone
→ Growth hormone releasing hormone is transported to the anterior pituitary
→induces release of growth hormone
Describe the pathway of CRH
→ Corticotropin releasing hormone is a peptide released to the capillary system
→ it gets transported to the anterior pituitary
→ this releases ACTH and acts on the adrenal cortex
→ induces the release of cortisol
Describe the pathway of thyrotropin releasing hormone?
→ thyrotropin releasing hormone is delivered to the anterior pituitary
→ causes the release of TSH
→ TSH acts on the thyroid and induces the release of thyroxine
Describe the pathway of vasopressin
→ Magnocellular neurons release vasopressin in the posterior pituitary
→ Vasopressin acts on the kidney and has an anti-diuretic effect
Describe the pathway of oxytocin
→ Magnocellular neurons release oxytocin in the posterior pituitary
→ oxytocin acts on the uterus to induce uterine contraction
→ oxytocin acts on mammary glands to induce milk ejection
Describe the ACTH axis to release cortisol
→ Stress activates the HPA axis
→ Stress activates the hypothalamus to release corticotrophin releasing hormone (CRH)
→This acts on receptors in the anterior pituitary which releases ACTH
→ ACTH is released in the blood
→ ACTH acts on receptors in the adrenal cortex to release cortisol
What is the effect of ACTH?
→ Mobilises energy for fight or flight
Describe how cortisol release is terminated
→ Cortisol induces a negative feedback effect
→ It acts on cortisol receptors in the pituitary or in the hypothalamus to inhibit CRH or ACTH
What happens to the HPA axis during chronic stress?
→ Cortisol levels are increased
→ Hypersensitivity of the axis leads to high levels of basal cortisol
→ Depression and anxiety related disorders
Describe the TSH axis
→ Thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release TSH
→ TSH acts on the thyroid to increase T4/T3 secretion
→Thyroxine exerts negative feedback on the pituitary receptors
What is the most potent thyroid hormone?
→ T3
What is the effect of TSH?
→ Targets tissues containing a deiodinase enzyme to convert T4 to T3
What does hyperthyroidism lead to?
→ Anxiety
What does hypothyroidism lead to?
→ Cognitive impairments such as apathy
Where is prolactin released from?
→ Lactotrophs in the anterior pituitary
What is the relationship between dopamine and prolactin?
→ Dopamine is released into the bloodstream and inhibits prolactin release
Where are vasopressin and oxytocin synthesized in?
→ Supraoptic and paraventricular nuclei in the hypothalamus
What is the difference between vasopressin and oxytocin?
→ 2 peptides
same length
What is vasopressin release stimulated by?
→ Changes in the activity of the osmoreceptor complex in the hypothalamus
What is the function of vasopressin?
→ Controls plasma osmolality by regulating water excretion
→ Stimulates vascular smooth muscle contraction in the DCT to reduce water loss and raise blood pressure
When is oxytocin elevated?
→ Parturition
→ Lactation
→ Mating
What is oxytocin released in stimulus to?
→ Peripheral stimuli of cervical stretch receptors and suckling at the breast
What is the function of oxytocin?
→ Regulates contraction of smooth muscle
→ uterus during labour
→stimulates placenta to make prostaglandins
→ contraction of vas deferens during ejaculation
Describe the RAAS system
→ Kidneys release renin
→ Renin cleaves angiotensinogen from the liver to angiotensin I
→ Angiotensin I → angiotensin II in the lungs
→ Angiotensin II constricts vessels
→ It acts on the subfornical organ which gives the signal to release vasopressin from the pituitary
→ blood pressure increases
Describe the role of oxytocin during childbirth
→ During labour the baby starts pushing on the uterus which stretches
→ There is stimulation of neurons that signal the magnocellular neurons to release oxytocin
→ Oxytocin acts on its receptors in the uterus to cause contractions
→ Baby moves forward which causes stretching
→ stretching causes further oxytocin release which is +ve feedback
What makes voles monogamous?
→ High levels of oxytocin in the reward centres of the brain
What does oxytocin deficiency lead to?
→ Antisocial behaviour
What stimuli activate the RAAS system?
→ dehydration
→ low blood pressure due to blood loss
What are the 4 receptor mechanisms?
→ Dimerisation pathway
→ G protein/adenylate cyclase pathway
→ DAG/IP3 pathway
→ Nuclear and steroid pathways
How does the dimerisation pathway work?
→ Binding of insulin and growth hormone to cell surface receptors leads to dimerisation of receptors
→ this recruits tyrosine kinases which phosphorylates target proteins and induces biological responses
How does the G protein pathway work?
→ binding of a hormone to GPCR results in conformational changes in the receptor
→ leading to GTP exchange for GDP and activation of adenylate cyclse
→Stimulation of adenylate cyclase leads to an increase in cAMP
→ cAMP activates PKA which phosphorylates target proteins to initiate gene expression
What 3 hormones use G proteins?
→ TSH and ACTH
→ Oxytocin is Gq coupled
How does the DAG/IP3 pathway work?
→ Oxytocin binds to GCPRS
→ stimulates phospholipase C
→ PIP2 → IP3 and DAG
→ IP3 stimulates Ca2+ release from intracellular stores
→ DAG activates PKC
→ PKC stimulates the phosphorylation of proteins and alter enzyme activities to initiate a biological response
How do nuclear and steroid pathways work?
→ Steroid hormones diffuse across the plasma membrane of target cells and bind to intracellular receptors in the cytoplasm
→ The receptors function as hormone regulated transcription factors controlling gene expression
What do nuclear receptors share?
→ Transcriptional activation domain
→ Zn2+ finger DNA binding domain
→ Ligand binding domain
What are the 6 effects of a pituitary adenoma?
→ loss of visual field (pressure on optic nerve)
→ too much GH (acromegaly)
→ Hypogonadism and infertility
→ Hypopituitarism
→ Hyperprolactinemia
→ Cushing syndrome (too much ACTH)
What are the symptoms of hypothyroidism?
→ cold hands
→ cold feet
→ lack of energy
