Neurohormones Flashcards
what are neurohormones?
produced by specialised nerve cells called neurosecretory cells and released into the blood rather than the synaptic space
- diffuse out of capillaries and act on receptors
- can have an effect on cells some distance away
- can act as neurotransmitters or autocrine/paracrine messengers
a small amount of hormone released from a neurones can have what kind of effect?
a widespread effect all around the body
Nervous system (Neuron point to point communication) vs Endocrine (Neurohormone communication)
ENDOCRINE SYSTEM
- Mediators travel within blood vessels
- Utilises chemical mediators (hormones)
- Slow communication
- Effects can be long-lasting
NERVOUS SYSTEM
- Signalling along nerve fibres
- Transmission of electrical impulses
- Fast communication
- Effects are generally short-acting
name 3 types of hormones
- peptides/proteins
- can be synthesised in a large precursor
- can be post-transcriptionally modified - amino acid derivatives
- tyrosine derived
- neurotransmitter that can also act as a hormone - steroid hormones
- derived from cholesterol
principal endocrine organs
- hypothalamus
- pituitary gland
- thyroid gland
- parathyroid gland
- adrenal gland
- pancreas
- ovary
- testes
anterior and posterior pituitary
- there are neurons which basically project from the hypothalamus and release neurohormones directly in the portal system, and they are transported to the anterior pituitary, where they bind to receptors and cause a different neurohormones to be released into circulation
- magnocellular neurons project from the hypothalamus directly to the posterior pituitary
different endocrine rhythms
- circadian - 24 hr clock, GH, cortisol (cortisol levels highest in the morning)
- pulsatile - periodicity longer than 24 hrs gonadotrophins
- infradian - periodicity less than 24 hrs menstrual cycle
key nuclei involved with neurosecretory cells
medial pre-optic, the arcuate and the paraventricular nuclei
what causes cortisol release?
CRH, a peptide released directly in the capillary system, is transported to anterior pituitary and causes ACTH release. ACTH travels to adrenal cortex and induce the release of cortisol.
what causes thyroxin release?
TRH induces release of TSH from the anterior pituitary. TSH acts on thyroids and induces release of thyroxin.
what does the posterior pituitary release?
VP and oxytocin from magnocellular neurons
what are Parvocellular neurosecretory cells?
- smallneuronsin the hypothalamic
- axons of these cells project to themedian eminence, at the base of the brain, where the nerve terminals release peptides thehypothalamo-pituitary portal system
- peptides carried to anterior pituitarygland, where they regulate the secretion of hormones into the systemic circulation
what is another name for growth hormone release inhibiting hormone? and what is its effect?
somatostatin
-inhibits GH release, VIP, glucagon, insulin, TSH
list the specialised cells of the anterior pit and their role?
gonadotroph cells - control LH/FSH secretion
somatotroph cells - control GH secretion
corticotroph cells - control ACTH secretion
thyrotroph cells - control TSH secretion
lactotroph cells - control prolactin secretion
ACTH
ACTH is derived from POMC
- stress activates hypothalamic neurones to release CRH, stimulating pituitary corticotrophs to release ACTH
- ACTH stimulates cortisol production from adrenal cortex, glucocorticoids and sex hormone production from zona fasiculata
- Cortisol will induce a negative feedback effect, acting on cortisol receptors in the pituitary/hyp inhibiting CRH or ACTH release
- There is a problem is cortisol is higher for a prolonged period of time eg. periods of chronic stress/repetitive exposure to stressful stimuli (eg. abuse)
- as the HPA is hyperstimulated, which will lead to high levels of basal cortisol levels, leading to depression/anxiety disorders
TSH regulation
TRH - hypothalamus
portal system
TSH - pituitary
TSH acts on TSH receptors in the thyroid to release T3 and T4. T3 is more potent, deiodinase enzyme converts T4 to T3
huge surge of thyroxin = negative feedback - thyroxin receptors in the pituitary and hypothalamus inhibit TSH/TRH release
prolactin
release by lactotrophs in anterior pit
- increased in pregnancy and lactation
- increases milk production
- under the control of dopamine - released from the hypothalamus directly into blood circulation, and it inhibits prolactin release
where are VP and oxytocin synthesised and released?
supraoptic and para ventricular nuclei in the hypothalamus, released in posterior pit
Kidneys and hypothalamus
thirst
kidney starts releasing renin - cleaves angiotensinogen into Ang I.
Ang I converted into ang II, which causes vasoconstriction
Ang II acts on the subfornicular organ in the brain, giving a signal to release vasopressin from the pituitary (Subfornical organ projects to vasopressin cells in lateral hyp)
VP will induce thirst but will also act on the kidney to induce water retention
mechanisms of action of peptide hormones?
eg. GH
peptide binding leads to receptor dimerisation, tyrosine kinase recruitment
-causes phosphorylation of target proteins, induce secondary messenger systems
mechanisms of action of GPCR’s?
- can be gs/gi/gq coupled
- GTP exchange for GDP
- gi/gs - stimulate or inhibit AC, cAMP, PKA. eg. TSH, ACTH
- gq - phospholipase C, PIP2- DAG and IP3, eg. oxytocin and GnRH
mechanisms of action of nuclear/cytoplasmic receptors?
eg. cortisol acts on nuclear receptors
cortisol has gets inside the cell because it’s lipophilic), binds to the nuclear receptor and translocates into the nucleus
-once inside the nucleus, the complex will bind to DNA to increase gene expression
what does a pituitary adenoma cause?
- too much GH - gigantism and acromegaly
- hypogonadism
- loss of visual field
- too much ACTH, XS cortisol secretion, Cushings syndrome
- hypopituitarism
- too much PRL
hypothyroidism
- too little thyroid hormone
- most common cause is Hashimoto’s disease
- autoimmune disease where immune system makes antibodies to the thyroid
- can cause mental retardation,
slow growth, cold hands and feet, and lack
of energy
