The stress axis Flashcards
What is the function of the hypothalamus?
Ancient, conserved, central regulator of homeostasis
What are the main nuclei in the hypothalamus?
Paraventricular nucleus (PVN)
Arcuate nucleus (Arc)
Ventromedial nucleus (VMN)
Where do the neurons in the Arc and VMN nuclei develop from?
Progenitors that either:
1) Transiently express shh
2) Respond to shh
What neurons are in the Arc nucleus?
What do these neurons do?
PomC+ neurons
Make precursor protein (Pomc) that is made into many hormones - some that impact on CRH
–> Makes ATCH
What neurons are in the VMN?
What do these neurons do?
SF1+ neurons
Help to regulate the maturation of steroidal hormones –> form active hormones
Where do the neurons in the PVN develop from?
What neurons are born in the PVN?
EARLY where Shh is REPRESSED in progenitors
CRH/CRF neurons
What is useful about the hypothalamus in simpler vertebrates?
SAME organisation of hypothalamus and the SAME neurons in the SAME nuclei SAME neurotransmitters and neurohormones
Can investigate problems happening in humans
Why need very high numbers of model organisms to do behavioural work?
Why is the zebrafish a good model for the hypothalamus?
Many VARIABLES in behaviour
Zebrafish - good model can link genetics to behaviour (screen MANY fish)
What neurons are important in the production of the stress hormone and therefore the regulation of stress?
- Pomc neurons (arc)
- SF1 neurons (VMN)
- CRH neurons (PVN)
Where do the CRH neurons project to?
What happens here?
Medial eminence
HPA axis:
1) End feet release CRH - taken up by the portal capillaries –> pituitary gland
2) In PG - regulates an endocrine cell –> release ACTH (adrenocorticotrophic hormone)
3) ACTH acts on the ADRENAL GLANDS –> release cortisol
What are the CRH neurons responsible for?
CRH released in response to stress
What is the main stress hormone?
Where is it released from?
Cortisol
Released from the adrenal gland when stimulated by ACTH
What is the DISC1 gene?
Results of this gene?
Deleted in Schizophrenia 1
High risk of mental illness
When was DISC1 identified?
Through genetic analysis of an inbred family with high-risk of mental problems:
- Each sibling/cousin had inherited a copy of the allele where this gene was deleted
What is the first question must ask if there is a phenotype resulting from a deleted gene?
WHERE is the gene NORMALLY expressed?
WHY lack of function –> mental illness
Where is DISC1 normally expressed?
How can see this?
Normally in the hypothalamus, with expression overlapping with cells expressing Fgf+ and Shh+ (early progenitors)
SPECIFIC to the hypoT - not in the rest of the body
Seen in zebrafish using IN SITU HYBRIDISATION
Why is it hard to determine what the DISC1 protein does?
Likely to interact with many proteins due to its structure:
- Large protein
- Globular domain at N terminus
- Alpha-helix domain
- Coiled coil domains
How was a disease model of DISC1 created in the zebrafish?
Create 2 lines of zebrafish mutants (with different point mutation in the globular domain - N terminus)
Fish are homozygous viable but DO NOT make the DISC1 mRNA/protein
Why were 2 zebrafish transgenic lines made in the disease model for DISC1?
To increase confidence in results
How are the WT and DISC1 mutant fish different to each other?
How can this been seen?
DISC1 mutants:
- Not as many progenitors and progenitors differentiate PREMATURELY (Interferes with the balance of stem/progenitor/differentiated cells)
- Hypothalamic neurons (especially ones involved in stress regulation) differentiate abnormally
Seen using ISH of zebrafish mutants/WT
How does DISC1 interfere with progenitor levels?
We are not sure
How do the hypothalamic neurons of the zebrafish DISC1 mutant differentiate abnormally?
Pomc+ (Arc) neurons REDUCED
Sf1+ (VMN) neurons INCREASED
Crh+ (PVN) neurons INCREASED
What do the differences between the WT and the DISC1 mutant fish show?
Imbalance in early progenitors has a KNOCK-ON effect and leads to the ABERRANT DIFFERENTIATION of HYPOTHALAMIC NEURONS that are involved in STRESS REGULATION
How can we measure an output to see if the difference in neurons has an impact on the physiology and/or behaviour of the fish?
1) Behavioural analysis
2) Assays - look at endocrine function
What is the normal neuroendocrine function?
Upregulation of neurohormones and cortisol after stress
How is the behaviour of the fish assayed?
With response to Shreckstoff or increased NaCl concentraion
What is Shreckstoff?
Pheramone released from damaged fish (eg. by predator) to warn other fish
Causes a stress response in the receiving fish
What happens to the behaviour of the fish when they are stressed?
Turn around and swim away quickly and erratically
Why does NaCl evoke a stress response in zebrafish?
Zebrafish from the Ganges (close to the sea - zebrafish don’t survive in the sea)
Sea - higher salt conc
When detect higher salt conc, zebrafish turn around and swim away
What is different about the stress response in WT and DISC1 mutant zebrafish?
DISC1 mutant - no stress response
What is different about the neuroendocrine function in WT and DISC1 mutant zebrafish?
In the WT - increase in cortisol and ff1b (sf1)
In the mutant - no increase
How can determine neuroendocrine function in zebrafish?
Expose to stress factor
Kill fish and extract cortisol to measure the levels
Why is it important to test both behaviour and endocrine function of the fish?
Test things in different ways:
Same result –> more believable outcome
What does cortisol do?
Why?
Triggers a feedback loop that down regulates the stress response
Hypothalamus works through systems that let you know when deviating away from the optimum set point
Why is it important to have oscillations in levels in early life?
How may this be altered in humans with DISC1 mutant?
To be able to understand where the MEDIAL LEVEL is
May be altered in DISC1 mutant humans: may noe be able to set up optimal set point –> Knock on effect in ability to process info in a way that is not stressful
How do we think DISC1 works in humans?
Effects the cells in the stress circuit and their differentiation –> crucial for the regulation of normal cortisol and behaviour
What is the stress axis AKA?
What is this modulated by and how?
The hypothalamus pituitary adrenal axis
Modulated by emotional responses as:
- Emotional responses modulate the ANS, that is responsible for the fight/flight response
- Fight/flight response carried out by the HPAA
Describe the stress axis
…
What is the function of CRH from the PVN in stress?
1) Released from the endfeet of CRH+ neurons in the PVN onto portal capillary
2) Portal capillary –> corticotroph cell in the pituitary gland
3) Corticotroph releases ATCH into GENERAL CIRCULATION
What is the role of ACTH
Made from POMC precursor
Released from corticotroph in the pituitary gland
–> Adrenal gland to stimulate the release of the stress hormone CORTISOL
What does cortiso do when released from the adrenal gland?
Acts on cells to produce a coordinated response (eg. run away)
Acts on the hypothalamus in a NEGATIVE FEEDBACK mechanism –> Return to baseline optimal level
