9. Adrenal cortex and HPA axis

What are the costs of the stress response?

- energetically expensive

- hyperactivated immune system

- osmorespiratory compromise in fish

What is the function of the adrenal cortex?

Produces corticosteroids to turn off the emergency immune response

Where is the adrenal cortex found?

On the outside of the adrenal glands (medulla on the inside)

What are glucocorticoids?

Vertebrate stress hormones, increase glucose (cholesterol precursor from food)

Cortisol

- almost all fish

- most mammals

Corticosterone

- amphibians

- reptiles

- birds

- rodents

What is aldersterone?

Mineralocorticoid (produced by further processing of corticosterone

Key to survival

What is ACTH?

Adrenocorticotropic hormone

Activates steroid producing enzymes

What is the key different between catecholamines/protein hormones and steroids?

Steroids aren't stored but are produced and released on demand (contrast to adrenal medulla that stores adrenaline). Instead, the cells are cholesterol ester rich (precursor) for short notice production

- close to capillary

- slightly slower to respond (not immediately involved)

What can we see in fish similar to the adrenal glands of humans?

Head kidney produces cells that are the homolog of the cortex and medulla

What do we see in fish blood glucose (associated with adrenaline) vs. cortisol levels at time points after being exposed to a stressor?

Glucose: fast increase, almost doubles and peaks at about 10-30 minutes

Cortisol: takes more time to build up and then to return to baseline. Peaks just before 1hr

What do we see when looking at seahorse blood cortisol after being taken out of water

Have a freeze response (instead of salmon fight response)

- no response to acute holding stress

- when exposed to chronic stress their blood cortisol does increase

- have hardware to produce the stress response

How is the HPA axis stimulated?

Signal to hypothalamus. If seen as a threat to homeostasis the hypothalamus releases corticosteroid-releasing hormone (CRH) at the median eminence. This travels through the portal vessels to the pituitary.

What is the role of the pituitary in the HPA axis?

Releases adrenocorticotropin hormone (ACTH) into the bloodstream

Where does ACTH act?

Stimulates the adrenal cortex to produce glucocorticoids which feed back negatively on the hypothalamus

How is the corticosteroid signal transduced?

Fat

Binds to nuclear receptor

How could we deduce the action of cortisol experimentally?

- surgery and replacement

- chemically (knock down receptor or prevent enzyme production)

- gene therapy

- pharmaceutical to mimic the chemical

What do we see in salmon exposed to ACTH vs. glucocorticoid agonist (mimic)?

ACTH: cortisol level increased

Agonist: cortisol decreases

What can we see in cytokine WBC gene expression in fish exposed to cortisol? What conclusion can we draw?

Exposed fish have much lower levels of cortisol. This shows that it can interact with the immune system as a potent anti-inflammatory

- bring back to homeostatic state

What are the two important sides to the stress response?

The response and then a way to return to baseline

What are the main and other roles of glucocorticoids?

RESTORATION: termination of stress response, immunosuppressant

Permissive/facilitating: free up glucose, needed after lots used up in response

Restore mineral imbalance in fish

What can arise from a prolonged stressor?

- reproductive issues

- anxiety/ depression

- insulin resistance

- hypertension

- gastrointestinal dysfunction

- immune suppression

- obesity/weight loss

What are the directions an alarm response can go?

Enters resistance phase which can lead to habituation or, if the stressor is too prolonged or too expensive, exhaustion