lecture - hormonal regulation of responses to environmental challenges

what is ELHS

state that animals may enter in response to significant environmental challenges

priorities shift towards survival only

how are circulating glucocorticoids are thought to be involved in expression of an ELHS

Cort usually binds to mineralocorticoid receptors, but when there are extremely high CORT levels they bind to GCrs

Fully saturating both MRs and GCrs → ELHS

describe the mineralocorticoid receptor curve

rises and plateaus fast

MRs have very high affinity for CORT so they saturated very fast even with low levels of CORT

describe the glucocorticoid receptor curve

rises slower and only reaches saturation when CORT levels are very high

GCrs have low affinity and so CORT levels must be high enough to saturate MRs to bind to GCrs

why would GCs rise to level B and level C

might rise to level B in times of feeding, passes baseline as there is a bit more CORT than needed to saturate MRs

might rise to level C in times of great stress fully saturating MRs and GCrs → ELHS

how can one test the effects of whether it is GCrs or MRs being saturated that causes ELHS

use drug to block GCr and MRs in different birds during severe snow storm and see who enters ELHS and leaves

why does it make sense to look for GCs over catecholamines

ELHS is a state not a reflex and GCs would regulate this

it is a state with distinctive levels

it is based on a threshold event meaning it is a slow burn, not a quick change like in catecholamines

what is CBG

corticosteroid binding globulin

where is CBG made

this protein is made in the liver

what does CBG do

circulates in plasma and selectively binds CORT

high CBG means...

low CORT in blood

low CBG means...

high CORT in blood

variation in CBG levels are important as...

only free CORT can enter target tissues

what is the evidence that it is free CORT in blood causing ELHS

birds with higher free CORT left earlier as there is less CBG to bind CORT allowing it to saturate MRs and GCrs faster

~50% of variation in time to leave can be explained by free CORT in blood