Wednesday, March 21, 2012

Stress I : The Adrenocortical Axis

1Basics¶

• Stress: adjustment to challenges
• 2 arms of stress response: neuroendocrine (HPA) and sympathetic nervous system activation
  • HPA: PVN secretes CRF --> hypophyseal portal system --> ACTH is secreted at ant. pituitary --> adrenal gland secrete cortisol, which negative feedbacks onto PVN and pituitary.
    • PVN has many inputs such as projections from arcuate nucleus.
  • sympathetic: salivation down, heart rate increases, digestion down, NE and E secretion from adrenal gland up, gut secretion down, bladder relax. airway relax, etc.
• stress responses:
  • ACTH stimulate adrenal gland to secrete E and NE (from medulla) and Glucocorticoids (from cortext) and brings about some changes:
    • increase in blood pressure, alertness and cardiovascular tone
    • blood flow to muscle and transient enhancement of immunity
    • inhibition of long-term energy-costly processes: growth and reproduction
      • children from Romanian orphanage have short statues due to inhibition of growth in early childhood.
• Projection to PVN:
  • direct:
    • brainstem : example - in the event of injury (viscerosensory reflex)
    • hypothalamic input: arcuate nucleus, dorsalmedial hypothalamus, perifornical LHA
  • indirect:
    • via BNST (Bed Nucleus of the Stria Terminalis): (all emotiona-related) amygdala, cortex, ventral subiculum, lateral septal complex
    • hypothalamic input: VMH, LHA

2CRF¶

• 41aa peptide, fairly conserved among species, processed from propeptide.
• produced in PVN by parvocellular neurons in stress response. increase ACTH level and consequently glucocorticol.
• also produced in other areas in the brain such as BNST, CeA, cortext, hippocampus, etc.
• IN situ hybridization shows that stress induces level of CRH mRNA
• CRF fibers run either laterla or medial to the fornix and end on portal capillaries in the median eminence
  • some fibers travel dorsally to PVN to brain stem and regulate heart rate and blood pressure (not our focus)
• central CRF mediate various behavior responses (on different areas of brain)
  • reduce eating, reproduction, sexual behavior
  • increase neophobia, locomotor activity in a familiar environment, behavioral despair, firing rate of catecholamine(NE and E)-producing neurons in locux coerules (which send projection to frontal cortex), arousal)

2.1CRF receptors locations¶

• Hippocampal formation
• Bed nuclues stria terminalis
• central nucleus of the amygdala (emotion processing)
• PVN of the hypothalamus (neuroendocrine transmission)
• locus coeruleus (arousal, alertness)
• prefrontal cortext: process stress, assessment, cognitive function

2.2CRF receptor types¶

• 2 types : CRF-R1 and CRF-R2
  • G protein coupled protein with 7 transmembrane domain
  • structure is mainly in extracellular domain (esp CRF-R1)
  • CRF-R1: coupled to PKA, adenycyclase system
  • CRF-R2: coupled to PKC, etc
• Urocortin (UCN): analog of CRF
  • UCN1 binds both CRF-R1/2
  • UCN2 and UCN3: binds only CRF-R2
  • stronger affinity to CRF-Rs than CRF.
    • CRF-R1 and R2: UCN > CRF
    • CRF also preferentially binds CRF-R1
• CRF-BP : binding protein for CRF, like buffer, suppress CRF activity
  • distribution : where CRF is expressed (ex: prefrontal cortex, septum, PVN, medulla, BST)
  • affinity order: UCN > hCRF > sauvagine (frog analog)
  • along w/ its ligands, a target for drug development for mental illness

3Stress Mediators¶

• CRF is not the only neuropeptide that is secreted in event of stress.
• in hypophysial portal circulation, various "cocktails" of neuropeptides and neurotransmitters are observed under different types of stress.
• ex: vasopression, oxytoxin are also secreted.
• synergy between the peptides potentiate CRF effect on ACTH secretion
  • CRF then stimulate 10% of the ant. pittuary cells, corticotropes, to synthesize and process POMC into ACTH.
  • POMC is processed in ant. and intermediate lobes.
    • ACTH can be further cleaved into CLIP and a-MSH (has to do w/ food intake)
    • b-endorphin(pain regulator) can also be derived from POMC

4Adrenal Gland¶

• many zones:
  • Cortex
    • Zona glomerulous --> aldoserone: salt balance and water retention
    • zona fasiculata -> glucocorticoids: stress regulation, metabolism, inflammation
    • zona reticularis: glucocorticoids and sex steroids: reproduction, neuronal control of behaviours,etc
  • Medulla --> adrenaline and noradrenaline (aka epinephrine and norepinephrine : E and NE)
• zona reticularis and medulla are under sympathetic innervation, so is affected by stress.

5glucocorticoid¶

5.1production¶

• produced from cholesterol and converted by a series of enzymes
• can be converted to cortisone (inactive)
• can be converted to aldosterone
• hydrophobic, so is often bound to (serve as buffer system)
  • albumin (low affinity and high capacity) or
  • CBG (cortisol binding globulin, produced by liver, high affinity and low capacity)
    • production is regulated by glucocorticoids, thyroid hormones and estrogens
• dexamethasone: synthetic glucocorticoids, doesn't bind to CBG, used as anti-inflammatory agents

5.2ACTH Diurnal rhythm¶

• highest in early day in human, prior to onset of active phase
• nadir (opp of zenith) is at onset of inactive phase
• diurnal rhytem of CRF and ACTH is governed by SCN (input into PVN). also has to do with adrenal sensitivity
• cortisol level follows ACTH's level closely.
• cortisol secretion amplitude differs in men and women, but period is the same in both.
• stressing animal at rising phase of ACTH --> largest response (ACTH and neuroactivity shoots up)
• stressing animal at falling phase --> changes are smaller
• corticosterone inhibits CRF mRNA production
  • adrenaletomized (ADX) mice leads to higher expression of CRF because of lack of negative feedback.
  • in ADX mice, more corticosterone replacement corresponds to lower CRF mRNA in PVN.

6Glucocorticoid Receptor Types¶

• two types of cytoplasmic receptors mediate genomic effects:
  • Type 1 (MR- mineralcorticoid receptor) :
    • prefers binding aldosterone over corticosterone
    • distribution: mostly in hippocampus - limbic system, brainstem motor nuclei
    • 10X higher affinity for corticosterone than GR. (making hypocampus very sensitive to glucocorticoid)
  • Type 2 (GR - glucocorticoid receptor):
    • higher affinity for corticosterone than aldosterone
    • distribution: everywhere in brain and periphery.
    • stronger affinity for dexamethasone (synthetic corticosterone) than MR.
• maybe one additional membrane receptor that mediates non-genomic effects:
  • ex: action potential, Ca+ entrance, etc.
• both MR and GR mediate the feedback effect on the HPA axis. Receptors on membrane might also mediate some feedback via the stimulation of endocannabinoids release.
• memory:
  • sensory integration is MR-dependent
  • consolidation process to retrieval memory is GR-dependent.
  • However, too much of GR occupied would lead to lower retrieval ability.
    • high level of cortisol leads to lower memory ability. happens a lot in old people.
• MR and GR have different/opposite effects
  • firing activity of hippocampal CA1 neurons increase when ADX mice are treated w/ aldosterone (agonist of MR)
  • opposite effect when RU28362 (agonist of GR) is used.
  • conclusion: as GC level goes up, GR starts to get occupied (since MR are saturated) and that leads to inhibition. a self-regulatory mechanism.

7Action of Glucocorticoid(GC)¶

• General mechanisms: (genomic effects) - hours to day
  • GC freely diffuse into cell after dissociated w/ binding globulin.
  • bind to receptor complex and free GR from hsp90
  • activated receptor (GR+glucocorticoid) goes into nucleus, dimerize, and act as a transcription factor or suppress transcription
  • they can also modulate other transcription factors (indirectly affect transcription of some genes)
• Nongenomic but membrane mediated effects - seconds to minutes
  • seen in rapid action potential, channel modification, entrance of Ca+
• Access of glucocorticoid to brain sites and GC receptors is regulated by 3 factors:
  • CBG (corticosteroid binding globulin): act as buffer. it cannot bind synthetic dexamethasone though.
  • Multiple drug resistance P-glycoprotein(MDRpG): gating protein. actively transport steroids nad synthetic steroid such asdexamethasone out of brain at the site of BBB (blood brain barrier).
    • retards entry of cortisol (rodent glucocorticoid) into the brain in rodents
    • found in circumventricular region, epithelial cells.
    • doesn't affect corticosterone (human glucocorticoid), which passes BBB readily.
  • Metabolism by 11HSD-1: converts corticosterone (inactive) back to parent steroid (glucocorticoids), so reactivate the steroid.
    • 11bHSD2 does the opposite: it converts active steorid to inert steroid

• Mechanism of how GC inhibits CRF neuron:

  • GC enters CRF and act via GR to produces CB (cannabinoid)
  • CB inhibits glutamate neuron and stimulates GABA neuron act those neurons' presynaptic terminals
  • by taking out the stimulatory and increase inhibitory signals to CRF neurons, CB inhibits CRF neuron.
  • this is a fast feedback action of GC on PVN CRF neurons

• GC effects on neuronal morphology and firing

  • on hippocampal granule neurons:
    • ADX mice's neurons have less dendrites.
    • ADX mice administered with GC : neurons have more dendrites.
    • Effects are opposite on hippocampal paramedial cells.
• High GC level leads to neural damage
  • mediated via increased level of glutamate.
  • APV , a NMDA receptor agonist, can alleviate neural damage under high level of GC.

8preventions of negative impact of high GC level¶

• patients at the event of stroke often has high glucocorticoid, which leads to neuronal damage.
• Gene delivery approaches:
  • overexpress 1BHSD2: turn corticosterone into 11-dehydro-corticosterone (inactive)
  • transfect cells with inactive GRb : receptor for corticosterone that does nothing.
  • divert functions by putting estrogen (which binds to GR) so instead of glucocorticoid action we have estrogen action.
  • expression of BDNF: brain-derived neurotrophic factor. decrease cell death in presence of kainic acid and corticosterone.

9level of GC¶

• diurnal rhythm in adrenocortical system:
  • fasted state: glucocorticoids are rising to prepare for food ingestion :catabolic
  • Basal/fed state: glucocoritcoids are lowest. digestive system, glycogen storage, insulin, protein synthesis, increase. (anabolic)
  • stressed (like fasted state): cataboilc
    • high corticosteroid,
    • down: protein sysntehsis + insulin, glucose uptake
    • up: protein breakdown, lipolysis, gluconeogenesis (--> could lead to diabete)
• 11BHSD1: reactivate inert cortisone to cortisol
  • high in visceral adipose tissue and some brain reason :
    • so visceral fat is dangerous. fat people can easily get diabete.
    • subcutaneous fat doesn't do that.
  • if we could inhibit 11BHSD1 specifically in adipose tissue that would be great!