Neurosteroids Flashcards
With use of a fish model, what is the action of progesterone metabolites in terms of effect on brain activity?
Rapid induction of GA
What is the mode of action of alphaxalone?
This synthetic anaesthetic steroid enhances GABAAR function
What is the action of 5 alpha, 3 alpha?
Highly potent GABAA modulator at physiological human levels
How are neurosteroids formed?
Glial cells convert cholesterol or peripherally derived progesterone to neuroactive steroids
Brains major inhibitory receptors are under the fine control of locally produced steroids
Act on synaptic receptors to prolong mIPSCS mediated by synaptic and greatly enhance extrasynaptic current (tonic inhibition)
Is there a physiological role of neurosteroids?
Examples of conditions whereby steroid levels are raised:
Acute stress
Pregnancy
Describe the role of neurosteroids in acute stress
Synaptic GABAAR of CRH-releasing PVN neurons are highly neurosteroid-sensitive
Neurosteroids prolong the mIPSP, inhibit the decay
Decreases the frequency of action potential firing frequency. In acute stress, the neurosteroids play a feedback role
Acute stress; release of glucocorticoids. The levels of neurosteroids increase which act back to decrease their excitability and release of CRH
Describe the role of neurosteroids in pregnancy
Up to the day before giving birth there are very high neurosteroid levels (100 nanomolar). This inhibition is sufficient to prevent the firing of oxytocin neurons
Parturition; 5a3a decreases, the GABAARs become neurosteroid insensitive. This results in increased neuronal firing, increased oxytocin release and therefore increase uterine contraction and lactation
How does the change in neurosteroid pharmacology happen in just a few hours after birth?
The enzyme protein kinase C is implicated in the changes to GABAAR function during pregnancy
The GABAAR beta subunit contains consensus sequences for phosphorylation by PKC/PKA on the M3 to M4 intracellular loop.
Serine 408 to serine 409
How is phosphorylation increased in the pregnancy period in terms of neurosteroids?
Increased phosphorylation can be achieved by:
Inhibiting phosphatases
Stimulation of PKC
Adding oxytocin - linked to G protein receptors, increasing PKC
This results in changes in synapses and neurosteroid insensitivity
How can phosphorylation be decreased in terms of neurosteroid sensitivity and what is the impact of that?
Stimulating phosphatases
Inhibition of PKC
Addition of oxytocin antagonist
Acts to create neurosteroid sensitive synapses
Describe the role of neurosteroids on behaviour in terms of specific GABAAR isoforms
Alpha 1 = sedation Alpha 2, 3 = anxiolytic, analgesic Beta 3 = immobility/ anaesthesia Beta 2 = sedation/ anaesthesia Alpha 5 = cognition
Are drosophila’s GABAAR neurosteroid sensitive?
No
Where is the neurosteroid binding site on GABAARS?
TM1
Describe the characteristics of the alpha 2 knockout mice
Inhibitory GABAAR mediated mIPSCs of alpha 2 Q241W mice exhibit a reduced duration compared with WT
Alpha 2 Q241W mice exhibit an anxiogenic phenotype, with reduced sensitivity to anxiolytic effects of administered 5a3a
Collectively, these observations suggest that the presence of an endogenous neurosteroid tone is sufficient to influence synaptic alpha 2 GABAARs and behaviour
Do neurosteroids play a role in CNS develeopment?
Yes; early in development, synaptic GABAA receptors of thalamic VB neurons are influenced by an endogenous neurosteroid tone
Furthermore, finasteride bath on early development brain slices reduces neural inhibition
Does indomethacin administration have an impact on neurosteroid synthesis?
Yes; the synaptic inhibition enhancement effects of 5a-DHP in P20 VB neurons is prevented by indomethacin administration
What is a trigger of neurosteroid synthesis?
NMDA receptor activation in the hippocampus increases neurosteroid synthesis
Are there drugs which work to change neurosteroid levels?
Alcohol
If you incubate hippocampal slice in ethanol there is no effect on GABAA immediately but after around 30 mins there was a prolongation of mIPSCs which was blocked by finasteride. Furthermore, mutation of the alpha 2 GABAAR (steroid insensitive mice) reduced chronic ethanol intake
Fluoxetine
In depression, neurosteroids are abnormally low. At lower doses required for antidepressant effects, fluoxetine will increase neurosteroid production (mechanism unknown)
Describe GABAAR expression and neurosteroid levels in pregnancy
Greatly raised levels of allopregnanolone
Reduced expression of GABAARs to prevent sedation, but to maintain anxiolytic/ analgesic effects
Describe the GABAAR expression and neurosteroid levels in the post partum period
Dramatic decrease in allopregnanolone
GABAAR expression recovery from control is compromised in PPD
Describe the role of brexanolone in PPD?
Brexanolone( 5a3a) will improve mood over 2 weeks after treatment
Brexanolone 60hrs iV infusion in patients (18-45 yrs) <6 months PP
Mood assessed at 3, 7 and 30 days following brexanolone treatment
Improved mood reported even at 30 days i.e. over 2 weeks after treatment
Describe the effect of ORG (agonist at progesterone GPCR) on GABA transmission/ inhibition
ORG (agonist of progesterone GPCR) has no ACUTE effect on tonic, or phasic inhibition
In contrast to allopregnanolone in mouse hippocampal dentate gyrus granule cells, ORG has no acute (5 mins) effect upon GABA responses mediated by tonic or synaptic receptors.
Prolonged incubation with ORG however will enhance the tonic current mediated by extrasynaptic receptors with NO effect on phasic current
The ORG induced increase of the tonic current is blocked by GFX (protein kinase C inhibitor)
How does ORG produce this delayed enhancement of the tonic current?
Prolonged incubation with ORG enhances surface expression of the GABAAR beta 3 subunit in cell lines
What aspect of the beta 3 subunit does ORG and allopregnanolone target?
Phosphorylate the serine residues
This effect is blocked by inhibition of PKC
The GABAAR beta subunit contains consensus sequences for phosphorylation by PKC/PKA on the M3 to M4 intracellular loop (serine 408 to serine 409)
