GABA Anaesthesia Flashcards
What is the most commonly used IV induction agent?
Propofol; 200mg/20ml
Death of MJ
What is a common property of all GA drugs?
Lipophilic
Study preformed on tadpoles to determine how much GA was requried before the riting reflex was inhibited (tadpole put on back and will automatically right itself)
The amount of GA drug required directly correlated with the degree of lipophilic solubility (ability to dissolve in olive oil)
Do GAs act non-specifically via the membrane?
The more lipid the GA; the increasing anaesthetic aqueous potency
GA act by perturbing membrane structure; the anaesthetic dissolves into the lipid membrane (nerve cells) and will perturb the structure. IN perturbing the structure of the neural membrane; to produce a state of unconsciousness
Describe the alcohol cut off phenomenon in terms of GA potency
As you increase chain length of alcohol (potency as anaesthetic increases and lipid solubility increases)
However, as you start to go above C9; decanol etc the relationship begins to fall away. Lipid solubility will increase by anaesthetic potency will not
Not all anaesthetics with a potency are highly lipid soluble; particularly anaesthetic enantiomers
What is an enantiomer?
Mirror image molecules - useful compound is tomidate
Identical lipid solubility and ability to perturb membrane demonstrate a different anaesthetic potency
Can anaesthetics bind to proteins?
Yes; fire fly luciferase study demonstrated anaesthetic ability to bind to a pure protein preparation
Why protein (s) do anaesthetic bind and work to?
Transmitter gated ion channel
5 transmembrane crossing subunit
For nicotinic receptors; ACh binds extracellularly, believed 2 molecules are required for opening of the channel (cation influx)
ACh; cation influx
GABA; anion (chloride) influx
Describe the most common cys-loop receptors (nicotinic ACh)
Pentamer; 4 transmembrane proteins (TM1-4). Large N terminal, cysteine cysteine loop, extracellular carboxyl tail Nicotinic ACh - cation influx 5-HT3 - cation influx GABAA - anion (inhibitory) influx Glycine - anion (inhibitory) influx
Describe the glutamate receptor family?
Tetramers; N terminal, internal carboxyl tail
NMDA
AMPA
Kainate
Described via agonist pharmacology
What is the particular importance of TM2 of the cys-loop receptor family?
Lines the ion channel conduction region of the receptor
Describe the action of propofol on GABA inhibitory reading via patch clamp
Increases it
Describe the action of etomidate on 6 major receptors:
Inhibitory; GABAA, glycine
Excitatory; AMPA/kainate, NMDA (+ glycine), alpha4beta2 nicotinic, 5-HT3
GABAA; large increase
No action on glycine, AMPA/ kainate, NMDA, nicotnic or 5-HT3
Describe the action of propofol on 6 major receptors:
Inhibitory; GABAA, glycine
Excitatory; AMPA/kainate, NMDA (+ glycine), alpha4beta2 nicotinic, 5-HT3
GABAA; large increase
Glycine; small increase
No action on AMPA/kainate or NMDA
Decreases nicotinic moderately, slightly decreases 5-HT3
Describe the action of alphaxalone (steroid) on 6 major receptors:
Inhibitory; GABAA, glycine
Excitatory; AMPA/kainate, NMDA (+ glycine), alpha4beta2 nicotinic, 5-HT3
GABAA; large increase
No action on glycine, AMPA/kainate, NMDA ot 5-HT3
Will moderately decrease nicotinic action. Important to note that betaxolone is equieffective in this role (however this is not an anaesthetic suggesting that this action on nicotinic receptors is not involved with anaesthetic action)
Describe the action of pentobarbitone on 6 major receptors:
Inhibitory; GABAA, glycine
Excitatory; AMPA/kainate, NMDA (+ glycine), alpha4beta2 nicotinic, 5-HT3
GABAA; large increase No action at glycine receptors Large decrease at nicotinic receptors Moderate decrease of AMPA/kainate Slight decrease at NMDA and 5-HT3
Does ketamine act at GABAA?
No; inhibits NMDA receptors
Does etomidate exist as an enantiomer?
Yes; R+ and S-
R+ is used as the hypnodate
Differ in anaesthetic potency; this is via ability to modulate GABA activity
R+; as conc is increased there is a dose dependent enhancement of GABA response peaking close to GABA maximum
S- can enhance GABA but it requires much higher concentrations, with a much smaller maximum effect
Describe the GABA mimetic effect of etomidate; comparing R+ and S-
At higher concentrations required to enhance GABA; they can directly open GABAA receptor channel to cause an inward current in the absence of GABA
GABAmimetic effect much higher for R+ than S-
Describe GABAAR competitive antagonists
Bicuculline, picrotoxin
Proconvulsant and anxiogenic
Where is alpha 5 predominantly expressed?
Hippocampus
Where is the delta subunit predominantly expressed?
Cerebellum
Hippocampus
Thalamus
Does etomidate show selectivity across GABAAR subtypes?
Yes; shown to be beta subunit dependent
Specifically beta 2/3 (limited impact at beta 1)
Why does etomidate show beta subunit selectivity?
Beta 1 - no action
Beta 2 - yes
Beta 2/1 chimera; N terminal from beta 2 and intracellular/ transmembrane loop from beta 1 = no
Beta 1/2 chimera; beta 1 N terminal, transmembrane/ intracellular loop from beta 2 = YES
Critical region of TM2; single amino acid (N/S - asparagine to serine) will dictate pharmacology of etomidate
What governs the beta subunit selectivity of etomidate?
A single amino acid; N (asparagine) to S (serine) inhibits etomidate action
Is there a specific IV GA transmembrane binding site?
May concentrate in the membrane close to the transmembrane site located between beta-alpha subunit interface
Describe the binding sites for etomidate and propofol at the GABAAR
Etomidate binds at the b-a interface with the phenyl ring packed against the N265 of the beta2 subunit
Propofol binding overlaps with etomidate but due to the small nature of the molecule; makes fewer connections with the receptor
What is the influence of etomidate and propofol at the inter-subunit binding cavities?
In the presence of GABA there are 5 interfacial sites; which are all open
In the presence of etomidate (alpha-beta2 cavity) it will close 2 of the cavities (unbound)
Propofol also binds at inter-subunit binding cavities; but again due to the small nature of the molecule it fails to close any interfacial sites
What did the work surrounding the beta unit sensitivity of etomidate lead to?
Creation of a beta subunit “knock-in” etomidate insensitive mouse
In vitro; Beta 2 (N265S) = reduced etomidate sensitivity Beta 3 (N265M) = reduced etomidate and propofol sensitivity
Does etomidate produce sedation in beta N265S mice?
No - look at locomotor activity in mice as a surrogate measure of sedation
Still possible at high doses to produce hypnosis
Can be argued that locomotor activity proves a limited surrogate for sedation
Slow wave sleep in beta2 mutant mice by etomidate was greatly reduced
What is the loss of righting reflex (LORR) used for as a model?
Surrogate measure for unconsciousness “anaesthesia” in humans
Are the LORR effects via etomidate affected by mutant B2 or B3 mice?
Yes; the LORR by etomidate is reduced in both the beta 2 and beta 3 mutant mice
Therefore for LORR; etomidate acts at beta 2 and beta 3 receptors