Glutamate Anaesthetics Flashcards
How can the properties of glutamate receptors be recorded?
Transfect epithelial cells with the cDNA encoding for the ionotropic glutamate receptor subunits and for GFP
1-2 days later, record from the GFP cells using whole cell patch clamp technique
Agonist-induced inward current
What are the 3 types of glutamate receptors?
AMPA
Kainate
NMDA
What is the main flow of current at -60mV in terms of a neurone and glutamergic receptors
Na+
Ca2+
Under these conditions, the Mg+ enters and blocks the NMDA open ion channel
Describe the NMDA receptor
Conducts Na+, Ca2+ and K+ BUT is blocked by Mg 2+
The block by Mg2+ is both voltage and agonist dependent
For the NMDAR to open it needs to be activated by glutamate (or NMDA) and by the co-agonist glycine
Phencyclidine (PCP) and ketamine are blockers of the NMDAR ion channel
Describe the main flow of current at +20 mV in terms of a neuron and glutamatergic receptors
Outward mainly carried by K+ leaving the cell
Under these conditions, Mg2+ does not enter or block the channel
Does NMDA require just glutamate to activate?
No; requires glutamate and the co-agonist glycine (D-serine)
What are the binding sites for glutamate and glycine on NMDA?
Glutamate at GluN2
Glycine at GluN1
Describe the impact of Mg+, APV (NMDA antagonist) and CNQX (AMPAR antagonist) on EPSCSs
Standard solution (containing Mg2+) there is a fast ESPC This is not influenced by APV but is eliminated by CNQX
In a Mg2+ free solution, the EPSCs exhibit a slow component. This slow component is inhibited by APV, however CNQX, the fast component is inhibited but there is no effect on the slow component of EPSC
Describe the synergistic interplay of synaptic AMPARs and NMDARs at an excitatory synapse
Neurally released glutamate activates synaptic AMPAR, but although glutamate binds to NMDARs, the assoc ion channel does not conduct due to blockage via Mg2+
The Na+ influx results in a depolarisation of the neuronal spine. If the presynaptic glutamergic nerve fires at high frequencies, then the depolarisation may be sufficient to unblock the Mg2+ NMDAR; allowing for a slow prolonged synaptic depolarisation
Describe the conductance of NMDARs
Inward Na+/ Ca2+
Outward K+
At neg membrane potential, blocked by Mg2+
What is EPSP integration?
Neurons require many excitatory postsynaptic potentials to summate to produce an EPSP sufficient to fire an action potential
Describe the difference between temporal and spatial summation
Spatial; 2 or more presynaptic inputs are simultaneously active causing their individual EPSPs to summate
Temporal; the same presynaptic fibre fires action potentials in quick succession, causing the individual EPSPs to summate
What do NMDA, AMPAR and KA receptors have in common?
Composed of 4 transmembrane subunits
Large N terminal domain, crosses membrane at TM1, re-entrant loop, Q/R site, TM2, extracellular loop, TM3, intracellular carboxyl tail
NMDA; GluN1, GluN2 (A,B,C or D), GluN (3A or B)
AMPA: GluA (1,2,3,4)
KA: GluK (1,2,3,4,5)
Describe the ability of AMPAR to be calcium permeable or impermeable
Majority of AMPARs are heteromers containing GluA2 subunit
AMPARs containing GluA1, 3 or 4 are calcium permeable
The GluA2 subunit is critical in determining receptor function. Incorporation of GluA2 impairs calcium permeability
The permeability to calcium is dictated by a single amino acid located on the ion conducting pore (glutamine Q (permeable), or arginine R (impermeable)
The GluA2 RNA may be subject to RNA editing, which will determine the GluA2 will contain a Q (calcium permeable) or an R (impermeable) at this key ion channel location
Describe the role of RNA editing and ion permeability of the AMPA receptor
GluA2 undergoes Q to R editing (adenosine deamination)
CAG = Q CGG = R
In adult; 95% of GluA2 is edited i.e. GluAR
GluA2Q = Ca2+ permeable GluA2R = Ca2+ impermeable
Note, GluA1 (Q), GluA3 (Q), GluA4 (Q) and unedited GluA2 (Q) are ALL Ca2+ permeable
Where is the glutamate binding site on AMPAR glutamate receptors?
S1 and S2
Evidence:
1) S1 and S2 show sequence homology to bacterial amino acid binding proteins
2) bacterial proteins bind amino acids between 2 lobes of a clam shell that are in a dynamic equilibrium between open and closed states; ligand binding stabilises the closed state
3) swapping S1 and S2 domains between GluA3 (AMPAR) and GluK2 (KAR) causes appropriate changes to agonist pharmacology
Describe the structure of NMDA receptors
Mostly composed of GluN1 and GluN2
The NMDAR subunits have a similar topology to AMPA and KA subunits
GluN1 subunit binds Glycine/ D-serine
GluN2 binds glutamate
At the equivalent AMPAR AMPAR Q/R site, NMDARs have an asparagine residue (N) - site of Mg2+ block
Describe the effects of ketamine upon IV injection
Dissociative anaesthetic state; marked sensory loss, amnesia and analgesia without complete loss of consciousness
Structurally related to PCP
Little to no effect on GABAARs
Acts as a “use-dependent” voltage-dependent blocker of NMDAR assoc cation channel
Describe the blockade of NMDA via ketamine
Blocks the channel in the open state (2ARD*)
Voltage dependent; the more negative the membrane, the more likely the blockade
Agonist dependent; channel has to be in open state for ketamine to access binding site
Describe the uses for nitrous oxide
Occasional inhalational anaesthetic
Analgesic in short, painful procedures; labour, dentistry
Evidence emerging for ability to demonstrate rapid antidepressant properties
Rapid onset and offset
What is the basic mode of action of NO?
Non-competitive NMDAR antagonist
Describe xenon as a GA
Rapid induction, short duration
Lack of CV effects
Powerful analgesic effects
Exhibits neuroprotective effects
Unfortunately, very expensive therefore limiting use
What is the mode of action of xenon?
NMDA receptor antagonist by inhibiting the glycine binding site on the NR1 subunit
