ligand gated ion channels Flashcards
How does Po change in ligand gated ion channels?
changes with binding of the ligand or with signalling pathways/protein protein interactions
Give steps to ion movement in ligand gated ion channels?
- binding of ligand
- conformational change in protein (structure altered/aa move
- opening/closing of gate
- ion movement
what functional roles do ligand gated ion channels have?
- fast chemical synapses (inhibitory and excitatory)
- nociception
- mechanosensation
- -autocrine and paracrine signalling
found in excitatory cels mainly in muscle and nerve (but also some epithelial cells
what does GABA stand for?
gamma aminobuteric acid
what are GABA and glycine receptors?
- chlorine Cl- channels so drives cell towards nernst for Cl- (negative)
- causes inhibition of CNS (hyperpolarisation)- less likely for cell to fire an AP as further away from threshold
(activated by binding of GABA or glycine)
what is an ionotropic glutamate receptors (GluRs)?
excitatory channel
- glutamate is the ligand
- cation non-selective
- sodium in, K out
- 0mv potential (as non selective)
how many subunits do glycine receptors have?
5 subunits (3 alpha and 2 beta)
-one pore and one binding site
what type of transmissions do Glycine receptors cause?
-fast inhibitory synaptic transmission
what are symptoms of hyperekplexia in infants?
- hypertonia (rigidity) in infants - runs of APs?
- enhanced startle reflex caused by auditory and tactile stimuli
- apnoea (tempry cessation of breathing)
- is life threatening
What are the symptoms of hyperekplexia in adults?
- hypertonia disappears
- enhanced startle reflex remains
- falls and injuries
- no longer life threatening
what can be used to treat hyperekplexia?
clonezepam - activates GABA (inhibitory) receptors
how is hyperekplexia caused?
- enhanced motor response
- lack of dampening down/inhibition of the reflex
where in GlyR are the mutations found that cause hyperekplexia?
found in alpha subunit (gene GLRA1)
- areas where mutations are clustered
- B8-B9 EC loop (scaefer)
where is the N46K mutation in GlyR?
- asparagine to lysine
- away from cluster
- by the agonist binding site
what does N46K mutation do to Cl- current?
- decrease in Cl- current - deactivation and desensitization
- lower Cl current = less Cl- in cell - less negative - closer to threshold - more likely to fire an AP
- higher conc of glycine needed to elict same response/Cl current as WT
what happens to the glycine dose response curve of N46K ?
- shifts to right
- need a higher concentration (10x) of glycine to elict same response /same Cl- current as WT
- for a given concentration of glysine N46K elicts a smaller response
define efficacy
the tendency of a ligand to activate receptor once bound
define affinity
the tendancy of a ligand to bind to a receptor
how easy does it bind
how can you tell if affinity or efficacy is affected in GlyR mutants?
- if affinity is effected dose response curve shifts sideways
- if efficacy is effected FULL agonist shift in dose response curve
-efficacy partial agonist - sideways shift in dose response curve and lower maximal response
Does the N46K mutation affect the affinity or efficacy of the GlyR channel?
- there is a right sideways shift in dose response but no change in maximal response when partial agonist is used (not due to efficacy)
- as mutation changes the affinity of receptor for glycine - glycine is less lekily to bind, so a higher conc (10x) is required for the same response as WT
what are the full and partial agonists of GlyR?
- full = glycine
- partial = taurine
how can you see deactivation and desensitisation of GlyR?
if you have a longer time exposure to glycine , current shows both deactivation and desensitisation
how does the N46K mutation effect deactivation and desensitisation of GlyR?
- activation is normal
- deactivation occurs faster in N46K compared to WT
Explain the mouse model which was used to examine treatment of hypekplexia?
- shaky mice used - Q177K- mutation in extracellular B8-B9 loop
- motor defects from 2 weeks onwards
- hind feet/limb clenching
- shaky mice die early (all dead by 6 weeks-not representable of human )
- righting time (how long take to get up after falling over) measured
- time shaky mice spend on rotating rod is lower than WT
- when mice treated with GABAr they can spend longer time on the rod
