ligand gated ion channels

Question 1

How does Po change in ligand gated ion channels?

Answer 1

changes with binding of the ligand or with signalling pathways/protein protein interactions

Question 2

Give steps to ion movement in ligand gated ion channels?

Answer 2

• binding of ligand
• conformational change in protein (structure altered/aa move
• opening/closing of gate
• ion movement

Question 3

what functional roles do ligand gated ion channels have?

Answer 3

• 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

Question 4

what does GABA stand for?

Answer 4

gamma aminobuteric acid

Question 5

what are GABA and glycine receptors?

Answer 5

• 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)

Question 6

what is an ionotropic glutamate receptors (GluRs)?

Answer 6

excitatory channel

• glutamate is the ligand
• cation non-selective
• sodium in, K out
• 0mv potential (as non selective)

Question 7

how many subunits do glycine receptors have?

Answer 7

5 subunits (3 alpha and 2 beta)

-one pore and one binding site

Question 8

what type of transmissions do Glycine receptors cause?

Answer 8

-fast inhibitory synaptic transmission

Question 9

what are symptoms of hyperekplexia in infants?

Answer 9

• hypertonia (rigidity) in infants - runs of APs?
• enhanced startle reflex caused by auditory and tactile stimuli
• apnoea (tempry cessation of breathing)
• is life threatening

Question 10

What are the symptoms of hyperekplexia in adults?

Answer 10

• hypertonia disappears
• enhanced startle reflex remains
• falls and injuries
• no longer life threatening

Question 11

what can be used to treat hyperekplexia?

Answer 11

clonezepam - activates GABA (inhibitory) receptors

Question 12

how is hyperekplexia caused?

Answer 12

• enhanced motor response

- lack of dampening down/inhibition of the reflex

Question 13

where in GlyR are the mutations found that cause hyperekplexia?

Answer 13

found in alpha subunit (gene GLRA1)

• areas where mutations are clustered
• B8-B9 EC loop (scaefer)

Question 14

where is the N46K mutation in GlyR?

Answer 14

• asparagine to lysine
• away from cluster
• by the agonist binding site

Question 15

what does N46K mutation do to Cl- current?

Answer 15

• 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

Question 16

what happens to the glycine dose response curve of N46K ?

Answer 16

• 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

Question 17

define efficacy

Answer 17

the tendency of a ligand to activate receptor once bound

Question 18

define affinity

Answer 18

the tendancy of a ligand to bind to a receptor

how easy does it bind

Question 19

how can you tell if affinity or efficacy is affected in GlyR mutants?

Answer 19

• 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

Question 20

Does the N46K mutation affect the affinity or efficacy of the GlyR channel?

Answer 20

• 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

Question 21

what are the full and partial agonists of GlyR?

Answer 21

• full = glycine

- partial = taurine

Question 22

how can you see deactivation and desensitisation of GlyR?

Answer 22

if you have a longer time exposure to glycine , current shows both deactivation and desensitisation

Question 23

how does the N46K mutation effect deactivation and desensitisation of GlyR?

Answer 23

• activation is normal

- deactivation occurs faster in N46K compared to WT

Question 24

Explain the mouse model which was used to examine treatment of hypekplexia?

Answer 24

• 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

Question 25

where is the location of mutant GlyR (Q177K)?

Answer 25

• more GlyR mutant protein abundant in the cell as less traffiked to the membrane
• synaptic location if disrupted
• reduction in overlap between gephryn (post synaptic membrane marker) in the mutant - less GlyR at the membrane

Question 26

why do mutant GlyR receptors have effect?

Answer 26

• mutant channels/receptors struggle to be traffiked to the post synaptic membrane
• when reach membrane they are difficult to open as need higher concentration of glycine to open them and elict same response/get same current
• smaller magnitude of current (can be seen in brainstem recordings)
• when are activated by glycine they close/inactuvate quickly

(decreased IPSPs, increased EPSPs - less inhibition- more likely to fire an AP)