L14 - Neuronal and Muscle Toxins Flashcards
What toxins is the neuronal Na channel affected by?
Tetrodotoxin
Saxitoxin
What toxins is the muscle Na channel affected by?
Tetrodotoxin
Saxitoxin
Conotoxin
What toxins is the Ca channel affected by?
Conotoxin
What toxins is the Kv K channel affected by?
Dendrotoxin
What toxins is the AchR channel affected by?
Tubocurarine
What toxins is Ach release affected by?
Tetanus toxin
Botulinum toxin
What is tetrodotoxin?
Guanidinium neurotoxin
100 times more lethal than spider toxins
10,000 times more lethal than cyanide
Only nM levels required
What does tetrodotoxin inhibit?
Voltage gated Na channels
What is the source of tetrodotoxin?
Produced by marine bacteria found in invertebrates, amphibians, fish
- Puffer fish, blue ring octopus
What are the symptoms of ingestion of tetrodotoxin?
Numbness of lips and tongue Facial paraesthesia - abnormal sensation Headache Nausea Dizziness Diarrhoea Vomiting Increasing paralysis and respiratory paralysis - death
What is the rate of onset of symptoms following ingestion of tetrodotoxin?
20 mins to 8 hours
What are the symptoms of a bite containing tetrodotoxin?
Same as ingestions but without facial effects
Faster death
What are the treatment for tetrodotoxin poisoning?
Mechanical ventilation
No anti-venom - tetrodotoxin binds too strongly to site of action
In tetrodotoxin what does the inhibition of voltage gated Na channels lead to?
Failure of neurotransmission – reduction in Ach release at NMJ
- Loss of sensation and then muscle paralysis
- Because sensory neurons meet the toxin first
- Respiratory paralysis and death
What are some examples of tetrodotoxin sensitive channels?
Nav 1.1 SCN1A – CNS and heart Nav 1.2 SCN2A - CNS Nav 1.3 SCN3A – CNS and heart Nav 1.4 SCN4A - skeletal Nav 1.6 SCN8A - CNS and PNS Nav 1.7 SCN9A - PNS
What are some examples of tetrodotoxin insensitive channels?
Nav 1.5 SCN5A - heart
Nav 1.8 SCN10A - sensory
Nav 1.9 SCN11A - PNS
Which channel highlighted the variable sensitivity to tetrodotoxin?
Rat Nav type II
Rat Nav type II tetrodotoxin sensitivity?
Wildtype rat channel - tetrodotoxin sensitive
Mutations in E387Q in pore region of rat channel
- Glutamate –> glutamine
- Tetrodotoxin insensitive
- Shows that tetrodotoxin sensitivity is based on one amino acid substitution in pore region
Heart Nav 374 tetrodotoxin sensitivity?
Heart Nav 374 – cysteine - tetrodotoxin insensitive
Change heart 374 to tyrosine - tetrodotoxin sensitive
What animals shows variable sensitivity to tetrodotoxin?
Garter snakes
- Difference geographical areas
- Different tetrodotoxin sensitivity
- Different Nav sequence IV domain
Both eat salamanders which can contain tetrodotoxin
- Only 1 snake population affected
- The snake population that survived was able to reproduce and pass on this gene
Where are dendrotoxins found?
Snake neurotoxins found in Mamba family
Only nM levels required
What do dendrotoxins inhibit?
Inhibit Kv channels
- Different types block different Kv subtypes
- Inhibition of repolarisation
What are the symptoms of infection with dendrotoxins?
Early weakness and numbness in the bitten extremity
Systemic responses 30 minutes to 1 hour after bite
Ptosis - drooping eyelids
Ophthalmoplegia – paralysis of eye muscles
Disphagia - difficulty swallowing
Paresis - mild paralysis
Respiratory failure - death
In dendrotoxin what does the inhibition of voltage gated K channels lead to?
Delays repolarisation of NMJ
Excess release of Ach from pre-synaptic membrane leading to Ach depletion
Failure of subsequent neurotransmission
Problems with muscle control - respiratory paralysis and death
Where is conotoxin found?
Found in cone snails
What are the different types of conotoxin?
Over 8000 different conotoxins
- 19 major gene families
Multiple and diverse targets - many channel types
Varying lethality
What are conotoxins being investigated for/
Parkinson’s, MS, lung cancer and chronic pain
What are the symptoms of infection with conotoxin?
Burning pain Swelling at site of toxin injection Numbness Cardiac and respiratory distress Muscle weakness Loss of coordination Ptosis Headache Nausea Stomach cramps
What are the symptoms of infection with conotoxin in severe cases?
Lips became stiff
Blurred vision
Paralysis
Coma
What are the two conotoxin pain targets?
Found in nociceptive nerve endings
Ca 2.2
- Found in pre-synaptic membrane and sensory neurones
- Mediates the release of neurotransmitter and therefore propagate pain
- Inhibited by Eu1.6
Na 1.8
What does Eu1.6 inhibit in HEK cells?
Cav2.2
In control as cell depolarised the Ca currents increase
In Eu1.6 the Ca currents are much smaller –> inhibits Cav2.2 –> fewer vesicles fuse –> less neurotransmitter
What does Eu1.6 inhibit in rate DRG cells?
Cav2.2
Looking at Cav2.2 in a cell where it is normally expressed
Used barium – do not get the negative feedback seen with calcium which inhibits the channel
In Eu1.6 the currents are much smaller than in control - blocks 1/3 of the channels - 30% inhibition
What experiment was used to see if Eu1.6 suppresses pain?
Rat partial nerve ligation
- Rat anaesthetised and sciatic nerve exposed
- Pain model – 1/3rd of the sciatic nerve tied off
- Application of pressure to hind paw until withdrawal
What were the 3 groups tested during the rat partial nerve ligation?
No pain relief – saline
- Feel pain with a low amount of pressure
- Positive control - morphine and gabapentin
- After injection able to withstand higher pressure
Eu1.6
- After injection able to withstand higher pressure
What is the overall effect of Eu1.6?
Inhibition of voltage gated Ca2+ channels
Reduction in release of neurotransmitter at synaptic cleft
Reduction in pain signalling
What are two examples of tarantula toxins?
µ-TRTX-Tp1a and GpTx-1
What do µ-TRTX-Tp1a and GpTx-1 do?
Inhibit Na+ currents through Nav1.7 –> can’t generate action potentials in sensory nerves
- Potential to block pain signals
Open probability is less in the presence of the toxin
What experiment was undertaken to see if GpTx-1 was an analgesic in mice?
Pain model - 40 µl of OD1 injected into hindpaw
GpTx-1 then blocks the Na channels that OD1 activated
What is OD1?
Scorpion toxin
What does OD1 causer?
40 µl of OD1 injected into hindpaw
Activates Nav1.7/1.6
Activates action potential firing in A and C fibres –>channels stay open for longer
- Pain felt in paw – shown by lifting, shaking and licking
