Lecture Course 2: Peripheral Neural Transmission Flashcards
* Botulinum toxin (general) *
- Prevents neurotransmitter release
- Preferential for cholinergic neurones
- Heavy chain C terminus -> ganglioside receptor
- N terminus translocates peptidase light chain into cell/vesicle
- Peptidase cleaves target snare
- Symptoms
- Somatic muscle weakness
- Loss of autonomic cholinergic activity
- (constipation, blurred vision, dry skin)
* Botulinum toxins B, D, F and G *
- Prevent neurotransmitter release
- Preferential for cholinergic neurones
- Cleaves v-SNARE Synaptobrevin
- Symptoms
- Somatic muscle weakness
- Loss of autonomic cholinergic activity
*Botulinum toxins A and E*
- Prevent neurotransmitter release
- Preferential for cholinergic neurones
- Cleaves t-SNARE SNAP-25
- Symptoms
- Somatic muscle weakness
- Loss of autonomic cholinergic activity
* Botulinum toxin C1 *
- Prevent neurotransmitter release
- Preferential for cholinergic neurones
- Cleaves v-SNARE **Synaptobrevin **and t-snare SNAP-25
- Symptoms
- Somatic muscle weakness
- Loss of autonomic cholinergic activity
* Tetanus toxin *
- Blocks vesicle release in inhibitory interneurones
- Retrogradedly transported to cell body, then transferred to inhibitory interneurone
- Targets t-snare synaptobrevin whichprevents inhibitory neurotransmitter release
- Motor neurones become more excitable
- => tetanus
* Hemicholinium *
- Blocks sodium-choline cotransporter, responsible for reuptake of choline into the cell to form Ach
Triethylcholine
Competitive substrate for Ach at transporters.
Released in place of Ach- a false transmitter.
Vesamicol
Non competitive reversible blocker of the vesicular ACh transporter.
This prevents vesicles from being filled with ACh.
* Beta Bungarotoxin *
- Blocks ACh release.
- Potassium channel binding region
- Phospholipase A2 activity
* Alpha latrotoxin *
- Binds to neurexins on plasma membrane and causes mass release of ACh by forming calcium permeable channels.
- Can also inhibit potassium channels.
- Black widow spider toxin
Alpha bungarotoxin
- Irreversibly blocks n.m.j.
- Does not block ganglions.
Trimetaphan
- Competitive antagonist for ganglionic nAChR
- Used for controlled lowering of blood pressure during surgery.
* Nicotine *
- nAChR agonist
- Selective for ganglionic nAChR
- Phase I- depolarising block. Neuron cannot be stimulated further.
- Phase II - desensitisation of nicotinic receptor - sodium channel inactivation. Neuron can be stimulated electrically directly but not via presynaptic neurone.
* Hexamethonium *
- Non competitive ganglion blocker.
- Use dependent blockade.
- Causes loss of sympathetic and parasympathetic systems - ‘hexamethonium man’.
* D-tubocurarine *
- Nicotinic receptor antagonist.
- Non selective between ganglionic and nmj.
- Leads to flaccid paralysis.
- Cannot cross intestinal epithelium or placenta.
* Atracurium *
- Competitive nAChR antagonist.
- Also may block nicotinic autoreceptors => tetanic fade
- Not orally active
- Used in anaesthesia
- Selective for white muscle
- Short half life.
- Ester - Broken down by plasma esterases.
* Pancuronium *
- Competitive nAChR antagonist.
- Also may block nicotinic autoreceptors => tetanic fade
- Not orally active
- Used in anaesthesia
- Selective for white muscle
- Long half life
- Not broken down by plasma esterases.
Decamethonium
- Depolarising nmj blockade.
- Phase I - spastic paralysis.
- Prolonged in multiply innervated muscles.
- Deepened by anticholinesterases, reversible with non depolarising blockers
- Prolonged in multiply innervated muscles.
- Phase II- flaccid paralysis.
- Desensitisation block.
- Reversed by anticholinesterases, deepened by curare like drugs.
- Phase I - spastic paralysis.
Suxamethonium
- Depolarising nmj blockade.
- Used for short term muscle relaxation
- eg. Intubation.
- Can cause dangerous increases in plasma K+.
- Phase I - spastic paralysis.
- Prolonged in multiply innervated muscles.
- Deepened by anticholinesterases, reversible with non depolarising blockers
- Phase II- flaccid paralysis.
- Desensitisation block.
- Reversed by anticholinesterases, deepened by curare like drugs.
Acetylcholine
- Non selective agonist for muscarinic and nicotinic receptors.
- Broken down by AChE and BuChE
Carbachol
- Non selective agonist for nicotinic and muscarinic receptors.
** * Muscarine ***
Muscarinic receptor agonist
Methacholine
- Non selective muscarinic agonist.
- Two isomers:
- +-Methacholine is broken down by AChE and 200x more potent than - isomer
Cevimeline
- Selective agonist for M3 muscarinic receptors (on glands and smooth muscle).
- Used to treat dry mouth in Sjögren’s syndrome.
* Atropine *
- Non selective antagonist for muscarinic receptors.
- Used to dilate eye (opthalmic examination) - although too long lasting
- Decrease bronchial and salivary secretions, bronchodilatation, increase heart rate, decrease GI motility
Pirenzepine
- Selective antagonist for muscarinic M1 receptors (on the PNS and CNS).
- Used to decrease gastric acid secretion.
* Darifenacin *
- Selective antagonist for muscarinic M3 receptors (on glands and smooth muscle and oxytinic cells).
- Used in cases of urinary incontinence.
M1 and M3 muscarinic receptors
Mechanism:
- couple through Gq/11 - generation of IP3 and DAG through cleavage of PIP2
- M1 only also inhibits potassium channels
- M3 only also increases intracellular calcium concentration
M2 muscarinic receptors
Mechanism:
- Couple to Gai
- inhibits adenylyl cyclase.
- cAMP decrease.
- decreased VG calcium channel activation, lower heart excitability and less neurotransmitter release
- beta gamma subunit opens potassium channel in SAN, decreasing heart rate
* Bethanechol *
- Non selective agonist for muscarinic receptors.
- Poorly absorbed from GI tract.
- Used for systemic treatment for urinary retention.
* Pilocarpine *
- Non selective agonist for muscarinic receptors.
- Poorly absorbed across GI tract.
- Used topically for glaucoma - absorbed through cornea and contracts cillary muscle.
* Edrophonium *
- Short acting Anticholinesterase.
- Reversible ionic interaction with AChE.
- Used to diagnose myasthenia gravis.