Neuromuscular Junction - BIOL2048/2049 PDF

BIOL2048/2049 The neuromuscular junction Dr Yihua Wang [email protected] Learning Outcomes By the end of this session you should be able to: give examples of drugs that act on the neuromuscular junction and describe their effects and their modes of action The neuromuscular j...

BIOL2048/2049 The neuromuscular junction Dr Yihua Wang [email protected] Learning Outcomes By the end of this session you should be able to: give examples of drugs that act on the neuromuscular junction and describe their effects and their modes of action The neuromuscular junction 1. What is NMJ? How does it work? 2. Diseases associated with NMJ. 3. Drugs working on NMJ. The neuromuscular junction 1. What is NMJ? How does it work? 2. Diseases associated with NMJ. 3. Drugs working on NMJ. Acetyl-coenzyme A (CoA) NM - embryonic form, α1, β1, δ and γ subunits (2:1:1:1); adult form, α1, β1, δ and ε subunits (2:1:1:1) Acetylcholine (ACh) and receptor Acetyl--------choline The Neuromuscular Junction (NMJ): a Specialized form of synaptic transmission: communication between neurons and skeletal muscle Patellar reflex or knee-jerk reflex Neuromuscular junction https://www.youtube.com/watch?v=CLS84OoHJnQ Major Events in Neuromuscular Transmission Motor neuron depolarization causes action potential to travel down the nerve fiber to the neuromuscular junction (1). Depolarization of the axon terminal causes an influx of Ca2+ (2) which triggers fusion of the synaptic vesicles (3) and release of neurotransmitter (Acetylcholine; ACh) (4). ACh diffuses across the synaptic cleft and binds to post- synaptic ACh receptor (AChR) located on the muscle fiber at the motor end-plate (5). Binding of ACh to AChRs opens the channels causing an influx of Na+ (5), depolarization of the sarcolemma that travels down the t-tubules (6) and ultimately causes the release of Ca2+ from the sarcoplasmic reticulum - CONTRACTION. Unbound ACh in synaptic cleft defuses away or is hydrolyzed (inactivated) by acetylcholinesterase (AChE) (7). The neuromuscular junction 1. What is NMJ? How does it work? 2. Diseases associated with NMJ. 3. Drugs working on NMJ. Neuromuscular Junction Disorders Myasthenia gravis Lambert-Eaton myasthenic syndrome Neuromyotonia (Isaac’s syndrome)  Myasthenia Gravis “grave muscle weakness” Autoantibodies to the nicotinic AChR on the motor end-plates of muscles.  Binding of ACh is blocked and muscle activation is inhibited.  The autoantibodies also induce complement-mediated degradation of the AChRs, resulting in progressive weakening of the skeletal muscles. Autoantibodies to MuSK, which is important for the tight clustering of AChRs at the neuromuscular junction. Myasthenia Gravis Myasthenia Gravis Lambert-Eaton myasthenic syndrome Autoantibodies to presynaptic voltage-gated calcium channel (VGCC) These antibodies interfere with the calcium- dependent release of ACh from the presynaptic membrane and subsequently cause a reduced endplate potential on the postsynaptic membrane, resulting in NMJ transmission failure.  Neuromyotonia (Isaac’s syndrome) Autoantibodies to presynaptic voltage-gated potassium channel (VGKC) Autoimmune neuromyotonia is typically caused by antibodies that bind to potassium channels on the motor nerve resulting in continuous/hyper- excitability. https://vevox.app/m#/157529992 The neuromuscular junction 1. What is NMJ? How does it work? 2. Diseases associated with NMJ. 3. Drugs working on NMJ. PHARMACOLOGY – Vol. II - Drugs on Skeletal Muscle - Michael W. Nott SR: sarcoplasmic reticulum PHARMACOLOGY – Vol. II - Drugs on Skeletal Muscle - Michael W. Nott SR: sarcoplasmic reticulum Top: The action of the normal agonist, acetylcholine, in opening the channel. Bottom, left: A nondepolarizing blocker, eg, rocuronium, is shown as preventing the opening of the channel when it binds to the receptor. Bottom, right: A depolarizing blocker, eg, succinylcholine, both occupying the receptor and blocking the channel. Normal closure of the channel gate is prevented and the blocker may move rapidly in and out of the pore. Depolarizing blockers may desensitize the end plate by occupying the receptor and causing persistent depolarization.  These toxins bind with high affinity to nicotinic cobratoxin acetylcholine receptor Causes a postsynaptic block at the NMJ a-bungarotoxin  NMJ Blocking Agents Effects: paralysis - small rapidly moving muscles (eyes, fingers), then limbs, last is respiratory muscles (recovery in reverse order) Competitive (non-depolarizing) agents (curare) compete with ACh for binding to receptor flaccid, relaxed paralysis non-NMJ effects: ganglia, muscarinic blocking, histamine release NMJ block CAN be reversed by AChE inhibitors Non-competitive (depolarizing) agents Competitive (non-depolarizing) Blocking Agents – Curare Tubocurarine, dimethyltubocarine (metocarine) no effect on nerve transmission muscle can still be stimulated 5-10mg (iv) produces flaccid paralysis 10-20mg (iv) can produce apnea, not active orally can cause histamine release (mast cells) can block ganglionic receptors [high concentration] Competitive (non-depolarizing) Blocking Agents – Others Pancuronium more potent than tubocurarine (x5) reduced histamine release than curare lack of ganglionic blockade Gallamine: also some muscarinic block Mivacurium: short acting, hydrolysis by AChE Atracurium: hydrolysis by AChE PHARMACOLOGY – Vol. II - Drugs on Skeletal Muscle - Michael W. Nott SR: sarcoplasmic reticulum  Acetylcholinesterase inhibitors Natural: venoms and poisons Weapons: nerve agents, insecticides Drugs: – To treat myasthenia gravis – To treat Alzheimer’s disease – To treat Lewy body dementia – To treat glaucoma – Antidote for anticholinergic poisoning  Cholinesterase inhibitors Increase the availability of acetylcholine (ACh), partially overcome the decreased receptor availability Reverse non-depolarising muscle blockade First line for ocular myasthenia Edrophonium, Neostigmine, Pyridostigmine, Distigmine (in order of increasing duration of action) Adjunct to immunosuppression for generalised myasthenia Organophosphates － irreversible bonds to the enzyme  What are the symptoms after over-stimulation of the NMJ?? Cholinergic crisis Excess of acetylcholine (ACh) – Nerve gas – Disease (e.g., Myasthenia gravis patients taking too much medication) – Surgery (too much ACh inhibitor to reverse surgical paralysis) The muscles stop responding to the excess ACh – Vasodilatation of blood vessels – Sweating – Salivation – Bronchial secretions (mucus) – Miosis (constriction of eye pupil) – Flaccid paralysis – Respiratory failure  Nerve gas poisoning Overstimulation of muscles, organs and glands Symptoms: Ataxia (lack of muscle control), slurred speech, areflexia (loss of reflexes), generalized convulsions, respiratory failure, death ATROPINE IS AN ANTIDOTE! Atropine binds to the acetylcholine receptors and prevents acetylcholine from binding to the receptors Oximes can also be co-administered – regenerate the enzyme  March 20th 1995, Sarin in Tokyo subway Sarin interacts with AChE: ACh builds up, firing off ACh receptors repeatedly  Organophosphate insecticides (parathion) Organophospates are irreversible AChE inhibitors - The phosphorous atom covalently binds to a serine hydroxyl group in the active site of acetylcholinesterase (AChE or ACE) - Enzyme no longer functional (inactive) DIFP: diisopropyl fluorophosphate Figure 10-9 Action of anticholinesterase drugs. Reversible anticholinesterase (neostigmine): recovery of activity by hydrolysis of the carbamylated enzyme takes many minutes. Irreversible anticholinesterase (dyflos): reactivation of phosphorylated enzyme by pralidoxime.  Neostigmine AChE inhibitors permit build-up of Ach. More intensive and prolonged activation Action of ACh that is liberated by cholinergic impulses or spontaneous leak from the nerve is enhanced Time prolonged to interact with receptors that are not blocked by auto-antibodies (in Myasthenia Gravis).  Edrophonium N-ethyl-3-hydroxy-N,N- dimethylbenzenaminium Trade names Tensilon, Enlon and Reversol. Edrophonium is a readily reversible AChE inhibitor that prevents breakdown of ACh True competitive inhibition of AChE at the NMJ Rapid dissociation Used in diagnosis myasthenia gravis Tensilon test: differentiate myasthenic crisis from cholinergic crisis  Edrophonium Used in diagnosis myasthenia gravis Tensilon test: differentiate myasthenic crisis form cholinergic crisis myasthenic crisis: not enough neuromuscular stimulation: Edrophonium will reduce the muscle weakness by effectively supplying more ACh Cholinergic crisis: too much neuromuscular stimulation: Edrophonium will make the muscle weakness worse by inducing a depolarizing block  Physostigmine also known as eserine from éséré, Calabar bean: witchcraft Reversible pseudo-competitive AChE inhibitor Clinical uses Physostigmine is used to treat myasthenia gravis, glaucoma, Alzheimer’s disease and delayed gastic emptying. It can cross the blood brain barrier and can be used to treat effects of atropine and other anticholinergic drug overdoses  AChE inhibitors in dementia Cholinergic hypothesis: Cognitive decline is linked to the loss of cholinergic transmission in hippo and cortex Loss of choline acetyl-transferase (CAT) (biochemistry and PET) and loss of cholinergic (presyn.) neurons in forebrain Enhance central cholinergic function Acetylcholine: short half life Muscarinergic agents ineffective and poorly tolerated Nicotinergic agonists vascular side effects  AChE inhibitors in dementia Tacrine (1993) centrally active non-competitive reversible AChE inhibitor. 20-30% showed improvement, side effects, 4x/day Donepezil (1997) piperidine-based reversible AChE inhibitor (less ButrylACh), long half life, side effects Rivastigmine and neostigmine(1998), carbamate AChE inhibitors. Inactivates enzyme for 10h producing a pseudo-irreversible inhibition. Metabolised in liver. Galatamine (snowdrops). Phenonthrene alkaloid. Reversible, competitve AChE PHARMACOLOGY – Vol. II - Drugs on Skeletal Muscle - Michael W. Nott SR: sarcoplasmic reticulum  food-borne botulism wound botulism infant botulism Symptomes similar to Myasthenia Gravis  Botox or Botulinum toxin type A Botox cleaves specific SNARE proteins. This blocks the vesicles from releasing Ach https://www.google.co.uk/search?q=botox+and+neuromuscular+junction&client=firefox-a&hs=zkB&rls=org.mozilla:en-GB:official&channel=sb&source=lnms&tbm=isch&sa=X&ei=LqlDVNf8BKOa7gaYuoC4DQ&ved=0CAgQ_AUoAQ&biw=1366&bih=631#facrc=_&imgdii=swtI_Bk6ajmJ9M%3A%3B2nLh6iUo6Kz3HM %3BswtI_Bk6ajmJ9M%3A&imgrc=swtI_Bk6ajmJ9M%253A%3BlL_IKZ3aPS_9YM%3Bhttp%253A%252F%252Fimageshack.us%252Fa%252Fimg96%252F2191%252Fbotoxmoapost.png%3Bhttp%253A%252F%252Fwww.proskinmd.com%252Fservice.php%253Fservice%253DBotox%3B346%3B215 Botox Uses For Both Medical And Cosmetic Reasons Learning Outcomes By the end of this session you should be able to: give examples of drugs that act on the neuromuscular junction and describe their effects and their modes of action https://vevox.app/m#/157529992