Neuromuscular Junction - Pharmacology

Neuromuscular Junction - Pharmacology1. Some pharmacological principles, illustrated by ACh at the NMJ2. Targets for drug action at the NMJ3. Drugs affecting synthesis, storage and release of ACh4. Drugs affecting ACh Esterase (inactivation)5. Drugs affecting ACh Receptors (activation)•competitive and depolarising block6. Clinical uses of drugs acting at the NMJReadingRang et al. Pharmacologyhttp://www.dns.ed.ac.uk/~rrr/NWPrrr.html

General Principle of Pharmacology: 1Most drugs are effectivebecause they bind to proteins : Enzymes Carriers Ion channels Receptors

General Principle of Pharmacology: 2Drugs are useful as: Experimental tools Clinical treatments

General Principle of Pharmacology: 3The concept of the receptor, and mostkinds of receptors, have been identifiedby the selective effects of specific drugs

General Principle of Pharmacology: 4Drug assays are based on: Ligand binding (->affinity, K A ) Biological response (->efficacy, EC50)

General Principle of Pharmacology: 5When “doing drugs” a good pharmacologist needs to know:•Target specificity/Ligand specificity• Agonist/Antagonist• Dose/Response• Affinity (K A )/Efficacy (EC50)• Competitive/Non-competitive• Clinical/Non-clinical uses• Side-effects

In neuromuscular pharmacology, the targets are:SynthesisStorage Release Action Inactivation

The biological responses normally measured in skeletalmuscle are: muscle contraction (twitch/tetanus); transmitter release (EPCs/EPPs); receptor response (nAChR channels) ion channel function (Action potentials/Na or K channels)

0 Ca Direct +Ca +4AP +Mg +4AP DirectTTX

XAction potential…add µ-conotoxin…add d-tubocurarine

EPPs in µ-conotoxin/d-tubocurarineEPP in low Ca/high MgMg 2+Ca 2+5 mV10.00 ms5 mV10.00 ms

Ligand-gated(eg AChR)Na +Voltage-gated(e.g. NaV)Na +ACh+ +K +

V-gated Ion-channel PharmacologyChannelNa channelsAntagonist/blockertetrodotoxin (TTX)saxitoxin (STX)µ-conotoxinlignocaineK channels Cs +tetraethylammonium (TEA)4-aminopyridine (4-AP)Ca channels Cd 2+ , Mg 2+N-typeω-conotoxinP-typeω-agatoxinL-typedihydropyridines (DHP)

Acetylcholine+Quaternary nitrogen

K channel blockers and some toxins enhance transmitter releaseα-latrotoxin7.06.56.05.55.04.54.03.51ACmV3.02.52.01.51.00.50.0-0.5-1.0-1.50 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200sBEFOREDuringAFTER

Drugs that inhibit transmitter synthesis/storageCholine uptakexxVesicle filling

Drugs that inhibit transmitter releaseCa 2+ channel blockersBotulinum toxins = ‘SNARE’ blockersCa 2+xx

Drugs that block ACh esterasexAntidote:

The “nicotinic” ACh Receptorδαεαchannel poreβACh binding

General scheme for Agonist-Receptor InteractionACh + R ACh-R ACh-R*boundactivatedACh-R 0“desensitized”

Drugs that stimulate ACh Receptors (agonists)

Drugs that block ACh Receptors (antagonists)

Atropined-Tubocurarine

Drugs that block ACh Receptors (antagonists)Example:d-tubocurarineExample:α-bungarotoxin

Drugs that block ACh Receptors (antagonists)Controlα-bungarotoxin10µm α-BTX

xACh antagonists that are used clinically

SuxamethoniumAcetylcholine22 Not broken down by AChE (nmj) Broken down by BuChE (blood plasma) Short lasting Not counteracted by cholinesterase inhibitor Used clinically for brief muscle relaxation “Depolarising blocker”

dTC dTC neo sux sux sux neo direct

Na + Na +SuxK + + +

Na +Na +Sux+K + +

Na + Na +Sux+K +

SuxNa +SuxK +

nACh Receptor Pharmacology - Neuromuscular JunctionAgonist Antagonist/blocker Cholinesteraseinhibitorsnicotine d-tubocurarine (curare) parathionacetylcholine α-bungarotoxin sarincarbachol atracurium ecothiopatedecamethonium pancuronium edrophoniumsuxamethonium (I) suxamethonium(II) neostigmine

Targets of drug action at the neuromuscular junctionvesamicolbotulinium (A-D)hemicholiniumα-bungarotoxind-tubocurarineatracuriumsuxamethoniumedrophoniumneostigminesarin

Summary1. Drugs with effects on neuromuscular synapses may be tested in many ways: bioassayof muscle contractions, iontophoresis during intracellular recording; or patch-clamprecording from single receptors and channels2. Every step in the cycle of ACh synthesis, storage, release, activation and inactivationis a potential target for drug action at the NMJ.3. Drugs affecting storage and release:•4-AP, TEA, latrotoxin, botulinum toxin, agatoxin•hemicholinium, vesamicol4. Drugs that block ACh Esterase:•edrophonium, prostigmine, neostigmine, sarin5. Drugs affecting ACh Receptors:•carbachol, decamethonium, suxamethonium•tubocurarine, a-bungarotoxin, atracurium6. Clinical uses of drugs acting at the NMJ:•muscle relaxants as adjunct to anaesthesia in surgery•treatment of neuromuscular disease