HuBio 543 September 19, 2007 Neil M. Nathanson K-536A, HSB 3-9457 nathanso@u.washington.edu Drugs acting at the neuromuscular junction

Autonomic Alternative Katzung, “Basic and Clinical Pharmacology” 10th ed. , 2007 (accessmedicine.com) Autonomic Amusements Lewis & Elvin Lewis, “Medical Botany” Davis, “The Serpent and the Rainbow”

Motor neurons cell bodies are in the ventral horn of the spinal cord

SynapticTransmission at Cholinergic Synapses ACh Choline + AcetylCoA ACh + CoA Acetyltransferase Ca ++ AChR Na+ AChE Ch +Ac

The Neuromuscular Junction

NMJ at the EM level Nerve Muscle Basal Lamina

Choline + AcetylCoA ACh + CoA Ch +Ac AChE AChR Na+ Acetyltransferase ++ AChR Na+ AChE Ch +Ac

Synaptic vesicles release ACh and then recycle and get refilled

Synaptic Vesicles Releasing ACh

Drugs that Act on Cholinergic Terminals 1. Hemicholinium-3: blocks choline uptake, depletes ACh 2. Vesamicol: inhibits ACh transport into synaptic vesicle 3. Black widow spider toxin: damages terminal; massive release and depletion of ACh 4. Botulinum toxins: taken up into terminal and blocks release of ACh

Bot Toxin A (Botox®): Cleaves a protein on the nerve terminal Bot Toxin B (Myobloc®): Cleaves a protein on the vesicle

Duration of Improvement After Botulinum Toxin Hemifacial Spasm Blepharo- spasm Cervical Dystonia Spasmatic Dysphonia Hand 2 4 6 8 10 12 14 16 Duration of Improvement (Weeks)

Botulinum toxin relieves axillary hyperhidrosis 150 100 Sweat Production (mg/min) 50 2 12 24 Weeks after injection into axilla

Uses of Botulinum Toxin Persistent muscle spasms Poststroke spasticity Treatment of hyperhidrosis Healing of anal fissure May be useful in tension and migraine headaches Elimination of facial wrinkles

Elimination of facial wrinkles by botulinum toxin Before After

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ACh RECEPTORS NICOTINIC MUSCARINIC

Nicotinic ACh Receptor: A Ligand-Gated Ion Channel Na+, Ca2+ ACh ACh

ACh initiates a local depolarization at the synaptic region

Desensitization of nAChR Muscle Depolarization Squirt ACh Bolus ACh

Rclosed + ACh Ropen-ACh Rdesen-ACh

Denervation Supersensitivity Muscle Fiber R Denervate Re-innervate

Therapeutic Uses of Neuromuscular Blockers Bind to nAChR and block neuromuscular transmission Muscle relaxants during surgery Facilitate manipulations during realignment of fractures Decrease physical trauma during electroshock therapy

Competitive Neuromuscular Blockers mivacurium d-tubocurarine

California Addresses Death Penalty Concerns May 16, 2007 California Addresses Death Penalty Concerns By JENNIFER STEINHAUERS State officials said Tuesday that they had addressed the plethora of serious concerns raised by a federal district judge last year over how California executed condemned inmates by lethal injections. These were among the concerns that Judge Fogel raised: -Not using consistent and reliable staff members for executions. -A lack of training and supervision for those staff members. -Poorly maintained records. -Improperly mixed drugs. -A poorly lighted execution area, a former gas chamber, preventing the proper monitoring of prisoners during executions.

Reversal of d-tubocurarine block of muscle contraction by neostigmine

Depolarizing Blockers Partial agonists- first activate, then desensitize

Apnea After Cessation of Administration of SuCh 40 80 120 160 100 200 Dose (mg) Duration of Apnea (min.) Normals Low pChE

ACh Interacts With AChE Acylated Enzyme Intermediate Active Enzyme

Edrophonium N CH2CH3 CH3 OH + Associates with anionic site of enzyme Hydrogen bonding group

Carbamate Esters

Neostigmine Interacts With AChE Carbamylated Enzyme Intermediate Regenerated Enzyme

DFP Interacts With AChE Phosphorylated Enzyme

Regenerated Enzyme Phosphorylated Enzyme Pralidoxime Regenerated Enzyme

Aging of phosphorylated AChE H C – O 7 3 P – O ENZYME HO Phosphorylated enzyme Aged enzyme The “aged” enzyme can no longer be reactivated by 2-PAM

Noncompetitive inhibitors of AChE Bind to a site distinct from the catalytic site to inhibit enzyme activity Used to improve/delay loss of cognition in patients with Alzheimer’s (to counteract the effects of the loss of cholinergic neurons) Tacrine- first drug approved for treatment of Alzheimer’s; many actions in addition to AChE inhibition; high incidence of liver toxicity Donepezil- more specific for inhibition of AChE; delays progression of symptoms; side effects- diarrhea, etc.- expected based on cholinergic stimulation-

Myasthenia Gravis Scherer et al., JAMA 293, 1906 (2005)

Myasthenia Gravis Autoimmune Disease- Neuromuscular Weakness Antibodies vs. nAChR- decrease nAChR # and Function Treat with: AChE Inhibitors- increase [ACh] at the NMJ- improves efficiency of NM transmission (neostigmine, pyridostigmine) Also: use a muscarinic receptor antagonist (e.g., atropine) to counter effects of increased ACh at muscarinic synapses