Overview - Indirect cholinergic agonism (AchE inhibition) - Muscarinic antagonism (emphasis on drugs and organ effects) - Nicotine-Ach receptor (emphasis on drugs and therapeutics)

Indirect cholinergic agonists Inhibitors of acetylcholinesterase Increase acetylcholine concentration and lifetime by inhibiting degradation Act by binding to AchE active site causing reversible (non covalent) or long lasting (covalent modification)

Mechanisms of indirect agonism - Quaternary alcohols (ephodronium) – reversible binding (limit acetylcholine access) Non covalent – enzyme-inhibitor complex lifetime (2 - 10 minutes) Carbamate esters – neostigmine, physostigmine – reaction with AchE active site Covalent carbamoylation – enzyme-inhibitor complex lifetime (0.5 - 6h) Organophosphates – Parathion, Sarin, Soman - phophorylates AchE active site Covalent phosphorylation – very stable enzyme-inhibitor complex, days (especially after aging)

AchE Inhibitors (quaternary alcohols and carbamates)

Organophosphates

Aging

Organ effects/therapeutic uses - Effects are due to acetylcholine accumulation and are both sympathetic and parasympathetic USES Approx Duration ALCOHOLS Edrophonium Myasthenia gravis 5 – 15 minutes arrythmias CARBAMATES Neostigmine Myasthenia gravis 0.5 – 2h Pyridostigmine Myasthenia gravis 3 – 6h Physostigmine Glaucoma 0.5 – 2h Demecarium Glaucoma 4 – 6h Organophosphates Echotiophate Glaucoma 100 h (> 4 days)

Treatment of organophosphate poisoning 1 - maintenance of vital signs (respiration particularly important) 2 - Decontamination (to avoid further absorption) 3 - Atropine parenterally (to minimize muscarinic effects) as required 4 - Rescue of AchE activity with Hydroxylamines (Pralidoxime, Diacetylmonoxime)

Muscarinic antagonism Attropa belladona

Muscarinic Antagonists ATROPINE SCOPOLAMINE

Muscarinic Antagonists ATROPINE Attropa belladona SCOPOLAMINE Atropine and Scopolamine are belladona alkaloids (competitive inhibitors) Drugs differ in their CNS effects, scopolamine permeates the blood-brain barrier At therapeutic doses atropine has negligible effects upon the CNS, scopolamine even at low doses has prominent CNS effects.

Mechanism of drug action Competitively block muscarinic receptors - Salivary, bronchial, and sweat glands are most sensitive to atropine - Smooth muscle and heart are intermediate in responsiveness In the eye, causes pupil dilation and difficulty for far vision accomodation Relaxation of the GI, slows peristalsis

History/sources Atropa belladona - used in the renaissance Deadly nightshade - used in the middle ages to produce prolonged poisoning Jimson plant leaves burned in India to treat Asthma (1800) purification of atropine (1831)

Effect of muscarinic inhibitor in the eye Pupil dilation vs accomodation

Effect of muscarinic inhibition in the heart and salivary glands Increases the heart rate after a transient bradychardia at the low dose Diminishes gland excretory function

Graphic summary of atropine effects

Organ effect – drug review Antidotes ORGAN DRUG APPLICATION CNS Benztropine Treat Parkinson’s disease Scopolamine Prevent/Reduce motion sickness Eye Atropine Pupil dilation Bronchi Ipatropium Bronchodilate in Asthma, COPD GI Methscopolamine Reduce motility/cramps GU Oxybutinin Treat transient cystitis Postoperative bladder spasms

Toxicity of muscarinic antagonists “DRY AS BONE, RED AS A BEET, MAD AS HATTER.” Dry is a consequence of decreased sweating, salivation and lacrimation Red is a result of reflex peripheral (cutaneous) vasodilation to dissipate heat (hyperthermia) Mad is a result of the CNS effects of muscarinic inhibition which can lead to sedation, amnesia (hypersensitivity), or hallucination

Nicotinic – Acetylcholine Receptor polarized Relaxation depolarized contraction

Signaling through Ach-nicotinic receptor (competitive and depolarizing blockers)

Competitive/depolarizing Binds and locks the receptor open Competitive Physically blocks Ach binding INHIBITOR

Examples of competitive/depolarizing drugs Mivacurium Tubocurarine Depolarizing AchE Butyrylcholinesterase Sensitive sites Succinylcholine

Clinical uses Adjuvant use in surgical anesthesia (muscular relaxation) Advantage – much lighter levels of anesthesia required Other uses: muscular relaxation for orthopedics (correction of dislocation/alignment of fractures) (short duration) – facilitate intubation, laryngoscopy, bronchoscopy, esophagoscopy Control of muscular spasms, strabism, hemifacial spasms, oromandibular and cervical dystonia, spasms of the lower esophageal sphincter Cosmetic – Bottox (Botulinum toxin A) Paralytic action on skeletal muscle

Agents/Features/Duration AGENT CLASS PROPERTY ONSET DURATION Succinylcholine Dicholine ester Depolarization 1 min 5 – 8 min Tubocurarine Alkaloid Competitive 5 min 80 – 120 min Atracurium Benzylisoquinoline Competitive 3 min 30 – 60 min Mivacurium Benzylisoquinoline Competitive 3 min 12 – 18 min Pancuronium Ammonio Steroid Competitive 5 min 120 – 180 min Vecuronium Ammonio Steroid Competitive 3 min 60 – 90 min Hydrolysis by esterases Liver clearance/renal elimination Both

Precautions/Toxicity - Prolonged apnea, cardiovascular collapse Sequence of paralysis : Eye muscles, Jaw, Larynx, limbs and trunk, intercostal muscles and the dyaphragm Generally caused by diminished esterase activity, renal malfunction, liver insufficiency, poor circulatory function. Special caution in patients with electrolyte imbalance (K+) Antidote : Neostigmine/Ephodronium to increase Ach, and atropine to block Ach muscarinic stimulation. Malignant hyperthermia – results from a discharge of Ca2+, exacerbated muscular action – tachycardia, hyperthermia, acidosis and rigidity (mutations of RYR1, central core disease) treated with Dantrolene, preservation of respiration

Summary Ach pilocarpine Muscarine Bethanechol Neostigmine** Edrophonium** Atropine Scopolamine Tubocurarine Mivacurium Tetrodoxin Batrachotoxin X X Hemicholinium Botulinum toxin X Curare alkaloids Snake venom α X Dantrolene X AchE inhibitors X Hydrolysis Ach ** Indirect

Movie http://www.youtube.com/watch?v=yd46Hs7pTow Nicotine in the brain