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Cholinergics
Drug
28
Biology
Graduate
03/15/2012

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Term
Latrotoxin
Definition
Black widow venom
Destroys synaptic vesicles, resulting in flooding of Ach and synaptic failure
Presentation: Localized diaphoresis, weakness, myoclonus, local paresthesias, abdominal pain, can result in CV collapse, pulmonary edema, ileus
Term
Beta Bungarotoxin
Definition
Tropical snake venom
Destroys synaptic vesicles, resulting in flooding of Ach and synaptic failure
Presentation: Localized diaphoresis, weakness, myoclonus, local paresthesias, abdominal pain, can result in CV collapse, pulmonary edema, ileus
Term
Botulinum Toxin
Definition
Botulism
Caused by ingestion of C. botulinum spores
Found in poorly canned foods, honey (infants susceptible)
Toxin inhibits vesicle release  paralysis
Local tx for spastic disorders, wrinkles (Botox)
Term
3,4-diaminopyridine
Definition
Increase acetylcholine release by activating K channels
Tx used for Lambert-Eaton myasthenic syndrome
RARE, first defined by Mayo Drs Lambert, Eaton in 1957
Autoantibodies to presynaptic Ca channels block Ach release (often part of paraneoplastic syndrome)
Presentation: Slowly progressive prox muscle weakness
Autonomic dysfunction often seen: Dry mouth, constipation, impotence in men
Tx includes 3,4-diaminopyridine (Side effects often mild - paresthesias)
Term
Nicotinic “Receptor”
Definition
Ligand-gated sodium channel
Ach  Increase Na+ influx  depolarization
Term
Muscarinic
Definition
G-protein coupled receptor
α (αs, αi, αq, and αo) → binds GTP, main effector
Term
Nicotine
Definition
- Preferentially targets ganglionic nAchRs, agonist short term then antagonist chronically
- Effects mainly in CNS
- Peripheral: Mixture of sympathetic/parasymp symptoms
Effects decline with repeated dosage
CV: Tachycardia / ↑ CO, BP
↓GI motility, nausea, vomiting
Diaphoresis
Term
Hexamethonium
Definition
Ganglionic Antagonists
- First effective antihypertensive
- No longer used clinically
Term
Trimethaphan
Definition
Ganglionic Antagonists
- Competitive ganglionic nAchR blocker
- Short-acting (must be given IV)
- Uses : Emergent perioperative lowering of BP
Term
Muscarine
Definition
- Poison derived from mushrooms
- Bind muscarinic receptors
- Effects: Ach-like
Term
Ach derivatives
Definition
Have more targeted effects
More resistant to degradation by ACE
Acetylcholine
Bethanechol
Carbachol
Pilocarpine
Term
Bethanecol
Definition
Bowel & Bladder
- More stable than carbachol
- Selective for muscarinic receptors
- Uses: Urinary retention (eg. postop)
Neurogenic ileus
Term
Pilocarpine
Definition
- Selective for muscarinic receptors
- Very stable – effects last one day
- Uses: Local for open/closed angle glaucoma
Ciliary contraction opens Schlemm’s canal for drainage of aqueous humor
- Side effects:
- Can enter CNS and cause disturbances
- Profuse sweating and salivation
Term
Muscarinic Antagonists
Definition
Block muscarinic receptors (little or no effect on nicotinic “receptors” [nAchRs])

Much more clinically useful than muscarinic agonists

Inhibit muscarinic functions
- Parasympathetic
- Sympathetic innervation of sweat glands (mAchRs)
Term
Atropine
Definition
Muscarinic Antagonists
Competitively binds and inhibits mAchRs
Effects last up to 4 hours (days in eye)
Uses:
-Eye – mydriasis and cycloplegia for eye exams
- Antispasmodic for bladder/GI
- Cholinergic overdose (eg. Mushroom poisoning, organophosphate poisoning)
- Preop antisecretory agent for respiratory tract
Term
Atropine Side effects
Definition
- “Red as a beet” (cutaneous flushing)
- “Hot as a hare” (fever)
- “Mad as a hatter” (CNS disturbances)
- “Dry as a bone” (Dry mouth)
- Events in older pts with narrow angle glaucoma
Term
Scopolamine
Definition
Similar peripheral effects as atropine
Stronger CNS effects including short-term memory loss
Uses:
- Motion sickness
- Occasionally as amnesic in anesthesia
Side effects similar to atropine
Term
Nondepolarizing (competitive) blockers
Neuromuscular Antagonists
Definition
- Inhibit AchRs at the motor endplate
- Causes flaccid paralysis
- Can be overcome with anticholinesterases
Term
Depolarizing agents
Neuromuscular Antagonists
Definition
- Activate AchRs repeatedly (agonism!)
- Produces first fasciculations followed by paralysis
Term
Neuromuscular Antagonists : Competitive - Curare
Definition
Derived as an alkaloid from S.A. plants (darts)
Causes flaccid paralysis
-Small, rapid then respiratory muscles
Derivatives used as adjuvant muscle relaxants in anesthesia (various durations – 15 min – hrs):
-Tubocurarine
-Pancuronium
-Vecuronium
-Atracurium
Term
Neuromuscular Antagonists : Depolarizing - Succinylcholine
Definition
Binds and activates nAch (Resists degradation by ACE)
Mechanism:
- Phase I – Depolarization  fasciculations
- Phase II – Channels inactivate upon chronic stimulation (minutes)  paralysis
SHORT action (< 10 min) – given IV
Uses : Short procedures like endotracheal intubation during induction of anesthesia
Side effects:
- Malignant hyperthermia with halothane (Dantrolene)
- Apnea
Term
Anticholinesterases
Definition
Inhibit ACE activity to increase Ach signaling
Reversible
Irreversible
“SLUDGE” Drugs
Term
Myasthenia Gravis
Definition
Most common NM transmission disorder (still uncommon)
Autoantibodies to nAchR and degrade the receptor
Symptoms : Ocular (ptosis, diplopia), Bulbar (fatigable chewing), Respiratory (myasthenic crisis)
Tx – Anticholinesterases to prolong effect of Ach:
- Edrophonium – Diagnosis
- Pyridostigmine, Neostigmine – Symptom management
- Immunosuppressives, possible thymectomy
Term
Edrophonium
Definition
Anticholinesterases: Reversible
Short-acting (10-20 min)
Diagnosis of myasthenia gravis (MG)
Competes with autoantibodies to the nAchR
(Currently being phased out as a diagnostic test)
Term
Neostigmine
Definition
Medium-acting
Anticholinesterases: Reversible
(2-4 hrs) – Oral symptomatic tx of MG
– Neurogenic ileus and urinary retention postop
Term
Pyridostigmine
Definition
Medium-acting
Anticholinesterases: Reversible
Symptomatic tx of MG
Term
Physostigmine
Definition
Anticholinesterases: Reversible
Medium-acting
(2-4 hrs) – Topical tx of glaucoma
– Antidote to atropine poisoning
Term
Organophosphates
Definition
Anticholinesterases: Irreversible
Mechanism:
- Irreversibly binds ACE active site
- Alkyl group is released (irreversible aging)
- PRALIDOXIME rescues ACE ONLY if tx before aging occurs
Cholinergic stimulation followed by paralysis
Uses :
- Insecticides
- Nerve gas (DFP, soman : “age” in seconds – minutes)
- Echothiophate – open-angle glaucoma
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