Lec 9

• Cholinergic neurotransmission

  Acetylcholine is a major neurotransmitter
• Locations of acetylcholine as a neurotransmitter

  • Somatic nervous system: at neuromuscular junction
  • Autonomic nervous system: both sympathetic and parasympathetic preganglionic neurons, parasympathetic postganglionic neurons
  • Central nervous system: memory and behavior
• Acetylcholine


  • Onium group: trimethylammonium optimal for fitting pocket with Asp, hydrophobic regions
  • Ester group: H bond formation
  • Size (five-atom rule): ethylene bridge (no more, no less) between onium and ester group is critical
• Modifications on the ethylene group of acetylcholine

  • α substitution: decreased overall activity, but more for M receptors
  • β substitution: decreased N receptor activity
• Neurotransmission


  1. Biosynthesis from choline and acetyl CoA

  2. Storage in membrane-bound vesicles/granules

  3. Release of neurotransmitter

  4. Binding with receptor

  5. Breakdown by acetylcholinesterase (AChE)

  6. Reuptake of choline for recycling
• Acetylcholinesterase inhibition

  Prolonging the cholinergic effect on the post-synaptic nerve
• Cholinergic receptors

  • N receptors: coupled to ion channels (rapid responses)
  • M receptors: coupled to G protein
• N receptors

  • N1/NM - at neuromuscular junction; related to muscle contraction; neurotransmission translates to direct activity
  • N2/NN - at autonomic ganglia; more related to neurotransmission (conveying of message or activity)
• M receptors


  • M1 (Gq): neural - memory and learning (in CNS), gastric secretion
  • M2 (Gi): cardiac - heart relaxation
  • M3 (Gq): glandular - smooth muscle contraction (lungs, exocrine glands), vasodilation
• Acetylcholine


  • Use: intraocular miotic
  • Not given systemically: lacks specificity when given, DOA too short due to rapid hydrolysis
• Methacholine


  • Addition of methyl group in the beta position makes it specific for muscarinic receptors and protects the ester bond from hydrolysis, making it more stable than acetylcholine
  • Previously used to diagnose bronchial asthma
• Carbachol

  • Choline chloride carbamate: carbamate more stable than ester, has longer duration
  • Use: topical miotic for glaucoma
• Bethanechol


  • β-methylcholine chloride carbamate: more specific for M3 receptors, combined steric and electrostatic effects allow for longer duration of action
  • Use: treatment of urinary retention and stomach distention after surgery
• Pilocarpine


  • Imidazole alkaloid, adopts conformation similar to muscarine which is similar to methacholine, agonist at M3 receptors
  • Use: miotic for glaucoma
• Cholinesterase inhibitors

  • Prevent the breakdown of acetylcholine in the synapse, prolonging its effects
  • Two types: AChE inhibition causes indirect cholinergic agonist effect, Butyrylcholinesterase (BuChE) has broader substrate specificity
• Binding of cholinesterase inhibitors


  • Reactive groups: acetyl, carbamyl, phosphoryl groups
  • Bond strength determines usefulness and toxicity: carbamates have slower enzyme dissociation than acetyl groups (reversible), organophosphates have even slower dissociation (irreversible)
• Physostigmine/eserine


  • Indole alkaloid, carbamate group provides slow dissociation, doesn't necessarily have to fit in the active site, just needs to reach the binding regions and block the enzyme
  • Use: topical miotic, systemic antidote for atropine
• Neostigmine


  • Derivatized from physostigmine, has quaternary ammonium and carbamate group in meta position for better acetylcholinesterase inhibition
  • Use: treatment of myasthenia gravis, antidote to neuromuscular blocking agents
• Pyridostigmine


  • Further improvement on neostigmine structure, carbamate AChE inhibitor, one-fifth as toxic as neostigmine due to quaternary ammonium limiting CNS effects
  • Use: treatment of myasthenia gravis
• Edrophonium


  • Structure related to neostigmine but has no carbamate group, results in rapid onset and short duration of action
  • Use: specific antidote for curare poisoning, diagnostic agent for myasthenia gravis
• Curare


  Damages the synapses of the neurons in patients, resulting in a decrease in the number of receptors present
• Neostigmine


  Chance to activate the little neurons still present in patients with curare-induced damage
• Curare


  Active in blocking the cholinergic effects at the nicotinic muscular receptor
• Quaternary ammonium compounds


  • Unable to cross BBB, limited CNS toxicity unlike physostigmine due to the relative permanent positive charge
• Pyridostigmine


  Further improvement in the structure of neostigmine, carbamate AChE inhibitor, used for treatment of myasthenia gravis, one-fifth as toxic as neostigmine
• Edrophonium


  Structure related to neostigmine, but has no carbamate group, resulting in rapid onset and short duration of action, used as a specific antidote for curare poisoning and diagnostic agent for myasthenia gravis
• Malathion, parathion


  Organophosphate esters, irreversible inhibitors of acetylcholinesterase, permit toxic acetylcholine accumulation
• Organophosphates


  • A and X (not necessarily a halogen) must increase the electrophilic character of the phosphorus atom, the phosphorus atom must be highly electrophilic, increasing the electronegativity of phosphorus promotes a nucleophilic reaction, A is usually O or S but can be Se, R groups provide lipophilicity and dermal absorption
• Organophosphates


  • Sarin (warfare agent), malathion (insecticide, less toxic to humans), parathion (more toxic insecticide)
• Activation of phosphorothioates


  Malathion to malaoxon, parathion to paraoxon - sulfur group is replaced by oxygen, resulting in a higher electrophilic phosphorus atom, more capable of binding with the acetylcholinesterase enzyme
• Cholinergic blockers


  Bind to receptor but do not change its structure in a manner similar to agonists
• Cholinergic blockers


  • Cationic nitrogen is critical for agonism, charge diffusion creates antagonist, ester group is needed for hydrogen bonding and may be absent, distance between nitrogen and ester group is still important, addition of a cyclic aromatic group promotes binding to cholinergic receptors
• Atropine


  Tropane alkaloid, competitive muscarinic blocker, used as mydriatic, to decrease secretions prior to surgery, and as an antispasmodic, can cross the blood-brain barrier and cause CNS toxicity
• Scopolamine/hyoscine


  Tropane alkaloid, acts as a CNS depressant (vs CNS stimulant atropine), used for prevention of motion sickness, can induce twilight sleep
• Ipratropium


  Quaternary ammonium derivative of atropine, permanently ionized and cannot cross the blood-brain barrier, used for inhalation in acute asthma and COPD
• Synthetic cholinergic blockers


  • Modifications to the tropane ring and tropic side to produce specific blockade of antispasmodic, antisecretory, and mydriatic effects, quaternization increases potency and may introduce ganglionic blockade
• Cyclopentolate, tropicamide


  Aminoalcohol ester and aminoamide respectively, used as mydriatics
• Dicyclomine/dicycloverine


  Aminoalcohol ester, tropane ring modification specific for M1 and M3 muscarinic receptors (gastric and secretory), tropic acid modification reduces neurotropic effects, used as a GI antispasmodic
• Biperiden


  Aminoalcohol with a tertiary ammonium group to allow blood-brain barrier crossing, used as an anticholinergic agent for Parkinson's disease
• Neuromuscular blocking agents


  Act at the neuromuscular junction, where there is no membrane sheath, allowing quaternary ammonium compounds to easily reach the nicotinic receptors