Mattingly - Cholinergic Pharmacology II

Question 1

What drug can inhibit sodium-dep choline transporter (CHT)?

Answer 1

Hemicholinium

Question 2

What does ChAT do?

Answer 2

Acetylcholine is synthesized from choline and acetyl Co-A (AcCoA) by the enzyme choline acetyltransferase (ChAT).

Question 3

What does VAT do? What inhibits VAT?

Answer 3

ACh is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol.

Question 4

What are SNAPs and VAMPs?

Answer 4

SNAPs, synaptosome associated proteins; VAMPs, vesicle-associated membrane proteins]

Question 5

General Cholinergic Synapse: 7 steps

Answer 5

1. Choline transported into presynaptic nerve terminal by Na-dependent choline transporter (CHT)
   o Blocked by hemicholinium drugs
2. ACh is synthesized in the cytoplasm from choline and acetyl CoA by ChAT
3. ACh transported into the storage vesicle by the vesicle associated transporter (VAT)
   o VAT inhibited by vesamicol
4. Peptides, ATP and proteoglycan also stored in the vesicle
5. Voltage sensitive Ca++ channels in terminal membrane open, Ca++ enters nerve terminal
6. Increase in Ca++ causes fusion of vesicles with surface of membrane and exocytosis of ACh and co-transmitters
   o Blocked by botulinum toxin
   o Receptors on presynaptic nerve terminal regulate transmitter release (SNAPs, VAMPs)
7. ACh action terminated by AChE

Question 6

Direct Acting Agonist: (3)

Answer 6

Direct Acting Agonist: mimic ACh at AChR (produce same conformational changes in AChR)

o Muscarinic: pilocarpine, methacholine, bethanechol

o Muscarinic and Nicotinic: ACh, carbachol

o Nicotinic: nicotine, varenicline (partial nicotinic agonist)

Question 7

Direct Acting Agonist:

Muscarinic (3)

Answer 7

Muscarinic: pilocarpine, methacholine, bethanechol

Question 8

Direct Acting Agonist: 2

Muscarinic and Nicotinic (2)

Answer 8

Muscarinic and Nicotinic: ACh, carbachol

Question 9

Direct Acting Agonist: 2

Nicotinic (2)

Answer 9

Nicotinic: nicotine, varenicline (partial nicotinic agonist)

Question 10

Black widow spider venom

Answer 10

• Promote release of NT by acting presynaptically:

Black widow spider venom: promotes sustained release of ACh causing skeletal muscle twitching; if systemic, produces other signs of cholinergic activity (sweating, abdominal cramps, bradycardia)

Question 11

Anticholinesterase:

Answer 11

Anticholinesterase (Cholinesterase Inhibitors/Indirectly-Acting Cholinomimetics): agents occupy ACh binding sites on cholinesterases to prevent destruction of ACh

Question 12

Anticholinesterase:

Reversible (8 Examples)

Answer 12

Reversible:

Metabolized or removed from binding sites within minutes to a few hours

Ex:
Physostigmine, rivastigmine
Neostigmine, pyridostigmine

Simple, competitive inhibitors of ACh binding to active site

Ex:
Edrophonium*, tacrine, donezepil, galantamine

Question 13

Anticholinesterase:

Irreversible (3 Examples)

Answer 13

Irreversible: stable covalent binding to esterases and remain on the binding site for hours to days

Echothiophate
Sarin (nerve gas)
Malthion, parathion (insecticides)

Question 14

AChR Antagonists:

Answer 14

AChR Antagonists: agent with little/no intrinsic activity occupies AChR, preventing ACh from reaching it

Question 15

Reversible Anti-Muscarinic:

Exs:

Answer 15

Reversible Anti-Muscarinic: competitive antagonist

Atropine*, scopolamine, tolterodine, oxybutynin, tropicamide, benzotropine, iptratropium, tiotropium, propantheline

Question 16

Reversible Anti-Nicotinic:

Exs:

Answer 16

Reversible Anti-Nicotinic: competitive antagonist

D-tubocurarine, atracurium, pancuronium, vecuronium, rocuronium
Mecamylamine

Question 17

What blocks synthesis of ACh presynaptically?

Answer 17

Blocking synthesis of ACh presynaptically:

Hemicholinium 3: blocks the transport of choline into the nerve terminal, causing gradual depletion of ACh (no therapeutic use)

Question 18

What prevents release of NT from presynaptic site? (3)

Answer 18

Botulinum Toxin A
Aminolycoside Abx
High serum Mg

Question 19

Botulinum Toxin A (Botox): only therapeutic agent for this use

Local injection to treats:

Effects last:

Answer 19

Prevent release of NT from presynaptic site
Botulinum Toxin A (Botox): only therapeutic agent for this use

Local injection to treat spastic disorders (ie. strabismus or spasmodic torticollis) and wrinkles

Effects last a few months, with restoration of neurotransmission via establishment of new neuroeffector junctions

Subsequent treatments may be less effective due to production of neutralizing Abs

Question 20

Amingoglycoside Abx:

Answer 20

Prevent release of NT from presynaptic site

Inhibit Ca++ movement through cholinergic nerve terminals, reducing exocytosis of ACh

Question 21

High Serum Mg++:

Answer 21

Prevent release of NT from presynaptic site

High Serum Mg++: due to competitive interference with Ca++ entry into presynaptic terminal

Question 22

Depolarizing Blockade:

Examples

Answer 22

Excessive cholinergic agonist stimulation of nicotinic R at NMJ results in depolarization of motor end plate and blocks transmission to the skeletal muscle sarcolemma (paralysis)

Succinylcholine: used therapeutically for this reason

Nicotine: depolarizing blockade can be a toxic effect

Question 23

Muscarinic Pharmacology:

Effect of ACh:

Effect of longer acting analogs
Types:

Answer 23

ACh: direct agonist with limited therapeutic use because of rapid hydrolysis in plasma by psedocholinesterases

Longer acting analogs of ACh: metabolized more slowly and can be employed for systemic effects

• Tertiary Amines: can be employed orally for systemic effects
• Quaternary Amines: need to be applied directly to the appropriate site or administered parenterally for systemic effects

Question 24

Muscarinic pharmacology in the CNS:
Muscarinic Agonists:

Toxicity:

Clinical Trials:

Answer 24

Muscarinic Agonists: most of the clinically used agonists are quaternary amines and do no cross the BBB

Toxicity: CNS symptoms may occur in poisoning with mushrooms containing muscarine

Clinical Trials: selective M1 agonist that crosses BBB for used in AD

Question 25

Atropine: MOA: CNS effects: Toxic? Effects due to:

Answer 25

Atropine: prototype; natural alkaloid; tertiary amine MOA: competitive antagonist of ACh at muscarinic receptor sites (can therefore be displaced by muscarinic agonists) CNS Effects: only seen at toxic levels (coma and death may follow) Significantly toxic (“red as a beet, dry as a bone, blind as a bat, mad as a hatter”) Effects due to blocking of MR in brainstem and cortex (has a less depressing effect on ascending reticular core than scopolamine)

Question 26

Atropine: use and pharmacokinetics

Answer 26

Use: Treatment of mushroom poisoning (muscarine) and organophophate poisoning Alleviates some motor symptoms of Parkinson’s Disease Pharmacokinetics: Absorption: well absorbed orally, parenterally, and topically (through skin) Distribution: widely ( to all body compartments) Metabolism: liver Excretion: some excreted unchanged in the urine

Question 27

Scopolamine: MOA: CNS effects: (4)

Answer 27

Scopolamine: natural alkaloid; tertiary amine MOA: same as atropine CNS Effects: has effects at therapeutic levels (not just toxic levels) 1. Effects due to blocking MR in brainstem and cortex (has 100x more depressing effect on ascending reticular core than atropine) 2. Hypnotic activity with amnesia 3. Occasionally produce delirium and hallucinations at therapeutic levels 4. Very effective anti-motion sickness drug (transdermal patch) o Note: site of action in brain stem between semicircular canals of inner ear and emetic center in medulla; will not block emetic action of agents triggering CTZ

Question 28

Scopolamine: pharmacokinetics

Answer 28

Absorption: well absorbed orally, parenterally; very good topical absorption Distribution/Metabolism/Excretion: same as atropine

Question 29

Benzotropine: Use:

Answer 29

Use: adjunct in oral treatment of Parkinsonian tremor and ridgidity (including those produced by anti-psychotics) Used a lot in nursing homes (possibly due to sedative action)

Question 30

Muscarinic pharmacology in the eye Muscarinic Agonists: Primary target: What does contraction result in? Exs:

Answer 30

Primary Therapeutic Target: sphincter pupillae muscle of the iris (circularly-oriented and controlled by PS innervation) Acts in concert with radially-oriented group of muscle fibers controlled by SS neurons to control the amount of light admitted into the posterior chamber of the eye Contraction (ie. due to application of muscarinic agonist) results in pupillary constriction (miosis) to reduce light entering the eye Piliocarpine and Carbachol

Question 31

Pilocarpine: Structure: Application: Use: standard and emergency Mechanism of Use in Glaucoma:

Answer 31

Muscarinic Agonist Structure: tertiary amine from a plant Application: primarily topical for ophthalmology application Use: - Standard cholinergic direct agonist for management of open-angle glaucoma (still second like to other drugs) - Emergency therapy of a narrow-angle glaucoma attack Mechanism of Use in Glaucoma: enhance drainage of intraocular fluid from anterior chamber of the eye to reduce abnormally high intraocular pressure - Contraction of sphincter pupillae opens canal of Schlemm and other drainage routes around the periphery of the iris

Question 32

Carbachol: Structure: Activity: Metabolism: Use: What is used as a miotic agent during eye surgery?

Answer 32

(aka carbamylcholine) Muscarinic Agonist Structure: synthetic quaternary amine Activity: agonist activity at ALL cholinoreceptors (muscarinic and nicotinic) Due to widespread activity, usually only employed topically as a miotic agent Metabolism: not degraded by cholinesterases Use: Miotic agent during eye surgery such as lens replacement (acetylcholine used for this as well; NOT pilocarpine)

Question 33

Muscarinic pharmacology in the eye Muscarinic Antagonists: Primary target: Cholinergic agonists vs antagonists

Answer 33

Primary Thearapeutic Target: ciliary muscle that controls the shape of the lens Contracts to increase convexity of lens to focus on near objects Cholinergic AGONISTS can produce transitory blurring of distance vision Cholinergic ANTAGONISTS (ie. atropine) cause inability to focus on near objects

Question 34

Muscarinic pharmacology in the eye Muscarinic Antagonists: Actions: Uses:

Answer 34

Action: Cause cycloplegia (paralysis of the ciliary muscle) resulting in a FIXED LENS that only focuses at a DISTANCE - Cause mydriasis (dilation of the pupil) Use: - Prescribing corrective lenses for the eye (short-acting agent tropicamide)

Question 35

Muscarinic pharmacology in the eye Muscarinic Antagonists: Issues, side effects, and contraindications

Answer 35

General Issues: Atropine and scopolamine are READILY absorbed through sclera and cornea (topical), but produce long-lasting effects (several days) - Oral or parenteral administration produces lesser and shorter duration effects on the eye Side Effects: - Photophobia (dilated pupil) - Difficulty with near vision - Potentially elevated intracocular pressure to dangerous levels* Contraindications: - Do not use long acting agents in people with glaucoma or a predisposition to glaucoma (ie. shallow anterior eye chamber) - This is why atropine/scopolamine are not routinely uses to examine the eyes for corrective lenses (use tropicamide instead)

Question 36

What is routinely used instead of atropine and scopolamine in the eye?

Answer 36

tropicamide

Question 37

Muscarinic Agonists and the Heart: Type of receptors used: Results:

Answer 37

M2 | Decreases cellular activity when stimulated

Question 38

Mechanism of M2-R in heart Effects of Gi beta/gamma: alpha:

Answer 38

Increase plasma membrane K+ permeability (hyperpolarization) Gi beta/gamma subunits act directly on K+ channels Gi alpha subunit inhibits adenylate cyclase to decrease cAMP (counteracts stimulator effects of beta-adrenoceptor stimulation

Question 39

Muscarinic Agonist Action on the Heart: Targets: Effects of M2 activation

Answer 39

Targets: SA node, atria and AV node Effects of M2 activation: • slows heart rate (negative chronotropic action) • decreases the force of contraction (negative inotropic action) • suppresses AV conduction (negative dromotropy).

Question 40

Muscarinic agonists in the heart: chronotropic, inotropic, dromotropic effects

Answer 40

Negative Chronotropic Action: slows heart rate o Effects on pacemaker cells due to effects on AP and diastolic depolarization phases of the cycle o Reduced slope of diastolic depolarization (due to hyperpolarization because of K+ permeability) o Shortened AP (due to increased K+ permeability during repolarization) Negative Inotropic Action: decreases force of contraction o Shortened AP --> less Ca++ influx and release --> less Ca++ available for contractile elements in the cell --> weaker contraction Negative Dromotropic Action: suppresses AV conduction

Question 41

Muscarinic Agonists and the Vascular System Vascular Effects: Cholinergic nerve stimulation of vascular system limited to: All vascular beds have:

Answer 41

Muscarinic Agonists and the Vascular System: Vascular Effects: most prominent with ACh administered by IV (experiemental) Cholinergic nerve stimulation of vascular system limited to ONLY sympathetic fibers that supply resistance vessels of skeletal muscle However, all vascular beds have M3 receptors on endothelium that promote vessel dilation.

Question 42

Mechanism of Vessel Dilation: Activation of M3 receptors via: Gq protein activation causes Ca+ stimulated NO synthase to produce:

Answer 42

Muscarinic agonists in the vascular system: Mechanism of Vessel Dilation: Activation of M3 receptors via ACh in blood stream Gq protein activation causes release of intracellular Ca++ Ca+ stimulated NO synthase to produce NO/EDRF (endothelial-derived relaxing factor)

Question 43

Muscarinic agonists in the vascular system: Result of ACh Administration: low dose, higher dose, even higher dose:

Answer 43

Low Dose by IV: metabolized quickly by plasma cholinesterases so the only actions are on the vascular system - Vasodilation --> Decreased blood pressure --> Triggers baroreceptor reflex --> Increase in SS tone to the heart --> Reflex tachycardia Higher Dose by IV: causes cardiac depression via direct action of ACh on the cardiac M2 receptors Even Higher Doses by IV: complex results because of additional cardiac stimulation from activation of SS ganglia nAChR and release of NE

Question 44

If Atropine Given before ACh in the vascular system:

Answer 44

If Atropine Given First: will always get tachycardia, regardless of dose Atropine blocks all the muscarinic receptors, leaving only nicotinic receptors for ACh to act on ACh stimulates nicotinic receptors to cause release of catecholamines

Question 45

Use of Acetylcholine in Diagnostic Angiography:

Answer 45

Direct intracoronary injection of ACh Normal Response: vasodilation subsequent to NO/EDRF release from endothelium Patients with Vasospastic Angina Pectoris: results in spasm of coronary artery

Question 46

Atropine (Muscarinic Antagonist) Cardiac and Vascular Effects:

Answer 46

Atropine (Muscarinic Antagonist) Cardiac and Vascular Effects: Cardiac Effects: produces prominent and prolonged tachycardia and increased AV conduction - Blocks all cardiac effects of the vagus - Blocks all depressant effects of exogenously administered cholinomimetic drugs - Recall that low doses of atropine may cause paradoxical bradycardia

Question 47

Cardiac Uses of Atropine: (2)

Answer 47

Sinus bradycardia occurring after MI Individuals with hyperactive carotid sinus reflexes (rare)

Question 48

Vascular Effects of Atropine: Location: Sign of toxicity:

Answer 48

Vascular Effects: has little effect on BP but does promote some cutaneous vasodilation via reflex mechanism (red as a beet) Location: blush areas and upper thorax (more pronounced and more deadly in children) Sign of toxicity: at toxic levels of atropine, skin becomes very red, warm and dry (“atropine flush”); related to thermoregulation because atropine blocks formation of sweat