Neurology: Pharmacology - Drugs acting on the autonomic nervous system

Question 1

List four types of cholinergic nerve fibres within the peripheral nervous system

Answer 1

1. Preganglionic efferent autonomic fibres
2. Most postganglionic parasympathetic fibres (some use NO or peptides as primary or cotransmitters)
3. Some postganglionic sympathetic fibres, e.g. to apocrine glands
4. Somatic motor fibres to skeletal muscle

Question 2

Outline the steps involved in cholinergic transmission

Answer 2

1. Acetyl-CoA is synthesised by mitochondria
2. Choline is transported from ECF into cell by Na+-dependent choline transporter (CHT)
3. Choline acetyltransferase (ChAT) catalyses the synthesis of ACh from choline and acetyl-CoA
4. ACh is transported from the cytoplasm into vesicles by vesicle-associated transporter (VAT), driven by proton efflux
5. Most of vesicular ACh is stored bound to vesicular proteoglycan (VPG)
6. Vesicles are concentrated on the inner surface of the nerve terminal close to the synapse via the interaction of SNARE proteins on the vesicle (vesicle-associated membrane proteins, VAMPs) and on the inside of the terminal cell membrane (synaptosomal nerve-associated proteins, SNAPs)
7. ACh release is triggered when action potential reaches the terminal and there is sufficient Ca2+ influx via N-type Ca2+ channels; Ca2+ interacts with VAMP synaptotagmin to produce fusion of the vesicular and nerve terminal membranes
8. ACh is released from VPG and along with other transmitters enters the synaptic cleft
9. ACh binds to and activates cholinoceptors
10. All released ACh eventually diffuses within range of acetylcholinesterase (AChE) enzymes which break down ACh to acetate and choline to terminate transmission (usually a rapid process; ACh half-life within cholinergic synapses is a fraction of a second)
11. ACh release is also modulated by cholinoceptors on the presynaptic membrane

Question 3

List three drugs affecting cholinergic transmission and how each works

Answer 3

1. Hemicholiniums: block CHT
2. Vesamicol: block VAT
3. Botulinum toxin: blocks vesicle release through enzymatic cleavage of two amino acids from one or more fusion (SNARE) proteins

Question 4

What five features of neurotransmitter function pose potential targets for pharmacologic therapies?

Answer 4

1. Synthesis
2. Storage
3. Release
4. Termination of transmitter action
5. Receptor effects

Question 5

Where are adrenergic nerve fibres found within the peripheral nervous system?

Answer 5

Most postganglionic sympathetic fibres

Question 6

What transmitters are released by adrenal medullary cells and why?

Answer 6

Adrenal medullary cells are embryologically analogous to postsynaptic sympathetic neurons and release a mixture of NA and adrenaline

Question 7

Outline the steps involved in adrenergic transmission

Answer 7

1. Tyrosine is transported from ECF via Na+-dependent carrier
2. Tyrosine is converted to dopa by tyrosine hydroxylase (rate-limiting step of catecholamine synthesis)
3. Dopa is converted to dopamine by dopa decarboxylase (in dopaminergic neurons, catecholamine synthesis stops here)
4. Dopamine is transported into vesicle via vesicular monoamine transporter (VMAT): in most adrenergic neurons, dopamine is then converted to NA via dopamine B-hydroxylase (and in the adrenal medulla and parts of the brain, NA is then converted to adrenaline)
5. Vesicles are concentrated on the inner surface of the nerve terminal close to the synapse via the interaction of SNARE proteins on the vesicle (vesicle-associated membrane proteins, VAMPs) and on the inside of the terminal cell membrane (synaptosomal nerve-associated proteins, SNAPs)
6. Catecholamine release is triggered when action potential reaches the terminal and there is sufficient Ca2+ influx via N-type Ca2+ channels; Ca2+ interacts with VAMP synaptotagmin to produce fusion of the vesicular and nerve terminal membranes
7. Catecholamine and cotransmitters (including ATP, dopamine B-hydroxylase and peptides) are released into the synaptic cleft
8. NA binds and activates adrenoceptors (or dopamine to dopamine receptors)
9. Termination of transmission results from: 1) simple diffusion aware from receptor site (catecholamine later metabolised in plasma or liver), and 2) reuptake into nerve terminal via norepinephrine transporter (NET)
10. Regulatory receptors are present on the presynaptic membrane
11. High activity of MAO in the nerve terminal results in significant NA turnover even in resting neuron (metabolic products are then excreted in the urine, so 24-hr urinary metanephrines can be used to provide an indication of level of catecholamine turnover); note this is not the primary mechanism of transmission termination which is described above

Question 8

What is the rate-limiting step of catecholamine synthesis? What drug blocks this step and what is it used for?

Answer 8

Conversion of tyrosine to dopa by tyrosine hydroxylase
Metyrosine: used to treat phaeochromocytoma

Question 9

Why does measurement of 24-hr urinary metanephrines provide an indication of catecholamine turnover?

Answer 9

High activity of MAO in the nerve terminal results in significant NA turnover even in resting neuron
Metabolic products are then excreted in the urine, so 24-hr urinary metanephrines can be used to provide an indication of level of catecholamine turnover

Question 10

List 9 drugs affecting adrenergic transmission and give their mechanism of action

Answer 10

1. Metyrosine: inhibits tyrosine hydroxylase
2. Reserpine: inhibits VMAT to deplete transmitter stores
   3-4. Bretylium and guanethidine: prevent SNARE protein interaction to block vesicle exocytosis
   5-6. Cocaine and TCAs: inhibit NET to increase transmitter activity in synaptic cleft
   7-9. Tyramine, amphetamine and ephedrine (indirectly-acting sympathomimetics): taken up by NET and transported into vesicles by VMAT, resulting in displacement of NA which is then released into the synaptic cleft by reverse transport via NET (i.e. process does not involve vesicle exocytosis)

Question 11

What is the difference between the iris circular and radial muscles?

Answer 11

Iris circular muscle = iris sphincter muscle = pupillary constrictor (contraction produces miosis)
Iris radial muscle = pupillary dilator (contraction produces mydriasis)

Question 12

Compare and contrast the effects of sympathetic and parasympathetic effects on the eye

Answer 12

Iris radial muscle: adrenergic produces contraction via a1-adrenoceptors (causes mydriasis), cholinergic no effect
Iris circular muscle: cholinergic produces contraction via M3-cholinoceptors (causes miosis), adrenergic no effect
Ciliary muscle: adrenergic relaxes via B-adrenoceptors (decreases accommodation), cholinergic contracts via M3-cholinoceptors (causes accommodation for near-vision)
Aqueous humour: adrenergic increases secretion via B-adrenoceptors (raises IOP), cholinergic increases outflow of aqueous humour via ciliary muscle contraction (lowers IOP)

Question 13

What is the effect of cholinomimetics on the eye? Give an example

Answer 13

Miosis, cyclospasm (via ciliary muscle contraction), and decreased IOP
E.g. organophosphate cholinesterase inhibitors

Question 14

What is the effect of muscarinic blockers on the eye? Give an example

Answer 14

Reverse or prevent cholinomimetic changes (i.e. miosis, cyclospasm, decreased IOP)
E.g. atropine

Question 15

What is the effect of alpha agonists on the eye? Give an example

Answer 15

Mydriasis
E.g. phenylephrine

Question 16

What is the effect of B-blockers on the eye? Give an example

Answer 16

Decreases aqueous humour secretion to decrease IOP
E.g. timolol

Question 17

Draw a flowchart that outlines the autonomic and hormonal feedback mechanisms controlling CV function

Question 18

Compare the pharmacokinetics of choline esters, tertiary alkaloids, and muscarine. Give examples of each

Answer 18

All are direct-acting cholinomimetics
Choline esters (e.g. ACh, methacholine): have a permanently charged quaternary ammonia group making them hydrophilic and therefore poorly absorbed and distributed, are all hydrolysed in GIT (and therefore less active via oral route) but specific esters vary markedly in their susceptibility to hydrolysis by cholinesterase (very rapid for ACh)
Tertiary alkaloids (e.g. pilocarpine, nicotine, lobeline): more lipid soluble, well-absorbed from most sites of administration (even topically for nicotine), and primarily renal clearance (accelerated by acidification of urine due to pH partitioning)
Muscarine: quaternary alkaloid (less well-absorbed from GIT compared with tertiary alkaloids)

Question 19

Outline the pharmacodynamics of direct-acting cholinomimetics (specifically their effects on the following organ systems: eye, heart, blood vessels, respiratory, GIT, GU and secretory glands)

Answer 19

Eye: iris circular muscle contraction (miosis), ciliary muscle contraction (accomodation), increased aqueous humour drainage as a result of the preceding (decreased IOP)
Heart: decreased chronotropy (slowed SA pacemaker potential), decreased dromotropy (slowed AV conduction velocity), decreased inotropy and refractory period in atria, to lesser extent decreased ventricular inotropy (less parasympathetic innervation in ventricles compared with atria)*
* overall net effect is complex and relates to local concentration of agonist in heart/vessels and degree of reflex responsiveness (e.g. minimally effective dose of ACh causes vasodilation with reflex tachycardia, large doses cause hypotension and bradycardia)
Blood vessels: vasodilation via EDRF (NO) released from endothelium, vasoconstriction (high-dose direct effect via M3 activation in vascular smooth muscle
Respiratory: bronchoconstriction (likely via M3), increased bronchial gland secretion
GIT: increased motility, increased gland secretion (especially salivary and gastric, to lesser extent pancreatic and small intestine), sphincter relaxation
GU: detrusor contraction, trigone and sphincter relaxation
Secretory glands (lacrimal, apocrine, nasopharyngeal): increased secretion

Question 20

Describe the three types of indirect-acting cholinomimetics and provide an example of each

Answer 20

1. Quaternary alcohols (e.g. edrophonium)
2. Tertiary or quaternary carbamates (e.g. neostigmine)
3. Organophosphates (e.g. echothiophate, sarin)

Question 21

Compare and contrast the pharmacokinetics of quaternary carbamates, tertiary carbamates, and organophosphates

Answer 21

Quaternary carbamates: poorly absorbed (large PO doses required), negligible distribution into CNS, duration of action largely determined by stability of inhibitor-enzyme complex and not metabolism/excretion, metabolised by cholinesterase as well as other non-specific esterases
Tertiary carbamates: well-absorbed from all sites (can be used topically in the eye), reaches CNS (so more toxic than quaternary carbamates), duration of action related to stability of inhibitor-enzyme complex, metabolised by cholinesterase or other non-specific esterases
Organophosphates: well-absorbed (but less stable than carbamates when dissolved in water so limited half-life in the environment), echothiophate highly polar and more stable than other organophosphates, distributed to all parts of body including CNS, malathion rapidly metabolised to inactive form by humans (parathion less so)

Question 22

Compare and contrast the mechanism of action of quaternary alcohols, carbamate esters, and organophosphates

Answer 22

Quaternary alcohol: reversible bind AChE (short-lived action: 2-10mins)
Carbamate esters: undergo 2-step hydrolysis sequence similar to ACh, however covalent bond of carbamoylated enzyme is resistant to second step so breakdown process is prolonged (duration of action 0.5-8hrs)
Organophosphates: bind AChE to form an extremely stable covalent phosphorus-enzyme bond (this bond takes hundreds of hours to hydrolyse); may also undergo “aging” which further strengthens the phosphorus-enzyme bond (strong nucleophiles e.g. pralidoxime may break this bond if given before aging occurs - but aging can occur rapidly e.g. within 10mins for soman)

Question 23

Outline the pharmacodynamics of indirect-acting cholinomimetics

Answer 23

CNS: subjective alerting response at low concentrations, seizure at higher concentrations (may lead to coma and respiratory arrest)
Eye, respiratory, GI, GUT: as for direct cholinomimetics
CV: decreased CO as a result of decreased chronotropy (bradycardia), inotropy (atrial > ventricular) and dromotropy; increased TPR and BP (due to activity at sympathetic ganglia or central sympathetic centres) at moderate doses, hypotension at higher doses
Neuromuscular function: increased strength of contraction (due to prolongation of ACh effect), fibrillation/fasciculation at high doses, depolarising +/- non-depolarising blocked with marked AChE inhibition (some quaternary carbamates have direct nicotinic agonist effect)

Question 24

List 9 clinical applications of cholinomimetics and give examples of drugs used for each

Answer 24

1. Glaucoma: pilocarpine (direct-acting)
2. Post-op ileus: bethanechol (direct-acting), neostigmine (AChE inhibitor)
3. Post-op or post-partum urinary retention, neurogenic bladder: bethanechol, neostigmine
4. Dry mouth (e.g. in Sjogren’s or post radiotherapy to glands): pilocarpine
5. Myaesthenia gravis: pyridostigmine (AChE inhibitor), edrophonium (used for diagnosis and to distinguish severe myaesthenia from excessive drug therapy which may cause paradoxical weakness due to nicotinic depolarising blockade), immunosuppressants
6. Reversal of neuromuscular blockade in anaesthesia: neostigmine, edrophonium (AChE inhibitor)
7. SVT: edrophonium (now rarely used)
8. Antimuscarinic toxicity (e.g. atropine or TCA overdose): physostigmine (only if extreme hyperthermia or SVT as physostigmine carries its own CNS risks)
9. Alzheimer’s disease: donepezil, rivastigmine (selective AChE inhibitors)

Question 25

What symptoms are seen with toxicity due to direct-acting muscarinic stimulants? How is this treated?

Answer 25

Causes GI upset (nausea, vomiting, diarrhoea), urinary urgency, hypersalivation, sweating, bronchoconstriction
Treated with atropine

Question 26

What symptoms are seen with toxicity due to direct-acting nicotinic stimulants? How much is fatal in acute toxicity? How is this treated?

Answer 26

Acute toxicity: fatal dose 40mg (1 drop of pure liquid) - this is the same amount in 2 cigarettes but most is destroyed by burning)
Acutely causes GI upset (nausea, vomiting), CNS stimulation (seizure, coma, respiratory arrest), skeletal muscle end plate depolarisation (respiratory paralysis), hypertension, cardiac arrhythmias
Rapidly metabolised and excreted so usually a full recovery if survival to 4hrs without hypoxic brain injury
Treatment largely symptomatic/supportive (can give atropine to treat muscarinic excess)
Chronic toxicity causes addiction and likely contributes to increased risk of vascular disease and sudden cardiac death

Question 27

What symptoms are seen with toxicity due to cholinesterase inhibitors? How is this treated?

Answer 27

Causes miosis, hypersalivation, sweating, bronchoconstriction, GI upset (vomiting, diarrhoea), CNS effects (cognitive disturbance, convulsion, coma), depolarising neuromuscular blockade (due to peripheral nicotinic effects)
Treatment involves: vital sign maintenance (especially decreased RR), decontamination of skin/clothing to prevent further absorption, and large doses of atropine +/- treatment with pralidoxime and benzodiazepines

Question 28

What is used as a preventative therapy against organophosphate poisoning and how does this work?

Answer 28

Pyridostigmine
Impedes organophosphate binding AChE through competitive agonist activity

Question 29

What is the effect of chronic exposure to organophosphates?

Answer 29

Delayed neuropathy due to axonal demyelination

Question 30

What is the duration of action of edrophonium?

Answer 30

5-15mins

Question 31

What is the duration of action of neostigmine, physostigmine, pyridostigmine, ambenonium and demecarium?

Answer 31

Neostigmine and physostigmine: 0.5-2hrs
Pyridostigmine: 3-6hrs
Ambenonium: 4-6hrs
Demecarium: 4-8hrs

Question 32

Describe the pharmacokinetics of antimuscarinic drugs

Answer 32

Chemistry: may be tertiary amine alkaloid esters (e.g. atropine), quaternary amines (produce more peripheral and less CNS effects)
Absorption: tertiary amines well-absorbed from gut and conjunctival membranes, quaternary poorly absorbed from GIT
Distribution: tertiary agents widely distributed (significant CNS levels in 30-60mins) especially scopolamine
Metabolism/excretion: atropine elimination has rapid phase (half-life 2hrs) and slow phase (half-life 13hrs), ~50% of atropine is excreted unchanged in urine (rest as hydrolysis and conjugation products), effects on eye persist >/= 72hrs

Question 33

Describe the mechanism of action of atropine

Answer 33

Acts as inverse agonist
Reversible muscarinic blockade (highly selective for muscarinic receptors with little activity at nicotinic receptors)
Does not distinguish between receptor subtypes

Question 34

Describe the pharmacodynamics of antimuscarinic drugs

Answer 34

CNS: drowsiness and amnesia (with therapeutic doses of scopolamine), excitation/agitation/hallucinations/coma in toxicity, decreased Parkinsonian tremor, decreased motion sickness
Eyes: mydriasis (unopposed sympathetic dilator activity), cycloplegia (inability to focus), decreased lacrimal secretion (dry eyes)
CV: bradycardia at low doses (due to blockade of presynaptic regulatory receptors), tachycardia at higher doses (blockade of vagal slowing), cutaneous vasodilation at toxic doses
Respiratory: bronchodilation and decreased bronchial gland secretion
GIT: decreased salivary and gastric gland secretion, decreased motility
GU: detrusor relaxation
Apocrine glands: suppression of thermoregulatory sweating (“atropine fever”)

Question 35

What is the risk of antimuscarinics related to their ophthalmological effects?

Answer 35

May induce acute glaucoma in patients with narrow anterior chamber angle

Question 36

List 10 clinical applications of antimuscarinics and give examples of drugs for each

Answer 36

1. Parkinsons disease: e.g. benztropine, orphenadrine (used as adjuncts)
2. Motion sickness: e.g. scopolamine
3. Ophthalmic uses: used for mydriasis when cycloplegia or prolonged action is required (otherwise alpha-agonists preferred); e.g. atropine, scopolamine, tropicamide
4. Bronchodilators: SAMAs (e.g. ipatropium) and LAMAs (e.g. tiotropium, umeclidinium)
5. Decreased airway secretion and decreased risk of laryngospasm with older inhalation anaesthetics (ethers): e.g. atropine
6. Bradyarrhythmias: e.g. atropine
7. Antidiarrhoeal: e.g. atropine (usually combined with opioid receptor agonist)
8. Overactive bladder: e.g. oxybutynin (somewhat M3-selective); may precipitate retention in BPH
9. Cholinergic poisoning: e.g. atropine (opposes muscarinic effects), pralidoxime (displaces organophosphate from organophosphate-cholinesterase complex)
10. Hyperhidrosis

Question 37

Describe the features of anticholinergic toxicity

Answer 37

1. Blind as a bat: mydriasis, cycloplegia
2. Mad as a hatter: delirium, agitation
3. Dry as a bone: dry mouth, urinary retention
4. Red as a beet: flushed skin (cutaneous vasodilation)
5. Hot as a desert: hyperthermia
6. Fast as a hare: tachycardia

Question 38

What is the mechanism of action of ganglion-blocking drugs?

Answer 38

Competitively block action of ACh and similar agonists at neuronal nicotinic receptors of both sympathetic and parasympathetic ganglia
I.e. they are non-depolarising competitive antagonists of nicotinic receptors

Question 39

Briefly outline the pharmacodynamics of ganglion-blocking drugs

Answer 39

CNS: sedation, tremor, choreiform movements, mental aberrations
Eye: cycloplegia, moderate pupillary dilation (parasympathetic usually dominates pupil size so when all innervation is inhibited there is slight dilation)
CV: decreased BP and contractility, increased HR (SA usually dominated by parasympathetic)
GIT: decreased gland secretion and contractility (causes marked constipation)

Question 40

Give three examples of ganglion-blocking drugs

Answer 40

TEA
Hexamethonium
Mecamylamine
* limited clinical use, primarily used in research to block all autonomic outflow