Pharmacology of efferent nervous system

Shi-Hong Zhang ( 张世红 ), PhD Dept. of Pharmacology,

School of Medicine, Zhejiang University shzhang713@zju.edu.cn

Nervous System

PeripheralNervous

System (PNS)

CentralNervous

System (CNS)

Organization of the nervous system

Sympathetic

PeripheralNervous

System (PNS)

EfferentDivision

AfferentDivision

AutonomicSystem (ANS)

SomaticSystem

Parasympathetic

Enteric

The Enteric Nervous System (+SNS/PSNS)

Sympathetic stimulation causes:

• stimulates heartbeat • raises blood pressure • dilates the pupils • dilates the trachea and bronchi • stimulates the conversion of liver glycogen into glucose • shunts blood away from the skin and viscera to the

skeletal muscles, brain, and heart • inhibits peristalsis （蠕动） in the gastrointestinal (GI)

tract • inhibits contraction of the bladder and rectum

Parasympathetic stimulation causes:

• slowing down of the heartbeat

• lowering of blood pressure

• constriction of the pupils

• increased blood flow to the skin and viscera

• peristalsis of the GI tract

Organization of the nervous system

Sympathetic

Nervous System

PeripheralNervous

System (PNS)

CentralNervous

System (CNS)

EfferentDivision

AfferentDivision

AutonomicSystem (ANS)

SomaticSystem

Parasympathetic

Enteric

Drugs that produce their

primary therapeutic effect by

mimicking or altering the

functions of autonomic

nervous system are called

autonomic

drugs.

Neurotransmitters• Synthesis• Storage• Release• Degradation

Receptors• Activation• Blockade

Drug actions and classification

(1) Mimetics - direct-acting: receptor agonists

- indirect-acting: increasing amounts and/or effects of transmitters

(2) Antagonists

- direct-acting: receptor antagonists

- indirect-acting: decreasing amounts and/or effects of transmitters

•Cholinergic Pharmacology

•Adrenergic Pharmacology

Autonomic Pharmacology

CASE STUDY• In mid-afternoon, a coworker brings 43-year-old JM to

the emergency department because he is unable to continue picking vegetables. His gait is unsteady and he walks with support from his colleague. JM has difficulty speaking and swallowing, his vision is blurred, and his eyes are filled with tears. His coworker notes that JM was working in a field that had been sprayed early in the morning with a material that had the odor of sulfur. Within 3 hours after starting his work, JM complained of tightness in his chest that made breathing difficult, and he called for help before becoming disoriented.

• Choline Uptake→

• ACh Synthesis Choline + AcCoA → ACh ChAT

• ACh Storage

• ACh Release

• ACh Effects- Postsynaptic- Presynaptic

• ACh inactivationACh → Choline + Acetate AChE

Cholinergic Terminal

Regulation

- by autoreceptors ACh acting on presynaptic m2-cholinergic receptors

- by heteroreceptors NE acting on presynaptic alpha2-adrenergic receptors

- by metabolism (extraneuronal)

Acetylcholine Release

ACh inactivation Cholinesterases

Acetylcholinesterase is located at cholinergic synapses and in erythrocytes (does not hydrolyze succinylcholine)

Pseudocholinesterase (synonyms: plasmacholinesterase or butyrylcholinesterase 丁酰胆碱脂酶 ) occurs mainly in plasma, liver and in glia (hydrolyzes succinylcholine)

Cholinergic Receptors

• Muscarinic receptors (M receptors)

M1, 3, 5 (smooth muscles); M2, 4(heart)

G-protein Coupled

End Organs

• Nicotinic receptors (N receptors)

NN (N1) receptors; NM(N2 ) receptors

Ligand-gated Ion Channels

NMJ & Ganglia

M receptors : G-protein Coupled

MuscarinicReceptorSignalingPathways

• Depression of the heart (heart rate, conduction)

• Contraction of smooth muscles (sensitive: GI tract, bronchial, urinary bladder; insensitive: uterine, blood vascular)

• Exocrine glands (sensitive: sweat, tear, salivary; insensitive: GI tract);

• Eye (contraction of sphincter muscle of iris: miosis; contraction of ciliary muscle 睫状肌 : contraction for near vision)

M receptors:

Cholinergic Vasodilation

• The response of an isolated blood vessel to ACh depends on whether the endothelium is intact (unrubbed) or missing

• When the endothelium is present, ACh causes smooth muscle relaxation by stimulating the production of nitric oxide (NO) in the endothelium

• In the absence of the endothelium, a small amount of vasoconstriction is observed

• NN receptors （ N1 receptors ） - Sympathetic and parasympathetic ganglia

- Adrenal medulla

• NM receptors （ N2 receptors ） - The Neuromuscular Junction (NMJ)

(Contraction of skeletal muscles)

N receptors

• At the NMJ, N receptors pentameric with four

types of subunits, two

subunits bind ACh for

ligand gating

• All other nAChRs,

including those at the

peripheral ganglia, have

2 ’s and 3 ’s

N receptors : Ligand-gated Ion Channels

Ganglionic Neurotransmission

N = Nicotinic AChR

M = Muscarinic AChR

EPSP = Excitatory Postsynaptic Potential

IPSP = Inhibitory Postsynaptic Potential

AA BB

The Neuromuscular Junction (NMJ)

Myasthenia Gravis• This means “serious disorder the NMJ”

• This is an autoimmune disease

• Antibodies against the subunit of the nAChR

• The ability of ACh to activate the nAChRs is blocked by the antibodies

• As many autoimmune diseases, stress can make the symptoms worse

• Treatment is to potentiate cholinergic signaling and to remove the antibodies (blood dialysis)

1. Cholinomimetics (1) Direct-acting drugs: Cholinoceptor agonists

M, N receptor agonists: acetylcholine

M receptor agonists: pilocarpine

N receptor agonists: nicotine

(2) Indirect-acting drugs: Cholinesterase inhibitors (Anticholinesterases)

Reversible: neostigmine 新斯的明 Irreversible: organophosphates 有机磷酸酯类 Cholinesterase reactivators: pralidoxime iodide

碘解磷定

Drug classification

2 Cholinergic antagonists

(1) Cholinoceptor antagonists

M cholinoceptor antagonists atropine (Antimuscarinic drugs)

N cholinoceptor antagonists NN cholinoceptor antagonists: mecamylamine

(Ganglionic blocking drugs, rarely used)

NM cholinoceptor antagonists: succinylcholine

(Neuromuscular blocking drugs )

(2) Botulinum Toxin (Botox, blocks ACh release)

Drug classification

Cholinomimetics

Ach derivatives （胆碱酯类）Natural muscarinic agonists ( 生物碱类 M

受体激动剂 )

Nicotinic receptor agonists (N 受体激动剂 )

Direct-acting drugs

AChE resistant

Bond cleaved by AChE

醋甲胆碱 氯贝胆碱

卡巴胆碱乙酰胆碱

ACh Derivatives

ACh Derivatives

Bethanechol 氯贝胆碱 is most commonly used, particularly post-op for the treatment of paralytic ileus and urinary retention

Natural Muscarinic Agonists

Nicotinic potency

• Arecoline: areca or betal nuts (India,E. Indies)• Pilocarpine: pilocarpus (S. Amer. shrub)• Muscarine: amanita muscaria (mushroom)

槟榔碱 毛果芸香碱 毒蕈碱

• Poisoning causes muscarinic

overstimulation ：- salivation, lacrimation, visual

disturbances;

- abdominal colic and diarrhea

- bronchospasm and bradycardia

- hypotension; shock

• Treatment is with atropine

Atropa belladonna

Amanita muscaria

““Food” PoisoningFood” Poisoning

颠茄

伞形毒蕈

(1) Eyes

• Miosis ( 缩瞳 ): contraction of sphincter muscle of iris• Lowing intraocular pressure: enlarging angle of anterior

chamber, increasing drainage of aqueous humor• Spasm of accommodation ( 调节痉挛 ): contraction of

ciliary muscle, contraction for near vision

• Ophthalmological 眼科 uses

Glaucoma 青光眼 : narrow (closed)- or wide (open)-

angles used for the emergency lowering of intraocular pressure

Iritis: miotics 缩瞳药 /mydriatics 扩瞳药

Pilocarpine

pilocarpine

atropine lens

miosis

mydriasis

paralysis of

accommodation

near sight

spasm of accommodation

far sight

iris

Ciliary muscle (contraction)

Anterior chamber

zonule

zonulePosterior chamberAnterior chamber

Ciliary muscle (dilation)

Canal of Schlemm

Circulation of aqueous humor

Glaucoma• Open-angle glaucoma ：

disease of the aging eye - increased intraocular pressure, degeneration of the optic head, and restricted visual field

• Obstruction of the aqueous drainage leads to elevated intraocular pressure (IOP), and may result in glaucomatous damage to the optic nerve

Glaucoma

• Glaucoma management involves lowering IOP by- Decreasing aqueous production by the ciliary

body - Increasing aqueous outflow through the

trabecular meshwork and uveal outflow paths

• Pilocarpine: increase aqueous outflow by contraction of the ciliary muscle to increase tone and alignment of the trabecular network

（ 2 ） Promoting secretion of exocrine

glands, especially in sweat, salivary and tear

glands

• Systemic use

Antidote 解毒剂 for atropine poisoning

• Adverse effects

M-like syndrome

Pilocarpine

Actions at ganglia, NMJ, brain are complex and frequently unpredictable, because of the variety of neuroeffector sites and because nicotine both stimulates and desensitizes effectors.

Periphery: HR, BP, GI tone & motility

CNS: stimulation, tremors, respiration, emetic effects

The addictive power of cigarettes is directly related to their nicotine content.

N receptor agonists:

Nicotine

1. Cholinomimetics (1) Direct-acting drugs: Cholinoceptor agonists M, N receptor agonists: acetylcholine M receptor agonists: pilocarpine N receptor agonists: nicotine(2) Indirect-acting drugs: Cholinesterase inhibitors

(Anticholinesterases) Reversible: neostigmine 新斯的明 Irreversible: organophosphates 有机磷酸酯类 Cholinesterase reactivators: pralidoxime iodide 碘解磷定

Drug classification

Acetylcholinesterase (AChE) Activity

A. Competitive (reversible)

B. Carbamates (氨甲酰类 slowly reversible)

C. Organophosphates (irreversible)

AChE Inhibitors

neostigmineThese agents are reversible and are

used medically (glaucoma or MG)

These agents are irreversible and

are used as pesticides or for

glaucoma毒扁豆碱

新斯的明依酚氯铵

Acetylcholinesterase Inhibitors: Reversible

Edrophonium ( 依酚氯铵 )Rapidly absorbed; A short duration of action (5-15min);Competitive (reversible)

Used in diagnosis of myasthenia gravis.

Excess drug may provoke a cholinergic crisis, atropine is the antidote.

Other reversible ACHEI: tacrine 他克林 , donepezil 多奈哌齐

Acetylcholinesterase Inhibitors: Carbamates

Inhibitory Effects are slowly reversible

Representative Drugsneostigmine (quaternary amine 季铵 )pyridostigmine (quaternary amine) physiostigmine (tertiary amine 叔胺 )

quaternary amines effective in periphery onlytertiary amines effective in periphery and CNS（ fat-soluble ）

Pharmacological effects • AChE(-), ACh release↑, stimulating NMR

• stronger effect on skeletal muscles

• effective on GI tract and urinary bladder

• more polar and can not enter CNS

• relatively ineffective on CVS, glands, eye

Neostigmine 新斯的明

Clinical uses1. Myasthenia gravis: symptomatic treatment

overdose: cholinergic crisis

胆碱能危象：大量出汗，大小便失禁，瞳孔缩小，睫状肌痉挛，心动过缓，低血压，肌无力，呼吸困难

2. Paralytic ileus 麻痹性肠梗阻 urinary retention: post

operative abdominal distension and urinary retention

3. Paroxysmal superventricular tachycardia （ rarely use ）4. Antidote for tubocurarine ( 筒 箭 毒 碱 ) and related drug

poisoning

5. Glaucoma

Neostigmine

Adverse effects• Cholinergic effects: muscarinic and nicotinic effects,

treated with atropine (muscarinic)

• Contraindications ： mechanical ileus （机械性肠梗阻） urinary obstruction

bronchial asthma

poisoning of depolarizing skeletal muscle relaxants

(e.g. succinylcholine, 琥珀酰胆碱 )

Neostigmine

Acetylcholinesterase Inhibitors: Irreversible

Bond is hydrolyzed in binding to the enzyme

For ophthalmic use

乙磷硫胆碱 梭曼

对硫磷 对氧磷

马拉硫磷 马拉氧磷

Dichlorvos 敌敌畏

Dimethoate 乐果

(1) Toxic symptoms

Acute intoxication• Muscarinic symptom:

eye, exocrine glands, respiration, GI tract, urinary tract, CVS

• Nicotinic symptoms:

NN: elevation of BP, increase of HR;

NM: tremor of skeletal muscles

• CNS symptoms:

excitation, convulsion; depression (advanced phase)

Organophosphates

CASE STUDY• In mid-afternoon, a coworker brings 43-year-old JM to

the emergency department because he is unable to continue picking vegetables. His gait is unsteady and he walks with support from his colleague. JM has difficulty speaking and swallowing, his vision is blurred, and his eyes are filled with tears. His coworker notes that JM was working in a field that had been sprayed early in the morning with a material that had the odor of sulfur. Within 3 hours after starting his work, JM complained of tightness in his chest that made breathing difficult, and he called for help before becoming disoriented.

(1) Toxic symptoms

Chronic intoxication

• usually occupational poisoning

• plasma ChE activity ↓,

• weakness, restlessness, anxiety, tremor, miosis, ……

Organophosphates

(2) Detoxication• Elimination of poison; Supportive therapy• Antidotes

Atropine － antagonizing muscarinic effects; early, large dose, and repeated use

Cholinesterase reactivators － reactivation of phosphated AChE; moderate-severe patients, early use (More effective on tremor), combined with atropine– Pyraloxime methoiodide (PAM ，碘解磷定 )– Pralidoxime chloride （氯解磷定） : safer than PAM– Obidoxime chloride （双复磷） : two active oxime groups

Organophosphates

Organophosphates

Pralidoxime （解磷定） can restore AChE activity if administered soon after toxin exposure.

• Conjugating with organophosphate by oxime group;

• Conjugating with free organophasphates

Why isn’t this ACHEI pesticide neurotoxic to humans?

Insects and mammals metabolize the ‘prodrug’ differently

Mammals – esterase activity: hydrolyzes the molecule into inactive metabolites

Insects - P450 metabolism: P-S bond converted to P-O bond: now, the molecule, malaoxon, is an active inhibitor

Malathion 马拉硫磷

glaucoma (e.g. physiostigmine 毒扁豆碱 , echothiophate 乙磷硫胆碱 )

myasthenia gravis (e.g. Edrophonium, neostigmine, pyridostigmine )

reverse neuromuscular blockade from competitive antagonists (neostigmine)

Alzheimer’s disease (tacrine & donepezil, galanthamine)

chemical warfare agents

insecticides

Summary: ACHEI Applications

Pharmacological Actions: Increases ACh concentrations at cholinergic synapses, thereby increasing cholinergic activity.

2 Cholinergic antagonists

(1) Cholinoceptor antagonists

• M cholinoceptor antagonists– atropine (Antimuscarinic drugs)

• N cholinoceptor antagonists– NN cholinoceptor antagonists: mecamylamine

(Ganglionic blocking drugs, rarely used)

– NM cholinoceptor antagonists: succinylcholine

(Neuromuscular blocking drugs )

• Botulinum Toxin (botox, blocks ACh release)

Drug classification

Muscarinic Antagonists (Antimuscarinic drugs)

Tertiary amines （叔铵） Quaternary amines （季铵）

异丙托铵 噻托溴铵

东莨菪碱

Atropa belladonna颠茄

Datura stramonium曼陀罗

Datura sp.洋金花 山莨菪

Henbane Seed

1. Pharmacological effects

Atropine

(1) Inhibition of exocrine gland secretion

salivary, sweat glands

tear, respiratory tract glands

relatively ineffective: GI tract

(2) Eye

mydriasis 瞳孔散大 rise in intraocular pressure

paralysis of accommodation 调节麻痹

pilocarpine

atropine lens

miosis

mydriasis

paralysis of

accommodation

near sight

spasm of accommodation

far sight

iris

Ciliary muscle (contraction)

Anterior chamber

zonule

zonulePosterior chamberAnterior chamber

Ciliary muscle (dilation)

Canal of Schlemm

1. Pharmacological effects

(3) Antispasmodic action on smooth muscle • sensitive: GI, urinary bladder (spasmodic state)

• relatively insensitive: bile duct, urinary tract,

bronchial tract

• insensitive: uterus

Atropine

1. Pharmacological effects(4) Cardiovascular system: dose dependent • Lower therapeutic doses: HR↓(bradycardia); Blood

vessels and blood pressure: no effect• Moderate to high therapeutic doses / high vagal tone:

HR↑ (tachycardia); A-V conduction ↑• Larger doses: cutaneous vasodilatation

(5) CNS stimulation ：• sedation, memory loss, psychosis (high dose)

Atropine

2. Clinical uses

(1) Ophthalmology

Measurement of the refractive errors ( 屈光不正 ): children

Acute iritis or iridocyclitis: mydriatics/miotics

(2) Antispasmodic agent

GI, biliary or renal colic, enuresis

(3) Inhibiting exocrine gland secretion

Preanesthetic medication 麻醉前用药(4) Bradycardia

sinus or nodal bradycardia, A-V block

(5) Antidote for organophosphate poisoning

(6) Septic shock 感染性休克

Atropine

3. Adverse effects(1) Side effects dry mouth, blurred vision, “sandy eyes”

(2) toxicity Lethal dose: 80~130 mg (adult), 10 mg (child)• Low: xerostomia (dry mouth); anhidrosis (dry skin),

tachycardia• Moderate: above plus mydriasis, cycloplegia ( 睫状肌

麻痹 ); difficulty speaking, swallowing & urinating; and hot, red, dry skin

• High: above plus ataxia, hallucinations 幻觉 & delirium 谵妄 ; coma

Atropine

3. Adverse effects

(3) Detoxication

Supportive treatment

Symptomatic treatment: e.g. diazepam for CNS

symptoms.

Antidote: Physostigmine or pilocarpine

(4) Contraindications

glaucoma, prostatauxe 前列腺肥大 , fever

Atropine

• Actions and clinical uses

– Peripheral effects are similar to atropine; but has stronger central effects (depression)

– Pre-anesthetic medication, prevention of motion sickness, Parkinson’s disease

Scopolamine 东莨菪碱

Anisodamine (654-1,2)

• Actions and clinical uses

– Peripheral effects, similar to atropine; lower toxicity

– Septic shock and visceral colic (relieve spasm of vascular smooth muscles)

• Tropicamide 托吡卡胺 : mydriatic, cycloplegic

shorter duration (1/4 day)• Propantheline 丙胺太林，普鲁本辛 poor absorption (po) and BBB penetration

antispasmodic effects in GI, treatment of peptic ulcer

• Ipratropium 异丙托铵 : asthma• Benztropine 苯托品 : Parkinson’s disease• Trihexyphenidyl 苯海索• Pirenzepine 哌仑西平： M1 selective, peptic

ulcer, asthma

Synthesized surrogates

CASE STUDYJH, a 63-year-old architect, complains of urinary symptoms

to his family physician. He has hypertension and the last 8 years, he has been adequately managed with a thiazide diuretic and an angiotensin-converting enzyme inhibitor. During the same period, JH developed the signs of benign prostatic hypertrophy, which eventually required prostatectomy to relieve symptoms. He now complains that he has an increased urge to urinate as well as urinary frequency, and this has disrupted the pattern of his daily life. What do you suspect is the cause of JH’s problem? What information would you gather to confirm your diagnosis? What treatment steps would you initiate?

Nicotinic receptor antagonists

• Acting on sympathetic and parasympathetic

ganglionic cells; reducing blood pressure by

inhibiting sympathetic ganglia ( have been

abandoned for clinical use, due to their lack of

selectivity)

• Short-acting; tachyphylaxis ( 快速抗药反应 )

• Used for treatment of hypertension

─ Trimethaphan( 咪噻芬 )

– Mecamylamine ( 美加明 )

NN receptor antagonists(Ganglionic blocking drugs)

• Two classes:

Depolarizing: succinylcholine 琥珀酰胆碱

Non-depolarizing: drugs act as competitive antagonists

d-tubocurarine 筒箭毒碱Note: Belong to Skeletal Muscle Relaxants. It is important to realize that muscle relaxation does not ensure unconsciousness, amnesia, or analgesia.

NM receptor antagonists

(Neuromuscular blocking drugs )

1. Depolarizing neuromuscular blockers (Non-competitive)

(depolarizing skeletal muscle relaxants) act as acetylcholine (ACh) receptor agonists the depolarized membranes remain depolarized and unresponsive

to subsequent impulses (ie, they are in a state of depolarizing block).

not metabolized by AChE - diffuse away from the neuromuscular junction and are hydrolyzed in

the plasma and liver by pseudocholinesterase (nonspecific cholinesterase, plasma cholinesterase, or butyrylcholinesterase) and elimination by kidney

NM receptor antagonists

(Neuromuscular blocking drugs )

Succinylcholine (Scoline 司可林 )

Succinylcholine is the only depolarizing agent used clinically (t1/2= 2-4 min).

Properties of actions:• initially transient fasciculations （肌束震颤）• anti-AChE potentiates their effects• tachyphylaxis after repeated uses• no ganglion-blocking effects at therapeutic doses• the drugs are highly polar, poor bioavailability; i.v. • as quaternary compounds, do not enter CNS

acetylcholinesuccinylcholine

• Main pharmacological effects

– Transient excitation (fasciculations), and then inhibition (relaxation)

– Relax Skeletal Muscles in neck, limbs > face, tongue, throat; less effective on breath muscles at therapeutic doses

Succinylcholine (Scoline)

• Clinical uses

– An adjuvant in anesthesia or operation– Intubation of trachea, esophagus, etc.– Prevention of trauma during electroshock therapy （无

抽搐电休克疗法）

– Contraindicated in awake patients, should use under anesthesia

Succinylcholine (Scoline)

• Adverse effects

(1) Apnea (respiratory paralysis)

overdose or hypersensitive patients;

neostigmine potentiates the toxic effects

(2) Muscle spasm

muscular pain after operation

Succinylcholine (Scoline)

(3) Elevation of K+ in plasma

contraindicated in patients with a tendency of hyperkaleemia

(4) Malignant hyperthermia

genetic abnormality, treated by dantrolene (Ca2+ release inhibitor)

(5) Others

rise in intraocular pressure (glaucoma);

histamine release

Succinylcholine (Scoline)

Genetic Variation: Effects on Duration of Action of Succinylcholine

• Duration of action is prolonged by high doses or by abnormal metabolism. The latter may result from hypothermia (decreases the rate of hydrolysis), low pseudocholinesterase levels, or a genetically aberrant enzyme.

• Low pseudocholinesterase levels generally produce only modest prolongation of succinylcholine's actions (2-20 min).

• One in 50 patients has one normal and one abnormal (atypical) pseudocholinesterase gene, resulting in a slightly prolonged block (20-30 min).

• Even fewer (1 in 3000) patients have two abnormal genes (homozygous atypical) that produce an enzyme with little or no affinity for succinylcholine and have a very long blockade (e.g., 4-8 h) following administration of succinylcholine.

• Scoline apnea

• Drug interactions- Thiopental （强碱性，可分解 scoline ）- ChE inhibitors:

AChE inhibitors, cyclophosphamide, procaine, etc.

- Some antibiotics: kanamycin, polymyxins, etc. (synergism in

neuromuscular blocking)

Succinylcholine (Scoline)

2. Nondepolarizing neuromuscular blockers (Competitive) (nondepolarizing skeletal muscle relaxants)

Tubocurarine ( 筒箭毒碱 )

Reversibly bind to the nicotinicreceptor at the neuromuscularjunction (competitive antagonists)

NM receptor antagonists (Neuromuscular blocking drugs)

• Effects: competitive blockade of NM receptors

• Uses: adjuvant medication for anesthesia or

operations, eg. tracheal intubation

• Adverse effects: Respiratory paralysis: can be reversed by neostigmine

Enhancing histamine release: BP , bronchoconstriction,

salivary secretion

Blocking ganglion: BP Contraindications: myasthenia gravis, bronchial asthma,

shock, child (< 10 y)

Tubocurarine

• Benzylisoquinolines （ 苄 基 异 喹 啉类）

atracurium （阿曲库铵） doxacurium （多撒库铵） mivacurium （米库铵）• Ammonio steroids （类固醇铵类） pancuronium （潘库铵） vecuronium （维库铵） pipecuronium （哌库铵） rocuronium （罗库铵）

Other nondepolarizing neuromuscular blockers

Botulinum Toxin 肉毒杆菌毒素

- Skeletal muscle relaxants

- blocks ACh release from cholinergic terminals

- selective for ACh terminals

- results in irreversible flaccid paralysis ( 松弛性瘫痪 ) in

muscles

Acts by cleaving SNAP proteins → inhibits ACh release

Botulinum Toxin

- an anaerobic bacillus, clostridium botulinum can multiply in

preserved food

- it synthesizes a protein that can be absorbed (pinocytosis or

transport?) from the GI tract to reach the systemic circulation

- penetrates tissues to reach cholinergic nerve terminals

- then, it is uptaken (pinocytosis) and internalized in vesicles

whose lumen becomes acidified

- the low pH of the vesicles splits the inactive molecule into 2

active enzymes that have proteolysis functions

Applications

• Strabismus (lack of parallelism of eyes 斜视 ),

blepharospasm (eyelid spasm), dystonia (abnormal tonicity).

• Excessive sweating

• Cosmetic procedures ( “frown lines” or “crow’s feet” 鱼尾纹 )

Note: effects can last for ~3-6 months.

Botulinum Toxin

• 杨世杰主编《药理学》人民卫生出版社 2010第二版

• Katzung BG, Basic & Clinical Pharmacology (10th edition), 2007.

• Lipincott’s illustrated reviews—Pharmocology (2nd edition), 2002

参 考 书 目参 考 书 目