Cholinergic Agonists and Antagonists M1,M3,M5(Gq coupled) Muscarinic R (mAChR) M2,M4(Gi coupled) Cholinergic R Nicotinic R NM(neuromuscular,or muscle type) (nAChR) NN(neuronal,or ganglion type) 1,2 Adrenergic R B1,B2,B3 Dopamine R D1,D2,D3D4,D5 Other receptors(receptors for NANC transmitters, e.g.nitric oxide,vasoactive intestinal peptide,neuropeptide Y) 1
1 Cholinergic Agonists and Antagonists Cholinergic R Adrenergic R Dopamine R Muscarinic R Nicotinic R M1, M3, M5 (Gq coupled) M2, M4 (Gi coupled) NM (neuromuscular, or muscle type) NN (neuronal, or ganglion type) β1, α1, α2 β2, β3 D1, D2, D3, D4, D5 Other receptors (receptors for NANC transmitters, e.g. nitric oxide, vasoactive intestinal peptide, neuropeptide Y) (mAChR) (nAChR)
Cholinergic receptors:Nicotinic vs.Muscarinic CH2-CH2 CH-0 The"Nicotinic actions"--similar to those induced by nicotine;action mediated by nicotinic cholinergic receptors stimulation of all autonomic ganglia(NN) stimulation of voluntary muscle (NM) secretion of epinephrine from the adrenal medulla(NN) Nicotinic ACh receptor Ligand-gated ion(Na)channel Acetylcholine binds to the a subunits at the boundary of a and y,and a and subunits. .Channel opening requires binding of 2 acetylcholine Exterior molecules,with positive cooperativity. Structurally and functionally similar to the sodium channel,which can be blocked by local anesthetics 28两 Acetylcholine binds to the a-subunits of the receptor making the membrane more permeable to cations(sodium)and causing a local depolarization. The local depolarization spreads to an action potential,or leads to muscle contraction when summed with the action of other receptors.The ion channel is open during the active state. In addition to a-bungarotoxin,there are other blocking agents for autonomic ganglions that include hexamethonium,tetraethylammonium,mecamylamine,and trimethaphan. Blocking at this level stops all autonomic outflow and produces a broad effect. 2
2 The “Nicotinic actions” -- similar to those induced by nicotine; action mediated by nicotinic cholinergic receptors • stimulation of all autonomic ganglia (NN) • stimulation of voluntary muscle (NM) • secretion of epinephrine from the adrenal medulla (NN) Cholinergic receptors: Nicotinic vs. Muscarinic Nicotinic ACh receptor • Ligand-gated ion (Na+) channel • Acetylcholine binds to the α subunits at the boundary of α and γ, and α and δ subunits. • Channel opening requires binding of 2 acetylcholine molecules, with positive cooperativity. • Structurally and functionally similar to the sodium channel, which can be blocked by local anesthetics • Acetylcholine binds to the α-subunits of the receptor making the membrane more permeable to cations (sodium) and causing a local depolarization. The local depolarization spreads to an action potential, or leads to muscle contraction when summed with the action of other receptors. The ion channel is open during the active state. • In addition to α-bungarotoxin, there are other blocking agents for autonomic ganglions that include hexamethonium, tetraethylammonium, mecamylamine, and trimethaphan. Blocking at this level stops all autonomic outflow and produces a broad effect
Cholinergic receptors:Nicotinic vs.Muscarinic The"Muscarinic actions"--reproduced by injection of muscarine,from Amanita muscaria. Similar to those of parasympathetic stimulation Multiple(5)muscarinic cholinergic receptors distributed in different tissues: Neural(M1):CNS,PNS,gastric parietal cells(excitatory;Gq) Cardiac(M2):atria conducting tissue;presynaptic(inhibitory;Gi) Glandular(M3):exocrine glands;smooth muscle (excitatory;Gq) Muscarinic receptors couple to G proteins Agonist t GEF Downstream D effectors GaGDP GaGTP PLC-B Ion channels Adenylyl cyclase GAP Kinases Pi Others 3
3 The “Muscarinic actions” -- reproduced by injection of muscarine, from Amanita muscaria. Similar to those of parasympathetic stimulation • Neural (M1): CNS, PNS, gastric parietal cells (excitatory; Gq) • Cardiac (M2): atria & conducting tissue; presynaptic (inhibitory; Gi) • Glandular (M3): exocrine glands; smooth muscle (excitatory; Gq) Cholinergic receptors: Nicotinic vs. Muscarinic Multiple (5) muscarinic cholinergic receptors distributed in different tissues: Gα•GDP GAP GEF Gα•GTP Muscarinic receptors couple to G proteins βγ βγ Pi Downstream effectors PLC-β Ion channels Adenylyl cyclase Kinases Others Agonist
Three Major Muscarinic Receptors(there are five) Agonist Agonist Agonist M1 M2 M3 “neural" “cardiac" "glandular' Gq Gi Gq ↑Inositol phosphates ↓cAMP Inositol phosphates (1P3) (IP3) Diacyl glycerol(DAG) ↓Calcium channels ↑Diacyl glycerol(DAG) ↓K'conductance 个K*conductance Intracellular calcium Depolarization Mostly excitatory Mostly inhibitory Mostly excitatory (decrease of M1 activity in (responsible for the (stimulation of glandular CNS may be a cause of vagal inhibition of the secretion,contraction of dementia) heart) visceral smooth muscle) Classes of cholinergic stimulants Direct-acting Indirect-acting Receptor agonists Cholinesterase inhibitors Carbamates Choline esters PHYSOSTIGMINE Phosphates NEOSTIGMINE ISOFLUROPHATE ACETYLCHOLINE Alkaloids PYRIDOSTIGMINE Antidote BETHANECOL PILOCARPINE EDROPHONIUM PRALIDOXIMINE 4
4 Agonist Three Major Muscarinic Receptors (there are five) Agonist Agonist M1 “neural” M2 “cardiac” M3 “glandular” Gq Gi Gq ↑ Inositol phosphates (IP3) ↑ Diacyl glycerol (DAG) ↑ Inositol phosphates (IP3) ↑ Diacyl glycerol (DAG) ↑ Intracellular calcium ↓ cAMP ↓ Calcium channels ↑ K+ conductance ↓ K+ conductance Mostly excitatory (decrease of M1 activity in CNS may be a cause of dementia) Mostly inhibitory (responsible for the vagal inhibition of the heart) Mostly excitatory (stimulation of glandular secretion, contraction of visceral smooth muscle) Depolarization Classes of cholinergic stimulants Direct-acting Receptor agonists Choline esters ACETYLCHOLINE BETHANECOL Alkaloids PILOCARPINE Cholinesterase inhibitors Carbamates PHYSOSTIGMINE NEOSTIGMINE PYRIDOSTIGMINE EDROPHONIUM Phosphates ISOFLUROPHATE Antidote PRALIDOXIMINE Indirect-acting
AChE Susceptibility Muscarinic Effect hc是。-0-G-Gh-w二 CH CH Highest (++++ High Acetylcholine CH Low(+) Highest(++++) (cy) w。-otow-r< CH Negligible Medium (++ Negligible Medium(++) Cholinergic effects: Adrenergic effects: Contraction of pupillary constrictor muscle .Contraction of pupillary dilator muscle --miosis --mydriasis Contraction of ciliary muscle-bulge of lens Stimulation of ciliary epithelium --near vision,T outflow of acqueous humor --T production of aqueous humor lris、 Cornea Pathway for aqueous humour Canai of Schlemm Trabecular meshwork (opened by pilocarpine) Lens Suspensory ligaments Secretion of acqueous humor (B) Fig.6.5 The anterior chamber of the eye,showing the pathway for secretlon and drainage of the aqueous humor. 5
5 AChE Susceptibility Muscarinic Effect Highest (++++) High Low (+) Highest (++++) Negligible Medium (++) Negligible Medium (++) Lens Pupillary dilator muscle (α1) Pupillary constrictor muscle (M3) Secretion of acqueous humor (β) (M3) Cholinergic effects: Adrenergic effects: • Contraction of pupillary constrictor muscle -- miosis • Contraction of ciliary muscle - bulge of lens -- near vision, ↑ outflow of acqueous humor • Contraction of pupillary dilator muscle -- mydriasis • Stimulation of ciliary epithelium -- ↑ production of aqueous humor Trabecular meshwork (opened by pilocarpine)
Pilocarpine,a cholinomimetic alkaloid Chewing pilocarpus causes salivation.It also induces sweating Action chielly muscarinic HO HC 0 CH2-'N-CH H Muscarine H C-CH2 Pilocarpine Used clinically for treating dry mouth(xerostomia)after radiation therapy of head and neck tumor;also for treatment of glaucoma. Organ effects of cholinergic agonists Eyes:contraction of ciliary muscle and smooth muscle of the iris sphincter(miosis)-aqueous humor outflow, drainage of the anterior chamber Cardiovascular:Bradycardia(possibly preceded by tachycardia),vasodilation(all vascular beds including pulmonary and coronary-M3)and hypotension, reduction of the contraction strength(atrial and ventricular cells,Ikcdiastolic depolarization,NO- inhibitable ATP),negative chronotropic effect (inhibition of adrenergic activation). Respiratory:Bronchoconstriction,mucus secretion 6
6 Pilocarpine, a cholinomimetic alkaloid • Chewing pilocarpus causes salivation. It also induces sweating. • Used clinically for treating dry mouth (xerostomia) after radiation therapy of head and neck tumor; also for treatment of glaucoma. Organ effects of cholinergic agonists • Eyes: contraction of ciliary muscle and smooth muscle of the iris sphincter (miosis) – aqueous humor outflow, drainage of the anterior chamber • Cardiovascular: Bradycardia (possibly preceded by tachycardia), vasodilation (all vascular beds including pulmonary and coronary – M3) and hypotension, reduction of the contraction strength (atrial and ventricular cells, IK+ , ICa2+ diastolic depolarization , NOinhibitable ATP), negative chronotropic effect (inhibition of adrenergic activation). • Respiratory: Bronchoconstriction, mucus secretion
Organ effects of cholinergic agonists GI-increases in tone,amplitude of contractions,and peristaltic activity of the stomach and intestines; enhances secretory activity of the gastrointestinal tract. Urinary bladder-increase ureteral peristalsis,contract the detrusor muscle of the urinary bladder,increase the maximal voluntary voiding pressure,and decrease the capacity of the bladder. Other effects-Increased secretion from all glands that receive parasymphatetic enervation(salivary,lacrimal, tracheobronchial,digestive and exocrine sweat glands) Muscarinic receptor antagonists ATROPINE OH SCOPOLAMINE 00 OH 7
7 • GI - increases in tone, amplitude of contractions, and peristaltic activity of the stomach and intestines; enhances secretory activity of the gastrointestinal tract. • Urinary bladder - increase ureteral peristalsis, contract the detrusor muscle of the urinary bladder, increase the maximal voluntary voiding pressure, and decrease the capacity of the bladder. • Other effects – Increased secretion from all glands that receive parasymphatetic enervation (salivary, lacrimal, tracheobronchial, digestive and exocrine sweat glands) Organ effects of cholinergic agonists Muscarinic receptor antagonists ATROPINE SCOPOLAMINE
Muscarinic Antagonists Attropa be -Atropine and Scopolamine are belladona alkaloids (competitive inhibitors) -Drugs differ in their CNS effects,scopolamine permeates the blood-brain barrier -At therapeutic doses atropine has negligible effects upon the CNS, scopolamine even at low doses has prominent CNS effects. Mechanism of drug action Competitively block muscarinic receptors Salivary,bronchial,and sweat glands are most sensitive to atropine Smooth muscle and heart are intermediate in responsiveness In the eye,causes pupil dilation and difficulty for near vision accommodation Relaxation of the Gl,slows peristalsis 8
8 Muscarinic Antagonists Attropa belladona - Atropine and Scopolamine are belladona alkaloids (competitive inhibitors) - Drugs differ in their CNS effects, scopolamine permeates the blood-brain barrier - At therapeutic doses atropine has negligible effects upon the CNS, scopolamine even at low doses has prominent CNS effects. Mechanism of drug action - Competitively block muscarinic receptors - Salivary, bronchial, and sweat glands are most sensitive to atropine - Smooth muscle and heart are intermediate in responsiveness - In the eye, causes pupil dilation and difficulty for near vision accommodation - Relaxation of the GI, slows peristalsis
Effect of muscarinic inhibitor in the eye Pupil dilation vs.accomodation 10 Pupil 2- Mydriasis 6 Scopolamine at 0 and 30 min 4 2 Accommodation Cycloplegia(paralysis of ciliary muscle) 0 01530456090 12 6 10 Time(minutes) (days) Organ effect of muscarinic antagonists ORGAN DRUG APPLICATION CNS Benztropine Treat Parkinson's disease Scopolamine Prevent/Reduce motion sickness Eye Atropine Pupil dilation Bronchi Ipatropium Bronchodilate in asthma,COPD GI Methscopolamine Reduce motility/cramps GU Oxybutinin Treat transient cystitis Postoperative bladder spasms 9
9 Effect of muscarinic inhibitor in the eye Pupil dilation vs. accomodation Scopolamine at 0 and 30 min Cycloplegia (paralysis of ciliary muscle) Mydriasis Organ effect of muscarinic antagonists ORGAN DRUG APPLICATION CNS Benztropine Treat Parkinson’s disease Scopolamine Prevent/Reduce motion sickness Eye Atropine Pupil dilation Bronchi Ipatropium Bronchodilate in asthma, COPD GI Methscopolamine Reduce motility/cramps GU Oxybutinin Treat transient cystitis Postoperative bladder spasms
Indirect cholinergic stimulants (cholinesterase inhibitors,anticholinesterases) Acetylcholinesterase(AChE) Bound to the basement membrane in the synaptic cleft at cholinergic synapses Also present as a soluble form in cholinergic nerve terminals;function unknown Binds Ach,forming a transient intermediate which then becomes acetylated AChE and choline:spontaneous and rapid deacylation follows,regenerating the enzyme Butyrylcholinesterase(BChE,pseudocholinesterase) .Widely present,including a presence in soluble form in the plasma Not particularly associated with cholinergic synapases Broader substrate specificity than AChE;hydrolyses butyrylcholine more efficiently than acetylcholine;function remains largely unknown Together with AChE,keeps acetylcholine concentration in plasma nearly undetectable Reversible cholinesterase inhibitors (compared to acetylcholine) 6c-c-0-CHg-CH2-NC Acetylcholine -CH HC一NH一C Carbaryl CHa Physostigmine Edrophonium 10
10 Indirect cholinergic stimulants (cholinesterase inhibitors, anticholinesterases) Acetylcholinesterase (AChE) • Bound to the basement membrane in the synaptic cleft at cholinergic synapses • Also present as a soluble form in cholinergic nerve terminals; function unknown • Binds Ach, forming a transient intermediate which then becomes acetylated AChE and choline; spontaneous and rapid deacylation follows, regenerating the enzyme Butyrylcholinesterase (BChE, pseudocholinesterase) • Widely present, including a presence in soluble form in the plasma • Not particularly associated with cholinergic synapases • Broader substrate specificity than AChE; hydrolyses butyrylcholine more efficiently than acetylcholine; function remains largely unknown • Together with AChE, keeps acetylcholine concentration in plasma nearly undetectable Reversible cholinesterase inhibitors (compared to acetylcholine) Ester of carbamic acid Quaternary ammonium
