Peripheral Nervous System: Neurotransmitters and Receptors The Peripheral Nervous System Efferent nervous system Somatic nervous system (non-autonomic,voluntary) Skeletal muscle Autonomic nervous system Heart.blood vessels. (vegetative,visceral,involuntary glands,other visceral enteric nervous system) organs,smooth muscle Somatic and visceral afferent nerves Anatomic classification:sympathetic(fight or flight) parasympathetic(rest and digest) Neurotransmitter-based classification:adrenergic,cholinergic,dopaminergic 1
1 Neurotransmitters and Receptors Peripheral Nervous System: Efferent nervous system Somatic nervous system (non-autonomic, voluntary) Skeletal muscle Autonomic nervous system (vegetative, visceral, involuntary; enteric nervous system) Heart, blood vessels, glands, other visceral organs, smooth muscle The Peripheral Nervous System Somatic and visceral afferent nerves Anatomic classification: sympathetic (fight or flight) parasympathetic (rest and digest) Neurotransmitter-based classification: adrenergic, cholinergic, dopaminergic
Otto Loewi(Nobel Laureate,1936) He discovered that stimulation of the vagus of a frog heart causes release of a substance that.when transferred to a second heart,could slow heart rate.He called this"Vagusstoff",demonstrating the chemical basis of neurotransmission. He also found that atropine can prevent the inhibitory action,but not the release of the "Vagusstoff". Incubation of the"Vagusstoff"with frog heart homogenate inactivates it. Physostigmine enhances the effect of vagus stimulation on the heart,and prevents the destruction of"Vagusstoff. On mature consideration,in the cold light of the morning,I would not have done it.After all,it was an unlikely enough assumption that the vagus should secrete an inhibitory substance;it was still more unlikely that a chemical substance that was supposed to be effective at very close range between nerve terminal and muscle be secreted in such large amounts that it would spill over and,after being diluted by the perfusion fluid,still be able to inhibit another heart.(Loewi 1921) 2
2 • He discovered that stimulation of the vagus of a frog heart causes release of a substance that, when transferred to a second heart, could slow heart rate. He called this “Vagusstoff”, demonstrating the chemical basis of neurotransmission. Otto Loewi (Nobel Laureate, 1936) • He also found that atropine can prevent the inhibitory action, but not the release of the “Vagusstoff”. • Incubation of the “Vagusstoff” with frog heart homogenate inactivates it. • Physostigmine enhances the effect of vagus stimulation on the heart, and prevents the destruction of “Vagusstoff”. On mature consideration, in the cold light of the morning, I would not have done it. After all, it was an unlikely enough assumption that the vagus should secrete an inhibitory substance; it was still more unlikely that a chemical substance that was supposed to be effective at very close range between nerve terminal and muscle be secreted in such large amounts that it would spill over and, after being diluted by the perfusion fluid, still be able to inhibit another heart. (Loewi 1921)
Neurotransmitter: A chemical that transmits signals from one neuron to another or from a neuron to an effector cell. Chemical (intracellular messengers) Electrical Stimulation ·Chemical Physiological (impulse） (neurotransmitter） functions Electrical (membrane ion channels) Definition of synapse: A junctional connection between two neurons,across which a signal can pass Pre-synaptic neuron:Where a neurotransmitter is synthesized,stored and released upon cell activation. Post-synaptic neuron or effector cell:Where neurotransmitter is detected and its action translated into cellular activities. 3
3 Electrical Stimulation (impulse) Chemical (neurotransmitter) Neurotransmitter: A chemical that transmits signals from one neuron to another or from a neuron to an effector cell. Chemical (intracellular messengers) Electrical (membrane ion channels) Physiological functions Pre-synaptic nerve cell Post-synaptic nerve cell Synaptic cleft Ca2+ Na+ Precursors (choline/tyrosine) Definition of synapse: A junctional connection between two neurons, across which a signal can pass Precursor Neurotransmitter Storage Release Recognition by receptors Metabolic disposition Pre-synaptic neuron: Where a neurotransmitter is synthesized, stored and released upon cell activation. Post-synaptic neuron or effector cell: Where neurotransmitter is detected and its action translated into cellular activities
CNS Pre-ganglionic Ganglion Post-ganglionic Effectors lelueJo Parasympathetic Ach Cardiac smooth muscles,gland cells Nicotinic Muscarinic nerve terminals Sympathetic NE Cardiac smooth muscles,gland cells, Adrenergic nerve terminals Ach (c.B) Sympathetic Ach Sweat glands Nicotinic Muscarinic Sympathetic D Renal vascular smooth muscle D1) nergic Sympathetic(adrenal medulla) Ach Epi Released into blood Nicotinic leJoes Motor(somatic) Skeletal muscle Nicotinic Ach=acetylcholine D=dopamine Epi=epinephrine NE norepinephrino Pharmacological division of cholinergic vs.adrenergic neurotransmission All preganglionic and parasympathetic postganglionic neurons use acetylcholine as neurotransmitter.Ach is the neurotransmitter at ganglia, nmj,and muscarinic tissue synapses. Most postganglionic sympathetic neurons use norepinephrine which is an adrenergic neurotransmitter. There are exceptions:Cholinergic transmission in sympathetic system- all ganglia,adrenal medulla,sweat glads(muscarinic).Dopaminergic innervation in sympathetic system-renal blood vessels. 4
4 Thoracolumbar Cranial Sacral CNS Pre-ganglionic Ganglion Post-ganglionic Parasympathetic Ach Nicotinic Ach Nicotinic Ach Nicotinic Ach Nicotinic Ach Nicotinic Epi Sympathetic Sympathetic Sympathetic Sympathetic (adrenal medulla) Motor (somatic) Ach Ach Muscarinic Muscarinic NE Adrenergic (α, β) D Dopaminergic (D1) Ach Nicotinic Cardiac & smooth muscles, gland cells, nerve terminals Cardiac & smooth muscles, gland cells, nerve terminals Sweat glands Renal vascular smooth muscle Released into blood Skeletal muscle Ach = acetylcholine D = dopamine Epi = epinephrine NE = norepinephrine Effectors • All preganglionic and parasympathetic postganglionic neurons use acetylcholine as neurotransmitter. Ach is the neurotransmitter at ganglia, nmj, and muscarinic tissue synapses. • Most postganglionic sympathetic neurons use norepinephrine which is an adrenergic neurotransmitter. Pharmacological division of cholinergic vs. adrenergic neurotransmission • There are exceptions: Cholinergic transmission in sympathetic system – all ganglia, adrenal medulla, sweat glads (muscarinic). Dopaminergic innervation in sympathetic system – renal blood vessels
Key Steps in Neurotransmission: Synthesis Storage Metabolism Action potential Recognition (action) Strategies for Pharmacological Intervention: Block synthesis and storage: Usually rate-limiting steps;produce long-term effects Block release: Rapid action and effective Interfere with metabolism: Can be reversible or irreversible:blocking metabolism increases effective neurotransmitter concentrations Interfere with action: Receptor antagonists agonists;high specificity Synthesis of acetylcholine: Choline Acetylcholine CH、 CH3 0 CH3-N'-CHz-CHz-OH Choline CH3-N+-CHz-CHz-O-C-CH3 CH3 acetyltransferase CH3+ 0 CoA-SH CoA-S-C-CH3 Acetyl-CoA CoA 5
5 Synthesis & Storage Action potential Metabolism Recognition (action) Key Steps in Neurotransmission: Strategies for Pharmacological Intervention: Block synthesis and storage: Usually rate-limiting steps; produce long-term effects Block release: Rapid action and effective Interfere with metabolism: Can be reversible or irreversible; blocking metabolism increases effective neurotransmitter concentrations Interfere with action: Receptor antagonists & agonists; high specificity Release Synthesis of acetylcholine: CH3 CH3 CH3 N+–CH2–CH2–OH CoA–S–C–CH3 O Choline Acetyl-CoA + Choline acetyltransferase CH3 CH3 CH3 N+–CH2–CH2–O–C–CH3 O CoA-SH + CoA Acetylcholine
Synthesis,storage and release of acetylcholine: Na' 0斯 Choline Synaptic Ao-CaA ChAT Ach Antiporter Pre-synaptic cell CAT choline acetyltransferase AchE Post-synaptic AchE acetylcholinesterase cell Degradation of acetylcholine: HO 0 Choline Acetic acid (CH),N-CH,-CH,-O-C-CHa AchE (CHj)3 N+-CHz-CHz-OH+CHaCOOH AchE Trp-86 600.000 Ach molecules/AchE min Ser-203 tumnover time of 150 microseconds Steps involved in the action of acetylcholinesterase: 1.Binding of substrate(Ach) 2.Formation of a transient intermediate (involving-OH on Serine 203,etc.) 3.Loss of choline and formation of acetylated enzyme 4.Deacylation of AchE(regeneration of enzyme) 6
6 Synthesis, storage and release of acetylcholine: Pre-synaptic cell Post-synaptic cell Ach Ca2+ Na+ Choline (10 µM) Choline Recognition by receptors Ca2+ Ach Ach Ach Nerve impulse NN NM Ach Ac-CoA ChAT Ach AchE AchE choline + acetic acid CAT = choline acetyltransferase AchE = acetylcholinesterase Synaptic cleft Antiporter + CH3COOH AchE (CH3)3 N+–CH2–CH2 (CH3)3 N+–CH2–CH2–O–C–CH3 –OH O H2O (-) (H) AchE Trp-86 Glu-334 His-447 Ser-203 Degradation of acetylcholine: Steps involved in the action of acetylcholinesterase: 1. Binding of substrate (Ach) 2. Formation of a transient intermediate (involving -OH on Serine 203, etc.) 3. Loss of choline and formation of acetylated enzyme 4. Deacylation of AchE (regeneration of enzyme) 600,000 Ach molecules / AchE / min = turnover time of 150 microseconds Choline Acetic acid
Julius Axelrod (Nobel Laureate,1970) His discoveries concern the mechanisms which regulate the formation of norepinephrine in the nerve cells and the mechanisms which are involved in the inactivation of this important neurotransmitter. Synthesis of Catecholamines Tyrosine hydroxylase Phenylethanolamine- N-methyl transferase Dopa decarboxylase (L-amino acid 0\ (L-AAD decarboxylase) Adrenal medulla Dopamine B-hydroxylase Regulation of Norepinephrine Synthesis and Metabolism: Uak1 E 的 Post-synaptic Pre-synaptic Ca NENE ● Diffusion, Normetanephrine(NMN) metabolism 7
7 HO HO CH2 NHCH3 OH CH Epinephrine HO HO CH2 NH2 OH CH Norepinephrine HO HO CH2 NH2 CH2 Dopamine HO HO HC NH2 CH2 DOPA COOH HO HC NH2 CH2 Tyrosine COOH TH DD (L-AAD) PNMT DBH Adrenal medulla Synthesis of Catecholamines Tyrosine hydroxylase Dopa decarboxylase (L-amino acid decarboxylase) Dopamine β-hydroxylase PhenylethanolamineN-methyl transferase 1 3 Julius Axelrod (Nobel Laureate, 1970) His discoveries concern the mechanisms which regulate the formation of norepinephrine in the nerve cells and the mechanisms which are involved in the inactivation of this important neurotransmitter. Pre-synaptic Post-synaptic Ca2+ Na+ Tyrosine Cellular messengers and effects Diffusion, metabolism Tyrosine Dopa TH DD Dopamine (DA) NE DBH ATP Ca2+ NE DBH ATP NE NE COMT αR βR α2R NE (-) Signal Regulation of Norepinephrine Synthesis and Metabolism: Uptake-1 Normetanephrine (NMN)
Degradation of Catecholamines: AO COM Pargyline, Nialamide 0A COMT=catechol-O-methyltransferase Diagnosis of pheochromocytoma MAO=monoamine oxidase MAO:associated with outer surface of mitochondia,including those within the terminals of adrenergic fibers. COMT:located mostly in cytoplasm.Rich in liver,kidney;not found in adrenergic neurons. E E 的 Post-synaptic Pre-synaptic NENE Diffusion, Normetanephrine(NMN) metabolism VMA:vanillylmandelic acid(3-Methoxy-4-hydroxymandelic acid) 8
8 Degradation of Catecholamines: HO HO CH2 NHCH3 OH CH Epinephrine HO HO CH2 NH2 OH CH Norepinephrine HO HO CH2 NH2 CH2 Dopamine HO HO COOH OH CH Dihydroxymandelic acid (DOMA) HO CH3O COOH OH CH 3-Methoxy-4-hydroxymandelic acid (VMA)* CH3O HO CH2 NHCH3 OH CH Metanephrine HO CH2 NH2 OH CH Normetanephrine (NMN) CH3O HO HO COOH CH2 Dihydroxyphenylacetic acid (DOPAC) CH3O HO CH2 NH2 CH2 3-Methoxytyramine CH3O HO COOH CH2 Homovanillic acid (HVA) COMT COMT COMT COMT COMT MAO MAO MAO MAO MAO MAO PNMT DBH Adrenal medulla COMT = catechol-O-methyltransferase MAO = monoamine oxidase *Diagnosis of pheochromocytoma ( - ) Pargyline, Nialamide Pre-synaptic Post-synaptic Ca2+ Na+ Tyrosine Cellular messengers and effects Diffusion, metabolism Tyrosine Dopa TH DD Dopamine (DA) NE DBH ATP Ca2+ NE DBH ATP NE NE COMT Normetanephrine (NMN) αR βR α2R NE (-) Signal MAO: associated with outer surface of mitochondia, including those within the terminals of adrenergic fibers. COMT: located mostly in cytoplasm. Rich in liver, kidney; not found in adrenergic neurons. VMA: vanillylmandelic acid (3-Methoxy-4-hydroxymandelic acid)
Drug intervention-- Cholinergic transmission (Rate-limiting) Precursor transport ...........Hemicholiniums ↓：Stimulatory Synthesis ⊥：Inhibitory Cholinergic antagonists Solid:Agonistic 0 Dotted:Antagonistic Atropine (anti-M) hexamethonium Storage Vesamicol (anti-NM) Trimetaphan 0 (anti-NN) Release Botulinum toxin Cholinergic agonists (direct acting) AntiChE Carbachol Ach Oxotremorine Reversible (neostigmine) Irreversible (organo- phosphate) Receptor Degradation +action by AchE Definition of Agonist and Antagonist: Agonist:(1)A natural ligand that activates a receptor.(2)A drug that has properties similar to a natural ligand in activating the same receptor. Antagonist:(1)A receptor-specific blocker.(2)A molecule,such as a drug(e.g.. enzyme inhibitor)or a physiologic agent(e.g.,hormone),that diminishes or prevents the action of another molecule. Mode of Action: Direct-acting: Molecule that physically binds to the target for its effect. Example:carbachol activates cholinergic receptors. Indirect-acting:Molecule that exerts effect on the target by interacting with another molecule. Example:neostigmine blocks AchE,causing Ach accumulation. Mode of action and agonism are different concepts.For example,a direct- acting molecule can be either agonistic or antagonistic. 9
9 Drug intervention -- Cholinergic transmission Precursor transport Synthesis Hemicholiniums Storage Vesamicol Release Botulinum toxin Degradation by AchE Receptor + action Ach Cholinergic agonists (direct acting) Carbachol Oxotremorine (Rate-limiting) AntiChE Reversible (neostigmine) Irreversible (organophosphate) ↓ : Stimulatory ⊥ : Inhibitory Solid: Agonistic Dotted: Antagonistic Cholinergic antagonists Atropine (anti-M) hexamethonium (anti-NM) Trimetaphan (anti-NN) Agonist: (1) A natural ligand that activates a receptor. (2) A drug that has properties similar to a natural ligand in activating the same receptor. Antagonist: (1) A receptor-specific blocker. (2) A molecule, such as a drug (e.g., enzyme inhibitor) or a physiologic agent (e.g., hormone), that diminishes or prevents the action of another molecule. Direct-acting: Molecule that physically binds to the target for its effect. Example: carbachol activates cholinergic receptors. Indirect-acting: Molecule that exerts effect on the target by interacting with another molecule. Example:neostigmine blocks AchE, causing Ach accumulation. Definition of Agonist and Antagonist: Mode of Action: Mode of action and agonism are different concepts. For example, a directacting molecule can be either agonistic or antagonistic
Drug intervention-- Adrenergic transmission ↓：Stimulatory Tyrosine ⊥：Inhibitory Solid:Agonistic (Rate--limiting） TH… Metyrosine Dotted:Antagonistic Dopa-→DA ☐k… Reserpine Adrenergic antagonists Vesicle(DA-→NE) Phentolamine (a-blocker) Propranolol (B-blocker) 0 Amphetamine,tyramine, Release ephedrine Adrenergic agonists Bretylium,guanethidine (direct acting) Cocaine Isoproterenol NE Tricyclic antidepressants Albuterol (e.g.imipramine) Receptor Recapture +action by Uptake-1 Autonomic Receptor Classification: M1,M3,M5(Gq coupled) Muscarinic R (mAChR) M2,M4(Gi coupled) Cholinergic R Nicotinic R了 NM(neuromuscular,or muscle type) (nAChR) L NN(neuronal,or ganglion type) C1,02 Adrenergic R B1,β2，β3 Dopamine R D1,D2,D3,D4,D5 Other receptors(receptors for NANC transmitters, e.g.nitric oxide,vasoactive intestinal peptide,neuropeptide Y) 10
10 Drug intervention -- Adrenergic transmission Tyrosine Dopa→DA Metyrosine Vesicle (DA→NE) Reserpine Release Bretylium, guanethidine Recapture by Uptake-1 Receptor + action NE Adrenergic agonists (direct acting) Isoproterenol Albuterol (Rate-limiting) Cocaine Tricyclic antidepressants (e.g. imipramine) Adrenergic antagonists Phentolamine (α-blocker) Propranolol (β-blocker) TH Amphetamine, tyramine, ephedrine ↓ : Stimulatory ⊥ : Inhibitory Solid: Agonistic Dotted: Antagonistic Autonomic Receptor Classification: 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)