Autonomic Nervous System:Introduction to neurotransmitter and receptor specificity

Thomas GuenthnerProfessor of PharmacologyCollege of MedicineTel. 996-7635Room E418, CMWE-mail: tmg@uic.edu Thanks to Dr. Richard Ye for Powerpoint concepts and slides

Identify the key conceptual similarities and differences between autonomic cholinergic and adrenergic pathways including receptor subtypes, neurotransmitters, transmitter synthesis, storage, and release, and relative specificities of drugs that stimulate or inhibit each branch or activity.

Knowledge objectives introduced by these two lectures:

List the major systems or organs innervated by the autonomic cholinergic and adrenergic systems.

Describe the organ system effects of cholinergic and adrenergic stimulation or antagonism.

Relate the tissue expression profiles of cholinergic and adrenergic receptors to their specific functions.

• 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. adrenergicneurotransmission

• There are exceptions: Cholinergic transmission in sympathetic system – all ganglia, adrenal medulla, sweat glads use Ach (nicotinic or muscarinic). Dopaminergic innervation in sympathetic system – renal blood vessels.

Pre-synapticnerve cell

Post-synapticnerve cell

Synapticcleft

Ca2+

Na+

Precursors(choline/tyrosine)

Synapse – site most amenable to pharmacologic manipulation:

Precursor

Neurotransmitter

Storage

Release

Recognitionby receptors

Metabolicdisposition

Manipulation possible at pre-synaptic neuron, where neurotransmitter is synthesized, stored and released upon cell activation, or at post-synaptic neuron or effector cell, where neurotransmitter is detected and its action is translated into cellular activities.

Synthesis & Storage

Actionpotential

Metabolism

Recognition(action)

Key Steps in Neurotransmission:

Strategies for Pharmacological Intervention:

Block synthesis and storage: Usually rate-limiting steps; produce long-term effectsBlock release: Rapid action and effectiveBlock reuptake increases synaptic neurotransmitter concentrations Can be selective or non-selectiveInterfere with metabolism: Can be reversible or irreversible; blocking metabolism

increases effective neurotransmitter concentrationsInterfere with recognition: Receptor antagonists & agonists; high specificity

Release

Reuptake

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 withanother non-target site.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 direct- acting molecule can be either agonistic or antagonistic.

•Discovered that stimulation of the vagus of a frog heart causes release of a substance that, when applied to a second heart, could slow heart rate. He called this “Vagusstoff”, demonstrating the chemical basis of neurotransmission.

Otto Loewi (Nobel Laureate, 1936)

• Also found that atropine can prevent the inhibitory action, but not the release, of “Vagusstoff”.

• Exposure of “Vagusstoff” to frog heart homogenate inactivates it.

• Physostigmine enhances the effect of vagus stimulation on the heart, and prevents the destruction of “Vagusstoff”.

Synthesis of acetylcholine:

CH3

CH3

CH3

N+–CH2–CH2–OH

CoA–S–C–CH3

O

Choline

Acetyl-CoA

+

Cholineacetyltransferase

CH3

CH3

CH3

N+–CH2–CH2–O –C–CH3

O

CoA-SH

+

CoA

Acetylcholine

Synthesis, storage and release of acetylcholine:

Pre-synapticcell

Post-synapticcell

Ach

Ca2+

Na+Choline(10 M)

Choline

Recognitionby receptors

Ca2+

Ach

Ach

Ach

Nerveimpulse

NN

NM

AchAc-CoA

ChAT

Ach

AchE

AchE

choline+ acetic acid

CAT = choline acetyltransferaseAchE = acetylcholinesterase

Synapticcleft

Antiporter

CH3COOH+AchE

(CH3)3 N+–CH2–CH2–OH(CH3)3 N+–CH2–CH2–O –C–CH3

OH2O

OH(-)AchE

Glu202Tyr337

Ser203Glu334His447

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

Drug intervention -- Cholinergic transmission

Precursor transport

Synthesis

Hemicholinium

Storage Vesamicol

Release Botulinum toxin

Degradationby AchE

Receptor+ action

Ach

Cholinergic agonists(direct acting)

CarbacholPilocarpine

(Rate-limiting)

AntiChE

Reversible (neostigmine)Irreversible (organo- phosphate)

: Stimulatory : InhibitorySolid: AgonisticDotted: Antagonistic

Cholinergic antagonists

Atropine (anti-M)Succinylcholine (anti-NM)Trimethaphan (anti-NN)

Physostigmine’s effect on acetylcholine receptor is indirect. This effect is mediated through the inhibition of cholinesterase, which causes an increase in the local concentration of acetylcholine. The net effect is agonistic on acetylcholine receptor.

An example of indirect-acting pharmacological agents:

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

COOHHO HC

NH2

CH2

Tyrosine

COOHTH

DD (L-AAD)

DBHPNMT

Adrenal medulla

Synthesis of Catecholamines Tyrosine hydroxylase

Dopa decarboxylase (L-amino acid decarboxylase)

Dopamine -hydroxylase

Phenylethanolamine-N-methyl transferase

13

Julius Axelrod (Nobel Laureate, 1970)

His discoveries concern the mechanisms which regulate the formation of norepinephrinein the nerve cells and the mechanisms which are involved in the inactivation of this important neurotransmitter.

Pre-synapticPost-synaptic

Ca2+

Na+

Tyrosine

Cellular messengersand effects

Diffusion, metabolism

Tyrosine

Dopa

THDD Dopamine

(DA)

NE

DBH

ATP

Ca2+

NE

DBH

ATP NE

NE

COMT

R

R

R

NE

(-)

Signal

Regulation of Norepinephrine Synthesis and Metabolism:

Uptake-1

Normetanephrine (NMN)

Drug intervention -- Adrenergic transmission

Tyrosine

DopaDA

Metyrosine

Vesicle (DANE)

Reserpine

ReleaseBretylium, guanethidine

Recaptureby Uptake-1

Receptor+ action

NE

Adrenergic agonists(direct acting)

IsoproterenolAlbuterol

(Rate-limiting)

CocaineTricyclic antidepressants (e.g. imipramine)

Adrenergic antagonists

Phentolamine (-blocker)Propranolol (-blocker)

TH

Amphetamine, tyramine,ephedrine

: Stimulatory : InhibitorySolid: AgonisticDotted: Antagonistic

PNS Receptor Functions

PNS Receptors - Pharmacological 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)

1,

21,

2,

D1, D2, D3, D4, D5

Other receptors (receptors for NANC transmitters,e.g. nitric oxide, vasoactive intestinal peptide, neuropeptide Y)

(mAChR)

(nAChR)

Thor

acol

umba

rC

rani

alS

acra

l

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()

D

Dopaminergic(D1)

Ach

Nicotinic

Cardiac & smoothmuscles, gland cells,nerve terminals

Cardiac & smoothmuscles, gland cells,nerve terminals

Sweat glands

Renal vascularsmooth muscle

Released intoblood

Skeletal muscle

Ach = acetylcholine NE = norepinephrineEpi = epinephrineD = dopamine

Effectors

Adrenergic receptors

Classification of adrenergic receptors by agonist potency

-- NE Epi > Iso

-- Iso > Epi > NE

NE = norepinephrineEpi = epinephrineIso = isoproterenol

HO

HO

CH2

NHCH3

OH

CH

Epi

HO

HO

CH2

NH2

OH

CH

NE

HO

HO

CH2

NH

OH

CH

Iso CH(CH3)2

Agonist

Signaling properties of adrenergic receptors

AgonistAgonist1 2 1,2,3

Gq Gi Gs

Inositol phosphates (IP3)

Diacyl glycerol (DAG)

cAMP cAMP

Calcium channels

K+ conductance

Mostly excitatory Mostly inhibitory Mostly excitatory

NorepinephrineEpinephrinePhenylephrine

NorepinephrineMethyl NEClonidine

IsoproterenolAlbuterol (2)Dobutamine (1)

Gs and Gi proteins have different functionsGs and Gi proteins have different functions

Agonist

s

Agonist

iACAC

s

i

Gs = stimulatory G protein

Gi = inhibitory G protein

AC = adenylyl cyclase (convert ATP to cAMP)

1: postsynaptic effector cells, especially smooth muscleVasoconstriction, relaxation of gastrointestinal smooth muscle, hepaticglycogenolysis

2 presynaptic adrenergic nerve terminals (autoreceptor), platelets, lipocytes, smooth muscle

Inhibition of transmitter release, platelet aggregation, contraction ofsmooth muscle

1 postsynaptic effector cells: heart, lipocytes, brain, presynaptic adrenergic / cholinergic terminals

Increased cardiac rate & force, relaxation of gastrointestinal smooth muscle

2 postsynaptic effector cells: smooth muscle, cardiac muscleBronchodilation, vasodilation, relaxation of visceral smooth muscle, hepaticglycogenolysis

postsynaptic effector cells: lipocytesLipolysis

Distribution and functions of adrenergic receptors:

O

OH

NH

CH3

CH3

Propranolol

NH

O

OH

NH

CH3

CH3

Pindolol

O

OH

NH

CH3

CH3

NHO

CH3

CH3

Acebutolol

O

OH

NH

CH3

CH3

OH

OH Nadolol

O

OH

NH

CH3

CH3

OCH3

Metoprolol

O

OH

NH

CH3

CH3

NH2

O

Atenolol1 antagonistic half-life

1 and 2 antagonistic

half-life 2 antagonistic effect

1 antagonisticPartial 2 agonistic

1 antagonisticPartial 2 agonistic

Dopaminergic receptors

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

COOHHO HC

NH2

CH2

Tyrosine

COOHTH

DD (L-AAD)

DBHPNMT

Tyrosine hydroxylase

Dopa decarboxylase (L-amino acid decarboxylase)

Dopamine -hydroxylase

Phenylethanolamine-N-methyl transferase

13

Dopaminergic receptors in the periphery

Dopamine receptors play important roles in CNS. Notably, dopamine neurotransmission is involved in several diseases including Parkinson’s disease, schizophenia, and attention deficiency disorder.

There are 5 types of dopamine receptors (D1 – D5). In periphery, D1 dopamine receptor mediates renal vasodilation, and increased myocardial contractility.

Agonist AgonistD2,3,4D1,5

GiGs

cAMP cAMP

Cholinergic receptors

“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

Nicotiana tabacum(cultivated tobacco)

Nicotinic acetylcholine receptor: Function

Ligand-gated ion (Na+) channel

- an “Ionotropic Receptor”

• Acetylcholine binds to the -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.

• Nicotine in small doses stimulates autonomic ganglia and adrenal medulla. When large doses are applied, the stimulatory effect is quickly followed by a blockade of transmission.

Nicotinic receptor antagonistsCompetitive vs. depolarizing

CompetitivePhysically blocks Ach binding

INHIBITOR

DepolarizingBinds and locks the receptoropen

“Muscarinic actions” -- reproduced by injection of muscarine, from Amanita muscaria (fly agaric). Similar to those of parasympathetic stimulation

• Neural/enteric (M1): CNS, ENS, gastric parietal cells (excitatory; Gq)• Cardiac (M2): atria & conducting tissue; presynaptic (inhibitory; Gi)• Glandular/endothelial (M3): exocrine glands, vessels (excitatory; Gq)• Neural (M4): CNS (inhibitory; Gi)• Neural (M5): CNS (excitatory; Gq)(Sites of primary expression are listed; all are found in CNS)

Cholinergic receptors: Muscarinic

Multiple muscarinic cholinergic receptors distributed in different tissues. Therefore, the “muscarinic actions” are dependent on the receptors in different tissues and cells.

Agonist

Muscarinic acetylcholine receptors –G Protein-Coupled Receptors (“Metabotropic” Receptors)

Agonist

M1(enteric, neuronal)

M2(cardiac)

M3(glandular, vascular )

Gq Gi IP3, DAG

(Depolarization)

(Stimulation) Intracellular Ca2+

cAMP Ca2+ channel

K+ conductance K+ conductance

Mostly excitatoryCNS excitationGastric acid secretionGastrointestinal motility

Mostly inhibitoryCardiac inhibitionPresynaptic inhibitionNeuronal inhibition

Glandular secretionContraction of visceral smooth muscleVasodilation (via NO) (Slow IPSP)

(Inhibition)

M5(CNS)

M4(CNS)

Intracellular signaling triggered by acetylcholine in the Heart

Main molecular players: M2, heterotrimeric G Protein Gi, Adenylyl cyclase

Clinical manifestation of excessive cholinergic effects

D – DefecationU – Urination M – MiosisB – BradycardiaE – EmesisL – Lacrimation S – Salivation

(DUMBELS)

Effects of muscarinic antagonists

“DRY AS BONE, RED AS A BEET, MAD AS HATTER.”• Decreased sweating, salivation and lacrimation• Reflex peripheral (cutaneous) vasodilation to

dissipate heat (hyperthermia) • CNS effects of muscarinic inhibition -- restlessness,

delerium, hallucination ALSO:BronchodilationTachycardiaMydriasis (pupil dilation) and Cycloplegia (loss of focus)GI and Bladder atony

Physiological Effects of ANSStimulation and Inhibition

Receptor distribution and effects in the autonomic nervous system:

Organ Receptor Parasympathetic Receptor

HeartRateForce Automaticity

Automaticity Force

Rate Force Conduction velocity AV block

M2M2M2

Arterioles

SA nodeAtrial muscleAV node

Ventricular muscle

Blood vessels

CoronarySkeletal muscleVisceraSkinBrainErectile tissueSalivary gland

ContractionRelaxationContractionContractionContractionContractionContractionContractionRelaxation

RelaxationRelaxation

Vein

M3M3

Sympathetic

(Continued, next page)

M3

Organ Sympathetic Receptor Parasympathetic Receptor

Relaxation

Motility Contraction

ContractionRelaxation

Viscera

Bronchiolar SMC GlandsGI track Smooth muscle Sphincters Glands

Uterus

Secretion

Motility RelaxationSecretionGastric acid secretion

Variable

M3

M3M3M3M1

Skin Pilomotor SMC Contraction (piloerection)

Salivary glands Secretion Secretion M3

Lacrimal glands Secretion M3

Kidney Renin release

Liver GlycogenolysisGluconeogenesis

Fat Lipolysis

M3Contraction

From: Rang et al. Pharmacology, 6th Ed. p. 169. Also, see Katzung, Basic & Clinical Pharmacology, 10th Ed. p.86.

Cardiovascular Pharmacology(Blood Pressure)

Cardiovascular effects of intravenous infusion of epinephrine, norepinephrine, and isoproterenolin man. Norepinephrine (predominantly -agonist) causes vasoconstriction and increased systolicand diastolic BP, with a reflex bradycardia. Isoproterenol (-agonist) is a vasodilator, but stronglyincreases cardiac force and rate. Mean arterial pressure falls. Epinephrine combines both actions.

Two kinds of effects produced by Ach. A. Ach causes a fall in BP due to vasodilation.B. A larger dose of Ach also produces bradycardia, further reducing BP.C. Atropine blocks the effect of Ach in lowering BP.D. Still under the influence of atropine, a much larger dose of Ach causes a rise in BP and tachycardia.

Sir Henry Hallett Dale(Nobel laureate, 1936)

A, B: Muscarinic effects of Ach (M3, M2)C: Muscarinic antagonistic effect (M)D. Stimulation of sympathetic ganglia (NN)

(Arterial pressure of ananesthetized cat wasmeasured)

Intracellular signaling triggered by acetylcholine in the endothelium

eNOS

●NO

L-Arg

L-Citruline

Major molecular players: M3, heterotrimeric G Protein Gq, Ca(2+)-CaM, eNOS, NO

eNOS Nitric oxide synthase

Nitric oxide (NO) signaling pathway for SMC relaxation

Secondmessenger

Inhibition of PDE causes sustained levelof cGMP that maintains SMC relaxation.

Sildenafil (Viagra)is an inhibitor forPDE 5.

Pulmonary Pharmacology(Asthma and COPD)

Ocular Pharmacology(Glaucoma)

Lens

Pupillary dilator muscle ()Pupillary constrictor muscle (M3)

Secretion of aqueous humor ()(M3)

Cholinergic effects: Adrenergic effects:

• Contraction of pupillary constrictor muscle-- miosis• Contraction of ciliary muscle - bulge of lens-- near vision, outflow of aqueous humor

• Contraction of pupillary dilator muscle-- mydriasis• Stimulation of ciliary epithelium-- production of aqueous humor

Trabecular meshwork(opened by pilocarpine)