Introduction to Autonomic Pharmacology

Gloanne C. Adolor, RPh, MD, MS, MBA, FPCP

Outline

• Review of ANS (favorite YL5 subjects)

• Drugs Acting on the ANS– Cholinoceptor-Activating and

Cholinesterase-Inhibiting Drugs– Cholinoceptor-Blocking Agents

Main Reference: Basic and Clinical Pharmacology by Katzung (11th ed).

Autonomic Nervous System (ANS)

• The ANS consists of motor neurons that: – Innervate smooth and cardiac muscle and

glands– Make adjustments to ensure optimal

support for body activities– Operate via subconscious control– Have viscera as most of their effectors

Functions and origins of the ANS

ANS branches• cholinergic fibers - acetylcholine• adrenergic fibers noradrenaline (norepinepherine

NE)

ANS Versus Somatic Nervous System (SNS)

• The ANS differs from the SNS in the following three areas– Effectors– Efferent pathways– Target organ responses

Comparison of Somatic and Autonomic Systems

Efferent Pathways

• Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector

• Axons of the ANS are a two-neuron chain– The preganglionic (first) neuron has a

lightly myelinated axon– The ganglionic (second) neuron extends to

an effector organ

Effectors

• The effectors of the SNS are skeletal muscles

• The effectors of the ANS are cardiac muscle, smooth muscle, and glands

Efferent (motor) nerves• Two systems

– Autonomic nerves (unconscious)• Eg cardiac output, respiration, etc

– Somatic nerves (voluntary)

Interactions of the Autonomic Divisions

• Most visceral organs are innervated by both sympathetic and parasympathetic fibers

• This results in dynamic antagonisms that precisely control visceral activity

• Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination

• Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes

Role of the Parasympathetic Division

• Concerned with keeping body energy use low• Involves the D activities – digestion, defecation, and

diuresis• Its activity is illustrated in a person who relaxes after

a meal– Blood pressure, heart rate, and respiratory rates are low– Gastrointestinal tract activity is high– The skin is warm and the pupils are constricted

Role of the Sympathetic Division

• The sympathetic division is the “fight-or-flight” system• Involves E activities – exercise, excitement, emergency,

and embarrassment• Promotes adjustments during exercise – blood flow to

organs is reduced, flow to muscles is increased• Its activity is illustrated by a person who is threatened

– Heart rate increases, and breathing is rapid and deep– The skin is cold and sweaty, and the pupils dilate

Visceral Reflexes

Figure 14.7

Action of ANS drugs

• Drugs to block ANS chemical transmission

• Drugs to mimic ANS action• ANS drugs can modify a variety of

effector tissues– Cardiac muscle– Blood pressure– Exocrine glands

Cholinergic transmission

• Acetylcholine is at motor neuron and CNS nerve terminals

• Synthesized from – Acetyl coA (mitochondria) – Choline (dietary)– Catalyzed by choline acetyl transferase (ChAT)

• Release is dependent on Calcium (Ca2+)• Causes muscle contraction

Acetylcholine

• Identified 1921

• Present at all NMJ and also CNS

• Synthesized in the axon terminal

• Diffuses across synaptic cleft

• Two receptor subtypes– Nicotinic ACh receptors– Muscarinic ACh receptors

Neuromuscular Junction

Synaptic End Plate

T.Caceri Veterinary Histology 2003

1999 Sinauer Associates Inc

Acetylcholine and NMJ

Characteristics of a neurotransmitter

• Synthesized in (or transported to) presynaptic terminal

• Stored in vesicles

• Regulated release

• Receptor located on postsynaptic membrane

• Termination of action

Neurotransmitter Effects

• All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect

• In the ANS:– Preganglionic fibers release ACh– Postganglionic fibers release norepinephrine or ACh and the

effect is either stimulatory or inhibitory– ANS effect on the target organ is dependent upon the

neurotransmitter released and the receptor type of the effector

Synaptic vesicles at the NMJ (EM)

Heuser and Heuser

Presynaptic events

• Calcium influx releases synaptic vesicles from microtubules

• Movement of synaptic vesicles to sites of action

• Interaction of specific proteins• Vesicle docking• Membrane fusion• Calcium dependent exocytosis

Synthesis and release of neurotransmitters

Synaptic Transmission in: Basic Neurochemistry 6 th Edition

Fusion proteins regulate neurotransmitter release

• Vesicle proteins– Synaptobrevin

• Presynaptic membrane proteins– Syntaxins– SNAP-25

The SNARE hypothesis

SNARE (Soluble N’ethylmalemide sensitive fusion Attachment protein REceptor)

A. Pestronk www.neuro.wustl.edu/neuromuscular 2003

Many presynaptic proteins regulate neurotransmitter release

Synaptic Transmission in: Basic Neurochemistry 6 th Edition

Vesicular transport of NT – drug implications

• Toxins targeting neurotransmitter release– Spider venom (excess ACh release)– Botulinum (blocks ACh release)

• Tetanus

Postsynaptic events

• Boutons have multiple nerve terminals• Simultaneous release• Stimulation of contraction via AP• Acetylcholine degraded after action

– ACETYLCHOLINESTERASE (AChE)

RECEPTORS

Cholinergic receptors

• Two classes for acetylcholine• Nicotinic and muscarinic

– Nicotinic are ion channelsIonotrophic

– Muscarinic are G-protein coupled

Metabotrophic

Nicotinic Receptors

• Nicotinic receptors are found on:– Motor end plates (somatic targets)– All ganglionic neurons of both sympathetic

and parasympathetic divisions– The hormone-producing cells of the

adrenal medulla

• The effect of ACh binding to nicotinic receptors is always stimulatory

Muscarinic Receptors

• Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers

• The effect of ACh binding: – Can be either inhibitory or excitatory– Depends on the receptor type of the target

organ

Ionotropic AChR

• Consist of five polypeptide subunits

• Receptors vary in:– subunit structure – agonist sensitivity – distribution

• Mediate fast synaptic transmission

Nicotinic AChR are sodium channels

1999 Sinauer Associates Inc

Metabotropic AChR

• Five muscarinic AChR subtypes

• G protein coupled

• Slower synaptic transmission via intracellular signaling cascade

Muscarinic AChR activate G-proteins

1999 Sinauer Associates Inc

Many G proteins for many usesG alpha class Initiating signal Downstream

signal

G alpha s b-Adrenergic amines, glucagon, parathyroid hormone, many others

Stimulates adenylate cyclase

G alpha i Acetylcholine, a-adrenergic amines, many neurotransmitters

Inhibits adenylate cyclase

G alpha t Photons Stimulates cGMP phosphodiesterase

G alpha q Acetylcholine, a-adrenergic amines, many neurotransmitters

Increases IP3 and intracellular calcium

G alpha 13 Thrombin, other agonists

Stimulates Na+ and H+ exchange

G proteins activate enzymes, most commonly the following:

• Adenylyl cyclase - converts ATP into cyclic AMP (cAMP)

• Phospholipase C - cleaves a lipid (inositol phospholipid) into inositol-1,4,5-trisphosphate (IP3, a hydrophilic sugar) and diacylglycerol (DAG, a lipid in the membrane)

Phospholipase C activates 2 signaling pathways

The Organization of the Sympathetic Nervous System

Martini,Fundamentals of Anatomy and Physiology, 5th Edition,Prentice Hall 2001

The Organization of the Sympathetic Nervous System

ThoracicLumbar

Martinit,Fundamentals of Anatomy and Physiology, 5th Edition,Prentice Hall 2001

Sympathetic Synapses

Note location of synapse

The Organization of the Sympathetic Nervous System: The Adrenal Medulla

The Adrenergic Synapse

The Adrenergic Synapse

From: Basic and Clinical Pharmacology 8th edition, B.G. Katzung; Lange 2001

Adrenergic receptors

• Four receptor subtypes 1, 2, 1, 2

• G protein linked– Bind either norepinephrine or epinephrine

Adrenergic Receptors

• The two types of adrenergic receptors are alpha and beta

• Each type has two or three subclasses (1, 2, 1, 2 , 3)

• Effects of NE binding to: – receptors is generally stimulatory– receptors is generally inhibitory

• A notable exception – NE binding to receptors of the heart is stimulatory

Adrenergic Receptors

It will be important to understand how alpha and beta receptorsand the subtypes are distinguished:•Originally by relative potency•Now by cloning

From: Basic and Clinical Pharmacology 8th edition, B.G. Katzung; Lange 2001

Pharmacologic Demonstration of Adrenoreceptor Types

• The existence of alpha and beta receptors was originally proposed by Ahlquist in 1948

• Latter Lands et al 1967, suggested the beta1 beta2 distinction

Adrenergic Receptors

Adrenergic transmission• Catecholamines are the neurotransmitters• Complex synthesis• Secretion at nerve terminals and adrenal glands• Adrenal glands

– Two adrenal glands – Consist of cortex (outer) medulla (inner)

medulla secretes:– Epinephrine (adrenaline) – Norepinephrine

NE and E are released at nerve

terminals and secreted by the adrenal medulla

Norepinephrine and epinephrine

• Catecholamines• Synthesized from dopamine• Present in CNS and sympathetic nerves• Widely distributed, general behavioral

arousal eg raise blood pressure etc• Stress increases release of norepinephrine

Epinephrine targets a G-protein coupled receptor

Beta-adrenergic receptor pathway 1

• On binding of ligand, the receptor activates a G protein.

Dual Innervation

• Most of viscera receive nerve fibers from both parasympathetic and sympathetic divisions

• Both divisions do not normally innervate an organ equally

Dual Innervation

• Antagonistic effects – oppose each other– exerted through dual innervation of same effector

• heart rate decreases (parasympathetic)• heart rate increases (sympathetic)

– exerted because each division innervates different cells

• pupillary dilator muscle (sympathetic) dilates pupil• constrictor pupillae (parasympathetic) constricts pupil

Dual Innervation• Cooperative effects seen when 2 divisions act on different

effectors to produce a unified effect– parasympathetics increase salivary serous cell secretion– sympathetics increase salivary mucous cell secretion

• ANS cooperation is best seen in control of the external genitalia– Parasympathetic fibers cause vasodilation and are responsible for

erection of the penis and clitoris– Sympathetic fibers cause ejaculation of semen in males and reflex

peristalsis in females

Dual Innervation of the Iris

Without Dual Innervation• Some effectors receive only sympathetic

– adrenal medulla, arrector pili muscles, sweat glands and many blood vessels

• Sympathetic tone – a baseline firing frequency– vasomotor tone provides partial constriction

• increase in firing frequency = vasoconstriction• decrease in firing frequency = vasodilation• can shift blood flow from one organ to another as

needed– sympathetic stimulation increases blood to skeletal and cardiac

muscles -- reduced blood to skin

Sympathetic and Vasomotor Tone

Sympathetic division prioritizes blood vessels to skeletal muscles and heart in times of emergency.

Blood vessels to skin vasoconstrict to minimize bleeding if injury occurs during stress or exercise.

Regulation of ANS• Autonomic reflexes control most of activity of

visceral organs, glands, and blood vessels.

• Autonomic reflex activity influenced by hypothalamus and higher brain centers, but it is the hypothalamus that has overall control of the ANS.

• Sympathetic and parasympathetic divisions influence activities of enteric (gut) nervous system through autonomic reflexes.

Levels of ANS Control

• The hypothalamus is the main integration center of ANS activity

• Subconscious cerebral input via limbic lobe connections influences hypothalamic function

• Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

Hypothalamic Control

• Centers of the hypothalamus control:– Heart activity and blood pressure– Body temperature, water balance, and

endocrine activity– Emotional stages (rage, pleasure) and

biological drives (hunger, thirst, sex)– Reactions to fear and the “fight-or-flight”

system

Levels of ANS Control

Figure 14.9