Pharmacology of the Autonomic Nervous System

Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
Pharmacology of the Autonomic Nervous System
Lecture Outline
I. Introduction
II. Anatomy
III. Biochemistry
A. Neurotransmitters
B. Nonclassical Neurotransmitters
C. Synthesis and Metabolism of Acetylcholine
D. Synthesis and Metabolism of Catecholamines
E. Summary of Intervention Mechanisms
IV. Norepinephrine, Epinephrine and Dopamine
A. Adrenoreceptors
B. Alpha Agonists
1
C. Alpha Antagonists
1
D. Alpha Agonists
2
E. Alpha Antagonists
2
F. Beta Agonists
G. Beta Antagonists
H. Dopamine Agonists and Antagonists
I. Indirectly Acting Phenylethylamines
V. Acetylcholine
A. Acetylcholine Receptors
B. Muscarinic Agonists
C. Muscarinic Antagonists
D. Nicotinic Agonists
E. Nicotinic Antagonists (Ganglionic Blockers)
F. Cholinesterase Inhibitors
G. War Gases
VI. Skeletal Muscle Relaxants
VII. Bibliography
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
Learning Objectives: Autonomic and Neuromuscular Pharmacology
1)
An understanding of the clinical physiology of the autonomic nervous system
a)
Key structures in central cardiovascular control
b)
Neurotransmitters involved in major central and peripheral neuronal pathways
c)
Synthesis and metabolism of norepinephrine (NE) and acetylcholine (Ach)
2)
An understanding of a and b adrenoreceptors, their subtypes and the clinical spectrum of
their general and selective stimulation and blockade
a)
Key uses and side effects of major drugs in each category
b)
Clinical circumstances where these agents may be beneficial
3)
An understanding of muscarinic agonists and antagonists, and cholinesterase inhibitors
4)
An understanding of agents that stimulate or relax skeletal muscle, including the cholinergic
neuromuscular agonists and antagonists as well as the neuromuscular agents acting at
noncholinergic sites.
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
Figure 1: Schematic diagram of the sympathetic and parasympathetic divisions of the peripheral autonomic nervous system.
The paravertebral chain of the sympathetic division is illustrated on both sides of the spinal outflow in order to demonstrate
the full range of target structures innervated. Although the innervation pattern is diagrammatically illustrated to be direct
connects between preganglionic outflow and postganglionic neurons, there is overlap of innervation such that more than one
spinal segment provides innervation to neurons within the ganglia.
I. Introduction

The CNS receives diverse internal and external stimuli. These are integrated and expressed
subconsciously through the autonomic nervous system to modulate the involuntary
functions of the body. This overlies a strong circadian rhythm of autonomic function. The
somatic nerves that innervate voluntary skeletal muscle are not part of the autonomic
system, but will be discussed in the final lecture.

The autonomic nervous system consists of two large divisions (Figure 1):
∑ sympathetic (thoracolumbar) outflow, and
∑ parasympathetic (craniosacral) outflow.

The two divisions are defined by their anatomic origin rather than by their physiological
characteristics.
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
II. Anatomy
A. Central
The circadian rhythm of autonomic function originates in the suprachiasmatic nucleus
(SCN) in the hypothalamus, and is entrained by light falling on melanopsin-containing
retinal ganglion cell dendrites (not rods or cones) in the eye and transmitted to the SCN
by the retinohypothalamic tract. The integration of autonomic outflow to the
cardiovascular system lies in the medulla. Stretch-sensitive mechanoreceptors in the
blood vessels of the thorax and neck relay information about blood pressure and blood
volume through the glossopharyngeal (from carotid arteries) and vagus (from aorta)
nerves to the nucleus tract solitarii (NTS) in the posterior medulla.

Excitatory neurons from the NTS innervate the dorsal motor nucleus of the vagus,
where parasympathetic outflow is regulated. Inhibitory neurons, using gamma-
aminobutyric acid (GABA) as neurotransmitter, innervate areas in the ventrolateral
medulla from which sympathetic outflow is regulated. The most important such site is
the rostral ventrolateral medulla (RVLM).

Destruction of the NTS or its afferent input in experimental animals (Figure 2) or
by tumors, radiation or infarction in patients can lead to the syndrome of
baroreflex failure. A family in Nashville with a genetic defect leading to tumors in
the carotid body and other paragangliomas has taught us most of what we know
about baroreflex failure. There is an acute period of dramatic hypertension during
which stroke or pulmonary edema may occur. This is followed by a syndrome of
wide swings in blood pressure, from hypotensive to hypertensive levels, with
pressure determined by anxiety (pressor), sedation (depressor), noise (pressor),
and sunlight (pressor).
Figure 2: Contrast between clinical effects of NTS (afferent) destructive lesions on left and RVLM (efferent) destructive
lesions on the right. The contast is due to an inhibitory neuron that communicates from the NTS to the RLM.


Efferent parasympathetic outflow to the cardiovascular system goes through the vagus
nerve. Efferent sympathetic outflow from the RVLM travels in the bulbospinal tract
to the intermediolateral column of the spinal cord.

Glossopharyngeal neuralgia (glossopharyngeal syncope) is a disorder occurring
in patients whose 9th cranial nerve becomes damaged (usually by neck tumor).
Paroxysms of severe throat pain associated with hypotension and bradycardia
occur. Attacks are due to massive spontaneous afferent discharges of the
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
glossopharyngeal nerve, providing excessive input into the NTS, and eliciting
parasympathetic activation and sympathetic withdrawal. Although a pacemaker
may be helpful in preventing bradycardia, the hypotension is sometimes so severe
that surgical section of the glossopharyngeal nerve is required.
Figure 3: Efferent sympathetic outflow
Legend: AP, area postrema; NTS, nucleus tractus solitarii; RVLM, rostral ventrolateral medulla (C1 area);
intermediolateral column of the spinal cord; glutamate-releasing neuron; GABA, g-aminobutyric acid
releasing neuron; ACh, acetylcholine-releasing neuron; NA, norepinephrine-releasing neuron
The vasomotor neurons of the bulbospinal activate preganglionic cells sympathetic
nerves.
III. Biochemistry
A. Neurotransmitters

The primary neurochemical mediator of both sympathetic and parasympathetic
preganglionic neurons is acetylcholine (ACh). The primary mediator of sympathetic
postganglionic fibers is usually norepinephrine (NE), but at least some sympathetic
postganglionic fibers to sweat glands are cholinergic (acetylcholine). The mediator of
parasympathetic postganglionic fibers is acetylcholine. Epinephrine is found in the
adrenal medulla, the central nervous system and the para-aortic bodies (organs of
Zuckerkandl). Dopamine is a neurochemical mediator in the central nervous system
and probably also in some neurons in the superior cervical ganglion and the kidney.
Norepinephrine, epinephrine and dopamine are sometimes collectively referred to as
catecholamines. Outside the United States, norepinephrine is often called
noradrenaline, and epinephrine, adrenaline. These endogenous compounds plus drugs
that resemble them functionally and structurally are also called sympathomimetic
amines.
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
B. Nonclassical Neurotransmitters

Although it was initially assumed that each neuron would have one and only one
neurotransmitter, it is now clear that multiple neurotransmitters commonly exist within
one neuron, and they may be differentially released. It is not fully understood why such
cotransmission occurs, but autonomic nerve stimulation may consist of several phases
with distinctive time courses, each of which is mediated by a different cotransmitter.
Furthermore, cotransmitters usually interact at various levels, ranging from modulation
of neurotransmitter release to the regulation of calcium concentrations of the effector
cells.

Thus, cotransmission may be a fundamental mechanism employed by autonomic
neurons to achieve efficient and precise control of their target tissues over a range of
functional demands.
Figure 4: Complexity of neurotransmission.

The complexity of cotransmission is well-illustrated in selected sympathetic neurons. It
can be seen that in various neurons, norepinephrine (NE), neuropeptide Y (NPY),
dynorphin 1-8 (DYN 1-8), and dynorphin 1-17 (DYN 1-17), are all involved,
presumably in addition to ATP (not shown in the figure) which is a cotransmitter in
almost all sympathetic neurons.

The neurotransmitter role of ATP deserves special consideration. ATP is localized, and
released from, sympathetic, parasympathetic, enteric, and even sensory neurons. ATP
acts at prejunctional or postjunctional sites, either directly as ATP on purinergic
receptors, or after metabolism to adenosine on adenosine receptors.

Purinergic receptors are categorized into P (sensitive to adenosine) and P (sensitive
1
2
to ATP), but many people subcategorize P receptors as adenosine receptors (A , A ,
1
1
2
and A ). These adenosine receptors will be detailed by other lecturers in this course.
3
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.
C. Synthesis and Metabolism of Acetylcholine

Acetylcholine (ACh) is synthesized by choline acetyltransferase, a soluble cytoplasmic
enzyme that catalyzes the transfer of an acetyl group from acetylcoenzyme A to choline.
The activity of choline acetyltransferase is much greater than the maximal rate at which
ACh synthesis occurs. Choline acetyltransferase inhibitors have little effect to alter the
level of this bound ACh. ACh is stored in a bound form in vesicles. Choline must be
pumped into the cholinergic neuron, and the action of the choline transporter is the rate-
limiting step in ACh synthesis.

Upon the arrival of an action potential in the cholinergic neuron terminal, voltage-
sensitive calcium channels open and ACh stores are released by exocytosis to trigger a
postsynaptic physiological response. The release of acetylcholine can be blocked by
botulinum toxin, the etiologic agent in botulism. Fortunately, botulinum toxin has been
used to treat dystonias and spastic disorders, and has turned out to be uniquely effective.
Although it requires local injection, it is often effective for weeks or months.

Much of the ACh released into the synapse is transiently associated with ACh receptors.
Figure 5: Characteristics of transmitter synthesis, storage, release, and termination of action at cholinergic and
noradrenergic nerve terminals are shown from the top downward. Circles with rotating arrows represent transporters;
ChAT, choline acetyltransferase; ACh, acetylcholine; AChE, acetylcholinesterase; NE, norepinephrine.
This action is terminated by the rapid hydrolysis of ACh into choline and acetic acid, a
reaction catalyzed by the enzyme acetylcholinesterase. The transient, discrete,
localized action of ACh is due in part to the great velocity of this hydrolysis. The
choline liberated locally by acetylcholinesterase can be reutilized by presynaptic
reuptake (by the high-affinity system described above) and resynthesis into ACh.
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.

In addition to acetylcholinesterase (true cholinesterase) which is found near cholinergic
neurons and in red blood cells (but not in plasma), there is also a non-specific
cholinesterase (pseudocholinesterase or butyrylcholinesterase) which is present in
plasma and in some organs but not in the red blood cell or the cholinergic neuron.

The genetic abnormalities in pseudocholinesterase can result in a marked deficiency.
One in 30,000 people are homozygotes for the most common functionally abnormal
variant, dibucaine-resistant pseudocholinesterase (3.8% of people are therefore
heterozygotes for this gene). Cholinergic nerve activity in such people is normal but
some drugs such as succinylcholine (used during anesthesia) which are normally broken
down by pseudocholinesterase, are very poorly metabolized by this variant enzyme.
Such patients may have prolonged muscle paralysis from succinylcholine.
D. Synthesis and Metabolism of Catecholamines

Tyrosine in the bloodstream is taken up into nerves and converted into catecholamine.
The five main enzymes whose functions are critical to the formation of catecholamines
are discussed below (Figure 6).

Tyrosine hydroxylase (tyrosine to dopa) is the rate-
limiting step in NE synthesis and is located in the
cytoplasm. Catecholamines act as feedback inhibitors
of this enzyme. During increased sympathetic
stimulation, dopa production is increased in two
ways: a) more enzyme is synthesized, and b) the
physical properties of the enzyme are altered
(allosteric activation) so that affinity for tyrosine is
increased and affinity for end products like NE is
reduced. A clinically useful inhibitor of this enzyme
is metyrosine (a-methyl-p-tyrosine).

Dopa decarboxylase (dopa to dopamine) is found in
the cytoplasm of many nonneural as well as neural
tissues and had been called "aromatic-L-amino acid
decarboxylase" because of its broad substrate
specificity. Peripheral (non-neuronal) dopa Figure 6: Metabolic pathway of
decarboxylase can be inhibited by carbidopa when catecholamine synthesis.
one is trying to prevent formation of peripheral
dopamine during dopa therapy of Parkinsonism. This limits dopamine production to the
central nervous system during dopa therapy, thus limiting peripheral side effects.

Dopamine-ß-hydroxylase (dopamine to norepinephrine) is a copper-containing enzyme
located primarily within the membrane of amine storage granules.

Some individuals have been found to have dopamine-ß-hydroxylase deficiency.
They present with lifelong orthostatic hypotension, and ptosis of the eyelids. Their
sympathetic neurons contain large quantities of dopamine, but little or no
norepinephrine. They can be treated with the drug dihydroxyphenylserine (DOPS),
which is decarboxylated directly into norepinephrine by dopa decarboxylase, thus
restoring the appropriate neurotransmitter.
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.

Phenylethanolamine-N-methyltransferase (norepinephrine to epinephrine) is restricted
to the adrenal medulla, the brain and the organ of Zuckerkandl, with only trace amounts
in other locations. It is strongly inhibited by physiological concentrations of
epinephrine providing feedback regulation of enzyme synthesis. Glucocorticoid
increases enzyme activity.

Much neuronal NE is located in neuronal vesicles. They store NE and protect it from
breakdown by monoamine oxidase (MAO) in the surrounding cytoplasm. These
vesicles are subsequently transported to the neuron terminal region for release.

Release occurs when acetylcholine liberated from preganglionic neurons induces
depolarization of postganglionic sympathetic neurons by acting on a nicotinic receptor
(see below). The influx of calcium stimulates migration of vesicles to the cell
membrane for excretion, by membrane fusion and exocytosis.

Local synaptic concentrations of catecholamines modulate their own release by
interacting with presynaptic a -receptors to reduce release of additional
2
norepinephrine and presynaptic b -receptors to increase release of norepinephrine
2
(more about this below).

The reason for this bidirectional control is not known with certainty, but may function to
stabilize synaptic neurotransmitter levels. In addition, other substances may
increase (angiotensin, and acetylcholine via a nicotinic receptor) and decrease
(dopamine, histamine, serotonin, adenosine, PGD , PGE and acetylcholine via a
2
2
muscarinic receptor) norepinephrine release in selected tissues.

Released catecholamines may a) be retaken up into the neuron (norepinephrine
transporter or uptake I), b) be taken up by the extraneuronal tissue (uptake II), or c)
be washed into the extracellular fluid and ultimately into the circulation. Termination
of action of released catecholamine varies with organ site. Thus, heavily innervated
tissues like the heart with narrow synaptic clefts tend to rely heavily on the
norepinephrine transporter (90% uptake of released NE), while tissues such as the aorta
with wide synaptic clefts and less dense innervation tend to rely on it less. The
catecholamines may be metabolized by one of two enzymes.

Monoamine oxidase occurs in two forms (A and B). Monoamine oxidase is located in
the outer membrane of mitochondria as well as extraneuronally. It converts
catecholamines to their corresponding aldehydes. Inhibitors include pargyline,
tranylcypromine, and selegiline (Deprenyl®) that will be covered in part 2 of the course
with COMT inhibitors below.

Catechol-o-methyltransferase (COMT) converts NE into normetanephrine and
epinephrine into metanephrine. It is found especially in liver and kidney.

Uptake into the neuron terminal by the NE transporter is very efficient. Fully half of an
intravenous infusion of NE is taken up and stored in neurons, primarily in heart, spleen,
and blood vessels, The structure of the NE transporter and elucidation of its regulation
was achieved by Vanderbilt’s Dr. Randy D. Blakely. The uptake mechanism is an
energy requiring, saturable membrane transport system, that can be blocked tricyclic
antidepressants, amphetamine and cocaine (more from Dr. Sanders-Bush, next section).
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Pharmacology 501
January 10 & 12, 2005
David Robertson, M.D.

In healthy persons at rest, plasma NE is about 250 pg/ml and plasma A is 25 pg/ml.
Normally, plasma norepinephrine level is doubled by standing, but it is several fold
elevated by running, and in myocardial infarction (MI), delirium tremens (DT’s), and
pheochromocytoma (pheo).
E. Summary of Intervention Mechanisms
1. Cholinergic neurotransmission can be modified at several sites, including:
a) Precursor transport blockade
hemicholinium
b) Choline acetyltransferase inhibition
no clinical example
c) Promote transmitter release
choline, black widow spider
venom (latrotoxin)
d) Prevent transmitter release
botulinum toxin
e) Storage
vesamicol prevents ACh storage
f) Cholinesterase inhibition
physostigmine, neostigmine
g) Receptors
agonists and antagonists

2. There are also many sites at which pharmacological alteration of sympathetic
noradrenergic function can take place (Figure 4). They are reviewed below:
a) Precursor transport blockade
no clinical example
b) Tyrosine hydroxylase inhibition
metyrosine, used to treat
pheochromocytoma
c) Dopa decarboxylase inhibition
carbidopa
d) Dopamine-ß-hydroxylase inhibition
disulfiram
e) Monoamine oxidase inhibition
pargyline, tranylcypromine,
selegiline
f) Storage
reserpine prevents NE storage
g) Release
guanethidine, guanadrel cause
initial release of NE leading to
depletion of catecholamine;
bretylium blocks NE release
h) Receptors
a-and b-agonists and antagonists
i) Norepinephrine transporter
cocaine, tricyclic antidepressants
(Uptake I) blockade
j) Catechol-o-methyltransferase
entacapone
inhibition
k) Uptake II
glucocorticoids

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