Download ANS - Anesthesiology, Pharmacology and Therapeutics

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PCTH 300-305 AUTONOMIC NERVOUS SYSTEM (ANS)
Please note: This is an extensive set of notes for a first exposure to pharmacology. Not all handouts will be so voluminous.
The intent is to make you read, and think about pharmacology. Memorizing so many drug names in Pharmacology is
intimating and it is not possible to know them all, but it is best to know those in bold type.
Brief overview of anatomy and function of the ANS:
The peripheral nervous system is outside the Central Nervous System (brain and spinal cord). It consists
of the autonomic nervous system (ANS) and the motor (somatic) nervous system. The ultimate control
of both systems lies within the CNS in ‘command and control’ centres. However, while the ANS has
considerable autonomy, CNS imperatives drive the motor (skeletal muscle) nervous system. Thus one
consciously walks, talks, etc. as a result of central commands from the CNS but digestion, heart rate, etc.
are not so directly controlled. The ANS serves to regulate the internal organs and function of the body,
while the motor system serves skeletal muscle (locomotion) functions.
Basic anatomy of the autonomic nervous system (ANS)

Anatomically the peripheral ANS is comprised of three (3) anatomic and functional divisions:
sympathetic, parasympathetic and enteric.

The basic pattern for both sympathetic(S) and parasympathetic sections (PS) of the ANS consists
of pre-ganglionic neurons arising from the CNS (where there are ANS centres) that travel to
peripheral ganglia from where postganglionic nerves innervate target organs.

The peripheral parasympathetic CNS outflows are to:
o Cranial nerves (cranial nerves III, VII, IX and X)
o Sacral nerves

Parasympathetic ganglia usually lie close to, or within, the target organ

Sympathetic nerves leave the CNS via thoracic and lumbar spinal roots to synapse at
sympathetic ganglia which form two (2) paravertebral chains, plus some midline ganglia

The enteric nerves arise from neurons in the intramural plexi of the gastrointestinal tract. This
enteric system receives input from both sympathetic and parasympathetic nerves, but can act
autonomously to control motor and secretion functions of the intestine.
The following diagrams outline the peripheral ANS – both structure and function. The two major
systems (PS and S), to a degree, act as counterparts to each other. Thus, parasympathetic nerve activity
slows heart rate while sympathetic activity increases heart rate. However, such simplification cannot be
taken too far. For example, via effects on heart and blood vessels sympathetic nerve activity increases
blood pressure whereas parasympathetic nerve activity has little or no effect on blood pressure. An
accelerator/brake analogy has been used for the roles of the PS and S, but it overly simplistic. Each and
every organ in the body has different degrees of sympathetic and parasympathetic innervation.
The pharmacology of the ANS mainly concerns drugs acting on efferent side of the ANS since few
drugs influence ANS afferent activity going to the CNS, or the ANS centres within the CNS. For the
greater part, most ANS drugs either mimic, accentuate, or block responses to efferent ANS activity.
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The role of the ANS is to maintain homeostasis and ensure that internal organs respond to the
requirements of the body. The following Diagrams (1-3) indicate which organs are innervated by the
ANS. For convenience in Diagram 1 para-sympathetic activity is on the left and sympathetic on the
right.
Diagram 1 An incomplete overview of the ANS
Diagram 2 provides more detail including the location of ganglia for both parasympathetic (left) and
sympathetic (right) pathways and more information as to organ responses to PS and S nerve activity.
Note: parasympathetic ganglia are located close to, on, or in, the organ they innervate, whereas the
sympathetic ganglia are mainly located in close proximity to the spinal column and, most importantly
communicate with each other particularly via the paravertebral sympathetic chain that lies just outside
the spinal column, but within the thorax. This anatomical pattern allows for an overall coordinated
activation of the sympathetic system, as opposed to the directed and localized activation of single organs
by parasympathetic nerves.
Diagram 2 ANS – more detail
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To complete the boredom quotient Diagram 3 further amplifies the anatomical and functional
differences between the PS and S sections of the ANS but, this time, with the parasympathetic section
on the right. Diagram 3 better illustrates the peripheral wiring of the ANS and the central role of the
parasympathtic vagus nerve in controlling internal organs. Note: the role of the cranial nerves in eye,
lacrymal and salivary glands in the head, and the sacral nerves in the lower part of gastrointestinal and
urinary organs. This diagram exemplifies the ‘wiring’ differences in the the two sections inasmuch that
most PS ganglia lie on or within the organ they innervate. On the otherhand, the diagram also shows the
important paravertebral sympathetic ganglion chain which forms a network with few ‘external way
stations’ such as the coeliac and superior and inferior mesenteric ganglia. The left side diagrams in
insets illustrate how sympathetic nerves innervate blood vessels and skin. Sweating is an important
function of the sympathetic nervous system as well as control of body hair and skin blood vessels.
When we are hot we sweat, body hair stands upright and skin flushes - all driven by sympathetic
activation BUT, as seen later, sweating in particular involves post ganglionic cholinergic activity.
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ANS:- Neurotransmitter Molecules
Acetylcholine (Ach) is the principal transmitter molecule at ganglia and at post-ganglionic
parasympathetic nerve endings.
Norepinephrine NA or NE (noradrenaline) is the principal neurotransmitter at post ganglionic
sympathetic nerve endings.
There are a few special exceptions to the above:
sympathetic nerves to sweat glands release ACh
the adrenal medulla (which is a sympathetic ganglion) releases mainly epinephrine (adrenaline).
Cholinergic refers to sites at which acetylcholine is the primary neurotransmitter.
Adrenergic refers to sites at which norepinephrine is the primary neurotransmitter.
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Overall summary of the physiological function of the ANS

The autonomic nervous system controls: smooth muscle (visceral and vascular) activity;
exocrine (and some endocrine) secretions; rate and force of the heart; certain metabolic processes
(e.g. glucose utilization)

Sympathetic and parasympathetic systems sometimes have opposing actions in certain situations
(e.g. control of the heart rate, gastrointestinal smooth muscle); but not in others (e.g. salivary
glands, ciliary muscle of the eye)

Sympathetic activity increases in stress (fight, fright, flight responses) whereas parasympathetic
activity predominates during satiation and repose. Both systems exert continuous physiological
control of specific organs under normal conditions.

In addition to the cholinergic and adrenergic ANS some parts of the ANS are Non Adrenergic
Non Cholinergic (NANC). The relative importance of the NANC system in physiology and
pathology is still being resolved.
A simplified?? table of actions of the ANS
Organ
Heart
Atria
Ventricle
AV node
Blood vessel
PSymp
Symp
Importance
Bradycardia
No effect
Slowed transmission
Very little
Tachycardia
Positive force
Increased
Constrict
P=PS
S>>>PS
Equal
S>>>>PS
Contract (usually
limited)
Secretions increased
Narrow iris
Contracts lens
Relax (usually
limited)
Drying of secretions
Dilate pupil
Nose
Salivary glands
Reduces patency
Watery secretions
Increases patency
Thick mucoid
secretions
Tear glands
Tears
PS
Adrenal glands
Nicotinic ganglion
PS (celiac ganglion)
Lung
Bronchial
muscle
Bronchial glands
Eye
Kidney
Liver
PS>S
PS>>S
PS>>>S
PS
S>>PS
PS>>S
Stimulates gall
bladder
Glycogenolysis
Contracts
Relaxes rectum
Contracts
Contracts rectum
Relaxes
PS>>>S
Stomach
Spleen
Bladder
Acid release
Contracts
Slows digestion
Contracts
Relaxes
PS>>>S
S
PS>>S
Genitalia
Male – tumescence
Detumescence
PS for up, S for down
Pancreas
Large intestine
Small intestine
Outcome
Cardiac modulation
Raise blood
pressure
Bronchial muscle
tone
Dominant effect
Accommodation of
vision
Dry mouth if PS
blocked
Dry eyes if PS
blocked
Release of
adrenaline
S liver
PS gall bladder
PS controls
defecation
Enteric nerves
important
PS controls
micturition
Involves NO
ANS Peripheral Neurotransmitter molecules
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
The principal transmitter at post-ganglionic parasympathetic endings is acetylcholine (ACh) and
also at all ganglia. Norepinephrine - NE (noradrenaline NA) is the major transmitter in post
ganglionic sympathetic nerve endings.

Preganglionic neurons of both PS and S sections are cholinergic and ganglionic transmission is
primarily via stimulation of post synaptic ganglionic nicotinic ACh receptors on post synaptic
neurones (some muscarinic receptors are also present in ganglia)

Postganglionic parasympathetic nerve endings are cholinergic with ACh acting on muscarinic
receptors (MChR) located on the effector organs.

Postganglionic sympathetic neurons are mainly noradrenergic A few special sympathetic nerves
are cholinergic (e.g. sweat glands) and ACh acts on muscarinic ACh receptors

Transmitters other than noradrenaline and acetylcholine (NANC transmitters) also occur in
the autonomic system. The main ones are nitric oxide (NO) and VIP (parasympathetic), ATP
and NPY (sympathetic). Others, such as 5-HT, GABA and dopamine, also play a role.

Co-transmission involving co-release of more than one transmitter is a common phenomenon
CHOLINERIC TRANSMISSION
Some ‘definitions’ used in Pharmacology
AGONISTS – such as neurotransmitters, hormones, natural or synthetic molecules that activate (i.e. agonize) the receptors to
which they bind, and produce a functional response.
RECEPTORS - are drug targets, some of which are plasma membrane proteins, to which drugs bind, and subsequently do, or
do not, produce a functional action.
ANTAGONISTS – bind selectively to a receptor without activating it thereby denying agonist access to their receptor
binding site.
PARTIAL AGONISTS – Stimulate a receptor inefficiently, but will produce a functional response when the receptor is not
occupied by a full agonist and reduce the actions of a full agonist
INVERSE AGONISTS – reduce activity in receptors that are spontaneously active in absence of agonist.
ANTAGONISTS – occupy receptors at or near the agonist binding site and do so without agonizing them. Thus they
antagonize agonists in a selective receptor specific manner. They may act reversibly or irreversibly.
ALLOSTERIC MODULATORS – do not bind to an agonist recognition site but their binding at an allosteric site modulates
receptor activity.
Receptors agonized (stimulated) by ACh (known as cholinoceptors or cholinergic receptors) exist as
various subtypes, whose presence and density varies with tissue type. There are two major types:
MUSCARINIC and NICOTINIC. Various subtypes of each of these two main types have been
molecularly identified, and are associated with different tissues. Drugs are invented (discovered) to
selectively target such sub-types so as to produce tissue specific functional actions.
Muscarinic Receptors: M1, M2, M3, (M4&5) are GPCRs (G Protein Coupled Receptors)
found peripherally, and in the CNS.
Nicotinic Receptors The two main peripheral types of nicotinic receptors are those on skeletal
muscle (Nm) and those on nervous tissue such as ganglia (Nn). Both are part of ACh-gated ion channels
which, when opened by ACh, have selective permeability to both Na and K ions. Their opening
produces depolarization of the post synaptic (post junctional) membrane. There are further sub- types of
Nicotinic receptors within the CNS.
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Synthesis and Metabolism of ACh
Acetylcholine is synthesized in cholinergic nerve endings from the amino alcohol - choline, and the
carboxylic acid - acetic acid, both common in tissues. The enzyme choline acetylase forms the ester,
acetylcholine. Synthesized acetylcholine is taken into and stored in vesicles in cholinergic nerve
endings. Vesicles are mobilized by calcium ions entering via voltage-opened Ca++ channels (N type
calcium channels) as a result of a sodium dependent action potential at the nerve terminal. The
mobilized vesicle fuses with the pre-synaptic nerve membrane and releases ACh into the synaptic cleft
where it diffuses to post-synaptic (post-junctional) cholinergic receptor. Released acetylcholine is
rapidly hydrolyzed by acetylcholinesterase into choline and acetate, thereby terminating its action.
Acetate is free to participate in intermediate metabolism while choline is re-cycled - taken up into the
pre-synaptic nerve ending. Drugs are available to interfere with the whole process, from ACh synthesis,
to vesicle storage, to Ca++ dependent release, to choline re-uptake. However, not many of such drugs
have therapeutic utility. Vesamicol is a reversible blocker of the intracellular transporter responsible for
ensuring storage of ACh into vesicles. N type calcium channel blockers (ω-Conotoxins, Ziconotide,
Caroverine and others) prevent opening of neuronal calcium channels. Such blockers interfere with
neuronal release of all transmitters that rely on calcium for activation of their vesicles. This ubiquitous
action on all nerve endings limits their utility.
Hemicholinium blocks the uptake of choline into the nerve ending
Botulinus toxin (from the anaerobe bacterium Clostridium botulinum) depletes nerve endings of ACh).
It is a two-chain polypeptide whose light chain is a protease that disables a fusion proteins (SNAP-25)
that prevents vesicle functioning. Function is restored slowly by synthesis of new vesicles, rather than
by re-filling. Of the above drugs, only botulinus toxin (botox) is routinely used therapeutically (in
neuromuscular junction pharmacology, skeletal muscle dystonias and cosmetics).
MUSCARINIC AGONISTS
The discovery in plants, fungi, and later in animals, of chemicals that had parasympathetic-like actions
was central to the analysis of cholinergic actions and analysis of the ANS. One fungal chemical was
muscarine (Amanita muscaria). This had actions that mimicked stimulation of the parasympathetic
system. Nicotine from plants, including tobacco (genus Nicotiana) stimulates nicotinic receptors in
ganglia resulting in the stimulation of the peripheral parts of the ANS. Atropine from the family
Solanaceae (nightshade)
Muscarinic agonists are of limited therapeutic use in medicine
MUSCARINIC AGONIST & their receptors (MAChR: M1-5)
All muscarinic receptors are GPCRs (G protein coupled receptors):
M1,3 &5 via Gq/11 family; M2 & 4 through Gi and Go when receptor is agonized so as to:
- Activate phospholipase C to form inositol triphosphate (IP3) and diacylglycerol (DAG) – which are
intracellular second messengers
- inhibit adenylate cyclase to reduce intracellular cAMP
- inhibit opening of calcium channels
- activate potassium channels to generate an inward potassium current - iKMACh
MAChRs- are present on effector tissue at cholinergic post ganglionic nerve endings, and sympathetic
sweat glands. They also occur in ganglion where they are of lesser importance than nicotinic receptors
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Subtypes of muscarinic receptors
Note: There is a lack of really selective (100:1 or more) agonists and/or antagonists for the
subtypes of M receptors, and a lack of clarity regarding their individual functional importance.
Different muscarinic agonists have different profiles of action in intact animals due to different
potencies on the various sub-types of muscarinic receptors, but we have still to fully elucidate the role of
each the sub-types. The following is an approximate summary.

M1 on neuronal tissue in ANS ganglia, CNS, exocrine glands - involve Gq, IP3 and DAG to
elevate Ca++, produces "slow" excitation in ganglia. Agonists that are partially selective:- McN
–A- 343, muscarine. Antagonists, again are only partially selective:- atropine, oxybutynin,
ipratropium, mamba toxin muscarinic toxin 7, pirenzepine.

M2 on cardiac tissue (PS nerves innervate atria, atrioventricular and sinus nodes, but not
ventricles) - decrease heart rate and reduced atrial contraction via Gi (decrease in cAMP) and K
channels. Inhibit synaptic transmission. Generally inhibitory effects. No really selective
agonists (maybe bethanechol), nor antagonists (dimetindene, otenzepad)

M3 increases glandular secretion (e.g. sweat glands), lungs, contract gut, bronchial and blood
vessel smooth muscles. However also relax blood vessel smooth muscles via releasing the
relaxant nitric oxide (NO) from endothelium of blood vessels. Few, if any selective agonists
carbachol, oxotremorine, pilocarpine (in eye) or antagonists (tolterodine, oxybutynin,
ipratropium, darifenacin, tiotropium).

M4 in CNS and acts as a regulatory ‘auto receptor’ – inhibits adenyl cyclase- regulatory role in
CNS dopamine transmission. Agonist not so selective (oxotremerine, carbachol), antagonists
(tropicamide – some 2-5x), AFDX-384, dicycloverine)

M5 down regulates cAMP and protein kinase A. No really selective agonists (milameline,
sabcomeline) nor antagonists (VU compounds)
All M receptors equally stimulated by the agonist ACh and blocked by the general muscarinic receptor
antagonist - ATROPINE
MUSCARINIC/nicotinic AGONISTS
Many are acetylcholine (ACh) analogs. The relationships between chemical structure and actions (agonists or
antagonist) are known as Structure Activity Relationships (SAR).
Acetylcholine:
Stimulates all types of
cholinoceptors (nicotinic and
muscarinic)
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Muscarine:
Agonist at muscarinic receptors
only. Methacholine (MCh)
stimulates only muscarinic
receptors.
SAR
Critical chemical centres in acetylcholine are the positive Nitrogen and the carbonyl oxygen (-C=O).
Those with an ester (acid + alcohol) linkage, such as ACh and methacholine MCh (methacholine =
acetylbetamethylacetylcholine) are metabolized by acetylcholinesterase (AChE). It might be surprising
that very small changes in acetylcholine’s structure produces methacholine and loss of nicotinic activity.
Knowledge of SAR potentially allows one to invent selective agonists and antagonists, but this has not
been totally successful with muscarinic receptors, despite our long knowledge of the cholinergic system.
This is a pity since the above list of receptors should provide an avenue to new drugs with specificity for
specific disease via selective action (agonist or antagonist) on sub-types of muscarinic receptors.
Therapeutic uses of Muscarinic Agonists (see above for details regarding sub types of the M receptor)
There are two pharmacological ways of way of mimicking the activity of the PS system: 1) by
muscarinic agonists, or 2) by anticholinesterase drugs. The therapeutic value of activation are
relatively limited, although there has been intense interest in the use of muscarinic agonists to treat
symptoms of Alzheimer’s disease. M1 receptors play a role in cognition and may have a potential
therapeutic role.
Classic agonists are methacholine, bethanecholine and carbamoylcholine (carbachol).
Bethanecholine is muscarinic receptor selective, methacholine is more selective for muscarinic versus
nicotinic, carbachol more nicotinic
Pilocarpine, a M3 agonists, has been used in glaucoma as has aceclidine. Other uses include dry mouth
in diseases such as Sjögren's syndrome.
Arecoline is a natural alkaloid fund in betel nut which if chewed aids digestion and gastrointestinal
function.
CHOLINESTERASE INHIBITORS
Cholinesterase are enzymes in tissue and plasma. They have their own characteristic profiles with
respect to factors such as: specificity, substrate site, inhibitors, and types of inhibition.
The main type - acetylcholinesterase – has high selectivity for ACh and is present at cholinergic
junctions and synapses. It has a clearly defined substrate site and has active sites which recognize: 1) N+
and 2), the ester linkage of acetylcholine. Anticholinesterase drugs can bind to either site, or both, and
act as reversible or irreversible inhibitors. Non-specific plasma and tissue esterases breakdown
esters and lack selectivity for ACh.
ACTIONS OF CHOLINESTERASE INHIBITORS
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The actions of reversible cholinesterase inhibitors are principally due to inhibition of
acetylcholinesterase and resulting accumulation of ACh at cholinergic synapses resulting in excessive
muscarinic and nicotinic receptor stimulation. In addition, some have additional direct agonist effects
on the receptor. Irreversible inhibitors such as the organophosphate compounds can have long term toxic
actions that are not due to inhibition of cholinesterases. Such inhibitors are lipid soluble, enter the brain,
and have profound effects at CNS cholinergic sites. Inhibitors with a charged nitrogen (N+) in their
structure do not penetrate the CNS to a significant degree.
Chemical types of Cholinesterase Inhibitors
1. Amino alcohols (edrophonium) bind reversibly at N+ site and are shorter acting.
2. Carbamates (neostigmine) are longer lasting and bind at both sites
3. Organophosphates (diisopropyl flurophosphate – DFP) bind irreversibly at the esteratic site
Edrophonium is very short acting and can be used to diagnose myasthenia gravis (see later) – increases
muscle strength shortly after i.v. administration is a positive test for myasthenia gravis. Also used to test
adequacy of treatment of myasthenia gravis with a longer acting drug (e.g. pyridostigmine) where if
edrophonium increases muscle strength more intensive anticholinesterse treatment is required.
Organophosphates covalently phosphorylate enzyme at the ester site and, unless quickly reversed, this
becomes irreversible. Used as insecticide and poison gases (vapours) e.g. chlorpyrifos, malathion and
diisofluorophosphonate (nerve gas) with over 100 organophosphate & 20 carbamate insecticides
available that cause many cases of poisoning in agriculture. The toxicity of such compounds includes
CNS effects, e.g. convulsions, GI, respiratory, urinary effects – stimulatory, like direct-acting agents,
cardiovascular effects - both sympathetic & parasympathetic stimulation, fibrillation of skeletal muscle
fibers, depolarizing blockade and muscle paralysis.
Symptoms of poisoning: Salivation, Lacrimation, Urination, Defecation, Gastric Emptying - hence
acronym, SLUDGE
Treatment of poisoning with cholinesterase inhibitors.
In addition to general supportive medical care, specific treatment includes atropine (or atropinic drugs)
to block muscarinic receptors in the periphery and brain. Atropine is most important in blocking
excess stimulation of muscarinic receptors in the periphery and in the CNS. Recent exposure (hours) to
organophosphates can be reversed by oximes such as PAM2 and DAM. Reversible anticholinesterase
inhibitors have been used for prophylaxis via “protection” of the active site of the enzyme from
organophosphates. The military around the World have antidote/treatment packs involving all the above
drugs; these sometimes include a reversible anticholinesterase to deny the irreversible anticholinesterase
access to active sites on the enzyme.
MUSCARINIC ANTAGONISTS
The original definitions of drugs as being antagonists were based upon the ability of certain drugs
(derived from plants and herbs) to selectively antagonize (inhibit/prevent) the actions of the ANS,
neurotransmitters, and their analogues.
The most simplistic view of an antagonist for a particular type of receptor is a drug which
occupies the agonist recognition site without agonizing (activating) the receptor. Thus antagonists
"deny" agonists access to their receptor to prevent endogenous or exogenous agonists from binding.
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This model is analogous to the substrate/inhibitor models for enzymes. This simplistic view works well
in many cases.
Conventionally, by Mass Action kinetics, the reaction is as follows:A+R = AR which, after suitable transduction, gives a response
I+R = IR which gives no response
Where A is an agonist, and I antagonist (inhibitor)
By forming the IR complex, a sufficient concentration of I will prevent A having an action by
competition for R, while a high enough concentration of A will prevent I forming IR by competition for
R: thus, we have Competitive Antagonism.
ATROPINE is the prototypical muscarinic receptor competitive antagonist
Atropine, from a plant Atropa belladonna - extracts widened the pupils and so "beautified" medieval
Italian ladies. Other muscarinic antagonists are found in other plants, e.g. Datura stramonium. Many
synthetic atropinics have been made in attempts to try and achieve selectivity for subtypes of
muscarinic receptors.
The primary action of muscarinic receptor antagonists is to block responses due to activation of
peripheral parasympathetic nerves or cholinergic muscarinic nerves in the CNS although some
atropinics have other, ancillary, pharmacological actions.
The formula for atropine fits the SAR for muscarinic receptor agonists and antagonists.
Note: atropine is larger than acetylcholine, but parts of its structure (N and ester linkage) mimic
elements of ACh. It binds to the ACh binding site on the muscarinic receptor, but fails to activate it.
Note: it has a greater lipophilic nature (phenyl ring) than acetylcholine.
THERAPEUTIC USE OF COMPETITIVE MUSCARINIC ANTAGONISTS
Many similar drugs have been found, or synthesized, following the discovery of atropine. Most of their
actions are principally a reflection of blockade of muscarinic cholinoceptors and the action of
endogenous ACh. However, some have other (extra) pharmacological actions. As a class, competitive
antimuscarinics will have in vivo actions only when there is parasympathetic activity while any other
pharmacological actions could be a possible bonus for certain drugs of this class. It is also important to
recognize that other classes of drugs can have competitive muscarinic antagonist actions. One useful
aide memoire for the functional actions of atropine, and by implication parasympathetic cholinergic
importance, are the symptoms of Atropine Poisoning
Dry as a bone - no sweating
Blind as a bat – no ocular accommodation
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Mad as a hatter – CNS symptoms. In the past the use of mercury by hatters resulted in CNS toxicity
hence the expression.
Red as a beet – a result of vasodilation and no sweating - in children atropine poisoning produces
dangerous hyperpyrexia
Atropinic drugs include:
Alkaloids (naturally occurring) – atropine, scopolamine (hyoscine)
Tertiary amines – dicyclomine, benztropine
Quaternary amines – ipratropium
Examples of the use of this class of drugs in medicine:
CARDIOVASCULAR - atropine and related drugs to reverse sinus node bradycardia. Heart rate is
under continuous sympathetic and parasympathetic activity. At rest, and when asleep, parasympathetic
nerves are dominant, whereas during exercise the sympathetic nerves are most important.
OPTHALMIC - tropicamide or cyclopentolate to dilate pupils. Control of the muscles of the iris and
lens is dominated by parasympathetic nerves therefore their blockade with a muscarinic antagonist will
dilate the pupil and paralyze accommodation for near vision (making reading difficult). Used for their
ophthalmic actions and eye examinations, but have a possible side effect of glaucoma.
LUNGS – atropinics can produce bronchodilation in some asthmatics: ipratropium or similar drugs by
inhalation. Such drugs are helpful for some asthmatics, and in chronic obstructive pulmonary disease
(COPD) since some bronchoconstriction and hypersecretion in asthma and COPD is dependent upon
muscarinic receptor stimulation.
MOTION SICKNESS – can be alleviated with hyoscine (scopolamine) acting via a CNS action. The
CNS mechanism(s) responsible for this effect has not been fully elucidated. Hyoscine has been used as
a so-called ‘truth serum’ to produce CNS confusion and thereby reveal ‘truths’.
PARKINSONISM - benzhexol and benztropine to limit movement disorders due to other classes of
AntiParkinson drugs via a CNS actions.
GASTOINTESTINAL CONDITIONS - e.g. with dicycloverine to facilitate G.I. endoscopy and in
hyperirritable bowel conditions,. Their old use in peptic ulcer has been discontinued.
ANAESTHESIA – atropinics are sometimes used to reduce troublesome secretions which are normally
due to parasympathetic stimulation, or to prevent bradycardia.
Despite our current knowledge of the different sub types of muscarinic receptors relatively little success
has met attempts to synthesize muscarinic antagonists with selectivity for the different M receptor
subtypes. It is an area which has still to be fully explored.
Examples:
Pirenzipine: selective (more potent one) M1 (5 times)> M2 20 times> M3.
Gallamine: some selectivity for M2
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Others have some relative selectivity for M3 (HHSD) and M2 (AF-DX116)
Mamba toxins (from snakes) for M1 and M4
As an example of ANS pharmacology consider the pharmacology of the Eye
The eye is mainly controlled by parasympathetic nerves and cholinergic innervation: sympathetic nerves have a
minor role. Thus, muscarinic antagonists can have marked actions in the eye - dilate the pupil and prevent
accommodation. By interfering with drainage of the eye such drugs can “cause” or exacerbate glaucoma.
The extraocular skeletal muscle fibres that control movement of the eye are skeletal muscles but, very unusually,
have multiple skeletal motor nerve innervation on each muscle fibre.
Glaucoma (increased intraocular pressure that can destroy the eye’s function) is a mismatch of secretion of
ocular fluids, and their drainage. Drugs for glaucoma can increase drainage, or reduce formation of fluid.
Drugs and Glaucoma
Muscarinic antagonists can exacerbate glaucoma. Muscarinic agonists and anticholinesterases can alleviate,
glaucoma. Various other drugs can be useful in glaucoma, including beta blockers which appear to decrease
secretion, as well as prostaglandins.
Nicotinic Cholinergic Transmission
There are various types of nicotinic receptors in the periphery and the CNS. There are two main
families: nicotinic skeletal muscle and nicotinic ganglionic cholinoceptors (see Google for the
biochemical characteristics of nicotinic receptors, location, etc.)
Nicotinic receptors (NAChR) are found on the:
Post junctional membranes of the skeletal neuromuscular endplate on a 1:1 relationship with
acetylcholinesterase enzyme.
Skeletal muscle spindle (afferent physiological receptor in skeletal muscle-extrafusal fibres).
At the ganglion of the sympathetic, parasympathetic and enteric ANS
Within CNS at various synaptic sites - both pre and post synaptic
Functionally there are at least two main peripheral types of nicotinic receptors (NAChR): skeletal muscle
and ganglionic (with at least two different CNS types).
NAChR is a ligand-gated ion channel where 2 molecules of the neurotransmitter ACh bind so as to open the
channel. NAChR receptors in skeletal muscle are generally "protected" from overstimulation with ACh by
adjacent AChE molecules. In the neuromuscular junction ACh released from the nerve generally only binds once
to a NAChR before it is hydrolyzed by acetylcholine into choline and acetate.
Structure of NAChR (Nicotinic receptor): Transmembrane spanning proteins (ligand-gated channel) are "gated"
by ACh to open to allow trans-membrane movement of ions - Na+ (Inwards) and K+ (Outwards) resulting in
depolarization of the post-synaptic end plate membrane.
Different subtypes of AChR nicotinic receptor are composed of alpha and beta components in various
proportions. In the skeletal muscle form of the nicotinic receptor 2 (two) ACh molecules have to bind to two
recognition sites in order to open the channel (co-operativity).
Ganglionic nicotinic receptors
Agonists: ACh, CCh, nicotine (after which the receptor is named), lobeline and DMPP
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All "agonize" receptor but high and continuous occupancy desensitizes the receptor to the action of further ACh – this results
in so-called desensitizing block. Nicotine first activates receptor, and then blocks it. This type of block also occurs in the
skeletal muscle receptor.
Antagonists (ganglion blockers): Hexamethonium (C6), trimethaphan
Therapeutic Uses: Nowadays rarely, if ever, used. For an understanding of the effects of nicotinic ganglionic blockade (and
incidentally which aspects of the autonomic nervous system are functional important in normal life) see "hexamethonium man"
as described by Paton
.
W. D. M. Paton, Pharm. Rev. 6, 59 (1954)
Hexamethonium man: is a pink complexioned person, except when he has stood for a long time, when he may get pale
and faint. His handshake is warm and dry. He is a placid and relaxed companion; for instance he may laugh, but he can’t cry
because the tears cannot come. Your rudest story will not make him blush, and the most unpleasant circumstances will fail to
make him pale. His socks and his shirt stay very clean and sweet. He wears corsets(!!) and may, if you meet him out, be
rather fidgety (corsets to compress his splanchnic vascular pool, fidgety to keep the venous return going from his legs). He
dislikes speaking much unless helped with something to moisten his dry mouth and throat. He is long-sighted and easily
blinded by bright light. The redness of his eyeballs may suggest irregular habits and in fact his head is rather weak. But he
always behaves as a gentlemen and never belches or hiccups. He tends to get cold, and keeps well wrapped up. But his
health is good; he does not have chilblains and those diseases of modern civilization, hypertension and peptic ulcers, pass
him by. He is thin because his appetite is modest; he never feels hunger pains and his stomach never rumbles. He gets
rather constipated so his intake of liquid paraffin is high. As old age comes on he will suffer from retention of urine and
impotence, but frequency, precipitancy, and strangury will not worry him. One is uncertain how he will end, but perhaps if he is
not careful, by eating less and less and getting colder and colder, he will sink into a symptomless, hypoglycemic coma and die,
as was proposed for the universe, a sort of entropy death.
Skeletal muscle nicotinic receptors (Neuromuscular-Skeletal Muscle Junction)
Action potentials travelling down ‘motor’ nerve axons (nerve impulse) reach the nerve ending
and open N-type calcium channel allowing Ca++ entry into the nerve ending and this entry facilitates
mobilization of ACh vesicles, their binding to nerve ending membrane, and release of ACh. ACh binds
to the NAChR (nicotinic skeletal muscle receptor) which opens to increase permeability to K+ and Na+
so producing depolarization at the endplate region. The depolarization, if large enough, will trigger an
action potential at the plasma membrane of the surrounding skeletal muscle fibre. The muscle action
potential spreads across the rest of the muscle to induce contraction via triggering Ca++ entry into the
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cell, and releasing Ca++ onto the actin-myosin contractile elements. The effector tissue (neuromuscular
endplate) is a matrix of NAChR and acetylcholinesterase (AChE) molecules closely packed together.
As a result, any free ACh is quickly inactivated. A low level of spontaneous (resting release) of ACh
vesicles always occurs. Each of such spontaneously released vesicles causes a miniature end plate
potential, but no activation of the skeletal muscle.
In most skeletal muscles in most mammalian skeletal muscle each muscle fibre has one single
end plate on which a single motor nerve axon terminates. A few special skeletal muscles have multiple
end plates with multiple axonal innervations. This exceptional pattern of multiple innervation in man is
found in the extra-ocular muscles of the eye, and on muscle spindles (part of the skeletal muscle
feedback control system embedded in muscles).
The ‘reserve’ for transmission from nerve to skeletal muscle is high - up to 10 times more ACh
released than is necessary to elicit an action potential in the muscle cell. There is also an excess of
NAChRs on the end plate itself. The extent of this ‘receptor reserve’ is apparent in the rare autoimmune
disease, myasthenia gravis, where over 90% of the end plate nicotinic receptors have to be inactivated
by endogenous autoimmune antibodies before neuromuscular transmission is impaired.
Myasthenia gravis is rare (200–400 cases per million, mostly females). If antibodies reduce functional
receptors by more than 90%, muscle weakness occurs – first apparent in certain muscle groups (ptosis of
eyelid). Diagnosis includes detection of NAChR antibodies as well as functionally by a short-lived
increase in muscular strength after i.v. injection of an ultra-short acting anticholinesterase,
edrophonium (a ‘provocative’ test). Treatment of myasthenia gravis includes immunosuppressive
drugs, thymectomy, and longer-acting anticholinesterases (together with atropine).
NmAChR Agonists: Classically include ACh, CCh (carbamycholine) and succinylcholine.
Succinylcholine is ester of a dicarboxylic acid (succinic acid) with two acetates. Succinylcholine
"agonizes" the NAChR, but at high enough concentrations producing continuous receptor occupancy
and desensitizes them resulting in a depolarizing block (which may be followed by a desensitizing
block).
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Antagonists (neuromuscular blockers):
The classic (first available) neuromuscular blocker was d-tubocurarine (from a S. American arrow
poison) followed by decamethonium (C10 - the first synthetic compound) and succinylcholine. Many
synthetic curare-like molecules have since been invented.
Pharmacology of Neuromuscular Blocking drugs.
The major use of neuromuscular blockers drugs is in anaesthesia/surgery. A short acting blocker
will produce paralysis of the laryngeal muscles allowing easy and rapid endotracheal intubation. In
surgery, skeletal muscle relaxation reduces the needs for high concentrations of anaesthetic drugs.
Anaesthestic drugs at high concentrations abolish most reflex skeletal muscle movements but carry a
higher risk.
Available neuromuscular blockers all produce skeletal muscle paralysis, but none are ideal. Ease
of reversal of blockade with anticholinesterases is important, as is a lack of interactions with other drugs
at the level of drug metabolism, and elsewhere. Other than in anaesthesia, such blockers are also used
selectively to alleviate certain spastic skeletal muscle conditions.
Succinylcholine (suxamethonium) - a special historical case
The depolarizing n.m. blocker succinylcholine is normally very short acting via rapid hydrolysis
due to plasma esterases. A rare genetic absence of cholinesterases results in a long duration of action (37 hours versus 3-7 minutes). Given i.v. it produces very rapid and normally short-lived relaxation –
hence its use for endotracheal intubation. However, it can cause skeletal muscle spasm and muscle
fasciculation - especially in extra ocular muscles – with subsequent muscle aches. Serum potassium
rises as a result of muscle damage due to muscle fasciculation. There is no real antidote for this drug.
Non depolarizing neuromuscular blockers (competitive antagonists at NAChR)
D-tubocurarine, was isolated from curare, a South American dart poison. Classic experiments
(C. Bernard, 1800s) identified the neuromuscular junction as its site of action. However, decades passed
before it was used in medicine. Since then newer “curare” drugs have been invented since dtubocurarine’s pharmacology is not “ideal”; it releases histamine, and can lower blood pressure (at
high concentrations it blocks nicotinic receptors on ganglia).
Synthetic curare-like drugs come from two chemical classes: 1) steroid derivatives; 2) isoquinoline derivatives (e.g. d-tubocurarine). Drugs in both classes have a nitrogen that is charged at
physiological pH; therefore they are poorly orally absorbed (curare was used as an arrow poison to
obtain monkeys for food).
Steroid derivatives with the ending ‘curonium’ include pancuronium, vecuronium, and rocuronium.
They are similar in their clinical actions and in pharmacology, except rocuronium and vecuronium
which are shorter acting. Pancuronium, the oldest, has more side effects.
Isoquinolone derivatives: with the ending ‘curium’ include atacurium, mivacurium and doxacurium.
Mivacurium is short acting and broken down by plasma esterases.
Uses: intravenous to produce neuromuscular blockade to aid surgery – rarely in any other conditions.
Reversal of neuromuscular blockade
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Neuromuscular blockade due to competitive neuromuscular blockers can be reversed (within
limits) by intravenous anticholinesterase drugs, such as neostigmine and pyridostigmine. These block
cholinesterase and thereby raise ACh concentrations in the junctional cleft. The elevated ACh activates
the NAChRs that are not occupied by the neuromuscular blocking drug.
However, neuromuscular
blockade in vivo is a dynamic situation and direct competition for a NAChR receptor site between the
elevated ACh and competitive blockers is not so important. Anticholinesterase drugs will elevate
acetylcholine levels throughout the body to a sufficient level as to produce muscarinic effects and an
atropinic drug will block such unwanted muscarinic effects.
Excessive neuromuscular blockade with the competitive non-depolarizing drugs cannot easily be
reversed by an anticholinesterase nsince the junctional cleft concentration of ACh cannot be raised
sufficiently by AChE inhibitors to functionally compete with the blocker and thereby activate NACHRs.
Sugammadex is a drug that complexes with steroidal neuromuscular blockers thereby inactivating
them.
Assessment of neuromuscular blockade: is with peripheral nerve stimulation while recording the
resulting muscle contractions using trains of 4 stimuli, double pulses, or post tetanic stimulation.
Other Drugs Acting on Skeletal Muscle Activity
Spasmolytic drugs act on spinal cord reflexes to reduce inappropriate skeletal muscle activity. They
include diazepam and baclofen. Izanidine is an imidazoline derivative that relaxes by way of central
(CNS) mechanisms.
Dantrolene which acts on excitation-contraction mechanisms in skeletal muscle is somewhat selective
in reducing inappropriate skeletal muscle activity. It is used in the inherited condition, malignant
hyperthermia, a condition triggered by some general anaesthetics and succinylcholine where poor
control of intracellular calcium concentrations brings about excessive activation of skeletal muscle
activity and metabolism.
Botulinum toxin (a protein) directly depletes cholinergic nerves of ACh. Once ACh is depleted with
botulinus toxin it takes months for the nerve ending to recover. Botulinus toxin (botox) given locally
can produce local skeletal muscle blockade via such depletion and therefore is used therapeutically to
block inappropriate muscle contractions in various motor nerve and muscle diseases (e.g. strabismus).
Non-therapeutic use of botulinus toxin (“botox”) is cosmetic, to remove the lines of worry, age, too
much sun, booze and tobacco. Such lines are due to underlying contraction of fine facial skeletal
muscles. Botox provides that ‘smoother] look, but of course repeated treatments are necessary (see
above). More importantly botulinus toxin is also used therapeutically to treat muscle spasms, upper
motor neuron syndrome, excessive sweating (hyperhydrosis due to parasympathetic cholinergic nerves)
and cervical dystonia.
Botulinus protein has two forms, A & B, both produced naturally by an anaerobic bacterium
(Clostridium botulinum) and can occur in poorly canned and preserved foods. It is absorbed orally and
is responsible for occasional poisoning. In such potentially lethal poisoning the patient has to be
artificially ventilated until the skeletal motor axons regain their function.
MjW2013