What are the major subdivisions of the autonomic nervous system?
It is divided into the sympathetic and the parasympathetic nervous systems.
What is the major neurotransmitter of the parasympathetic autonomic nervous system?
Acetylcholine (ACh). ACh is released into the synaptic clefts from the pre-and the postsynaptic neurons of the parasympathetic nervous system.
In the sympathetic nervous system, what neurotransmitter is released from the preganglionic neuron into the synaptic cleft?
ACh. Remember that while the postganglionic neurotransmitters may differ between the sympathetic and parasympathetic branches of the autonomic nervous system, the preganglionic neurotransmitter released into the synaptic cleft is identical—ACh.
Where are sympathetic preganglionic fibers located?
In the paravertebral chains on either side of the spinal column or the prevertebral ganglia on the ventral surface of the aorta. Sympathetic preganglionic fibers are short.
Where are parasympathetic preganglionic fibers located?
In or near the wall of the organ they innervate. Parasympathetic preganglionic fibers are very long.
Where are nicotinic receptors located?
Postsynaptic neurons in ganglia of both the parasympathetic nervous system (PNS) and sympathetic nervous system (SNS); adrenal medulla; neuromuscular junction (NMJ); central nervous system (CNS)
Where are muscarinic receptors located?
Organs innervated by the PNS; thermoregulatory sweat glands innervated by the SNS; CNS (cortex, hippocampus)
What does the PNS do to heart rate?
It decreases the heart rate. Remember that at rest the heart is constantly under parasympathetic tone to slow the heart rate from the intrinsic rate set by the sinoatrial (SA) node at about 80 beats per minute.
What enzyme catalyzes the reaction between choline and acetyl-CoA to form ACh?
Choline acetyltransferase (CAT)
The neuronal release of ACh into the synapse is inhibited by what toxin?
What organism produces botulinum toxin?
Clostridium botulinum (anaerobic, spore forming, gram-positive rod)
The venom of which spiders result in the release of stored ACh into the synapse?
Any spider of the genus Latrodectus (widow spiders) of which the black widow is the most common species found in North America. They produce α-latrotoxin which causes the release of ACh from the preganglionic neuron into the synaptic cleft.
What enzyme degrades ACh?
What are the breakdown products of ACh?
Choline and acetate
Where is AChE located in the autonomic nervous system?
In the synaptic cleft
What is muscarine?
It is an alkaloid found in various poisonous mushrooms.
Where are each of the following types of muscarinic receptors found in the body?
Nerves; gastric parietal cells
Nerves; cardiac cells; smooth muscle
Smooth muscle; exocrine glands; lungs; gastrointestinal (GI) tract; eye; bladder
For each of the following muscarinic receptor types, name the type of G-protein it is coupled to and the second messenger system responsible for execution of its activity upon stimulation:
Gq coupled; inositol triphosphate (IP3), diacylglycerol (DAG) cascade
Gi coupled; inhibition of cyclic AMP (cAMP) production, activation of potassium channels
Gq coupled; IP3, DAG cascade
Gi coupled; inhibition of cAMP production
Gq coupled; IP3, DAG cascade
Does the PNS directly innervate the vasculature?
No. Vascular tone is primarily determined by the degree of stimulation of adrenergic receptors of the sympathetic nervous system which directly innervate the vascular smooth muscle cells. However, there are muscarinic receptors located on the vasculature.
How can ACh lower blood pressure?
ACh binds to ACh receptors in the vasculature leading to increased synthesis of nitric oxide (NO) via second messenger pathways. An increase in NO leads to vasodilation.
NO is also known as what?
Endothelial-derived relaxation factor (EDRF)
What amino acid is a precursor to NO synthesis?
Does AChE have a high affinity for ACh?
Does ACh increase or decrease the following (in other words, what is the effect of parasympathetic stimulation of the following)?
Decreases (both arterial and venous dilation via NO)
Decreases (via M2 receptors)
Increases (via M3 receptors)
Increases (via M3 receptors)
Increases (via sympathetic stimulation of muscarinic cholinergic receptors)
Increases (via M3 receptors)
Increases (via M3 receptors)
Miosis (constriction of pupil)
Increases (via M3 receptors)
Bladder detrusor muscle tone
Increases (via M3 receptors)
Bladder sphincter tone
Decreases (in combination with increased detrusor tone this leads to increased urination also via M3receptors)
Decreases (via M3 receptors)
What does ACh do to the ciliary muscle of the eye?
Increased contraction which leads to increased accommodation
How does ACh cause miosis?
Increased contraction of the circular muscle in the iris
Does bethanechol have muscarinic activity?
Does bethanechol have nicotinic activity?
Does AChE have a high affinity for bethanechol?
No (zero affinity). This gives bethanechol a long duration of action.
What is a clinical use for bethanechol?
Nonobstructive urinary retention as can result from denervation of the urinary sphincter in conditions such as diabetes or spinal cord injury. Bethanechol can also be used for gastroesophageal reflux disease (GERD). As a cholinergic drug, it will increase detrusor tone and GI motility.
Does carbachol have muscarinic activity?
Yes, it is a muscarinic agonist.
Does carbachol have nicotinic activity?
Yes, it is also a nicotinic agonist.
Does AChE have a high affinity for carbachol?
No, the enzyme has zero affinity for carbachol.
What is carbachol used for?
It is a miotic agent to reduce intraocular pressure (IOP) in emergency settings of narrow-angle and open-angle glaucoma.
Does pilocarpine have muscarinic activity?
Yes, it is a muscarinic agonist.
Does pilocarpine have nicotinic activity?
Does AChE have a high affinity for pilocarpine?
No, the enzyme has zero affinity for pilocarpine.
What is pilocarpine used for?
It is the miotic drug of choice to lower IOP in emergency settings of narrow-angle and open-angle glaucoma.
Can pilocarpine cross the blood-brain barrier (BBB)?
Yes. Because it is a tertiary, uncharged amine.
Give examples of reversible AChE inhibitors:
Neostigmine; pyridostigmine; physostigmine; edrophonium; rivastigmine; donepezil; galantamine; tacrine
What are donepezil, galantamine, rivastigmine, and tacrine used for?
Alzheimer-type dementia. They are AChE inhibitors, thereby increasing the levels of ACh in the brain.
What two AChE inhibitors are quaternary ammonium compounds and therefore cannot cross the BBB?
As a result, these drugs will not reverse the central nervous system effects of cholinergic toxicity.
What short-acting AChE inhibitor is used to diagnose myasthenia gravis and is also used to differentiate myasthenic from cholinergic crisis?
Edrophonium. The trade name of edrophonium is Tensilon. This test is commonly referred to as the Tensilon test.
Which reversible AChE inhibitor is used as an antidote in atropine overdose?
Physostigmine, a tertiary amine, is able to cross the BBB to act on the CNS.
Give examples of irreversible AChE inhibitors:
Echothiophate; isoflurophate; sarin; malathion; parathion
Name an irreversible AChE inhibitor that is used as nerve gas:
Which two AChE inhibitors are used as insecticides?
What is another name for the irreversible AChE inhibitors?
How do organophosphates irreversibly inhibit AChE?
The phosphate group covalently binds to serine hydroxyl group in the active site of AChE, thereby rendering the enzyme permanently inactive.
What is used to counteract the muscarinic and CNS effects of organophosphate poisoning?
Atropine via competitive inhibition. Atropine binds the muscarinic receptors, outcompeting the increased levels of ACh thereby preventing overstimulation.
What agent is used to reactivate inhibited AChE during organophosphate poisoning?
Pralidoxime (2-PAM). It is critical to initiate treatment with pralidoxime early along with atropine to prevent the process of aging where AChE is irreversibly inactivated by the organophosphates.
What are the signs and symptoms of organophosphate poisoning?
SLUDGE: salivation; lacrimation; urination; diaphoresis; GI motility (diarrhea); emesis. Basically, parasympathetic overstimulation.
Does atropine block nicotinic receptors, muscarinic receptors, or both?
It blocks muscarinic receptors.
What are the pharmacologic actions of atropine?
Mydriasis; cycloplegia; tachycardia; sedation; urinary retention; constipation; dry mouth; dry eyes; decreased sweating; hallucinations; sedation; hyperthermia; delirium; blurred vision; coma (high doses). Basically, anticholinergic/sympathetic overstimulation.
What class of drugs can be used to counteract atropine overdose?
Name three drug classes that may cause antimuscarinic adverse effects:
Low-dose (<0.5-1 mg) atropine does what to heart rate?
Decreases heart rate (unknown paradoxical vagalmimetic effect)
High-dose (>0.5-1 mg) atropine does what to heart rate?
Increases heart rate (parasympatholytic effect)
What is belladonna?
A perennial plant also known as “deadly nightshade” due to the toxic effects of its foliage and berries from which atropine is derived. Other toxins include scopolamine and hyoscyamine. The name belladonna derives from the cosmetic enhancing effects of dilated pupils, blushing of the cheeks, and reddening of the lips for which the plant was originally used.
How does scopolamine differ from atropine?
Scopolamine has a longer duration of action, more potent CNS effects, and is able to block short-term memory.
What is the main therapeutic indication of scopolamine?
Giving drugs with anticholinergic activity can precipitate an emergent situation in patients with what medical condition?
Patients with narrow-angle glaucoma
What are the signs and symptoms of acute-angle-closure glaucoma?
General distress; pain; headache; red eye; photophobia; increased IOP; visual changes; malaise; nausea; vomiting
What two anticholinergic agents are quaternary ammonium compounds and used for the treatment of asthma and chronic obstructive pulmonary disease (COPD)?
Tiotropium has a longer half-life compared to ipratropium.
Does ipratropium effect airway secretions?
No (unlike atropine, which decreases airway secretions)
Name three ganglionic blocking agents:
What are ganglionic blocking agents primarily used for?
Lowering blood pressure; blocking autonomic nervous system reflexes; smoking cessation due to blockade of central nicotine receptors
Why can ganglionic blockers cause a marked postural hypotension?
Since sympathetic tone to the blood vessels is blocked, both arterial and venous dilation occur, lowering blood pressure. Moreover, the ganglionic blockers prevent the sympathetically mediated baroreceptor response to a sudden decrease in blood pressure, such as that occurs with a rapid change in position from sitting to standing.
Neuromuscular blocking agents (NMBs) can be grouped into what two general categories?
Do NMBs work at muscarinic or nicotinic receptors?
Nicotinic receptors (remember the NMJ has nicotinic receptors).
How many subunits is the nicotinic receptor made of?
Five subunits. Two α:- and three β-subunits make up this transmembrane ligand-gated ion channel.
Which subunit of the nicotinic receptor does ACh bind to?
Between the two α-subunits
Binding of ACh to the nicotinic receptor at the NMJ is required to open which type of ion channel?
What is the most commonly used NMB?
Succinylcholine, the only depolarizing NMB. This is an ideal drug for endotracheal intubation due to its fast onset of action and short duration of action.
How does succinylcholine work at the NMJ?
It behaves as a cholinergic agonist that remains bound to the ACh receptor for a prolonged period.
What happens during each of the following phases of succinylcholine activity at the NMJ?
The receptor becomes depolarized and transient fasciculations are observed as various motor units depolarize.
The receptor becomes resistant to depolarization and a flaccid paralysis ensues.
What are the two main uses of succinylcholine?
Is succinylcholine short or long acting?
It is short acting with a duration of 4-8 minutes because of rapid hydrolysis by plasma cholinesterase.
What are the adverse effects of succinylcholine?
Malignant hyperthermia; apnea; hypertension; hyperkalemia
What are the signs and symptoms of malignant hyperthermia?
Muscular rigidity; increased oxygen consumption; increased carbon dioxide production (usually the first sign detected during surgery); tachycardia; hyperthermia is a late finding
How is malignant hyperthermia treated?
What is the mechanism of action of dantrolene?
It inhibits calcium release from the sarcoplasmic reticulum of muscle cells, thereby relaxing muscle tone and reducing heat production.
Succinylcholine may have a prolonged half-life in what type of patients?
Patients with a genetic deficiency or altered form of plasma cholinesterase
What is the mechanism of action of nondepolarizing NMBs?
Competitive antagonists of ACh at the NMJ
Which drug is the prototype of the nondepolarizing NMBs?
What antidote is used in tubocurarine overdose?
AChE inhibitor (increases ACh concentration which competes with tubocurarine at ACh receptors at the NMJ)
List in order, from first to last, the muscles that are paralyzed by nondepolarizing NMBs:
Which antimicrobial class of drugs may act in synergy with nondepolarizing NMBs by inhibiting release of ACh from nerve endings by competing with calcium ions, thereby increasing neuromuscular blockade?
Aminoglycosides (most likely to occur with high doses; patients with hypocalcemia, hypomagnesemia, or neuromuscular disorders)
Give examples of nondepolarizing NMBs:
Tubocurarine; atracurium; mivacurium; rocuronium; vecuronium; pancuronium; pipercuronium
What is the only nondepolarizing NMB that does not require dosage reduction in patients with renal failure?
Atracurium, which excreted in bile, not in urine
What nondepolarizing NMB has the most rapid onset of action?
Rocuronium. Think: ROcuronium—Rapid Onset
In what situations are nondepolarizing NMBs used?
Adjunct to general anesthesia to facilitate endotracheal intubation and to relax skeletal muscles during surgery; to facilitate mechanical ventilation in ICU patients
What are the major neurotransmitters of the SNS?
Epinephrine; norepinephrine; dopamine
What amino acid is the precursor to dopamine, epinephrine, and norepinephrine?
What are the steps, in order, to the synthesis of epinephrine starting from tyrosine?
Tyrosine is converted into DOPA by tyrosine hydroxylase (rate-limiting step); DOPA is converted into dopamine by DOPA decarboxylase; dopamine is converted into norepinephrine by dopamine β-hydroxylase; norepinephrine is converted into epinephrine by methylation in the adrenal medulla.
What two enzymes metabolize norepinephrine?
What is the mechanism of action of reserpine?
It inhibits the transport of norepinephrine from the neuronal cytoplasm into the synaptic vesicles.
What are the common side effects of reserpine?
What breakdown products of norepinephrine are excreted in the urine and can be measured to help diagnose pheochromocytoma?
Vanillylmandelic acid (VMA); metanephrine; normetanephrine
What are the two major classes of adrenergic receptors?
What neurotransmitters are metabolized by MAO type A?
Norepinephrine; epinephrine; serotonin; tyramine; dopamine
What neurotransmitters does MAO type B metabolize?
Dopamine. Dopamine is metabolized by both the A and B type of the enzyme.
How does cocaine increase norepinephrine levels in the synaptic cleft?
It inhibits the reuptake of neurotransmitter back into the presynaptic neuron.
How do amphetamine, ephedrine, and tyramine increase norepinephrine levels?
They act as indirect sympathomimetic agents by entering the presynaptic neuron releasing stored norepinephrine into the synaptic cleft.
Where are α1-receptors found?
Vascular smooth muscle; papillary dilator muscle; pilomotor smooth muscle; prostate; heart
Where are α2-receptors found?
Postsynaptic CNS adrenoceptors; pancreatic β-cells; platelets; adrenergic and cholinergic nerve terminals; vascular smooth muscle; fat cells
Where are β1-receptors found?
Heart; juxtaglomerular cells
Where are β2-receptors found?
Respiratory, uterine, and vascular smooth muscle; skeletal muscle; liver
Where are D1-receptors found?
Where are D2-receptors found?
Compare and contrast the local versus the systemic effects of α2-receptor activation.
Local infusion of an α2-agonist will activate the α2-receptors in the vasculature, causing vasoconstriction. Systemic administration will activate the central α2-receptors in the locus ceruleus which inhibits norepinephrine release and sympathetic activation. The central effects overwhelm the local effects leading to decreased blood pressure.
Give examples of α2-receptor agonists:
Clonidine; α-methyldopa; guanabenz; guanfacine; dexmedetomidine
What are the therapeutic indications of clonidine?
Hypertension; severe pain; heroin withdrawal; nicotine withdrawal; ethanol dependence; clozapine-induced sialorrhea; prevention of migraines
What is dexmedetomidine used for?
Sedation of intubated and mechanically ventilated patients; prolongation of spinal anesthesia
With drugs that activate both α- and β-receptors, which receptors are generally activated first (which receptors are more sensitive)?
Activation of what receptor type in the eye will lead to contraction of the radial muscle and subsequently lead to mydriasis?
For each of the following receptor types, name the type of G-protein it is coupled to:
Name the major effects mediated by each of the following receptor types:
Mydriasis; vasoconstriction → increased blood pressure; decreased urination; increased glycogenolysis; decreased renin release; ejaculation
Inhibition of norepinephrine release (central effect); inhibition of insulin release; platelet aggregation
Increased heart rate; increased conduction velocity; increased force of heart contraction; increased renin release
Vasodilation; bronchodilation; relaxation of uterine, respiratory, and vascular smooth muscle; increased insulin secretion; increased potassium uptake; increased glycogenolysis
Vasodilation of coronary, renal, and mesenteric vasculature; increased glomerular filtration rate (GFR); increased renal blood flow (RBF); increased sodium excretion
Will α2-receptor activation in the pancreas cause insulin secretion to increase or decrease?
It will cause insulin secretion to decrease.
Will β2-receptor activation in the pancreas cause insulin secretion to increase or decrease?
It will cause insulin secretion to increase.
Which receptor type does epinephrine preferentially bind to at low doses?
β-Receptors (vasodilation in vasculature)
Which receptor type does epinephrine preferentially bind to at high doses?
α-Receptors (vasoconstriction in vasculature)
What is the drug of choice in patients with type 1 (immediate) hypersensitivity reactions?
What is the dose of epinephrine given for anaphylaxis?
0.1-0.5 mg. Note: The EpiPen (epinephrine auto-injector) that many patients with a history of anaphylaxis carry is 0.3 mg.
What is the concentration of epinephrine used for anaphylaxis?
What is the concentration of epinephrine used for advanced cardiac life support (ACLS) protocol?
Why is epinephrine often given in combination with local anesthetics?
Epinephrine causes a vasoconstriction, thereby inhibiting the local anesthetics redistribution away from its site of action, so it increases the duration of local anesthesia.
What is the concentration of epinephrine when given in combination with local anesthetics?
State whether each of the following cardiovascular effects increases or decreases with low-dose epinephrine:
Peripheral vascular resistance
Systolic blood pressure
Diastolic blood pressure
What receptors are activated by isoproterenol?
β1 = β2
What receptors are activated by dopamine?
D > β > α
What receptors are activated by dobutamine?
β1 > β2
What receptors are activated by phenylephrine?
α1 > α2
Does norepinephrine activate β2-receptors?
Activation of dopamine receptors will cause what type of response in the mesenteric and renal vasculature?
What is dopamine metabolized to?
Homovanillic acid (HVA)
What is dobutamine used for?
Increases cardiac output in congestive heart failure (CHF) without affecting RBF (unlike dopamine)
Tyramine is a breakdown product of which amino acid?
Where is tyramine found?
Examples of foods and beverages which contain tyramine include: beer, ale, robust red wines, chianti, vermouth, homemade breads, cheese, sour cream, bananas, red plums, figs, raisins, avocados, fava beans, Italian broad beans, green bean pods, eggplant, pickled herring, liver, dry sausages, canned meats, salami, yogurt, soup cubes, commercial gravies, chocolate, and soy sauce.
What enzyme is responsible for the breakdown of tyramine?
MAO type A
What can potentially happen if a patient on a monoamine oxidase inhibitor (MAOI) consumes large amount of fermented cheese?
Hypertensive crisis due to tyramine in the cheese which leads to the release of norepinephrine from storage vesicles in presynaptic neurons
What is phenylephrine and pseudoephedrine used to treat?
What are mixed action adrenergic agonists?
Substances that release stored norepinephrine from nerve terminals and also directly stimulate α- and β-receptors
What are some examples of mixed action adrenergic agonists?
Ephedrine; pseudoephedrine; metaraminol
Which drug is a nonselective, competitive antagonist at both α1- and α2-receptors?
Which drug is a nonselective, irreversible antagonist at both α1- and α2-receptors?
What are phentolamine and phenoxybenzamine mainly used for?
To achieve α-receptor blockade before surgical removal of pheochromocytoma, to achieve perioperative blood pressure control, and to prevent intraoperative hypertension from release of catecholamines during surgical manipulation
What is the mechanism of action of prazosin?
What are prazosin, terazosin, and doxazosin used to treat?
Benign prostatic hyperplasia (BPH); hypertension
Does the “H” in BPH stand for hypertrophy or hyperplasia?
Hyperplasia, though hypertrophy is still sometimes erroneously used. There is an actual increase in the number of prostatic cells in BPH. The cells do not simply increase in volume as do, for example, skeletal or cardiac muscle cells in response to increased used.
This drug is a selective α1A-receptor antagonist, used in the treatment of BPH, and has less cardiovascular side effects versus traditional α1-antagonists.
What advantages do selective α1-antagonists have over nonselective α-antagonists?
Less reflex tachycardia
What CNS prejunctional α2-receptor antagonist is used to treat postural hypotension and erectile dysfunction (impotence)?
What CNS prejunctional α2-receptor antagonist is used to treat depression?
Give examples of β1-selective antagonists:
Acebutolol; atenolol; bisoprolol; betaxolol; esmolol; metoprolol
Give examples of nonselective β-antagonists:
Propranolol; timolol; pindolol; nadolol
Give examples of mixed-a (Xj/P ntagonists:
What is the name of a β-antagonist that also blocks potassium channels and is used as an antiarrhythmic?
What is intrinsic sympathomimetic activity (ISA)?
Drugs act as partial agonists and only work when there is increased sympathetic drive such as with exercise; less bradycardia; less effects on lipid metabolism
Which two β-antagonists have ISA?
What happens to exercise tolerance in patients being treated with β-blockers?
Decreased exercise tolerance
What are the main therapeutic indications of β-blockers?
Angina; arrhythmias; hypertension; CHF (not all β-blockers); thyrotoxicosis; glaucoma (ophthalmic formulations)
What are some noncardiovascular uses of propranolol?
Migraine prophylaxis; performance anxiety “stage fright”
β-Blockers can inhibit the majority of effects caused by thyrotoxicosis except for what sign?
Diaphoresis. Remember that sweat glands have muscarinic receptors and are cholinergic rather than adrenergic.
β-Blockers can inhibit the majority of effects caused by hypoglycemia except for what sign?
Diaphoresis. β-Blockers are contraindicated in diabetic patients treated with oral hypoglycemics for this reason.
What does propranolol do to serum triglycerides?
Increases serum triglycerides
What does propranolol do to serum low-density lipoprotein (LDL)?
Increases serum LDL
Why does propranolol cause vivid dreams?
Crosses the BBB
Why should β-blockers be tapered down instead of abruptly discontinued?
Chronic therapy leads to upregulation of β-receptors; therefore, abrupt discontinuation may lead to life-threatening cardiovascular rebound effects (tachycardia; hypertension; arrhythmias; death)
A mother brings her 4-month-old infant to the pediatrician. The child developed what appeared to be a viral febrile illness which rapidly progressed. The infant now appears listless and has decreased muscle tone. The mother reports decreased feeding and bowel movements and says that the child’s cry sounds different from normal. The only treatment she has initiated is warm milk with honey to help the child’s immune system. What is the most likely diagnosis?
While the child may have initially had a viral upper respiratory tract infection (URI), ingestion of honey exposed the infant to C. botulinum spores which are able to colonize the immature pediatric small intestines. Botulinum toxin is then released into the systemic circulation which blocks the release of ACh from the synaptic cleft, leading to the flaccid paralysis known as “floppy baby syndrome.”
A patient has been found to inconsistently have elevated blood pressure during various office visits over the past several years. A decision is made to try propranolol to control the patient’s blood pressure. On return visit 2 weeks later, the patient’s blood pressure is markedly elevated and the patient reports headaches and blurred vision. What is the most likely diagnosis?
Pheochromocytoma. The patient most likely had sporadic release of catecholamines from the tumor leading to periods of increased blood pressure. The most common neurotransmitter released from pheochromocytomas is norepinephrine. Nonselective blockade of the β-adrenergic receptors by propranolol leads to unopposed α-adrenergic receptor activation by the norepinephrine released by the pheochromocytoma leading to a hypertensive emergency.
A 62-year-old man comes to his primary care physician for a regular checkup. His blood pressure has been mildly elevated on the last two office visits. Today’s reading is 140/92. He is also complaining of some difficulty initiating urination as well as getting up two or more times per night to urinate. What is the best pharmacotherapy to initiate at this time?
Prazosin or another selective α1-antagonist such as terazosin or doxazosin. Prazosin blocks α1-receptors found in the base of the bladder and prostate, preventing constriction and decreasing the resistance to urine flow. It also relaxes vascular smooth muscle leading to decreased blood pressure. Always try to minimize the amount of medications you prescribe for patients. Here, the dual action of prazosin makes it an ideal choice for this patient with two separate health concerns.