Classes of Autonomic Nervous System Agents

Cholinergic Agents

Drugs that stimulate receptor sites mediated by acetylcholine

How do Cholinergic agents achieve their effects?

By mimicking the action of acetylcholine (direct-acting) or inhibiting it breakdown (indirect-acting)

Cholinergic agents are also called

Parasympathomimetic because their effects resemble those produced by stimulating parasympathetic nerves

Clinical uses of cholinergic agents

• Aid in the diagnosis of myasthenia gravis (a neuromuscular disease that causes muscle weakness and breathing problems)

• Reduce the intraocular pressure of glaucoma

• Stimulate GI motility

• Treat urinary retention

• Control vomiting

• Act as an antidote for neuromuscular blockers

Direct-Acting cholinergics

• Acetylcholine

• Carbamylcholine

• Bethanechol

• Pilocarpine

• Metoclopramide

Acetylcholine

Seldom used clinically because it’s broken down so rapidly by acetylcholinesterase

Carbamylcholine

Has been used to treat atony of the GI tract and to stimulate uterine contractions in swine

Bethanechol

Used to treat GI and urinary tract atony

Pilocarpine (Isopto Carpine)

Reduces intraocular pressure associated with glaucoma and is used to treat keratoconjunctivitis sicca

Metoclopramide (Reglan)

Used to control vomiting and to promote gastric tract emptying

Indirect-Acting Cholinergic (Anticholinesterase) Agents

• Edrophonium

• Neostigmine

• Physostigmine

• Organophosphate compounds

• Demecarium

• Pyridostigmine

Edrophonium

Primarily used in the presumptive diagnosis of myasthenia gravis

Neostigmine (Prostigmine)

Used to treat urine retention and GI atony and are used as an antidote to neuromuscular blocking agents

Physostigmine (Antilirium)

Uses of this product are similar to those of neostigmine

Organophosphate compounds

Commonly used as insecticide dips and may result in toxicity if used inappropriately

Demecarium (Humorsol)

Used in the preventative management of glaucoma

Pyridostigmine (Mestinon)

Drug used for the treatment of myasthenia gravis

Adverse side effects of cholinergic drugs

bradycardia, hypotension, heart block, lacrimation, diarrhea, increases intestinal activity, intestinal rupture, and increased bronchial secretions

Cholinergic blocking agents (anticholinergic)

Drugs that block the action of acetylcholine at muscarinic receptors of the parasympathetic nervous system

Cholinergic blocking agents also called

Parasympatholytic because their effects reduce the activity of the parasympathetic nervous system

Clinical uses of cholinergic blocking agents

• Treatment of diarrhea and vomiting via a decrease in GI motility

• Drying of secretions and prevention of bradycardia before anesthesia

• Dilation of the pupils for ophthalmic exam

• Relief of ciliary spasm of the eye

• Treatment of sinus bradycardia

Dosage forms of cholinergic blocking agents

• Atropine

• Methscopolamine

• Glycopryyolate

• Aminopentamide

• Propantheline

• Pralidoxime

Atropine

Used as a preanesthetic to dry secretions and prevent bradycardia; to counteract organophosphate poisoning; to dilate the pupils for ophthalmic exam; to control ciliary spasms of the eye; to treat sinus bradycardia; and to slow a hypermotile gut

Methscopolamine

An ingredient of Biosol-M and is used to control diarrhea

Glycopyrrolate

A quaternary ammonium compound with actions similar to atropine and it provides longer duration of action that atropine and is used primarily as a preanesthetic

Aminopentamide

Used to control vomiting and diarrhea in dogs and cats

Propantheline

Used to treat diarrhea, urinary incontinence, and bradycardia and to reduce colonic peristalsis in horses to allow rectal exam. Is a cholinesterase reactivator used to treat organophosphate intoxication

Adverse effects of cholinergic blockers

They’re dose related. Overdoses can cause drowsiness, disorientation, tachycardia, photophobia, constipation, anxiety, and burning at the injection site

Adrenergic (sympathomimetic) agents

They bring action at receptors mediated by epinephrine or norepinephrine; mimic the “fight or flight” response

Adreneric agents may be classified as

Catecholamines or noncatecholamines, and either category can also be classified according to the specific receptor types activated (alpha-1, alpha-2, beta-1, beta-2)

Alpha receptor activity adrenergic agents

Causes an excitatory response (except in the GI tract)

Beta stimulation adrenergic agents

Causes an inhibitory response (except in the heart)

Clinical uses of adrenergic (sympathomimetic) agents

• To stimulate the heart beat during cardiac arrest

• To reverse the hypotension and bronchoconstriction of anaphylactic shock

• To strengthen the heart during congestive heart failure

• To correct hypotension through vasoconstriction

• To reduce capillary bleeding through vasoconstriction

• To treat urinary incontinence

• To reduce mucous membrane congestion (vasoconstriction) in allergic conditions

• To prolong the effects of local anesthetic agents

• To treat glaucoma

Dosage forms of adrenergic (sympathomimetic) agents

• Epinephrine

• Norepinephrine

• Isoproterenol

• Phenylephrine

• Dopamine

• Phenylpropanolamine

• Dobutamine

• Albuterol

Epinephrine

Stimulates all receptors to cause an increase in heart rate and cardiac output, constriction of the blood vessels in the skin, dilation of the blood vessels in muscle, dilation of the bronchioles, and an increase in metabolic rate

Norepinephrine

Mostly an alpha stimulator with some beta stimulation. Its primary influence is that of a vasopressor (to raise blood pressure)

Isoproterenol (Isuprel)

A pure beta stimulator and its primary use is for bronchodilation

Phenylephrine (Neo-Synephrine)

An alpha stimulator that’s used as a nasal vasoconstrictor and also increases blood pressure

Dopamine

A precursor of epinephrine and norepinephrine. Its action is dose dependent. It’s used to treat shock and congestive heart failure and to increase renal perfusion. It increases blood pressure and cardiac output

Phenylpropanolamine

Used to treat urinary incontinence in dogs, it’s also used as a decongestant and brnchodilator

Dobutamine

A beta-1 agonist that’s used for short-term treatment of heart failure

Albuterol

A beta agonist and the main use is bronchodilation

Adverse side effects of adrenergic agents

Tachycardia, hypertension, nervousness, and cardiac arrhythmia. Hypertension, arrhythmia, an pulmonary edema may occur with an overdose

Adrenergic (Sympatholytic) blocking agents

Are used to disrupt/inhibit that activity of the sympathetic nervous system

Adrenergic blocking agents classified according to

The site of their action as an alpha blocker, beta blocker, or ganglionic blocker. Drugs usually block only once category of receptor

Alpha blockers dosage forms

• Phenoxybenzamine

• Acepromazine

• Prazosin

• Yohimbine

• Atipamezole

Phenoxybenzamine

A hypotensive (vasodilator) agent

Acepromazine

This tranquilizer acts as an alpha blocker and causes vasodilation. It’s used as a sedative, tranquilizer, and preanesthetic adjunct

Prazosin

Is a hypotensive agent, used to treat urethral spasms in cats and dogs secondary to urethral onstruction

Yohimbine (Yobine)

Used as an antidote for xylazine toxicity

Atipamezole (Antisedan)

Is a reversal agent for dexmetomidine

Adverse side effects of alpha blockers

Hypotension (phenoxybenzamine, tranquilizers, prazosin), tachycardia (phenoxybenzamine), muscle tremors (yohimbine), and seizures (acepromazine)

Beta blockers

Are used to treat glaucoma, arrhythmias, and hypertrophic cardiomyopathy

Beta blockers dosage forms

• Propranolol

• Timolol

• Atenolol

Propranolol

Used to treat cardiac arrhythmias and hypertrophic cardiomyopathy. It decreases the heart rate, decreases blood pressure, and decreases cardiac output

Timolol

An ophthalmic preparation used to treat glaucoma

Atenolol

Used in a similar way to propranolol

Adverse side effects of beta blockers

Bradycardia, hypotension, worsening of heart failure, bronchoconstriction, heart block, and syncope

Ganglionic blockers

Are seldom used in vet med