Anticholinergic Poisonings

Chapter 90 Anticholinergic Poisonings





INTRODUCTION


Neurotransmission within the autonomic nervous system may be divided functionally into the sympathetic and parasympathetic systems, or chemically into the adrenergic and cholinergic systems. The cholinergic division consists of all synapses at which acetylcholine is the chemical mediator released. The cholinergic portion of the autonomic nervous system consists of: (1) all preganglionic neurons in the sympathetic and parasympathetic systems, (2) all postganglionic neurons of the parasympathetic system, (3) postganglionic sympathetic neurons that innervate sweat, salivary, lacrimal, and nasopharyngeal glands, and (4) postganglionic sympathetic neurons that innervate skeletal muscle and blood vessels and induce vasodilation when stimulated. The responses of various effector organs to cholinergic stimulation are shown in Table 90-1. Generally, cholinergic neurons can be considered those responsible for an anabolic or vegetative state, stimulating digestion, decreasing cardiac work, and relaxing vascular smooth muscle.


Table 90-1 Responses of Effector Organs to Cholinergic Stimulation
















































































Effector Organs Cholinergic Response
Ophthalmic
Iris sphincter Contraction (miosis)
Ciliary muscle Contraction (near vision)
Cardiac
SA node Decrease heart rate
Atria and ventricles Decrease contractility
AV node and His-Purkinje system Slow conduction
Vascular
Coronary, skin, mucosa, skeletal muscle, cerebral, pulmonary, salivary arterioles Dilation
Systemic veins No effect
Pulmonary
Bronchioles Constriction
Bronchial glands Secretion
Gastrointestinal
Motility and tone Increase
Mucous glands Secretion
Pyloric sphincter Increase tone
Intestinal sphincters Decrease tone
Gall bladder and ducts Contraction
Urinary
Detrusor Contraction
Trigone and sphincter Relaxation
Ureter motility and tone Increase
Endocrine / Glandular
Adrenal medulla Secretion of epinephrine and norepinephrine
Pancreatic acini and islet cells Increase secretion
Salivary, lacrimal and nasopharyngeal glands Increase secretion

AV, Atrioventricular; SA, sinoatrial.


Modified from Brunton LL: Goodman & Gilman’s the pharmacological basis of therapeutics, ed 11, New York, 2005, McGraw-Hill.


Postsynaptic cholinergic receptors can be further subdivided into two pharmacologic classes, muscarinic and nicotinic. Muscarinic receptors, named because of the selective action of muscarine, an alkaloid found in toadstools, are postsynaptic receptors found within the central nervous system (CNS) and on smooth muscle and glands, but not within autonomic ganglia. Muscarinic receptors are further divided into five subtypes (M1 to M5), each located within a specific part of the body, and each having its effect via a second messenger G-protein system.


Nicotinic receptors are found within the autonomic ganglia as well as on skeletal muscle and within the CNS. They were named because they selectively respond to the application of nicotine, which causes opening of an ion channel that allows passage of sodium, calcium, potassium, and other small cations. Because these receptors do not use second messenger systems, responses to stimulation are rapid in onset and short in duration, and they are generally excitatory in nature. Each receptor is made up of five subunits that have varying binding affinities for cholinergic agonists and antagonists, and the subunits vary depending on the tissue in which the receptor is located.


Many plants contain anticholinergic compounds, as do numerous drugs. Box 90-1 contains a listing of plants and drugs that can cause anticholinergic toxicity.1 The drugs are further divided into those in which anticholinergic activity causes the primary toxic effect and those in which anticholinergic effects occur but other toxic effects are more likely to be life threatening. Although exposure to toxic plants with anticholinergic effects is less common in small animals than in large animals, case reports of natural exposures exist.2 The classes of drugs most commonly associated with anticholinergic toxicity in small animals include antihistamines (e.g., diphenhydramine) and tricyclic antidepressants (e.g., amitriptyline, clomipramine). In addition, it is important to assess the possibility of exposure to human medications, such as anti-Parkinsonian drugs and antinausea medications (e.g., promethazine).


Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Anticholinergic Poisonings

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