7. drugs affecting the nervous system

Chapter 7

Drugs Affecting the Nervous System

Copyright © 2011 Delmar, Cengage Learning

Basic Anatomy and Physiology

• Basic unit of the nervous system = neuron– Sensory– Associative– Motor

• Parts of the neuron– Cell body– Dendrite– Axon

• Two parts of the nervous system– CNS (central): brain

and spinal cord– PNS (peripheral):

cranial nerves, spinal nerves, autonomic nervous system


Autonomic Nervous System

• Sympathetic (Adrenergic): “fight or flight”– Increases heart rate, respiration rate, and blood flow

to muscles; decreases GI function; causes pupillary dilation

– Preganglionic synapse: ACh; postganglionic synapse: epi or norepi


Autonomic Nervous System

• Parasympathetic (Cholinergic): “homeostatic”– Brings heart rate, respiration rate, and blood flow to

muscles back to normal levels; returns GI function to normal; constricts pupils to normal size

– Pre- and postganglionic synapse: ACh


Anticonvulsants

• help prevent seizures by suppressing the spread of abnormal electric impulses from the seizure focus to other areas of the cerebral cortex

• All anticonvulsants are CNS depressants and may cause ataxia, drowsiness, and hepatotoxicity


Anticonvulsants

• Barbiturates– CNS depressant– Grouped by duration of action– Phenobarbital, pentobarbital, primidone


Anticonvulsants

• Benzodiazepines– Potentiate effects of GABA, cause muscle

relaxation, relieve anxiety – Diazepam, lorazepam, clorazepate


Anticonvulsants

• Potassium bromide– Adjunct to anticonvulsant therapy– Depressive effect on neuron excitability – Long half life; given as a loading dose


Anticonvulsants

• Add-ons: used for refractory seizures– Levetiracetam

• Dogs and cats• Side effects include sedation, ataxia, and anorexia

– Zonisamide• Works by blocking calcium and sodium channels in

the brain• Side effects include ataxia and sedation


Anticonvulsants

• Add-ons: used for refractory seizures– Gabapentin

• Works by inhibiting calcium channels resulting in decreased excitatory neurotransmission

• Side effects include sedation, ataxia, and the potential for hepatotoxicity

– Felbamate• Used in dogs that to control seizures• Side effects include hepatotoxicity, reversible

blood dyscrasias, and keratoconjunctivitis sicca


Calming Agents

• Tranquilizers: used to calm animals; reduce anxiety and aggression

• Sedatives: used to quiet excited animals; decrease irritability and excitement

• Anti-anxiety drugs: lessen anxiousness, but do not make animals drowsy


Calming Agents

• Phenothiazine derivatives – Unknown mechanism, believed to block

dopamine and alpha-1 receptors– Causes sedation, relieves fear and anxiety – Doesn’t produce analgesia – Side effects: hypotension, lower seizure

threshold, protrusion of nictitating membrane, paraphimosis


Calming Agents

• Butyrophenones– Similar to phenothiazines– Side effects: salivation, panting, shivering

• Benzodiazepines– Anti-anxiety, produce muscle relaxation, reduce

anxiousness

• Alpha-2 agonists– Some analgesia, some muscle relaxation, decreased

response to stimuli – Side effects: bradycardia and heart block


Analgesics

• Pain is classified as:– Physiologic: the bodies protective mechanism to

avoid tissue injury– Pathologic: arises from tissue injury and inflammation

or from damage to nervous system• Can be furthered divided into nociceptive (peripheral tissue

injury) or neuropathic (damage to the peripheral nerves or central nervous system)

– Acute: arising from a sudden stimulus such as surgery

– Chronic: persisting beyond the time normally associated with tissue injury


Analgesics

• Signs of pain in animals– Change in personality– Abnormal vocalization– Licking, biting, scratching

pain area– Change in appearance of

hair coat– Change in

posture/ambulation– Change in activity level

– Change in appetite – Change in facial

expression– Excessive sweating or

salivation– Oculonasal discharge– Teeth grinding – Change in bowel

movement or urination


Analgesics

• Analgesics: drugs that relieve pain

• Analgesics are categorized as non-narcotic (Chapter 16) or narcotic

• Narcotic analgesics are used for moderate to severe pain


Analgesics

• Narcotics refers to opiate (natural from opium poppy seeds) or opioid-like (synthetic)– Opioids produce analgesia and sedation and relieve

anxiety– Side effects of opioids include respiratory depression

and excitement if given rapidly


Opioids

• Do not produce anesthesia; patients still respond to sound and sensation

• Produce analgesia and sedation, and relieve anxiety

• Side effects: respiratory depression, excitement if given too rapidly

• Produce their effects by the action of opioid receptors– Mu = found in the brain– Kappa = found in the cerebral cortex and spinal cord– Sigma = found in the brain


Opioids

– Opium– Morphine sulfate– Meperidine– Hydromorphone– Butorphanol

– Hydrocodone– Fentanyl– Etorphine– Buprenorphine– Pentazocine


Opioid Antagonist

• Block the binding of opioids to their receptors• Used to treat respiratory and CNS depression of

opioid use• Examples include naloxone and naltrexone• Naloxone is a opioid antagonist used to reverse

respiratory depression following narcotic overdose– Relatively free of side effects


Neuroleptanalgesics

– Combination of an opioid and a tranquilizer or sedative

– Can cause a state of CNS depression and analgesia and may or may not produce unconsciousness

– Combination products may be prepared by veterinarian

– Examples include acepromazine and morphine; xylazine and butorphanol


Anesthetics

– Anesthesia means without sensation– Anesthetics interfere with the conduction of nerve

impulses– Anesthetics produce loss of sensation and muscle

relaxation, and may cause loss of consciousness– General anesthetics affect the CNS, produce loss of

sensation with partial or complete loss of consciousness

– Local anesthetics block nerve transmission in the area of application with no loss of consciousness


Local Anesthetics

– Block pain at the site of administration or application in the PNS and spinal cord

– May be used as nerve blocks, aid in endotracheal tube placement, and ease skin irritation

– Applied topically to mucous membranes and the cornea by infiltration of a wound or joint, by IV, and around nervous tissue

– Examples include lidocaine, proparacaine, tetracaine, mepivacaine, bupivacaine


General Anesthetics

• Injectable general anesthetics:– Barbiturates: CNS depressants derived from

barbituric acid. Used mainly as anticonvulsants, anesthetics, and euthanasia solutions

– Side effects: potent cardiovascular and respiratory depression

– May be long-acting, short-acting, or ultra-short acting– May vary in structure and be classified as an

oxybarbiturate or thiobarbiturate– Examples: phenobarbital, pentobarbital, thiopental,

methohexital


General Anesthetics

• Injectable general anesthetics (cont.):– Dissociatives: belong to the cyclohexamine family– Cause muscle rigidity (catalepsy), amnesia, and mild

analgesia– Work by altering neurotransmitter activity– Used for restraint, diagnostic procedures, and minor

surgical procedures– Side effects: cardiac stimulation, respiratory

depression, and exaggerated reflexes– Examples include ketamine and tiletamine


General Anesthetics

• Injectable general anesthetics (cont.):– Miscellaneous:

• Guaifenesin: skeletal muscle relaxant used in combination with an anesthetic drug to induce general anesthesia in horses

• Propofol: short-acting injectable anesthetic agent that produces rapid and smooth induction when given IV (lasts 2–5 minutes)

– Side effects include cardiac arrhythmias and apnea


General Anesthetics/Analgesics

• Inhalant general anesthetics: inhalant anesthetics are halogenated hydrocarbons– Halothane:

• Nonflammatory, inhalant anesthetic administered via a precision vaporizer

• Can cause hepatic problems, malignant hyperthermia, cardiac problems, and tachypnea

• Contraindicated in cases of gastric dilatation, pneumothorax, and twisted intestines

• Leave animals on 100% oxygen following surgery to prevent diffusion hypoxia


General Anesthetics/Analgesics• Inhalant general anesthetics (cont.):

– Isoflurane:• Nonflammatory, inhalant anesthetic administered via a

precision vaporizer• Causes rapid induction of anesthesia and short recoveries

following anesthetic procedures• Does not cause the cardiac arrhythmia problems of

halothane• Vigilant monitoring is needed because the animal can

change anesthetic planes quickly• Masking of animals with isoflurane is difficult because it

irritates the respiratory system• Side effects include respiratory depression and malignant

hyperthermia



• Inhalant general anesthetics (cont.):– Isomers of isoflurane:

• Nonflammable and have fewer cardiovascular side effects than other inhalants

• Quickly enter the bloodstream and escape to the brain, making them good for mask inductions

• Examples:– Enflurane: increases intracranial pressure (do not use if

animal has seizure history)– Desflurane: cannot be delivered by standard vaporizers

and can reduce blood pressure– Sevoflurane: profound respiratory depressant; close

monitoring is needed



• Inhalant general anesthetics (cont.):– Sevoflurane:

• A non-flammable inhalant anesthetic• The main isomer of isoflurane• Produces rapid induction and rapid recoveries• Produces fewer cardiovascular side effects than the other

inhalant anesthetics• Quickly enters the bloodstream and escapes to the brain,

also is eliminated quickly from the body• Has low tissue solubility, resulting in rapid elimination of the

drug by the body



• Inhalant general analgesics (cont.):– Nitrous oxide:

• Inhalant analgesic that diffuses rapidly throughout the body• Can enter gas-filled body compartments (increases pressure

in these compartments)• Contraindicated in cases of gastric dilatation, pneumothorax,

and twisted intestines• Leave animals on 100% oxygen following surgery to prevent

diffusion hypoxia


CNS Stimulants

• Reverse CNS depression caused by CNS depressants– Doxapram: stimulates brainstem to increase

respiration in animals with apnea or bradypnea; commonly used when animals have C-sections

– Methylxanthines: bronchodilators that have adverse effect of CNS stimulation; include caffeine, theophylline, and aminophylline; side effects include gastrointestinal irritation and bronchodilation


Euthanasia Solutions

• Used to humanely end an animal’s life

• Usually contain pentobarbital

• When pentobarbital is the only narcotic agent present, it is a C-II controlled substance

• When pentobarbital is in combination with other agents, it is a C-III controlled substance


Autonomic Nervous System Drugs

• Autonomic nervous system drugs work either by acting like neurotransmitters or by interfering with neurotransmitter release

• Two groups of drugs affect the parasympathetic nervous system

• Two groups of drugs affect the sympathetic nervous system


Cholinergic Drugs

• Mimic the action of the parasympathetic nervous system (refer to Table 7-5 in your textbook)

• Examples include bethanechol, metoclopramide, pilocarpine, edrophonium, neostigmine, demecarium, and organophosphates


Anticholinergic Drugs

• Inhibit the actions of acetylcholine by occupying the acetylcholine receptors (refer to Table 7-6 in your textbook)

• Examples include atropine, glycopyrrolate, aminopentamide, and propantheline


Adrenergic Drugs

• Simulate the action of the sympathetic nervous system (refer to Table 7-7 in your textbook)

• Side effects include tachycardia, hypertension, and cardiac arrhythmias

• Examples include epinephrine, norepinephrine, isoproterenol, dopamine, dobutamine, phenylpropanolamine, isoetharine, albuterol, terbutaline, ephedrine, and xylazine


Adrenergic Blocking Agents

• Block the effects of the adrenergic neurotransmitters (refer to Table 7-7 in your textbook)

• Examples of alpha-blockers include phenoxybenzamine, prazosin, and yohimbine

• Examples of beta-blockers include propranolol, metoprolol, and timolol


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