Fundamentals of Pharmacology for Veterinary Technicians

Chapter 7

Drugs Affecting the Nervous System

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

• 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

CNS Drugs

• 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

– Examples:• Phenobarbital (short-acting barbiturate)• Primidone (structurally similar to phenobarbital)• Diazepam (used IV to treat status epilepticus)• Clorazepate (adjunct anticonvulsant)• Potassium bromide (adjunct anticonvulsant)

CNS Drugs

• 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

• Examples in these groups:– Phenothiazine derivatives (acepromazine,

chlorpromazine)– Benzodiazepines (diazepam)– Alpha-2 agonists (xylazine, detomidine, medetomidine)

CNS Drugs

• Analgesics: drugs that relieve pain

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

• Narcotic analgesics are used for moderate to severe pain

• Narcotic refers to opioid (natural) or opioid-like (synthetic) products

CNS Drugs

• 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

CNS Drugs

• Examples of opioids:– Opium– Morphine sulfate– Meperidine– Hydromorphone– Butorphanol– Hydrocodone– Fentanyl– Etorphine– Buprenorphine– Pentazocine

CNS Drugs

• Opioid antagonists:– Block the binding of opioids to their

receptors– Used to treat respiratory and CNS

depression of opioid use– Examples include naloxone and

naltrexone

CNS Drugs

• 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

CNS Drugs

• 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

• 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)

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

General Anesthetics/Analgesics

• 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

General Anesthetics/Analgesics

• 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 Drugs

• 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

CNS Drugs

• 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 (see Table 7-5)

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

Anticholinergic Drugs

• Inhibit the actions of acetylcholine by occupying the acetylcholine receptors (see Table 7-6)

• Examples include atropine, glycopyrrolate, aminopentamide, and propantheline

Adrenergic Drugs

• Simulate the action of the sympathetic nervous system (see Table 7-7)

• 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 (see Table 7-7)

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

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