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Questions and Answers
Which non-sedating drug can be used for allergic rhinitis and urticaria?
Which non-sedating drug can be used for allergic rhinitis and urticaria?
What is the primary indication for pitolisant?
What is the primary indication for pitolisant?
Which of the following drugs is associated with sedation as an adverse effect?
Which of the following drugs is associated with sedation as an adverse effect?
Which formulation of antihistamine is useful for treating severe drug hypersensitivity reactions?
Which formulation of antihistamine is useful for treating severe drug hypersensitivity reactions?
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What can increase the risk of cardiac toxicity and arrhythmia when using second-generation antihistamines?
What can increase the risk of cardiac toxicity and arrhythmia when using second-generation antihistamines?
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Which of the following correctly describes an adverse effect associated with 1st and 2nd generation antihistamines?
Which of the following correctly describes an adverse effect associated with 1st and 2nd generation antihistamines?
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How do H3 receptor antagonists like pitolisant work in the treatment of narcolepsy?
How do H3 receptor antagonists like pitolisant work in the treatment of narcolepsy?
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What type of reaction should be avoided in allergic asthma regarding antihistamines?
What type of reaction should be avoided in allergic asthma regarding antihistamines?
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What differentiates inverse agonists from standard antagonists regarding H1 receptors?
What differentiates inverse agonists from standard antagonists regarding H1 receptors?
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Which of the following drugs is classified as a first-generation antihistamine?
Which of the following drugs is classified as a first-generation antihistamine?
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What is a common side effect of first-generation antihistamines due to their anticholinergic properties?
What is a common side effect of first-generation antihistamines due to their anticholinergic properties?
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Which antihistamine generation primarily is used to avoid sedation?
Which antihistamine generation primarily is used to avoid sedation?
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What enzyme is primarily responsible for the metabolism of many antihistamines in the liver?
What enzyme is primarily responsible for the metabolism of many antihistamines in the liver?
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What is the primary manifestation of an overdose of first-generation antihistamines?
What is the primary manifestation of an overdose of first-generation antihistamines?
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What is the expected duration of action for many antihistamines?
What is the expected duration of action for many antihistamines?
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Which of the following is NOT a commonly recognized first-generation antihistamine?
Which of the following is NOT a commonly recognized first-generation antihistamine?
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Which histamine receptor is primarily responsible for stimulating gastric acid secretion?
Which histamine receptor is primarily responsible for stimulating gastric acid secretion?
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What effect does histamine have when acting on the H1 receptor?
What effect does histamine have when acting on the H1 receptor?
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Which physiological action of histamine involves promoting wakefulness?
Which physiological action of histamine involves promoting wakefulness?
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In the case of allergic reactions, which receptor antagonists are commonly used to control itching?
In the case of allergic reactions, which receptor antagonists are commonly used to control itching?
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What is the primary consequence of histamine released during an inflammatory response?
What is the primary consequence of histamine released during an inflammatory response?
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Which physiological reaction does histamine induce when injected intradermally?
Which physiological reaction does histamine induce when injected intradermally?
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Which type of histamine receptor is linked to the modulation of both innate and acquired immune responses?
Which type of histamine receptor is linked to the modulation of both innate and acquired immune responses?
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Which mechanism is responsible for the synthesis of histamine?
Which mechanism is responsible for the synthesis of histamine?
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Study Notes
Lesson 29: Pharmacology of Histamine
- Histamine is a basic amine found in most tissues, but highly concentrated in tissues exposed to the environment (lungs, skin, gastrointestinal tract)
- Histamine released from mast cells during inflammatory or allergic reactions
- Histamine metabolized by histaminase and/or a methylating enzyme
- Histamine synthesis: L-histidine → histamine (via L-histidine decarboxylase)
Histamine Receptors
- Four types of histamine receptors (H1-H4) are G protein-coupled receptors (GPCRs)
- H1 receptor implicated in inflammatory response; associated with [Ca2+]
- H2 receptor implicated in inflammatory response and associated with [AMPc] increase in parietal cells, involved in acid secretion
- The four receptor types are implicated in inflammatory responses
Physiological Actions of Histamine (Summary of Effects)
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Mediated through H1 receptor:
- Contraction of most smooth muscles (except blood vessels)
- Vasodilation
- Bronchoconstriction
- Increased vascular permeability
- Stimulation of sensory nerve fibers (itching, etc.)
- Influences waking or sleep (H1/H3 involved)
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Mediated through H2 receptor:
- Stimulation of gastric secretion
- Cardiac stimulation
- Implicated in the pathogenesis of peptic ulcers, stimulating gastric acid secretion
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Effects on Skin (H1 receptor):
- Injected intradermally, histamine causes the "triple response": Reddening, weal, and flare
- H1 antagonists used to control itching caused by allergic reactions and insect bites
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Sleep/Wake Homeostasis (H3 receptor):
- H3 receptor antagonists can be used to treat narcolepsy
Antihistamines
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H1 receptor antagonists and H1 inverse agonists (2nd generation) used to treat inflammatory and allergic conditions
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Antagonists simply block the H1 receptor, preventing histamine binding
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Inverse agonists block histamine binding and reduce basal H1 receptor activity, leading to a more potent antiallergic effect
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Classified as:
- First generation: cross blood-brain barrier, can cause sedation (chlorphenamine, diphenhydramine, etc.)
- Second generation: unlikely to cause sedation (cetirizine, fexofenadine, etc.)
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Duration of action ranges from 6 to 24 hours (differing by type), facilitating once daily use
Clinical Uses
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Allergic reactions: non-sedating second-generation antihistamines (fexofenadine, cetirizine) for rhinitis and urticaria. Topical preparations used for allergic conjunctivitis/insect bites. Injectable antihistamines and adrenaline used for severe reactions. Azelastine for nasal allergic symptoms.
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Antiemetics: Prevention of motion sickness (promethazine, doxylamine) and other causes of nausea
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Narcolepsy treatment: Pitolisant (H3 receptor antagonist/inverse agonist) for excessive daytime sleepiness
Adverse Effects
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Mostly metabolized in the liver, excreted in the urine
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Sedative CNS effects are unwanted when treating allergies
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Contraindications (1st and 2nd generation):
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Alcohol
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Anticholinergics
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Digestive obstruction
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Glaucoma
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Contraindications (2nd generation):
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Arrhythmias
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CYP3A4 inhibitors (drug interaction)
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Adverse Drug Reactions (ADRs):
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Sedation: dry mouth, blurred vision, constipation, and retention of urine; digestive disturbances
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Second-generation Drugs: cardiac toxicity/arrhythmia, increased risk when taken with grapefruit juice or agents that inhibit cytochrome P450 in the liver
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Drug-induced photosensitivity
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Description
Explore the pharmacology of histamine including its synthesis, metabolism, and various receptors. This lesson covers the physiological actions of histamine, focusing on the roles of H1 to H4 receptors in inflammatory responses. Test your understanding of histamine's effects in the body and its relevance in allergic reactions.
