Source: Hindu

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Context

• Currently, there is a lot of research ongoing on H3 and H4 receptors, in particular to alleviate the effects of neurological and immunological disorders, respectively, using specialised drugs.

Details

• Antihistamines are a class of medications used to treat allergic reactions and symptoms by blocking the action of histamine, a chemical released by the immune system during an allergic reaction.
• When histamine binds to specific receptors in the body, it causes symptoms such as sneezing, itching, runny nose, and watery eyes.

Receptors

• Histamine performs various functions in the body, mediated by different histamine receptors known as H1, H2, H3, and H4 receptors.
• Each receptor type is found in different locations and serves distinct purposes:

• H1 Receptors: Found in the inner surface of blood and lymphatic vessels, neurons, and smooth muscle cells. H1 antihistamines block these receptors and are used to alleviate symptoms of minor allergies such as sneezing, itching, and hives.

• H2 Receptors: Located in certain cells of the gastric glands, where they stimulate the release of gastric acid for digestion. H2 antihistamines are used to reduce gastric acid secretion and treat conditions like peptic ulcers and gastroesophageal reflux disease (GERD).
• H3 Receptors: Found in cells in the central nervous system, where they regulate the release of neurotransmitters such as dopamine and serotonin. Research on H3 receptors is ongoing to develop drugs that modulate neurotransmitter release, potentially treating neurological disorders such as Alzheimer's disease and attention deficit hyperactivity disorder (ADHD).
• H4 Receptors: Located on cells involved in the body's inflammatory response, including immune cells. H4 receptors regulate inflammation, including allergic reactions. Ongoing research aims to develop drugs targeting H4 receptors to alleviate the symptoms of various immunological disorders.

Mechanism of Action

• Histamine Receptors: Histamine acts on specific receptors in the body, mainly H1 receptors (involved in allergic reactions) and H2 receptors (involved in gastric acid secretion).
• Antagonistic Action: Antihistamines block the H1 receptors, preventing histamine from binding to them and eliciting an allergic response.

Classification

• First-Generation Antihistamines: Older antihistamines that readily cross the blood-brain barrier, causing sedation.
• Second-Generation Antihistamines: Newer antihistamines designed to have reduced sedative effects due to their limited penetration of the blood-brain barrier.

Common First-Generation Antihistamines:

• Diphenhydramine (Benadryl): Often used for treating allergies, itching, and insomnia.
• Chlorpheniramine (Chlor-Trimeton): Used for allergies, hay fever, and the common cold.
• Hydroxyzine (Atarax, Vistaril): Used for itching, anxiety, and nausea.

Common Second-Generation Antihistamines:

• Cetirizine (Zyrtec): Used for allergies, hay fever, and hives.
• Loratadine (Claritin): Used for allergies and allergic rhinitis.
• Fexofenadine (Allegra): Used for allergies and chronic idiopathic urticaria.

Indications

• Allergic Rhinitis: Relief of sneezing, itching, rhinorrhea, and nasal congestion.
• Allergic Conjunctivitis: Alleviation of itching and redness of the eyes.
• Urticaria (Hives): Treatment of itching and rash associated with hives.
• Anaphylaxis: Adjunctive therapy in severe allergic reactions.
• Motion Sickness: Some antihistamines have antiemetic effects and can be used to prevent motion sickness.

Side Effects

• First-Generation: Sedation, drowsiness, dry mouth, blurred vision, constipation, urinary retention, and confusion.
• Second-Generation: Less sedation compared to first-generation, but may still cause drowsiness in some individuals, as well as headache and dry mouth.

Dosage Forms

• Oral Tablets/Capsules: Most antihistamines are available in oral forms.
• Liquid Formulations: Available for pediatric patients or individuals who have difficulty swallowing pills.
• Topical Preparations: Some antihistamines are available in creams or ointments for skin conditions like itching and urticaria.
• Nasal Sprays: Certain antihistamines are formulated as nasal sprays for allergic rhinitis.

Sources:

Hindu

PRACTICE QUESTION Q.  Discuss the mechanism of action, classification, and therapeutic applications of antihistamines. How do they contribute to the management of allergic reactions and related symptoms? (250 words)