Autacoids, Histamine, and Antihistamines: Downloadable Resources (PDF & PPT)
Download comprehensive notes on autacoids, histamine, and antihistamines in PDF and PPT formats. These materials cover the definition of autacoids, the role of histamine in the body, the different types of histamine receptors, and the use of antihistamines in treating allergic reactions and other conditions. Suitable for students, healthcare professionals, and anyone interested in pharmacology. Download for offline study.
Keywords: Autacoids, Histamine, Antihistamines, PDF, PPT, Download, Pharmacology, Allergy, H1 Receptors, H2 Receptors, H3 Receptors, H4 Receptors, Anaphylaxis, Inflammation, Allergic Rhinitis, Urticaria, Cetirizine, Loratadine, Fexofenadine, Diphenhydramine.
Autacoids, Histamine, and Antihistamines: A Detailed Overview
This document provides a comprehensive overview of autacoids, with a particular focus on histamine and antihistamines. It covers their roles in the body, mechanisms of action, and clinical applications.
Autacoids: Definition and Classification
Autacoids, also known as "local hormones," are a diverse group of signaling molecules that act locally at the site of their synthesis. Unlike hormones that are transported through the bloodstream, autacoids exert their effects near the cells that produce them. They are involved in a wide range of physiological and pathological processes, including inflammation, pain, allergic reactions, and regulation of blood flow. Examples of autacoids include:
- Histamine
- Serotonin (5-HT)
- Prostaglandins
- Leukotrienes
- Kinins (e.g., Bradykinin)
- Platelet-Activating Factor (PAF)
- Endothelins
Histamine: Synthesis, Storage, and Release
Histamine is a biogenic amine derived from the amino acid histidine. It is synthesized by the enzyme histidine decarboxylase. Histamine is stored in granules within mast cells (found in tissues) and basophils (found in blood). It is also found in enterochromaffin-like (ECL) cells in the stomach and in certain neurons in the brain.
Histamine release is triggered by various stimuli, including:
- Immunological Reactions (IgE-mediated): Allergens bind to IgE antibodies on the surface of mast cells and basophils, leading to degranulation and histamine release. This is the primary mechanism in allergic reactions.
- Physical Injury: Trauma, heat, or cold can cause mast cell degranulation.
- Drugs: Certain drugs, such as morphine and some antibiotics, can directly stimulate histamine release.
- Venoms and Toxins: Some venoms and toxins contain substances that trigger histamine release.
Histamine Receptors
Histamine exerts its effects by binding to specific receptors on target cells. There are four known histamine receptor subtypes: H1, H2, H3, and H4. Each receptor subtype has a different distribution in the body and mediates distinct physiological effects:
- H1 Receptors: Found in smooth muscle, endothelium, and the central nervous system (CNS). Activation of H1 receptors mediates:
- Vasodilation (widening of blood vessels)
- Increased vascular permeability (leading to edema or swelling)
- Bronchoconstriction (narrowing of airways)
- Pruritus (itching)
- Smooth muscle contraction (e.g., in the gut)
- Wakefulness and alertness (in the CNS)
- H2 Receptors: Found in gastric parietal cells, vascular smooth muscle, and some immune cells. Activation of H2 receptors primarily stimulates:
- Gastric acid secretion
- Vasodilation (to a lesser extent than H1)
- Cardiac stimulation (increased heart rate and contractility)
- H3 Receptors: Primarily found in the CNS, particularly in the brain. They act as autoreceptors, inhibiting histamine release from neurons. They also modulate the release of other neurotransmitters.
- H4 Receptors: Found mainly in immune cells (e.g., mast cells, eosinophils, T cells) and in the bone marrow. They play a role in chemotaxis (movement of cells) and inflammation.
Antihistamines: Classification and Mechanism of Action
Antihistamines are drugs that block the action of histamine at its receptors. They are primarily used to treat allergic reactions. Antihistamines are classified based on which receptor subtype they primarily block:
- H1-Receptor Antagonists (H1 Antihistamines): These are the most commonly used antihistamines. They are used to treat allergic conditions such as:
- Allergic rhinitis (hay fever)
- Urticaria (hives)
- Conjunctivitis (eye allergies)
- Insect bites and stings
- Anaphylaxis (as an adjunct to epinephrine)
H1 antihistamines are further divided into first-generation and second-generation drugs:
- First-Generation H1 Antihistamines: These older drugs readily cross the blood-brain barrier, causing sedation and other CNS effects. Examples include:
- Diphenhydramine (Benadryl)
- Chlorpheniramine (Chlor-Trimeton)
- Hydroxyzine (Atarax)
- Second-Generation H1 Antihistamines: These newer drugs have less ability to cross the blood-brain barrier, resulting in significantly less sedation. Examples include:
- Cetirizine (Zyrtec)
- Loratadine (Claritin)
- Fexofenadine (Allegra)
- Desloratadine (Clarinex)
- Levocetirizine (Xyzal)
- H2-Receptor Antagonists (H2 Antihistamines): These drugs primarily block H2 receptors in the stomach, reducing gastric acid secretion. They are used to treat:
- Gastroesophageal reflux disease (GERD)
- Peptic ulcers
- Ranitidine (Zantac) *Note: Ranitidine has been withdrawn from the market in many countries due to concerns about NDMA contamination.*
- Famotidine (Pepcid)
- Cimetidine (Tagamet)
- Nizatidine (Axid)
- H3 and H4 Receptor Antagonists: These are primarily used in research settings and are not currently available for widespread clinical use. There is ongoing research into their potential therapeutic applications.
Side Effects of Antihistamines
- First-generation H1 antihistamines: Sedation, dry mouth, blurred vision, urinary retention, constipation, dizziness.
- Second-generation H1 antihistamines: Generally well-tolerated, with less sedation. Headache and dry mouth can occur.
- H2 antihistamines: Generally well-tolerated. Headache, diarrhea, and dizziness can occur. Cimetidine can cause drug interactions and hormonal effects.
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