Enlist Histamine Receptor and Neurochemistry Notes
Keywords: Download PDF, Notes, PPT, Histamine Receptor, Neurochemistry, Histamine, Antihistamines, H1, H2, H3 Receptors
This document provides detailed information on histamine receptors, neurochemistry, and the role of histamine in biological systems. It also covers the synthesis, release, and function of histamine, as well as the mechanism of action of antihistamines.
Enlist Histamine Receptor and Neurochemistry Notes
INTRODUCTION
Histamine, [2-(imidazol-4-yl) ethylamine], is biosynthesized by the decarboxylation of the basic amino acid histidine. It is found in all organs and tissues of the human body. Histamine is stored in the secretory granules of mast cells (pH 5.5) as positively charged and ionically complexed with negatively charged acidic groups on other secretory granules, which constitute heparin. The principal target cells of immediate hypersensitivity reactions are mast cells and basophils, which generate IgE antibodies that bind to FC€ receptors on the granule surface. This leads to transmembrane activation of tyrosine protein kinase, which phosphorylates and activates phospholipase. Phosphatidylinositol biphosphate is converted into inositol triphosphate, triggering the intracellular release of calcium ions.
The calcium ions cause the exocytic release of histamine with the transfer of Na+ ions from the extracellular space. The released histamine targets histaminergic receptors (H1, H2, and H3) to elicit its actions.
Role of Histamine
Histamine is an important chemical messenger, communicating information from one cell to another, and is involved in a variety of complex biological actions. It is mainly stored in an inactive bound form, from which it is released as a result of an antigen–antibody reaction, initiated by different stimuli such as venoms, toxins, proteolytic enzymes, detergents, food materials, and numerous chemicals. Systemically, histamine contracts smooth muscles of the lungs and the gastrointestinal system, causes vasodilation, lowers blood pressure, and increases heart rate. It also causes symptoms such as itching, sneezing, watery eyes, and a runny nose.
Histamine Receptors
Histamine exerts its biological function by interacting with at least three distinct specific receptors: H1, H2, and H3. Historically, the term antihistamine has been used to describe a drug that acts on H1 and H2 receptors. An antihistaminic agent should ideally prevent the production or release of these autocoids by inhibiting the response of sensitized mast cells and basophils to specific antigens.
Antihistamines
- Antihistamines are drugs that competitively block the H1 receptors.
- Antihistamines antagonize the stimulant action of histamine on the smooth muscles of the gastrointestinal tract (GIT), uterus, and blood vessels, and inhibit histamine-augmented salivary secretion.
- H1-receptor antagonists have been used clinically to treat various allergic disorders, such as seasonal or perennial allergic rhinitis and chronic urticaria.
Release and Function of Endogenous Histamine
Histamine is released due to the interaction of an antigen with IgE antibodies on the mast cell surface and plays a central role in immediate hypersensitivity reactions.
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