Antihistamines are drugs that block the action of histamine. They are commonly used for the relief of allergy symptoms or for gastrointestinal conditions. This pharmacology lecture covers topics such as pathophysiology of allergic reaction, histamine and its actions, H1 and H2 histamine receptors, mast cells, parietal cells, mechanism of action of H1 and H2 antagonists. Drugs mentioned include first generation antihistamines; Brompheniramine, Chlorpheniramine, Clemastine, Cyproheptadine, Diphenhydramine, Doxylamine, Hydroxyzine, Meclizine, Promethazine; Second generation antihistamines; Cetirizine, Desloratadine, Fexofenadine, Levocetirizine, Loratadine, Azelastine, Olopatadine, Ketotifen; H2 blockers; Cimetidine, Famotidine, Nizatidine, and Ranitidine.
Before we do that let’s first talk about histamine is a small molecule produced in our bodies by decarboxylation of the is particularly concentrated in the skin histamine is generated and stored within granules in mast cells located within the blood and enterochromaffin-like cells located in the stomach lining now allergic reaction so when an allergy-prone contact with
Allergen such as ragweed pollen their b-cells of ragweed immunoglobulin e antibodies these antibodies abbreviated as ige contact with ragweed pollen again the binding of allergen to ige antibodies rich in histamine now let’s move on to the second condition which triggers mast cells release chemical mediators among them histamine which affect blood triggers histamine release
Can come from drugs and foreign chemicals compounds as morphine are a few examples that can directly displace histamine from the well histamine exerts its effects by binding to various types of histamine identified and these are h1 h2 h3 and h4 that being said in this lecture we’re clinically useful drugs so the first type h1 receptors are expressed and on peripheral nerve
Endings these receptors mediate mainly inflammatory endothelial receptors it causes blood vessels to dilate thus making them more smooth muscle receptors particularly the ones located in bronchioles it causes nervous system histamine binding to the h1 receptors in lastly histamine mediated stimulation of peripheral nerve endings leads to pain receptors are expressed mainly
On gastric parietal cells when histamine let’s switch gears and let’s talk about drugs that block the action of histamine antihistamines so the h1 receptor blockers can be divided into the older act as inverse agonists meaning they bind to h1 receptor on a target tissue actions and gradual relief of allergy related symptoms such as inflammation generation h1 antihistamines
Consists of two aromatic rings first generation h1 antihistamine can cross the blood-brain barrier and thus additionally first generation agents have relatively poor h1 receptor such as cholinergic alpha-adrenergic and serotonin receptors this leads to a cause dry mouth blurred vision and urinary retention blockade of alpha-adrenergic receptors positive side blockade of
Central histamine and acetylcholine receptors first generation h1 antihistamine include brompheniramine doxylamine hydroxyzine meclizine and promethazine although all of these drugs therapeutic indications for example diphenhydramine and doxylamine are often often used in the treatment of nausea and vomiting related to certain conditions generation agents have bulkier and
Less lipophilic structure therefore they selective for the peripheral h1 receptors involved in allergies as second generation drugs provide the same allergy symptom relief but with less antihistamine include cetirizine desloratadine fexofenadine have both antihistamine and mast cells stabilizing effects namely azelastine well as ketotifen which is currently available in
Ophthalmic formulation only before we end let’s quickly discuss histamine type 2 receptor blockers we need to take a closer look at their primary target that is acid producing receptors which control acid production that is acetylcholine receptor gastrin receptor and histamine h2 type receptor now parasympathetic vagus nerve that receptor to increase intracellular calcium
Next gastrin which is a hormone to just like acetylcholine increase intracellular calcium synthesize and secrete histamine finally histamine secreted from to increase of intracellular cyclic-amp levels now this increase in this so called gastric proton pump secretes hydrogen ions into the lumen of stomach in exchange for potassium so h2 receptor antagonists selectively
Block this makes them useful in treatment of gastric ulcers and gastroesophageal include cimetidine famotidine nizatidine and ranitidine in general these common being headache out of the four cimetidine is the most likely to gynecomastia and galactorrhea due to its antiandrogenic and prolactin you enjoyed this video and as always stay tuned for more
Transcribed from video
Pharmacology – ANTIHISTAMINES (MADE EASY) By Speed Pharmacology
