Antihistamines
There are two types of histamine antagonists namely H1 receptor antagonists (classical type) and H2 receptor antagonists (used in peptic ulcer). Only H1 receptor antagonists are described below and 112 receptor antagonists will be discussed along with other drugs used for the treatment of peptic ulcer.
H1 antihistamines antagonists act by competitively blocking the histamine receptors. They do not control the release of histamine. They block the triple response and antagonize the smooth muscle effects of histamine. They do not block the gastric secretory and cardiac stimulant actions of histamine.
Some of the actions of these drugs may not be related to blockade of H1-receptors. They may be as a result of their effects at other receptors such as 5-HT receptors, a1-adreno- ceptors, and acetylcholine muscarinic receptors, both peripherally and in the central nervous system. So the other actions of H1-receptor antagonists are:
• The older antihistamines produce variable degree of sedation. This depends on their ability to cross blood-brain barrier and their affinity for the central H1-receptors.
• Certain H1-antihistamines (e.g. promethazine, diphenhydramine, dimenhydrinate, cyclizine, and meclizine) possess antimotion sickness properties.
• Promethazine also controls vomiting of pregnancy and other causes.
• Promethazine and few other antihistamines reduce tremors, rigidity, and sialorrhoea of Parkinsonism due to their anticholinergic and sedative properties.
• Some of the antihistamines such as chlorpheniramine, diphenhydramine, and promethazine are also effective anitussives.
• Many H1-blockers antagonize muscarinic actions of acetylcholine. So they enhance the anticholinergic actions of atropine or its congeners, phenothiazines, tricyclic antidepressants and disopyramide.
• Some H1-blockers such as mepyramine and antazoline have strong membrane stabilizing property while others have weak action. Due to this, antazoline may be used as antiarrhythmic agent. However, they are not used as local anesthetics because they cause irritation when injected s.c.
• On i.v. injection, most of the antihistamines cause a fall in blood pressure.
• Many H1-blockers inhibit neuronal uptake of noradrenaline.
The newer long acting and least sedative H1- blockers are astemazole, ceterizine, loratadine, desloratadine, terfenadine, fexofenadine, azelastine, mizolastine, ebastine. At present, the use of astemazole and terfenadine has been stopped because they induce potentially fatal arrhythmia and torsades de pointes.
Currently loratadine, desloratadine and fexofenadine of this class are used because they are devoid of the above mentioned side effects. New azelastine has good topical activity. So it is used for seasonal and perennial allergic rhinitis by nasal spray to provide quick symptomatic relief which lasts for 12 hours.
On oral administration, antihistamines are rapidly absorbed and the peak effect is observed in one hour. The duration of action is for 4 hours in case of old antihistaminic. They are metabolized in liver and excreted in the urine. The important side effects are drowsiness, dry mouth, and blurring of vision. They also potentiate the action of other central nervous system depressants.
Uses of antihistamines
• They are extensively used to prevent allergic reactions or to treat their symptoms such as allergic rhinitis, hay fever and urticaria.
• Promethazine is used as antiemetic agent.
• Diphenhydramine, dimenhydrinate, buclizine, cyclizine and medicine are used for the prevention and treatment of motion sickness. However, their use in nausea and vomiting (morning sickness) of pregnancy is controversial.
• Antihistamines with central nervous system depressant action such as pyrilamine and diphenhydramine have been used as sedative and to induce sleep, especially in children.
• They are also employed in the management of cardiac arrhythmia, Parkinsonism, idiopathic pruritus, cough, vertigo and acute muscle dystonia caused by antiemetic antipsychotic drugs.
• Antihistamines with antimuscarinic effects, E.g. dimenhydrinate (50—100 mg oral), meclizine (25—50 mg oral) or promethazine (25 mg oral) are used for short control of salivation (as in occlusal adjustment or impression making in dentistry) 30 mm before these procedures.
These are used when patient is taking calcium carbonate containing antacid which decreases the absorption of scopolamine, glycopyrrolate and propantheline used to control excessive salivation in dentistry in routine.
Antihistamines is widely distributed in almost alibody tissues. So far H1 (smooth muscles of bronchi and intestine), H2 (cardiac and gastrointestinal tract) and H (neurons in brain) histamine receptors have been described.
Although histamine has no therapeutic use, it is used to diagnose hypochiorhydria and achiorhydria.
H1 histamine antagonists act by competitively blocking the H1 receptors and are extensively used to prevent allergic reactions or to treat their symptoms, viz. allergic rhinitis, hay fever and urticaria.
Promethazine, diphenhydramine, dimenhydrinate, buclizine, cyclizine, and meclizine are used for the prevention and treatment of motion sickness. Promethazine is also useful antiemetic antihistamines.
Pyrilamine and diphenhydramine have been used as sedative and hypnotic, especially in children. Long-acting non-sedative antihistaminics (loratadine and cetirizine) can be used in allergic asthma and as an adjunct to the standard treatment of bronchial asthma.
Serotonin (5-hydroxytryptamine) is believed to act as a putative neurotransmitter in central nervous system and plays an important role in the peristalsis of gastrointestinal tract. However serotonin as such has no clinical use as a antihistamines.
Cyproheptadine is a serotonin antagonist with additional H1 antihistaminic, mild anticholinergic and sedative properties. It is used in allergy, pruritus, carcinoid tumor, postgastrectomy dumping syndrome and migraine headache prophylaxis.
Methysergide is useful for migraine prophylaxis, carcinoid tumor and postgastrectomy dumping syndrome. Ketanserin and mianserin are useful in the treatment of migraine, carcinoid syndrome and peripheral vascular diseases. Ondansetron is very effective antiemetic antihistamines.
Buspirone and ipsapiron are useful for generalized anxiety, phobia and panic attacks. Sumatriptan is effective in migrain and anxiety.
