Histamine is a local hormone that is produced and stored in mast cells, basophils, and certain neurons. It binds to four types of histamine receptors, H1-H4, to produce various effects. H1 receptor activation causes allergic reactions, bronchoconstriction, increased secretions, wakefulness, vasodilation, and positive effects on the heart. H2 receptor activation stimulates gastric acid secretion. Antihistamines are drugs that block H1 receptors, divided into sedating first generation drugs and less sedating second generation drugs. They are used to treat allergies, motion sickness, vertigo, pre-anesthesia, cough, and other conditions involving histamine. H2

1. Antihistaminics
Dr. Pramod Bhalerao

2. • Histamine is categorised as autacoids or local
hormones. (autos=self; akos=remedy).

3. Histamine Biosynthesis

4. Histamine storage
• Mast cells
• Basophil
• Histaminocytes in stomach
• Histaminergic neurons in brain.

5. Histamine receptors
• H1
• H2
• H3
• H4

6. Pharmacological actions of Histamine

7. H1 receptor mediated actions
• Stimulant of sensory nerve endings
• Pain, itch.
• Urticaria response to insect bites and stings.

8. H1 receptor mediated actions
• Smooth muscle
• Bronchoconstriction
• Contraction of intestinal smooth muscles

9. H1 receptor mediated actions
• Increased secretions of exocrine glands in
bronchioles, pancreas, salivary, and lacrimal
glands.

10. H1 receptor mediated actions
• Effect in CNS
• wakefulness

11. H1 receptor mediated actions
• Dilatation of arterioles
• Dilatation of post-capillary venules
• Fall in BP.
• Headache
vasodilatation

12. H1 receptor mediated actions
• Positive chronotropic
• Positive ionotropic
• These effects may occur reflexly due to fall in
BP.

13. H1 receptor mediated actions
• When histamine is injected intradermally
• Triple response
• Flush-reddening of skin
• Wheal- formation of oedematous patch
• Flare- red irregular halo surrounding wheal.

14. H1 receptor mediated actions
• Severe hypersensitivity reactions may also
occur following injection of histamine.
• Anaphylaxis.

15. H2 receptor mediated actions
• Gastric acid secretion by activation of H2
receptors on gastric parietal cells.

16. H1- Receptor antagonists

17. Clinical classification
• First generation drugs- short duration of
action, more sedating,
antimuscarinic(anticholinergic) side effects.
• Second generation drugs- long duration of
action, least sedating, devoid of
antimuscarinic side effects.

18. First generation antihistaminics
• Highly sedative
• Diphenhydramine
• Dimenhydrinate
• Promethazine
• Hydroxyzine
• Moderately sedative
• Pheniramine
• Meclizine
• Buclizine
• Cinnarizine
• Mild sedative
• Chlorpheniramine
• Dexchlorpheniramine
• Cyclizine
• Clemastine

19. Second generation antihistaminics
• Fexofenadine
• Loratadine
• Desloratadine
• Cetirizine
• Levocetirizine
• Azelastine
• Second generation drugs- long duration of
action, least sedating, devoid of antimuscarinic
side effects.

20. Side effects of antihistaminics
• Sedation (Patients should be cautioned not to
operate motor vehicles or machinery requiring
constant attention).
• Dryness of mouth, constipation, urinary
retention can be ascribed to anticholinergic
property.

21. Uses of H1 antihistaminics

22. Allergic disorders
Urticaria
Angioedema of lips
Allergic conjunctivitis
Itching

23. Other conditions involving histamine
ivy poisoning insect bite
blood/saline infusion induced rigor.

24. Common cold
• Symptomatic relief by anticholinergic (reduce
rhinorrhoea) and sedative actions.

25. Motion sickness
• Promethazine, diphenhydramine,
dimenhydrinate and meclozine.
• Should be taken one hour before starting
journey.

26. • Other uses
• morning sickness,
• drug induced vomitting
• postoperative vomitting,
• radiation sickness.

27. Vertigo
• Vertigo is a sensation of spinning while
stationary.
• Cinnarizine

28. Preanaesthetic medication
• Promethazine has been used for its
anticholinergic and sedative properties.

29. Cough
• They have no selective cough suppressant
action.
• may afford symptomatic relief by
• sedative and
• anticholinergic property.

30. Parkinsonism
• Promethazine and some others afford mild
symptomatic relief in early cases based on
• anticholinergic
• sedative property.

31. Acute muscle dystonia
• Caused by antipsychotic drugs is promptly
relieved by parenteral promethazine,
diphenhydramine or hydroxyzine.
• Based on central anticholinergic action of the
drugs.

32. H2 antagonist
• Cimetidine
• Ranitidine,
• Famotidine,
• Roxatidine
• Used in peptic ulcer disease.

33. Thank You