2. Histamine
• physiologically active amine C5H9N3
• plant and animal tissue
• Released from mast cells as part of an allergic
reaction (IgE / PG / LT / TNF-A / SP/ NKA
Complement-mediated)
-Gastric Secretion
-Capillary Dilatation
-Bronchoconstriction
-Hypotension Imidazoline
ring and amine gp
4. Mechanism of Action
• Combining with specific cellular receptors on
cells viz H1-H4 ; Leucocyte diapedesis
Neutrophil Chemotaxis Mast Cell
Degranulation
• Primarily H1-mediated in Lewis’ Triple
Response / UV
• H2-H4 blockers / Atypical Antihistaminics
found largely ineffective
5. Type Location Function
H1 Found on smooth muscles,
endothelium, and central
nervous system tissue
Causes vasodilation, bronchoconstriction
bronchial smooth muscle contraction, separation
of endothelial cells (responsible for hives),
and pain and itching due to insect stings; the
primary receptors involved in allergic rhinitis
symptoms and motion sickness; sleep regulation.
H2 Located on parietal cells Primarily stimulate gastric acid secretion
And we use antiacidic drugs for this receptor
H3 Found on central nervous
system and to a lesser
extent peripheral nervous
system tissue
Decreased neurotransmitter release:
histamine, acetylcholine, norepinephrine,
serotonin
H4 Found primarily in
the basophils and in
the bone marrow. It is also
found onthymus, small
intestine, spleen, andcolon.
Plays a role in chemotaxis.
6. 1st
Antihistaminic
• Piperoxan
• Discovered in 1933 by Jeff Forneau and Daniel
Bovent while developing a guinea pig animal
model of anaphylaxis
• They received the Nobel Prize in 1957
7. Antihistaminics
• Pharmaceutical agents that inhibit the action of
histamine
• blocking its attachment to histamine receptors
(Classic Antihistaminics)
• inhibiting the enzymatic activity of
histidine decarboxylase (atypical antihistaminics)
• The term antihistamine only refers to H1 receptor
antagonists
