Histamine is released during allergic reactions and binds to four types of histamine receptors (H1-H4) throughout the body. H1 receptors are responsible for many allergic symptoms and are targeted by antihistamines. First generation antihistamines like diphenhydramine are sedating but newer second generation ones like loratadine are less sedating. Antihistamines treat allergies, nausea, and insomnia. H2 receptor blockers like ranitidine reduce stomach acid and treat ulcers. Mast cell stabilizers prevent histamine release for asthma prevention.

1. Histamine and Antihistamines

2. A. Histamine Synthesis

3. Type Location Function
Causes:
vasodilation
Bronchoconstriction
bronchial smooth muscle
contraction separation of
endothelial cells
Found on: (responsible for hives),
H1 histamine • Smooth muscle and pain and itching due
receptor • Endothelium to insect stings;
• CNS tissue
the primary receptors
involved in allergic rhinitis
symptoms and motion
sickness;
sleep regulation.

4. H2 histamine Located on parietal Primarily stimulate
receptor cells gastric acid secretion
Decreased
Found on central
neurotransmitter
H3 histamine nervous system and
release: histamine,
to a lesser extent
receptor acetylcholine,
peripheral nervous
norepinephrine,
system tissue
serotonin
Found primarily in
the basophils and in
H4 histamine the bone marrow. It
Plays a role in
is also found on
receptor chemotaxis.
thymus, small
intestine, spleen, and
colon.

5. B. Release of histamine can occur by two
processes:
1. Energy- and Ca2+-dependent
degranulation reaction. The release of
histamine from mast cells is induced by:
A. immunoglobulin E (IgE) fixation to
mast cells (sensitization) and
subsequent exposure to a specific
antigen;
B. complement activation (mediated by
immunoglobulin G or immunoglobulin
M) may also induce degranulation.

7. 2. Energy- and Ca2+-independent release
(displacement).
• Displacement is induced by:
A. drugs such as morphine, tubocurarine,
guanethidine, and amine antibiotics.
B. mast cell damage, which is caused by noxious
agents such as venom or by mechanical
trauma, can release histamine.

8. Mechanism of action
A. Histamine (H1)-receptors
– H1-receptors are found in the brain, heart,
bronchi, gastrointestinal tract, vascular
smooth muscles, and leukocytes.
– H1-receptors are membrane bound and
coupled to G-proteins, specifically Gq/11, and
their activation causes:
– increase in phospholipase A2 and D activity
– increases in diacylglycerol and intracellular Ca2+
– increased cyclic guanosine 5′-monophosphate (cGMP)

9. – Activation of H1-receptors in the brain
increases wakefulness.
– Activation of H1-receptors in vessels causes
vasodilation and an increase in permeability.
– Activation of H1-receptors typically stimulates
nonvascular smooth muscle.

10. B. Histamine (H2)-receptors
1. H2-receptors are membrane bound; they are
found in the brain, heart, vascular smooth
muscles, leukocytes, and parietal cells.
2. The response of H2-receptors is coupled via
Gαs to increased cyclic AMP (cAMP)
production.
3. Activation of H2-receptors:
• increases gastric acid production
• causes vasodilation
• generally relaxes smooth muscles.

11. C. Histamine (H3)-receptors
1. H3-receptors are found in the central nervous
system (CNS) and peripheral nervous system
(PNS) at presynaptic nerve terminals.
2. H3-receptors are membrane bound and
coupled to Gi/o; their activation increases
intracellular Ca2+ and decreases cAMP.

12. 3. Stimulation of H3-receptors
– on nerve cells causes a decrease in
histamine release
– in the CNS, stimulation of H3 modulates the
release of dopamine, acetylcholine,
serotonin, and norepinephrine.
– Activation of H3-receptors on the vagus
nerve decreases acetylcholine (ACh) release.

13. D. Histamine (H4)-receptors
1. H4-receptors are found on hematopoietic cells
and in the spleen, thymus, and colon.
2. Stimulation of H4 receptors increases
chemotaxis of mast cells and leukocytes cells
toward sites of inflammation.
3. H4 receptors are coupled to Gi/Go and thereby
inhibit the production of cAMP and increase
intracellular Ca2+

14. Histamine agonists
• Histamine, betazole, and impromidine.
– Betazole has approximately tenfold greater
activity at H2-receptors than at H1-receptors.
– Impromidine is an investigational agent; its
ratio of H2 to H1 activity is about 10,000.
– Methimepip is an H3-specific agonist.

15. • The uses of histamine agonists are primarily
diagnostic.
• These agents are used:
1. in allergy testing to assess histamine sensitivity
2. in the test of gastric secretory function

16. • The adverse effects of these agents can
be quite severe; they include:
• flushing
• a burning sensation
• hypotension
• tachycardia
• bronchoconstriction.

17. Clinical Uses of Antihistamines
• Allergic rhinitis (common cold)
• Allergic conjunctivitis (pink eye)
• Allergic dermatological conditions
• Urticaria (hives)
• Angioedema (swelling of the skin)
• Puritus (atopic dermatitis, insect bites)
• Anaphylactic reactions (severe allergies)
• Nausea and vomiting (first generation H1-
antihistamines)
• Sedation (first generation H1-antihistamines)

18. Histamine (H1)-receptor antagonists
• Competitive inhibitors.
• Classification:
1. First-generation agents
2. Second-generation agents

19. First-generation agents
• Groups:
1. Alkylamines
2. Ethanolamines
3. Ethylenediamines
4. Piperazines
5. Tricyclics

20. First-generation agents
1.Alkylamines
– Alkylamines include
– Chlorpheniramine
– Brompheniramine
– These agents produce slight sedation.

21. 2. Ethanolamines
– Include
– diphenhydramine
– doxylamine
– clemastine
– dimenhydrinate
(combination of diphenhydramine and 8-
chlorotheophylline)
– Ethanolamines produce marked sedation;
– doxylamine is marketed only as a sleeping
aid.
– Ethanolamines also act as antiemetics.

22. 3. Ethylenediamines
– Include:
– pyrilamine and antazoline.
– Ethylenediamines produce
moderate sedation and can cause
gastrointestinal upset.

23. 4. Piperazines
– include meclizine and cyclizine.
– Piperazines produce marked adverse
gastrointestinal effects and moderate
sedation.
– These agents have
A.antiemetic
B.antivertigo activities.

24. 5. Phenothiazines
– include promethazine.
– Phenothazines produce marked sedation.
– These agents have antiemetic activity.
– Phenothiazines are also weak α-
adrenoceptor antagonists.

25. 6. Methylpiperidines
– include cyproheptadine.
– have antihistamine, anticholinergic, and
antiserotonin activities.

26. 2. Second-generation agents
2. Piperidines
Loratadine [Claritin]
Desloratadine [Clarinex]
– Poor CNS penetration: reduced sedation
– Little or no anticholinergic activity
– Desloratadine:
• is the active metabolite of loratadine
• has about 15-fold greater affinity for the H1
receptor than the parent compound

27. Fexophenadine
– is structurally different than the other
piperidine antihistamines,
– sedative activity is low but dose dependent.

28. 2. Clemastine
• is a second-generation ethanolamine
• longer duration of action than
dimenhydramine
• it has some antiemetic activity.
3. Alkylamines:
A. acrivastine.
• Acrivastine is not associated with cardiac
effects.

29. B. Cetirizine [Zyrtec]
– Cetirizine is not associated with cardiac
abnormalities.
– Cetirizine has poor penetration into the
CNS.
– Cetirizine is less sedating;
– it is ineffective for motion sickness or
antiemesis.

30. Pharmacologic properties
of Histamine (H1)-receptor antagonists
• well absorbed after oral administration.
1st 2nd
generation generation
Onset 30 min 30 min
Duration 3-8 hrs 3-24 hours

31. • H1-receptor antagonists are lipid soluble; most first-
generation agents cross the blood— brain barrier.
• H1-receptor antagonists are metabolized in the liver;
• many induce microsomal enzymes and alter their own
metabolism and that of other drugs.

32. Pharmacologic Actions
• Many H1-receptor antagonists, especially the
ethanolamines, phenothiazines, and
ethylenediamines, have muscarinic—cholinergic
antagonist activity.
• Most of these agents are effective local anesthetics,
probably because of a blockade of sodium channels in
excitable tissues.
• Dimenhydrinate and promethazine are potent local
anesthetics.

33. • H1-receptor antagonists relax histamine-induced
contraction of bronchial smooth muscle and have some
use in allergic bronchospasm.
• These agents block the vasodilator action of histamine.
• H1-receptor antagonists inhibit histamine-induced
increases in capillary permeability.
• These agents block mucus secretion and sensory nerve
stimulation.

34. • H1-receptor antagonists, especially the first-generation
agents, frequently cause CNS depression (marked by
sedation, decreased alertness, and decreased appetite).
• In children and some adults, these agents stimulate the
CNS.

35. Therapeutic Uses
1. Treatment of allergic rhinitis and conjunctivitis.
• Clemastine is approved for the treatment of rhinorrhea.
• Many antihistamines are used to treat the common cold,
based on their anticholinergic properties, but they are
only marginally effective for this use.
• Diphenhydramine also has an antitussive effect not
mediated by H1-receptor antagonism.

36. 2. Treatment of urticaria and atopic dermatitis,
including hives
3. Sedatives. Several (doxylamine, diphenhydramine)
are marketed as over-the-counter (OTC) sleep aids.
4. Prevention of motion sickness
5. Appetite suppressants

37. Adverse effects
• (significantly reduced with second-generation agents)
• Sedation, dizziness, and loss of appetite.
• These agents can cause gastrointestinal upset, nausea,
and constipation or diarrhea.
• H1-receptor antagonists produce anticholinergic effects
(dry mouth, blurred vision, and urine retention).
• Two second-generation H1 antagonists, astemizole and
terfenadine (a prodrug of fexofenadine) were
discontinued or removed from the market because they
were associated with Q-T prolongation and ventricular
tachycardias.

39. Histamine (H2)-receptor antagonists
• Cimetidine [Tagamet]
• Ranitidine [Zantac]
• Famotidine [Pepcid AC]
• Nizatidine [Axid]
Competitive antagonists at the H2-receptor,
which predominates in the gastric parietal cell.

41. • Used in the treatment of:
1. Gastrointestinal disorders, including
heartburn and acid-induced indigestion.
2. These agents promote the healing of gastric
and duodenal ulcers.
3. Used to treat hypersecretory states such as
Zollinger-Ellison syndrome.

42. Pharmacokinetics
• The bioavailability of H2-antagonists goes from
50% for ranitidine and famotidine to
approximately 90% for nizatidine and advised
dosages take this into account.
• They are taken especially in the evening to
reduce night gastric acidity.
• Their elimination is primarily renal.
• Cimetidine inhibits cytochrome P-450 and
increases the concentrations and the effects of
many other drugs.

43. Adverse effects
• H2 antagonists are generally well-tolerated, except for
cimetidine where all of the following adverse drug
reactions (ADRs) are common.
• Infrequent ADRs include hypotension.
• Rare ADRs include: headache, tiredness, dizziness,
confusion, diarrhea, constipation, and rash.
• Additionally, cimetidine may also cause gynecomastia in
males, loss of libido, and impotence, which are
reversible upon discontinuation.

44. The chromones:
• Cromolyn [Intal]
• Nedocromil sodium [Tilade]

45. • These are administered by inhalation.
• They inhibit the release of histamine and other
autocoids from the mast cell.
• Each is used prophylactically in the treatment of
asthma
• they do not reverse bronchospasm.

46. • Adverse effects:
– confined to the site of application
– Include:
–sore throat
–dry mouth.
• Nedocromil sodium
– more effective in reducing bronchospasm
caused by exercise or cold air.