Histamine is a biogenic amine found in many tissues that is involved in allergic and inflammatory processes as well as gastric acid secretion and neurotransmission. It is synthesized and stored in mast cells and basophils and released during allergic reactions. Histamine exerts its effects through four receptor subtypes (H1-H4), with H1 and H2 receptors having drugs that target them clinically. H1 receptor antagonists are used to treat allergic rhinitis, chronic urticaria, and motion sickness, while H2 receptor antagonists suppress gastric acid secretion. New drugs targeting H3 and H4 receptors may provide treatments for neurological and inflammatory conditions, respectively.

1. histamine
Moderator: Dr. Pooja Shukla
Resident: Fariha Fatima JR-2

2. Overview:
• Introduction
• Synthesis, Storage, and Release
• Clinical manifestations of histamine pathophysiology
• Histamine Receptors
• Drugs acting on histamine receptors

3. Introduction:
Histamine is a biogenic amine found in many tissues, including
mast cells, basophils, lymphocytes, neurons, and gastric
enterochromaffin-like cells.
 It is an autacoid—that is, a molecule secreted locally to
increase or decrease the activity of nearby cells.

4. Histamine is a major mediator of allergic and inflammatory
processes.
 It also has significant roles in the regulation of gastric acid
secretion, neurotransmission, and immune modulation.

5. Histamine Synthesis, Storage, and
Release:

6. Histamine synthesis and storage can be divided into two “pools”:
a slowly turning over pool
a rapidly turning over pool
The slowly turning over pool is located in mast cells and basophils.
Histamine is stored in large granules in these inflammatory cells, and the
release of histamine involves complete degranulation of the cells.

7. The rapidly turning over pool is located in gastric ECL cells
and in histaminergic CNS neurons.
These cells synthesize and release histamine as required for
gastric acid secretion and neurotransmission, respectively.

8. Clinical manifestations of
histamine pathophysiology:

13. Receptors:

14. Actions of Histamine

15. Tissue and Organ System
Effects of Histamine

16. The “triple response”
Intradermal injection of histamine causes a characteristic red spot,
edema, and flare response that was first described many years ago.
The effect involves three separate cell types: smooth muscle in the
microcirculation, capillary or venular endothelium, and sensory nerve
endings.

17. At the site of injection, a reddening appears owing to dilation of small
vessels, followed soon by an edematous wheal at the injection site and a
red irregular flare surrounding the wheal.
The flare is said to be caused by an axon reflex.
A sensation of itching may accompany these effects.

18. HISTAMINE ANTAGONISTS
The effects of histamine released in the body can be reduced in several
ways.
Physiologic antagonists , especially epinephrine, have smooth muscle
actions opposite to those of histamine, but they act at different receptors.
This is important clinically because injection of epinephrine can be
lifesaving in systemic anaphylaxis.

20. H-1 Antihistamines:
The basic structure of 1st generation antihistamines
consists of 2 aromatic rings linked to a substituted
ethylamine backbone.
These drugs are divided into 6 subgroups based on
their substituted side chains:
Ethanolamines,Ethylenediamines,
Alkylamines, Piperazines,
Phenothiazines & Piperidines.

21. The H 1 antagonists are conveniently divided into :
first-generation second-
generation
 Neutral at physiological pH
 Cross bbb where they block
the actions of histaminergic
neurons in the CNS.
 Ionised at
physiological pH
 Do not cross bbb

23. Pharmacokinetics:
These agents are rapidly absorbed after oral
administration, with peak blood concentrations occurring in
1–2 hours.
They are widely distributed throughout the body, and the
first-generation drugs enter the central nervous system
readily.
 Some of them are extensively metabolized, primarily by
microsomal systems in the liver.

24. Most of the drugs have an effective duration of action of 4–6
hours following a single dose,
 meclizine and several second-generation agents are longer-
acting, with a duration of action of 12–24 hours.
The newer agents are considerably less lipid soluble than the
first-generation drugs and are substrates of the P-glycoprotein
transporter in the blood-brain barrier; as a result they enter the
central nervous system with difficulty or not at all.

25. Pharmacodynamics:

27. The first-generation H 1 -receptor antagonists have
many actions in addition to blockade of the actions of
histamine.
Some of these actions are of therapeutic value and
some are undesirable.

28. • Sedation
• Anti nausea and antiemetic actions
• Anti parkinsonism effects
• Anti cholinoceptor actions
• Adrenoceptor-blocking actions
• Serotonin-blocking action
• Local anesthesia

29. Other actions
Certain H 1 antagonists, eg, cetirizine, inhibit mast cell release
of histamine and some other mediators of inflammation.
This action is not due to H 1 -receptor blockade and may reflect
an H 4 -receptor effect
A few H 1 antagonists (eg, terfenadine, acrivastine) have been
shown to inhibit the P-glycoprotein transporter found in cancer
cells, the epithelium of the gut, and the capillaries of the brain.

30. Clinical Uses:
First-generation H 1 -receptor blockers are among the
most extensively promoted and used over-the-counter
drugs.
The prevalence of allergic conditions and the relative
safety of the drugs contribute to this heavy use.
The fact that they do cause sedation contributes to heavy
prescribing and over-the-counter use of second-generation
antihistamines.

31. 1.Allergic Reactions
The H 1 antihistaminic agents are often the first drugs used to
prevent or treat the symptoms of allergic reactions.
In allergic rhinitis (hay fever), the H 1 antagonists are second-
line drugs after glucocorticoids administered by nasal spray.
In urticaria, in which histamine is the primary mediator, the H 1
antagonists are the drugs of choice and are often quite effective
if given before exposure.

32. For atopic dermatitis, antihistaminic drugs such as
diphenhydramine are used mostly for their sedative
side effect, which reduces awareness of itching.
The second-generation H 1 antagonists are used
mainly for the treatment of allergic rhinitis and chronic
urticaria.

33. 2.Motion Sickness and
Vestibular Disturbances
Scopolamine and certain first-generation H 1
antagonists are the most effective agents available for
the prevention of motion sickness.
The antihistaminic drugs with the greatest
effectiveness in this application are diphenhydramine
and promethazine.

34. The piperazines (cyclizine and meclizine) also have
significant activity in preventing motion sickness and
are less sedating than diphenhydramine in most
patients.

35. 3.Nausea and Vomiting of
Pregnancy
Doxylamine, an ethanolamine H 1 antagonist, was
promoted for this application as a component of
Bendectin, a prescription medication that also
contained pyridoxine.

36. Toxicity:
Less common toxic effects of systemic use include
excitation and convulsions in children, postural
hypotension, and allergic responses.
Overdosage of astemizole or terfenadine may induce
cardiac arrhythmias; the same effect may be caused at
normal dosage by interaction with enzyme inhibitors.

37. Drug Interactions:
Lethal ventricular arrhythmias occurred in several patients
taking either of the early second-generation agents,
terfenadine or astemizole, in combination with ketoconazole,
itraconazole, or macrolide antibiotics such as erythromycin.
These antimicrobial drugs inhibit the metabolism of many
drugs by CYP3A4 and cause significant increases in blood
concentrations of the antihistamines.

38. H 2 -RECEPTOR ANTAGONISTS:
H 2 receptor antagonists (also called H 2 blockers ) reversibly
and competitively inhibit the binding of histamine to H 2
receptors, resulting in suppression of gastric acid secretion.
H 2 receptor antagonists also indirectly decrease gastrin and
acetylcholine-induced gastric acid secretion.

40. All four drugs are well tolerated in general.
Occasional minor adverse effects include diarrhea,
headache, muscle pain, constipation, and fatigue.
H 2 receptor antagonists may induce confusion and
hallucinations in some patients.

41. H 3 & H 4 RECEPTOR
ANTAGONISTS:
Till date no drug directed against H3 &H4 have been approved
for clinical use.
H3 receptors are thought to provide feedback inhibition of
certain effects of histamine in CNS and in ECL use.
In animal studies, H3 receptor antagonists include wakefulness
and improves attention.
Drugs used in animals are:
Thioperamide, Clobenpropit,Ciproxifan, Proxyfan.

42. H 4 receptor antagonists represent a promising area
of drug development to treat inflammatory conditions
that involve mast cells and eosinophils.

43. Conclusion:
The more recent elucidation of the H 3 and H 4
receptor subtypes has renewed interest in the role of
histamine in CNS-related disorders.
H 3 -specific receptor targeting may provide new
therapies for a number of cognitive, neuroendocrine,
and neuropsychiatric conditions.

44. The H 4 receptor is also an exciting molecular target for drug
development, as it is thought to play an important role in
inflammatory conditions involving mast cells and eosinophils.
Agents directed against H 4 receptors might one day be
employed to treat a variety of inflammatory conditions, such
as asthma, allergic rhinitis, inflammatory bowel disease and
rheumatoid arthritis.