Histamine Receptor & Classification of antihistamine

• The histamine receptors are a subclass of G protein-coupled receptors that bind histamine as their main endogenous ligand.

• There are four known histamine receptors:

1. H1 receptor

2. H2 receptor

3. H3 receptor

4. H4 receptor

Comparison

		Histamine receptors
Mechanism	Function/Effects	Location
Stimulate phospholipase C	ileum contraction modulate circadian cycle itching systemic vasodilatation bronchoconstriction (allergy-induced asthma) IgE production	Smooth muscle of respiratory and GIT CNS neurons T-cells, B-cells, Neutrophils, eosinophils Heart cell
Stimulate adenyl cyclase and increase cAMP	speed up sinus rhythm Stimulation of gastric acid secretion Smooth muscle relaxation Inhibit antibody synthesis Vasodilation	Gastric parietal cell Vascular smooth muscles CNS Heart uterus
Act on Gi protein alpha subunit	Decrease Acetylcholine, Serotonin and Norepinephrine Neurotransmitter release in CNS Presynaptic auto receptors (inhibition of histamine release)	Mostly found in CNS
Act on Gi protein alpha subunit	mediate mast cell chemotaxis. Modulate immune function	Hematopoietic cells

Classification of antihistamine:

Classification of antihistamine

1.   H1-Antagonist First-Generation- 

• are used as sedative agents, antiallergic agents, to cure motion sickness, and an antiemetic agent. These drugs have strong sedative action and anticholinergic side effects.

  • Diphenhydramine hydrochloride*

  • Dimenhydrinate

  • Doxylamines succinate

  • Clemastine fumarate

  • Diphenylphyraline hydrochloride

  • Tripelenamine hydrochloride

  • Chlorcyclizine hydrochloride

  • Meclizine hydrochloride

  • Buclizine hydrochloride

  • Chlorpheniramine maleate

  • Triprolidine hydrochloride*

  • Phenidamine tartarate

  • Promethazine hydrochloride

  • Trimeprazine tartrate

  • Cyproheptadine hydrochloride

  • Azatidine maleate

2.   H1-Antagonist Second-Generation- 

• are used as antiallergic agents, or adjuvant treatment for anaphylactic shock. These drugs are non-sedative or mildly sedative.

  • Loratadine,

  • Cetirizine

  • levocetirizine

  • Astemizole
    

3.  H1-Antagoinst mast cell stabilizer-

• Cromolyn sodium

4.  H2-Antagonist Gastric acid inhibitor- 

• are used to reduce the production of stomach acid

  • Ranitidine

  • Cimetidine

  • Famotidine

5.   Gastric proton pump Inhibitor-

• Omeprazole

• Lansoprazole

• Rabeprazole

• Pantoprazole

classification of Antihistaminic Agents

Here is the diagram for the classification of Antihistaminic Agents: