Histamine Receptors are a class of G protein-coupled receptors with histamine as their endogenous ligand. There are four known histamine receptors: H1 receptor, H2 receptor, H3 receptor, H4 receptor. The H1 receptor is a histamine receptor belonging to the family of Rhodopsin-like G-protein-coupled receptors. This receptor, which is activated by the biogenic amine histamine, is expressed throughout the body, to be specific, in smooth muscles, on vascular endothelial cells, in the heart, and in the central nervous system. H2 receptors are positively coupled to adenylate cyclase via Gs. It is a potent stimulant of cAMP production, which leads to activation of Protein Kinase A. Histamine H3 receptors are expressed in the central nervous system and to a lesser extent the peripheral nervous system, where they act asautoreceptors in presynaptic histaminergic neurons, and also control histamine turnover by feedback inhibition of histamine synthesis and release. The Histamine H4 receptor has been shown to be involved in mediating eosinophil shape change and mast cell chemotaxis.
Histamine is an organic nitrogenous compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter.
Mepyramine maleate, a first generation antihistamine, is an antagonist of histamine H1 receptor, with Kds of 0.8 nM, 5200 nM and >3000 nM for H1, H2, and H3 receptor, respectively, and a pKd of 9.4 for H1 receptor.
Clemizole hydrochloride is an H1 histamine receptor antagonist, is found to substantially inhibit HCV replication. The IC50 of Clemizole for RNA binding by NS4B is 24±1 nM, whereas its EC50 for viral replication is 8 µM.
Ciproxifan maleate(FUB-359 maleate) is a highly potent and selective histamin H3-receptor antagonist with IC50 of 9.2 nM, with low apparent affinity at other receptor subtypes.
Promethazine Hcl(NSC-231688) is the first-generation antihistamine; strong antagonist of the H1 receptor and moderate mACh receptor antagonist, moderate affinity for 5-HT2A, 5-HT2C, D2 and α1-adrenergic receptors.
Cetirizine 2Hcl, a second-generation antihistamine, is a major metabolite of hydroxyzine, and a racemic selective H1 receptor inverse agonist used in the treatment of allergies, hay fever, angioedema, and urticaria.
