(5) Because these drugs act as competitive antagonists at the histamine receptors, they are not useful in anaphylaxis where the H1 receptors
are already well occupied by histamine.
The preclinical data found that lurasidone binds weakly to histamine H1 receptors
, which have been implicated in weight gain and sedation, to the 5HT2C receptor, which has also been implicated in weight gain, and to muscarinic receptors, which might be involved in some of the cognitive effects associated with antipsychotics, said Dr.
Second-generation agents are selective for H1 receptors
and are less sedating than first-generation agents.
We still do not have enough data on loratadine and other newer antihistamines to reach the same conclusion with certainty, however, even though these agents are members of the same class and presumably work on the same histamine H1 receptors
The pharmacologic agents that are available as ophthalmic solutions, used in the treatment of allergic conjunctivitis belong to diverse classes: (1) Antihistamines - which block H1 receptors
, e.g., levocabastine, azelastine, emedastine, bepotastine, and alcaftadine; (2) Mast cell stabilizers - which increase the calcium influx to the mast cell and prevent changes in the membrane permeability resulting in the stability of membrane decreasing degranulation of mast cells,  e.g.
mediate actions on brain activity, and classic antihistamines such as CPA act as H1 antagonists (1).
Release of histamine and its effects on H1 receptors
Aripiprazole exhibits high affinity for dopamine D2 and D3, serotonin 5-HT1A and 5- HT2A receptors (Ki values of 0.34, 0.8, 1.7, and 3.4 nM, respectively), moderate affinity for dopamine D4, serotonin 5-HT2C and 5-HT7, alpha1-adrenergic and histamine H1 receptors
(Ki values of 44, 15, 39, 57, and 61 nM, respectively), and moderate affinity for the serotonin reuptake site (Ki=98 nM).
The ratio of affinity for H1 receptors
to cerebral acetylcholine receptors is more than 10 times greater for hydroxyzine than for diphenhydramine.