# Histamine H4 receptor

The histamine H$_4$ receptor, like the other three histamine receptors, is a member of the G protein-coupled receptor superfamily that in humans is encoded by the HRH4 gene.

## Discovery

Unlike the histamine receptors discovered earlier, H$_4$ was found in 2000 through a search of the human genomic DNA data base.

## Tissue distribution

H$_4$ is highly expressed in bone marrow and white blood cells and regulates neutrophil release from bone marrow and subsequent infiltration in the zymosan-induced pleurisy mouse model. It was also found that H$_4$ receptor exhibits a uniform expression pattern in the human oral epithelium.

## Function

The Histamine H$_4$ receptor has been shown to be involved in mediating eosinophil shape change and mast cell chemotaxis. This occurs via the βγ subunit acting at phospholipase C to cause actin polymerization and eventually chemotaxis.
The histamine H$_4$ receptor has been identified as a vital regulator of the immune system, involved in eosinophil migration, mast cell recruitment, dendritic cell activation, and T cell differentiation. The discovery of this receptor has brought it to increasing attention for its therapeutic use in inflammatory diseases such as allergy, asthma, chronic itch, and autoimmune diseases.

## Structure

The 3D structure of the H$_4$ receptor has not been solved yet due to the difficulties of GPCR crystallization. Some attempts have been made to develop structural models of the H$_4$ receptor for different purposes. The first H$_4$ receptor model was built by homology modelling based on the crystal structure of bovine rhodopsin. This model was used for the interpretation of site-directed mutagenesis data, which revealed the crucial importance of Asp94 (3.32) and Glu182 (5.46) residues in ligand binding and receptor activation.
A second rhodopsin based structural model of the H$_4$ receptor was successfully used for the identification of novel H$_4$ ligands.
Recent advancements in GPCR crystallization, in particular the determination of the human histamine H$_1$ receptor in complex with doxepin will likely increase the quality of novel structural H$_4$ receptor models.

## Ligands

Although the effectiveness of H4 receptor ligands has been studied in animal models and human biological samples, further research is needed to understand genetic polymorphisms and interspecies differences in their actions and pharmacological characteristics.

### Agonists

* 4-Methylhistamine
* VUF-8430 (2-[(Aminoiminomethyl)amino]ethyl carbamimidothioic acid ester)
* OUP-16
* Clozapine
* JNJ 28610244

### Antagonists

* Toreforant (JNJ 38518168)
* Mianserin (also a H1 and H3 antagonist)
* Thioperamide (also a selective H3 antagonist)
* JNJ 7777120 (discontinued)
* JNJ 39758979 (discontinued)
* ZPL389
* VUF-6002 (1-[(5-Chloro-1H-benzimidazol-2-yl)carbonyl]-4-methylpiperazine)
* A987306
* A943931
* Pimozide

### Therapeutic potential

The available data support the H4 receptor as a promising new drug target for modulating histamine-mediated immune signaling and offer optimistic prospects for developing new therapies for inflammatory diseases.
H$_4$ receptor antagonists could be used to treat asthma and allergies.
The highly selective histamine H$_4$ antagonist VUF-6002 is orally active and inhibits the activity of both mast cells and eosinophils in vivo, and has anti-inflammatory and antihyperalgesic effects.