SIGNALLING PATHWAY OF HISTAMINE RECEPTORS
Histamine bind to a different type of histamine G-coupled protein receptors and exerts its action. The four histamine receptors which are identified by researchers are H1, H2, H3 and H4 (Figure 5) , where the first two receptors i.e. H1 and H2 are important targets for clinically effective drugs. In some human tumours, these histamine receptors are recognized by genomic-based methods for example colorectal cancer, melanoma, and breast cancer, vaginal, cervical and ovarian cancer. The H1 receptors are on smooth muscles, endothelial cells, as well as brain and, are also found in peripheral nerve endings and mediate vascular permeability at the site of inflammation. H1 receptor activation causes anaphylactic reactions such as bronchoconstriction, hypotension, flushing and tachycardia . When histamine bind to the H1 receptor in the brain elevates other things like wakefulness and appetite suppression. In the peripheral nerve, histamine mediates stimulation which leads to pain and itching sensations. The H2 receptors are mainly located in gastric parietal cells and cause gastric secretions. In histaminergic neurons, H3 receptors are mostly found which regulate 5-HT3, dopamine, Norepinephrine, Acetylcholine and histamine. Its mRNA is situated in the peak of the spinal cord, bone marrow and peripheral hematopoietic cells play a vital role in the variation of myoblast and promyeloblast along with chemotaxis .