Localized expression of histamine H1 receptors in syncytiotrophoblast cells of human placenta

The previous Northern blot analysis and in situ hybridization studies showed that histamine H-1-receptor (H1R) mRNA is expressed in human placenta and suggested that H1R plays some roles in the function of placenta in pregnancy. To investigate further, it is essential to show the precise location of H1R in the placenta. In the present study, we investigated H1R expression in human placenta by radioligand binding assay and immunohistochemical study using an antibody against human H1R. Placentas were obtained from normal uncomplicated deliveries. Membranes prepared from the tissue exhibited saturable [H-3]mepyramine binding (K-d = 4.0 +/- 0.6 nM and B-max = 91.4 +/- 4.9 fmol/mg of protein). Stereoisomers of chlorpheniramine inhibited [H-3]mepyramine binding; d-chlorpheniramine inhibited more potently than 1-chlorpheniramine, K-i values being 1.1 +/- 0.4 and 270 +/- 170 nM, respectively. The placenta tissues were positively immunostained with anti-H1R antibody only in the region of the syncytiotrophoblast of chorionic villus. The tissues were double stained with anti-H1R antibody and an antibody against human chorionic gonadotoropin (hCG) that is solely expressed in placental syncytiotrophoblast cells. The results showed that H1R and hCG were expressed on the same cells, that is, syncytiotrophoblast cells. These results indicate that H(1)Rs are specifically expressed in syncytiotrophoblast cells of human placenta organ.