Localized Expression of Histamine H₁ Receptors in Syncytiotrophoblast Cells of Human Placenta

Abstract

The previous Northern blot analysis and in situ hybridization studies showed that histamine H₁-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 [³H]mepyramine binding (K_d = 4.0 ± 0.6 nM and B_max = 91.4 ± 4.9 fmol/mg of protein). Stereoisomers of chlorpheniramine inhibited [³H]mepyramine binding; d-chlorpheniramine inhibited more potently than l-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 H₁Rs are specifically expressed in syncytiotrophoblast cells of human placenta organ.