Muscarinic cholinergic receptor subtypes expression by human placenta

The presence of a cholinergic system in the placenta is suggested by several data, but no information is available concerning cholinergic receptor expression by placenta. The present study was designed to investigate muscarinic cholinergic receptors in sections of human placenta using a radioligand binding technique with [³H]N-methyl scopolamine ([³H]NMS) as a ligand. [³H]NMS was bound to sections of human placenta in a manner consistent with the labelling of muscarinic cholinergic receptors. The dissociation constant (K(d)) value was 0.1 ± 0.03 nM and the maximum density of binding site (B(max)) value was 10.82 ± 0.09 fmol/mg of tissue. The binding was time-, temperature- and concentration-dependent, belonging to one class of high affinity sites. Analysis of [³H]NMS displacement curves by compounds acting on the different subtypes of muscarinic cholinergic receptor subtypes suggests that human placenta expresses the four subtypes (M₁-M₄) of muscarinic cholinergic receptor assayable with radioligand binding assay techniques. The demonstration of muscarinic cholinergic recognition sites in human placenta may contribute to define the possible significance of placental cholinergic system. Moreover, human placenta can be used as an easily obtainable human source of M₁-M₄ muscarinic cholinergic receptor subtypes.