Rat amnion: a novel site of oxytocin production

Fetal membranes, in addition to serving as passive barriers, possess secretory capacities and have a role in amniotic fluid production. In the present study, we demonstrate that rat amniotic/chorionic membranes have the capacity to synthesize oxytocin (OT). Analysis of RNA extracted from rat amnion/chorion membranes by RNA blotting and polymerase chain reaction (PCR) revealed the presence of a single species of OT mRNA. The transcript was smaller than the hypothalamic OT transcript and equaled the size of uterine and placental OT mRNA. The abundance of amniotic OT mRNA decreased by 37% from Day 14 to Day 21 of pregnancy. Over the same time, amniotic/chorionic OT immunoreactivity, as assessed by RIA, increased from 0.17 +/- 0.03 to 1.36 +/- 0.18 ng/g tissue weight. HPLC analysis of amniotic/chorionic membrane extracts revealed two peaks of immunoreactive OT (ir-OT). The first peak (36% of total ir-OT) co-eluted with synthetic OT, while the second peak (64%) corresponded to a noncovalent complex, most likely between OT and neurophysin-I. Using the selective OT receptor ligand, 125I-d(CH2)5 [Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT, we detected high numbers of OT binding sites in rat uterine tissues, but no OT binding sites in amniotic/chorionic or placental membranes. We conclude that the rat amniotic/chorionic membranes represent an extra-hypothalamic site of OT gene expression in the rat during pregnancy and thus represent a possible source of OT present in amniotic fluid.