The neuropeptide, head activator, in human placenta and serum from pregnant women.

: The hydra neuropeptide, head activator, was detected in human placenta using radioimmunoassay. The placenta contained 11-68 fmol/g wet weight of head activator. The slope of the inhibition curve by placental extract in radioimmunoassay was identical to that of synthetic head activator. The molecular weight of the major immunoreactive head activator in placental extract corresponded to the synthetic head activator after Bio-Gel P-2 column chromatography. The peak of immunoreactive head activator in the extract emerged at the same retention time as that of the synthetic head activator on reversed-phase high-performance liquid chromatography. Head activator was localized histologically in trophoblasts of placenta by peroxidase-antiperoxidase staining techniques. The head activator staining was inhibited by the excessive synthetic head activator. The serum head activator level during pregnancy ranged from 9 to 31 pmol/l. A positive linear correlation was found between serum head activator content and gestational weeks.