Identification of a gonadotropin-releasing hormone-like factor in the rabbit fetal placenta

The biological and immunological gonadotropin-releasing hormone (GnRH)-like activities in rabbit fetal placentas collected at Day 18 of gestation were investigated. Both crude and partially purified acid extracts of placental tissue were tested. A similarly prepared liver extract served as a control. Immunological GnRH-like activity, determined through a GnRH radioimmunoassay was 1.3-2.0 pg/mg protein for the crude placental extract, 7.1-9.2 pg/mg protein for the partially purified placental extract and was nondetectable for liver extract. Both the crude and partially purified placental extracts increased (P less than 0.01) luteinizing hormone (LH) release by dispersed rabbit pituitary cells, whereas the liver extract had no effect. The (Ac-D-p-Cl-Phe1,2, D-Trp3, D-Arg6, D-Ala10)-GnRH antagonist was used to determine whether the biological GnRH-like activity in the placental extract was mediated through GnRH receptors. All three doses of antagonist (10, 100 and 1000 ng) suppressed the biological GnRH-like activity in the placental extracts. Molecular sieve chromatography of the partially purified placental extract showed that the immunoreactive GnRH-like factor eluted in the same fractions as the GnRH standard. These data indicate that the rabbit fetal placenta has both immunological and biological GnRH-like activity.