Late Pregnancy and Nocturnal Prolactin Prolactin Release* Rat Choriocarcinoma Cells Inhibit Surges and Serotonin-Induced

The purpose of the present study was to determine the effect of hormonal secretion by trophoblast cells on serotonin-induced release of PRL in both pregnant and nonpregnant rats. In the first experiment, three compounds that effectively lead to stimulation of serotonin receptors were injected ip or intraarterially between 0900-1200 h on day 8 or 16 of pregnancy. These included the serotonin precursor 5-hydroxytryptophan (20 mg/kg BW); a releaser of serotonin, fenfluramine (10 mg/kg BW); and a serotonin S2 receptor agonist, DOI (2,5-dimethoxy-4-iodophenyl-2-aminopropane-HCk 500 pg/kg BW). When injected on day 8, these treatments significantly (P < 0.01) increased the level of plasma PRL within 30 min after the injection. However, on day 16 the same treatments could not induce any change in the plasma PRL level. In the second experiment, rat choriocarcinoma (Rcho) cells, which secrete placental lactogen I in