Radioactive [14C]5-hydroxytryptamine (5-HT, serotonin) creatinine sulphate was injected intrapleurally into female rats treated with oestrogen and/or progesterone which had received, immediately before the injection of 5-HT, homogenates of placenta, foetus, uterus or plasma, taken from pregnant rats on the 19th day of pregnancy. The placental homogenate produced a significant increase in 5-HT uptake by the myometrium, when the rats had been primed with moderate doses of oestrogen plus progesterone. Higher doses prevented the increased uptake, and oestrogen treatment alone did not induce 5-HT uptake. The highest level of 5-HT accumulation in the uterus was produced by placental extract after pretreatment with 0·5 mg. oestradiol plus 10 mg. progesterone/rat/day.
These results suggest that the placenta contains a 'trans-serotonin' system which is dependent on the oestrogen—progesterone balance and serves to accumulate 5-HT in the placenta and myometrium. Shifts of the hormonal balance may contribute to the release of 5-HT and thus promote uterine contractions at any stage of pregnancy.
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