PREVENTION BY ANTAGONISTS OF THE TOXIC ACTION OF 5-HYDROXYTRYPTAMINE ON PREGNANCY.

As 5-hydroxytryptamine can interrupt pregnancy in all its stages in mice and as pregnancy can be protected from the action of 5-hydroxytryptamine by either progesterone or by prolactin only in early (days 1 to 6) but not in late pregnancy (day 14), the present experiments with two antagonists of 5-hydroxytryptamine, namely methysergide and cyproheptadine, were carried out. Each compound antagonized the action of 5-hydroxytryptamine in late pregnancy. The toxic effects produced by 5-hydroxytryptamine are probably due to a direct action on the placental circulation which is prevented by the antagonists. In early pregnancy, methysergide prevented the effect of 5-hydroxytryptamine, which is believed then to act on the central nervous system, interfering with luteal activity. Cyproheptadine alone interrupted early pregnancy, an action which was prevented by progesterone.