Effects of restraint stress on luteal function in rats during mid-pregnancy

This study was undertaken to investigate the relationship between the inhibitory effect of restraint stress and the protective effect of the feto-placental luteotrophic factors on luteal function during mid-pregnancy in rats. The number of conceptuses was adjusted to one (1C group) or more than ten (FC group) on day 7 of pregnancy, and each rat received restraint stress from day 12 to day 17 of pregnancy. Restraint stress consisted of placing a rat individually in a small plastic holder three times a day for 1 h each time. Restraint stress significantly decreased serum progesterone concentration on day 17 of pregnancy in the 1C group, but not in the FC group. Restraint stress also decreased serum progesterone concentration on day 17 of pregnancy in the 1C group which received bilateral adrenalectomy on day 12 of pregnancy. The number of animals with fetal resorption in this group of rats (10 out of 14 animals) was significantly greater than in any other group of rats. The number of animals with fetal resorption in the adrenalectomized 1C group was significantly lower after daily injections of 4 mg progesterone from day 12 to day 17 of pregnancy. In the FC group of rats, even in adrenalectomized rats, restraint stress did not cause any changes in serum progesterone concentration or fetal loss. These data indicate that restraint stress is luteolytic and causes fetal loss during mid-pregnancy; this effect can be blocked by some factors from conceptuses, as occurred in the FC group.