PHOTOPERIODIC DISSECTION OF ENDOCRINE EVENTS AT PARTURITION

The time of parturition in rats is determined in part by the photoperiod, and an 8 h advance in the timing of the light-dark cycle before mid-pregnancy delays parturition by about 12 h. This effect of the photoperiod was eliminated by lesions placed in the anterior hypothalamus, whereas pinealectomy had no effect. In a cross-sectional study, rats under control and time-advanced photoperiods were killed at 3 h intervals up to the predicted time of parturition. The decline in plasma progesterone, the rise in prolactin and the appearance of 20αOHSDHase activity in the ovaries were correlated with the time of parturition. All three events were delayed in rats subjected to the 8 h advance in the photoperiod. Plasma levels of LH and FSH, and the prepartum fetal growth spurt, were more closely related to gestational age than the predicted hour of birth. There was a marked circadian rhythm in plasma corticosterone reaching peak levels shortly before the onset of darkness. Parturition occurred when the plasma levels of corticosterone were low. Various pharmacological treatments designed to influence the levels or actions of prolactin, oestrogen and corticosterone in pregnant rats under both control and time-advanced photoperiods were investigated. These studies indicate that a sequence of three key endocrine events leads to parturition in the rat: 1) luteal regression, creating basal levels of progesterone for the last 24 h of pregnancy, 2) a phase during which the uterus is prepared for parturition under the influence of oestrogen and relaxin, and 3) a delivery phase during which the uterus is stimulated to contract, possibly by the production of a time-dependent signal. © 1979.