A CRITICAL PERIOD FOR POSITIVE FEEDBACK IN IMMATURE (21-DAY-OLD) RATS

Abstract

The concentrations of LH and prolactin in the serum in response to a single s.c. injection of oestradiol benzoate (OB, 1 μg) were studied in 21-day-old Wistar rats of different sexual status. In intact female rats, the total concentration of oestrogen in the serum reached a maximum level within 30 min of the injection, whereas the level of LH fell and remained low until a surge occurred 54 h later. However, the amount of prolactin in the serum increased within 1 h of injection of OB and thereafter showed diurnal fluctuations with evening peaks. Male rats castrated within 24 h of birth showed increases in the serum level of both LH and prolactin in response to OB, which were similar in timing to those seen in intact female rats. However, intact male rats failed to show an increase in the level of either LH or prolactin after treatment with OB, while neonatally androgenized female rats, although failing to show an increase in the level of LH, exhibited a prolactin response which was similar to that observed in intact female rats. A further study was undertaken in rats which had received an initial injection of OB and a subsequent injection of progesterone 5 h before the expected female-type LH surge. In intact female rats, progesterone augmented the oestrogen-stimulated surge of LH, but this effect was not observed in either intact male or neonatally androgenized female rats. In all three groups, however, the level of prolactin in the serum was increased when compared with control animals which had received OB only. The effect of progesterone, injected at various times after priming with OB, on the output of LH and prolactin was also studied in female rats killed 5 h after the second injection. Up to 21 h after injection of OB, progesterone had no effect on the level of LH or prolactin. If, however, progesterone was then injected during the middle of the light period, but not in the dark period, the concentrations of LH and prolactin increased. Thus there are critical times during a 24 h period when the gonadotrophin output of the oestrogen-primed female rat is responsive to the stimulatory effects of progesterone.