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Injection of sodium pentobarbitone (Nembutal) into rats between 12.00 and 13.00 h on the day of pro-oestrus was fully effective in blocking the expected ovulation. In 75% of the rats, ovulation of the present generation of large follicles occurred 24 h later (delayed ovulation). Injection of Nembutal between 12.00 and 13.00 h on the day of pro-oestrus and at the same time on the subsequent day was fully effective in blocking the ovulation twice: in only two rats out of 13 did ovulation of the present generation of follicles still occur. When unilateral ovariectomy was performed immediately after injection of a single dose of Nembutal into pro-oestrous rats, delayed ovulation was significantly inhibited. However, after inhibition of ovulation by either two injections of Nembutal or one injection and unilateral ovariectomy, delayed ovulation could be induced by treatment with a small dose of oestradiol benzoate during the Nembutal-induced anaesthesia. It thus seemed that delayed ovulation failed because of disruption of oestrogen production after administration of Nembutal. The concentration of oestradiol-17β in the plasma of Nembutal-treated pro-oestrous rats decreased rapidly during the 24 h after treatment. It is concluded that this decrease in the concentration of oestradiol is due to the inherent ageing of preovulatory follicles manifesting itself when exposure to the ovulatory surge of LH is inhibited.
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Department of Endocrinology, Growth and Reproduction, Erasmus University, Medical Faculty, P.O. Box 1738, Rotterdam, The Netherlands
(Received 23 October 1975)
Normal cyclic ovarian activity stops in old female rats: cyclicity may be replaced by absence of ovulation and persistent vaginal cornification (Everett, 1939; Aschheim, 1964/5). In the present study, factors were examined which could contribute to the change from cyclicity to absence of ovulation. Vaginal cyclicity, ovarian microscopy, and the pro-oestrous surge of luteinizing hormone (LH) were compared between old and young rats. Locally bred (R × U)F 1 hybrid females were kept under standard lighting conditions (lights on at 05.00 h; off at 19.00 h). Three- to 5-month-old rats were all cyclic and showed predominantly 5-day cycles. From 7 months onwards a rapid increase occurred in the number of animals with persistent vaginal cornification; cyclic animals kept 5-day cycles (Table 1).
In the first experiment sodium pentobarbitone (Nembutal) was injected
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Adult male rats which had been castrated at birth and treated with the non-aromatizable androgen dihydrotestosterone propionate (DHTP) showed incomplete copulatory behaviour. When tested with oestrous female rats during treatment with testosterone propionate (TP) they readily mounted these females and showed frequent penile intromissions but rarely ejaculated. In a long series of observations the proportion of ejaculating rats in tests of 30 min did not exceed 50%. Neonatally castrated rats treated with DHTP during infancy thus seemed to be capable of ejaculation in adulthood during treatment with TP, but the threshold for the occurrence of the ejaculatory reflex seemed to be higher than in normal male rats.
By replacing treatment in adulthood with TP by a combined treatment with DHTP and oestradiol benzoate (OB), the frequency of ejaculation was not increased. It was concluded that the incomplete copulatory behaviour was not due to reduced efficiency of aromatization of androgen within the brain of these rats.
The addition of OB to DHTP during the neonatal period of treatment enhanced the frequency of ejaculation in adulthood. The combined treatment of 0·1 mg DHTP on days 1, 3 and 5 with 0·01 mg OB on day 1 made adult copulatory behaviour during treatment with TP indistinguishable from that of rats castrated on day 10 or rats castrated at birth and treated with TP during infancy.
It was concluded that the masculine organization of systems and structures involved in the display of male copulatory behaviour occurs under the influence of both non-aromatizable androgen and oestrogen, oestrogen being most likely the substance required to 'organize' the central nervous aspects of the regulation of this behaviour. The absence neonatally of nonaromatizable androgen and/or oestrogen results in specific deficiencies in adult copulatory behaviour as compared with the behaviour of normal male rats.
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In rats with 4- or 5-day reproductive cycles various responses to the blockade of ovulation with sodium pentobarbitone at pro-oestrus (or at pro-oestrus and on the next day) were compared. The blood concentrations of oestradiol decreased rapidly during the 24 h period after injection of sodium pentobarbitone at pro-oestrus in rats with 5-day cycles. In those with 4-day cycles this response took almost a day to develop. Injection of sodium pentobarbitone at pro-oestrus and on the next day interfered with ovulation of the present crop of large follicles in rats with 5-day cycles. In 4-day cyclic rats this procedure delayed ovulation of these follicles by 48 h. Receptive behaviour was absent on the day after the second injection of sodium pentobarbitone in rats with 5-day cycles; some receptivity was, however, induced by the injection of gonadotrophin. The latter injection resulted in the release of a low number of eggs; fertilization of these eggs, however, did not occur. In 4-day cyclic rats receptive behaviour was recorded on the day after the second injection of sodium pentobarbitone: fertilization of delayed ovulated eggs took place normally but pregnancy was seen only rarely.
The results indicated clear differences in responses to blockade of ovulation with sodium pentobarbitone between rats with 4- or 5-day cycles. The differences most probably result from a more advanced 'age' of preovulatory follicles at pro-oestrus of 5-day cycles compared with those of 4-day cycles. Experimental delay of ovulation reveals ageing changes and the probable onset of atresia at an earlier time after blockade in 5-day cyclic than in 4-day cyclic rats.
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SUMMARY
Female rats which showed 5-day ovarian cycles spontaneously, but 4-day cycles for periods of variable length after induction of pseudopregnancy, were examined. While the time of onset of the ovulatory LH surge at pro-oestrus was the same in both 4- and 5-day cycles, dioestrous progesterone concentrations were dissimilar. The most prominent feature was a sharp increase during the night of the first day of dioestrus of the 4-day cycles. The results are interpreted as suggesting that dioestrous steroid concentrations rather than pro-oestrous LH concentrations are important in establishing cycle duration.
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In rats with 5 day reproductive cycles, 'anovulatory' cycles were induced by blockade of ovulation with sodium pentobarbitone injected at pro-oestrus. Such anovulatory cycles were characterized in the ovaries by gradual atresia of the large follicles present at the time of the injection and the growth of a new cohort destined for the next ovulation. In the vaginal smears, anovulatory cycles were indistinguishable from normal ovulatory cycles.
The serum concentrations of progesterone remained at baseline levels during dioestrus of anovulatory cycles whereas increased concentrations of progesterone were observed during dioestrus of ovulatory cycles. It is concluded that the 'non-functional' corpora lutea of the cycle are the source of dioestrous progesterone.
The length of anovulatory cycles after a single injection of pentobarbitone was 5 days despite the absence of any increase in progesterone concentrations during dioestrus. It is concluded that progesterone production during dioestrus plays no major role in the control of the duration of 5 day reproductive cycles.
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The present study was concerned with the control of luteal activity in female rats which had been treated neonatally with 1·25 mg testosterone propionate (TP). Treatment of such rats in adulthood with 15 i.u. human chorionic gonadotrophin induced ovulation followed by a period of luteal activity. The two daily surges of prolactin secretion, typical for a period of luteal activity in the normal female rat, were not observed in TP-treated females. Instead, higher basal levels of prolactin were observed in TP-treated females than in normal female rats. Furthermore, uterine traumatization at 5 days after ovulation did not result in the formation of decidual tissue.
In intact TP-treated females luteal activity, induced and temporarily sustained by an ectopic pituitary transplant, persisted after removal of the pituitary graft. In contrast, in TP-treated females which had been ovariectomized on day 25 of age and had received an ovarian transplant before induction of the luteal phase, luteal activity ended within a week after removal of the ectopic pituitary gland. Females treated with TP which had been ovariectomized on day 25 of life had lower plasma levels of prolactin and higher levels of dopamine in hypophysial stalk plasma than intact TP-treated females when measured at 4 months of age. Treatment of ovariectomized rats with oestradiol-17β increased levels of prolactin in plasma and lowered levels of dopamine in hypophysial stalk plasma.
It is concluded that the control of luteal activity in TP-treated females shows 'male' characteristics. However, the presence of the ovaries in such rats leads to decreased hypothalamic release of dopamine and increased plasma levels of prolactin, probably due to increased oestrogen levels. These increased levels of prolactin are sufficient to maintain luteal activity.
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Rats with 5-day ovarian cycles were injected daily with 1 mg bromocriptine. This treatment resulted in a change of cycle length from 5 to 4 days and a rapid increase in ovarian weight. The increase in ovarian weight resulted from the accumulation of large numbers of corpora lutea. Normal numbers of corpora lutea were formed during each cycle but luteal bodies did not disappear subsequently. Luteolysis affected only minor foci of luteal tissue and the majority of luteal tissue remained histologically intact throughout the further period of study. The reduction of cycle length from 5 to 4 days occurred when bromocriptine was administered from the day of ovulation only. If treatment was commenced at a later time during the cycle it was not effective.
Treatment with bromocriptine appeared to affect the concentrations of progesterone in the blood during dioestrus. During treatment the rats showed the pattern characteristic for 4-day cycles: typically, the high concentrations of progesterone on the day after metoestrus remained absent. These data suggest (1) that the latter part of the production of progesterone during dioestrus by 'non-functional corpora lutea' is dependent on prolactin and (2) that prolongation of high progesterone production after metoestrus plays an important role in changing the length of the cycle from 4 to 5 days.
Treatment with bromocriptine did not significantly affect the rate of maturation of follicles destined for the next ovulation. It is possible that follicular maturation is not among the critical variables which determine whether normal ovulatory cycles will last for 4 or 5 days.
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The suckling stimulus exerts an inhibitory action on the release of gonadotrophins during lactation. The possible involvement of the adrenal glands in this process was examined by studying the plasma levels of gonadotrophins in lactating rats ovariectomized on the day after parturition. It appeared that the suppression, throughout suckling, of the rise in levels of gonadotrophins in blood after ovariectomy occurred to the same extent in adrenalectomized and in sham-operated animals. It thus seems unlikely that adrenocortical hormones, albeit secreted in larger quantities during lactation, exert an inhibitory effect on the release of gonadotrophins.
Adrenalectomy had a marked effect on the plasma concentrations of prolactin during the second half of lactation. Whereas plasma concentrations of prolactin in the first half of lactation were similar in adrenalectomized and sham-operated rats, the concentrations in adrenalectomized rats did not undergo the reduction found in sham-operated rats. Adrenal hormones may thus be involved in the reduction of blood levels of prolactin observed in rats and in other mammals as lactation progresses.
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Corpora lutea formed after post-partum ovulation in the rat become functionally active under the influence of prolactin released as a result of suckling. During this period of luteal activity (lactational pseudopregnancy) ovulations do not occur. Despite continued suckling plasma prolactin declines gradually during lactation but this gradual decrease was not observed when adrenalectomy was performed on day 2 of lactation. The prolongation of lactational pseudopregnancy after adrenalectomy is probably associated with this observation. Daily treatment of adrenalectomized lactating rats with 5 mg corticosterone acetate, but not with 1·5 mg, reduced the duration of lactational pseudopregnancy to that of controls.
Removal of litters of five pups on day 13 of lactation was followed by resumption of ovulation 3 days later in control animals. However, in adrenalectomized rats the interval between removal of the five-pup litter and the next ovulation was prolonged to 10 or more days because of the persistence of luteal activity. This prolonged interval in adrenalectomized rats was not caused by an acute effect of the absence of adrenal hormones since it was not observed in rats which had been adrenalectomized 3 days before removal of the litter. Furthermore, the increase in the interval between removal of the five-pup litter and resumption of ovulation was also not due to the absence of the main glucocorticoid in the rat, since daily treatment with corticosterone from the day of adrenalectomy failed to prevent the occurrence of the long delay. Adrenal transplants could, however, prevent the effect induced by adrenalectomy. Since the medulla of these transplants had become necrotic, it seems that factors of adrenocortical, but not of adrenal medullary origin, are important in preventing the occurrence of the prolonged interval.
It is concluded that the adrenal glands affect the regulation of prolactin secretion during lactation and, as a consequence, are important in establishing the duration of the anovulatory state during lactation.
