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  • Author: I. Rothchild x
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The luteolytic effect of prolactin in the rat seems to be exerted on corpora lutea (CL) which have lost their capacity to secrete progesterone (Desclin, 1949; Malven, Cousar & Row, 1969). Since pregnant rats show signs of progesterone secretion for about 3 days after hypophysectomy and hysterectomy on day 12 (Rothchild, 1972; Rothchild, Billiar, Kline & Pepe, 1973), it seemed worth while to compare their response to prolactin treatment with that of pseudopregnant rats, in which the same operations cause a rapid cessation of progesterone secretion (Rothchild, 1972; Rothchild et al. 1973).

Regularly cyclic 250 g Holtzman (Sprague—Dawley) rats were made pseudopregnant by mechanical stimulation of the cervix; deciduoma formation was induced in some of these by uterine scratching on day 5 (day 1 = last day of cornified vaginal smears). Pregnant rats had been mated in our laboratory (day 1 = day of insemination). Each rat was hypophysectomized and hysterectomized on

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The mechanism of action of the uterus on the corpus luteum of the rat is still incompletely understood. Silbiger & Rothchild (1963) suggested that hysterectomy reduced the luteolytic effect of luteinizing hormone (LH) (Rothchild, 1965a) by reducing its secretion, but Christian, Yuan & Rothchild (1968) failed to demonstrate any difference in parameters of LH secretion between intact and hysterectomized pseudopregnant rats. It seemed worth-while, therefore, to test the possibility that the luteolytic action of LH depended on the presence of the uterus.

Regularly cyclic adult Sprague-Dawley rats (200–250 g) were hysterectomized or sham-hysterectomized; a few weeks later, each rat was hypophysectomized and its pituitary transplanted beneath its left kidney capsule. (The corpora lutea, in this preparation, remain functional for several months (Everett, 1956; Rothchild, 1965b).)

Fourteen days later, both the hysterectomized and the sham-hysterectomized rats were divided into two groups: one was injected with 0·9% NaCl solution, the

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Factors affecting luteinizing hormone (LH) secretion in response to stimulation of the preoptic area (POA) of the forebrain in rats were explored by determining serum LH levels after electrochemical stimulation of the POA. In rats made anovulatory by exposure to constant light (CLA rats), peak concentrations of LH in serum were found 2 h after stimulation with 5–15 mC, and 1 h after stimulation with 0·5–1 mC. The peak levels increased with increasing doses between 0·5 and 15 mC. The incidence of rats ovulating and the mean number of ovulations/rat were roughly proportional to the stimulating dose, but a plateau was reached between 5 and 10 mC. A threshold level of serum LH seemed to be necessary for ovulation, and the incidence of ovulations of six ova or more/rat increased with the increase in peak serum LH level.

Preoptic-roof section, which cuts dorsal afferents to the POA, enhanced the increase in serum LH in response to POA stimulation in CLA rats, while sodium pentobarbitone anaesthesia decreased the response. In both cases, the incidence of ovulation and the number of ovulations/rat were not different from values found in POA-stimulated control CLA rats showing the same peak serum LH level.

In normal cyclic rats the response of serum LH to stimulation was much greater on the morning of pro-oestrus than on that of oestrus; at prooestrus a second rise occurred between 17.00 and 19.00 h. Three days after ovariectomy the basal level of LH increased; these ovariectomized rats showed a small increase in response to a dose of 5 mC. Treatment with 20 μg oestradiol benzoate at the time of ovariectomy, however, resulted in a lowered basal LH level, but the peak response to 5 mC was almost as great as that found in similarly stimulated intact CLA rats. In intact males and in neonatally androgen-treated females the peak levels of serum LH in response to doses of 5 or 15 mC were equivalent to those in CLA females in response to doses of only 1–5 mC.

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On day 9 of dioestrus the uteri of pseudopregnant rats bearing deciduomata were either slit lengthwise to curette out the decidual tissue or were removed in toto to determine if the prolonging effect of decidualization on pseudopregnancy could be eliminated. Both operations significantly reduced the length of the pseudopregnancy dioestrus (combined mean of 19·3 ± 0·56 days v. 22·2 ± 0·77 days for rats with intact deciduomata; P < 0·01). Conversely, the control experiment of slitting the non-decidualized uterus on day 9 significantly prolonged pseudopregnancy in comparison with control groups (19·2 ± 0·92 days v. 13·5 ± 0·37 days for pseudopregnant controls which were not subjected to operation, or had a laparotomy or uterine traumatization performed on day 9; P < 0·001). The effect of slitting the uterus on other days of pseudopregnancy was investigated. The mechanism through which slitting prolongs pseudopregnancy is unknown.

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The activities of 20α-hydroxysteroid dehydrogenase and 3β-hydroxysteroid dehydrogenase in rat corpora lutea during the second half of pregnancy were measured. In luteal tissue of the intact pregnant rat, 20α-hydroxysteroid dehydrogenase activity was undetectable between days 12 and 18 of pregnancy but appeared slowly after hypophysectomy and hysterectomy on day 12. Treatment of the hypophysectomized and hysterectomized animal with oestradiol delayed this increase until day 17, at which time a rapid induction of this enzyme occurred.

In the normal pregnant rat mean luteal 3β-hydroxysteriod dehydrogenase activity increased between days 12 and 18 (P <0·05, Student's t-test) but fell rapidly after hypophysectomy and hysterectomy on day 12. Oestradiol treatment prevented this fall in activity and enzyme activity was not distinguishable from that of the intact rat.

Progesterone secretion correlated well with the activities of these two enzymes in the three conditions examined.

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To test the hypothesis of Raj & Moudgal (1970) that luteinizing hormone (LH) is the essential luteotrophin during pregnancy in the rat, pregnant rats were hypophysectomized and hysterectomized on either day 12 or day 15 of pregnancy, and the changes in peripheral serum progesterone level measured. The serum progesterone level remained at approximately the day-12 value for 3 days after hypophysectomy and hysterectomy on day 12, but fell drastically and remained low after the same operation on day 15, or in pseudopregnant rats operated on on day 12, or after removal of the ovaries from pregnant rats on day 12. Oestrogen treatment increased the serum progesterone level slightly in the pregnant rats after hypophysectomy and hysterectomy, but not after ovariectomy; it had no effect in the pseudopregnant rats, with or without deciduomata, or in lactating rats nursing litters of seven to nine pups.

The corpora lutea stopped growing or slowly regressed soon after hypophysectomy—hysterectomy in all except the pregnant rats operated on on day 12 and treated with oestrogen, and in these growth was very slight. The luteal content of progesterone did not change for 3 days after hypophysectomy—hysterectomy on day 12 of pregnancy, and fell slightly thereafter. The metabolic clearance rate of progesterone was not significantly changed by hypophysectomy—hysterectomy.

It thus appears that true secretion of progesterone continues in pregnant rats for about 3 days after day 12 in the absence of the pituitary and placentas, but at a much lower rate than that found in intact, or in day-12 hypophysectomized pregnant rats (Pepe & Rothchild, 1972a). The placental luteotrophin thus seems to increase the rate of progesterone secretion in the absence of LH. The results do not seem to fit with the hypothesis that LH is essential for progesterone secretion.

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J. E. Sánchez-Criado, K. Ochiai and I. Rothchild


Adult female rats were hypophysectomized and their pituitary glands autotransplanted beneath the left kidney capsule on day 2 (day 1 was the day of ovulation). In such rats the pituitary secretes prolactin fairly constantly and the corpora lutea secrete progesterone for several months. To induce the luteolytic effect of prolactin the rats were first injected s.c. with 2-bromo-α-ergocryptine (CB-154) on cycle days 12, 13 and 14 (i.e. 10, 11 and 12 days after operation) to depress prolactin secretion, and then with CB-154 vehicle (70% ethanol) daily until cycle day 21, to allow prolactin secretion to resume. One ovary was removed from each rat on day 15 and the remaining one on day 22. The mean (± s.e.m.) weight of the corpora lutea on day 15 was 1·46±0·06 mg and 0·98±0·07 mg on day 22 (n = 17). In contrast, rats in which the CB-154 treatment was maintained to day 21 had corpora lutea which weighed 1·31 ±0·09 on day 15 and 1·47 ±0·08 mg on day 22 (n = 15). To investigate whether indomethacin, a prostaglandin synthesis inhibitor, affected the luteolytic action of prolactin, the experiment was repeated, but on day 15 (after the removal of one ovary) the groups in which CB-154 treatment was stopped, as well as the group in which CB-154 treatment was maintained, were each divided into two groups. In one, indomethacin-containing silicone elastomer wafers and, in the other, blank silicone elastomer wafers, were placed within the bursa of the remaining ovary. There were no differences in corpus luteum weight on day 15 among any of these groups and the two groups of the first experiment.

There was no significant difference in corpus luteum weight between day 15 and day 22 in any of the six groups except for the two groups treated with the CB-154 vehicle and not with indomethacin. Thus, treatment with indomethacin prevented the fall in corpus luteum weight associated with the discontinuation of CB-154 treatment.

Serum prolactin levels fell until day 15 in all rats and remained low in those in which the CB-154 treatment was maintained to day 21, but returned to control values in those treated with vehicle after day 14. Serum progesterone levels fell and remained low in all groups. Indomethacin treatment had no effect on the levels of either serum prolactin or progesterone. We conclude that some of the pharmacological effects of indomethacin are to prevent prolactin-induced luteolysis, and we suggest that prolactin induces rapid regression of the corpus luteum by stimulating intraluteal prostaglandin production or by being necessary for the effect of luteolytic prostaglandins.

J. Endocr. (1987) 112, 317–322