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The changes in adenohypophysial and hypothalamic content and in hypothalamic release of dopamine and thyrotrophin-releasing hormone (TRH) into the hypophysial portal system during the suckling-induced release of prolactin were investigated. An increase in peripheral plasma levels of prolactin was induced by mammary nerve stimulation in urethane-anaesthetized and by suckling in unanaesthetized lactating rats. In the unanaesthetized rat, suckling caused a decrease of dopamine levels in hypothalamus and adenohypophysis and a short-lasting small increase in hypothalamic TRH. Mammary nerve stimulation induced a transient decrease in dopamine levels and an increase in TRH levels in hypophysial stalk blood. To assess the significance of the observed changes in dopamine and TRH levels for prolactin release, these changes in dopamine and TRH were mimicked in lactating rats anaesthetized with urethane and pretreated with α-methyl-p-tyrosine (AMpT, a competitive inhibitor of catecholamine synthesis). Reducing hypothalamic dopamine secretion by treatment with AMpT increased peripheral plasma levels of prolactin from 15 to 477 ng/ml; an infusion with dopamine, resulting in plasma levels similar to those measured in hypophysial stalk plasma, reduced plasma levels of prolactin to 127 ng/ml. Neither a 50% reduction in dopamine infusion rate for 15 min nor administration of 100 ng TRH caused an appreciable change in plasma prolactin levels. However, when dopamine infusion was reduced by 50% for 15 min just before TRH was injected, then an increase in plasma levels of prolactin from 172 to 492 ng/ml was observed. Thus, the effectiveness of TRH in releasing prolactin in the lactating rat was enhanced when a transient decrease of dopamine levels occurred before treatment with TRH. It is concluded that the changes observed in dopamine and TRH levels in hypophysial stalk blood are involved in the suckling-induced prolactin release in an important manner.
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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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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.
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ABSTRACT
The changes in hypothalamic release of dopamine and thyrotrophin-releasing hormone (TRH) into the hypophysial portal vascular system during an oestrogen-stimulated surge of prolactin in ovariectomized rats were investigated.
A single injection of 5 μg oestradiol benzoate resulted in a reliable increase in the plasma levels of prolactin during the afternoon 3 days later. Anaesthesia did not block this afternoon surge of prolactin, although its magnitude was only half of that of unanaesthetized rats. Before and during this surge, hypophysial stalk blood was collected into methanol to analyse the hypothalamic release of dopamine and TRH. Immunoreactive TRH in these methanolic extracts eluted as a single peak with the same retention time as authentic TRH on reverse-phase high performance liquid chromatography. In comparison to the morning values, levels of dopamine decreased and those of TRH increased in hypophysial stalk blood by 50 and 240% respectively. These data indicate that hypothalamic dopamine and TRH may be involved in the afternoon surge of prolactin.
Daily treatment with parachlorophenylalanine, an inhibitor of serotonin synthesis, reduced the hypothalamic release of TRH by 50%, but did not prevent the afternoon surge of prolactin and TRH induced by oestradiol benzoate.
J. Endocr. (1985) 105, 107–112
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SUMMARY
Serum LH, FSH, prolactin and progesterone concentrations and follicular size were measured in rats during pseudopregnancy after unilateral ovariectomy (ULO) on day 1. This operation did not affect the duration of pseudopregnancy. Following pseudopregnancy compensatory ovulation occurred.
After ULO progesterone concentrations remained low as compared with control values but the progesterone secretion per ovary was increased slightly. After ULO, LH concentrations were significantly increased on days 3 and 4 of pseudopregnancy. FSH concentrations were increased 5 h after the operation and again on days 4–7 of pseudopregnancy when compared with sham-operated control animals. During pseudopregnancy prolactin secretion increased each night. Increased prolactin levels were observed 18 and 36 h after ULO. The number of medium-sized follicles had increased 24 h after ULO.
From the present study it is concluded that progesterone secretion by the corpora lutea can be increased to a limited degree by the increased release of gonadotrophins. Furthermore, it is concluded that a rapid increase in FSH concentrations after ULO is responsible for the initiation of the compensatory follicular growth.
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SUMMARY
Pseudopregnant rats were treated early in pseudopregnancy with 1 or 10 mg medroxyprogesterone acetate (MPA). Serum FSH, LH and progesterone concentrations were determined on days 2–20 of pseudopregnancy in treated and control rats.
The mean duration of pseudopregnancy was 13·5 days in the control animals, but when animals were treated with 1 mg MPA a dioestrous period of 21·4 days was observed. A period with leucocytic vaginal smears of at least 2 months was observed after treatment with 10 mg MPA.
Injection with MPA on day 3 of pseudopregnancy did not affect the serum FSH concentrations during the subsequent days. The progesterone pattern was alike in the three groups of animals, i.e. the duration of the activity of the corpora lutea was similar in all groups. However, 10 mg MPA slightly lowered progesterone concentrations on days 4–8 of pseudopregnancy. In the saline-treated rats, LH concentrations decreased from days 2–5, and remained low until they increased after day 11 of pseudopregnancy. This increase was delayed until day 20 in the animals treated with 1 mg MPA, and was not observed in the animals treated with 10 mg MPA.
It is argued that the increase of LH concentration at the end of pseudopregnancy is not instrumental in the decrease of peripheral progesterone concentration but rather that the decrease in the progesterone concentration leads to the increase in the LH concentration.
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The present study was concerned with a possible involvement of LH in the process of functional luteolysis in the pseudopregnant rat.
Daily injections with 2 μg ovine LH during pseudopregnancy reduced peripheral and ovarian levels of progesterone in intact and hysterectomized rats and in hypophysectomized rats with a pituitary transplant under the kidney capsule. However, a daily dose of 10 μg LH did not alter the levels of progesterone. A short-lasting decrease in plasma progesterone occurred when endogenous levels of LH were temporarily raised in pseudopregnant rats by a single injection of LH releasing hormone (LH-RH). Treatment with LH or LH-RH, however, did not shorten the duration of pseudopregnancy.
Daily treatment of pseudopregnant rats with 5 or 20 ng oestradiol benzoate, but not with 1000 ng, decreased plasma levels of progesterone. On the other hand, daily treatment with oestradiol benzoate did not affect plasma progesterone in pseudopregnant rats which were hypophysectomized and had an ectopic pituitary gland. Plasma levels of LH were not increased in the animals receiving 5 or 20 ng oestradiol benzoate daily, suggesting that the effect of oestradiol benzoate on plasma progesterone is not through an enhanced secretion of LH. Treatment with oestradiol benzoate did not affect the duration of pseudopregnancy.
In conclusion, low doses of LH can reduce peripheral levels of progesterone during pseudopregnancy, but it seems improbable that LH is involved in the process of functional luteolysis. Furthermore, low doses of oestradiol benzoate can also decrease plasma progesterone, but the mechanisms involved are still not understood.
