W. J. De Greef

Evidence for the involvement of corticotropin-releasing factor in the inhibition of gonadotropin release induced by hyperprolactinemia.

The hypothesis was tested that corticotropin-releasing factor (CRF) is involved in the inhibition of gonadotropin secretion during chronic hyperprolactinemia. Two models of hyperprolactinemia were used, namely inoculation with the prolactin (PRL)-secreting tumor 7315b and implantation of isogenic pituitary glands. Gonadectomized, adrenalectomized male rats received a testosterone capsule and a corticosterone pellet and were inoculated subcutaneously with tumor 7315b. Similar rats without tumor served as controls. The rats were studied 3-4 weeks later while anesthetized with urethane. Plasma testosterone and corticosterone were similar in the two groups of rats. Compared to controls, the tumor-bearing rats had significantly higher plasma levels of PRL (100-fold increase) and adrenocorticotropin (ACTH; 3-fold increase), whereas plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) had significantly decreased to 15 and 40%, respectively. CRF release into hypophysial stalk plasma was higher in rats with tumor 7315b than in controls (298 +/- 23 vs. 197 +/- 28 pg/h), and hypothalamic CRF content had increased from 3.0 +/- 0.3 to 4.3 +/- 0.3 ng. Male rats received 3 pituitary glands under the kidney capsule. Sham-operated rats served as controls. They were studied 5-7 weeks later while anesthetized with urethane. Compared to controls, pituitary-grafted rats had larger adrenals (49 +/- 4 vs. 34 +/- 2 mg), higher plasma PRL (156 +/- 18 vs. 52 +/- 8 ng/ml), ACTH (0.46 +/- 0.05 vs. 0.22 +/- 0.02 ng/ml) and corticosterone (455 +/- 39 vs. 268 +/- 14 ng/ml), and lower plasma levels of LH (21 +/- 2 vs. 41 +/- 6 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Nocturnal Prolactin Surges in the Pregnant Rat by Incubation Medium Containing Placental Lactogen

Abstract Rats hysterectomized on Day 7 or 8 of pregnancy continued to have nocturnal prolactin surges 1 day later. Conditioned medium obtained from incubation of Day 11 placentas infused via the jugular vein completely blocked this nocturnal surge, indicating a negative feedback of placental secretions on prolactin. Infusion of an ultrafiltrate of the conditioned medium which only contained molecules with M r above 10,000 also blocked the prolactin surge. Next, it was determined whether this feedback of placental secretions on prolactin may work by way of hypothalamic dopamine. Levels of dopamine in hypophysial stalk blood from pregnant rats on Day 12, a time when secretion of placental lactogen is high, were not different from those in rats in which placental lactogen was absent. It is concluded that termination of prolactin surges at midpregnancy may be due to feedback of placental secretions, possibly placental lactogen, on the hypothalamus and/or pituitary. However, these experiments do not support the hypothesis that this inhibition is mediated by alteration in hypothalamic dopamine secretion.

Effect of Suckling on the in vivo Release of Thyrotropin-Releasing Hormone, Dopamine and Adrenaline in the Lactating Rat

The present study was concerned with the effect of suckling on the hypothalamic release of thyrotropin-releasing hormone (TRH), dopamine and adrenaline in lactating rats as estimated by push-pull perfusion of the median eminence-arcuate nucleus area. The push-pull cannula was implanted on day 15 of pregnancy. This surgery did not interfere with pregnancy, time of delivery or lactation. Push-pull perfusion was performed on day 8 or 14 of lactation and 30 out of 42 perfusions were successful. On the day of perfusion mothers and young were separated. Six hours later push-pull perfusion was begun and 6 samples at 15-min intervals were collected. In control animals, not allowed to nurse pups during perfusion, the release of TRH, dopamine and adrenaline did not change during the 90-min period. In experimental animals, reunited with their litter after 30 min of perfusion, the hypothalamic release of adrenaline did not change. However, both on day 8 and 14 suckling induced a 50% decrease in the release of dopamine (p less than or equal to 0.025) which lasted for 15-30 min. Suckling on day 14 did not affect the concentration of TRH in the perfusate, but on day 8 the TRH output gradually decreased for 45 min after the onset of suckling.

L-type lipase activity in ovaries of superovulated rats Relation to cholesterol homeostasis

The impact of lowering the ovarian L(iver)-type lipase activity on cholesterol homeostasis in the ovaries was studied in superovulated rats. L-type lipase activity increased rapidly after injection with chorionic gonadotrophin (HCG) (day 0), its activity remained high between days 3 and 8. During this period plasma progesterone and 20 alpha-hydroxyprogesterone were raised. The ovarian content of unesterified cholesterol remained constant during this period while cholesterol esters increased. Lowering of the L-type lipase activity by in vivo treatment with anti-liver lipase (ALLA) during 4-5 h did not affect plasma hormones or ovarian cholesterol contents. However, de novo cholesterol synthesis in the ovaries was significantly increased by about 40%. After pretreatment of the rats with aminogluthetimide, ALLA administration led to a 250% increase in de novo cholesterol synthesis in the unesterified cholesterol fraction, but was without effect on plasma hormones and on the ovarian cholesterol content. Administration of the cholesterol synthesis inhibitor Simvastatin led to a 25% lowering in ovarian cholesterol synthesis without effect on plasma hormones or ovarian cholesterol content. Additional administration of ALLA affected only the plasma progesterone (-30%). These results indicate that L-type lipase is involved in ovarian cholesterol homeostasis.

Plasma Levels of Luteinizing Hormone during Hyperprolactinemia: Response to Central Administration of Antagonists of Corticotropin-Releasing Factor

Since high concentrations of prolactin (PRL) enhance the hypothalamic release of corticotropin-releasing factor (CRF), and CRF decreases the hypothalamic secretion of luteinizing hormone (LH)-releasing hormone (LHRH), it could be that CRF is involved in the suppressed secretion of LH during hyperprolactinemia. The aim of this study was to explore this possibility in hyperprolactinemic male rats. Hyperprolactinemia, induced by insertion of 3 pituitary glands under the kidney capsule, decreased plasma LH levels by 68% and caused a 2-fold increase in plasma corticosterone. Intracisternal administration of the CRF antagonist alpha-helical CRF(9-41) induced both in pituitary-grafted rats and in normoprolactinemic controls a 2 to 3-fold increase of LH in the plasma sample taken 1 h after injection of alpha-helical CRF(9-41). Plasma levels of LH in pituitary-grafted rats were 2-3 times higher during intracerebroventricular infusion for 7 days with CRF antiserum than during saline infusion. Furthermore, after infusion of CRF antiserum for 7 days into the lateral brain ventricle plasma LH levels had increased by 270% in normoprolactinemic male rats. These results indicate that hypothalamic CRF is involved in the control of LH release in male rats. To further investigate whether CRF is involved in the effect of PRL on LH secretion, we infused PRL, alone or together with CRF antiserum, for 7 days into the lateral brain ventricle of normoprolactinemic male rats. After 7 days of PRL infusion, LH levels had decreased by 45%, whereas plasma corticosterone was 150% higher. This action of PRL on LH and corticosterone was prevented when besides PRL also CRF antiserum was infused.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of liver-type lipase in rat ovaries and its activity during the estrous cycle and lactation

The conditions for an in vitro assay of liver-type lipase, i.e. an enzyme resembling the lipase releasable from the liver by heparin (liver lipase), in rat ovaries were established. The liver-type lipase activity in the ovaries was almost completely (greater than 95%) located in the corpora lutea and its activity ranged from 0.44 to 0.77 mU per corpus luteum of (pseudo)pregnant rats. Preovulatory ovarian follicles contained very low lipase activity. During the estrous cycle the pattern of lipase activity was similar to that of serum progesterone levels (maximal at diestrus 1 and minimal at diestrus 2). In the individual rats liver-type lipase activity in the ovaries was strongly correlated with serum progesterone and 20 alpha-hydroxyprogesterone. The activity of liver-type lipase also varied during lactation. It was relatively low at an early stage (2-3 days) but increased during later stages of lactation. The serum progesterone level was relatively low in rats lactating for 2-3 or 22-24 days. During the intervening time, its concentrations was elevated. Since serum 20 alpha-hydroxyprogesterone levels varied inversely to progesterone, the total amount of progestagens in blood during lactation remained constant. The cholesterol content of the corpora lutea of the lactating rats was initially high and decreased during the lactation.

Hepatic lipase gene expression is transiently induced by gonadotropic hormones in rat ovaries

Hepatic lipase (HL) gene expression was studied in rat ovaries. A transcript lacking exons 1 and 2 could be detected by reverse transcription-polymerase chain reaction (RT-PCR) in the ovaries of mature cyclic females and of immature rats treated with pregnant mare serum followed by human chorionic gonadotropin (hCG) to induce superovulation. By competitive RT-PCR the HL transcript was quantified. Low levels of HL mRNA were detected in ovaries of mature cyclic females and of immature rats. During superovulation HL mRNA was several fold higher than in mature cyclic rats and transiently increased to a maximum at 2 days after hCG treatment. Pulse-labelling of ovarian cells and ovarian slices with [35S]methionine followed by immunoprecipitation with polyclonal anti-HL IgGs showed de novo synthesis of a 47 kDa HL-related protein. Expression of the protein was transiently induced by gonadotropins with a peak at 2 days after hCG treatment. Induction of liver-type lipase activity occurred only after HL mRNA and synthesis of the HL-related protein had returned to pre-stimulatory levels. We conclude that in rat ovaries the HL gene is expressed into a variant mRNA and a 47 kDa protein. The expression of the HL gene in ovaries is inducible and precedes the expression of the mature, enzymatically active liver-type lipase.

Receptivity, proceptivity and attractivity of pregnant and pseudopregnant rats

In the present study the effects of pregnancy and of pseudopregnancy on oestrogen-induced receptivity, proceptivity and attractivity in female rats were investigated. Receptivity and proceptivity were determined in heterosexual pair tests. Attractivity was determined using a new method: a residential plus-maze. It was found that during the second half of pregnancy oestrogen treatment did not induce the above-mentioned behaviours. However, during the second half of pseudopregnancy oestrogen treatment resulted in high levels of these behaviours. Also, combined oestrogen plus progesterone treatment showed similar high frequencies of these behaviours in pseudopregnant rats.

Prolactin and delay of implantation in lactating adrenalectomized rats.

Adrenalectomy before pregnancy in rats caused the persistence of high blood levels of prolactin (PRL) throughout the ensuing postpartum lactation. The persistence of hyperprolactinaemia was without ef

Hysterectomy, decidualization, LH concentrations and corpus luteum function in the rat*

Abstract In 1923 Loeb described that the uterus plays an important role in the control of corpus luteum function in the guinea pig: removal of the uterus resulted in a prolongation of the luteal phase. Similar findings have been reported for several other mammalian species (Anderson, 1972). In the rat, hysterectomy does not alter the duration of the estrous cycle (Perry and Rowlands, 1961), but the duration of pseudopregnancy (the equivalent of the luteal phase in the reproductive cycle of other mammalian species) is extended after hysterectomy. The fact that hysterectomy leads to a prolonged luteal phase in certain mammalian species, pointed to the existence of a uterine luteolytic factor (Schomberg, 1969). Some evidence for the existence of a uterine luteolytic substance is available for the rat. It was shown that after removal of the endometrium the duration of pseudopregnancy was extended to almost that observed in hysterectomized rats (Waynforth, 1965). Hechter, Fraenkel, Lev and Soskin (1940) could reduce in the rat the duration of pseudopregnancy with uterine transplants. Furthermore, the duration of pseudopregnancy was shortened after injections of endometrial suspensions (Bradbury, Brown and Gray, 1950) or aqueous endometrial extracts (Dobrowolski, Snochowski and Stupnicki (1974). These observations suggest that the endometrium is the source of the luteolytic factor, and recently evidence was provided that a prostaglandin could be the uterine luteolytic factor (Pharriss, Tillson and Brickson, 1972; Labhsetwar, 1974). Studies in sheep (McCracken, Carlson, Glew, Goding, Baird, Green and Samuelsson, 1972) and in the guinea pig (Blatchley, Donovan, Horton and Poyser, 1972) have provided some evidence for this concept. Also in the rat a prostaglandin may be the uterine luteolytic factor: treatment with prostaglandin F 2α terminates pseudopregnancy in the rat by reducing ovarian and peripheral progesterone concentrations (Pharriss and Wyngarden, 1969; Behrman, Yoshinaga and Greep, 1971). The fact, however, that only a slight increase in F prostaglandins around day 7 of pseudopregnancy and no increase in E prostaglandins was observed (Saksena, Watson, Lau and Shaikh, 1974) throws some doubt upon the idea that a prostaglandin is the luteolytic factor in the rat. On the other hand, the finding that in a bioassay the dose-response curve of endometrial extract from pseudopregnant rats runs parallel with that of prostaglandin F 2α ; (Dobrowolski et al., 1974) could be considered as evidence that a prostaglandin is the uterine luteolytic factor in the rat. In the present paper the effects of hysterectomy and decidualization on certain aspects of pseudopregnancy in the rat are discussed.