R.P. Deis

Studies concerning the hormonal induction of lactogenesis by prostaglandin F2α in pregnant rats

Abstract Factors involved in the induction of lactogenesis by prostaglandin F 2 α in pregnant rats have been studied. PGF 2 α-tromethamine salt (150 μg × 2) injected on day 18 of pregnancy, induced a significant increase in ovarian 20α-hydroxysteroid dehydrogenase activity with maximal values 24 h after treatment. Simultaneously, serum progesterone concentration starts to decrease significantly 4 h after PGF 2 α administration. These ovarian changes are related to the significant increase in serum prolactin which occur between 12 and 24 h after prostaglandin treatment. At mammary gland level, specific prolactin binding sites were significantly higher 4 and 8 h after PGF 2 α administration followed by a rapid decrease to normal values at 12 and 24 h. The effect on placental function was determined by measurement of the chorionic mammotrophin in plasma. No modification was found in chorionic mammotrophin levels in the first 24 h after PGF 2 α treatment, being the values similar to those in the control rats. Lactogenesis visualized by an oxytocin test, occurred in the pregnant PGF 2 α treated rats 24 h after the administration of the drug. The described hormonal changes occurring after PGF 2 α treatment, resemble the physiological events that take place in the control rats before parturition. These results suggest that prostaglandin may play a physiological role in the induction of lactogenesis in pregnant rats.

Lactogenesis induced by ovariectomy in pregnant rats and its regulation by oestrogen and progesterone

Ovariectomy on day 19 of pregnancy augmented galactosyl transferase activity 24 h after surgery preceding by 6 h the significant alpha-lactalbumin accumulation. Progesterone, injected immediately after ovariectomy showed a clear inhibitory effect on both galactosyl transferase and alpha-lactalbumin concentration, measured 30 h after ovariectomy. However, once the synthesis of lactose has been induced, progesterone is no longer inhibitory. Oestrogen induced a significant increase in lactose synthetase activity but no effect was obtained on galactosyl transferase activity. Progesterone, in a time and dose dependent relationship, was capable of preventing the effect of estrogen on lactogenesis. The lactogenic action of oestrogen in ovariectomized pregnant rats might be due to a direct effect at the mammary gland level facilitating the action of prolactin or through an indirect effect mediated via an increase on prolactin release.

Regulation of pituitary DNA synthesis during different reproductive states in the female rat: role of estrogens and prolactin

We studied DNA synthesis in the rat adenohypophysis during the estrous cycle, pregnancy and lactation. During the estrous cycle, DNA synthesis was 3 times higher on the morning of estrus than on the other days. This peak was abolished completely by ovariectomy or pentobarbital, which also blocked the preovulatory surges of LH and prolactin. Methallibure , which blocked the LH but not the prolaction surge, had a partial effect on DNA synthesis. An acute and significant decrease in pituitary DNA synthesis occurred between days 0 (estrus) and 1 of pregnancy, followed by a less pronounced diminution until parturition. After delivery, DNA synthesis increased steeply on day 1 of lactation, returning to low values by day 3, under normal suckling conditions. Thelectomy , which blocked suckling-induced prolactin release, or antiestrogen treatment, which did not decrease prolactin secretion, diminished pituitary DNA synthesis on day 1 of lactation. Estrogen administration to intact or ovariectomized rats on days 9-11 of lactation stimulated (100%) DNA synthesis. Ovariectomy had no effect. In conclusion, in the different reproductive states studied, pituitary DNA synthesis is related to prolactin release in the presence of estrogens.

INHIBITORY EFFECT OF PROGESTERONE ON THE LACTOGENIC AND ABORTIVE ACTION OF PROSTAGLANDIN F2α

The effect of ovariectomy, progesterone and prolactin treatment on the action of prostaglandin F2alpha (PGF2alpha) was determined in pregnant rats. PGF2alpha (150 mug times 2) injected i.p. on day 1. or 18 of pregnancy induced lactogenesis about 25 h later and abortion on days 20 and 21 of pregnancy. Treatment with PGF2alpha (100 mug times 2 or 50 mug times 2) on day 19 induced lactogenesis around 22 or 38 h later, respectively, and abortion on day 21. PHF2alpha treatment on day 17 was less effective. Unilateral ovariectomy on day 17 of pregnancy induced lactogenesis 32 h later but not abortion. PGF2alpha (150 mug times 2) given on the day of surgery advanced lactogenesis 12 h and rats aborted on day 19. Bilateral overiectomy on day 17 induced abortion between days 20 to 21, but if a single dose of PGF2alpha (300 mug) was injected on day 18. all the ovariectomized rats aborted on day 19. Progesterone (10 mg) injected into rats treated with PGF2alpha (150 mug times 2) on day 18, prevented abortion and delayed lactogenesis. Prolactin (1 mg times 4) treatment delayed only abortion. Serum prolactin levels were significantly higher 12 h after the last dose of PGF2alpha (150 mug times 2) in rats treated on days 17, 18 or 19 of pregnancy. Pretreatment with progesterone prevented the rise in prolactin concentration. These result suggest that the lactogenic and abortive action of PGF2alpha may be dependent on the uterine and plasma concentration of progesterone.

Hormonal regulation of casein synthesis at the end of pregnancy

Ovariectomy or ovariohysterectomy on day 18 of pregnancy augmented mammary beta-casein content 28 h later. Progesterone injected immediately and 12 h after ovariectomy showed a clear inhibitory effect on casein synthesis. Estrogen induced a significant increase in mammary beta-casein content when injected 12 h after surgery. Treatment with CB-154 to prevent prolactin release did not affect the increase of casein induced by ovariectomy. When CB-154 was injected to ovariohysterectomized pregnant rats, significant reduction of casein synthesis was obtained. According to these findings, rat placental lactogen in the absence of prolactin and progesterone induces beta-casein synthesis. Therefore prolactin, ovarian and placental hormones interplay at the end of pregnancy for full expression of the mammary gland genome.

Inhibition of milk ejection by a visual stimulus in lactating rats: implication of the pineal gland.

The effect of visual and auditory stimuli on milk ejection during suckling was studied in normal and pinealectomized lactating rats. The photic and auditory stimuli were applied to each mother for 10 s every 20 s during the 30 min suckling period. Both stimuli inhibited milk ejection without altering the nursing behavior. In mothers kept in complete darkness or in which the visual stimulus shone continuously during the suckling period, milk ejection was not affected. The inhibition of milk ejection is therefore produced by the light on-off sequence. In lactating rats exposed to the stimulus during 3 consecutive days, a significant inhibition of milk ejection was obtained each day. A normal milk-ejection response occurred in both non-stimulated pinealectomized and in stimulated pinealectomized lactating rats. Pinealectomy did not prevent the inhibitory effect of the sound stimulus. Treatment with methysergide prevented the inhibition of milk ejection induced by the visual stimulus but did not prevent the inhibitory effect of the auditory stimulus. It seems that the pineal gland mediates an inhibitory visual reflex acting on oxytocin release and milk ejection.

Antiovulatory Effect of ICI 33,828 (Methallibure) Without Affecting Prolactin Release in Normal Rats

The acute effect of 1-alpha-methylallylthiocarbamoyl-2-methylthiocarbamoylhydrazine (methall ibure) on the release of luteinizing hormone (LH) and prolactin on the day of proestrus was studied in normal rats. 21 rats received 5 mg methallibure/100 gm body weight the day before proestrus when blood samples were obtained for LH and prolactin determinations. The rats were sacrificed the examined for ova. 8 methallibure-treated rats were injected with 10 following morning (first day of estrus) and the oviducts were mcg LH/100 gm body weight on the day of estrus following blood extraction. Oviducts were examined for ova the next day. The single dose of methallibure blocked LH release in 19 out of 21 rats. The 19 rats had mean LH values of 130.2 ng/ml, highly significant (p less than .0001) when compared with 392.7 ng/ml in the controls. Prolactin release was not affected by methallibure, since the mean prolactin level for treated rats was 166.8 ng/ml and 198.7 ng/ml for the controls. The serum prolactin peak on the afternoon of proestrus was confirmed in 7 normal (31 ng/ml) and in 17 estrous rats (28.3 ng/ml). In 4 treated rats, prolactin levels determined on the day of estrus were markedly higher (61 ng/ml; p less than .0001) than in normal estrous rats. None of the rats which had subnormal LH levels showed spontaneous ovulation. However, the 10 mcg LH/100 gm body weight in previously methallibure-treated rats induced ovulation. The number of ova per rat was similar in the LH treated (9.6) and in the controls (10.6). It is concluded that methallibure prevents ovulation by centrally blocking LH release either without affecting or by stimulating prolactin release.

A dual modulatory effect of progesterone on the LHRH-induced LH release

The role of progesterone on the release of LH induced by 25 or 50 ng of LHRH was studied in proestrus rats in which spontaneous preovulatory release of LH was prevented by sodium pentobarbitone. After the s.c. administration of progesterone (5 mg) at 18.00 h of diestrus day 2 or at 12.00 h of proestrus, serum LH was not detectable at 17.15 h of proestrus. Injections of 25 or 50 ng of LHRH at 17.00 h of proestrus induced a dose response release of LH 15 min after. However, the LH response to LHRH administration increased significantly when progesterone was injected at 12.00 h of proestrus. The potentiating effect of progesterone seems to be exerted at pituitary level. The effect of LHRH and the enhanced response of the pituitary after progesterone treatment was prevented by the administration of the antiestrogen Tamoxifen in diestrus day 2. The release of LH induced by 50 ng of LHRH on proestrus day was blocked by the previous injection of progesterone on diestrus day 2. The inhibition was maintained even though a second dose of progesterone was given at 12.00 h of proestrus. The simultaneous administration of estrogen and progesterone on diestrus day 2 did not prevent the inhibitory effect of progesterone. It is concluded that the facilitatory or inhibitory effect of progesterone on the release of LH induced by LHRH is dependent upon the previous sensitization of the pituitary to estrogen.