In the present study a very simple test has been developed to demonstrate the time of onset of lactation in pregnant rats. Pro-oestrous female albino rats were placed with a male rat overnight. The next morning, if spermatozoa were found in the vaginal smear, was counted as the first day of pregnancy. In our colony, rats usually deliver on day 22. Starting on day 20 of pregnancy the rats were tested as follows: under ether anaesthesia, 50 m-u. oxytocin were given i.p. and a nipple was cut superficially. If the mammary gland is ready for lactation milk will flow out 1 or 2 min. after oxytocin administration. The whole procedure takes less than 5 min. and the rat recovers quickly. Using this test 47 pregnant rats have been studied. The time when the first foetus was delivered was taken as the time of parturition. In none of the rats used
J. PRILUSKY and R. P. DEIS
The effect of prostaglandin F2α (PGF2α) on milk ejection and on oxytocin release during suckling for one or two periods of 30 min was studied in lactating rats. Doses of PGF2α (20 or 40 μg) were injected i.p. 15 min before the suckling period. Control rats were injected with physiological saline. An inhibition of milk ejection proportional to the dose of drug administered was obtained. A normal milk ejection response was induced with a small dose of oxytocin injected immediately before nursing to mothers treated with PGF2α, indicating that the blocking effect was not due to a lack of mammary gland response. Two groups of mothers were injected with 40 μg PGF2α 2 and 4 h respectively before suckling. In both groups milk ejection was partially but significantly inhibited. In rats pre-treated with sodium pentobarbitone (3·5 mg/100 g body wt) to prevent the release of oxytocin induced by suckling, PGF2α (10 or 20 μg) did not modify the inhibition of milk ejection indicating that PGF2α does not have milk-ejecting activity. The administration of oxytocin to anaesthetized rats, immediately before a second suckling period, induced a normal milk-ejection response while in the rats treated with PGF2α, oxytocin was less effective. The results indicate that PGF2α inhibited milk ejection by a central block on oxytocin release and that the lipid is not able to mimic peripherally the milk-ejecting activity of oxytocin.
R. P. DEIS and NIA ALONSO
An action of dopamine on kidney function has been reported by McDonald, Goldberg, McNay & Tuttle (1964) in man and by McNay, McDonald & Goldberg (1963) in dogs. We have studied the effect of dopamine (D) (3,4 dihydroxyphenylethylamine HCl; Nutritional Biochemical Co.) on diuresis in rats.
Male rats (200–300 g.) of the Instituto strain were used. They were trained for 3 days to minimize the effect of stress and observations made both during water diuresis and at normal rates of urine flow. Water diuresis was induced by giving tap water by stomach tube (4 ml./100 g. body wt). After placing the rats in a special container, urine was collected every 30 min. for 2 hr. and the bladder was gently pressed before and after each 30-min. period to ensure complete evacuation. Dopamine (2 mg./100 g. body wt) was administered i.p. in 0·2 ml. saline. In five rats with normal rates
J. PRILUSKY and R. P. DEIS
The effect of l-DOPA on milk ejection and on prolactin release during 30 min of suckling was studied in lactating rats. Various doses of l-DOPA (1·25, 2·5, 5 and 10 mg/100 g body wt) were injected i.p. 30 min before the suckling period. Control rats were injected with 0·9% NaCl solution only. An inhibition of milk ejection proportional to the dose of drug administered was obtained. The dose of 10 mg completely blocked milk ejection but 1·25 mg had no effect. A normal milk-ejection response was obtained with a small dose of oxytocin injected immediately before nursing into mothers treated with 10 mg l-DOPA, indicating that the blocking effect was not due to a lack of mammary gland response. In control mothers, serum prolactin levels increased from 67·2 ± 25·9 (s.e.m.) to 950·3 ± 118·7 ng/ml after a 30 min suckling period. l-DOPA (5 and 10 mg) prevented the release of prolactin induced by suckling, but 1·25 and 2·5 mg l-DOPA had no effect. The results indicate that oxytocin and prolactin release induced by suckling in lactating rats is inhibited by an increase of catecholamines at the hypothalamic-hypophysial axis.
R. P. DEIS and NIA ALONSO
The effect of synthetic thyrotrophin releasing factor (TRF) on serum prolactin and LH concentrations was determined by radioimmunoassay in male, cyclic and pseudopregnant female rats. A solution of TRF (0·1, 0·25, 0·5 and 1 μg/rat) was injected i.v. at 17.00 h into rats pretreated with sodium pentobarbitone at 13.00 h. A group of male rats was also treated with TRF at 11.00 h after pretreatment with sodium pentobarbitone at 07.00 h. Fifteen minutes after TRF administration, blood samples were obtained by heart puncture. Doses of 0·25, 0·5 and 1 μg TRF significantly increased the serum prolactin concentration in pro-oestrous rats. The mean serum prolactin level after the injection of 0·5 and 1 μg into oestrous rats and 0·5 μg TRF into dioestrous day 2 rats, was significantly greater than the control values. Injection of TRF on day 1 of dioestrus had no effect. Serum LH concentration was not significantly modified by the various doses of TRF administered. On day 3 of pseudopregnancy a significant increase of serum prolactin values was obtained with 0·5 and 1 μg TRF. On day 7 of pseudopregnancy a dose of 0·5 μg produced the same effect, but on day 10 of pseudopregnancy only 1 μg TRF significantly increased serum prolactin levels when compared with the control rats. In male rats serum prolactin concentration was significantly greater than the control values after TRF treatment either in the morning or the afternoon. The response was similar to that obtained in pro-oestrous rats. The results suggest that the ability of synthetic TRF to stimulate prolactin release exists in both female and male rats and that TRF does not affect LH secretion.
D. ZAMBRANO and R. P. DEIS
The ultrastructural effect of oestradiol implants into the hypothalamus on the adenohypophysis of adult female rats was studied. The main results are the demonstration that mammotrophs or prolactin cells, and to a lesser extent somatotrophs, increased their secretory activity after bilateral implants into the median eminence and arcuate nuclei; in the periventricular area the reaction was weaker and in the anterior hypothalamus the response was negative. In animals with implants in the basal tuberal region, mammotrophs showed an asynchronous secretory activity. Mitosis has been found to occur in fully differentiated mammotrophs, a finding which can be correlated with the increased number of such cells after basal tuberal implants. The gonadotrophs appeared to be atrophied.
R. P. DEIS and NIA ALONSO
The specific action of synthetic thyrotrophin releasing factor (TRF) on the release of thyrotrophin, both in vivo and in vitro, has been established in several species including human subjects (Bowers, Schally, Schalch, Gual, Kastin & Folkers, 1970; Fleischer, Burgus, Vale, Dunn & Guillemin, 1970; Hall, Amos & Garry, 1970; LaBella & Vivian, 1971). In spite of the physiological and chemical evidence that hypothalamic factors may be specific for each of the pituitary hormones, an effect of synthetic TRF on the release of prolactin was recently described in man (Jacobs, Snyder, Wilber, Utiger & Daughaday, 1971), lactating cows (Convey, Tucker, Smith & Zolman, 1972), and in rat somatotrophic-mammotrophic tumour cell cultures (Tashjian, Barowsky & Jensen, 1971).
This study was undertaken to see if synthetic TRF could induce the release of prolactin when injected into intact female rats. Virgin albino rats weighing between 200 and 250 g were used. A solution of
SILVIA CATALÁ and R. P. DEIS
Ovariectomy of rats on day 20 of pregnancy impaired parturition and lactation. All rats showed prolonged delivery with many foetuses born dead and the mothers were unable to rear the young. Treatment with oestradiol benzoate (0·5 or 1 μg) in a single dose permitted normal delivery and lactation when the hormone was injected immediately after ovariectomy. Oestrogen administered the day after ovariectomy was not effective. The administration of prolactin and corticotrophin to ovariectomized pregnant rats did not prevent abnormal parturition but lactation improved in 50% of the mothers. A group of animals ovariectomized on day 20 was treated with oxytocin every 2 h on day 22 starting at 08.00 h. This treatment did not facilitate parturition and lactation. Maternal behaviour, which was always present in the other groups, was also impaired by this treatment. When ovariectomy was performed on the evening of day 21 of pregnancy, a partial impairment of parturition was observed but lactation was normal in seven out of eight rats. The results indicate that oestrogen is essential near term for normal parturition and lactation. The absence of the steroid may make the uteri less sensitive to oxytocic substances and also affect milk ejection.
M Soaje and R P Deis
It is well known that the fall in serum progesterone concentrations during late pregnancy induces prolactin secretion in rats. On day 19 of pregnancy, administration of 10 mg of the antiprogesterone RU-486/kg induced a small but significant increase in serum prolactin. A lower dose (2 mg/kg) was not effective. Administration of naloxone (2 mg/kg) to pregnant rats on day 19 of pregnancy did not modify circulating prolactin but, after RU-486 treatment, a notable increase in serum prolactin was obtained 30 min after naloxone was given. The lack of effect of naloxone-methobromide in pregnant rats pretreated with RU-486 may indicate that the opioid-induced prolactin suppression acts centrally, most probably at the hypothalamic level. During day 21 of pregnancy, the time-course of prolactin secretion, measured at 0900, 1400, 1900 and 2200 h, was inversely correlated with circulating progesterone levels. At 0900 h, serum prolactin was very low with high serum progesterone concentrations but a significant increase in serum prolactin occurred at 2200 h; this was coincident with a significant decrease in the steroid. The stimulatory effect of naloxone on prolactin secretion was clearly dependent on the circulating progesterone level. Thus, at 1900 h of day 21, naloxone induced a significant increase in serum prolactin but, at 2200 h, the opioid antagonist dramatically enhanced the circulating level of prolactin. The serum prolactin increase induced by naloxone at 1900 h was prevented by the s.c. administration of 5 mg progesterone given 7 h earlier. Similarly, the large increase in serum prolactin levels at 1800 h on day 19 of pregnancy, after administration of RU-486 plus naloxone, was completely abolished by treatment with CB154. The lack of effect of RU-486 and naloxone on serum prolactin levels in virgin rats on the day of pro-oestrus demonstrates that the effect of naloxone on prolactin in pregnant rat is peculiar to the end of pregnancy.
In conclusion, the attenuation of the central inhibitory action of progesterone facilitates the release of prolactin which is dramatically enhanced by naloxone treatment. These results provide an important new insight into the existence of a neuromodulatory regulation of prolactin secretion by the opioid system showing a paradoxical opioid-induced prolactin suppression at the end of pregnancy.
Journal of Endocrinology (1994) 140, 97–102
G. A. Jahn and R. P. Deis
The part played by the adrenergic system on the release of prolactin and lactogenesis induced by prostaglandin F2α and the antiprogesterone RU 486 was studied in pregnant rats. Two doses of prostaglandin F2α (150 μg) administered at 08.00 and 12.00 h on day 19 of pregnancy induced, at 12.00 h on day 20 (24 h after administration), a significant increase in the serum concentration of prolactin, with a significant decrease in serum progesterone levels. These hormonal changes significantly augmented casein and lactose levels in the mammary gland. Treatment with RU 486 (2 mg/kg) at 08.00 h on day 19 augmented casein and lactose concentrations in the mammary gland at 12.00 h on day 20 without modifying serum concentrations of prolactin and progesterone. The adrenergic antagonists, propranolol (3 mg/kg), metoprolol (10 mg/kg), ICI 118 551 (200 μg/kg), idazoxan (100 μg/kg) and prazosin (10 mg/kg), were administered s.c. at 12.00 and 20.00 h on day 19 and 08.00 h on day 20 of pregnancy to intact rats or to rats previously treated with RU 486 or prostaglandin F2α. These adrenergic antagonists did not modify serum prolactin or progesterone levels in intact or RU 486-treated rats, but serum prolactin levels in the prostaglandin F2α-treated group were significantly reduced by treatment with propranolol, metoprolol or prazosin. In addition, propranolol and ICI 118 551 also decreased the casein and lactose concentrations in the mammary glands of RU 486- and prostaglandin F2α-treated rats, while the other compounds had no effect. We also studied the effect of adrenergic antagonists on the release of prolactin and lactogenesis induced by the physiological decrease in progesterone at the end of pregnancy. On day 21 of pregnancy at 18.00 h, serum progesterone levels in intact rats were lower than 40 nmol/l, while serum prolactin and casein and lactose concentrations in the mammary gland were higher compared with values measured at 12.00 h on day 20. Treatments with propranolol, metoprolol or prazosin administered at 20.00 h on day 20 and 08.00 and 14.00 h on day 21 of pregnancy were capable of significantly reducing serum prolactin concentrations while only propranolol decreased mammary casein and lactose. The effect of propranolol was not mediated through a reduction in serum placental lactogen measured by Nb2 lymphoma cell bioassay.
These results show that the adrenergic system participates, through α1 and β1 receptors, in the regulation of prolactin release induced by the decrease in progesterone in pregnant rats. They also show that β2-adrenergic receptors play a role in the induction of casein and lactose synthesis in the mammary gland.
Journal of Endocrinology (1991) 129, 343–350