C. S. Nicoll

Prolongation of lactation in the rat by litter replacement.

Summary Lactation was prolonged in 16 rats up to 70 days after parturition by providing them with fresh litters every 10 days. Milk production, as judged by litter weight gains, declined about 1/3 after first 20 days and continued at this level for the remainder of 70-day period. The lobule-alveolar system of most rats showed little or no evidence of involution during entire period. This suggests that the decline in milk yield was due either to decrease in secretion of galactopoietic hormones or to reduction in secretory capacity of alveolar cells, or both.

PROLACTIN SECRETION IN VITRO: EFFECTS OF GONADAL AND ADRENAL CORTICAL STEROIDS.

Summary Effects of estradiol (E), progesterone (P), testosterone (T), cortisol (F) and corticosterone (B) on prolactin production by rat adenohypophysis in vitro were examined. Addition of P (1 and 2 μg/ml), T (1 and 2 μg/ml), B (1 and 20 μg/ml) or F (0.5 and 5.0 μg/ml to culture medium did not influence prolactin secretion; however, F at 10 μg/ml significantly depressed prolactin production in vitro. This depressant effect of F, however, cannot be regarded as physiological due to the high concentration used. Incorporation of E into medium at concentrations of 0.05 and 0.5 μg/ml significantly increased prolactin secretion whereas E at 2.0 μg/ml was without effect. These observations indicate that only E, of all steroids tested, can act at the pituitary level to influence prolactin secretion. The effects of other steroid hormones on prolactin secretion in vivo are probably mediated via pathways other than by a direct effect on the adenohypophy-sis. We are indebted to Roger Deuben for technical assistance.

Effects of reserpine and serotonin on milk secretion and mammary growth in the rat.

Summary 1. In mature virgin rats, daily subcutaneous injections of 20 μg reserpine or .5 mg serotonin creatinine sulfate base/kg BW for 5 days, following daily injections of 10 μg estradiol for 10 days, initiated milk secretion in 14 out of 15 rats and induced growth of mammary lobuloalveolar tissue. When either drug was injected with estradiol for 10 days, mammary growth was greater than with estradiol alone, but there was little evidence of milk secretion. Injections of reserpine or serotonin alone for 21 days failed to elicit mammary growth or secretion. 2. In rats following litter removal on 4th day after parturition, milk secretion ceased and mammary glands showed pronounced involution 10 days later. Injections of 10 or 20 μg reserpine or .5 mg serotonin creatinine sulfate base for 10 days following litter removal, retarded mammary involution and maintained secretory function. 3. It is concluded that in estrogen-primed virgin rats or in post-partum rats after litter removal, reserpine and serotonin stimulate secretion of prolactin and perhaps other factors by the pituitary favorable to mammary growth and lactation.

Initiation of Lactation in Rats with Hypothalamie or Cerebral Tissue.

Summary Rat tissue containing the hypothalamus, cerebral tissue, kidney or liver were injected subcutaneously in saline suspension into rats for 5 days, after they had previously been injected with estradiol for 10 days to develop their mammary glands. Saline alone was injected into control rats. The tissue containing the hypothalamus initiated lactation in 12 out of 15 rats; cerebrum in 4 out of 15 rats; and liver, kidney or saline in none out of 5 rats each. Tissue containing the hypothalamus was injected intradermally over crop sacs of 5 pigeons for 5 days and failed to induce a response, indicating that the active principle is not prolactin. It is suggested that rat hypothalamus and possibly cerebral tissue produce release of prolactin and probably ACTH from the anterior pituitary in amounts sufficient to initiate lactation.

Local Action of Oxytocin on Mammary Glands of Postpartum Rats After Litter Removal.

Summary 1. Since no convincing evidence exists that the action of oxytocin in maintaining mammary secretion in postpartum rats after litter removal is due to prolactin release, attempts were made to determine whether its effects were exerted directly on the mammary gland. After maintaining milk secretion in 25 such rats for 5 or 9 days by daily injections of prolactin or twice-daily injections of oxytocin, a mammary biopsy was taken from each rat. They were then given an intraperitoneal injection of 2 IU oxytocin or physiological saline, and 10 minutes later all except 5 rats were killed and the remainder of the previously biopsied mammary gland was removed. Mammary biopsies from the remaining 5 rats were taken after 10 minutes and also 1, 2, 4 and 8 hours later. 2. Histological examination revealed that prior to intraperitoneal oxytocin administration, most mammary alveoli contained considerable secretion and the ducts were relatively thin, whereas 10 minutes after oxytocin the alveoli appeared shrunken or collapsed with little or no secretion, and the ducts were widely distended with milk. No notable increase in alveolar filling followed until the 8th hour after oxytocin injection. Saline had no effect on mammary alveoli and they remained filled with secretion. 3. Ejection of milk from the alveoli into larger ducts by oxytocin is believed to remove the pressure on epithelial cells which secrete milk and on blood capillaries surrounding them, permitting synthesis of more milk by circulating prolactin and other hormones. Milk ejected into ducts is believed to be gradually reabsorbed into the circulation, and some may flow back into the alveoli. These local effects of oxytocin on the mammary gland are believed to account for its ability to maintain mammary secretion in postpartum rats after litter removal.

Induction of mammary growth and lactation in rabbits with epinephrine acetylcholine and serotonin.

Summary 1. Thirty-eight mature female, predominantly New Zealand White rabbits, were injected subcutaneously daily with 0.2 mg estradiol for 15 days to develop the mammary duct system. They were then divided into 4 groups and injected subcutaneously as follows for 5 days: 0.85% saline; 0.25 mg epinephrine in oil twice daily; 12.5 mg acetylcholine iodide/kg BW twice daily; 5 mg serotonin base/kg BW once daily. On 21st day the rabbits were killed and mammary glands examined for growth and lactation. 2. Only 1 of 11 saline-injected rabbits showed gross lactation and lobulo-alveolar development, in contrast to 6 of 8 epinephrine-treated rabbits, 5 of 8 acetylcholine-injected rabbits and 9 of 11 rabbits treated with serotonin. The latter drug induced the greatest, and acetylcholine the least degree of mammary growth and lactation. 3. These results demonstrate that the 3 drugs in pharmacological doses can induce prolactin release from the anterior pituitary of rabbits.

Induction and Maintenance of Lactation in Rats by Electrical Stimulation of Uterine Cervix.

Summary 1. The uterine cervix of virgin, sexually mature rats was stimulated electrically 2 or 3 times daily for 5 days, following daily injections of 10 μg estradiol for 10 days to develop the mammary glands. Milk secretion was initiated in 8 out of 10 rats by this treatment, whereas 5 saline-injected controls showed no secretion and mammary involution. Twice daily injections of 70 mg morphine sulfate/kg BW prior to uterine stimulation did not inhibit lactation in 5 rats, and it was actually more intense in these animals. Twice daily injections of 1 IU oxytocin or 1 IU vasopressin for 5 days failed to initiate mammary secretion, showing that these hormones are not responsible for prolactin release in the cervical-stimulated rats. 2. Following litter removal on 4th postpartum day, injections of saline into mother rats for 10 days resulted in cessation of lactation and pronounced mammary involution. Twice daily electrical stimulation of the uterine cervix or injections of oxytocin for 10 days maintained secretory activity and retarded mammary involution, whereas vasopressin was much less effective in these respects. 3. It is concluded that electrical stimulation of the uterine cervix initiates lactation through sympathetic and hypothalamic pathways, resulting in prolactin release from the anterior pituitary. Oxytocin and to a lesser extent, vasopressin, are believed to maintain mammary secretion in postpartum, non-suckled rats by ejecting accumulated secretory material from the alveolar lumina and smaller ducts into the larger ducts and stromal tissue spaces, where it can be readily absorbed into the circulation.

Failure of Oxytocin to Initiate Mammary Secretion in Rabbits or Rats.

Summary Oxytocin failed to initiate lactation when injected 4 times daily into 45 mature female New Zealand White rabbits initially primed with estradiol or made pseudopregnant to develop their mammary glands. Prolactin injections initiated copious milk production in pseudopregnant rabbits. 2. Neither oxytocin nor vasopressin inhibited mammary involution or induced secretion in estrogen-primed rats, whereas injections of prolactin alone retarded mammary involution and addition of ACTH initiated mammary secretion. 3. It is concluded that oxytocin does not induce prolactin release in rabbits or rats.