Abstract
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.
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