Friday 9 September 2016

Pulsatile but not Tonic Secretion of Oxytocin Plays the Role of Anti-Precancerous Lesions of the Mammary Glands in Rat Dams Separated from the Pups during Lactation

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Lactation interruption can increase maternal stress and breast tumorigenesis; disorders in hypothalamic oxytocin(OXT)- secreting system and lactation failure could account for these effects. The question is whether abnormal activity of the OXT -secreting system is causally related to precancerous lesions of maternal mammary glands that could underlie breast tumorigenesis. To answer this question, we observed effects of lactation interruption on maternal behaviors, milk availability, OXT neuronal excitability and the histological features of mammary glands of lactating rats that were intermittently separated from the pups. Separation of lactating dams from pups for four days with four hour consecutive contacts per day caused losses of maternal interests in the offspring and lactation failure. The separation also caused early involution of the mammary glands. By simulating OXT release during suckling in isolated rat mammary glands, we determined that the pulsatile pattern of OXT release was the most effective way to suppress hydroxide peroxide-induced expressions of phosphorylated extracellular signal-regulated protein kinase 1/2 and cyclooxygenase 2, two proliferative biomarkers. Dam-pup separation also disrupted the electrical activity of OXT neurons in the supraoptic nucleus and their responses to excitatory stimuli in brain slices. These findings indicate that lactation failure results from lack of pulsatile OXT release during suckling, which is causally related to a series of maternal mental disorders and precancerous lesions of the mammary glands.

Breastfeeding has many beneficial effects on mothers and the babies. Lactation failure is associated with the high incidence of breast cancer, type 2 diabetes and obesity in the mothers and autism, sudden death, and deficiency in maternal behaviors in the babies. Lactation failure is also associated with increased incidence of postpartum depression and premenopausal breast cancer. Successful lactation is based on coordinated actions of milk production, secretion and ejection controlled by a series of hormonal events. These hormones synchronize the development of mammary glands and hypothalamic plasticity to meet requirements of lactation. Milk ejections are the most sensitive event in the lactation and are achieved through the milk-ejection reflex (MER). During suckling, afferent inputs from nipples are integrated in the brain into intermittent activation of hypothalamic oxytocin (OXT) neurons in forms of burst firing. Synchronized bursts among OXT neurons cause release enough OXT to trigger a milk-ejection from the mammary glands. OXT neuronal activity is directly modulated by various cellular components and neurochemical around OXT neurons. During lactation, the supraoptic nucleus (SON) undergoes significant plastic changes which form the basis of burst discharges.

Somatodendritic release of OXT is another dramatic feature in the SON, which plays permissive and auto-regulatory roles in neurochemical modulation of OXT neuronal activity. OXT receptors (OXTRs) are localized on both neurons and astrocytes in the supraoptic nucleus (SON). By activation of OXTRs, OXT can change the morphology and functions of supraoptic cells by activation of Gαq/11 type of G protein coupled receptors and by mobilization of a series of cellular signals, e.g., cyclooxygenase 2 (Cox-2) and phosphorylated extracellular signal-regulated protein kinase (pERK) 1/2. Moreover, the findings that mother-baby separation can decrease blood OXT levels and OXT has anxiolytic effects suggest the involvement of the OXT-secreting system in the occurrence of lactation failure[18,19]. However, it remains a question about the critical loci in the MER pathway that are responsible for lactation failure. Moreover, it is not answered whether and how lactation failure is causally related to the breast tumorigenesis.

Lactation failure is associated with many factors, such as mother-baby separation, lacking social supports, obesity, babies’ sickness, poor breast conditions, cesarean section, mental disorders, and early usages of bottle feeding and milk substitutes, etc. Among them, mother-baby separation is the common cause of lactation failure. Thus, to establish causal relationship between lactation failure and postpartum health issues in women and to clarify neural mechanisms underlying lactation failure-associated breast cancer, we used a maternal separation model to observe effects of lactation interruption (L-I) on maternal behaviors, OXT neuronal excitability and proliferative activity of mammary glands of lactating rats. Lastly we examined different patterns of OXT applications on the expression of pERK 1/2 and Cox-2 of the mammary glands in response to oxidative stress in vitro. The preliminary results have been published in an abstract form. 

All procedures in these experiments were in accordance with the guidelines on the use and care of laboratory animals set by NIH and approved by the Institutional Animal Care and Use Committees of the University of California, Riverside, and Harbin Medical University, respectively.Primiparous Sprague-Dawley rats were used during lactation as described previously with minor modification. Within 24 h of parturition, litters were adjusted to 10 pups. Dams had 4 h contacts per day with their pups, beginning on lactation day 8-9; separated pups were nursed by foster mothers at all other times. On postpartum day 11-12, observations were made of the following: maternal behaviors (e.g., pup retrieval, ano-genital licking, and suckling), signs of anxiety and depression, litter and dam body weight gains, the latency and duration of MER. This model was also used in observations of firing activity of OXT neurons in whole animals and in brain slices, measurement of intramammary pressure (IMP), and examination of functional proteins as described in the followings. 

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