What is the use of oxytocin after childbirth?

In ancient times, if a pregnant woman had been pregnant for a long time or even exceeded the fertility standard, some methods of inducing labor would be chosen. However, because technology was not well developed at that time, some traditional methods were used, and it was very common for both mother and child to die at the same time. Nowadays, with the development of technology, we can use some medical methods to induce labor, such as injecting oxytocin. So, what is the use of oxytocin after childbirth?

Oxytocin has the effect of promoting milk discharge and stimulating uterine contraction.

1. Effect on mammary glands During lactation, mammary glands continuously secrete milk, which is stored in alveoli. When the alveoli are surrounded by contractile myoepithelial cells, the alveolar pressure increases, causing milk to be ejected from the alveoli through the lactiferous ducts and out of the nipple. Milk ejection is a typical neuroendocrine reflex. The nipple is rich in sensory nerve endings. The sensory information of sucking the nipple is transmitted to the hypothalamus through the afferent nerves, which excites the neurons that secrete oxytocin. The nerve impulses are transmitted to the neurohypophysis through the hypothalamic-pituitary tract, causing the stored oxytocin to be released into the blood and act on the myoepithelial cells in the mammary gland to cause them to contract, causing milk to be discharged. During the milk ejection reflex, the concentration of antidiuretic hormone in the blood does not change. Based on the milk ejection reflex, it is easy to establish a conditioned reflex. For example, seeing the baby or hearing its cry can induce conditioned reverse milk ejection. In addition to inducing milk secretion, oxytocin also has the function of preventing mammary glands from atrophy during lactation.

During the milk let-down reflex, the secretion of prolactin and oxytocin increases, while the release of GnRH decreases. Increased prolactin secretion promotes the secretion of GnRH, which is beneficial to the next milk emission. The decreased release of GnRH causes a decrease in the secretion of gonadotropin from the pituitary gland, which can lead to cessation of menstruation during lactation. The reduction in GnRH release may be due to the stimulation of nipple sucking, which causes the excitement of dopamine neurons in the hypothalamus and the release of dopamine, which can inhibit the release of GnRH; it may also be related to β-endorphin in the hypothalamus. It can both promote the secretion of prolactin and inhibit the release of GnRH.

2. Oxytocin acts on the uterus to promote uterine muscle contraction, but this information processing is related to the functional state of the uterus. Oxytocin has a weaker effect on the non-pregnant uterus and a stronger effect on the pregnant uterus. Estrogen can increase the sensitivity of the uterus to oxytocin, while progesterone has the opposite effect. Oxytocin can allow extracellular Ca to enter the uterine smooth muscle cells, increase the Ca concentration in the muscle cells, and induce muscle cell contraction possibly through the action of calmodulin and with the participation of protein kinase. Studies have shown that although oxytocin stimulates uterine contractions, it does not initiate further contractions of the uterus during labor.

Because the chemical structures of oxytocin and vasopressin are similar, their physiological effects overlap to a certain extent. For example, the antidiuretic effect of oxytocin on dogs is equivalent to 1/200 of that of vasopressin, while the contractile effect of vasopressin on the isolated uterus of rats is about 1/500 of that of oxytocin.