Oxytocin and prolactin

Definitions & Key takeaways

Oxytocin and prolactin are two hormones that are involved in a variety of physiological processes, including reproductive and lactation functions. During pregnancy, these hormones prepare the breasts for lactation, by stimulating the maturation of the milk-secreting alveoli.

Oxytocin is produced by the hypothalamus and released by the posterior pituitary gland. It plays a role in childbirth, lactation, and sexual behavior. It is also thought to have a role in social bonding and attachment. On the other hand, prolactin is produced by the anterior pituitary gland and plays a role in lactation and the production of breast milk. It is also involved in the regulation of the menstrual cycle and fertility.

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Introduction0:00–0:43

Oxytocin and prolactin are two hormones that are mostly involved in the production and release of milk from the breasts during the nursing period.
Prolactin - pro meaning “for” and “lactin” referring to milk - it stimulates the production of milk. Oxytocin, on the other hand, makes the smooth muscle cells around the glands in the breasts contract so that they release the milk.
Oxytocin also helps the muscle in the uterus contract during labor. Both of these hormones are synthesized all the way up in the brain, by two interconnected structures – the hypothalamus and the pituitary gland.

Oxytocin0:43–1:38

So, basically, the hypothalamus is a part of the brain made up of several nuclei – or clusters of neurons. Two of these nuclei, the paraventricular and supraoptic nuclei, contain neurons that secrete oxytocin, as well as other hormones like vasopressin, or antidiuretic hormone, which is involved in regulating the amount of water in our body.
When oxytocin is produced, it travels down the axons of these neurons, and reaches the posterior lobe of the pituitary gland.
Down the length of these axons, there are small dilations called Herring bodies – which store the oxytocin until it’s released in the blood.
Outside of pregnancy, oxytocin levels are low, but production increases just a bit during an orgasm - so here’s the reason for that lovely “afterglow”.

Prolactin1:38–2:36

Prolactin, on the other hand, is synthesized by special cells called lactotrophs, which are in the anterior lobe of the pituitary.
In women that are not pregnant or breastfeeding, and in men, prolactin levels are usually kept in check by the hypothalamus in two ways.
The first way is the most important, and it’s when the hypothalamus secretes a constant stream of dopamine - which is also called prolactin inhibiting factor.
Dopamine binds to specific receptors on the lactotrophs and inhibits the release of prolactin. The second way is when the hypothalamus secretes thyrotropin releasing hormone, also called prolactin releasing hormone, which can stimulate prolactin release.
Both oxytocin and prolactin bind to receptors on cells in the breasts - so let’s take a closer look at them. Each breast is made up of 15 to 20 lobules, and each lobule consists of clusters of hollow alveoli lined by a single layer of milk-secreting epithelial cells.

Pregnancy2:36–3:31

Each lobule continues with a lactiferous duct, which is also lined with contractile myoepithelial cells, and it opens at the nipple.
During pregnancy, oxytocin and prolactin, along with the estrogen and progesterone produced by the placenta, stimulate the growth of the milk-producing alveoli.
In turn, this makes the breasts themselves grow and prepare for lactation. However, the high levels of estrogen and progesterone required to maintain pregnancy inhibit the production of milk until the baby is born.
Now, oxytocin really comes into play during labor. Labor starts with the dilation of the cervix - the lowermost part of the uterus.

Labor3:31–4:15

When the cervix dilates, this sends signals to the hypothalamus to produce more oxytocin. In turn, oxytocin dilates the cervix more, so that the baby has enough room to pass through - this is an example of a positive feedback cycle, because the two events stimulate each other.
After the cervix has dilated enough, oxytocin makes the muscular layer of the uterus contract periodically to help push the baby out through the vagina, and into the world.

Postpartum4:15–5:18

After the baby is born, the placenta detaches from the uterine wall, and this leads to a drop in estrogen and progesterone levels.
This means that prolactin can finally stimulate the milk-secreting epithelial cells. Breastfeeding also increases prolactin levels and, in turn milk production for the duration of nursing.
Apart from milk production, high levels of prolactin also inhibit the release of gonadotropin releasing hormone from the hypothalamus, which results in decreased luteinizing hormone, or LH, and follicle stimulating hormone, or FSH, levels from the anterior pituitary.
FSH and LH normally induce the development of ovarian follicles and ovulation - which is why women typically don’t ovulate or have menstrual periods while breastfeeding.
Now, when the baby begins suckling at the breast, the tactile receptors in the nipple send a signal to the hypothalamus, which responds by also producing oxytocin in pulses – sometimes more and sometimes less.

Breastfeeding5:18–5:59

This oxytocin binds to oxytocin receptors on the myoepithelial cells, which leads to the ejection of milk in pulsatile pumping manner that promotes maximum emptying of milk – like repeatedly squeezing and releasing a wet sponge until there’s no water left in it.
But fear not - prolactin levels increase, so the breasts fill up with milk again just in time for the baby’s next meal. Alright, as a quick recap: oxytocin and prolactin are two hormones produced by the hypothalamus and the pituitary gland.

Review5:59–6:41

During pregnancy, these hormones prepare the breasts for lactation, by stimulating the maturation of the milk-secreting alveoli.
During childbirth, oxytocin dilates the cervix and helps the uterus contract to help the baby come out. After the baby is born, oxytocin and prolactin are the cornerstone of lactation: prolactin stimulates milk production, while oxytocin makes the muscle cells contract and push the milk out of the breasts during breastfeeding.