What are you going to learn?
- What are prolactin's effects on the organism?
- How is prolactin secreted?
- How is prolactin regulated?
- What happens if there's too much or too little of the hormone?
- terms: galactorrhea
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Prolactin has three major effects. The first one is breast development. During puberty, prolactin stimulates development of mammary ducts, the ducts milk is transported through, and also, during pregnancy, it stimulates development of mammary alveoli, which produce milk after childbirth. Prolactin also stimulates milk production by inducing the synthesis of its components and it also stimulates milk secretion during breastfeeding.
Prolactin can also inhibit ovulation by inhibiting the secretion of gonadotropin-releasing hormone in the hypothalamus. Gonadotropin-releasing hormone regulates the secretion of luteinizing and follicle-stimulating hormone. When the secretion of gonadotropin-releasing hormone is inhibited, secretion of luteinizing and follicle-stimulating hormone is also inhibited, and new oocytes cannot develop. This inhibition lasts until a few weeks after the end of nursing.
Secretion
Prolactin is synthesized by special cells called lactotrophs in the anterior pituitary. There’s not a lot of prolactin in the blood as its blood levels only increase during pregnancy and lactation. This, of course, only applies to women, because in men, its blood levels are low constantly. During pregnancy, prolactin's blood levels rise considerably. However, there are also high levels of estrogen and progesterone that down-regulate prolactin receptors in the breasts. (Down-regulation means that there’s less receptors of the hormone or the hormone doesn’t bind that easily to its receptor.) This inhibition is essential to make sure that there is no lactation until the child is born. When estrogen and progesterone levels decrease significantly during childbirth, prolactin then starts to stimulate milk production and lactation can start.
Regulation
Prolactin's most important inhibitory factor is dopamine, which is synthesized in neurons in the hypothalamus and which is transported to the anterior pituitary directly through hypothalamic-hypophysial portal vessels. Some amount of dopamine is also secreted by neurons in the posterior pituitary.
Its most important stimulatory factor is thyrotropin-releasing hormone, which stimulates the production and secretion of prolactin but also of thyroid-stimulating hormone. Of course, prolactin secretion is also stimulated by pregnancy and breastfeeding. During breastfeeding, sensory receptors in the nipple get stimulated and they transmit the signal to hypothalamus. Hypothalamus inhibits the secretion of dopamine and prolactin is then secreted.
Negative feedback also plays a role because prolactin stimulates secretion of dopamine.
Pathophysiology
Prolactin's deficiency results, obviously, in no lactation.
Excess of prolactin, which can happen when dopamine does not work properly, for example, leads to infertility. This is because high levels of prolactin inhibit secretion of gonadotropin-releasing hormone in the hypothalamus, which stimulates secretion of luteinizing and follicle-stimulating hormone. And if there’s no luteinizing and follicle-stimulating hormone and oocytes cannot develop properly. Another possible symptom is galactorrhea, which means that milk unexpectedly leaks from the breasts.
References:
Costanzo, L. S. (2018). Physiology. Elsevier.
Hall, J. E., Hall, M. E., & Guyton, A. C. (2021). Guyton and Hall Textbook of Medical Physiology. Elsevier.